Naghma Farooqi, MD, FACOG

• Assistant Professor and Clerkship Director
• Department of Obstetrics and Gynecology
• Texas Tech University Health Sciences Center
• Lubbock, Texas

Under normal conditions hiv infection rate swaziland discount lagevrio amex, we are not conscious of the fact that we are upright in the same sense that we are conscious that the sky is blue antiviral drug for herpes purchase 200 mg lagevrio with amex, the birds are singing hiv infection rates sub saharan africa buy generic lagevrio on line, or a cool breeze is in the air early symptomatic hiv infection symptoms discount lagevrio 200 mg overnight delivery. It appears that our perception of equilibrium adapts extraordinarily quickly hiv infection rates by country 2011 order lagevrio line, vanishing from our consciousness even more rapidly than does awareness of the touch of our clothes. We also do not think anything of the steady visual image that we see from moment to moment. Yet, under pathological conditions, including alcohol intoxication, vestibular dysfunction creates disturbing perceptions that, when present, dominate our conscious experience and imperiously grab our attention. The sensory epithelium for each canal is contained in a swelling called an ampulla. The ampullae of the anterior (A-A), posterior (P-A), and horizontal (H-A) semicircular canals are labeled. The functional division between the canals and otoconial organs is of great clinical importance. The peripheral vestibular system processes information from the two types of end organs separately and differently. Therefore, peripheral vestibular lesions of the otoconial organs or canals can give rise to problems that are restricted to either disequilibrium or vertigo, respectively, as well as to a combination of the two. To understand the two prominent vestibular symptoms, let us back up and look at the two types of accelerating forces. Linear head acceleration is acceleration of the head along any linear path of motion. Because of gravity, static head tilt, which is simply the position of the head with respect to gravity, continuously stimulates the otoconial end organs of the vestibulum (more on this later). A: Shaking your head to indicate "No" represents an angular rotation in the yaw plane. The center of the angular rotation is the midpoint of the head, and this is therefore an on-axis rotation. B: the semicircular canals also respond to off-axis angular rotations, such as riding a merry-go-round, in which the center of the rotation is not located at the center of the head. Pathological conditions of the peripheral vestibular system, such as alcohol intoxication, create disturbing perceptions of disequilibrium or vertigo. Disequilibrium refers to an impaired postural stability that stems from dysfunction of sensations arising from the otoconial masses. An individual with disequilibrium does not feel in balance, even when standing, sitting, or even lying down. Disequilibrium produces postural instability and often results in injurious falls. Fear of falling and the uncertainty associated with feeling unbalanced lead some individuals with disequilibrium to retreat from the world and social interactions. Vertigo refers to a perception of spinning, either that the world is moving around a stationary individual or that the self is spinning within a stationary environment. On the other hand, dizziness is a term that people use to refer to an uncomfortable sensation of lightheadedness, the feeling that one is about to faint, or to vertigo. In fact, dizziness is frequently caused by problems outside of the vestibular system, particularly problems with the cardiovascular system such as hypovolemia. Simply getting up quickly can sometimes produce a completely benign lightheadedness that patients may report as feeling "dizzy. Despite the distinct definitions of dizziness and vertigo, lay people often use these terms interchangeably. It is important therefore to clarify what an individual means to convey by the terms used. Lightheaded patients should probably see a cardiologist, whereas recurring episodes of vertigo may indicate a problem with the vestibular system. Along with sensory disturbances, vestibular dysfunction is also tightly associated with autonomic and motor problems. One familiar example is the feeling of nausea that accompanies a perception of vertigo. As discussed later, the vestibular-to-autonomic connection is key to motion sickness. A common motor companion to vestibular dysfunction is nystagmus, an oscillatory eye movement that occurs normally under certain conditions but also frequently accompanies peripheral or central vestibular pathologies (see much more in Chapter 19). This involuntary and abnormal movement of the eyes in turn alters visual perception, producing oscillopsia. Recall from Chapter 5 that oscillopsia is a failure of gaze control in which the visual scene moves continuously with every eye movement. Movies that intentionally use this device for effect make many viewers feel nauseated. In oscillopsia, the moving visual image persists far longer than the 90­120 minutes of a film; the effect is not only disturbing but also debilitating. Both systems depend on hair cells, and information from cochlear and vestibular afferents travels within the eighth cranial nerve. The commonalities between cochlear and vestibular end organs are reflected in conditions that are best thought of as inner ear disorders. Such patients sometimes elect to undergo either a surgical ablation of the vestibular apparatus or injections of the ototoxic antibiotic gentamicin into the affected labyrinth in order to rid themselves of the vertigo. Many congenital forms of deafness also involve at least some degree of vestibular impairment. Inflammation of the inner ear, termed labyrinthitis, or of the vestibular branch of the vestibulocochlear nerve, vestibular neuritis, causes a rapid onset of vestibular symptoms such as vertigo, disequilibrium, and nausea and may also involve hearing loss and tinnitus. Labyrinthitis and vestibular neuritis are thought to be caused by viral infections in most cases, with symptoms typically subsiding in days to weeks. The starts, stops, and changes in the direction of head movements produce accelerations and decelerations that vestibular hair cells respond to . To describe head movements, we use Hertz (Hz), or cycles/s, as was the case for airborne sound. In the case of a force acting on the head, a 2 Hz head movement would be one in which your head moves back and forth twice within a second, returning to the starting position every half second. In humans, the dominant frequencies of head motion are below 10 Hz, about 2­3 Hz during normal walking, a glacial frequency range compared to that of human cochlear hair cells (20 Hz­16 kHz in a young adult). Whereas a cochlear hair cell bundle may have less than 50 microseconds to respond, a 10 Hz head movement moves a vestibular hair bundle in one direction for 50 ms, an ionotropic ion channel "eternity. First, vestibular hair cells retain a kinocilium, a tall cilium that polarizes the stereocilia bundle during development. In the cochlea, the kinocilium is only present transiently before regressing over the course of normal development. Loss of the kinocilium may serve to increase bundle stiffness and therefore enable responses to the high-frequency displacements that occur in the cochlea. In contrast, vestibular hair bundles are not as stiff and cannot move as fast as cochlear ones. Second, there is only a small endolymphatic potential in the vestibulum (about +10 mV) as compared to the endocochlear potential of +90­100 mV Thus, the driving force in vestibular hair cells. Finally, vestibular hair cells do not express the molecular motor protein prestin; no stimulus amplification occurs in the vestibulum. For example, because prestin is present in the cochlea but not the vestibulum, deafness caused by mutations in prestin occurs without any vestibular dysfunction. Although both types of hair cells are susceptible to damage from ototoxic drugs, some drugs such as the aminoglycoside antibiotic gentamicin affect vestibular hair cells more than cochlear hair cells. Recall that the kinocilium is a tall cilium located at the tall end of the hair bundle. Bending the hair bundle orthogonal to the preferred-to-nonpreferred axis has no effect on the tip links and therefore does not elicit a hair cell response. Deflection of the stereocilia toward the kinocilium is the preferred direction of stimulation and results in hair cell depolarization. Deflection of the bundle away from the kinocilium is the nonpreferred direction of stimulation and results in hyperpolarization. Hair cells do not respond to deflection of the stereocilia in the orthogonal direction (into or out of the page). During linear accelerations in the vertical plane, the sacculus is displaced either farther downward during upward acceleration or upward during downward acceleration (B). When the sacculus floats up, as occurs during a downward acceleration, the resting effect of gravity on the sacculus is relieved momentarily, resulting in a feeling of "weightlessness. However, during static tilt (D) or linear accelerations (E), the otoconial mass shifts, and the stereocilia are deflected. The deflection of the utricular stereocilia can be the same during a static tilt and a linear acceleration, as is the case in the examples illustrated in D and E. Additional input from the semicircular canals, somatosensory afferents, and motor centers are used by central vestibular neurons to disambiguate these signals. In both the sacculus and utriculus, there is a dividing line called the striola (red arrows in A and C). Recall that the otoconial mass serves as a weight, or load, that can be displaced by linear forces. This comparison illustrates the importance of having a heavy object to sense linear acceleration. Essentially, vestibular function requires that the otoconial masses have sufficient mass upon which gravitational and other forces can act. Each otoconial mass contains about 200,000 tiny weights called otoconia (Greek for "ear dust") that consist of calcium carbonate within a matrix of glycosylated proteins called otoconins. Critically, most otoconins are only made during fetal development, and the mature otoconial masses form at that time. Since no replacements for the primary otoconins are available after birth, the loss of otoconia is irreversible. Unfortunately, the supply of otoconia does not always last for a human lifetime as the otoconial masses degenerate with age. Decalcification of the otoconia and loss of fibrils leads to a progressive degeneration of the otoconial masses that starts by the sixth decade. Over time, the pits expand until small pieces of the otoconial masses form, break loose, and eventually float off. Because the degeneration of otoconia occurs preferentially in the sacculus, the sacculus eventually may lack sufficient otoconia to sense gravity in affected individuals. In essence, when an otoconial mass resembles a feather more than a stone, accurately sensing gravity is no longer a possibility. Endolymph, present throughout the lumen of the membranous labyrinth, bathes the hair cell bundles and surrounds the otoconial mass. With age or trauma, pieces of the otoconial mass can break off, irreversibly diminishing the otoconial mass. Mild to moderate loss of otoconia from the otoconial masses may have little effect on equilibrium. However, a total or near-total loss of the otoconial masses adversely affects balance and contributes to disequilibrium. Because of the finite supply of otoconia, degeneration of the otoconial masses is a progressive problem. As discussed earlier, disequilibrium is a debilitating state that can lead both to falls and to social isolation born of the fear of falling. It may be that impairment in balance secondary to otoconial mass degeneration is as inevitable or likely as other age-related changes, such as presbyopia and presbyacusis. If it were not for the bulkiness of the term, we might talk of "presbyequilibrium" as we do of presbyopia and presbyacusis. They are made of calcium carbonate within a proteinaceous matrix, and they continue to grow, accreting more mass throughout the lifetime of a fish. Thus, even though the mammalian otoconial mass and fish otolith both serve as a load that enables the detection of gravitational force, the difference between the singleedition otoconial mass and the ever-growing otoliths is substantial and of great clinical significance in aging humans. The displacement of endolymph stimulates canal hair cells, resulting in a perception of head rotation. Hair cell stimulation occurs preferentially during particular head movements that bring the dislodged mass in contact with vestibular hair cells and is thus dependent on head position. Treatment is aimed at maneuvering the head in specific trajectories so that the otoconial mass fragments move from the canals to the vestibule containing the utriculus and sacculus. Since the macular hair cells are embedded in the otoconial mass, they do not respond to free-floating otoconia. Thus, thanks to the constant of the gravitational force, otoconial end organs respond to the static position of the head, whether it be upright or tilted. The sacculus and utriculus also respond to linear accelerations of the head caused by self-motion, falling, or vehicular movement. During linear accelerations, the heavy otoconial mass lags behind the macula (think of a car ornament lagging behind as the car accelerates forward), so that the hair cell stereocilia are displaced. In the upright person, utricular hair cells are arranged in such a way that they respond to linear acceleration in any horizontal direction, meaning accelerations to either side as well as accelerations forward and backward. During an upward acceleration, as occurs during the upward phase of a jump, the otoconial mass lags behind, displaced even more than normal by gravity alone. By sensing the displacement of the otoconial mass induced by gravity and other linear accelerations, hair cells in the utriculus and sacculus signal both the static position of the head and linear accelerations of the head. The canals are fluid-filled tori (torus is the singular form), meaning shapes that resemble inner tubes. The three canals, oriented orthogonal to one another in three planes of space, are: · the anterior, or superior, semicircular canal · the posterior, or inferior, semicircular canal · the horizontal semicircular canal Each semicircular canal begins and ends in the utriculus. This sensory epithelium is the crista ampullaris, or simply the crista, where the hair cells are situated, the canal equivalent to the macula.

Because the correspondence between cranial nerves and cranial nerve nuclei is far from one-to-one average time from hiv infection to symptoms cheap lagevrio 200 mg overnight delivery, the symptoms that arise from peripheral (cranial nerve) and central (cranial nerve nucleus) lesions are different hiv transmission rates from infected female to male order lagevrio 200mg without a prescription. In order to appreciate how certain collections of symptoms could only arise from cranial nerve damage hiv infection circumcision buy lagevrio on line amex, the cranial nerve nuclei are introduced here (Table 5-3); further detail is provided in Chapter 6 kleenex anti viral tissues reviews generic lagevrio 200mg mastercard. Since the Edinger-Westphal nucleus is a close neighbor of the oculomotor nucleus hiv infection probability buy generic lagevrio online, damage to one of these often damages the other as well. These nuclei are not of great clinical import because they are not injured in isolation, and, when lesioned, they account for the most minor of the resulting problems. The remainder of this chapter details the function of each cranial nerve, including the forebrain ones, along with the potential clinical consequences of damage to each nerve. The olfactory nerve carries the entire output of the olfactory epithelium into the brain. No other route exists by which chemosensory information regarding airborne chemicals (aka odors) can reach the brain. Indeed, a common cause of olfactory nerve damage is whiplash from a car crash or sports injury. The quick back-and-forth movement clips the thin olfactory axons as they pass through the cribriform plate. However, typically this is not the case, perhaps because scar tissue blocks the nascent sensory axons from entering into the olfactory bulb. The appearance of anosmia, the inability to smell, or hyposmia, a reduction in smell detection and discrimination, in young people is usually due to a peripheral lesion acquired through trauma, viral infection, or chemical injury from smoke, nasal sprays, or the like. Since olfactory testing is not common, congenital patients may not become aware of their sensory deficit until reaching their teens or early twenties. Central changes to the modulatory control of olfactory bulb processing appear to be responsible for the impairment of olfaction in neurodegenerative diseases. In Kallmann syndrome, congenital anosmia is coupled with a failure to enter puberty and therefore reach sexual maturity. In this syndrome, the axons of olfactory sensory cells fail to cross the cribriform plate to reach the olfactory bulb. Hormonal therapy allows patients with Kallmann syndrome to mature sexually but they will remain unable to smell. Mutations in a number of different genes involved in the development of olfactory sensory neurons and axon guidance of olfactory nerve fibers may cause Kallmann syndrome. Patients with acquired anosmia rarely present with the complaint that they cannot smell. Flavor is a compound sensation that integrates the primary senses of taste, smell, texture, and temperature. Flavor is even heavily influenced by vision-hence the garnish that adorns plates presented by top chefs-and contextual cues such as lighting, music, conversation, and the social milieu. The extreme dependence of flavor on non-oral cues has been used to successfully fool food critics into raving about fast food presented as fine, organic food. It can cause weight loss-without smell, food becomes far less appetizing-or weight gain-eating more is required to reach a sensation of fullness-and even alternating weight loss and gain. Food preferences may also shift to involve hot and spicy ingredients that contribute to flavor by engaging somatosensory (trigeminal) rather than olfactory pathways. Finally, patients with acquired anosmia may experience phantom smells, which are typically unpleasant. The emotional toll exacted by anosmia on congenital patients is usually less than it is for those with acquired anosmia. However, missing a sensory modality that everyone else has can be challenging both socially and medically. Moreover, the condition is sufficiently rare that awareness is low and patients often encounter disbelief and even hostility from teachers, friends, and strangers. The reader can participate in creating a more understanding and compassionate future for anosmic patients. Recall that the retina and optic nerve develop from the optic vesicle, itself an outpouching of the diencephalon (see Chapter 3). Nonetheless, the optic nerve leaves the cranium and therefore meets the most critical criterion for a cranial nerve. The nomenclature involving these axons is peculiar as the very same axons from the retina are called by two different names at different parts of their trajectory. As these same axons continue past the chiasm to the thalamus, they are called the optic tracts. At the optic chiasm, axons from the nasal hemiretina (gray) cross the midline while temporal retinal axons (blue) bypass the optic chiasm, destined for the lateral geniculate body (lgb) on the same side. Between the optic chiasm and the lateral geniculate, retinal axons continue into the optic tracts, which travel next to the cerebral peduncles (cp), a major fiber tract at the base of the midbrain. Despite the different names, the optic nerves and optic tracts contain the same retinal axons. On the right, the visual consequences of damage to either visual or gaze control pathways are diagrammed for a person looking at an italicized H. Moderate to severe damage to the retina or optic nerve may impair vision when both eyes are open. The same impairment will occur when the affected eye is open alone but not when the affected eye is closed. Visual losses that affect one part of the world, viewed through either eye, are due to retrochiasmal damage. A common example of this is a scotoma, a small region of visual loss, illustrated here as a chunk missing from the H. A scotoma is present when either eye is open alone as well as when both eyes are open together. Visual problems that do not appear when either the left or right eye is open alone are due to a problem with gaze pathways; these problems produce double vision or diplopia. Note that patients may not report their symptoms in medically accurate terms, for example referring to diplopia as blurry vision. Since the bulk of the visible world is viewed binocularly, through both eyes, vision may remain largely intact with loss of only a small monocular crescent on the side of the lesion (see Chapter 15). Optic nerve damage, such as that caused by optic neuritis, an inflammation of the nerve often found in patients who either have or go on to develop multiple sclerosis, may cause blurred or reduced vision of the eye-and sometimes blindness-on the affected side. Instead diplopia results from a misalignment of the eyes due to a problem with the motor control of eye position (see more later). As will be explored in more detail in Chapter 7, visual loss that affects the ability to see part of the world, or visual field, with either eye results from a retrochiasmal lesion distal to the optic chiasm. The optic nerve carries optical information that serves functions beyond visual perception. Of great clinical import, the optic nerve carries sensory information that drives the pupillary light reflex. This reflex links changes in ambient luminance, the overall level of light, to pupillary diameter: increasing the pupil size when light levels dim and decreasing pupillary diameter when the amount of light increases. Flashing a light onto the eyes in this condition will elicit pupillary constriction. The reflex depends on the optic and oculomotor cranial nerves, along with midbrain circuitry. After introducing the oculomotor nerve, the logic of pupillary light reflex testing will be explained. The direction of gaze is tightly controlled by the brain through a process known as fixation. Light emanating from every spot in our visual field is refracted topographically onto spots on the retina. Before diving into gaze control, it is critical that the reader understands the precise meanings of fixate and focus. Unfortunately, focus is often used incorrectly and fixate is not in the common vernacular, although it should be. Thus, when an individual fixates on an object, light from that object is focused onto the retina. At rest, the eye sits in the neutral position, looking straight ahead with the pupil located in the center of the palpebral fissure, the visible part of the eye. Extraocular muscles move the eye in the horizontal, vertical, and torsional planes. A horizontal movement of the eye toward the temple is called abduction, and one toward the nose is adduction. Up and down vertical eye movements are termed elevation and depression, respectively. Torsional movements are rotations of the eyes around the anterior-posterior axis that goes through the eye; intorsion is a rotation of the top of the eye toward the nose and extorsion, away from the nose. A: Moving the eyes up and down, termed elevation and depression, involves rotation of the eyes around a horizontal axis. B: Rotating the eyes around the optic axis that goes through the pupil is called torsion, with rotating the top of the eye toward the nose termed intorsion and away from the nose, extorsion. C: Moving an eye in the horizontal plane is accomplished by rotating the eye around a vertical axis. Movement of the eye toward the nose is termed adduction, and moving it toward the temple is abduction. This vergence, or convergence, is disconjugate (the two eyes move in different directions) with the left eye moving right and the right eye moving left. Vertical (A) and torsional (B) movements are always conjugate, meaning that the two eyes invariably move in the same direction. Horizontal movements can either be conjugate (C) or disconjugate in the case of vergence (D). E: For viewing far objects located straight ahead, the eyes relax back to the neutral position. Even so, eye movements do more of the work in determining gaze than do head movements. Small gaze shifts are accomplished using eye movements almost exclusively, and even large gaze shifts depend primarily on eye movements (see Chapter 19). Six extraocular muscles, muscles that attach to the eyeball, are responsible for moving the eyes and directing gaze. Before delving into the functions of these cranial nerves, a detour into how the extraocular muscles produce eye movements is warranted. The medial and lateral recti not only have one action each, but they accomplish that action solo. The remaining four extraocular muscles-inferior oblique, superior rectus, superior oblique, inferior rectus-are more complicated because they all have secondary actions as well as a primary action. B: Testing the cranial nerves innervating extraocular muscles can be accomplished by having a person trace an "H" with his gaze. In the adducted eye, the oblique muscles serve as the elevators and depressors whereas in the abducted eye, the recti muscles do so. A: the rectus muscles arise at the annulus of Zinn and insert at sites that are anterior to the globe equator. The inferior oblique inserts posterior to the equator as does the tendon connected to the superior oblique (I). B­E: the pulling directions of the inferior and superior rectus muscles are angled with respect to the axis of the eye and are at a right angle to the pulling directions of the inferior and superior oblique muscles. D: When the eye is in the neutral position, rotation of the inferior and superior rectus muscles results in adduction as well as depression or elevation. Conversely, the inferior and superior oblique muscles abduct the eye in addition to their primary action of shifting eye position vertically. There are also torsional consequences, with superior oblique muscle contraction causing intorsion and inferior oblique muscle contraction causing extorsion. E: When the eye is abducted, the inferior and superior rectus muscles rotate the eye purely around a horizontal axis, whereas the inferior and superior oblique muscles cannot contribute to depression or elevation because their pulling directions are parallel to the horizontal axis of the eye. F: the medial and lateral recti cause simple adduction and abduction, respectively. G: the superior rectus elevates, adducts, and intorts the neutrally positioned eye. I­J: the superior oblique depresses, abducts, and intorts the neutrally positioned eye but is a strong depressor of the eye in the far adducted position. Since the recti insert at sites anterior to the equator of the globe, their pulling directions are relatively straightforward, true to the etymology of rectus (Latin for "straight"). The oblique muscles differ substantially from the rectus muscles in both anatomy and action. The upshot of this is that the superior oblique rotates the eye forward around the horizontal axis. The forward rotation around the horizontal axis is responsible for the counterintuitive inversion whereby the superior oblique muscle depresses the eye and the inferior oblique elevates the eye. The eyes move together, in conjugate, during all vertical (and torsional) eye movements and most horizontal eye movements. This means that if one eye moves up, the other eye also moves up; when one eye abducts to the right, the other also moves to the right but adducts to do so. Conjugate eye movements employ pairing an extraocular muscle on one side with a different extraocular muscle on the other side. To accomplish a horizontal gaze shift, the lateral rectus muscle on one side and medial rectus on the opposite side are paired together. We can thus consider that each lateral rectus muscle is yoked to the contralateral medial rectus muscle. They are: · superior rectus + contralateral inferior oblique · inferior rectus + contralateral superior oblique For example, when looking down and to the left, the left superior oblique and the right inferior rectus are contracted together. The yoked movements of the three pairs of extraocular muscles support all conjugate eye movements.

However hiv infection in mouth lagevrio 200 mg on line, movement of the endolymph in the canals is opposed by frictional forces arising from the viscosity of the endolymph and the elasticity of the cupula antiviral used for h1n1 order lagevrio 200mg with amex. These opposing forces cause the movement of the endolymph and cupula to lag head acceleration and to approximate the velocity of the head hiv stories of infection discount lagevrio generic. Thus hiv infection mayo clinic cheap lagevrio online visa, vestibular hair cells require head acceleration to detect movement of the cupula but code for head velocity hiv infection when undetectable cheap 200 mg lagevrio otc. Another way to view this is that the lag of the endolymph and cupula integrate the acceleration stimulus into a velocity signal. Similarly, macular hair cells in the otoconial organs only respond to linear accelerations but have responses that are proportional to velocity. That hair cells only respond to acceleration is evident by the lack of any vestibular sensation during constant velocity motion, such as occurs in a car or a plane cruising at a constant speed. In contrast, during takeoff and landing of a plane and during car accelerations and decelerations, a clear sensation of slowing down or speeding up is perceived. Application of this type of movement decomposition into component vectors can be extended from exclusively rotational movements to truly natural head movements, which typically include both linear and angular acceleration. In this way, vestibular end organs respond separately to the individual vector components, angular and linear, that make up natural movements. A: A pitch forward (black arrow) is equal parts forward movement in the right anterior semicircular canal­left posterior semicircular canal plane (red) and forward movement in the left anterior semicircular canal­right posterior semicircular canal plane (blue). B: A roll right (black arrow) is equal parts forward movement in the right anterior semicircular canal­left posterior semicircular canal plane (red) and backward movement in the left anterior semicircular canal­right posterior semicircular canal plane (blue). The rightward pivot illustrated is composed of a rightward translation and a clockwise rotation. These movements evoke responses in hair cells in the utriculi and the horizontal semicircular canals. It is these primary vestibular afferents that take information from the inner ear to the brain. This depolarization results in constitutive release of glutamate, the hair cell neurotransmitter, which in turn activates primary vestibular afferents. Thus, vestibular afferents discharge at fairly high rates, in the range of 50­100 spikes per second, even when the head is not moving and even in the canals that do not respond to the everpresent gravity. The elevated resting potential that is found in inner ear hair cells allows for bidirectional sensory coding. Accelerations in the nonpreferred direction hyperpolarize hair cells and result in less glutamate release from the hair cell. Of course, hair cell depolarization, as occurs with accelerations in the preferred direction, increases the rate of glutamate release and therefore elicits an increased discharge rate in the vestibular afferents. This is useful particularly since the preferred and nonpreferred directions are equally meaningful. Bidirectional sensory responses are also present in vision, where dark and light are both informative. Resting discharge and therefore bidirectional responses are notably absent from the somatosensory system, which in turn accentuates deviations from the background quiet as notable. As with cochlear hair cells, vestibular hair cells respond to stimulation with graded potentials (blue line). As diagrammed on the left, hair cells release glutamate from a specialized type of active zone called a ribbon synapse (black) onto postsynaptic vestibular afferents (red). When the hair bundle is in the neutral position (top row), the hair cell membrane potential is about -50 mV (middle row), and vestibular afferents have a resting discharge of roughly 50­100 spikes per second (bottom row). In response to a stimulus that deflects the stereocilia in the preferred direction, the hair cell depolarizes and releases more glutamate. In contrast, in response to a stimulus that deflects the stereocilia in the nonpreferred direction, the hair cell hyperpolarizes and releases less glutamate than at rest. The depolarized rest potential in the hair cell and the resting discharge of the vestibular afferent enable the vestibular system to respond to stimulation in opposing directions. Vestibular afferents project to neurons in four vestibular nuclei within the hindbrain. A small number of vestibular afferents also project directly to the cerebellum, the only sensory afferents to enjoy such privileged access to the cerebellum. The hindbrain targets of vestibular afferents serve several functions that revolve almost exclusively around motor control. We may notice a warm sunny day or a beautiful bird singing a melodic song, but gravity and rotational forces are not prominent in our conscious life. Reflecting this reality, vestibular pathways to neocortex exist but are dwarfed by vestibular pathways to motor control centers and motoneurons. In contrast to its miniscule role in perception, the vestibular system plays a central and absolutely essential role in everyday motor control. As terrestrial animals, we battle gravity constantly in order to stay upright and keep the horizon horizontal. As will be elaborated in the next chapter, our mechanism for keeping gaze steady depends on vestibular rather than visual input. The words that best describe vestibular sensory functions (balance, equilibrium, steady) are all borrowed from the motor side. It is impossible to describe the feeling of falling in the same way that you could describe the color of a visual object, timbre of a musical note, or texture of cloth. Thus, despite the sensory systemstyle connections from the inner ear to the brainstem, the vestibular system serves a primarily motor function, guiding and modifying all movements in a healthy individual. Essentially, this reflex allows us to read as we walk, ride public transit, or sit in a moving train. As it turns out, central vestibular neurons get information about slow accelerations from the visual system rather than the inner ear. Midbrain neurons that receive retinal input respond to the overall movement of the entire visual field, or optic flow. For example, as you look out the window of a slowly moving vehicle, the world is not stationary but rather flows by, opposite to the direction of travel. Importantly, central vestibular neurons sum inner ear and optic flow inputs without distinguishing between them. Because of this free-for-all mixing, a slow (vestibular) acceleration of the self is simply no different (to a vestibular nuclear neuron) from a slow (optical) acceleration in the world. A common example of this is experienced when sitting in a train at a station; we are often fooled into thinking that our own train has begun to move and is accelerating ever so slightly when, in fact, the train on the next track is slowly moving. Throughout past evolutionary time, adult terrestrial animals traveled only when self-propelled. This situation has changed with the inventions of boats, trains, automobiles, and planes. However, the vestibular system has not evolved to catch up with the recent development of passenger vehicles. Consequently we are prone to suffer the affliction of motion sickness, which only occurs when one is moved through space rather than propelling oneself through space. Essentially, each motor command is accompanied by a message to the vestibular system to ignore the vestibular consequences of the intended movement. Along with the signal that initiates the twirling movement, motor control centers also send a message to suppress the excitation of vestibular nuclear neurons arising from the right horizontal canal. In other words, the predictable sensory consequences of an intentional action are suppressed. In the absence of a motor signal cancelling anticipated sensory inputs, head movements elicit responses in central vestibular neurons. For example, consider traveling through urban streets, replete with potholes, during the stop-and-start traffic of rush hour. The vestibular stimulation inherent in traveling this course would be canceled in a person walking or running through the streets. However, when a person traverses this same terrain as a vehicular passenger, innumerable accelerations and decelerations in vertical and horizontal planes gain access to central vestibular neurons. Consequently, the overall message from the vestibular system is one of head motion, and lots of it. This creates a mismatch between information from the vestibular system-lots of movement-and information from the visual system-the world is steady. When traveling on Earth, the mismatch involved is primarily between visual and vestibular signals, as just described. In space, mismatch between the various vestibular end organs occurs because of the microgravity environment. Additionally, somatosensory inputs, such as those along your back as you lie down, are altered in gravity-free space. The toxin detector hypothesis holds that, across evolutionary time, vestibular input has always gone haywire in response to the osmotic changes caused by ingested neurotoxins. For example, afferents from the left and right horizontal canals may discharge together after food poisoning. Accompanying the unpleasant and unnatural feeling of vertigo, there is an evolutionarily adaptive reaction of nausea and emesis (vomiting). Although everyone can experience motion sickness given severe enough conditions, some are more susceptible than others; the reason for the individual variation in susceptibility is not known. The nonpharmacological strategy to avoid motion sickness focuses on avoiding near fixation during passenger travel. This approach yields a moving visual image that is more in line with the vestibular input of motion. The most common pharmacological treatment for motion sickness is directed at the guts rather than at the brain or vestibulum. Scopolamine, a muscarinic receptor antagonist, reduces gastrointestinal motility while also blocking accommodation, nasal and oral secretions, and causing some degree of drowsiness. The lessons regarding sensory mismatch gained from thinking about motion sickness can inform our understanding and even treatment of vestibular disorders. Consider an older individual who experiences episodes in which she feels that objects rotate around her when she is still and even when her eyes are closed. Walking feels unsteady, as though the ground has been transformed into clouds that give way with every step. One simple way to ameliorate these symptoms is to provide a sensory input, beyond vision, that the world is steady. Thus, a person suffering from feeling unsteady can gain a perceptual advantage, beyond any physically stabilizing benefit, by touching a fixed object such as a wall. Slow accelerations do not excite vestibular afferent but are conveyed by midbrain neurons responsive to optic flow into the vestibular nuclei. And input arising from motor control centers during selfgenerated movements cancels the message from predictable vestibular inputs. Another difference between vestibular afferents and vestibular nucleus neurons is that only the latter can integrate input from more than one end organ. This is useful because the input from only one vestibular end organ or even from an end organ pair can be ambiguous. In this situation, vestibular nucleus neurons cannot use input from both the utriculi alone, but combined input from the sacculi and utriculi are informative and can be used to distinguish between the two stimuli. Somatosensory afferents, particularly from the neck, can also aid in distinguishing between ambiguous vestibular signals. In our example, neck proprioceptive input to the vestibular nuclei differs when the head is tilted versus being upright. A deceleration at constant altitude has virtually the same effect on vestibular afferents as does an upward climb; this ambiguity is just one of a myriad that can occur in an airplane. When the horizon is not visible, pilots can fall victim to spatial disorientation, and each year a number of pilots of small planes crash for this reason. First, visual perception is far more reliable than vestibular perception; without the former, we are at a disadvantage. Second, somatosensory inputs are normally too weak to counteract a vestibular illusion. And yet, despite its ambiguity, vestibular perception powerfully drives reactions, which can transform a minor confusion into a major and uncorrectable crisis within seconds. The only defenses against disorientation are flying during clear conditions or expertise at reading and using instruments to fly. Projections to the cervical ventral horn are important in coordinating head and shoulder movements needed for large shifts in gaze. Projections to the spinal ventral horn at all levels are critical to maintaining postural balance. Moreover, vestibular projections to the cerebellum also contribute to the motor coordination of postural balance and eye movements with head position and body movement. The secondary sensory neuron in the visual system that corresponds to a vestibular nucleus neuron is in the retina. Of course, no retinal (or geniculate or visual cortex) neuron projects to a motoneuron or even to a motor control center. The contrast between visual and vestibular pathways highlights the extreme motor emphasis of the vestibular system. Under normal, all-is-well conditions, the output of the vestibular system is entirely motor in character. However, when vestibular well-being is challenged, vestibular perception comes to the fore, along with autonomic distress. Putting on glasses with a poor prescription, spinning, or diving from a high board all can elicit an unpleasant perception of vertigo or disequilibrium. From thalamus, vestibular information reaches regions near the head representation in somatosensory cortex. Stimulation within "vestibular" cortical areas most frequently produces a sense of movement, imbalance, or vertigo, more evidence that vestibular perception is restricted to impaired conditions. Vestibular pathways also reach brainstem and cortical regions involved in homeostasis.

This communication typically occurs across long distances through the use of patterned sequences of action potentials hiv infection rates nyc lagevrio 200 mg low cost. Action potentials are then turned into chemical messages through the release of packets of neurotransmitters released upon the arrival of the action potential trigger antiviral shot cheap 200mg lagevrio otc. We also consider how the packets of neurotransmitters are formed and the mechanisms that terminate the actions of neurotransmitters so that every neural message has an end as well as a beginning hiv infection rates louisiana order discount lagevrio. Finally hiv primo infection symptoms buy lagevrio us, we consider how the binding of neurotransmitters to receptors on target cells is transformed into a received message hiv infection and seizures order lagevrio on line amex. Our treatment of neuronal communication takes us full circle, from the integration of inputs to sending a message, and, finally, to receiving that message. In the end, we stand in awe of the diverse electrochemical and molecular mechanisms that allow neurons to metaphorically whisper or shout, to cover their ears or turn up the volume on headphones. Here, we describe the electrical properties of the neuron under resting conditions. Neurons return again and again to this default, but critical, electrical potential. A failure of neurons to maintain an adequate resting membrane potential results in a complete disruption of function. In this chapter, we consider subthreshold synaptic inputs that cause graded responses but do not cause a neuron to fire an action potential. For those needing a brief reminder of fundamental principles and terms of electricity, consider the following analogies: · Electrical potential or voltage: Water at the top of a tall waterfall can fall a long way and thus has a large amount of potential energy. Water in a land-locked lake has no potential and thus represents zero potential or the ground state. Note that electrical potential, symbolized as E, and voltage, symbolized as V, are synonymous for our purposes. For this reason, firefighters use wide hoses rather than garden hoses to put out fires. In neuronal processes, resistance is inversely proportional to the caliber (diameter) of a dendritic or axonal process. As a very rough approximation, consider that water in one lake must fill a bucket before entering a stream. The water will reach the stream more slowly if a large intermediary bucket must be filled than if a small one needs to be filled. Thus, the large bucket transfer system has a higher capacitance than does the small bucket one. Capacitance impacts the rate of charge transfer-or water transfer in this case-but does not change the eventual outcome: given enough time, all the lake water will make its way into the stream, regardless of whether it is transferred by a small or large bucket. Although this analogy fails under scrutiny, the important point to remember is that voltage changes slowly across a highcapacitance membrane and rapidly across a low-capacitance membrane. Membranes that surround cells, including neurons, termed cellular or plasma membranes, separate the inside of a cell, the intracellular compartment, from the outside or extracellular space. The hydrophilic heads of the lipids face either the extracellular space or the intracellular cytosol. The action of specific enzymes, including flippases and scramblases, can alter the composition of the inner and outer leaflets, or layers, so that different lipids predominate in the inner leaflet (the one bordering the cytosol) and the outer leaflet (bordering the extracellular space). Two layers of tail-to-tail aligned lipids form a bilayer, so that a wide hydrophobic core is bounded on either side by shallow hydrophilic borders. B: Flipases and scramblases are membrane enzymes that move lipids between the two layers or leaflets of a biological membrane. As a result, the two leaflets of a plasma membrane typically contain somewhat different lipid compositions (not illustrated). Ultimately, membranes prevent the free diffusion of charged molecules, anions (A­) and cations (C +) while allowing gases and amphiphilic molecules, compounds that have both hydrophilic and hydrophobic regions, to freely move between the separated compartments. The hydrophobic core of biological membranes, formed by lipid tails, repels charged molecules. In contrast, gases and small amphiphilic substances, such as fats, cholesterol, and-importantly-most general anesthetics, diffuse easily through biological membranes. Although charged molecules cannot move across lipid bilayers, they do move across biological membranes through several specialized routes that extend into both the extracellular and intracellular compartments. The routes are formed by membrane proteins, proteins that are anchored within the membrane and span the bilayer. Typically, multiple protein subunits complex together and extend across the lipid bilayer to provide routes through which ions can cross a lipid bilayer or membrane. The pore is typically selective, so that only a particular ion or set of ions, distinguished by size and/or charge, passes through. Channel-opening is triggered or gated when the voltage difference across the membrane reaches a certain value (in the case of voltagegated channels) or when a ligand, such as a neurotransmitter, binds to the channel (in the case of ligand-gated channels; see later discussion and discussion in Chapter 13). Ligand-gated channels are also called ionotropic receptors; the two terms are synonymous. If one thinks of a channel as a door, then gating refers to the mechanism that opens that door. Ligand-gated channels are analogous to doors that open with a key, whereas voltage-gated channels can be thought of as swing or pocket doors that open with force. An ion channel, a transporter, and a gap junction, three different types of membrane proteins, are shown in cross section. In the closed configuration (left), ion channels do not allow ion movement across the membrane. In the open configuration (right), ion channels form a pore that allows ions to cross between the cytosol and the extracellular space. B: Transporters move ions and other small molecules across the membrane without ever forming a membrane-spanning pore. There are several different types of molecular transporters, only one of which is illustrated. C: Gap junctions form a conduit between the inside of two different cells (in this case, cell 1 and cell 2) through which a variety of ions (small dots) and large molecules (larger black stars) can move. At the site of a gap junction, the membranes of the two cells involved are closely juxtaposed, being separated by about 3 nm rather than the normal synaptic gap of 30 nm or so. Complementary membrane proteins, termed connexins, in the two cells join to form an actual pore. Unlike channels, transporters never form a pore that stretches from inside to outside the cell. Using a variety of mechanisms, transporters transfer molecules across the lipid bilayer membrane. In other words, a gap junction pore stretches from the cytosol of one cell to the cytosol of a closely adjacent cell. In the nervous system, ions, metabolites, and signaling molecules may all pass through gap junctions. The size and characteristics of gap junction channels can be modulated so that, under different circumstances, the same gap junction may pass nothing, only ions, or large signaling molecules as well as ions. Since gap junctions are physical connections, molecules cross from one cell to another virtually instantaneously. As with the rest potential of glial, epithelial, muscle, blood, and other types of cells, the potential of a neuron at rest is negative with respect to the extracellular fluid. To understand why the mechanisms matter so much more than the actual membrane potential, consider a nightclub at two different times: just before opening and just after closing. The inside of the club boasts the same number of workers and the same lack of patrons at both times. However, just after closing, the workers are tired, the liquor depleted, and the line of people outside is gone, whereas at opening, the workers are fresh, supplies replenished, and a long line of energetic people are lined up outside. We could say that the nightclub is in the same empty state at both times but that would not accurately reflect the huge difference between the nightclub at the two times. At opening time, another tick of the clock will usher in a noisy, crowded, vibrant social scene, a scene that is an impossibility after closing time. These mechanisms possess special significance because they determine the degree to which a neuron defends its resting membrane potential even in the face of inputs that cause deviations. Cells that deviate easily from rest potential can reach the threshold for an action potential quickly and thus are highly excitable, whereas those that deviate briefly and rarely from rest, nearly always returning quickly to the resting membrane potential, are far less excitable. Two neurons with the same rest potential, supported by different electrochemical mechanisms, may have very different levels of excitability. For this reason, it is important to not only remember that the resting membrane potential of most neurons is -60 to -70 mV but also to understand the electrochemical forces contributing to that rest potential, our task in this chapter. These ions include two positively charged ions, or cations, potassium (K+) and sodium (Na+), and one small anion, chloride (Cl-). Each of these ions is differentially distributed across the membrane of a neuron, with potassium ions more prevalent inside the cell and sodium and chloride ions more prevalent outside. When a cell is at rest, each ionic species exists in steady state, with the same number of ions leaving the cell as entering it. Before considering how all three ions arrive at steady state, we consider how just one ionic species, K+, reaches electrochemical equilibrium. However, since neurons are negative with respect to ground, electric forces attract potassium ions inward. Thus, chemical and electrical driving forces oppose one another, with the electrical driving force pushing potassium ions to the negative side, which is the intracellular side of the membrane, and chemical forces pushing potassium ions to the extracellular side where the potassium ion concentration is lower. In the case of potassium ions, the ionic concentration inside cells is roughly 30-fold higher than that in the extracellular fluid. Since cells are negative with respect to ground, electrical forces push the positively charged potassium ions in. If we consider potassium ions exclusively, the steady state potential predicted by the Nernst equation is about -92 mV. The electrical potential (E), where the chemical and electrical forces on any given ionic species, X, are exactly opposing is given by the Nernst equation. By calculating the value of a term encompassing several constants at human body temperature, the Nernst equation can be simplified to: where [X]0 and [X]i are the extracellular and cytosolic concentrations of the ionic species in question. For potassium ions and other monovalent cations, the valence is +1, and, for monovalent anions such as the chloride ion, z = -1. This brings us to an expression of the Nernst equation for potassium ions: where [K]0 and [K]i are the extracellular and intracellular concentrations of potassium ions. If we plug in physiological values for [K]0 and [K]i, 5 and 155 mM, respectively, we can solve the Nernst equation for potassium ions: Thus, when the neuron is at a potential of -92 mV the Nernst potential for potassium, ions-the electrochemical gradient for potassium ions-is at steady state, with the same number of potassium ions leaving as entering the cell. Let us now use our understanding of the rest potential to predict the consequences of an elevation in extracellular potassium ion concentration on neuronal membrane potential. Kidney failure, certain congenital conditions, or a number of drugs can cause hyperkalemia. Elevation of the extracellular potassium ion concentration has one critical consequence. The decrease in the chemical gradient results in more potassium ions entering than leaving the cell: until a new steady state potential is reached. As earlier, more potassium ions would enter than leave the cell until the electrical and chemical gradients once again were exactly opposite. For example, if [K]o were raised from a normal value of 5 mM (range of normal values is 3. Above the reversal potential, potassium ions leave the cell, and below the reversal potential, the net flow of potassium ions reverses, so that potassium enters the cell. Thus, when the membrane potential is more polarized (from ground), or hyperpolarized, than -92 mV, potassium ions enter the cell; when the membrane potential is less polarized, or depolarized, than -92 mV, potassium ions leave the cell. Using these two principles, we can predict the so-called passive responses of a cell, which are any responses that do not involve an action potential. The preceding only holds when potassium ions are the exclusive ionic species that crosses the membrane at rest, as is true of astrocytes and cardiac muscle cells. However, as we shall see in the next section, neurons at rest are permeable to sodium and chloride ions as well as to potassium ions. To understand the neuronal resting potential, we must take into account all three ionic species with permeability. The probability that a voltage-gated ion channel is open depends on the membrane potential. Only ion channels that open at rest potentials contribute to the resting membrane potential. For example, ion channels that allow potassium ions to pass have a high probability of opening at the astrocytic resting membrane potential. In contrast, there is no chance that ion channels that allow sodium ions to pass will open at the resting membrane potential of an astrocyte. Thus, potassium ions contribute to the resting membrane potential of an astrocyte, and sodium ions do not. In addition to channels permeable to potassium ions, channels permeable to sodium and chloride ions may also open at rest potential. Thus, the resting membrane potential, the default potential of a cell, depends on two factors: · the ionic species to which a neuronal membrane is permeable at rest: Permeability depends not only on the presence of ion channels through which an ion can pass but also on the conformation of the ion channel. As an analogy, there may be many doors into a nightclub, but if they are all locked and thus impermeable, then no one gains entrance. As mentioned earlier, three ionic species-potassium, chloride, and sodium-permeate neuronal membranes at rest. The most definitively established resting permeability values come from experiments using a very large axon found in the squid. For the squid giant axon, the permeabilities of potassium, chloride, and sodium ions are: Another way to view this is that potassium ions carry 60­70% of the current in a typical resting neuron, chloride ions carry 25­35%, and sodium ions carry only about 3­4% of the current. The influence of sodium ions on the resting potential is substantial despite the relatively low permeability of sodium ions at rest. This is because of the very large driving force that results from the positive Nernst potential for sodium ions (+67 mV). The number, selectivity, and conformational state of ion channels limit the movement of ions across the cell membrane just as the number, type, and state of doors limit access to a room.

Experimental animals left with only subcortical structures still seek and consume food hiv infection graph 200 mg lagevrio otc, groom themselves hiv infection symptoms nhs lagevrio 200 mg visa, and even take care of their offspring symptoms of hiv infection cheap lagevrio 200 mg overnight delivery. Thus anti viral ear drops buy lagevrio us, subcortical inputs to the basal ganglia are sufficient for many of the actions that mammals perform every day hiv infection symptoms after 2 years order discount lagevrio on-line. In fact, most of our daily activities are packages of motor behavior that have been chunked by the basal ganglia and are performed by rote without cognitive oversight. The notable exception to this rule is that eye movements are modulated by the oculomotor loop of the basal ganglia. In this chapter, we focus on the skeletomotor loop through the basal ganglia before briefly discussing loops through the basal ganglia that influence cognitive and emotional function. In order to select an action, the basal ganglia utilize information of three major types: · Efference copy that provides a running record of current and imminent actions, as well as those in the immediate past (see Chapter 24 for a refresher on efference copy if needed) · Sensory, cognitive, and affective information that conveys the current internal and external circumstances, as well as memories associated with those circumstances · Urgency or saliency of different actions, including the action being currently performed Efference copy from currently selected actions heavily biases basal ganglia selection. Thus, without a compelling reason to stop, we typically continue doing what we are currently doing. A person reading the newspaper at one moment is astronomically more likely to be reading the newspaper than to be doing sit-ups or any other action besides newspaper-reading a minute later. In this way, efference copy allows for behavioral continuity in our actions, lending great advantage to the current action over any other potential action. Of course, behavioral continuity also supports biological inertia, favoring the continuation of ongoing activity-or inactivity, as the case may be-for hours. In this regard, think of the great effort needed to start an activity, such as writing a term paper, in contrast to our ready ability to while away an entire afternoon continuing to "do nothing. By biasing action selection heavily toward the currently selected action, efference copy input to the basal ganglia promotes sticking with an action for as long as it takes to complete the action while also preventing us from perseverating with an action already completed. Consider how grocery clerks would behave if, instead of working 8-hour shifts, they worked 1,000-scan shifts; viz. One can imagine that, working under this regime, many more clerks would quickly train themselves to group scanning items into a chunk. Most people are far more likely to go bike-riding on a sunny day than on a rainy one, when their legs feel strong rather than when experiencing muscle cramps. We pick fights when angry or frustrated much more often than when we are happy or even sad. In sum, the solution to every instance of action selection-should I do a cartwheel, lift weights, or play with my cat The selection criteria employed by the basal ganglia are based on operational learning, an unconscious associative process continuously executed by the basal ganglia. Operational learning is the process by which we learn to associate actions with the immediate consequences of those actions. Operational learning is also called instrumental, procedural, operant, or reinforcement learning. It is an implicit type of learning in which we learn the effects of self-generated actions. This information is used to bias behavior toward actions that produce positive effects over those that cause negative effects. In this type of learning, an individual acts and then learns the consequences of that action. A cat meows in the morning and is fed, a child who cries is picked up and comforted, a person walks outside with bare feet and sustains a foot injury, a basketball player does not play defense and is benched. Thus, actions that produce favorable outcomes recur, and those that produce immediately adverse effects are rarely repeated. Operational learning works over a very short time frame so that the immediate consequences of an action are the only consequences taken into account. Consequences that occur some time later are not associated with actions through operational learning. Thus, a rat that presses a lever and receives an intravenous bolus of cocaine a second later learns to associate pressing the lever with the immediately positive feeling produced by the cocaine. Indeed, rats do just that and may press a lever for a drug such as cocaine to the exclusion of eating and other critical survival behaviors. Operational learning does not take the "long view" but rather continues to favor continued lever-pressing over the wiser choice of eating, drinking, and sleeping. Once drug-taking becomes a habit, it is no longer constrained by contingencies, as described earlier. An individual who wants to unlearn the ultimately counterproductive behavior of drug abuse faces a difficult biological obstacle. Rules learned through operational learning become the criteria and provide the evaluative structure that allows for selection between potential actions. However, after learning this operational rule, the only unexpected outcome would be not hearing a sound after pressing a doorbell. One idea posits that a phasic burst of dopamine release accompanies unexpected sensory events and facilitates the association between the preceding motor command and the resulting sensory outcome. All predictions emerging from operational learning exist unconsciously, although some rise to conscious levels as well. Some lessons derived from operational learning are both realistic and critical to survival-pet cats learn through operational learning that if they meow loud enough, they will be fed. Other lessons derived from operational learning are more hopeful than realistic-every time I touch my hand to my ear and then rub my chin, I make a foul shot. Because of the highly subjective nature of operational learning, the basal ganglia do not always make wise choices that optimize benefit and minimize danger. One person having fond memories of beach vacations may excitedly jump into the ocean to swim and play whereas another person, remembering a past pummeling by surf, runs from every incoming wave, frightened to even wet her feet. In sum, memories and associations formed by operational learning, more so than sensory details, tip the scales for or against candidate actions. For example, in the oral region of the putamen, neurons might receive inputs from various cortical regions involved in generating a smile of enjoyment, a smile used for greeting, whistling, and kissing, all actions that require mouth muscles. As should be clear by now, only one of the mouth movements listed can occur at a time, and the basal ganglia select the winner. In each graph, the amount of movement (y axis) for a number of discrete action possibilities (x axis) is plotted. Actions that occur simultaneously are typically both well practiced and use different muscles, for example, walking and chewing gum. B: When a high-priority action possibility arises, the first pathway engaged is the hyperdirect pathway. C: Immediately following the hyperdirect pathway, the direct pathway is engaged, leading to focal disinhibition of a salient action. D: Indirect pathways provide an annulus, or donut, of inhibition around the chosen action. Thus, the indirect pathways sharpen the disinhibition produced by the direct pathway. Dopamine facilitates the direct pathways (upward blue arrow) through D1 receptors and exerts a net inhibitory modulatory effect (downward blue arrow) on indirect pathways mediated by D2 receptors. I list the three pathways here with their general function in action selection; subsequent sections describe each pathway in more detail: · the hyperdirect pathway through the subthalamus provides a global stop signal that stops all movements. In addition to these three pathways, local circuits are instrumental in shaping the final output of the basal ganglia. Local circuits within the striatum, internal globus pallidus, and substantia nigra pars reticulata use lateral inhibition to facilitate the bids of leading candidates while inhibiting loser candidate actions-the neural equivalent of jumping off the sinking ship to get on the bandwagon. Lateral inhibition opposes indecision in action selection, preventing us and other animals from spending unproductive time deciding between candidate actions of nearly equal weights. Recall that neurons in the substantia nigra pars compacta are dopaminergic neurons that project to the caudate and putamen as the nigrostriatal dopamine pathway. Centered on the midline of the midbrain, between the left and right substantia nigra, is the ventral tegmental area that contains another group of dopaminergic cells. Dopaminergic cells in the ventral tegmental area supply dopamine to the nucleus accumbens (often called simply accumbens), also known as the limbic striatum. The accumbens occupies the ventral region present at the rostral pole of the striatum. Dopamine arising from the ventral tegmental area is released in the nucleus accumbens. This release is thought to be critically important to naturally rewarding stimuli such as food, drink, and sex. The nigrostriatal dopamine pathway from substantia nigra pars compacta to the striatum is critically important to movement. Dopaminergic nigral cells densely innervate the striatum, firing tonically all the time. The tonic release of dopamine within the striatum is necessary for movement just as oil is required for an engine to run. Mice engineered with an inactivated tyrosine hydroxylase gene do not make dopamine; these mice do not move around, do not feed, and die within a day or so of birth. Yet these mice breathe, evidence that dopamine affects certain movements but not others. Dopamine strongly promotes movement in a graded fashion: more dopamine promotes more movement. Moving less than normal is termed hypokinesia, whereas a person who stops moving altogether, becoming "frozen," exhibits akinesia (see Chapter 7). Treatment with the dopamine precursor, L-Dopa, increases dopamine levels (see Chapter 12) and facilitates basal ganglia­mediated movement in both dopamine-deficient mice and parkinsonian patients. Just as a lack of dopamine causes akinesia, surplus dopamine causes excess movements. As a consequence, drugs that artificially increase the tonic level of dopamine, such as amphetamines or cocaine, greatly increase motor activity. Users appear jumpy and, because of this excess motor activity, are readily perceived as being "on something. After receiving long-term treatment with dopamine receptor antagonists commonly used to treat psychiatric diseases, patients develop receptor supersensitivity to dopamine that operationally facilitates dopaminergic transmission. In addition to tonic firing, dopaminergic cells fire with a phasic burst of activity when something unexpected-a sound, a light, a touch, and so on-occurs. This phasic burst of activity plays an important and still incompletely understood role in basal ganglia learning, including assignment of reward value. This type of compulsive behavior, termed punding, takes a wide variety of expressions from taking apart and putting back together flashlights to weeding without breaks even to relieve oneself. Changes in therapeutic management are used to reverse these unfortunate side effects. For our purposes here, it is important to recognize that (1) dopamine is absolutely required for goal-directed movements, and (2) dopamine facilitates the learning of motor sequences as chunks, as well as the modification of those chunks according to changing circumstances. In contrast, inputs from motor cortex to the striatum are conducted through unmyelinated axons. Therefore, motor cortex excites the subthalamic nucleus before it reaches striatum. This difference in timing gives rise to the term hyperdirect in comparison to the direct pathway that depends on motor cortex projections to striatum. Subthalamic neurons are glutamatergic, the only glutamatergic neurons within the basal ganglia collective. A: Neurons in somatomotor cortex (M1) are normally inactive but discharge before initiating a movement. Since thalamic cells project to somatomotor cortex, the inhibitory effect of the hyperdirect pathway on thalamic firing is passed on to motor cortex. In sum, the hyperdirect pathway has an immediate but shortlasting effect of global suppression of somatomotor cortex. Activity in the hyperdirect pathway opposes action, providing a global interrupt signal. Such an interrupt signal would be useful for interrupting an action for any number of reasons. Perhaps something else really important, such as a mosquito bite screaming to be scratched, has suddenly come up. Or a jogger sees a careening bicyclist approaching and does a full stop to avoid a collision. A clerk stops scanning items when a customer asks a question or a supervisor calls out his or her name. A global stop signal may also be helpful in switching from one movement to another. Starting a movement from a state of not-moving is far easier than switching from one movement to another. For example, to switch from running to skipping is far more difficult than initiating skipping directly from standing. As a region that has the ability to make a "full-stop," the subthalamic nucleus biases decisions away from fast, impulsive actions. When the subthalamic nucleus is active, more support for a candidate action is needed in the direct pathway. Compulsive gambling, shopping, eating, punding, and sexual activity have all been reported in patients who showed no previous predilections for such behaviors. The emotional and financial cost of these behaviors to affected patients and their loved ones is enormous. Clearly, presurgical education of the potential risks can alleviate this unfortunate situation. Once alerted, physicians can adjust stimulation parameters and therapeutic drug dosages to minimize impulsive behavior while retaining as much therapeutic effect (on movement) as possible. It is characterized by uncontrolled flinging, flailing, or other ballistic movements of the proximal arm, leg, and often the face.

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