USMLE Step 1 Qs (11)

There is an increasing body of evidence that reductions in serotonin levels play an important role in depressive disorders. The raphe neurons, located along the midline of the brainstem, provide the basic sites of serotonergic neurons that project to all parts of the brain and spinal cord. The other choices refer to structures that concern motor and/or sensory functions mainly associated with cranial nerves. Since the raphe neurons were damaged, the neurotransmitter most likely responsible for the onset of depression in this instance is serotonin. It is possible that other transmitter systems, such as the catecholamines, may also play a role in this disorder; however, they would not be chiefly responsible for the disorder in this instance because of the restricted locus of the lesion. Recent practice has been to treat depression with serotonin reuptake inhibitors such as fluoxetine (Prozac), which has been found to be effective after several weeks of treatment. The other choices for question 102 would be inappropriate because they would likely have a depressant effect on CNS functions.

A tumor pressing on the optic chiasm will disrupt the optic nerve (and tract) fibers that cross to the opposite side. These fibers mediate vision associated with fibers arising from the nasal retina of each eye. Since the nasal retina of each eye is associated with the temporal visual field for each eye, the visual loss is referred to as a bitemporal hemianopsia.

A lesion of the subthalamic nucleus results in hemiballism, a form of dyskinesia in which the patient displays severe involuntary movements. It is believed to occur as a result of an imbalance in the output signals of the basal ganglia. There is a change in the relationship between efferent pathways associated with the two pallidal segments (i.e., a direct pathway from the medial pallidal segment to the VL and VA nuclei of the thalamus versus an indirect pathway, involving connections between the lateral pallidal segment, subthalamic nucleus, and substantia nigra). Thus, in hemiballism the indirect pathway is disrupted, resulting in a change in the output signals of the pallidum to the thalamus.

The primary regions shown to be affected by Alzheimer's disease include the basal nucleus of Meynert, (which contains cholinergic neurons that project widely to the forebrain, including the cerebral cortex), the hippocampal formation, and the cerebral cortex. The other choices included structures that have not been significantly implicated in this disorder. The neurotransmitter that has been most implicated in this disorder is ACh. Alzheimer's brains have been shown to have reduced levels of ACh and cholinergic markers, especially after damage to cholinergic neurons of the basal nucleus of Meynert. While reductions in other neurotransmitter levels may also occur, the other choices of neurotransmitters presented have not been clearly implicated in this disorder. One of the clearest neuropathological characteristics of Alzheimer's disease is the presence of amyloid deposits and neurofibrillary tangles in the cerebral cortex. In fact, there has been a new and promising strategy that has been applied for the treatment of Alzheimer's disease. It involves the attempt to administer small molecules that retard the aggregation of amyloid- peptides that form fibrillar amyloid plaques, which affect the normal functions of neurons.

Myasthenia gravis is an autoimmune disease that causes cranial nerve and limb muscle weakness by producing antibodies that act against the nicotinic receptor at the neuromuscular junction. The result is that the action of nerve fibers that innervate skeletal muscle are affected, producing loss of the effects of ACh at the neuromuscular junction. The net result is a reduction of the size of the action potential in the muscle, producing a weakness in the affected muscle. This disorder is reversed by administration of drugs that inhibit the enzyme, acetylcholinesterase, that degrades ACh. Multiple sclerosis, ALS, and combined system disease (see the chapter entitled "The Spinal Cord") involve damage to axons and/or nerve cells within the CNS, producing much more profound damage to motor functions and, in the case of combined system disease, damage to both motor and sensory systems. Muscular dystrophy is typically characterized, in part, by progressive weakness of muscles and degeneration of the muscle fibers. The other disorders listed all involve disorders affecting the CNS, and thus, the symptoms associated with these disorders differ significantly from those described in this case. Excessive release of ACh is not a realistic event that is likely to occur (except from the bite of a black widow spider). In theory, if it were to occur, there is no reason to believe that muscular weakness would be a symptom. Instead, there would be some rigidity and muscle spasms.

It has been discovered that one mechanism of neurodegeneration involves prolonged activation of neurons by glutamate. It is believed that if glutamate accumulates in the extracellular space and is not removed, the presence of glutamate will effectively stimulate the neuron to death. It has been shown that neurotoxicity is linked to cell death after a stroke, which causes brain ischemia and oxygen deprivation. Glutamate receptors are involved in ischemic cell damage in the following way: Glutamate released from the presynaptic terminal would normally activate NMDA and AMPA receptors in the postsynaptic membrane. This results in an increase in the intracellular concentration of Ca2+, which remains long after the initial stimulus is removed, and thus prevents the cell from reestablishing a resting membrane potential. The net effect here is to produce injury (or death) to the cell.

This large pituitary tumor is seen to compress the optic chiasm. Damage to the chiasm affects the crossing fibers of the nasal retina, which convey information from the temporal visual fields. This results in a bitemporal hemianopsia. Since some parts of the optic nerves are spared, pupillary reflexes are preserved. The neuroanatomic substrates for conjugate gaze (i.e., frontal eye fields; pontine gaze center; medial longitudinal fasciculus; and nuclei of cranial nerves III, IV, and VI) are unaffected by the tumor; the mechanism of conjugate gaze remains intact.

Sleep apnea can occur for several reasons. One common basis is an obstruction of the airways (called obstructive sleep apnea). In this case, as indicated in the statement of the question, the physicians ruled out this possibility. Another possible cause involves central sleep apnea. This is due to disruption of the mechanism involving chemoreceptors in the carotid body that monitors carbon dioxide and oxygen levels in the blood. Axons in the carotid body project via the glossopharyngeal nerve (IX) to the reticular formation of the medulla. Therefore, disturbances involving the carotid body could result in central sleep apnea. Here, inappropriate signals are sent to the medullary reticular formation, which, in part, projects caudally to ventral horn sites in the spinal cord, governing such muscles as those that regulate the diaphragm and, therefore, disrupt the normal breathing process.

The bipolar cell receives inputs from the receptor cells (i.e., rods and cones). The response of the bipolar cell to the receptor cell input is then mediated to the ganglion cell. Horizontal and amacrine cells connect neighboring receptor or bipolar cells; Golgi cells are not present in the retina; and optic nerve cells project out of the retina as indicated earlier.

This figure is a ventral view of the brainstem. Fibers that arise from the nucleus ambiguus exit the brain on the lateral side of the medulla as part of the vagus nerve (K) and innervate the muscles of the larynx and pharynx as special visceral efferents. The motor root (H) lies medial to the sensory root and innervates the muscles of mastication. The mamillary bodies (A), which lie on the ventral surface of the brain at the caudal aspect of the hypothalamus, project many of their axons to the anteroventral thalamic nucleus as the mamillothalamic tract. The facial nerve (C) exits the brain at the level of the ventrolateral aspect of the caudal pons and its special visceral efferent component innervates the muscles of facial expression.
The cerebral peduncle (G) is situated in the ventrolateral aspect of the midbrain and contains fibers of cortical origin that project to all levels of the neuraxis of the brainstem and spinal cord. Note that the selection of choice E, the pyramids, would not have been a correct choice since the fibers present at this level can only terminate within the medulla or spinal cord. First-order somatosensory fibers from the region of the face (I) enter the brain laterally at the level of the middle of the pons as the sensory root of the trigeminal nerve. The oculomotor nerve (B) exits the brain at the level of the ventromedial aspect of the midbrain and some fibers of the general somatic efferent component of this nerve innervate the medial rectus. Damage to this component results in a loss of ability for medial gaze. Another component of the oculomotor nerve, the GVE component, constitutes the preganglionic parasympathetic neuron in a disynaptic pathway whose postganglionic division innervates the pupillary constrictor muscles. Accordingly, damage to the preganglionic division results in loss of pupillary constriction, which normally occurs in the presence of light as well as in accommodation. The abducens nerve (J) exits the brain at a ventromedial position at the level of the medulla-pontine border, and its fibers innervate the lateral rectus muscle. Damage to this nerve results in a lateral gaze paralysis.
The optic chiasm (F) contains fibers that cross over to reach the lateral geniculate nucleus on the side contralateral to the retina from which they originated. Such fibers are associated with the temporal (i.e., lateral) visual fields. Therefore, damage to the optic chiasm will cause blindness in the lateral half of each of the visual fields. Such a deficit is referred to as bitemporal hemianopsia. First-order neurons from the labyrinth organs (i.e., semicircular canals, saccule, and utricle) convey information concerning the position of the head in space along the vestibular component of the eighth nerve into the central nervous system. This nerve enters the brain laterally at the level of the upper medulla. The hypoglossal nerve (L) exits the brain at the level of the middle of the medulla between the pyramid and the olive. These fibers innervate muscles that move the tongue toward the opposite side. For this reason, a lesion of the hypoglossal nucleus or its nerve will result in a deviation of the tongue to the side of the lesion because of the unopposed action of the contralateral hypoglossal nerve, which remains intact.

The primary regions shown to be affected by Alzheimer's disease include the basal nucleus of Meynert, (which contains cholinergic neurons that project widely to the forebrain, including the cerebral cortex), the hippocampal formation, and the cerebral cortex. The other choices included structures that have not been significantly implicated in this disorder. The neurotransmitter that has been most implicated in this disorder is ACh. Alzheimer's brains have been shown to have reduced levels of ACh and cholinergic markers, especially after damage to cholinergic neurons of the basal nucleus of Meynert. While reductions in other neurotransmitter levels may also occur, the other choices of neurotransmitters presented have not been clearly implicated in this disorder. One of the clearest neuropathological characteristics of Alzheimer's disease is the presence of amyloid deposits and neurofibrillary tangles in the cerebral cortex. In fact, there has been a new and promising strategy that has been applied for the treatment of Alzheimer's disease. It involves the attempt to administer small molecules that retard the aggregation of amyloid- beta peptides that form fibrillar amyloid plaques, which affect the normal functions of neurons.

The primary regions shown to be affected by Alzheimer's disease include the basal nucleus of Meynert, (which contains cholinergic neurons that project widely to the forebrain, including the cerebral cortex), the hippocampal formation, and the cerebral cortex. The other choices included structures that have not been significantly implicated in this disorder. The neurotransmitter that has been most implicated in this disorder is ACh. Alzheimer's brains have been shown to have reduced levels of ACh and cholinergic markers, especially after damage to cholinergic neurons of the basal nucleus of Meynert. While reductions in other neurotransmitter levels may also occur, the other choices of neurotransmitters presented have not been clearly implicated in this disorder. One of the clearest neuropathological characteristics of Alzheimer's disease is the presence of amyloid deposits and neurofibrillary tangles in the cerebral cortex. In fact, there has been a new and promising strategy that has been applied for the treatment of Alzheimer's disease. It involves the attempt to administer small molecules that retard the aggregation of amyloid-beta peptides that form fibrillar amyloid plaques, which affect the normal functions of neurons.

One of the strategies used effectively for the treatment of anxiety disorders is to use classes of drugs that suppress CNS activity. One such class includes benzodiazepine agonists, such as chlordiazepoxide. This drug enhances GABA transmission by binding to the benzodiazepine site on the GABAA-receptor benzodiazepine chloride ionophore complex. In this manner, it acts as a GABA agonist, producing anxiolytic, sedative, and anticonvulsant effects. The other choices for both questions relate to drugs that have opposite effects, namely, ones with excitatory effects on CNS neurons.

This figure is a ventral view of the brainstem. Fibers that arise from the nucleus ambiguus exit the brain on the lateral side of the medulla as part of the vagus nerve (K) and innervate the muscles of the larynx and pharynx as special visceral efferents. The motor root (H) lies medial to the sensory root and innervates the muscles of mastication. The mamillary bodies (A), which lie on the ventral surface of the brain at the caudal aspect of the hypothalamus, project many of their axons to the anteroventral thalamic nucleus as the mamillothalamic tract. The facial nerve (C) exits the brain at the level of the ventrolateral aspect of the caudal pons and its special visceral efferent component innervates the muscles of facial expression.
The cerebral peduncle (G) is situated in the ventrolateral aspect of the midbrain and contains fibers of cortical origin that project to all levels of the neuraxis of the brainstem and spinal cord. Note that the selection of choice E, the pyramids, would not have been a correct choice since the fibers present at this level can only terminate within the medulla or spinal cord. First-order somatosensory fibers from the region of the face (I) enter the brain laterally at the level of the middle of the pons as the sensory root of the trigeminal nerve. The oculomotor nerve (B) exits the brain at the level of the ventromedial aspect of the midbrain and some fibers of the general somatic efferent component of this nerve innervate the medial rectus. Damage to this component results in a loss of ability for medial gaze. Another component of the oculomotor nerve, the GVE component, constitutes the preganglionic parasympathetic neuron in a disynaptic pathway whose postganglionic division innervates the pupillary constrictor muscles. Accordingly, damage to the preganglionic division results in loss of pupillary constriction, which normally occurs in the presence of light as well as in accommodation. The abducens nerve (J) exits the brain at a ventromedial position at the level of the medulla-pontine border, and its fibers innervate the lateral rectus muscle. Damage to this nerve results in a lateral gaze paralysis.
The optic chiasm (F) contains fibers that cross over to reach the lateral geniculate nucleus on the side contralateral to the retina from which they originated. Such fibers are associated with the temporal (i.e., lateral) visual fields. Therefore, damage to the optic chiasm will cause blindness in the lateral half of each of the visual fields. Such a deficit is referred to as bitemporal hemianopsia. First-order neurons from the labyrinth organs (i.e., semicircular canals, saccule, and utricle) convey information concerning the position of the head in space along the vestibular component of the eighth nerve into the central nervous system. This nerve enters the brain laterally at the level of the upper medulla. The hypoglossal nerve (L) exits the brain at the level of the middle of the medulla between the pyramid and the olive. These fibers innervate muscles that move the tongue toward the opposite side. For this reason, a lesion of the hypoglossal nucleus or its nerve will result in a deviation of the tongue to the side of the lesion because of the unopposed action of the contralateral hypoglossal nerve, which remains intact.

One of the strategies used effectively for the treatment of anxiety disorders is to use classes of drugs that suppress CNS activity. One such class includes benzodiazepine agonists, such as chlordiazepoxide. This drug enhances GABA transmission by binding to the benzodiazepine site on the GABAA-receptor benzodiazepine chloride ionophore complex. In this manner, it acts as a GABA agonist, producing anxiolytic, sedative, and anticonvulsant effects. The other choices for both questions relate to drugs that have opposite effects, namely, ones with excitatory effects on CNS neurons.

Glaucoma is a condition of elevated intraocular pressure caused (perhaps by infection) when debris accumulates in the spaces that lead to Schlemm's canal. If not treated, it can rapidly lead to blindness because the pressure can block conduction along the optic nerve. In addition, glaucoma can also be associated with frontal headaches; the diagnosis can be identified by determining the intraocular pressure.

The classic appearance of a patient with a lesion of the cerebellar hemispheres is one in which voluntary and skilled movements are affected. They are uncoordinated and there are errors in the range, force, and direction of movement. The relationships between the cerebellum and the motor regions of the cerebral cortex have been disrupted. Lesions of other regions such as the flocculonodular lobe, vermal region of the anterior cerebellar cortex, or fastigial nucleus produce different symptoms (disturbances of balance, muscle tone, or nystagmus). Although pure lesions limited to the ventral spinocerebellar tract have not been reported, it is likely that such a lesion could not account for the symptoms indicated in this question. Information carried by this tract concerns activity of Golgi tendon organs of muscles of the lower limbs.

Disruption of optic tract fibers destined for the lateral geniculate nucleus will cause a homonymous hemianopsia because it affects fibers arising from the temporal retina of the ipsilateral side and from the nasal retina of the contralateral side. Since the damage occurred in the left optic tract, the loss of vision is reflected on the right visual field [i.e., the left temporal retina is associated with the nasal (or right) visual field of the left eye, and the right nasal retina is associated with the temporal (or right) visual field of the right eye]. Therefore, such a lesion would result in a right homonymous hemianopsia.

A number of recent studies have indicated that different olfactory glomeruli respond to different kinds of olfactory stimuli. In a sense, this represents a type of organization of the olfactory bulb that bears a functional similarity to the spatial organization that exists for other sensory systems. There is no evidence that such a spatial arrangement exists for other components of the olfactory system, nor is there any evidence that temporal summation plays any role in the process of olfactory discrimination.

There is an increasing body of evidence that reductions in serotonin levels play an important role in depressive disorders. The raphe neurons, located along the midline of the brainstem, provide the basic sites of serotonergic neurons that project to all parts of the brain and spinal cord. The other choices refer to structures that concern motor and/or sensory functions mainly associated with cranial nerves. Since the raphe neurons were damaged, the neurotransmitter most likely responsible for the onset of depression in this instance is serotonin. It is possible that other transmitter systems, such as the catecholamines, may also play a role in this disorder; however, they would not be chiefly responsible for the disorder in this instance because of the restricted locus of the lesion. Recent practice has been to treat depression with serotonin reuptake inhibitors such as fluoxetine (Prozac), which has been found to be effective after several weeks of treatment. The other choices for question 102 would be inappropriate because they would likely have a depressant effect on CNS functions.

It has been discovered that one mechanism of neurodegeneration involves prolonged activation of neurons by glutamate. It is believed that if glutamate accumulates in the extracellular space and is not removed, the presence of glutamate will effectively stimulate the neuron to death. It has been shown that neurotoxicity is linked to cell death after a stroke, which causes brain ischemia and oxygen deprivation. Glutamate receptors are involved in ischemic cell damage in the following way: Glutamate released from the presynaptic terminal would normally activate NMDA and AMPA receptors in the postsynaptic membrane. This results in an increase in the intracellular concentration of Ca2+, which remains long after the initial stimulus is removed, and thus prevents the cell from reestablishing a resting membrane potential. The net effect here is to produce injury (or death) to the cell.

This figure is a midsagittal section of the brain. A major portion of the anterior commissure (E) contains fibers that arise from the olfactory bulb and decussate to the contralateral olfactory bulb. The septum pellucidum (G) forms the medial wall of the lateral ventricle, which in fact separates the lateral ventricle on one side from that on the opposite side. The cingulate gyrus (H) is a prominent structure on the medial aspect of the cerebral cortex and constitutes a component of the limbic lobe. It receives a significant input from the anteroventral thalamic nucleus. The major output pathway of the hippocampal formation is the fornix system of fibers (A), which arises from cells in its subicular cortex and adjoining regions of hippocampus. These fibers are then distributed to the anterior thalamic nucleus, mamillary bodies, and septal area. The basilar portion of the pons (D) lies in the ventral half of this region of the brainstem. It receives inputs from each of the lobes of the cerebral cortex, which it then relays to the cerebellar cortex. The primary visual cortex lies on both banks of the calcarine fissure. Cells located on the lower bank receive inputs from the lateral geniculate nucleus that relate to either the nasal or temporal upper visual fields. Therefore, a lesion of this region would produce an upper quadrantanopia (i.e., loss of one-quarter of the visual field). The corpus callosum constitutes the major channel by which the cerebral cortex on one side can communicate with the cortex of the opposite side. The genu of the corpus callosum (F) contains fibers that pass from the frontal lobe of one side to that of the other.

This figure is a ventral view of the brainstem. Fibers that arise from the nucleus ambiguus exit the brain on the lateral side of the medulla as part of the vagus nerve (K) and innervate the muscles of the larynx and pharynx as special visceral efferents. The motor root (H) lies medial to the sensory root and innervates the muscles of mastication. The mamillary bodies (A), which lie on the ventral surface of the brain at the caudal aspect of the hypothalamus, project many of their axons to the anteroventral thalamic nucleus as the mamillothalamic tract. The facial nerve (C) exits the brain at the level of the ventrolateral aspect of the caudal pons and its special visceral efferent component innervates the muscles of facial expression.
The cerebral peduncle (G) is situated in the ventrolateral aspect of the midbrain and contains fibers of cortical origin that project to all levels of the neuraxis of the brainstem and spinal cord. Note that the selection of choice E, the pyramids, would not have been a correct choice since the fibers present at this level can only terminate within the medulla or spinal cord. First-order somatosensory fibers from the region of the face (I) enter the brain laterally at the level of the middle of the pons as the sensory root of the trigeminal nerve. The oculomotor nerve (B) exits the brain at the level of the ventromedial aspect of the midbrain and some fibers of the general somatic efferent component of this nerve innervate the medial rectus. Damage to this component results in a loss of ability for medial gaze. Another component of the oculomotor nerve, the GVE component, constitutes the preganglionic parasympathetic neuron in a disynaptic pathway whose postganglionic division innervates the pupillary constrictor muscles. Accordingly, damage to the preganglionic division results in loss of pupillary constriction, which normally occurs in the presence of light as well as in accommodation. The abducens nerve (J) exits the brain at a ventromedial position at the level of the medulla-pontine border, and its fibers innervate the lateral rectus muscle. Damage to this nerve results in a lateral gaze paralysis.
The optic chiasm (F) contains fibers that cross over to reach the lateral geniculate nucleus on the side contralateral to the retina from which they originated. Such fibers are associated with the temporal (i.e., lateral) visual fields. Therefore, damage to the optic chiasm will cause blindness in the lateral half of each of the visual fields. Such a deficit is referred to as bitemporal hemianopsia. First-order neurons from the labyrinth organs (i.e., semicircular canals, saccule, and utricle) convey information concerning the position of the head in space along the vestibular component of the eighth nerve into the central nervous system. This nerve enters the brain laterally at the level of the upper medulla. The hypoglossal nerve (L) exits the brain at the level of the middle of the medulla between the pyramid and the olive. These fibers innervate muscles that move the tongue toward the opposite side. For this reason, a lesion of the hypoglossal nucleus or its nerve will result in a deviation of the tongue to the side of the lesion because of the unopposed action of the contralateral hypoglossal nerve, which remains intact.

There is an increasing body of evidence that reductions in serotonin levels play an important role in depressive disorders. The raphe neurons, located along the midline of the brainstem, provide the basic sites of serotonergic neurons that project to all parts of the brain and spinal cord. The other choices refer to structures that concern motor and/or sensory functions mainly associated with cranial nerves. Since the raphe neurons were damaged, the neurotransmitter most likely responsible for the onset of depression in this instance is serotonin. It is possible that other transmitter systems, such as the catecholamines, may also play a role in this disorder; however, they would not be chiefly responsible for the disorder in this instance because of the restricted locus of the lesion. Recent practice has been to treat depression with serotonin reuptake inhibitors such as fluoxetine (Prozac), which has been found to be effective after several weeks of treatment. The other choices for question 102 would be inappropriate because they would likely have a depressant effect on CNS functions.

Damage to region C unilaterally will produce an upper quadrantanopia. This means that if the region C is damaged on one side of the brain, the person will lose vision in the upper quadrant of the visual field on the opposite side. This can occur due to a stroke or injury affecting the visual pathway in the brain.

The pathway for conscious proprioception from the body utilizes the ventral posterolateral nucleus as its thalamic relay. Conscious proprioception from the head utilizes the ventral posteromedial nucleus as its relay. The taste pathway utilizes the ventral posteromedial nucleus as well. The visual system utilizes the lateral geniculate nucleus, and the auditory system utilizes the medial geniculate nucleus. In contrast, the olfactory system can transmit olfactory information to the prefrontal cortex without engaging thalamic nuclei. Thus, olfactory information reaches the pyriform cortex and amygdala from the olfactory bulb and then is transmitted directly to the prefrontal cortex. However, it should be noted that olfactory information also can reach the prefrontal cortex by virtue of projections from the olfactory tubercle and pyriform cortex via the mediodorsal thalamic nucleus. Thus, the olfactory system may utilize a parallel processing mechanism in transmitting inputs to the prefrontal cortex.

From the lateral geniculate nucleus, there are two trajectories that the fiber pathways take en route to the visual cortex. One pathway passes dorsally through the parietal lobe and terminates in the upper bank of the calcarine fissure in the ipsilateral primary visual cortex. The second pathway takes a more circuitous (ventral) route—called the Meyer-Archambault loop—through the temporal lobe and terminates in the lower bank of the calcarine fissure in the ipsilateral primary visual cortex. The lower bank of the calcarine fissure is associated with the upper visual quadrants of the contralateral visual fields for both eyes, while the upper bank of the calcarine fissure is associated with the lower quadrants of the contralateral visual fields for both eyes. Thus, if there is a lesion of the left temporal lobe affecting the Meyer-Archambault loop, then the right upper quadrant for each eye will be affected. This deficit is referred to as a right upper quadrantanopia.

This figure is a ventral view of the brainstem. Fibers that arise from the nucleus ambiguus exit the brain on the lateral side of the medulla as part of the vagus nerve (K) and innervate the muscles of the larynx and pharynx as special visceral efferents. The motor root (H) lies medial to the sensory root and innervates the muscles of mastication. The mamillary bodies (A), which lie on the ventral surface of the brain at the caudal aspect of the hypothalamus, project many of their axons to the anteroventral thalamic nucleus as the mamillothalamic tract. The facial nerve (C) exits the brain at the level of the ventrolateral aspect of the caudal pons and its special visceral efferent component innervates the muscles of facial expression.
The cerebral peduncle (G) is situated in the ventrolateral aspect of the midbrain and contains fibers of cortical origin that project to all levels of the neuraxis of the brainstem and spinal cord. Note that the selection of choice E, the pyramids, would not have been a correct choice since the fibers present at this level can only terminate within the medulla or spinal cord. First-order somatosensory fibers from the region of the face (I) enter the brain laterally at the level of the middle of the pons as the sensory root of the trigeminal nerve. The oculomotor nerve (B) exits the brain at the level of the ventromedial aspect of the midbrain and some fibers of the general somatic efferent component of this nerve innervate the medial rectus. Damage to this component results in a loss of ability for medial gaze. Another component of the oculomotor nerve, the GVE component, constitutes the preganglionic parasympathetic neuron in a disynaptic pathway whose postganglionic division innervates the pupillary constrictor muscles. Accordingly, damage to the preganglionic division results in loss of pupillary constriction, which normally occurs in the presence of light as well as in accommodation. The abducens nerve (J) exits the brain at a ventromedial position at the level of the medulla-pontine border, and its fibers innervate the lateral rectus muscle. Damage to this nerve results in a lateral gaze paralysis.
The optic chiasm (F) contains fibers that cross over to reach the lateral geniculate nucleus on the side contralateral to the retina from which they originated. Such fibers are associated with the temporal (i.e., lateral) visual fields. Therefore, damage to the optic chiasm will cause blindness in the lateral half of each of the visual fields. Such a deficit is referred to as bitemporal hemianopsia. First-order neurons from the labyrinth organs (i.e., semicircular canals, saccule, and utricle) convey information concerning the position of the head in space along the vestibular component of the eighth nerve into the central nervous system. This nerve enters the brain laterally at the level of the upper medulla. The hypoglossal nerve (L) exits the brain at the level of the middle of the medulla between the pyramid and the olive. These fibers innervate muscles that move the tongue toward the opposite side. For this reason, a lesion of the hypoglossal nucleus or its nerve will result in a deviation of the tongue to the side of the lesion because of the unopposed action of the contralateral hypoglossal nerve, which remains intact.

To generate an excitatory focus with an inhibitory surround, three types of inhibition are present in the dorsal column nuclei. First-order neurons ascending in the dorsal columns make synaptic contact with different cells in the dorsal column nuclei and excite those cells. One such cell may be an inhibitory interneuron that makes synaptic contact with a neighboring dorsal column nuclear cell, thus inhibiting that cell (i.e., feed-forward inhibition). In addition, the dorsal column cell that is excited by the first-order neuron may make synaptic contact with another inhibitory interneuron (in addition to its classical ascending projection to the ventral posterolateral nucleus of the thalamus). This inhibitory interneuron makes synaptic contact with an adjacent dorsal column cell and inhibits that cell (i.e., feedback inhibition). Finally, a descending fiber from the postcentral gyrus can make synaptic contact with inhibitory interneurons that inhibit dorsal column cells. The figure below illustrates feedback, feed forward, and descending inhibition. Inhibitory neurons are depicted in black.

Tardive dyskinesia, a disorder involving involuntary movements of the mouth, face, and tongue, is caused by long-term treatment with antipsychotic drugs that block or decrease dopaminergic synaptic transmission. Such treatment eventually produces a hypersensitivity in dopamine receptors to dopamine. An imbalance is created between dopamine, GABA, and cholinergic systems within the striatum and this is believed to be responsible for the disorder.

The superior temporal gyrus is the primary auditory receiving area in the cerebral cortex. Accordingly, the primary afferent source to this region arises from the medial geniculate nucleus, which constitutes a specific thalamic relay for processing of auditory information.

The output of the retina is mediated by the ganglion cells. Ganglion cells receive inputs from photoreceptor and bipolar cells. In turn, ganglion cells give rise to optic nerve fibers, which project through the optic chiasm and optic tracts to the lateral geniculate nucleus of the thalamus. Other cells mentioned in this question only produce local connections within the retina.

The lesion will disrupt fibers of the medial lemniscus (lateral aspect of the lesion) and thus produce contralateral loss of conscious proprioception. It will also disrupt fibers passing from the cerebellum to the red nucleus and ventrolateral nucleus of the thalamus, which could account for a tremor of the contralateral limb. Note that there would be no ipsilateral motor loss because functions associated with the red nucleus are expressed on the contralateral side. Oculomotor palsy would also be present because the lesion disrupts root fibers of cranial nerve III. However, the lesion is too rostral to affect cranial nerve IV. There is no hearing loss because the auditory fibers are situated too far laterally. Since the taste pathway is essentially ipsilateral, if any fibers are damaged by the lesion, deficits in taste sensation would be ipsilateral.

This constellation of deficits, including paralysis of the lower right face, paralysis of the right lower limbs, and right deviation of the tongue, requires a lesion located in the left internal capsule. Since the motor fibers from the cortex that supply all three of these regions (i.e., limbs, lower face, and tongue) are all crossed, a lesion of the internal capsule will produce each of these deficits. Also, recall that the tongue will deviate to the side of the lesion when the lesion affects the lower motor neuron (LMN) (i.e., cranial nerve XII) directly. When it affects the UMN (i.e., fibers in the internal capsule), inputs into the contralateral nucleus of cranial nerve XII are affected. Thus, the tongue in this instance will deviate to the side opposite the lesion. A lesion of the pontine tegmentum will not affect descending corticospinal or corticomedullary fibers since these fibers are contained in the basilar part of the pons. A lesion of the medulla would be too caudal to affect cortical fibers that terminate on cells of the facial nucleus whose axons innervate muscles of the lower face.

Choreiform movements have generally been associated with damage to the neostriatum (the cortex and the globus pallidus have occasionally been implicated). Normally, there is a balance in what seems to be opposing effects of ACh, dopamine, and GABA in the neostriatum. In this disorder, the levels of ACh and GABA are significantly reduced. This creates an imbalance in which dopamine levels now become (relatively) too high. Accordingly, effective pharmacologic treatment involves the use of dopamine receptor blockers.

The primary regions shown to be affected by Alzheimer's disease include the basal nucleus of Meynert, (which contains cholinergic neurons that project widely to the forebrain, including the cerebral cortex), the hippocampal formation, and the cerebral cortex. The other choices included structures that have not been significantly implicated in this disorder. The neurotransmitter that has been most implicated in this disorder is ACh. Alzheimer's brains have been shown to have reduced levels of ACh and cholinergic markers, especially after damage to cholinergic neurons of the basal nucleus of Meynert. While reductions in other neurotransmitter levels may also occur, the other choices of neurotransmitters presented have not been clearly implicated in this disorder. One of the clearest neuropathological characteristics of Alzheimer's disease is the presence of amyloid deposits and neurofibrillary tangles in the cerebral cortex. In fact, there has been a new and promising strategy that has been applied for the treatment of Alzheimer's disease. It involves the attempt to administer small molecules that retard the aggregation of amyloid- peptides that form fibrillar amyloid plaques, which affect the normal functions of neurons.

A neuritis involving the optic disk would affect the size of the visual field loss around the optic disk, which corresponds to the blind spot. In general, this kind of neuritis would expand somewhat the size of the blind spot but would cause no further visual loss.

An arterial occlusion compromised the blood supply to the occipital lobe on the left side of the brain. Therefore, it would result in a right homonymous hemianopsia with no motor deficits (since no motor regions of the brain are affected).

The primary structures damaged by this lesion include the crus cerebri, which results in a UMN paralysis of the contralateral limbs, as well as a paresis of the lower facial and tongue muscles. The other outstanding syndrome present from this lesion is a paralysis that results from damage to cranial nerve III. Other structures may be marginally affected.

The largest synaptic potentials produced by cortical stimulation would most likely be seen in spinal motor neurons that innervate distal muscles. One of the primary functions of the corticospinal tract is to control the distal muscles of the hands and fingers. Penfield and others constructed a homuncular map from stimulation studies of the cortex. Such studies reveal that the region of the cortex that is associated with the hands and fingers is considerably larger than those regions that are associated with the proximal musculature. Accordingly, stimulation of the hand region of the cortex would activate more fibers than other cortical regions. It is likely that more ventral horn neurons (located in a lateral position) innervate distal musculature than neurons (located in a medial position) innervate proximal musculature. Since the size of the synaptic potential is a function of both the number of fibers that provide a converging input into a given region and the number of cells that discharge in response to that converging input, it is reasonable to conclude that the largest potentials would be observed following stimulation of the cortical regions associated with the distal musculature. Since neurons situated in the motor cortex project their axons to motor horn cells and interneurons but not to sensory neurons of the dorsal horn (although the component of the corticospinal tract that arises from the parietal lobe does project to the dorsal horn), stimulation of the motor cortex could only produce weak potentials at best among sensory neurons in the dorsal horn.

Primary nociceptive afferent fibers would have to release an excitatory transmitter in order for normal transmission to take place. Two excitatory transmitters have been identified in association with different classes of primary nociceptive afferents: (1) substance P and (2) excitatory amino acids. The best candidate as an excitatory amino acid is glutamate. Since enkephalins have been shown to be inhibitory transmitters in the pain system, they are not likely to be released from the primary afferents. Instead, other CNS neurons impinge upon the primary afferents, and enkephalins are released from those neurons.

This disorder is referred to as the Argyll Robertson pupil and occurs with CNS syphilis (tertiary). Although the precise site of the lesion has never been fully established, it is believed to be in the pretectal area. The reasoning is as follows: In the pupillary light reflex, many optic fibers terminate in the pretectal area and superior colliculus region and are then relayed to the autonomic nuclei of cranial nerve III. Impulses from this component of cranial nerve III then synapse with postganglionic parasympathetics that innervate the pupillary constrictor muscles, thus producing pupillary constriction. In the case of the accommodation reflex, retinal impulses first reach the cortex and are then relayed through corticofugal fibers to the brainstem. Some of these fibers are then relayed directly or indirectly to both motor and autonomic components of cranial nerve III, thus activating the muscles required for the accommodation reaction to occur, which includes pupillary constriction.

Cells in the lateral geniculate nucleus respond very much like ganglion cells in the retina because of the point-to-point projection pathway from the retina to the lateral geniculate. Accordingly, lateral geniculate cells have small concentric receptive fields that are either on-center or off-center in which the cells respond best to small spots of light that are in the center of the receptive field. On the other hand, cells in the visual cortex display a much greater complexity in their responses to images in the visual field. Instead of responding to small spots of light, they respond to lines and borders in the different areas of the visual field. In particular, the simple cell responds as a function of the retinal position in which the line-stimulus is located as well as its orientation (e.g., whether it is in a vertical or horizontal position). As a result, when a bar of light is positioned in the appropriate part of the visual field with the appropriate orientation, the cells in area 17 will respond maximally. When either of these parameters is altered, the firing pattern of the cell will be reduced or totally inhibited. Complex cells lack clear excitatory and inhibitory zones (i.e., these neurons respond to bars of light in a given orientation but they are not position-specific). Hypercomplex cells are stimulated by bars of light of specific lengths or by specific shapes.

In one form of retinitis pigmentosa, there is a genetic defect with respect to rhodopsin. The result of this defect is the production of defective opsin. As a consequence, rod cells are affected, leading to a reduced response to light. However, central vision is spared as are cone cells. Central nervous system (CNS) neurons such as those located in area 17 are not directly affected and vision is not totally lost.

REM sleep is characterized by a low-voltage EEG pattern typical of an alert person. For this reason, REM sleep is sometimes referred to as paradoxical sleep. Other EEG patterns, such as slow waves, high-voltage EEGs, and sleep spindles, occur at other stages of sleep. In addition, REM sleep is characterized by a general loss of skeletal muscle tone with the exception of the eye muscles, which govern the REMs.

The principal ascending pathway of the auditory system listed in this question is the lateral lemniscus. It transmits information from the cochlear nuclei to the inferior colliculus. The trapezoid body is a commissure that contains some of the fibers of the lateral lemniscus that cross from the cochlear nuclei of one side of the brainstem en route to the inferior colliculus of the other side. The trapezoid body is present at the level of the caudal pons. The brachium of the superior colliculus, trigeminal lemniscus, and medial lemniscus do not transmit auditory sensory information.

Experimental evidence indicates the prefrontal cortex is a key region for the conscious perception of smell. This conclusion is based upon two observations. First, the prefrontal cortex receives major inputs from the olfactory bulb by following routes: olfactory bulb to pyriform cortex to prefrontal cortex; or olfactory bulb to pyriform cortex (and olfactory tubercle) to mediodorsal thalamic nucleus to prefrontal cortex. Second, lesions of the prefrontal cortex result in a failure to discriminate odors. Olfactory functions are not known to be associated with any of the other choices. Instead, the primary auditory receiving area is located in the auditory cortex; the posterior parietal lobule is concerned with such processes as the programming mechanisms associated with complex motor tasks; the cingulate gyrus has been associated with such functions as spatial learning and the modulation of autonomic and emotional processes; and the prefrontal gyrus contains the primary motor area.

One of the most important features of the anterior lobe of the cerebellum is that it receives major inputs from structures that mediate information concerning muscle spindle and Golgi tendon organ activity (sometimes referred to as unconscious proprioception). The pathways that mediate unconscious proprioception include the dorsal and ventral spinocerebellar tracts and the cuneocerebellar tract. Accordingly, the cerebellar anterior lobe is sometimes referred to as the spinocerebellum. The fastigial and dentate nuclei receive their principal inputs from the cerebellar cortex, and their axons project out of the cerebellum. The posterior lobe receives few, if any, inputs from pathways that mediate unconscious proprioception information.

The VA nucleus has properties of both the specific and nonspecific thalamus. By this, it is meant that, on the one hand, the VA nucleus receives motor inputs from the basal ganglia, including the substantia nigra, and projects its axons to an important motor region of the brain—the premotor cortex. In this context, the VA nucleus functions as a relay nucleus for the transmission of information associated with motor functions. On the other hand, the VA nucleus also receives various inputs from other thalamic nuclei (such as the centromedian nucleus) and projects its axons in a widespread manner to other parts of the frontal lobe, including the prefrontal cortex. In this context, the VA nucleus can modulate a wide variety of neurons in the cerebral cortex, both directly and indirectly, which is characteristic of nonspecific thalamic nuclei.

A major target of efferent fibers from the hippocampal formation is the septal area. Fibers located in the precommissural fornix supply the septal area in an extensive and topographical manner. In turn, the septal area projects significant numbers of fibers to the lateral (and medial) regions of the hypothalamus. In this manner, the septal area serves as a relay for the transmission of signals from the hippocampal formation to the hypothalamus. The hippocampal formation does not project to the habenular nuclei, mediodorsal nucleus, or the bed nucleus of the stria terminalis. Moreover, the cingulate gyrus does not project directly to the hypothalamus.