USMLE Step 1 Qs (8)

There are three basic mechanisms by which the transmitter is removed from the synaptic cleft: (1) enzymatic degradation, (2) reuptake, and (3) diffusion. In the case of the neuromuscular junction, ACh (and not glutamate) is the neurotransmitter and the primary mechanism involves enzymatic degradation. The enzyme involved is acetylcholinesterase, which helps break down ACh into acetate and choline. Choline is then taken up by the presynaptic terminal. Concerning the other choices, choline acetyltransferase is the enzyme involved in the synthesis of ACh, glutaminase, and glutamine synthetase are involved in the formation of glutamate from glutamine and glutamine from glutamate, respectively. Serine hydroxymethyltransferase is the enzyme that converts serine into glycine.

A positive Babinski's sign, or dorsiflexion of the great toe when the lateral portion of the plantar surface of the foot is scratched, is a sign of corticospinal tract dysfunction, a tract consisting of UMNs. Peripheral nerve (including the sural nerve) lesions are LMN lesions.

Taste associated with the anterior two-thirds of the tongue is mediated by the facial (cranial nerve VII) nerve. The geniculate ganglion contains the cell bodies associated with the sensory (gustatory) component of the seventh nerve. The somatic motor component of the seventh nerve mediates the muscles of facial expression. Thus, the sensory and motor components of the seventh nerve affected in this individual can be characterized as special visceral afferent (because this afferent contains chemoreceptors) and special visceral efferent (because the motor component innervates skeletal muscle and is derived from a branchial arch), respectively.

The corticospinal tract mediates voluntary motor function. The fibers cross in the medullary pyramids, thus lesions below this structure cause ipsilateral weakness. The reflexes are brisk, since in a UMN lesion, there is a loss of inhibition to spinal reflexes. Muscle, dorsal root, and femoral nerve damage are all examples of lesions distal to the spinal cord. A frontal lobe lesion would not cause a sensory or motor level, and would probably cause problems more proximally, such as slurred speech.

The reflexes are lost because the LMNs, which are affected by this process, are unable to participate in the reflex arc necessary for a knee or ankle jerk to take place. These LMNs originate with stretch receptors in the tendons. Answers a, c, d, and e are all examples of UMN lesions, usually characterized by hyperactive reflexes.

Excitatory amino acids and, in particular, the glutamate family of compounds have long been thought to play an important role in epileptiform activity. Epileptiform activity typically includes AMPA-receptor activation. However, as the seizure becomes more intense, there is increased involvement of NMDA receptors. This is evidenced by the facts that NMDA antagonists can reduce the intensity and length of the seizure activity and that, following removal of human epileptic hippocampal tissue, there is an up-regulation of both AMPA and NMDA receptors. Metabotropic glutamate receptors have been shown to be present in the retina but have not yet been demonstrated to be present in regions of the brain that are typically epileptogenic. GABA and glycine are inhibitory transmitters; therefore, seizures would logically block such receptor activation. There has been no substantive evidence concerning the role of cortical nicotinic receptors in epilepsy.

In astigmatism, the shapes of the cornea and possibly the lens become oblong, resulting in differences in the curvature of the lens along the long and short axes. Thus, astigmatism is corrected with a cylindrical lens.

Tryptophan hydroxylase, tyrosine hydroxylase, and choline acetyltransferase are enzymes that are critical for the biosynthesis of serotonin, catecholamines, and ACh, respectively. Dopamine -hydroxylase converts dopamine to norepinephrine. Catechol-O-methyltransferase and monoamine oxidase are critical for the metabolic degradation of catecholamines

The tumor is situated in the lentiform nucleus and internal capsule. Therefore, corticospinal fibers will be affected, causing a UMN paralysis of the left side. Dyskinesia would not be seen because any effects normally seen in association with damage to the basal ganglia would be masked by the effects of the damage to the internal capsule. Since the cerebellum was not involved, there would be no intention tremor. Neither would there be any visual deficits from this glioma since optic nerve fibers are not involved. The following schematic diagram indicates the approximate extent of the tumor. Labeled are the caudate nucleus (C), the globus pallidus (GP), the internal capsule (IC), the putamen (P), and the tumor (T).

This is an example of a LMN problem. Answers a, b, and c are UMN structures. The trigeminal nerve is a cranial nerve that mediates sensation on the face and the muscles of mastication. Loss of diaphragmatic function causes respiratory distress.

Noradrenergic activity can be blocked by a number of mechanisms. Reserpine, for example, prevents the synthesis and storage of norepinephrine in sympathetic nerve terminals. Guanethidine sulfate affects noradrenergic transmission by blocking the release of norepinephrine at the sympathetic endings. Competitive alpha-receptor blockers include phenoxybenzamine hydrochloride and phentolamine, whereas metoprolol blocks beta1 receptors. Since ACh is the transmitter at preganglionic synapses of both the parasympathetic and sympathetic nervous systems, hexamethonium chloride is an effective ganglionic blocker at these synapses.

There are three basic mechanisms by which the transmitter is removed from the synaptic cleft: (1) enzymatic degradation, (2) reuptake, and (3) diffusion. In the case of the neuromuscular junction, ACh (and not glutamate) is the neurotransmitter and the primary mechanism involves enzymatic degradation. The enzyme involved is acetylcholinesterase, which helps break down ACh into acetate and choline. Choline is then taken up by the presynaptic terminal. Concerning the other choices, choline acetyltransferase is the enzyme involved in the synthesis of ACh, glutaminase, and glutamine synthetase are involved in the formation of glutamate from glutamine and glutamine from glutamate, respectively. Serine hydroxymethyltransferase is the enzyme that converts serine into glycine.

To walk down stairs, one has to have the ability to move the eyes down when they are in the medial position. This involves the use of cranial nerve IV (trochlear nerve), which innervates the superior oblique muscle (whose action is to pull the eye downward when in the medial position). If there is damage to this nerve on one side, the eyes will not be able to focus on the same visual field, thus producing double vision. Cranial nerve IV is classified as a general somatic efferent fiber because it innervates skeletal muscle and it is derived from somites.

To walk down stairs, one has to have the ability to move the eyes down when they are in the medial position. This involves the use of cranial nerve IV (trochlear nerve), which innervates the superior oblique muscle (whose action is to pull the eye downward when in the medial position). If there is damage to this nerve on one side, the eyes will not be able to focus on the same visual field, thus producing double vision. Cranial nerve IV is classified as a general somatic efferent fiber because it innervates skeletal muscle and it is derived from somites.

The central pathways mediating taste include the following: primary afferent taste fibers associated with taste receptors of cranial nerves VII, IX, and X synapse in the solitary nucleus. Many fibers from the solitary nucleus project to the ventral posteromedial nucleus of the thalamus, which, in turn, project to the ventrolateral aspect of the postcentral gyrus.

The largest numbers of excitatory synapses in the CNS are mediated by glutamate as it is believed that approximately half of the synapses in the brain release glutamate. For example, functions mediated by fibers that originate from the cerebral cortex and descend to such regions as the neostriatum, thalamus, brainstem, and spinal cord are generally believed to be mediated by glutamate. Many other neuronal systems throughout the brain and spinal cord utilize glutamate as well. Dopaminergic and noradrenergic neurons, while mostly excitatory, can also be inhibitory at some synapses and are less numerous than glutamate. Cholinergic and substance P synapses are also excitatory, but are likewise less numerous than glutamate.

Pacinian corpuscles best mediate vibration.

MPTP was discovered by accident when drug abusers who were using a synthetic heroin derivative developed signs of Parkinson's disease. It was discovered that their drug included the contaminant MPTP. As a consequence, MPTP has been applied systemically in a number of experimental animals, resulting in significant decreases in dopamine content of the brain due to the loss of dopaminergic neurons in the substantia nigra. These animals also developed symptoms similar to those seen in Parkinson's patients. For these reasons, this drug is currently being used for research purposes in order to develop a better understanding of this disease and to establish possible drug therapies for its treatment and eventual cure.

Recent studies have demonstrated that the suprachiasmatic nucleus controls the biologic clock of internal circadian rhythms. During the light phase of the light-dark cycle, metabolic activity (measured by 14C-2-deoxyglucose autoradiography) within the suprachiasmatic nucleus is significantly increased. In contrast, during the dark phase, there is very little metabolic activity.

This figure is a lateral view of the cerebral cortex. Cells in the "arm" area of the primary motor cortex (H) project their axons to the cervical level of the spinal cord. This area receives major input from the ventrolateral nucleus of the thalamus. The leg region of the primary somatosensory cortex (A) lies immediately caudal to the central sulcus, is almost devoid of pyramidal cells, and is referred to as a granulous cortex. Damage to the cells situated in the region of the dorsal border of the superior temporal gyrus and the adjoining area of the inferior parietal lobule (Wernicke's area) (C) causes impairment in the appreciation of the meanings of written or spoken words.
The primary, secondary, and tertiary auditory receiving areas in the cortex are located mainly in the superior temporal gyrus (D). It is the final receiving area for inputs from the medial geniculate nucleus, which represents an important relay in the transmission of auditory signals to the cortex. An additional area of the cortex governing speech (F) is called the motor speech area, or Broca's area. It is situated in the inferior aspect of the frontal lobe immediately rostral and slightly ventral to the precentral gyrus. Lesions of this region produce impairment of the ability to express words in a meaningful way or to use words correctly. The orbital frontal cortex (E) lies in a position inferior and rostral to Broca's motor speech area. This region governs higher-order intellectual functions and some aspects of emotional behavior.
The caudal aspect of the middle frontal gyrus (G) contains cells that, when activated, produce conjugate deviation of the eyes. This action is believed to be accomplished, in part, by virtue of descending projections to the superior colliculus, pretectal region, and horizontal gaze center of the pons. Lesions of the posterior parietal lobe (B) of the nondominant hemisphere will produce a disorder of body image, referred to as sensory neglect. The patient will frequently fail to recognize or neglect to shave or wash those body parts. The patient may even fail to recognize the presence of a hemiparesis involving that part of the body as well. The precentral gyrus (H) constitutes the primary motor cortex. Lesions of this region produce a UMN paralysis involving a contralateral limb.

Gary has a spinal cord syndrome called Brown-Séquard''s syndrome, or hemisection of the spinal cord. The lesion is not at the cervical level because motor functions of the upper limbs were considered normal. The examiner can pinpoint the location of the lesion by using the "sensory level," or level at which the loss of pain and temperature begin, by remembering that the lesion affects fibers that have entered the spinal cord one or two levels below it, and then cross to the contralateral side. Therefore, a loss of sensory function at the T10 level indicates a lesion at the T8 or T9 level. A level at which motor deficits begin can be helpful as well, but in lesions of the thoracic spinal cord, muscles innervated by thoracic nerves are difficult to test. The examiner still expects weakness in the lower extremities, and this helps to make the diagnosis. Brown-Séquard''s syndrome may occur as a result of different types of tumors or infections of the spinal cord.

The spinothalamic tract carries fibers mediating pain and temperature. The primary pain fibers enter the spinal cord and pass one or two segments in Lissauer''s marginal zone before making a synapse with neurons that form the lateral spinothalamic tract. Fibers of the lateral spinothalamic tract then cross to the contralateral side one or two segments above or before where the primary afferent fibers have entered the cord. Accordingly, pain and temperature are lost below the lesion on the contralateral side. The cuneate and gracile fasciculi mediate proprioception and vibration, and the corticospinal tract mediates voluntary motor function.

Taste associated with the anterior two-thirds of the tongue is mediated by the facial (cranial nerve VII) nerve. The geniculate ganglion contains the cell bodies associated with the sensory (gustatory) component of the seventh nerve. The somatic motor component of the seventh nerve mediates the muscles of facial expression. Thus, the sensory and motor components of the seventh nerve affected in this individual can be characterized as special visceral afferent (because this afferent contains chemoreceptors) and special visceral efferent (because the motor component innervates skeletal muscle and is derived from a branchial arch), respectively.

The smooth muscle of the bladder is innervated by postganglionic fibers of the sympathetic nervous system that arise from the inferior mesenteric ganglion. This ganglion, in turn, receives its inputs from T12–L2 of the intermediolateral cell column of the spinal cord. The smooth muscle of the bladder also receives inputs from postganglionic parasympathetic fibers that are innervated by preganglionic fibers arising from S2–S4. The external sphincter of the bladder (striated muscle) is innervated by ventral horn cells from the spinal cord. These ventral horn cells, in turn, receive inputs from supraspinal neurons that arise, in part, from the cerebral cortex. It is these neurons that form a part of the substrate for voluntary control over bladder functions.

Meissner's corpuscles, Merkel's receptors, and pacinian corpuscles respond to tactile, pressure, or possibly vibratory stimuli, while free nerve endings are associated with nociceptive stimuli. Joint capsules respond to movement of the limb, and the axons of these receptors contribute to the dorsal column–medial lemniscal system mediating the conscious perception of movement.

Since the flocculonodular lobe receives and integrates inputs from the vestibular system, it is understandable why lesions that disrupt this integrating mechanism for vestibular inputs would result in difficulties in maintaining balance. Indeed, this is a classic feature of lesions of the flocculonodular lobe but is not associated with lesions in the hemispheres of the posterior lobe, anterior limb of the internal capsule, or the dentate nucleus, which are functionally linked to the frontal lobe. Lesions of the anterior lobe also do not affect mechanisms of balance.

This patient does not have a UMN lesion (spinal cord or above) because of the absent reflexes and ascending paralysis bilaterally involving all of the extremities. Lesions in the brain almost always give unilateral findings, and spinal cord lesions give a distinct level. The damage cannot be in the muscle, because the patient has sensory involvement, as well. This case is an example of Guillain-Barré syndrome, or an inflammatory disease of the peripheral nerve resulting from demyelination. Inflammatory cells are found within the nerves, as well as segmental demyelination and some degree of wallerian degeneration. This damage can cause an ascending paralysis and sensory loss, affecting the arms, face, and legs. The CSF often has a high protein level, making a spinal tap a useful test for the diagnosis of Guillain-Barré syndrome. Nerve conduction studies are also helpful in making the diagnosis. Most neurologists believe Guillain-Barré syndrome to be an immunologic reaction directed against the peripheral nerve, and some patients have a history of having had some type of infection prior to developing Guillain-Barré syndrome. However, a clear-cut cause is rarely found. Despite a known cause, most patients recover from Guillain-Barré syndrome, although the speed of recovery varies. Treatment is currently available (administration of gamma globulin), and, if instituted early in the course of the disease, decrease in the length of the illness is possible.

Structures associated with sensory functions, such as the proper sensory nucleus and the spinal nucleus of cranial nerve V, are derived from the alar plate

Extensive myelination occurs in postnatal development. The failure of the pyramidal tracts to form myelin would account for the reduction in their size. In this particular situation, the size of the cerebral cortex was approximately normal, suggesting that there was no significant decrease in cortical cells. Variation in the numbers of synaptic contacts, transmitter formation, and glial cells would not account for a reduction in the size of the pyramidal tract

This figure is a lateral view of the cerebral cortex. Cells in the "arm" area of the primary motor cortex (H) project their axons to the cervical level of the spinal cord. This area receives major input from the ventrolateral nucleus of the thalamus. The leg region of the primary somatosensory cortex (A) lies immediately caudal to the central sulcus, is almost devoid of pyramidal cells, and is referred to as a granulous cortex. Damage to the cells situated in the region of the dorsal border of the superior temporal gyrus and the adjoining area of the inferior parietal lobule (Wernicke's area) (C) causes impairment in the appreciation of the meanings of written or spoken words.
The primary, secondary, and tertiary auditory receiving areas in the cortex are located mainly in the superior temporal gyrus (D). It is the final receiving area for inputs from the medial geniculate nucleus, which represents an important relay in the transmission of auditory signals to the cortex. An additional area of the cortex governing speech (F) is called the motor speech area, or Broca's area. It is situated in the inferior aspect of the frontal lobe immediately rostral and slightly ventral to the precentral gyrus. Lesions of this region produce impairment of the ability to express words in a meaningful way or to use words correctly. The orbital frontal cortex (E) lies in a position inferior and rostral to Broca's motor speech area. This region governs higher-order intellectual functions and some aspects of emotional behavior.
The caudal aspect of the middle frontal gyrus (G) contains cells that, when activated, produce conjugate deviation of the eyes. This action is believed to be accomplished, in part, by virtue of descending projections to the superior colliculus, pretectal region, and horizontal gaze center of the pons. Lesions of the posterior parietal lobe (B) of the nondominant hemisphere will produce a disorder of body image, referred to as sensory neglect. The patient will frequently fail to recognize or neglect to shave or wash those body parts. The patient may even fail to recognize the presence of a hemiparesis involving that part of the body as well. The precentral gyrus (H) constitutes the primary motor cortex. Lesions of this region produce a UMN paralysis involving a contralateral limb.

This figure is a lateral view of the cerebral cortex. Cells in the "arm" area of the primary motor cortex (H) project their axons to the cervical level of the spinal cord. This area receives major input from the ventrolateral nucleus of the thalamus. The leg region of the primary somatosensory cortex (A) lies immediately caudal to the central sulcus, is almost devoid of pyramidal cells, and is referred to as a granulous cortex. Damage to the cells situated in the region of the dorsal border of the superior temporal gyrus and the adjoining area of the inferior parietal lobule (Wernicke's area) (C) causes impairment in the appreciation of the meanings of written or spoken words.
The primary, secondary, and tertiary auditory receiving areas in the cortex are located mainly in the superior temporal gyrus (D). It is the final receiving area for inputs from the medial geniculate nucleus, which represents an important relay in the transmission of auditory signals to the cortex. An additional area of the cortex governing speech (F) is called the motor speech area, or Broca's area. It is situated in the inferior aspect of the frontal lobe immediately rostral and slightly ventral to the precentral gyrus. Lesions of this region produce impairment of the ability to express words in a meaningful way or to use words correctly. The orbital frontal cortex (E) lies in a position inferior and rostral to Broca's motor speech area. This region governs higher-order intellectual functions and some aspects of emotional behavior.
The caudal aspect of the middle frontal gyrus (G) contains cells that, when activated, produce conjugate deviation of the eyes. This action is believed to be accomplished, in part, by virtue of descending projections to the superior colliculus, pretectal region, and horizontal gaze center of the pons. Lesions of the posterior parietal lobe (B) of the nondominant hemisphere will produce a disorder of body image, referred to as sensory neglect. The patient will frequently fail to recognize or neglect to shave or wash those body parts. The patient may even fail to recognize the presence of a hemiparesis involving that part of the body as well. The precentral gyrus (H) constitutes the primary motor cortex. Lesions of this region produce a UMN paralysis involving a contralateral limb.

Since the lesion is restricted to the medial aspect of the basilar part of the pons, the corticospinal tract would be affected, producing paralysis of the contralateral limbs. Although other structures would also be affected and could produce additional deficits, such deficits are not listed in this question. The other dysfunctions listed would not occur because they are associated with structures situated in the pontine tegmentum, which is not included in this lesion.

The supraoptic nucleus, like the paraventricular nucleus, contains magnocellular neurons that synthesize vasopressin and oxytocin and transport these hormones down their axons to the posterior pituitary. For this reason, the supraoptic nucleus plays a significant role in the regulation of water balance. There is no evidence to support the notion that the supraoptic nucleus has a role in feeding behavior, temperature regulation, sexual behavior, or short-term memory functions.

This figure is a lateral view of the cerebral cortex. Cells in the "arm" area of the primary motor cortex (H) project their axons to the cervical level of the spinal cord. This area receives major input from the ventrolateral nucleus of the thalamus. The leg region of the primary somatosensory cortex (A) lies immediately caudal to the central sulcus, is almost devoid of pyramidal cells, and is referred to as a granulous cortex. Damage to the cells situated in the region of the dorsal border of the superior temporal gyrus and the adjoining area of the inferior parietal lobule (Wernicke's area) (C) causes impairment in the appreciation of the meanings of written or spoken words.
The primary, secondary, and tertiary auditory receiving areas in the cortex are located mainly in the superior temporal gyrus (D). It is the final receiving area for inputs from the medial geniculate nucleus, which represents an important relay in the transmission of auditory signals to the cortex. An additional area of the cortex governing speech (F) is called the motor speech area, or Broca's area. It is situated in the inferior aspect of the frontal lobe immediately rostral and slightly ventral to the precentral gyrus. Lesions of this region produce impairment of the ability to express words in a meaningful way or to use words correctly. The orbital frontal cortex (E) lies in a position inferior and rostral to Broca's motor speech area. This region governs higher-order intellectual functions and some aspects of emotional behavior.
The caudal aspect of the middle frontal gyrus (G) contains cells that, when activated, produce conjugate deviation of the eyes. This action is believed to be accomplished, in part, by virtue of descending projections to the superior colliculus, pretectal region, and horizontal gaze center of the pons. Lesions of the posterior parietal lobe (B) of the nondominant hemisphere will produce a disorder of body image, referred to as sensory neglect. The patient will frequently fail to recognize or neglect to shave or wash those body parts. The patient may even fail to recognize the presence of a hemiparesis involving that part of the body as well. The precentral gyrus (H) constitutes the primary motor cortex. Lesions of this region produce a UMN paralysis involving a contralateral limb.

Pain is mediated by C delta and A delta fibers in the skin.

The central pathways mediating taste include the following: primary afferent taste fibers associated with taste receptors of cranial nerves VII, IX, and X synapse in the solitary nucleus. Many fibers from the solitary nucleus project to the ventral posteromedial nucleus of the thalamus, which, in turn, project to the ventrolateral aspect of the postcentral gyrus.

The premotor areas play an important role in the programming or sequencing of responses that compose complex learned movements. They receive significant inputs for this process from the posterior parietal lobule and, in turn, signal appropriate neurons in the brainstem and spinal cord (both flexors and extensors). Lesions of the postcentral gyrus produce a somatosensory loss. Lesions of the precentral gyrus produce paralysis. Neither lesions of the prefrontal cortex nor those of the cingulate gyrus have been reported to produce apraxia.

Because one-half of the spinal cord is damaged, the dorsal columns are damaged, and the patient will have loss of proprioception and vibration ipsilateral to and below the level of the lesion. The loss must be ipsilateral because fibers mediating this type of sensation cross above the level of the lesion. The fasciculus gracilis carries fibers originating from the sacral, lumbar, and lower thoracic levels, and the fasciculus cuneatuse carries those from the upper thoracic and cervical levels. Lissauer''s tract carries pain and temperature fibers via the dorsal root entry zone. Brown-Séquard''s syndrome may occur as a result of different types of tumors or infections of the spinal cord.

Calcification of the internal carotid artery could serve to disrupt nerve fibers proximal to it. One such group of fibers includes parts of the optic nerve. In this case, the component of the right optic nerve affected includes the lateral aspect, or those fibers that mediate vision associated with the nasal visual field of the right eye. If the damage were more extensive and if it involved the entire nerve, then total blindness of the right eye would have occurred.

Specialized peripheral receptors, which specifically respond to changes in blood pressure, include the carotid sinus (associated with cranial nerve IX) and the aortic arch (associated with cranial nerve X). If these receptors (or the cell bodies associated with these receptors) are damaged, then one of the fundamental regulatory mechanisms for the control of blood pressure would be disrupted. The results of such a disruption would likely lead to increases and instability in blood pressure with evidence of spiking of blood pressure. Because these sensory receptors in these structures respond to increases in blood pressure, they are, in effect, stretch receptors and are consequently referred to as baroreceptors. The principal projection of the axons associated with these baroreceptors is the solitary nucleus of the medulla, which in turn, projects to autonomic nuclei such as the dorsal motor nucleus of the vagus nerve, ventrolateral medulla, and higher regions associated with autonomic functions, which include the PAG, hypothalamus, and limbic system.

The nerve affected by this disorder is cranial nerve VII (facial nerve). The cell bodies of origin, which innervate the muscles of facial expression (special visceral efferents), arise from the facial nucleus, which are located in the ventrolateral aspect of the lower pons. The preganglionic parasympathetic neurons, which synapse with postganglionic neurons in the submandibular and pterygopalatine ganglia, arise from the superior salivatory nucleus of the lower pons. The most likely locus of the defect is the geniculate ganglion. The region of the geniculate ganglion and regions adjacent to it contain sensory, skeletal, and visceral motor components of this nerve. Therefore, disruption of this nerve in the region of the geniculate ganglion will produce the constellation of deficits described in this case. The other choices are not appropriate. Cranial nerve IX is not involved. Neither are the regions of the reticular formation and facial nucleus, because lesions at either of these locations could not account for the totality of deficits described in this case. The lesion could not have involved the cerebral cortex because the motor effects were described as a flaccid facial paralysis. A cortical lesion does not produce flaccidity of these muscles.

Taste associated with the anterior two-thirds of the tongue is mediated by the facial (cranial nerve VII) nerve. The geniculate ganglion contains the cell bodies associated with the sensory (gustatory) component of the seventh nerve. The somatic motor component of the seventh nerve mediates the muscles of facial expression. Thus, the sensory and motor components of the seventh nerve affected in this individual can be characterized as special visceral afferent (because this afferent contains chemoreceptors) and special visceral efferent (because the motor component innervates skeletal muscle and is derived from a branchial arch), respectively.

The nerve affected by this disorder is cranial nerve VII (facial nerve). The cell bodies of origin, which innervate the muscles of facial expression (special visceral efferents), arise from the facial nucleus, which are located in the ventrolateral aspect of the lower pons. The preganglionic parasympathetic neurons, which synapse with postganglionic neurons in the submandibular and pterygopalatine ganglia, arise from the superior salivatory nucleus of the lower pons. The most likely locus of the defect is the geniculate ganglion. The region of the geniculate ganglion and regions adjacent to it contain sensory, skeletal, and visceral motor components of this nerve. Therefore, disruption of this nerve in the region of the geniculate ganglion will produce the constellation of deficits described in this case. The other choices are not appropriate. Cranial nerve IX is not involved. Neither are the regions of the reticular formation and facial nucleus, because lesions at either of these locations could not account for the totality of deficits described in this case. The lesion could not have involved the cerebral cortex because the motor effects were described as a flaccid facial paralysis. A cortical lesion does not produce flaccidity of these muscles.

The nerve affected by this disorder is cranial nerve VII (facial nerve). The cell bodies of origin, which innervate the muscles of facial expression (special visceral efferents), arise from the facial nucleus, which are located in the ventrolateral aspect of the lower pons. The preganglionic parasympathetic neurons, which synapse with postganglionic neurons in the submandibular and pterygopalatine ganglia, arise from the superior salivatory nucleus of the lower pons. The most likely locus of the defect is the geniculate ganglion. The region of the geniculate ganglion and regions adjacent to it contain sensory, skeletal, and visceral motor components of this nerve. Therefore, disruption of this nerve in the region of the geniculate ganglion will produce the constellation of deficits described in this case. The other choices are not appropriate. Cranial nerve IX is not involved. Neither are the regions of the reticular formation and facial nucleus, because lesions at either of these locations could not account for the totality of deficits described in this case. The lesion could not have involved the cerebral cortex because the motor effects were described as a flaccid facial paralysis. A cortical lesion does not produce flaccidity of these muscles.

The lesion involves the superior colliculus. This structure receives inputs from the cerebral cortex and optic tract and its neurons respond to moving objects in the visual field. It is considered essential for the regulation of tracking movements. Lesions of the superior colliculus have not been shown to produce any of the other deficits listed in this question. Nystagmus is not likely to occur because the lesion does not involve the medial longitudinal fasciculus or the pontine gaze center.

The centromedian nucleus is a classical nonspecific thalamic nucleus. It can modulate cortical activity by making local connections with specific thalamic nuclei, and therefore modify the specific thalamic inputs to different regions of the cerebral cortex. In addition, the centromedian nucleus also projects to the putamen. This projection is sometimes referred to as the thalamostriatal projection. Since the centromedian nucleus receives considerable inputs from the cerebral cortex, this connection to the putamen provides a basis by which the cerebral cortex can influence the basal ganglia in addition to its direct projections to the neostriatum.

The major transmitter released at terminals of neostriatal and paleostriatal fibers is GABA. Thus, the output of the basal ganglia is mainly inhibitory. This suggests that thalamic influences upon the cortex are generated through the process of disinhibition, whereby neurons of the basal ganglia are inhibited. The presence of glycine in striatal neurons has yet to be demonstrated. Enkephalins are released from terminals of neostriatal-pallidal fibers but not from other efferent neurons of the striatum. Dopamine is released from the brainstem and some adjoining hypothalamic neurons but certainly not from striatal neurons. The neostriatum receives cortical inputs that utilize glutamate, but the release of GABA from terminals of striatal efferent fibers has not been demonstrated.

Emma has had a stroke resulting from occlusion of medial branches of the left vertebral artery, presumably secondary to atherosclerosis (i.e., cholesterol deposits within the artery, which eventually occlude it). The resulting syndrome is called the medial medullary syndrome, because the affected structures are located in the medial portion of the medulla. These structures include: the pyramids, the medial lemniscus, the medial longitudinal fasciculus, and the nucleus of the hypoglossal nerve and its outflow tract. Emma's symptoms result from damage to the aforementioned structures, and may have been caused by the same process (atherosclerosis) that resulted in her heart disease. The weakness of her right side was caused by damage to the medullary pyramid on the left side. Her face was spared because fibers supplying the face exited above the level of infarct. However, a lesion in the corticospinal tract of the cervical spinal cord above C5 could cause arm and leg weakness, and spare the face, because facial fibers exit in the rostral medulla. A lesion in the inferior portion of the precentral gyrus of the left frontal lobe would cause right-sided weakness, but would include the face, because this area is represented more inferiorly than are the extremities. Her unsteady gait was a result of the weakness of her right side, but may also have been the result of the loss of position and vibration sense on that side from damage to the medial lemniscus (as demonstrated by the inability to identify the position of her toe with her eyes closed, and the inability to feel the vibrations of a tuning fork). Without position sense, walking becomes unsteady because it is necessary to feel the position of one's feet on the floor during normal gait. Damage to both the medial lemniscus and pyramids at this level causes problems on the contralateral side because this lesion is located rostral to the level where both of these fiber bundles cross to the opposite side of the brain. Damage to the descending component of the medial longitudinal fasciculus could only affect head and neck reflexes, but not gait. Gait is also unaffected by pain inputs. Deviation of the tongue occurs because fibers from the hypoglossal nucleus innervate the genioglossus muscle on the ipsilateral side of the tongue. This muscle normally protrudes the tongue toward the contralateral side. Therefore, if one side is weak, the tongue will deviate toward the side ipsilateral to the lesion when protruded. A lesion in the precentral gyrus causes protrusion of the tongue toward the side that is contralateral to the lesion, because it is rostral to the crossing of fibers into the hypoglossal nucleus. Emma's speech was dysarthric (slurred) because her tongue was weak on the left side. The physician saw this during the exam when her tongue deviated to the left when protruded. Since the weakness of the tongue is purely a motor problem, rather than an effect that is manifested by a lesion to higher centers in the cortex (which mediate the structure and function of speech), the grammar, content, and meaning of Emma's speech remained intact, as would be expected with an aphasia or agnosia.

Fibers from the left lateral geniculate destined for the upper bank of the calcarine fissure will mediate visual impulses associated with lower quadrants of the right visual fields for both eyes. This deficit is referred to as a right lower quadrantanopia.

Lesions of the lateral hypothalamus are likely to produce aphagia. Feeding behavior is elicited by stimulation of the lateral hypothalamus. Neurons in this region respond to the sight or taste of food. Since drinking is also associated with lateral hypothalamic functions, a lesion of this structure would also disrupt this behavior. Lesions of the lateral hypothalamus do not produce either hypertension or sexual behaviors. The neurons regulating these functions are elsewhere within the hypothalamus.