Answers and Explanations

1. Answer: C. Purkinje cells are inhibitory neurons of the cerebellar cortex that utilize GABA as their neurotransmitter. Their axons mostly do not leave the cerebellar cortex but synapse on cells of the deep cerebellar nuclei. They receive direct excitatory input from climbing fibers and indirect excitatory input from mossy fibers by way of axons of granule cells. Golgi and stellate cells are inhibitory intemeurons in the cerebellar cortex.

2. Answer: A. Dysmetria is the inability to stop a movement at the proper place.

3. Answer: C. Chronic alcohol abuse preferentially affects anterior vermis Purkinje cells; the vermis controls proximal musculature so that lesions produce gait ataxias.

4. Answer: C. A Romberg sign is indicative of a dorsal column somatosensory lesion, not a cerebellar lesion.

Spinal cord Vestibular system Cerebral cortex Olive

Deep cerebellar nuclei

Answer: C. Axons, which enter the cerebellum in the middle cerebellar peduncle, arise form neurons situated in pontine nuclei. Corticopontine axons, which arise from neurons located in the frontal lobe, are the primary source of input to these pontine neurons.

These axons will make up the optic nerve

Figure IV-7-2. Retina

At the optic chiasm, 60% of the optic nerve fibers from the nasal half of each retina cross and project into the contralateral optic tract (Figure IV-7-3). Fibers from the temporal retina do not cross at the chiasm and instead pass into the ipsilateral optic tract. The optic tract contains remixed optic nerve fibers from the temporal part of the ipsilateral retina and fibers from the nasal part of the contralateral retina. Because the eye inverts images like a camera, in reality each nasal retina receives information from a temporal hemifield, and each temporal retina receives information from a nasal hemifield. Most fibers in the optic tract project to the lateral geniculate nucleus. Optic tract fibers also project to the superior colliculi for reflex gaze, to the pretectal area for the light reflex, and to the suprachiasmatic nucleus of the hypothalamus for circa dian rhythms.

Visual Pathways

AJl lesions past the chiasm produce contralateral defects. Lesions of the optic tract result in a loss of visual input from the contralateral visual field. For example, a lesion of the right optic tract results in a loss of input from the left visual field. This is called a homonymous hemi-anopia; in this example, a left homonymous hemianopia.

Lesions of the visual radiations are more common than lesions to the optic tract or lateral geniculate body and produce visual field defects (a contralateral homonymous hemianopia) similar to those of the optic tract if all fibers are involved.

Lesions restricted to the lateral fibers in Meyer loop, usually in the temporal lobe, result in a loss of visual input from the contralateral upper quarter of the visual field. For example, a lesion of the temporal fibers in the right visual radiation results in loss of visual input from the upper left quarter of the field (a left superior quadrantanopia).

Lesions restricted to the medial fibers in the visual radiation in the parietal lobe result in a loss of visual input from the contralateral lower quarter of the field (an inferior quadrantanopia).

Lesions inside the primary visual cortex are equivalent to those of the visual radiations, resulting in a contralateral homonymous hemianopsia, except that macular (central) vision is spared.

Lesions of the cuneus gyrus are equivalent to lesions restricted to the parietal fibers of the visual radiation, with macular sparing.

Lesions of the lingula are similar to lesions of the Meyer's loop fibers except for the presence of macular sparing. The pupillary fight reflex is spared in lesions of the radiations or inside visual cortex because fibers of the pupillary light reflex leave the optic tracts to terminate in the pretectal area. The combination of blindness with intact pupillary reflexes is termed cortical blindness.

Note

Lesions to the visual radiations are more common than lesions to the optic tract

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