THE MESENCEPHALON The mesencephalon, or midbrain, is characterized by two large fiber bundles on the ventral surface called the cerebral peduncles. Caudally these peduncles, which carry cortical fibers to lower levels, pass into the pons and rostrally they become the internal capsule. 1. Relative to the cerebral peduncles (crus cerebri) the trochlear (C.N. IV) nerves appear at the lateral borders oculomotor nerves emerge at the lateral borders oculomotor nerves emerge between them optic tracts enter the interpeduncular fossa Correct. No. The first and third foils are correct! The trochlear nerve leaves the mesencephalon on the dorsal surface and curves around the peduncles to appear at the lateral borders of the peduncles. 2. The dorsal surface of the midbrain contains superior colliculus inferior colliculus trochlear (C.N.IV) nerve cerebral peduncles (crus cerebri) Right. No The first three foils are correct> The peduncles are on the ventral surface. 3. The tectum of the midbrain includes the main portion ventral to central gray basal portion on the ventral surface reticular nuclei superior and inferior colliculi Correct. No. The fourth foil is correct. The main central part of the midbrain is the tegmentum and the basal portion includes the cerebral peduncles (crus cerebri). The substantia nigra is a dark pigmented area between the cerebral peduncles and tegmentum. 4. At the level of the inferior colliculus, the tectum is the inferior colliculus. The afferent connections of the inferior colliculus include which of the following nuclei and tracts. lateral lemniscus ipsilateral medial geniculate cerebral cortex (primary auditory cortex) substantia nigra Right. No. The first three foils are correct. The inferior colliculus is a relay nucleus in the auditory pathway. It receives auditory impulses from lateral lemniscus and receives feedback impulses from auditory cortex and medial geniculate. 5. The inferior colliculus projects (efferent connections) to which of the following structures hippocampus superior colliculus auditory cortex medial geniculate via brachium of the inferior colliculus Right. No. The secons and fourth foils are correct. The medial geniculate body receives auditory impulses from the inferior colliculus and sends them on to the cortex. Impulses are also conveyed to the superior colliculus which establishes the reflex for turning the eyes in response to sound. In addition to these efferent connections, the cerebellum also receives fibers from the inferior colliculus via the anterior medullary vellum. The inferior colliculus may play a role in localizing sound. 6. At the level of the inferior colliculus, in the tegmentum (area ventral to central gray) the medial longitudinal fasciculus is lateral to the spinothalamic tract decussation of the superior cerebellar peduncle (brachium conjunctivum) occurs medial lemniscus lies along the midline rubrospinal tract is just dorsal to the medial lemniscus Right. No. The second and fourth foils are correct. At this level the medial lemniscus is more lateral and follows the contour of the substantia nigra. The medial longitudinal fasciculus always lies very medial, just ventral to the central gray. It coordinates the eye movements interconnecting C.N. III, IV, VI. At the level of the inferior colliculus there are important nuclei which are arranged around the aqueduct. They include locus ceruleus, mesencephalic nucleus, trochlear nucleus, dorsal tegmental nucleus, ventral tegmental nucleus and the more ventral interpeduncular nucleus. 7. The mesencephalic nucleus is in the auditory pathway is homologous to the dorsal root ganglia has neurons which convey pain and temperature sensations from the face has unipolar neurons with axons which convey proprioceptive impulses from muscles of mastication and peridontal membranes Correct. No. The second and fourth foils are correct. Pain and temperature from the face are carried by fibers that synapse with neurons in the spinal trigeminal nucleus. 8. The nucleus of the trochlear nerve (C.N.IV) lies in the V-shaped ventral part of the central gray project axons which arch around the central gray and cross in the dorsal mesencephalon is the only cranial nerve that crosses in the brainstem and only cranial nerve that emerges on the dorsal aspect of the brainstem projects axons which innervate the lateral rectus muscle of the eye Correct. No. The first three foils are correct. The trochlear nerve innervates the superior oblique muscle of the eye. A lesion of one nucleus causes paralysis of the contralateral superior oblique because of the crossing of the fibers. This causes a vertical diplopia (double vision). 9. The locus ceruleus ( nucleus pigmentosa) is located at the edge of the central gray neurons produce acetylcholine neurons produce norepinephrine cell bodies are clear and non-pigmented Correct. No. The first and third foils are correct. The locus ceruleus is believed to be the major norepinephrine producing nucleus in the brain with axons from the nucleus spreading throughout the brain. The nucleus contains pigmented cells containing melanin granules. 10. The cerebral peduncles (crus cerebri) are continuous with the internal capsule rostrally contain corticospinal neurons in the middle three fifths have corticopontine fibers on each side of the central corticospinal area are composed largely of non-myelinated fibers Correct. No. The first three foils are correct. The peduncles are mostly myelinated fibers, giving them a white appearance on cross section. 11. The substantia nigra is located between cerebral peduncles and the tegmentum contains a zona compacta and zona reticulata is involved in regulating motor activity is usually degenerated in patients with Parkinson's disease Right. No. All foils are correct. 12. The nigrostriatal tract projects from substantia nigra to thalamus contains cholinergic fibers contains adrenergic (norepinephrine producing) fibers contains dopaminergic fibers Right. No. The second and last foils are correct. The fibers connect the nigra with caudate nucleus. The cholinergic (acetylcholine producing) fibers facilitate activity of caudate nucleus and dopaminergic fibers inhibit its activity. The system is important in Parkinson"s disease where higher dopamine (L-dopa treatment) levels reduce the limb rigidity and tremor associated with the disease. 13. At the level of the superior colliculus in the midbrain,the superior colliculus is the tectum. The superior colliculus is involved in visual reflexes has a uniform appearance of gray matter receives retinal fibers corticollicular fibers arise only from the occipital lobe Right. No. The first and third foils are correct. The afferent connections of the superior colliculus include the retina and corticocollicular (corticotectal) fibers that arise from all over the cortex, but most come from the occipital lobe. Those from the frontal lobe (frontal eye fields) are concerned with conjugate eye movements. The superior colliculus is a laminated, light-dark, white matter-gray-matter, structure. 14. In addition to cortex and retina, the superior colliculus receives fibers directly from medial geniculate body spinal cord cerebellum inferior colliculus Right. No. The second and third foils are correct. The spinal cord fibers ascend as the spinotectal tract and may convey pain sensation. The inferior colliculus sends fibers to the superior colliculus via the tectotectal tract and this communication is believed involved in head and eye turning reflex toward a sound. 15. The efferent connections of the superior colliculus are tectospinal tract tectopontocerebellar tract tectoreticular tract spinotectal tract Right. No. The first three foils are correct. The tectospinal tract is made up of fibers from superior colliculus that decussate in the dorsal tegmentum, they descend near the medial longitudinal fasciculus to the cervical cord. They play a role in neck movement in response to visual stimuli. Rostral to the superior colliculus is the pretectal area which receives retinal fibers and is involved in pupillary reflexes. It projects fibers to the oculomotor nuclei. This area appears involved in vertical gaze because tumors of the overlying pineal gland often prevent vertical gaze by compressing or infiltrating this area. The aqueduct is obstructed early with pinealomas, causing increased intracranial pressure and dilated ventricles. These signs make up Parinaud's syndrome; primarily failure of conjugate eye movement above the horizontal. 16. The pretectal nucleus lies rostral to the superior colliculus between the mesencephalic -diencephalic junction on each side are connected by the posterior commissure sends fibers to homolateral and contralateral Edinger-Westphal nucleus stimulation causes pupillary dilatation Right. No. The first three foils are correct. The pretectal fibers project to the rostral portions of homolateral and contralateral Edinger-Westphal nuclei. The Edinger-Westphal nucleus is concerned with the light reflex. Since impulses set up by light in either eye can reach both Edinger-Westphal nuclei, strong light in one eye causes bilateral pupillary constriction. The fibers from the Edinger-Westphal nuclei pass through the oculomotor nerves to the ciliary ganglia (parasympathetic ganglion) and post ganglionic fibers then enter the sphincter (circular) muscle of the iris. 17. A pupillary light reflex can occur in an eye to which the optic nerve has been severed. True False Right. No the answer is True. 18. The nuclear groups in the tegmentum ( the central midbrain area ventral to the aqueduct) at the superior colliculus level include red nucleus oculomotor nucleus accessory oculomotor nuclei trochlear nucleus Right. No. The first three foils are correct. The trochlear nucleus (C.N. IV) is at the level of the inferior colliculus. 19. The red nucleus in the tegmentum is pink because of rich vascularity receives (synaptically) oculomotor fibers receives fibers from deep cerebellar nuclei receives cortical fibers mainly from the postcentral gyrus Correct. No. The first and third foils are correct. Oculomotor rootlet fibers pass through the red nucleus but don't stop. Fibers from dentate, globose and emboliform nuclei of the cerebellum pass in the brachium conjunctivum to the tegmentum of the inferior colliculus where they decussate and partially synapse in the red nucleus. Lesions in this pathway cause intention tremors-tremor when reaching for something. Most corticorubral fibers come from the precentral gyrus (motor cortex). 20. The efferent fibers from the red nucleus project to spinal cord cerebellum reticular formation inferior olive Right. No. All foils are correct. The red nucleus also projects to the thalamus. The red nucleus is essentially a synaptic relay station for pathways involved with movement cerebello- thalamic system, corticospinal system (Rubrospinal fibers) project to the same spinal cord laminae as the corticospinal tract The rubrospinal tract is considered an indirect corticospinal tract (cerebral cortex to red nucleus to spinal cord). 21. The oculomotor nucleus is ventral to the medial longitudinal fasciculus (MLF) is in the tegmentum at the level of the inferior colliculus projects General Somatic Afferent (GSA) fibers contains a medial visceral cell column that projects General Visceral Efferent (GVE) fibers Right. No, only the fourth foil is correct. The oculomotor nucleus (C.N. III) is dorsal to the MLF and it lies in the tegmentum of the midbrain at the level of the superior colliculus, not at the level of the inferior colliculus. The oculomotor nucleus is motor to external ocular muscles (the superior, medial and inferior recti, levator palpebrae superioris of the eyelid and the inferior oblique) thus the fibers are efferent (GSE) not afferent (GSA). A lesion of an oculomotor nerve causes ipsilateral drooping of the eyelid, deviation of the eye out and down, diplopia ( double vision) dilated pupil unresponsive to light and accommodation. 22. From the following symptoms, pinpoint the site of the patient's lesion. Deviation of the left eye down and out plus drooping eyelid and dilated, unresponsive (to light) pupil. Also contralateral upper motor neuron paralysis (spasticity). pretectal nucleus tectum at superior colliculus tegmentum on the left medial part of cerebral peduncle on the left oculomotor nerve after leaving the midbrain Right. No. Only the fourth foil is correct. 23. Outward and downward deviation of left eye including drooping eyelid, dilated non-responsive pupil and contralateral tremor on reaching for objects (intention tremor) pretectal nucleus tectum -sup.colliculus tegmentum on left medial aspect of left peduncle oculomotor nerve after leaving midbrain Right. No. The second and fourth foils are correct. 24. Outward, downward deviation of left eye with drooping eyelid and dilated unresponsive pupil. Normal vision in eye. pretectal nucleus tectum at superior colliculus tegmentum on left medial aspect of left peduncle left oculomotor nerve Right. No. The answer is E. 25. A light is directed into the left eye of a patient and the pupils do not constrict in either eye. The lesion is in the right oculomotor nerve left oculomotor nerve left tegmentum left optic nerve right optic nerve Right. No. The answer is D. If you have trouble, you might sketch the afferent and efferent fibers of the light reflex. 26. A light is directed into the left eye of a patient and only the right pupil constricts. The lesion is in the right oculomotor nerve or nucleus left oculomotor nerve or nucleus left tegmentum left optic nerve right optic nerve Right. No. The answer is B. 27. The accommodation-convergence reflex is a reflex which occurs when one looks at a near object. In accommodation - convergence reflex the ciliary muscle contracts, relaxing the suspensory ligament of the lens pupillary dilatation contraction of the medial rectus muscles (convergence) flattening of the lens Right. No. The first and third foils are correct. The ciliary muscles contract, relaxing the suspensory ligament of the lens. This allows the lens to return to a more round shape (thickening), allowing for a sharper focus on close objects. The pupil constricts during this reflex. 28. The pathway for the accommodation - convergence probably includes afferent fibers from retina to occipital cortex efferent fibers from occipital cortex to inferior colliculus efferent fibers to the nucleus of Perlia fibers from nucleus of Perlia, with oculomotor nerve to otic ganglia Right. No. The first and third foils are correct. The pathway is not known for sure but it is different from the light reflex since in syphilis of the nervous system an Argyll Robertson pupil may be present in which the pupil does not constrict with light but it does constrict during accommodation. The efferent fibers from the occipital cortex go to prectectal or tectal area (sup. colliculus). The parasympathetic preganglionic fibers from the Nucleus of Perlia project to the ciliary ganglia in the posterior orbital cavity. The nucleus of Perlia (central nucleus) is thought to be a midline cell group concerned with convergence. However, it is difficult to identify in man and monkey, and its existence and function remain in doubt. 29. The mesencephalic reticular formation merges rostrally with the zona incerta is a continuation of the pontine reticular nuclei major output is to diencephalon and cerebral cortex functions in behavioral and sleep mechanisms Right. No. All foils are correct.