THE PONS The PONS is the central part of the brain stem and lies rostral to the medulla and caudal to the midbrain (mesencephalon). The pons is easily identified by its ventral bulge, the pontine protuberance. 1. What artery lies against the pontine protuberance in the pontine groove? posterior cerebral basilar vertebral arteries posterior communicating posterior inferior cerebellar Very good! No, the answer is B. The vertebral arteries come together at the caudal margin of the pons to form the basilar artery. The basilar artery ascends in the pontine groove to the rostral end of the pons where it divides into its terminal branches, the posterior cerebral arteries. 2. On coronal section of the pons the pons can be divided into basis pontis ( basal or ventral) ventral pyramids tegmentum granular layer Very good! No. The first and third foils are correct. The pons can be divided into the ventral basis pontis and the dorsal tegmentum. The basis pontis corresponds to the pontine protuberance and contains the The basis pontis corresponds to the pontine protuberance and contains the pontine nuclei. The tegmentum is phylogentically older and consists largely of the reticular formation. 3. The fiber systems of the basis (ventral or basal) pontis are the corticospinal fibers corticobulbar fibers corticopontocerebellar tract spinocerebellar tract Very good! No. The first three foils are correct. The corticospinal fibers pass through the pons on going from cortex to spinal cord. The corticobulbar fibers descend from cortex and most synapse in a reticular nucleus before reaching cranial nerve nuclei (corticoreticularbulbar). The fibers in the corticopontocerebellar tract originate from wide areas of cortex, synapse on ipsilateral pontine neurons that cross the middle and form the middle cerebral peduncle enroute to the cerebellum. 4. The specific sensory lemniscal system in the pons lies in the dorsal tegmental area includes medial lemniscus, trigeminal lemniscus and spinothalamic tract carries pain and temperature from ipsilateral half of body lies horizontally along basal tegmentum just dorsal to the basis pontis Very good! No. The second and fourth foils are correct. The spinothalamic tract is the most lateral of this lemniscal system and carries pain and temperature from the contralateral half of the body. The trigeminal tract is just medial, carrying sensation from the face. 5. The transverse fibers of the trapezoid body arise from the cochlear nuclei intermingle with ascending fibers of the sensory lemniscal system gather in lateral pons to form the lateral lemniscus are part of the visual system Very good! No. The first three foils are correct. The trapezoid body and lateral lemniscus are in the auditory system. 6. Tracts which pass through the tegmentum of the pons include medial longitudinal fasciculus rubrospinal tract descending sypathetic fibers from the hypothalamus corticobulbar fibers enroute from basis pontis to cranial nerve nuclei Very good! No. All foils are correct. The medial longitudinal fasciculus and tectospinal tract are next to the midline just beneath the floor of the 4th ventricle. A lesion of dorsal pons will, among other things, cause a Horner's syndrome due to division of sympathetic fibers. The pontine reticular formation is a rostral continuation of the medullary reticular formation and is also divided into medial and lateral groups. Damage to the reticular formation usually results in unconsciousness. 7. The pontine reticulospinal tract arises from reticularis pontis caudalis raphe nucleus reticularis pontis oralis parvicellular reticular nucleus Very good! No. The first and third foils are correct. The oralis is a rostral extension of the caudalis and their output (reticulospinal tract) modulates extensor and flexor motor neurons as well as forms the ascending reticular activating fiber system. The raphe reticular nucleus is on each side of the midline and contains neurons that produce serotonin, very likely a neurotransmitter. Cranial nerves which leave the ventral surface of the pons include abducens (C.N. VI), facial (C.N. VII), cochleovestibular (C.N.VIII), and trigeminal (C.N. V.). 8. The cochleovestibular (C.N. VIII) nerve travels as one nerve from inner ear to pons. they take separate paths in the pons. The cochlear division of C.N. VIII is larger than the vestibular division contains central fibers of bipolar neurons in the spinal ganglia has peripheral processes which are linked to hair cells in the organ of Corti contains central fibers that project to the vestibular nuclei in the caudal pons Very good! No. The first three foils are correct. The central fibers of the cochlear nerve project to the dorsal and ventral cochlear nuclei. 9. The dorsal cochlear nucleus (located just dorsal to the restiform body) receives fibers mediating high frequency sound mediating low frequency sound from hair cells in basal turus of the cochlea from hair cells in apical turus of the cochlea Very good! No. The first and third foils are correct. The ventral cochlear nucleus receives the low frequency sound impulses from the apical turus of the cochlea. 10. The ventral acoustic stria in the pons is also known as the trapezoid body originates from the dorsal cochlear nucleus consists of second order neurons projects to the inferior olivary nucleus Very good! No. The first and third foils are correct. The intermediate and ventral acoustic stria are second order neurons (second neuron from the receptor site in the organ of Corti) from the ventral cochlear nucleus. These fibers project to the superior olivary nucleus and nucleus of the trapezoid body. Most third order neurons from these nuclei form the contralateral lateral lemniscus. Thus a lesion of the lateral lemniscus would cause greatest hearing loss in the contralateral ear. 11. On leaving the lateral lemniscus, auditory fibers continue to synapse in their ascent to the cortex. These relay stations include nucleus of the lateral lemniscus inferior colliculus medial geniculate body primary auditory cortex in the temporal lobe Very good! No. All foils are correct. A tonotopic organization is believed to exist throughout the auditory system. 12. The olivocochlear bundle carries high frequency impulses to the pons carries low frequency impulses is in the dorsal acoustic stria is efferent and causes feedback suppression of sound sensitivity in the organ of Corti Very good! No. Only the fourth foil is correct. The fibers in this bundle come from the counterlateral accessory olive and ipsilateral superior olive. This feedback inhibition regulates the activity in the auditory nerve. 13. Impulses along the cochlear nerve are controlled by displacement of Reissner's membrane tectorial membrane basilar membrane none of the above Very good! No, the answer is C. Displacement of the basilar membrane towards the scala tympany produces excitation and away from this membrane, inhibition. 14. Reflex movement of eyes and head toward a sound involve reflex pathways. One of these pathways include which of the following structures inferior colliculus superior colliculus tectobulbar and tectospinal pathways inferior olive Very good! No. The first three foils are correct. The inferior olive is not involved. The tectobulbar and tectospinal pathways project to the extraocular muscles and musculature of the neck. The second reflex pathway is from superior olive to the abducens (C.N. VI) nerve nucleus then medial longitudinal fasciculus to the other cranial nerves innervating extraocular muscles (C.N. IV) and C.N. III). 15. The bipolar cells in Scarpa's ganglion have peripheral fibers ending in semicircular canals, utricle and saccule have central fibers that form the vestibular nerve have central fibers that accompany from internal auditory meatus to pons have central fibers that project to the cochlear nuclei Very good! No. The first three foils are correct. The central fibers of bipolar cells in Scarpa's ganglion form the vestibular nerve. These fibers enter the pons at the pontomedullary junction (also called the cerebellopontine angle) and project largely to the four vestibular nuclei; a smaller portion goes to the cerebellum by way of the juxtarestiform body. 16. The vestibular nuclei include medial inferior lateral superior Very good! No. All foils are correct. The vestibular nuclei are in the tegmentum of the pons. 17. Besides receiving input from the vestibular nerve, the vestibular nuclei receive fibers from spinal cord cerebellum cortex (vestibular) thalamus Very good! No. The first three foils are correct. The output from the vestibular nuclei is similar to the input, ie, spinal cord and cerebellum, but fibers also project to the nuclei of the extraocular muscles, C.N. III, IV, VI. 18. The vestibular projection to the spinal cord is via the lateral vestibulospinal tract (from the lateral vestibular nucleus) superior vestibulospinal tract (superior nucleus) medial vestibulospinal tract inferior vestibulospinal tract Very good! No. The first and third foils are correct. Fibers of the lateral vestibulospinal tract facilitates extensor motor neurons, medial vestibulospinal tract facilitates flexor motor neurons. The medial medial vestibulospinal tract facilitates flexor motor neurons. The medial vestibulospinal tract also sends fibers to the dorsal motor nucleus of the vagus which accouonts for nausea and vomiting that may occur with motion sickness. 19. Vestibular output to nuclei of the extraocular muscles occurs via trapezoid body ascending medial longitudinal fasciculus (MLF) lateral lemniscus reticular formation Very good! No. The second and fourth foils are correct. 20. A lesion of the right MLF rostral to the abducens (C.N. VI) would cause adduction paralysis of right eye adduction paralysis left eye horizontal nystagmus of left eye horizontal nystagmus of right eye Very good! No. The first and third foils are correct. Lesions of the rostral MLF interfere with normal conjugate eye movements and this is called internuclear ophthalmoplegia. This condition is seen in multiple slerosis and in small infarcts (cerebral thromboses) in the pons. 21. The facial nerve (C.N.VII) is a mixed cranial nerve (both sensory and motor components). The sensory fibers are from external ear (exteroceptive) pharynx anterior two-thirds of tongue-taste fibers posterior one-third of tongue-taste fibers Very good! No. The first and third foils are correct. The GSA fibers from the external ear convey touch, pain and temperature senses. The cell bodies for those fibers are in the geniculate ganglion. The central processes these nerves project to neurons in the spinal trigeminal nucleus (similar fibers arrive here from C.N. IX and X). Cell bodies for the taste fibers (SVA) are also in the geniculate ganglion with the central fibers projecting to the nucleus solitarius (same as taste fibers of C.N. IX-post. one-third of tongue and C.N. X-epiglottic region). Some believe few fibers innervate the soft palate-these would be GVA. 22. The motor components of the facial nerve include general visceral efferent (GVE) general somatic efferent (GSE) special visceral efferent (SVE) A and C none of the above Very good! No, the answer is D. SVE fibers innervate the muscles of facial expression, the platysma, the buccinator and stapedius, stylohyoid and posterior belly of the digastric. The fibers arise from the motor nucleus and course around the abducens (C.N.VI) genu forming the facial colliculus on the floor of the fourth ventricle. 23. The special visceral efferent (SVE) fibers of the facial nerve (C.N.VII) arise from the basis pontis of the pons arise from the superior salivatory nucleus are postganglionic fibers are preganglionic fibers Very good! No. The first and third foils are correct. The SVE fibers are part of the parasympathetic division of the autonomic nervous system (craniosacral outflow). The fibers are preganglionic until they synapse in ganglia near the site of action. Those fibers carrying impulses for the lacrimal gland synapse in the pterygopalatine ganglion and the postganglionic fibers project to the lacrimal gland. Others synapse in the submandibular ganglia and postganglionic fibers innervate the submandibular and sublingual salivary glands. 24. The motor nucleus of the facial nere (C.N. VII) receives fibers from trigeminal system cerebral cortex basal ganglia superior olive Very good! No. All foils are correct. Fibers from cortex to C.N. VII are direct corticobulbar or corticoreticulobulbar. Cortical input is bilateral to the part of the nucleus supplying musclesto upper half of the face and only contralateral to nucleus supplying lower to upper half of the face and only contralateral to nucleus supplying lower half of the face. Thus, a cortical lesion on one side of the brain involving the facial motor area will only paralyze the lower, contralateral part of the face. The superior olive is involved in reflex grimacing in response to loud noise and the trigeminal system is part of the blinking reflex when particles get in the eye- stimulating nerve endings in the cornea. 25. The chorda tympani branches from the facial nerve and passes through the middle ear and then joins the lingual branch of the trigeminal (C.N. V). Damage to the chorda tympani in the middle ear would cause loss of pain and temperature to the external ear loss of taste to the ipsilateral anterior two-thirds of the tongue loss of lacrimation (tearing) in the ipsilateral eye loss of salivation from the ipsilateral submandibular and submaxillary glands. Very good! No. The second and fourth foils are correct. The chorda tympani separates from the facial nerve and carrying impulses to the submandibular ganglion (SVE) and impulses from taste buds in the anterior two-thirds of the tongue (SVA). All other afferent and efferent impulses are spared with a chorda tympani lesion, including movement of the muscles of facial expression. 26. A lesion proximal to the geniculate ganglion may cause Bell's palsy which includes paralysis of ipsilateral muscles of facial expression loss of taste to anterior 2/3 of tongue impaired lacrimation hyposensitivity of sounds Very good! No. The first three foils are correct. There is a hypersensitivity to sounds (hyperacusis) because the stapedius muscle is paralyzed and the magnitude of movement of the tympanic membrane is not reduced as much as it is normally, in response to loud noises. 27. A lesion of the facial nerve as it leaves the skull through the stylomastoid foramen may cause impaired lacrimation hyperacusis impaired taste of anterior 2/3 of the tongue paralysis of the muscles of facial expression ipsilateral to the lesion Very good! No. Only the fourth foil is correct. At the nerve exit, only motor fibers to the muscles of facial expression remain in the nerve. 28. The abducens (C.N. VI) nucleus lies in a paramedian location in the tegmentum of the pons receives fibers from the vestibular nuclei via the medial longitudinal fasciculus projects to the lateral rectus muscle sends out fibers that exit on the dorsal surface of the pons Very good! No. The first three foils are correct. The only cranial nerve that exits on the dorsal surface is the trochlear (C.N. IV). 29. Failure of both eyes to move to the right but move well to the left indicates a lesion of the left abducens nerve right abducens nerve left abducens nucleus right abducens nucleus Very good! No, the answer is D. A lesion of the abducens nerves causes failure of lateral gaze only ipsilateral eye. But a lesion in the abducens nucleus also causes paralysis of the medial rectus of the contralateral eye resulting in right horizontal gaze in both eyes. The nuclear lesion is believed to also interrupt reticular fiber passing through the nucleus on their way to the contralateral oculomotor nucleus (which innervates the medial rectus). 30. A patient has diplopia. You find he cannot abduct his right eye and he has a spastic paralysis of his left arm and leg. The most likely site of the lesion is pontine tegmentum on the right abducens (C.N. VI) after it leaves the ventral pons pontine tegmentum on the left basis pontis on the right basis pontis on the left Very good! No, the answer is D. This lesion affects both the corticospinal pathway and the C.N. VI. Since the right lateral rectus is involved, the lesion is in the right basis pontis and interrupts the abducens nerve fibers passing through. 31. A patient has diplopia and on examination cannot abduct the left eye and has loss of position sense and cannot localize touch on the right side. The lesion is most likely in the left basis pontis right basis pontis left pontine tegmentum right pontine tegmentum Very good! No, the answer is C. The medial lemniscus is horizontal and medially placed with rootlets of the 6th C.N. passing through it. A lesion in the medial lemniscus, on the left, would damage the rootlets and produce the findings described. 32. A patient has loss of horizontal gaze in the right eye and has paralysis of all right facial muscles. The lesion is in the right ventral tegmentum right dorsal tegmentum left ventral tegmentum right basis pontis Very good! No, the answer is B. A right dorsal tegmental lesion would damage the right abducens nucleus as well as fibers of the facial nerve genu as they arch over the abducens nucleus. 33. The efferent root of the trigeminal nerve (C.N. V) arises from the motor nucleus in the basis pontis midbrain tectum pontine tegmentum dorsal horns of cervical cord Very good! No. The answer is C. The efferent fibers supply the muscles of mastication and the tensor tympani, tensor palati, mylohyoid and anterior belly of the digastric. 34. The mesencephalic nucleus of the trigeminal nerve is homologous to the dorsal root ganglia of the spinal nerves is located at the pontine-midbrain junction receives proprioceptive fibers from deep structures of the face primarily receives fibers carrying pain, temperature and touch Very good! No. The first three foils are correct. This nucleus receives proprioceptive fibers from muscles and deep soft tissues and the second order neurons from this nucleus project to cerebellum, thalamus, motor nuclei of brain stem and reticular formation. 35. The general somatic afferent (GSA) fibers of pain and temperature have their cell bodies in the geniculate ganglion descend as far as the second or third cervical segment descend,cross the midline and synapse in the spinal trigeminal nucleus descend and synapse in the spinal trigeminal nucleus Very good! No. The second and third foils are correct. The descending pain and temperature fibers synapse all along their descent in the descending spinal tract of the trigeminal nerve in the spinal nucleus of the trigeminal. Axons from these cells cross the midline and ascend in the ascending trigeminal tract. These second order fibers send branches to C.N. XII,VII and V motor nuclei enroute to the thalamus. 36. Fibers in the afferent root of the trigeminal nerve carrying touch sensations from the face, enter the pons and bifurcate with descending fibers following same paths as pain and temperature bifurcate with ascending branches projecting to the main sensory nucleus of the trigeminal both of the above are correct neither are correct Very good! No, the answer is C.