The Brainstem o includes the medulla, pons, and mesence mesencephalon phalon (midbrain). o gives rise to cranial nerve (CN) III to CN XII. receives its blood supply from the vertebrobasilar system. o Medullar Oblongata (Myelenc (Myelencephalon) ephalon) o contains autonomic centers that regulate respiration, circulation, and gastrointestinal gastrointes tinal motility. gives rise to CN IX to CN XII. o o The nuclei of CN V and CN VIII extend caudally into the medulla. is connected to the cerebellum by the inferior cerebellar peduncle. o o Ascending sensory pathways and relay nuclei Fasciculus gracilis and fasciculus cuneatus convey dorsal column modalities. terminate in the nucleus gracilis and nucleus cuneatus. Nucleus gracilis and and nucleus cuneatus cuneatus contain second-orde second-orderr neurons of the dorsal column – medial lemniscus pathway. give rise to internal arcuate fibers. project via the medial lemniscus to the ventral posterolateral nucleus nucleus of the thalamus. thalamus. Internal arcuate fibers fibers arise from the nucleus gracilis and nucleus cuneatus contralateral al medial lemniscus. form the contralater Decussation of the medial lemniscus is formed by decussating internal arcuate fibers . Medial lemniscus conveys dorsal column modalities to the ventral posterolateral nucleus. Spinal lemniscus contains the lateral and ventral spinothalamic tracts and the spinotectal tract. o Descending motor pathways Pyramidal decussation decussation is located at the spinomedullary junction consists of crossing corticospinal fibers. Pyramids constitute the base of the medulla. contain uncrossed corticospinal fibers . o Cerebellar pathways and relay nuclei Accessory (lateral) cuneate cuneate nucleus contains second-order neurons of the cuneocerebellar tract. projects to the cerebellum via the inferior cerebellar peduncle. Inferior olivary olivary nucleus underlies the olive. is a cerebellar relay nucleus that projects olivocerebellar fibers via the inferior
cerebellar peduncl e to the contralateral cerebellar cortex and cerebellar nuclei. receives input from the red nucleus. Central tegmental tract extends from the midbrain to the inferior olivary nucleus contains rubro-olivary and reticulothalamic fibers. contains taste fibers. Lateral reticular nucleus is a cerebellar relay nucleus projects via the inferior cerebellar peduncle to the cerebellum. Arcuate nucleus Dorsal spinocerebellar tract mediates unconscious proprioception from the lower extremities to the cerebellum via the inferior cerebellar peduncle. Ventral spinocerebellar tract mediates unconscious proprioceptio n from the lower extremities to the cerebellum via the superior cerebellar peduncle . Inferior cerebellar peduncle connects the medulla to the cerebellum. Cranial nerve nuclei and associated tracts Medial longitudinal fasciculus (MLF) contains vestibular fibers of CN VIII coordinate eye movements via CN III, IV, and CN VI. mediates nystagmus and lateral conjugate gaze . Solitary tract receives general visceral afferent (GVA) input from CN IX and CN X. receives special visceral afferent (SVA) (taste) input from CN VII, CN IX, and CN Solitary nucleus projects GVA and SVA input ipsilaterally via the central tegmental tract to the parabrachial nucleus of the pons and to the posteromedial nucleus of the thalamus. Dorsal motor nucleus of CN X gives rise to vagal preganglionic parasympathetic general visceral efferent (GVE) fibers that synapse in the terminal (intramural) ganglia of the thoracic and abdominal viscera. Inferior salivary nucleus of CN IX gives rise to preganglionic parasympathetic (GVE) fibers that synapse in the otic ganglion. Hypoglossal nucleus of CN XII gives rise to general somatic efferent (GSE) fibers that innervate the intrinsic and extrinsic muscles of the tongue. Nucleus ambiguus of CN IX, CN X, and CN XI
o
represents a special visceral efferent (SVE) cell column whose axons innervate pharyngeal arch muscles of the larynx and pharynx . These fibers contribute to parts of CN IX, CN X, and CN XI Ventral horn of CN XI is located at the level of the pyramidal decussation. contains motor neurons of the spinal accessory nerve. Spinal trigeminal tract replaces the dorsolateral tract of Lissauer. contains first-order neuron general somatic afferent (GSA) fibers that mediate pain, temperature, and light touch sensations from the face via CN V, CN VII, CN IX, and CN X. projects to the spinal trigeminal nucleus. Spinal trigeminal nucleus replaces the substantia gelatinosa of the spinal cord. gives rise to decussating axons that form the ventral trigeminothalamic tract . This tract terminates in the ventral posteromedial nucleus of the thalamus. Inferior and medial vestibular nuclei of CN VIII receives proprioceptive (special somatic afferent [SSA]) input from the semicircular ducts, utricle, saccule, and cerebellum project to the cerebellum and MLF. Area postrema lies rostral to the obex in the floor of the fourth ventricle. is a circumventricular organ with no blood – brain barrier.
Pons o
consists of base
contains corticobulbar, corticospinal, and corticopontine tracts and pontine nuclei tegmentum, contains cranial nerve nuclei, reticular nuclei major ascending sensory pathways. is connected to the cerebellum by the middle cerebellar peduncle. contains auditory relay nuclei and vestibular nuclei vestibular nuclei regulate postural mechanisms and vestibuloocular reflexes. contains, in its caudal portion, the facial motor nucleus of CN VII it innervates the muscles of facial expression. contains, in the mid pons, the trigeminal motor nucleus of CN V its axons innervate the muscles of mastication. contains a center for lateral gaze . gives rise to CN V to VIII. Ascending sensory pathways and relay nuclei
o o
o
o
o o o
Dorsal and ventral cochlear nuclei receive auditory input from the cochlea through SSA fibers via the cochlear branch of CN VIII. are auditory relay nuclei give rise to the ipsilateral and contralateral lateral lemniscus. Trapezoid body is formed by decussating fibers of the ventral cochlear nuclei. contains the acoustic striae, medial lemnisci, exiting abducent (CN VI) fibers, and aberrant corticobulbar fibers. Superior olivary nucleus is an auditory relay nucleus at the level of the trapezoid body. • receives input from the cochlear nuclei. contributes bilaterally to the lateral lemniscus. Lateral lemniscus is a pontine auditory pathway extending from the trapezoid body to the nucleus of the inferior colliculus. conducts a preponderance of contralateral cochlear input. Medial lemniscus mediates contralateral dorsal column modalities to the ventral posterolateral nucleus of the thalamus. Spinal lemniscus contains lateral and ventral spinothalamic tracts and the spinotectal tract. Descending motor pathways (base of the pons ) Corticobulbar tract synapses in the motor nuclei of the cranial nerves except in the ocular motor nuclei of CN III, IV, and VI. Corticospinal tract (pyramidal tract) synapses in the ventral horn of the spinal cord. Corticopontine tract synapses in the pontine nuclei Cerebellar pathways and relay nuclei Central tegmental tract extends from the midbrain to the inferior olivary nucleus . contains rubro-olivary and reticulothalamic fibers. Juxtarestiform body forms part of the inferior cerebellar peduncle. contains vestibulocerebellar, cerebellovestibular, and cerebelloreticular fibers. Middle cerebellar peduncle contains pontocerebellar fibers. connects the pons to the cerebellum. Superior cerebellar peduncle connects the cerebellum to the pons and midbrain.
o
o
contains the dentatorubrothalamic fibers and the ventral spinocerebellar tract. Pontine nuclei are cerebellar relay nuclei in the base of the pons. give rise to pontocerebellar fibers that constitute the middle cerebellar peduncle. Cranial nerve nuclei and associated tracts Dorsal and ventral cochlear nuclei of CN VIII are found at the medullopontine junction. Medial, lateral, and superior vestibular nuclei of CN VIII receive proprioceptive (SSA) input from the semicircular ducts, utricle, saccule, and cerebellum. project to the cerebellum and the MLF. The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract. Medial longitudinal fasciculus contains vestibular fibers of CN VIII coordinate eye movements via CN III, CN IV, and CN VI. mediates nystagmus and lateral conjugate gaze. Abducent nucleus of CN VI underlies, in the caudal medial pontine tegmentum, the facial colliculus of the rhomboid fossa. projects exiting fibers through the trapezoid body gives rise to GSE fibers that innervate the lateral rectus muscle. gives rise to fibers that project via the MLF to the contralateral medial rectus subnucleus of the oculomotor nucleus of CN III . is the pontine center for lateral conjugate gaze , which receives commands from the contralateral frontal eye field (area 8). It innervates via the MLF the contralateral medial rectus muscle and via abducent fibers the ipsilateral lateral rectus muscle to execute conjugate lateral gaze. Facial nucleus of CN VII gives rise to SVE fibers that innervate the muscles of facial expression. receives bilateral input for upper facial muscles and contralateral input for lower facial muscles. contains neurons that project axons dorsomedially, encircle the abducent nucleus as a genu Superior salivatory nucleus of CN VII includes the lacrimal nucleus. gives rise to GVE preganglionic parasympathetic fibers that synapse in the pterygopalatine and submandibular ganglia. Spinal trigeminal tract and nucleus of CN V Motor nucleus of CN V lies in the lateral midpontine tegmentum
o
lies medial to the principal sensory nucleus of the trigeminal nerve. receives bilateral corticobulbar input. gives rise to SVE fibers that innervate muscles of mastication. Principal sensory nucleus of CN V lies lateral to the motor nucleus of CN V. receives discriminative tactile and pressure sensation input from the face. gives rise to trigeminothalamic fibers that join the contralateral ventral trigeminothalamic tract. gives rise to the uncrossed dorsal trigeminothalamic tract, which terminates in the ventral posteromedial nucleus of the thalamus. Mesencephalic nucleus and tract of CN V extend from the upper pons to the upper midbrain. contain pseudounipolar neurons. receive input from muscle spindles and pressure receptors (muscles of mastication and extraocular muscles). Locus ceruleus is a melanin-containing nucleus in the pons and midbrain. is an important nucleus of the monamine system that projects noradrenergic axons to all parts of the central nervous system (CNS). Mesencephalon (Midbrain) mediates auditory and visual reflexes. o o contains the oculomotor nerve (CN III) and the trochlear nerve (CN IV), which innervate the extraocular muscles of the eye. o contains a center for vertical conjugate gaz e in its rostral extent. o contains the substantia nigra, the largest nucleus of the midbrain; degeneration of this extrapyramidal motor nucleus results in Parkinson disease. o contains the paramedian reticular formation lesions of this formation result in coma. gives rise to two cranial nerves: CN III (oculomotor) and CN IV o (trochlear). consists dorsoventrally of three parts o the tectum the tegmentum the base (basis pedunculi) o Structures of the midbrain Tectum is located dorsal to the cerebral aqueduct . forms the roof of the midbrain including the superior and inferior colliculi. Tegmentum is located between the tectum and the base (basis pedunculi).
contains cranial nerve nuclei and sensory pathways. Basis pedunculi (crus cerebri) forms the base of the midbrain contains corticospinal, corticobulbar, and corticopontine tracts. Pedunculus cerebri (cerebral peduncle) includes the tegmentum and basis pedunculi. Pretectum (pretectal area) is located between the superior colliculus and the habenular trigone Inferior collicular level of the midbrain Inferior colliculus Nucleus of the inferior colliculus is an auditory relay nucleus that receives binaural input from the lateral lemniscus. projects to the medial geniculate body via the brachium of the inferior colliculus. Commissure of the inferior colliculus interconnects the inferior collicular nucleus and its opposite partner Brachium of the inferior colliculus conducts auditory information from the inferior collicular nucleus to the medial geniculate body. Lateral lemniscus projects binaural auditory information to the inferior collicular nucleus. Cerebral aqueduct is located between the tectum and tegmentum. is surrounded by the periaqueductal gray matter. interconnects the third and fourth ventricles. blockage (aqueductal stenosis) results in hydrocephalus. Periaqueductal gray matter is the central gray matter that surrounds the cerebral aqueduct contains several nuclear groups. Locus ceruleus Mesencephalic nucleus and tract Dorsal tegmental nucleus contains enkephalinergic neurons that play a role in endogenous pain control. Dorsal nucleus of raphe contains serotonergic neurons. Trochlear nucleus of CN IV gives rise to GSE fibers, which encircle the periaqueductal gray matter decussate in the superior medullary velum exit the midbrain from its dorsal aspect innervate the superior oblique muscle. Medial longitudinal fasciculus
o
contains vestibular fibers that coordinate eye movements. interconnects the ocular motor nuclei o f CN III, CN IV, and CN VI. Decussation of the superior cerebellar peduncles is most conspicuous structure of this level. Interpeduncular nucleus receives input from the habenular nuclei via the habenulointerpeduncular tract (fasciculus retroflexus of Meynert). Substantia nigra is divided into the dorsal pars compacta, which contains large pigmented (melanin) cells , and the ventral pars reticularis. receives gamma-aminobutyric acid – ergic (GABA-ergic) input from the caudatoputamen (striatonigral fibers). projects dopaminergic fibers to the caudatoputamen (nigrostriatal fibers). projects nondopaminergic fibers to the ventral anterior nucleus, ventral lateral nucleus, and mediodorsal nucleus of the thalamus (nigrothalamic fibers). Medial lemniscus mediates dorsal column modalities to the ventral posterolateral nucleus. Spinal lemniscus contains the lateral and ventral spinothalamic tracts and the spinotectal tract. Central tegmental tract contains rubro-olivary and reticulothalamic fibers. Basis pedunculi (crus cerebri) Superior collicular level of the midbrain Superior colliculus receives visual input from the retina and from frontal (area 8) and occipital (area 19) eye fields. receives auditory input from the inferior colliculus to mediate audiovisual reflexes. is concerned with detection of movement in visual fields , thus facilitating visual orientation, searching, and tracking. Commissure of the superior colliculus interconnects the two superior colliculi. Brachium of the superior colliculus conducts retinal and corticotectal fibers to the superior colliculus and to the pretectum, thus mediating optic and pupillary reflexes. Cerebral aqueduct and periaqueductal gray matter Oculomotor nucleus of CN III gives rise to GSE fibers that innervate four extraocular muscles (medial, inferior, superi- or recti, and inferior oblique) and the superior levator palpebrae.
o
projects crossed fibers to the superior rectus. projects crossed and uncrossed fibers to the levator palpebrae Edinger-Westphal nucleus of CN III gives rise to GVE preganglionic parasympathetic fibers that terminate in the ciliary ganglion. Postganglionic fibers from the ciliary ganglion innervate the ciliary body (accommodation) and the sphincter muscle of the iris (pupillary light reflex). Medial longitudinal fasciculus Central tegmental tract contains rubro-olivary and reticulothalamic fibers . Red nucleus is located in the tegmentum at the level of the oculomotor nucleus (the level of the superior colliculus). receives bilateral input from the cerebral cortex. receives contralateral input from the cerebellar nuclei. gives rise to the crossed rubrospinal tract. gives rise to the uncrossed rubro-olivary tract. exerts facilitatory influence on flexor muscles Medial lemniscus Spinal lemniscus Substantia nigra Basis pedunculi (crus cerebri) Posterior commissural level (pretectal region) is a transition area between the mesencephalon and the diencephalon Posterior commissure marks the caudal extent of the third ventricle. marks the rostral extent of the cerebral aqueduct. interconnects pretectal nuclei , thus mediating consensual pupillary light reflexes. Pretectal nucleus receives retinal input via the brachium of the superior colliculus. projects to the ipsilateral and contralateral EdingerWestphal nucleus, thus mediating the pupillary light reflexes.
o
Corticobulbar (Corticonuclear) Fibers o arise from precentral and postcentral gyri . may synapse directly on motor neurons or indirectly via interneurons o (corticoreticular fibers). innervate sensory relay nuclei (gracile, cuneate, solitary, and trigeminal). o o innervate cranial nerve motor nuclei bilaterally, with the exception of part of the facial nucleus (CN VII). The upper face division of the facial nucleus receives bilateral input ;
the lower face division of the facial nucleus receives only contralateral input innervate the ipsilateral spinal nucleus of CN XI , which supplies the sternocleidomastoid muscle innervate the contralateral spinal nucleus of CN XI , which innervates the trapezius muscle. The orbicularis oculi muscle receives a variable number of crossed and uncrossed fibers; the paresis therefore varies from patient to patient.
o
o
o
