Chapter 12

Neuroanatomy Overview

Spinal Cord


Spinal Cord divided incompletely by DMS and VMF. Central canal is continuous with ventricular system. Gray matter is central; white is on periphery.

Gray Matter

Forms a butterfly shape, with dorsal and ventral horns; lateral horn present in thoracic and upper lumbar segments -- it contains preganglionic sympathetic neurons.

Dorsal Horn

Afferent fibers enter dorsal roots and mostly terminate near point of entry; some divide into ascending and descending branches, running in Lissauer's tract; these establish synaptic contacts over several segments of gray matter. Cutaneous afferents tend to terminate in dorsal laminae (superficial) while proprioceptive and muscle afferents project to deeper laminae.

Tip of dorsal horn, a k a substantia gelatinosa, receives collaterals of group A gamma and group C afferents (nociceptive, unmyelinated). These neurons are excitatory and use glut. and subs. P. In SG, complex interactions occur -- ascending, descending, interneurons. May explain why rubbing sore may inhibit pain info transmission.

N. dorsalis of Clarke contains DSCT, which receives afferent input from m. spindles, GTO's, tactile and pressure receptors.

Ventral Horn

Contains alpha motor neurons (extrafusal) and gamma motor neurons (intrafusal). Alpha and gamma neurons are a k a lower motor neurons. Particularly well developed in cervical and lumbar spinal cord, where upper and lower limb motor neurons are found. Generally, axial m. neurons are medial and limb m. are lateral. C3-5 contains phrenic n. Cells in ventral horn receive input from certain dorsal root root afferent (muscle spindles in myotactic reflex). Also receive descending pathways for motor control.

Spinal Reflexes

General comments

Pathways consist of afferent neurons conveying sensory impulses to CNS (spinal cord/brainstem) and efferent neurons to effector organ. Also, interneurons within CNS modulate reflexes.

Myotactic reflex/gamma loop/reciprocal inn.

Intrafusal fibers (Group Ia afferents from nuclear bag/chain fibers) excite alpha motor neurons of same m., causing reflex. Important in anti-gravity mm., which must oppose stretch. To maintain posture.

In addition, gamma motor neurons (which are also under control of descending paths, which can lead to derangement), cause increase in tension to sensory endings, lowering threshold of stretch receptors and increase sensitivity of stretch reflex.

With reciprocal inn., Ia afferent fibers not only excite alpha motor neurons of same m., but also inhibit antagonistic m. (preventing greater stretch).

Flexor reflex (a k a paw withdrawal)/crossed extensor reflex

Primary afferent fibers activate flexor alpha neurons via interneurons. This occurs with nociceptive stimuli (like a pin). If descending pathways are out of whack, a non-painful stimuli may elicit flexor reflex. (like Babinski sign).

Activation of flexor in weight bearing limb causes reflex extension of contralateral limb. (CE reflex).

White matter

Consists of dorsal, lateral, and ventral funiculi. Nerve fibers sharing common origins, term. and functions are organized into tracts/fasciculi. Some fibers interconnect adj. or distant cord segments and permit intersegmental coordination; others connect spinal cord to brain. Intersegmental or propriospinal fibers are found in fasciculus proprious surrounding spinal gray.

Ascending. tracts

Carry pain, thermal, tactile, and proprioceptive info to brain; some goes to conscious levels (cerebral cortex), while some goes to subconscious centers (cerebellum).

Pathways share common char.:

Dorsal columns and spinothalamic tracts share this pattern.


Carries Proprioception and discriminative touch. Fasciculus gracilis is medial and contains T6 and below; fasciculus cuneatus. is lateral and contains T6 and above. Terminate in respective nuclei.(plus lateral cun. n. for C8 and up), where they decussate in medulla as internal arcuate fibers; ascend as medial lemniscus; terminate in VPL. Medial lemniscus in medulla is arranged with lower information located laterally. Third order neurons project to somatosensory cortex, with legs medial and arms lateral. ("Protect your gonads") Lesions lead to ataxia (due to loss of proprioceptive) and loss of discriminative touch.


Carries pain, temp, nondiscriminative touch. Lies lateral and ventral to ventral horn. Some divide into lateral/ventral spinothalamic. Cells synapse in dorsal horn and decussate through ventral white commissure. Axons carrying pain and temp. decussate within one segment (lateral spinothalamic); touch and pressure (ventral spinothalamic) may ascend/descend several levels before decussating. Spinothalamic fibers run near Medial lemniscus in spinal lemniscus. Majority of fibers go to VPL.

Spinothalamic tract is sometimes called neospinothalamic system. It is highly organized. somatotopically; it is thought to be route for fast pain. Spinoreticulothalamic system is phylogenetically older system for sensory. impulses to get to higher centers. Some 2nd order neurons from dorsal horn ascend to the ventrolateral region of cord and terminate in the reticular formation, particularly in the medulla. Reticulothalamic fibers go to intralaminar thalamus nuclei, which go to cortex. This is slow pain. Activation of spinothalamic/spinoreticular fibers is modulated by descending pathways. Lesions lead to impairment of sensation on contralateral side.

A 2-neuron path: Spinocerebellar tracts

Ascending. pathways which carry impulse to subconscious. level are spinocerebellar tracts.

DSCT and VSCT are located near the dorsolateral and ventrolateral surf. of the cord. They carry proprioceptive info. It consists of 2 neurons. 2nd order neuron contains cell bodies in base of dorsal horn and terminate in cerebellar cortex. Fibers of the DSCT are found in Clarke's column. The axons ascend ipsilaterally to cerebellum and enter through ICP. VSCT fibers decussate and ascend contralaterally to enter through SCP. Some axons then recross within cerebellar white matter. Lesions lead to ataxia.

Descending tracts

These tracts originate from cerebral cortex and brain stem. They are concerned with;

Corticospinal tracts

These are concerned with control of voluntary, discrete, skilled movements, esp. in distal parts of limbs. They arise from cell bodies in cortex (incl. precentral g. (primary motor cortex)). The large Betz cells give rise to the largest diameter corticospinal axons. Corticospinal axons pass though the internal capsule to enter the crus cerebri of the midbrain. In the medulla, they form two prominent columns: the pyramids (so corticospinal tracts is a k a pyramidal tract); in the caudal medulla 75-90% of fibers decussate and enter contralateral lateral corticospinal tract, which is located just ventrolateral to dorsal horn. The remainder of ipsilateral. fibers form ventral corticospinal tract, which is lateral to VMF. They also decussate near there termination so pyramidal fibers effectively inn. contralateral spinal cord. 55% terminate in cervical, 20% at thoracic, 25% at lumbosacral. Fibers terminate extensively in spinal gray; some which originate in motor cortex go to ventral horn, making monosynaptic contact with motor neurons.)

Rubrospinal tract

This is concerned with overall tone of flexor muscles, being excitatory to motor neurons of these mm. Originates from red nucleus of midbrain tegmentum. Fibers course ventromedially and cross at ventral tegmental decussation, after which they descending to spinal cord ventrolateral to lateral corticospinal tract. Red nucleus receive. afferent fibers from motor cortex and cerebellum. It thus represents a non-pyramidal route to influence spinal motor activity.

Tectospinal tract

This is associated with head/neck movements. Fibers arise from sup. colliculus of midbrain (at level of red nucleus and crus cerebri). Axons pass ventromedially around periaqueductal gray to dorsal tegmental decussation. Descending fibers in spinal cord lie near VMF and terminate in cervical segments. The sup. colliculus receive visual input and the tectospinal tract is though to mediate reflex movements in response to visual stimuli.

Vestibulospinal tract

Fibers arise from vestibular nuclei in pons and medulla in and near the floor of the fourth ventricle. The vestibular nuclei receive input from the vestibular nerve and the cerebellum.

Lateral Vestibulospinal Tract

Latvestibulospinal tract descend ipsilateral. from lateral vestibular n. on in middle of ventral-most of VH. It mediates excitatory influence on extensors and control extensor m. tone in anti-gravity maintenance of posture.

Medial Vestibulospinal Tract

Medial vestibular n. contributes fibers to ipsilateral. MLF is located near VMF.

Reticulospinal tracts

The reticular formation of the pons and medulla gives rise to these fibers, which influence. voluntary movement, reflex. activity and m. tone. They also mediate circulatory. system and help control breathing. Pontine (medial reticulospinal tract) fibers descend ipsilateral, while medullary (lateral reticulospinal tract) fibers descend bilaterally. Both run in ventral funiculus, basically hugging VH.


Lower Motor Lesions

Lesions to neurons in cord and brainstem and their axonal processes.

Poliomyelitis affects LMN to limb and respiratory. mm., esp. lower limbs.

Upper motor lesions

Lesions to neurons and axons of descending tracts which control activity of lower motor neurons.

ALS (degeneration of descending pathways)

`Pyramidal signs' account for loss of discrete movements and Babinski

Hyperreflexia and spasticity are probably due to other descending pathways than corticospinal tract.

Brain Stem (Ch. 6)

(Root for BS is bulb as in "-bulbar") Contains numerous ascending/descending tracts, as well as CN III-XII. Also, reticular formation, where a number of nuclei exist. The reticular formation has a number of important functions:

Dorsal BS

Peduncles attach to cerebellum. (Inf to medulla, middle to pons, sup to midbrain).

In caudal medulla, Fasciculus. gracilis, cuneatus running to their nuclei, visible on outside as gracile and cuneate tubercles.

In rostral medulla, central canal opening out into fourth ventricle. (From dorsal BS, caudal 1/3 of fourth ventricle is rostral medulla; the rest is caudal pons). fourth ventricle is widest at pontomedullary junction where lateral recess extends to lateral margin of BS and Foramina of Luschka are found. The lateral walls of the rostral part of fourth ventricle (pontine) are made by sup and inf cerebellar peduncles. In the rostral pons, the wall converge to form cerebral aqueduct.

The inf and sup colliculi lie rostral to the sup peduncle (at the midbrain level) and are parts of the auditory and visual systems, respectively. The trochlear nerve exits immediately caudal to inf colliculus (it is the longest of the CN, and the only to exit dorsally.)

Ventral BS

Pyramids are prominent columns running on either side of the VMF. Decussation of pyramids obscures VMF. Lat. to pyramid lies the olive, within which is the inf. olivary n., which is connected to cerebellum and controls movement.

Medullary CN include IX-XII. Med-Pons junction is marked by pontocerebellar fibers, which come from pontine nucl. and pass through the contralateral MCP to enter the cerebellar hemisphere. The pontine nuclei serve as connection between cerebral and cerebellar cortices and coordinates movement. The transverse fibers obscure the underlying corticospinal tract.

Ventral surface of midbrain consists of, on each side, a mass of descending fibers, the crus cerebri. In the midline, the crura are separated by the interpeduncular fossa. The crus cerebri is continuous rostrally with the internal capsule and consists of corticobulbar/spinal fibers which have left the cerebral hemisphere.


Caudal medulla

From spinal cord->medulla, ventral horn is attenuated. Dorsal horn replaced by caudal part of spinal nucl of V. Like the DH, SNofV receives general sensation fibers via trigeminal nerve. It extends the whole length of the BS and into the upper spinal cord. The caudal part of the trigeminal nucl. is associated with pain/temperature. CN V attaches at the pons, so these fibers descend (spinal tract of V), which is immediately superficial to nucl. (An exception to "ascending sensory" rule of thumb). In ventral medulla, pyramids decussate.

Mid medulla

Pyramids are prominent. Dorsal columns terminate in their nuclei. Internal arcuate fibers contain decussating fibers of 2nd order neurons which go to form medial lemniscus.

Rostral medulla (elephant)

Pyramids are still conspicuous. Immediate dorsal to the medial aspect of the pyramids is the medial lemniscus, which lies on the midline. The inf. olivary nucl is obvious, lying in the olive. It received afferent fibers from the motor and sensory cortices and the red nucl of the midbrain. Its main efferent connection to the cerebellum is the ICP, which is made up of climbing fibers which end in excitatory synapses with dentate nuclei and Purkinje cells in cerebellar cortex. The VTTT is pointed to by the inf olive. The spinal lemniscus (containing spinothalamic fibers) is lateral to medial lemniscus. Dorsally, the floor of the fourth ventricle is formed; immediately and deep to this floor lie a number of CN nuclei. XII nucl lies just lateral to midline. Dorsal motor nucl of X contains preganglionic parasympathetic. neurons + lies lateral to XII. (The area postrema lies at the most caudal aspect of the ventricular floor and is where blood-brain barrier is absent and where emetic drugs (vomit induced) operate). In the lateral part of the floor of the ventricle are the vestibular nucl, which receive afferent fibers from the vestibular nerve. Ventromedial to XII is the MLF, which links the vestibular nucl to the extraocular muscles. The dorsolateral rostral medulla is dominated by the ICP, or restiform body, which connects medulla and cerebellum. ICP contains

On dorsal and lateral rim of the ICP are the cochlear nuclei. Deep to the ventricular floor and just dorsal to the inf. olivary n. is the nucleus aMbiguus (Motor to IX, X, XI)


Can be divided into ventral (basilar pons) and dorsal portion (tegmentum). Ventral potion contains pontocerebellar fibers (which orig. from scattered pontine nuclei and pass to contralateral cerebellum through MCP). Corticospinal fibers (which continue into medullary pyramids) are small longitudinal fibers between pontine fibers. The medial lemniscus starts separating from midline, rotating and moving dorsally along with the spinal lemniscus and the VTTT.

Caudal pons

In the caudal pons, the trapezoid body is between the lemniscal fibers and the pontocerebellar fibers and indicates the crossing of the acoustic fibers from the cochlear nuclei, which ascend into the MB as the lateral lemniscus and terminate in the inf colliculus.

Beneath the floor of the ventricle (in the tegmentum), are CN nuclei for XI, VII motor nucl, and the motor nucleus of V. The sensory nucleus of V ends in the pons, near the origin of the trigeminal n.

Rostral pons

In the rostral pons, the SCP form the lateral walls of the fourth ventricle, the thin sup medullary velum spanning beneath them to form its roof. The SCP contains

SCP converge to midline as they pass into the MB.

MB (kissing dolphins? rostral = kissing elvi)

The aqueduct divides the MB into the dorsal tectum, which consists of the inf and sup colliculi, and the ventral tegmentum, which is bounded ventrally by the crus cerebri.

Caudal MB

In the caudal MB, the inf colliculus is part of the ascending auditory projection (ascending fibers run in lateral lemniscus -> inf coll -> MGN of thalamus -> auditory cortex of temp lobe). The lateral lemniscus. "hugs" the inf coll. The central tegmentum is dominated by the decussation of the SCP.

Rostral MB

In the rostral MB, the sup colliculus is part of the visual system (afferent corticotectal fibers come from visual cortex of occipital lobe and frontal eye field of frontal lobe). These inputs are used for

Some visual fibers in optic tract terminate just rostral to sup coll in the pretectal nucl, which connects with parasympathetic neurons that mediate the pupillary light reflex.

Ventral to the colliculi the cerebral aqueduct contains the periaqueductal gray, whence cometh the trochlear (look for the "T") and the oculomotor nuclei. Close to the nuclei is the MLF, which links them to the abducens nucl in the pons and is important in the control of gaze.

At the level of the sup coll, just rostral to the decussation of the SCP, is the red nucleus, where some of the SCP fibers end. The red nucleus is involved in motor control. Other afferents come from the motor cortex of the frontal lobe; efferent fibers leave the red nucl, cross in the ventral tegmental decussation and descend as the rubrospinal tract. The red nucl projects to the inf. olivary nucl. in the medulla via the central tegmental tract.

The most ventral part of the midbrain tegmentum is the location of the substantia nigra, pars compacta, which projects to the caudate and putamen. The crus cerebri lies ventral to the subs. nigra. and consists of corticobulbar (to motor CN nucl) and corticospinal (pyramidal) fibers. The temporopontine and frontopontine fibers lie dorsal and ventral to these fibers. They connect the cortex to the pontine nuclei, and thence to the cerebellum via the MCP, and are involved in coordination.

Reticular formation

Contains a number of nuclei and tracts; it runs the length of the BS. It is a phylogenetically old part of BS and is involved in survival functions.

The reticulospinal tracts originate from the pontine and medullary reticular formations. These influence muscle tone and posture.

Reticular activating system is ascending fibers in reticular formation; receive direct and indirect input from multiple sense sources. Through thalamus nuclei, they activate cerebral cortex and heighten arousal.

The raphe nuclei (serotonergic) lie along midline throughout BS;

locus coeruleus lies in the tegmentum of the caudal MB/rostral pons. It is the principal noradrenergic cell group in brain. It is implicated in sleep regulation (particularly REM sleep)

Afferent nuclei

trigeminal sensory nucleus at pons (big nucl)

vestibular and cochlear nuclei in medulla

solitary nucleus (taste from VII) in medulla

Efferent nuclei

Somatic efferent cell column

Lies near midline: III, IV, VI, and XII

III: periaqueductal gray of MB at sup coll

IV: periaqueductal gray of MB at inf coll

VI: caudal pons beneath fourth ventricle

XII: in medulla

Branchiomotor cell column

Inn. striated mm. derived from the branchial arches.

V: motor nucleus of V in tegmentum of mid-pons

VII: caudal pontine tegmentum (facial expression + stapedius)

XI, X, cranial XI: nucleus ambiguus in medulla

Parasympathetic cell column

Consists of preganglionic parasympathetic neurons: III, VII, IX and X

III: Edinger-Westphal n., midbrain periaqueductal gray near III n.; (ciliary ganglion -> sphincter pupillae and ciliary muscle of eye)

VII: sup. salivatory n. in pontine tegmentum (pterygopalatine ganglia->lacrimal, and submandibular ganglia ->submandibular and sublingual salivary glands)

IX: inf salivatory n. in pontine tegmentum caudal to above n. (so it doesn't go to VII, obviously.) (otic ganglia->parotid)

X: largest preganglionic cell group, dorsal motor nucleus of X; found in medulla beneath fourth ventricle, lateral to XII n.; (to thoracic and abdominal viscera)

Eye movements

Oculomotor n. (III)

From periaqueductal gray at level of sup coll, fibers course ventrally through MB tegmentum, many traversing red nucl , to exit on medial aspect of crus cerebri within interpeduncular fossa. Passes between PCA and SCA then runs ant along wall of cavernous sinus to superior orbital fissure.

Pupillary Light Reflex

Illuminate retina->direct light reflex (constriction of ipsilateral. pupil) + consensual light reflex (constriction. of contra. pupil via fibers from optic tract going through pretectal area rather than LGN. pretectal fibers project bilaterally to E-W nuclei, whose efferent fibers control constrictor.

Accommodation reflex

Look at nearby object. Involves visual cortex, with corticobulbar fibers activating E-W nuclei bilaterally.

Trochlear n. (IV)

Trochlear n. in MB periaqueductal gray at inf coll -> axons pass dorsally, cross midline->emerge caudal to inf coll->courses around cerebral peduncle to gain the ventral aspect of the brain, passing between the PCA and SCA with III->superior orbital fissure->SO4

Supplies contralateral superior oblique.

Abducens n. (VI)

N. in fourth vent. of caudal pons->ventral surf of BS at pontomedullary junction->ant. through cavernous sinus to superior orbital fissure->LR6

Trigeminal n.

Attaches as two adj. roots on ventrolateral pons (larger is sensory)


Sensory fibers are primary sensory neurons from V1 (ophthalmic), V2 (maxillary), V3 (mandibular). Touch, pressure, pain, temperature from face, scalp, nasal/oral cavities, dura, cornea. Proprioceptive. from muscles of mast and TMJ. Afferents (except proprioceptive) from trigeminal (semilunar) ganglion, located at convergence of V1-V3. The central processes of these cells terminate in V sensory nucl.

The sensory n. consists of three subnuclei:

  1. chief sensory nucl of V - in pontine tegmentum close to nerve entry; touch and pressure
  2. mesencephalic nucl of V - extends into midbrain; proprioception; only primary afferents to have cell bodies in CNS.
  3. spinal nucl of V - extend caudally to medulla and into cord, where it becomes continuous with subs gel.; pain and temp.; spinal tract is adj. to nucl and becomes continuous with Lissauer's tract which carries functionally homologous afferents from spinal nerves.

Second order neurons in V nucleus decussate to form contra. trigeminothalamic tract (trigeminal lemniscus), which ends in VPM of thalamus. (and thence to lateral sensory cortex) and in cerebellum, where reflex connections to certain motor cell groups of BS are formed. (For example, grimace and eye closure (corneal reflex) to noxious stimulation in territory of V).


The motor axons of V arise from motor nucl of V in pontine tegmentum medial to chief sensory nucl. Axons leave the pons in the motor root of V and join V3.

Facial n. (VII)

Joins brain at cerebellopontine angle in ventrolateral aspect of caudal pons. Lateral root (nervus intermedius) contains sensory and parasympathetic fibers, while medial root is motor.


Sensory fibers provide

Cell bodies of primary afferent neurons lie within the geniculate ganglion in the facial canal of the petrous temp. bone. Taste fibers end in the solitary nucleus in the medulla, which projects to the VP of the thalamus. Cutaneous sensory. fibers terminate in trigeminal nucl.


Motor fibers originate in facial motor nucl of caudal pontine tegmentum; axon initially passes dorsally, looping over VI n. beneath floor of fourth vent before leaving BS.

Reflex connections

Corneal reflex

(visual) sup coll and/or (tactile) trigeminal sensory nucl->VII->orbicularis oculi

Stapedius reflex

sup olivary nucl (central auditory pathway) informs->VII of loud noise so ->stapedius contracts

Lesions to VII

Corticobulbar fibers supply VII n. Those controlling motor neurons of upper face are bilateral; for lower motor neurons, contralateral. So UMN lesion = ??

lateral lower face LMN lesion =??

Needed for lacrimal and submandibular and sublingual glands.

Vestibulocochlear n. (VIII)

vestibular and cochlear divisions contain first-order axons whose dendrites go to hair cells. Both pass through int. acoustic meatus with facial and attach to BS at pontomedullary junction.

Vestibular nuclei

Hair cells in membranous labyrinth->vestibular ganglion in IAM->vestibular nucl of rostral medulla.

Vest nucl contact numerous regions for

Cochlear nuclei

Hair cells in organ of Corti within cochlear duct->cell bodies lie within cochlea and form spiral ganglion->BS at rostral medulla->fibers bifurcate to D/V cochlear nuclei, which lie about ICP. Ascending auditory pathway is somewhat more complex and variable than other senses.

There are several locations between the medulla and thalamus where axons may synapse and not all fibers behave in the same manner. Second order neurons ascend to the pons, some crossing at the trapezoid body. Some fibers terminate in sup. olivary. nucl. Olivocochlear fibers leave BS in VIII to serve inhibitory. role in organ of Corti. From sup. olivary. nucl., ascending fibers form lateral lemniscus which runs to the inf coll of the MB. Some axons of the lateral lemniscus terminate in a pontine nucleus. The sup olivary nucl and this nucl of the lateral lemniscus are though to establish reflexes with V and VII, mediating tensor tympani and stapedius reflexes. The inf coll sends axons to the medial geniculate n. of the thalamus. Axons from the MGN project through the internal capsule to the primary auditory cortex in the superior temporal gyrus (hidden within lateral fissure). Tonotopic info is maintained. Representation of cochlea is bilateral rostral to cochlear nuclei b/c some fibers do not decussate. Adj. to primary auditory cortex is auditory assoc. cortex, or Wenicke's area, which is important in language processing.

Acoustic neuromas (IX)



Ataxia and paralysis of CN (esp. V-VII) and the limbs

Glossopharyngeal (IX)

Attaches to BS lateral to olive in rostral medulla

Afferent fibers contain

Small motor component arises from rostral nucl ambig.. in medulla to stylopharyngeus.

Innervates parotid.

Vagus (X)

Attach at lateral medulla, caudal to IX

Afferent fibers contain

Motor fibers arise in nucleus ambiguus in medulla. Innervate soft palate, pharynx, larynx and upper esophagus. This nucl. is important for speech control and swallowing. The most caudal efferents from the nucl. are regarded as leaving the cranial roots of XI, but they transfer to X at the jugular foramen.

Accessory (XI)

Cranial part from lateral medulla carrying fibers from the caudal nucl. ambiguus.

Spinal root from motor neurons in VH C1-C5; the axons leave the cord via rootlets from the lateral aspect of the cord (not ventral). The rootlets course rostrally and coalesce and enter the foramen magnum. At the medulla, the spinal and cranial roots briefly join, but separate at the jug. for. The fibers supply the SCM and trapezius.

Hypoglossal (XII)

Motor to tongue. Axons in hypoglossal nucl. in floor of fourth ventricle in medulla->fibers exit ventrolaterally between pyramid and olive. Receives afferent from solitary nucl and trigeminal sensory nucl to mediate reflex movements of chewing, sucking and swallowing. Corticobulbar fibers from the contralateral motor cortex subserve voluntary tongue movements.


Functions are entirely motor. Overlies fourth ventricle.

Cerebellar cortex

Fibers enter via peduncles and are excitatory to cerebellar cortical neurons. Inhibitory modulation in the cerebellar cortex is provided by Golgi, basket and stellate cells.

Deep Cerebellar Nucl.

Found within cerebellar white matter above roof of fourth ventricle.

"Fat Guys Eat Donuts", med to lateral

Extracerebellar afferent come from vestibular nucl, reticular nucl, pontine nucl and spinocerebellar tracts -- primary though collaterals of mossy fibers to cerebellar cortex itself.

Purkinje fibers from cerebellar cortex for dense inn. These nuclei are primary efferent fibers for cerebellum to rest of brain.

Efferent fibers go to


Cerebellar lesions cause;


Thalamus is largest component of diencephalon (between BS and cerebral hemisphere)

Almost all thalamic nucl. have rich reciprocal connections with cerebral cortex.

Internal medullary lamina (IML) divides thalamus into ant, medial and lateral nuclear masses

Embedded within IML are intralaminar nuclei

Reticular nucl lies on the lateral aspect of the thalamus.

Lateral Nuclei

Specific thalamus nucl have well-defined sensory or motor functions and highly organized. connections with cerebral cortex. They all lie within ventral part of lateral nucl mass.

Non specific nucl connect with wider areas of cortex, including associative and limbic regions;

Other nuclei

Anterior nuclear group is part of limbic system. (AV, AM, AD). Receives fibers from mammillary body of hypothalamus, projects to cingulate g.

Medial nuclear group (MD a k a DM, nucleus reuniens, etc.). Afferent from hypothalamus, amygdala, intralaminar nucl, lateral nuclei; MD connects to frontal lobe cortex (mood/emotions.)

Intralaminar group. Afferent from reticular formation and ascending sensory system. Efferent to cerebral cortex and striatum; responsible for activating cerebral cortex.

Reticular nucl receives collaterals of thalamocortical and corticothalamic fibers.

Cerebral hemisphere and cerebral cortex

Histological structure;

Organization of cerebrum;

Corpus striatum

Includes caudate, putamen and globus pallidus

Primarily concerned with control of posture and movement

Caudate lies in the wall of the lateral ventricle.

The putamen and GP lie lateral to the internal capsule, deep to the cortex of the insula

The caudate and putamen are the input regions of the basal ganglia

Afferent come from cerebral cortex, intralaminar thalamus nucl, subs nigra pars compacta

Efferent go to globus pallidus and the pars reticulata of subs nigra

GP receives afferent fibers from striatum and subthalamic nucl

The GP consists of medial and lateral segments;

The thalamus sends fibers to motor areas of frontal lobe

When a movement is initiated from cerebral cortex, impulse discharge not only through corticospinal and CB paths but also through corticostriatal projection to neostriatum.

These glut fibers excite striatal neurons.

Striatum has two routes to control basal ganglia output neurons in medial pallidus and SN, PR.

Parkinson's affects direct pathway, because striatal fibers can no longer inhibit medial pallidum b/c of lack of dopamine. It also effects the indirect pathway; lateral pallidal neurons are inhibited, so subthalamic nucleus is disinhibited, which activates medial pallidal neurons, which ultimately induces akinesia.

Huntington's disease results from problem on indirect side. There is attrition of fibers that project to lateral pallidum. The lateral pallidal neurons are disinhibited, so the subthalamic nucleus is stuck in inhibition, so medial pallidal neurons are underactive and unwanted movements occur.

Autonomic NS

Enter and leave CNS through spinal and cranial nn.

Afferent fibers establish connections with ascending neurons so conscious. awareness of visceral function is achieved. Descending connections from hypothalamus have profound effect on autonomics

Efferent neurons differ from those of somatic nervous system b/c there are two neurons between CNS and structure: preganglionic neuron->autonomic ganglion->postganglionic neuron

Many structures receive fibers from both sympathetic + parasympathetic

Sympathetic division

Thoracolumbar in origin, preganglionic sympathetic neurons lie in lateral horn of spinal gray. They leave the cord in the ventral n. root and join the spinal nerve. Postganglionic neurons have their cell bodies in one of two locations:

1. Sympathetic chain of ganglia running along vertebral column. Ganglia are linked to those spinal nerves which contain sympathetic outflow by two small nerves, the rami communicantes. Preganglionic fibers pass into the chain via the white RC, so called because the fibers are myelinated.

Fibers concerned with inn. of head and thorax terminate in synaptic contact with postganglionic cell bodies in the sympathetic chain. The postganglionic cell fibers return to the spinal n. via the gray RC.

Fibers concerned with inn/ of pelvic and abdominal viscera pass uninterrupted through the chain and travel to the plexi containing their corresponding postganglionic cells.

2. Plexi (celiac, sup/inf mesenteric)


Sympathetic ACh/nicotinic receptor NE/adrenergic receptor*

ParasympatheticACh/nicotinic receptor ACh/muscarinic receptor

* Exception is sweat gland, which is cholinergic. In facial nerve lesion, sweat gland can be activated by parasympathetic NT release at mealtime -- gustatory sweating.

Parasympathetic division

Preganglionics associated with III, VII, IX, and X, as well as S2-S4.

Cell bodies of postganglionic PS neurons lie in ganglia close to structure to be innervated. Within the alimentary canal these neurons contribute to myenteric (Auerbach's) and submucosal (Meissner's) plexi -- these plexi are called `enteric NS' because these plexi also contain afferent neurons and interneurons. This allows sustained motility of the guy in the absence of CNS input. They can't be simply equated with parasympathetic NS b/c they get synaptic input from postganglionic sympathetic neurons.

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