The Spinal Cord and Central Nervous System

Every adjustment, every neurological screen and every red flag decision rests on knowing where a lesion sits inside the central nervous system. This chapter walks from the coverings of the cord to the cortex, linking each structure to the deficit it produces when damaged, because that is exactly how the board asks the question and how a patient presents in your office.

Section 1Meninges, cerebrospinal fluid and the ventricles

Three coverings, three spaces

The brain and spinal cord are wrapped by three connective tissue membranes. From outside in they are the dura mater, the arachnoid mater and the pia mater. A simple memory aid is "PAD" read from the inside out (pia, arachnoid, dura).

The cranial dura has two layers. The outer periosteal (endosteal) layer is simply the periosteum of the inner skull and is firmly attached at the sutures and the skull base. The inner meningeal layer is continuous with the spinal dura. Where the two layers separate they enclose the dural venous sinuses (superior and inferior sagittal, straight, transverse, sigmoid, cavernous and others), which drain venous blood and absorb CSF. The meningeal layer also folds inward to form the dural reflections: the falx cerebri between the hemispheres, the tentorium cerebelli between the occipital lobes and the cerebellum, the falx cerebelli between the cerebellar hemispheres, and the diaphragma sellae over the pituitary. The free edge of the tentorium forms the tentorial notch through which the midbrain passes; a supratentorial mass can push the medial temporal lobe (uncus) through this notch, compressing the oculomotor nerve and producing a "blown" ipsilateral pupil.

The spinal dura is a single meningeal layer. It is separated from the periosteum of the vertebral canal by the epidural space, which in the spine is a real space filled with fat and the internal vertebral (Batson) venous plexus. This valveless plexus communicates with pelvic, abdominal and thoracic veins and is the route by which prostate, breast and lung cancers seed the vertebral bodies. The spinal dura is anchored above to the margin of the foramen magnum and to the posterior surfaces of C2 and C3, and the dural sac ends at about S2. Below that the filum terminale picks up a dural covering (the coccygeal ligament, or filum terminale externum) and attaches to the dorsum of the coccyx. Each spinal nerve root carries a sleeve of dura into the intervertebral foramen, where it blends with the epineurium.

In the skull, by contrast, the epidural space is only a potential space because the dura is fused to bone. An epidural hematoma is usually arterial (classically the middle meningeal artery, torn by a temporal bone fracture at the pterion), forms a lens shaped (biconvex) collection that does not cross sutures, and often follows a lucid interval. A subdural hematoma forms between dura and arachnoid when bridging veins tear, is crescent shaped, crosses sutures, and is common in the elderly and in alcoholics with brain atrophy. A subarachnoid hemorrhage from a ruptured berry aneurysm produces the "worst headache of my life" with neck stiffness.

The arachnoid mater is a thin avascular membrane held against the dura by CSF pressure (the subdural space is normally only potential). Delicate trabeculae cross the subarachnoid space to the pia. This space contains CSF, the major arteries and veins of the brain surface, and the cranial and spinal nerve roots. Where the brain surface pulls away from the arachnoid, the space widens into cisterns: the cerebellomedullary cistern (cisterna magna), the pontine cistern, the interpeduncular cistern, the quadrigeminal cistern and, in the spine, the lumbar cistern between about L2 and S2, which contains the cauda equina and is the target of lumbar puncture.

The pia mater is a delicate vascular layer adherent to the surface of the brain and cord, following every sulcus. In the spine the pia forms two specializations. The denticulate ligaments are 21 or so pairs of tooth like lateral extensions that pass between the dorsal and ventral roots and attach to the dura, suspending the cord in the middle of the dural sac; the first is at the foramen magnum and the last lies between T12 and L1 roots. The filum terminale (internum) is a strand of pia that continues from the tip of the conus medullaris to the end of the dural sac at S2.

Cerebrospinal fluid: production, flow and absorption

The adult has roughly 150 mL of CSF at any moment and produces about 500 mL per day, so the whole volume turns over three to four times daily. Most CSF is secreted by the choroid plexus, tufts of fenestrated capillaries covered by a modified ependymal epithelium that projects into the lateral, third and fourth ventricles. The choroid epithelial cells are joined by tight junctions, and these form the blood CSF barrier (the capillaries themselves are leaky; the barrier is the epithelium). Secretion is an active process: Na+/K+ ATPase on the apical surface pumps sodium into the ventricle, carbonic anhydrase supplies bicarbonate, and chloride and water follow. This is why acetazolamide, a carbonic anhydrase inhibitor, reduces CSF production and is used in idiopathic intracranial hypertension.

Compared with plasma, normal CSF is clear and colorless, has much less protein (about 15 to 45 mg/dL), glucose about two thirds of serum glucose (roughly 45 to 80 mg/dL), slightly higher chloride and magnesium, lower potassium and calcium, and fewer than 5 white cells per microliter (all mononuclear). Normal opening pressure in the lateral decubitus position is about 70 to 180 mm H2O (some sources accept up to 200 to 250).

The flow pathway is a favorite board sequence:

1. Lateral ventricles (one in each hemisphere; anterior horn, body, atrium, posterior and inferior horns).
2. Through the paired interventricular foramina (of Monro) into the third ventricle, a midline slit between the two thalami.
3. Through the cerebral aqueduct (of Sylvius) in the midbrain to the fourth ventricle, which lies dorsal to the pons and upper medulla and ventral to the cerebellum.
4. Out of the fourth ventricle through the single midline median aperture (foramen of Magendie) and the two lateral apertures (foramina of Luschka) into the subarachnoid space (cisterna magna and pontine cistern). A memory aid: Magendie is Medial, Luschka is Lateral. A small amount also passes into the central canal of the cord.
5. Around the brain and cord in the subarachnoid space, then absorbed by the arachnoid granulations (villi), which herniate into the superior sagittal sinus and act as one way pressure dependent valves into venous blood.

Hydrocephalus and the ependyma

The ventricles are lined by ependymal cells, ciliated cuboidal glia whose cilia help move CSF. Hydrocephalus is an excess of CSF with ventricular enlargement. In noncommunicating (obstructive) hydrocephalus flow is blocked within the ventricular system, most often at the narrow cerebral aqueduct (aqueductal stenosis) or at the fourth ventricle outlets (as in Dandy Walker malformation or a posterior fossa tumor). Only the ventricles upstream of the block dilate. In communicating hydrocephalus CSF reaches the subarachnoid space but is poorly absorbed, typically because the arachnoid granulations are scarred by prior meningitis or subarachnoid hemorrhage; all ventricles enlarge. Normal pressure hydrocephalus in older adults causes the triad "wet, wobbly and wacky" (urinary incontinence, a magnetic broad based gait, and dementia) because the enlarged lateral ventricles stretch the corona radiata fibers that serve the legs and bladder.

Section 2Gross and internal anatomy of the spinal cord

Shape, length and enlargements

The spinal cord begins at the foramen magnum, continuous with the medulla, and in the adult ends as the cone shaped conus medullaris at about the L1 to L2 disc level. In the newborn it ends at about L3. The cord is roughly 42 to 45 cm long, about the thickness of a finger, and slightly flattened front to back. It has 31 segments defined by the 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. There are 8 cervical nerves but only 7 cervical vertebrae because C1 exits above the atlas and C8 exits below C7.

Two swellings mark the limb segments. The cervical enlargement spans roughly C4 to T1 and supplies the brachial plexus; the lumbosacral (lumbar) enlargement spans roughly L1 (or L2) to S3 and supplies the lumbosacral plexus. They are wider because they hold many more lower motor neurons and interneurons for the limbs.

Because the vertebral column grows longer than the cord during fetal life (the "ascent" of the cord), lower roots must travel downward inside the dural sac before reaching their foramina. Below the conus the lumbar, sacral and coccygeal roots form the cauda equina ("horse's tail") floating in the lumbar cistern, along with the filum terminale.

Segment to vertebra relations

Since the cord is shorter than the canal, a given cord segment lies above the vertebra of the same number. The classic working rule:

The other rule to master is root exit. In the cervical spine each nerve exits above the vertebra of the same number (C6 exits between C5 and C6), and C8 exits between C7 and T1. From T1 downward each nerve exits below its numbered vertebra (L4 exits the L4 to L5 foramen). A posterolateral disc herniation at L4 to L5 usually compresses the traversing L5 root, not the exiting L4 root, because the exiting root has already left high in the foramen; a far lateral (foraminal) herniation at the same level catches the exiting L4.

Surface features

The cord surface shows a deep anterior median fissure (with the anterior spinal artery in it), a shallow posterior median sulcus, and paired posterolateral sulci where the dorsal rootlets enter and anterolateral sulci where the ventral rootlets leave. Above T6 a posterior intermediate sulcus separates the fasciculus gracilis medially from the fasciculus cuneatus laterally. Each segment gives off dorsal (sensory) and ventral (motor) rootlets that fuse into roots and join just distal to the dorsal root ganglion in the intervertebral foramen to form the mixed spinal nerve.

Gray matter: horns, laminae and nuclei

In cross section the central gray matter forms an H or butterfly. Its parts are the dorsal (posterior) horn, which is sensory; the ventral (anterior) horn, which is motor; and, from T1 to L2 and from S2 to S4, an intermediate zone with autonomic cell columns. The lateral horn (intermediolateral cell column) is present only from T1 to L2 (or L3) and contains preganglionic sympathetic neurons. The sacral parasympathetic nucleus sits in a similar position at S2 to S4. The gray commissure around the central canal connects the two sides, and the anterior white commissure ventral to it carries crossing fibers, most importantly the spinothalamic fibers.

Bror Rexed divided the gray matter into ten layers, the Rexed laminae, that recur at every level:

Within lamina IX the motor neurons are arranged somatotopically: medial groups supply axial and trunk muscles and are present at all levels; lateral groups, found only in the enlargements, supply the limbs. Within the lateral group, flexor motor neurons lie dorsal to extensor ones, and those for distal muscles lie lateral to those for proximal muscles.

White matter: three funiculi

The white matter is divided by the roots into a posterior (dorsal) funiculus between the posterior median sulcus and the dorsal roots, a lateral funiculus between the dorsal and ventral roots, and an anterior funiculus between the ventral roots and the anterior median fissure. The posterior funiculus carries the dorsal columns (gracilis and cuneatus). The lateral funiculus carries the lateral corticospinal, rubrospinal, spinocerebellar and lateral spinothalamic tracts. The anterior funiculus carries the anterior corticospinal, vestibulospinal, tectospinal, reticulospinal and anterior spinothalamic tracts and the medial longitudinal fasciculus. Short propriospinal fibers, the fasciculus proprius, hug the gray matter at every level, and Lissauer's dorsolateral tract caps the dorsal horn, carrying pain fibers up or down one or two segments before they synapse.

The proportion of white to gray matter changes with level, and the board asks you to identify a section. The cervical cord has the most white matter (every tract is at full size), is oval, has large ventral horns in the enlargement and shows both gracilis and cuneatus. The thoracic cord is small and round with slender horns, a visible lateral horn and a prominent Clarke nucleus. The lumbar cord is round with large ventral and dorsal horns and relatively less white matter. The sacral cord is small with gray matter dominating and very little white matter.

Blood supply of the cord

Three longitudinal arteries run the length of the cord. The single anterior spinal artery forms from the union of branches of both vertebral arteries at the level of the foramen magnum, runs in the anterior median fissure and supplies the anterior two thirds of the cord: the ventral and lateral horns, the corticospinal and spinothalamic tracts. The paired posterior spinal arteries arise from the vertebral or posterior inferior cerebellar arteries and supply the posterior one third, mainly the dorsal columns and dorsal horns.

These longitudinal channels are too small to supply the whole cord alone, so they are reinforced by segmental medullary (radicular) arteries from the vertebral, ascending and deep cervical, posterior intercostal, lumbar and lateral sacral arteries. The largest is the great anterior segmental medullary artery of Adamkiewicz, usually arising on the left side between T9 and T12 (range T8 to L2). It supplies most of the lower two thirds of the cord. The upper thoracic cord (about T1 to T4) is a watershed zone with sparse collateral supply, so it is vulnerable to hypotension. Clamping or injuring the artery of Adamkiewicz during aortic aneurysm repair causes anterior spinal artery syndrome: bilateral paralysis and loss of pain and temperature below the lesion with sparing of vibration and position sense.

Section 3Ascending and descending tracts

General plan of a sensory pathway

Conscious sensory pathways use a chain of three neurons. The first order neuron has its cell body in a dorsal root ganglion (or a cranial nerve ganglion) and carries the signal into the CNS. The second order neuron lies in the dorsal horn or a brainstem nucleus, and its axon decussates (crosses the midline) and ascends to the thalamus. The third order neuron in the thalamus (ventral posterolateral nucleus, VPL, for the body; ventral posteromedial, VPM, for the face) projects through the posterior limb of the internal capsule to the primary somatosensory cortex in the postcentral gyrus. Where the second order axon crosses determines the side of the deficit, and that single fact solves most lesion questions.

Dorsal column medial lemniscus (DCML)

The DCML carries fine (discriminative) touch, two point discrimination, vibration, conscious proprioception (joint position sense) and stereognosis. Large myelinated A beta fibers from Meissner and Pacinian corpuscles, muscle spindles and joint receptors enter the medial division of the dorsal root and ascend ipsilaterally without synapsing in the cord. Fibers from below T6 (legs and lower trunk) form the medially placed fasciculus gracilis; fibers from T6 and above (arms and upper trunk) add laterally as the fasciculus cuneatus. The dorsal columns are therefore somatotopically layered with the sacral fibers most medial.

The first order neurons synapse in the nucleus gracilis and nucleus cuneatus in the lower medulla. Second order axons sweep across the midline as the internal arcuate fibers (the sensory decussation) and ascend as the medial lemniscus to VPL. Third order neurons go to areas 3, 1 and 2 of the postcentral gyrus. A lesion of the dorsal column in the cord produces ipsilateral loss; a lesion of the medial lemniscus above the decussation produces contralateral loss.

Testing: vibration with a 128 Hz tuning fork, joint position sense at the great toe, two point discrimination, and the Romberg test. A positive Romberg (stable with eyes open, falls with eyes closed) indicates loss of proprioceptive input, a sensory ataxia from dorsal column or large fiber peripheral nerve disease, not a cerebellar problem.

Anterolateral system: spinothalamic tracts

The lateral spinothalamic tract carries pain and temperature; the anterior spinothalamic tract carries crude (nondiscriminative) touch and pressure. First order neurons are small A delta (sharp, fast pain, cold) and C fibers (slow burning pain, warmth). They enter through the lateral division of the dorsal root, may travel one or two segments in Lissauer's tract, and synapse in the dorsal horn (laminae I, II and V). The transmitter is glutamate, with substance P and CGRP for C fibers. Second order axons cross within one or two segments through the anterior white commissure and ascend in the contralateral anterolateral funiculus to VPL. The tract is laminated with sacral fibers most lateral (dorsolateral) and cervical fibers most medial; this explains sacral sparing when an intramedullary lesion expands outward from the center.

Companion anterolateral pathways, the spinoreticular (arousal and the unpleasant quality of pain) and spinomesencephalic tracts, reach the reticular formation and periaqueductal gray, which drives descending pain inhibition via the raphe nuclei and locus ceruleus.

Spinocerebellar tracts: unconscious proprioception

These two neuron pathways inform the cerebellum about limb position and movement and end ipsilaterally in the cerebellum (some cross twice, so the net effect is still ipsilateral). The posterior (dorsal) spinocerebellar tract arises from Clarke nucleus (C8 to L3), ascends ipsilaterally in the lateral funiculus and enters through the inferior cerebellar peduncle; it carries leg and lower trunk information from individual muscle spindles and tendon organs. Its upper limb equivalent is the cuneocerebellar tract from the accessory (lateral) cuneate nucleus, also via the inferior peduncle. The anterior (ventral) spinocerebellar tract crosses in the cord, ascends, enters through the superior cerebellar peduncle and crosses back, reporting the activity of spinal interneurons and whole limb movement. The rostral spinocerebellar tract is its upper limb counterpart. Friedreich ataxia degenerates the spinocerebellar tracts, dorsal columns and corticospinal tracts together.

Descending motor tracts

The corticospinal tract is the principal voluntary motor pathway, especially for skilled distal movements. About a third of its fibers arise from the primary motor cortex (area 4, precentral gyrus, including the giant Betz cells), a third from premotor and supplementary motor areas (area 6), and a third from parietal areas 3, 1, 2 and 5. Fibers descend through the corona radiata, the posterior limb of the internal capsule, the middle three fifths of the cerebral peduncle (crus cerebri), the basilar pons and the medullary pyramids. At the medulla to cord junction about 85 to 90 percent cross in the pyramidal decussation to form the lateral corticospinal tract in the lateral funiculus, which synapses on lateral (limb) motor neurons. The remaining 10 to 15 percent continue uncrossed as the anterior corticospinal tract and cross at segmental levels through the anterior white commissure to reach axial motor neurons. In the lateral corticospinal tract the leg fibers are lateral and the arm fibers medial. The corticobulbar tract does the same job for cranial nerve motor nuclei; most of these receive bilateral input, the key exceptions being the lower face (facial nucleus portion for the lower face receives contralateral input only) and, in many people, the hypoglossal nucleus.

These tracts explain posturing. In decorticate posturing (lesion above the red nucleus) the rubrospinal tract is intact, so the arms flex while the legs extend: think "cor" means the hands come to the heart. In decerebrate posturing (lesion between the red nucleus and the vestibular nuclei, at the midbrain or upper pons) the rubrospinal influence is lost and unopposed vestibulospinal and pontine reticulospinal drive extends all four limbs. Decerebrate carries a worse prognosis.

Upper versus lower motor neuron lesions

Classic cord lesion patterns

Brown Sequard syndrome is a hemisection of the cord, often from a stab wound, lateral disc or tumor. Below the lesion it produces ipsilateral spastic weakness (lateral corticospinal), ipsilateral loss of vibration, position and fine touch (dorsal column) and contralateral loss of pain and temperature starting one or two segments below the lesion (spinothalamic, already crossed). At the level of the lesion there is ipsilateral lower motor neuron weakness and a band of ipsilateral total anesthesia from destroyed roots and dorsal horn. A lesion above T1 can also damage descending hypothalamospinal fibers, causing ipsilateral Horner syndrome.

Central cord syndrome, the most common incomplete cord injury, typically follows hyperextension of a spondylotic cervical spine in an older adult. The central gray and the medially placed arm fibers of the corticospinal tracts suffer most, so the arms are weaker than the legs, there is variable sensory loss below the lesion and often sacral sparing. Syringomyelia, a fluid cavity in the central cord commonly associated with Chiari I malformation, first damages the crossing spinothalamic fibers in the anterior white commissure, giving a bilateral cape like loss of pain and temperature over the shoulders and arms with preserved touch (dissociated sensory loss); as it expands it destroys ventral horn cells (hand wasting) and then the lateral tracts.

Section 4The brainstem and cranial nerve nuclei

Three parts and their landmarks

The brainstem connects the spinal cord to the diencephalon and contains ascending and descending tracts, the nuclei of cranial nerves III to XII, the reticular formation and vital cardiorespiratory centers. Each level has a basal part (descending motor fibers), a tegmentum (cranial nerve nuclei, reticular formation, ascending tracts) and, in the midbrain, a tectum (colliculi) dorsal to the aqueduct.

The medulla oblongata runs from the foramen magnum to the pontomedullary junction. Ventrally it shows the pyramids (corticospinal fibers) beside the anterior median fissure and the oval olives (inferior olivary nucleus, which sends climbing fibers to the cerebellum) lateral to them. Dorsally the closed lower medulla shows the gracile and cuneate tubercles; the open upper medulla forms the lower half of the floor of the fourth ventricle. The medulla contains the respiratory centers and cardiovascular (vasomotor) center, the nucleus of the solitary tract and the area postrema, a chemoreceptor trigger zone for vomiting that lacks a blood brain barrier.

The pons has a bulging basilar part of transverse pontocerebellar fibers that form the middle cerebellar peduncle, with corticospinal bundles scattered among the pontine nuclei. Its dorsal tegmentum forms the upper floor of the fourth ventricle, where the facial colliculus marks the place the facial nerve fibers loop around the abducens nucleus. The pons houses the pneumotaxic and apneustic respiratory areas and the locus ceruleus.

The midbrain (mesencephalon) surrounds the cerebral aqueduct. Ventrally the cerebral peduncles (crus cerebri) carry corticospinal and corticobulbar fibers, with the interpeduncular fossa between them where CN III exits. Behind the crus lies the pigmented substantia nigra (dopaminergic pars compacta). The tegmentum contains the red nucleus, the oculomotor and trochlear nuclei and the periaqueductal gray. The dorsal tectum holds the superior colliculi (visual reflexes) and inferior colliculi (auditory relay); CN IV is the only nerve to exit the dorsal brainstem, just below the inferior colliculus, and it is the only cranial nerve whose fibers fully decussate.

Organization of cranial nerve nuclei

In the embryo the sulcus limitans divides the alar plate (sensory, dorsolateral) from the basal plate (motor, ventromedial). In the floor of the fourth ventricle this pattern persists: motor nuclei lie medial and sensory nuclei lateral to the sulcus limitans. The nuclei form functional columns: general somatic efferent (III, IV, VI, XII) closest to the midline; special visceral (branchial) efferent (motor V, VII, nucleus ambiguus for IX, X and cranial XI) displaced ventrolaterally; general visceral efferent (Edinger Westphal, superior and inferior salivatory, dorsal motor nucleus of X); then general and special visceral afferent (solitary nucleus); then general somatic afferent (trigeminal); and special somatic afferent (vestibular and cochlear) most laterally.

The reticular formation

The reticular formation is a diffuse network of neurons in the core of the tegmentum, from the medulla to the midbrain. Its ascending component, the ascending reticular activating system, projects through the intralaminar thalamic nuclei to the cortex and maintains wakefulness; bilateral damage causes coma. Its descending reticulospinal output modulates muscle tone, posture and pain. Neurochemically distinct groups sit within or beside it: the raphe nuclei (serotonin, sleep and pain modulation), the locus ceruleus in the pons (norepinephrine, arousal and stress) and the ventral tegmental area and substantia nigra (dopamine). Medullary reticular neurons form the respiratory rhythm generator and the vasomotor center.

Brainstem vascular syndromes

Brainstem strokes produce crossed (alternating) findings: a cranial nerve deficit on the side of the lesion and a long tract deficit (hemiplegia or hemianesthesia) on the opposite body, because the cranial nerve has not yet crossed while the long tracts already have or will. The cranial nerve involved localizes the level.

Section 5Cerebrum, diencephalon, cerebellum and their blood supply

Lobes and functional areas

Each cerebral hemisphere has a frontal, parietal, temporal and occipital lobe, plus the insula buried beneath the lateral sulcus and the limbic lobe on the medial surface. The central sulcus separates frontal from parietal; the lateral (Sylvian) fissure separates temporal below from frontal and parietal above; the parieto-occipital sulcus marks the occipital lobe medially, and the calcarine sulcus holds primary visual cortex.

Broca and Wernicke areas are connected by the arcuate fasciculus; its lesion causes conduction aphasia with poor repetition. White matter fibers are classified as association fibers (within one hemisphere), commissural fibers (corpus callosum, anterior commissure, joining hemispheres) and projection fibers (to and from lower centers, through the internal capsule).

Internal capsule

The internal capsule is a compact band of projection fibers between the caudate and thalamus medially and the lentiform nucleus laterally. Its anterior limb (between caudate head and lentiform) carries frontopontine and thalamocortical fibers. The genu carries corticobulbar fibers. The posterior limb (between thalamus and lentiform) carries corticospinal fibers (arm anterior, leg posterior) and thalamocortical somatosensory fibers; its retrolenticular and sublenticular parts carry the optic radiations and auditory radiations. Because so many fibers converge in so small a space, a small lacunar infarct of the lenticulostriate branches of the middle cerebral artery can cause a pure motor hemiparesis of face, arm and leg equally.

Diencephalon: thalamus and hypothalamus

The thalamus is the relay station for nearly all sensory input to the cortex (smell is the exception) and for motor loops from the basal ganglia and cerebellum.

The hypothalamus, below the hypothalamic sulcus, controls autonomic, endocrine and homeostatic functions. The supraoptic nucleus makes mainly ADH and the paraventricular nucleus mainly oxytocin and CRH; both travel down the infundibulum to the posterior pituitary. The lateral nucleus drives hunger (lesion: anorexia), the ventromedial nucleus produces satiety (lesion: hyperphagia and obesity), the anterior hypothalamus promotes cooling and parasympathetic activity, the posterior promotes heat conservation and sympathetic activity, the suprachiasmatic nucleus sets circadian rhythm from retinal input, and the mammillary bodies participate in memory and are damaged in Wernicke Korsakoff syndrome (thiamine deficiency).

Basal ganglia and limbic system

The basal ganglia include the caudate and putamen (together the striatum), the globus pallidus (putamen plus globus pallidus form the lentiform nucleus), the subthalamic nucleus and the substantia nigra. They modulate movement through a loop: cortex to striatum to globus pallidus internus to VA and VL thalamus and back to cortex. The direct pathway disinhibits the thalamus and facilitates movement; the indirect pathway (via globus pallidus externus and subthalamic nucleus) inhibits it. Dopamine from the substantia nigra pars compacta excites the direct pathway through D1 receptors and inhibits the indirect pathway through D2 receptors, so loss of nigral dopamine in Parkinson disease produces bradykinesia, rigidity, resting tremor and postural instability. Huntington disease (CAG repeat, autosomal dominant) degenerates the caudate, causing chorea and dementia. A lesion of the subthalamic nucleus causes contralateral hemiballismus.

The limbic system governs emotion, memory and drives. Its core structures are the hippocampus (memory consolidation), amygdala (fear and emotional memory; bilateral damage gives Kluver Bucy syndrome), cingulate and parahippocampal gyri, fornix, mammillary bodies and anterior thalamus.

Cerebellum

The cerebellum coordinates movement, balance and motor learning. It has a midline vermis and two hemispheres, divided into three functional regions. The vestibulocerebellum (flocculonodular lobe) controls balance and eye movements; the spinocerebellum (vermis and paravermis) controls axial and proximal limb tone and posture; the cerebrocerebellum (lateral hemispheres) plans and times skilled limb movement. The cortex has three layers (molecular, Purkinje, granular). Purkinje cells are the only output of the cortex and are inhibitory (GABA) onto the deep nuclei: from medial to lateral, fastigial, globose, emboliform and dentate ("Fat Girls Eat Doughnuts"). Inputs arrive as climbing fibers from the inferior olive and mossy fibers from everywhere else. The inferior peduncle carries mostly input from the cord and vestibular system, the middle peduncle carries pontocerebellar input from the opposite cortex, and the superior peduncle carries most output, from the dentate to the contralateral red nucleus and VL thalamus.

Because the output crosses once to the opposite cortex and the corticospinal tract crosses back, a cerebellar hemisphere controls the ipsilateral body. Hemisphere lesions give ipsilateral limb ataxia, dysmetria (positive finger to nose), intention tremor, dysdiadochokinesia and hypotonia; vermis lesions give truncal ataxia and a wide based gait, typical of alcoholic degeneration.

Blood supply of the brain

Two internal carotid arteries (anterior circulation) and two vertebral arteries (posterior circulation) supply the brain. The vertebral arteries arise from the subclavians, ascend through the transverse foramina of C6 to C1, loop behind the lateral mass of the atlas in the suboccipital triangle, pierce the posterior atlanto-occipital membrane and dura, enter the foramen magnum and unite at the pontomedullary junction to form the basilar artery. Before joining they give the anterior spinal artery and the posterior inferior cerebellar artery (PICA). The basilar gives the anterior inferior cerebellar artery (AICA), the labyrinthine artery (often from AICA), pontine branches and the superior cerebellar artery, then divides into the two posterior cerebral arteries (PCA).

The circle of Willis joins the systems around the optic chiasm and pituitary stalk: the anterior communicating artery links the two anterior cerebral arteries (ACA), and each posterior communicating artery joins the internal carotid to the PCA. The most common site of a berry aneurysm is the anterior communicating artery; a posterior communicating aneurysm compresses CN III and dilates the pupil.

Words worth knowing