6.1 Central Nervous System and Related Structures

Why CNS anatomy is a scored Spinal Anatomy topic

Spinal Anatomy is 22% of NBCE Part I and is tested in Session 1 with General Anatomy and Physiology. Within that domain, anatomy of the central nervous system and related structures is 18%. The official bullets are cerebrum; brainstem and cerebellum; spinal cord and meninges; vascular supply and relationships; ventricles and cerebrospinal fluid; development; and histology. Items are identification and relation items: which internal-capsule limb carries the corticospinal tract, which aperture drains the fourth ventricle, which artery supplies the dorsal columns.

Cerebrum

The telencephalon is the cerebral hemispheres plus the basal nuclei derived with them. Each hemisphere has a cortex of gray matter, white matter (association, commissural, and projection fibers), and a lateral ventricle. The longitudinal fissure and falx cerebri separate the hemispheres; the corpus callosum (rostrum, genu, body/trunk, splenium) is the largest commissure. The anterior commissure links temporal lobes; the posterior commissure sits at the midbrain–diencephalon junction and is used as an imaging landmark.

Lobes are named from the bones that overlie them, with two that are not skull bones:

The motor and sensory homunculi are inverted and distorted. The contralateral lower limb and perineum occupy the paracentral lobule on the medial surface (anterior cerebral artery territory). Trunk, upper limb, hand, and face occupy the lateral convexity (middle cerebral artery). The face and tongue are nearest the lateral fissure; the hand has a large cortical territory. A stem that names paracentral cortex is asking about the foot, not the hand.

Basal ganglia (basal nuclei) used on the exam:

The internal capsule is compact projection white matter. Anterior limb sits between caudate head and lentiform nucleus (frontopontine fibers, anterior thalamic radiations). Genu carries corticobulbar fibers. Posterior limb sits between thalamus and lentiform nucleus and carries corticospinal fibers plus superior thalamic (somatosensory) radiations. Retrolenticular fibers include optic radiations; sublenticular fibers include auditory radiations. Do not confuse internal capsule with extreme capsule (between insula and claustrum) or external capsule (between claustrum and putamen).

Diencephalon (not a cerebral lobe, but tested with cerebrum because it is the core of the hemisphere): thalamus (relay; ventral posterolateral = body, ventral posteromedial = face), hypothalamus (autonomic and endocrine), epithalamus (pineal, habenula), subthalamus, and the third ventricle in the midline. The optic nerve and retina are diencephalic outgrowths—functionally CNS tracts, not peripheral nerves.

Brainstem and cerebellum

The brainstem is midbrain (mesencephalon), pons, and medulla oblongata, from rostral to caudal. Cranial-nerve attachments are the fastest way to place a lesion on a diagram.

Pyramidal (motor) decussation is at the caudal medulla: most corticospinal axons cross to form the lateral corticospinal tract. Internal arcuate fibers of the dorsal-column nuclei cross slightly more rostrally to form the medial lemniscus. Medial longitudinal fasciculus interconnects extraocular nuclei and vestibular nuclei—anatomy here, physiology in the neurophysiology chapter.

The cerebellum occupies the posterior fossa under the tentorium. Vermis is midline; hemispheres are lateral; flocculonodular lobe is the oldest (vestibular) division. Deep nuclei, medial to lateral: fastigial, globose, emboliform, dentate (interposed nuclei = globose + emboliform). Peduncles: inferior (restiform body; largely spinocerebellar and olivocerebellar afferents), middle (pontocerebellar afferents from contralateral pontine nuclei), superior (largely cerebellar efferents to red nucleus and VL thalamus). Cortex histology is a three-layer sheet: molecular, Purkinje (sole cortical output, inhibitory to deep nuclei), and granular. Climbing fibers from inferior olive and mossy fibers from spinal/pontine sources are the two afferent classes.

Spinal cord and meninges

The cord has 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) but is shorter than the vertebral column. Cervical enlargement (C5–T1 cord segments) supplies the brachial plexus; lumbosacral enlargement (about L1–S3 cord segments) supplies the lumbar and sacral plexuses. Conus medullaris typically ends at the L1–L2 disc in the adult (range T12–L3). Cauda equina is the bundle of lumbar, sacral, and coccygeal roots in the lumbar cistern. Filum terminale internum (pia) tethers conus to the end of the dural sac; filum terminale externum (coccygeal ligament) continues to the coccyx.

Because cord segments do not match vertebral levels below the cervical region, a vertebral disc herniation is named by bone while the compressed root is named by the nerve. In the lumbar spine the cord is gone: an L4–L5 disc typically threatens the L5 root in the lateral recess, not a cord segment. Cervical roots exit above the same-numbered vertebra (C8 above T1), so a C6–C7 disc typically threatens C7. Those relations are anatomy; disc-disease algorithms belong to later boards.

Rexed laminae organize gray matter:

White-matter tracts you must place in a cord cross-section:

Meninges. Cranial dura has periosteal and meningeal layers that split to form venous sinuses. Spinal dura is the meningeal layer only, so a true epidural space with fat and the internal vertebral venous plexus exists in the vertebral canal—the space used for epidural anesthesia. Arachnoid is avascular; subarachnoid space holds CSF and major vessels. Pia follows every contour, forms denticulate ligaments (about 21 pairs) that tether cord to dura between dorsal and ventral roots, and continues as filum. Potential subdural space is between dura and arachnoid. Cranial dural septa: falx cerebri, tentorium cerebelli (incisura for midbrain), falx cerebelli, diaphragma sellae.

Vascular supply and relationships

Anterior circulation is internal carotid. After the cavernous sinus, each ICA gives ophthalmic, posterior communicating, anterior choroidal, then bifurcates into anterior cerebral (ACA) and middle cerebral (MCA). ACA supplies medial hemisphere (paracentral lobule = contralateral foot). MCA supplies lateral convexity (arm and face homunculus, language areas when left). Anterior choroidal supplies parts of internal capsule, globus pallidus, and optic tract—small vessel, large clinical cost.

Posterior circulation is vertebral–basilar. Vertebral arteries typically enter the C6 transverse foramina, cross the posterior arch of C1, and enter the foramen magnum. They give posterior inferior cerebellar arteries (PICA) and join as basilar at the pontomedullary junction. Basilar gives anterior inferior cerebellar (AICA), pontine perforators, superior cerebellar (SCA), and terminal posterior cerebral arteries (PCA) (occipital lobe, visual cortex, parts of thalamus and midbrain). PCA versus SCA: CN III exits between them in the ambient cistern.

The cerebral arterial circle (of Willis) is an anastomotic hexagon in the interpeduncular/suprasellar cistern: ACom, both ACAs, both ICAs, both PComs, both PCAs. MCA is a lateral continuation of ICA and is not a circle segment. Posterior communicating arteries join ICA to PCA. Hypophyseal arteries from ICA supply the pituitary stalk.

Spinal arterial supply. One anterior spinal artery (ASA) forms from paired branches of the vertebral arteries and runs in the anterior median fissure. Sulcal (central) arteries from ASA enter the cord and supply anterior two-thirds, including ventral horns, lateral corticospinal tracts, and spinothalamic tracts. Paired posterior spinal arteries, from PICA or vertebral, supply the posterior third (dorsal horns and dorsal columns). Segmental radicular/medullary feeders reinforce both systems. The great anterior radicular artery (of Adamkiewicz) is usually a left-sided vessel entering between T8 and L1 (often T9–T12) and is the dominant caudal ASA reinforcement. ASA occlusion produces motor and pain/temperature loss with spared dorsal-column sensation—an anatomy item, not a stroke workup.

Venous sinuses are dural endothelial channels without valves. Superior sagittal sinus in the falx receives convexity veins and arachnoid granulations. Inferior sagittal sinus plus the great cerebral vein (of Galen) form the straight sinus. These meet at the confluence (torcular Herophili) and drain to transverse then sigmoid sinuses, then internal jugular vein at the jugular foramen. Cavernous sinus flanks the sella: ICA and CN VI run in the sinus proper; CN III, IV, V1, and V2 travel in the lateral wall. Emissary veins and the vertebral venous plexus (Batson) are valveless routes between extracranial and intracranial/spinal veins.

Ventricles and cerebrospinal fluid

Four ventricles communicate as one CSF space. Lateral ventricles (frontal, body, atrium, occipital, temporal horns) connect through interventricular foramina of Monro to the third ventricle (between thalami, with hypothalamic recesses). Cerebral aqueduct (of Sylvius) in the midbrain is a stenosis-prone bottleneck. Fourth ventricle sits between pons/medulla and cerebellum; its floor is the rhomboid fossa.

Choroid plexus in the lateral, third, and fourth ventricles secretes most CSF. Daily production is about 500 mL; total volume is about 150 mL, so the fluid turns over several times per day. Flow: choroid → lateral ventricles → Monro → third → aqueduct → fourth → median aperture of Magendie (unpaired, to cisterna magna) and lateral apertures of Luschka (paired, to pontine/cerebellopontine cisterns) → subarachnoid space over cord and convexities → arachnoid granulations into superior sagittal sinus. A minority of CSF is absorbed along nerve-root sheaths. Obstruction proximal to the apertures produces noncommunicating hydrocephalus; impaired absorption with open ventricles is communicating.

Lumbar cistern (cauda equina in CSF) is sampled by lumbar puncture below L2, commonly the L3–L4 or L4–L5 interspace, using the iliac crests (L4) as the surface landmark from axial osteology. For skull foramina that transmit these vessels and nerves, see /study-guides/nbce-part1/spinal-anatomy-axial/axial-osteology.

Development

Neuroectoderm thickens as the neural plate under notochord induction, folds into a neural groove, and closes as a neural tube. The cranial neuropore closes about day 24–25; the caudal neuropore about day 26–27. Failed cranial closure is anencephaly; failed caudal closure is open spinal dysraphism. Neural crest cells delaminate from the folds and form dorsal-root ganglia, autonomic ganglia, adrenal medulla, Schwann cells, and parts of leptomeninges and viscerocranium.

Primary vesicles: prosencephalon, mesencephalon, rhombencephalon. Secondary vesicles: telencephalon and diencephalon (from prosencephalon); mesencephalon unchanged; metencephalon (pons + cerebellum) and myelencephalon (medulla) from rhombencephalon. Flexures (cephalic, pontine, cervical) fold the tube into adult topography. The sulcus limitans divides alar plate (dorsal, sensory) from basal plate (ventral, motor) in brainstem and cord. Alar-plate neurons in the brainstem migrate to form some cranial-nerve sensory nuclei; basal plate forms motor nuclei in a medial-to-lateral visceral/somatic pattern.

Histology

A neuron has a soma with Nissl substance (rough ER) that stops at the axon hillock. Dendrites receive; the axon conducts. CNS synapses are mostly chemical. Astrocytes (GFAP-positive) form the glial limitans, induce the blood–brain barrier with endothelial tight junctions, and buffer potassium and transmitters. Oligodendrocytes myelinate multiple CNS internodes. Microglia are mesodermal phagocytes. Ependymal cells line ventricles; specialized ependyma plus capillaries form choroid plexus. CNS myelin is oligodendroglial; injury here is a CNS pattern. Peripheral myelin is Schwann-cell and belongs to the next section. Gray matter is neuron-rich; white matter is myelinated tract. Cerebral cortex is neocortex (six layers) except archicortex (hippocampus, three layers) and paleocortex (olfactory).

Membrane potentials, synapses, and tract physiology are tested under /study-guides/nbce-part1/physiology-neural-cardiovascular/neurophysiology; this section stops at named structure, blood supply, and tissue type.