Medical School · Year 2 · Neuroscience · includes a quiz and discussion video

Lecture 1: Neuroscience Overview and Neuroanatomy

Unit 2.5: Neuroscience

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the organization of the central and peripheral nervous systems
2. Explain the development of the nervous system from neural tube to mature brain
3. Describe the major divisions of the brain and their functions
4. Explain the cellular components of nervous tissue including neurons and glia
5. Describe the meninges, their spaces, and clinical significance of hemorrhage patterns
6. Apply neuroanatomical terminology to describe brain structures and lesion localization

1. Organization of the Nervous System

The nervous system serves as the body's master control and communication network, integrating sensory information, coordinating motor responses, and supporting higher cognitive functions. Understanding its fundamental organization provides the framework for all subsequent neurological learning.

The central nervous system encompasses the brain and spinal cord, representing the integrative and command center of neural processing. The peripheral nervous system comprises cranial nerves, spinal nerves, and associated ganglia that connect the CNS to the rest of the body. This anatomical distinction has important clinical implications, as diseases affect CNS and PNS tissues differently and regeneration capacity varies dramatically between the two systems.

Functional classification divides the nervous system based on the type of control and direction of information flow. The somatic nervous system governs voluntary control of skeletal muscle, allowing conscious movement and interaction with the environment. The autonomic nervous system manages involuntary control of visceral functions through its sympathetic, parasympathetic, and enteric divisions. Afferent pathways carry sensory information toward the CNS, while efferent pathways transmit motor commands away from the CNS to effector organs.

Gray matter contains neuronal cell bodies, dendrites, and synapses where neural processing occurs. In the cerebrum, gray matter forms the outer cortex and deep nuclei, while in the spinal cord, it occupies the central H-shaped region. White matter consists of myelinated axons organized into tracts that transmit information between different nervous system regions. Understanding this distinction helps interpret imaging findings and predict functional consequences of lesions.

<image>Panel A: A human figure with the brain and spinal cord highlighted in blue (CNS) and peripheral nerves in green (PNS), with cranial nerves, spinal nerves, and ganglia labeled. Panel B: A cross-section comparison showing gray matter (cell bodies, depicted as clusters of neurons with dark nuclei) versus white matter (myelinated axons shown as parallel bundled fibers with myelin sheaths). Panel C: A functional classification flowchart showing sensory afferents (receptors to CNS) and motor efferents (CNS to muscles/glands). Panel D: Somatic (voluntary skeletal muscle control) and autonomic (involuntary visceral control with sympathetic, parasympathetic, and enteric subdivisions) branches clearly distinguished.</image>

2. Development of the Nervous System

Neural development begins during the third week of gestation when the ectoderm thickens along the dorsal midline to form the neural plate. The plate invaginates to create the neural groove, whose edges elevate as neural folds. By the fourth week, these folds fuse to form the neural tube, the primordium of the entire central nervous system. Failure of neural tube closure results in devastating congenital defects ranging from spina bifida to anencephaly.

The primitive neural tube differentiates into three primary brain vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These subsequently subdivide into five secondary vesicles that give rise to adult brain structures. The prosencephalon divides into telencephalon, which becomes the cerebral hemispheres, and diencephalon, which forms the thalamus and hypothalamus. The mesencephalon remains undivided, forming the midbrain. The rhombencephalon splits into metencephalon, which becomes the pons and cerebellum, and myelencephalon, which forms the medulla oblongata.

Neural crest cells arise from the margins of the neural folds during closure and migrate extensively throughout the embryo. These multipotent cells give rise to numerous structures including dorsal root ganglia and autonomic ganglia of the PNS, chromaffin cells of the adrenal medulla, Schwann cells providing PNS myelination, melanocytes, and much of the craniofacial skeleton. The pia and arachnoid meninges also derive from neural crest, while the dura develops from mesoderm. Neural crest-derived tumors and developmental abnormalities represent important clinical entities.

The ventricular system develops from the lumen of the neural tube, with the lateral ventricles forming within the telencephalon, the third ventricle within the diencephalon, the cerebral aqueduct traversing the midbrain, and the fourth ventricle developing between the pons, medulla, and cerebellum. Understanding these embryological relationships helps explain patterns of congenital malformations and hydrocephalus.

<image>Panel A: Neurulation stages from week 3-4 showing neural plate (flat ectoderm thickening), neural groove (invaginating plate), neural folds (elevated edges approaching midline), and closed neural tube with neural crest cells migrating away (shown as orange cells dispersing bilaterally). Panel B: Primary vesicle formation with prosencephalon, mesencephalon, and rhombencephalon labeled on a simple tube diagram. Panel C: Secondary vesicle development with color-coded regions showing telencephalon (cerebral hemispheres) in purple, diencephalon (thalamus/hypothalamus) in blue, and mesencephalon (midbrain) in green. Panel D: Metencephalon (pons/cerebellum) in yellow and myelencephalon (medulla) in orange, each with arrows pointing to adult structure illustrations.</image>

3. Major Divisions of the Brain

The cerebrum represents the largest brain division and contains four lobes within each hemisphere. The frontal lobe, anterior to the central sulcus, houses the primary motor cortex in the precentral gyrus and governs executive functions including planning, judgment, problem-solving, and personality expression. The prefrontal cortex integrates complex behaviors, while Broca's area in the dominant hemisphere controls motor aspects of speech production.

The parietal lobe lies posterior to the central sulcus and contains the primary somatosensory cortex in the postcentral gyrus. This region processes tactile, proprioceptive, and spatial information, enabling body awareness and spatial reasoning. Lesions produce contralateral sensory deficits and spatial neglect syndromes. The temporal lobe, inferior to the lateral sulcus, houses primary auditory cortex, Wernicke's area for language comprehension, and hippocampal structures essential for memory formation. The occipital lobe at the posterior pole contains primary visual cortex along the calcarine sulcus.

The diencephalon occupies the central core of the brain, surrounded by the cerebral hemispheres. The thalamus serves as the major relay station for sensory information ascending to cortex and participates in consciousness and sleep-wake regulation. The hypothalamus, despite its small size, controls autonomic function, endocrine output through pituitary regulation, and homeostatic mechanisms including temperature, hunger, thirst, and circadian rhythms. The epithalamus contains the pineal gland producing melatonin, while the subthalamus participates in motor circuits.

The brainstem connects the cerebrum to the spinal cord and contains vital regulatory centers. The midbrain houses nuclei for eye movement control and serves as a relay for auditory and visual information. The pons bridges the cerebellum to the brainstem and contains respiratory control centers and multiple cranial nerve nuclei. The medulla oblongata contains cardiovascular and respiratory centers essential for life, along with nuclei for swallowing, vomiting, and other reflexes. The cerebellum coordinates movement, maintains balance, and contributes to motor learning.

<image>Panel A: A lateral view of the left cerebral hemisphere with lobes color-coded showing frontal (blue) with precentral gyrus and Broca's area labeled, parietal (green) with postcentral gyrus highlighted, temporal (yellow) with superior temporal gyrus and Wernicke's area marked, and occipital (red) at the posterior pole with major sulci (central, lateral, parieto-occipital) marked with arrows. Panel B: A midsagittal view showing diencephalon structures including thalamus (large central mass), hypothalamus (inferior), pineal gland (posterior), and pituitary (inferior projection). Panel C: The brainstem in posterior view with cerebellum removed showing midbrain (superior and inferior colliculi), pons (middle bulge), medulla (inferior tapered region), with cranial nerve exit points numbered. Panel D: The cerebellum from posterior view with vermis and hemispheres labeled.</image>

4. Cellular Components of Nervous Tissue

Neurons are the fundamental information-processing units of the nervous system, specialized for receiving, integrating, and transmitting electrochemical signals. The cell body or soma contains the nucleus and protein synthesis machinery, integrating thousands of synaptic inputs through its dendritic tree. Dendrites are branching extensions that receive signals from other neurons, with elaborate arborization patterns increasing receptive surface area. Dendritic spines further expand synaptic capacity and represent sites of considerable plasticity.

The axon emerges from the axon hillock, the trigger zone where action potentials initiate when the integrated signal exceeds threshold. Each neuron typically possesses a single axon that may extend considerable distances and branch into collaterals reaching multiple targets. Axon terminals or boutons contain synaptic vesicles filled with neurotransmitter, ready for calcium-triggered release. The cytoskeleton provides structural support and transport railways, with microtubules serving as tracks for motor proteins, neurofilaments determining axon caliber, and actin microfilaments enabling growth cone motility during development and regeneration.

Neurons exhibit diverse morphologies corresponding to their functions. Multipolar neurons with multiple dendrites and one axon predominate throughout the CNS, including motor neurons and most interneurons. Bipolar neurons possess one dendrite and one axon, found in specialized sensory systems including retina and olfactory epithelium. Pseudounipolar neurons of dorsal root ganglia have a single process that bifurcates into peripheral and central branches, efficiently transmitting sensory information without somatic integration.

Glial cells outnumber neurons and provide essential support functions. Astrocytes form the blood-brain barrier, regulate extracellular potassium and neurotransmitter concentrations, provide metabolic support, and guide development. Oligodendrocytes myelinate CNS axons, with each cell forming myelin segments on multiple adjacent axons. Microglia serve as the brain's resident immune cells, performing surveillance and phagocytosis. Ependymal cells line the ventricles, producing and circulating cerebrospinal fluid. In the PNS, Schwann cells myelinate axons in a one-to-one relationship, while satellite cells support ganglionic neurons.

<image>Panel A: A multipolar neuron with labeled structures showing soma with visible nucleus and Nissl substance, multiple branching dendrites with dendritic spines (shown in magnified inset), axon hillock transitioning to axon, myelinated axon segments with nodes of Ranvier, and synaptic terminals with vesicles. Panel B: Comparison of neuron morphologies showing multipolar (motor neuron with elaborate dendrites), bipolar (retinal bipolar cell with single dendrite and axon), and pseudounipolar (DRG neuron with T-shaped process). Panel C: Glial cells including astrocyte with star-shaped processes contacting a blood vessel (forming BBB), oligodendrocyte extending myelin to multiple axons, microglia with small ramified processes, and ependymal cells lining a ventricle surface with cilia. Panel D: Comparison box showing Schwann cell wrapping a single axon segment versus oligodendrocyte myelinating multiple axons.</image>

5. The Meninges

Three membranous layers collectively termed the meninges protect the brain and spinal cord. The dura mater, outermost and toughest, consists of dense fibrous connective tissue. Within the skull, the dura comprises two layers: the periosteal layer adhering to bone and the meningeal layer forming the inner surface. These layers separate at specific locations to create dural venous sinuses that drain cerebral blood. In the spinal canal, only the meningeal layer is present, creating a true epidural space filled with fat and the vertebral venous plexus.

Dural reflections project inward, compartmentalizing the cranial cavity. The falx cerebri extends vertically between the cerebral hemispheres within the longitudinal fissure, containing the superior and inferior sagittal sinuses. The tentorium cerebelli separates the cerebrum from the cerebellum horizontally, with the tentorial notch allowing passage of the brainstem. The falx cerebelli separates cerebellar hemispheres posteriorly, and the diaphragma sellae covers the sella turcica housing the pituitary. These reflections become important in herniation syndromes when expanding masses displace brain tissue.

The arachnoid mater is a delicate avascular membrane beneath the dura. Fine trabeculae extend from its inner surface across the subarachnoid space to the pia, resembling spider web architecture from which it derives its name. The subarachnoid space contains cerebrospinal fluid cushioning the brain and major cerebral arteries traversing the brain surface. Arachnoid granulations project into the superior sagittal sinus, providing the primary route for CSF reabsorption into venous blood.

The pia mater intimately adheres to the brain and spinal cord surface, following every gyral contour and dipping into each sulcus. This highly vascular membrane provides direct nutrient support to underlying neural tissue. Together with arachnoid, it forms the leptomeninges, commonly involved in meningitis. Understanding meningeal spaces is essential for interpreting hemorrhage patterns: epidural hematomas typically result from arterial bleeding (middle meningeal artery) and appear lens-shaped on CT, subdural hematomas arise from torn bridging veins and appear crescent-shaped, while subarachnoid hemorrhage from aneurysm rupture fills sulci and cisterns.

<image>Panel A: A coronal section through the skull and brain with meningeal layers labeled showing periosteal and meningeal dura (thick outer layer), arachnoid mater (thin middle layer with trabeculae crossing to pia), subarachnoid space (containing CSF and arteries), and pia mater (thin layer following brain surface into sulci), with an inset showing the superior sagittal sinus with arachnoid granulations projecting into venous blood. Panel B: Dural reflections showing falx cerebri between hemispheres, tentorium cerebelli separating cerebrum from cerebellum with tentorial notch around midbrain, and falx cerebelli posteriorly. Panel C: CT appearance of epidural hemorrhage (biconvex/lens-shaped, adjacent to temporal bone with middle meningeal artery marked) and subdural hemorrhage (crescent-shaped spanning convexity, with torn bridging vein illustrated). Panel D: CT appearance of subarachnoid hemorrhage (blood in sulci and basal cisterns, with ruptured aneurysm at circle of Willis depicted).</image>

6. Neuroanatomical Terminology

Precise anatomical terminology enables accurate description of nervous system structures and lesion locations. Directional terms in the neuraxis differ from standard anatomical position due to the brain's flexure during development. Rostral indicates toward the nose or front, while caudal means toward the tail or back. In the brainstem and spinal cord, rostral-caudal aligns with superior-inferior, but in the forebrain, rostral indicates anterior. Dorsal refers to the back or top (posterior in forebrain, superior in spinal cord), while ventral indicates the front or bottom.

Medial and lateral maintain their standard meanings, describing positions relative to the midline. The three standard anatomical planes apply throughout neuroanatomy: sagittal planes divide left from right, with the midsagittal plane passing through the midline; coronal or frontal planes divide front from back; axial or horizontal planes divide top from bottom and are commonly used for CT and MRI imaging. Understanding these planes is essential for interpreting neuroimaging and surgical approaches.

Specialized terminology distinguishes between CNS and PNS structures with equivalent functions. A nucleus refers to a collection of neuronal cell bodies within the CNS, while a ganglion describes the same in the PNS. A tract consists of bundled axons within the CNS (analogous to a nerve in the PNS). Commissures are fiber bundles crossing the midline connecting homologous structures, while decussations represent tract crossings that connect non-homologous regions.

Surface features define cortical anatomy. A gyrus is a ridge of cortex between sulci, while a sulcus is a shallow groove between gyri. A fissure represents a deeper cleft, with major fissures serving as lobe boundaries. The longitudinal fissure separates the hemispheres, containing the falx cerebri. Understanding these terms allows precise description of pathology location and prediction of resulting deficits based on functional localization.

<image>Panel A: Directional terms on a side view of the brain and upper spinal cord showing rostral (toward nose) and caudal (toward tail) arrows along the neuraxis, with the flexure at the midbrain-forebrain junction showing how dorsal shifts from posterior to superior, and medial and lateral shown on a coronal view. Panel B: The three anatomical planes on a head with brain visible showing sagittal (dividing left/right, shown as vertical slice), coronal (dividing front/back, shown as vertical slice perpendicular to sagittal), and axial (dividing top/bottom, shown as horizontal slice). Panel C: CNS versus PNS terminology comparison with illustrations showing nucleus (cluster of cell bodies in CNS) vs ganglion (in PNS), and tract (bundled axons in CNS) vs nerve (in PNS). Panel D: Commissure (corpus callosum crossing midline) and decussation (pyramidal decussation with fibers crossing to opposite side) diagrams.</image>

7. Surface Anatomy of the Brain

The cerebral surface exhibits a complex pattern of gyri and sulci that increase cortical surface area approximately threefold beyond what a smooth surface would allow. This folding pattern, while individually variable, follows consistent principles that enable reliable identification of functional regions. Major sulci serve as landmarks demarcating lobes and identifying eloquent cortex.

The central sulcus, also called the Rolandic fissure, represents the most important surface landmark. It separates the frontal lobe anteriorly from the parietal lobe posteriorly, running from the medial longitudinal fissure over the convexity to nearly reach the lateral sulcus. The precentral gyrus immediately anterior contains primary motor cortex with its somatotopic organization. The postcentral gyrus immediately posterior houses primary somatosensory cortex with corresponding somatotopy.

The lateral sulcus, or Sylvian fissure, separates the temporal lobe below from the frontal and parietal lobes above. It extends from the lateral brain surface deep into the insula, a cortical region buried within the sulcus with autonomic and interoceptive functions. The superior temporal gyrus along the upper bank of the lateral sulcus contains primary auditory cortex and, in the dominant hemisphere, Wernicke's area for language comprehension.

The parieto-occipital sulcus marks the boundary between parietal and occipital lobes on the medial brain surface. The calcarine sulcus courses horizontally through the occipital lobe, with primary visual cortex distributed along its upper and lower banks. The cingulate sulcus defines the superior border of the cingulate gyrus, a key limbic structure involved in emotion and cognitive control, which wraps around the corpus callosum on the medial surface.

<image>Panel A: A lateral view of the left hemisphere with major sulci marked as dark lines and labeled showing central sulcus (Rolandic) with precentral gyrus colored blue (motor) anteriorly and postcentral gyrus colored green (sensory) posteriorly, lateral sulcus (Sylvian) with superior temporal gyrus below it, and lobe boundaries indicated by dotted lines with Broca's and Wernicke's areas highlighted in the dominant hemisphere. Panel B: A medial view with corpus callosum visible in white, cingulate gyrus curving above it, cingulate sulcus defining its upper border, parieto-occipital sulcus separating parietal from occipital, and calcarine sulcus with primary visual cortex (V1) colored red along its banks. Panel C: A superior view with the longitudinal fissure between hemispheres and motor/sensory strip somatotopy illustrated. Panel D: A homunculus draped over the cortex showing the somatotopic organization of motor and sensory areas.</image>

8. Internal Brain Structures

Deep to the cortical mantle lie the basal ganglia, a collection of nuclei critically involved in motor control, habit formation, and reward processing. The caudate nucleus follows a C-shaped course paralleling the lateral ventricle, with its head bulging into the frontal horn and its tail extending into the temporal lobe to end near the amygdala. The putamen, lateral to the globus pallidus, receives most cortical input to the basal ganglia. Together, caudate and putamen comprise the striatum, named for the striated appearance created by fiber bundles traversing between them.

The globus pallidus lies medial to the putamen, separated from it by the external medullary lamina. It consists of external and internal segments with distinct connections and functions within basal ganglia circuitry. The putamen and globus pallidus together form the lentiform nucleus, visible as a lens-shaped structure on axial imaging. The subthalamic nucleus and substantia nigra, though anatomically in the diencephalon and midbrain respectively, function as integral basal ganglia components. Disorders of these structures produce characteristic movement abnormalities: Parkinson disease from substantia nigra degeneration, Huntington disease from caudate atrophy.

The limbic system comprises interconnected structures governing emotion, memory, and motivated behavior. The hippocampus, located in the medial temporal lobe, is essential for forming new declarative memories and spatial navigation. The amygdala, anterior to the hippocampus, processes emotional significance and fear responses. The cingulate gyrus participates in emotional processing and cognitive control. The fornix carries hippocampal output toward the mammillary bodies, which relay to the anterior thalamus in the Papez circuit underlying memory consolidation.

The internal capsule, a critical white matter structure, carries ascending sensory and descending motor fibers between cortex and brainstem/spinal cord. Its anterior limb lies between caudate and lentiform nucleus, carrying frontopontine and anterior thalamic radiations. The genu contains corticobulbar fibers to brainstem motor nuclei. The posterior limb, between thalamus and lentiform nucleus, carries corticospinal tract and sensory radiations. Small lacunar infarcts in the internal capsule produce pure motor or pure sensory stroke syndromes with profound deficits from tiny lesions.

<image>Panel A: An axial section at the level of the basal ganglia with structures labeled and color-coded showing caudate nucleus head (purple) bulging into lateral ventricle, putamen (blue) laterally, globus pallidus external and internal segments (green shades) medially, and internal capsule (white) between thalamus and lentiform nucleus with anterior limb, genu, and posterior limb labeled. Panel B: A coronal section showing the relationship of basal ganglia to lateral ventricles and thalamus, with the subthalamic nucleus and substantia nigra visible inferiorly. Panel C: The limbic system from a medial view showing hippocampus (seahorse-shaped, in temporal lobe), fornix (arching fiber bundle), mammillary bodies, amygdala (anterior to hippocampus), and cingulate gyrus with its connection through the Papez circuit indicated by arrows. Panel D: The internal capsule on an axial view with somatotopic organization showing face fibers in genu, arm in anterior posterior limb, and leg in posterior portion.</image>

9. Spinal Cord Overview

The spinal cord extends from the foramen magnum, where it continues from the medulla, to approximately the L1-L2 vertebral level in adults. This discrepancy between cord and vertebral column length results from differential growth rates during development, with the vertebral column elongating more rapidly. Below the conus medullaris (tapered cord terminus), the lumbar and sacral nerve roots descend as the cauda equina within the lumbar cistern, where lumbar puncture safely accesses CSF.

Two enlargements interrupt the cord's cylindrical profile. The cervical enlargement spanning C4-T1 contains motor neuron pools for upper extremity muscles and receives sensory input from the arms. The lumbar enlargement from L1-S2 similarly serves the lower extremities. Between and beyond these enlargements, the cord is narrower, corresponding to thoracic and sacral segments with less sensory and motor territory. The filum terminale, a thin filament of pia mater, anchors the conus to the coccyx, preventing excessive cord movement.

In cross-section, the spinal cord displays characteristic organization with central gray matter surrounded by peripheral white matter, opposite to the cerebral arrangement. The gray matter forms an H or butterfly shape, with dorsal horns receiving sensory input, ventral horns containing motor neurons innervating skeletal muscle, and lateral horns present only at T1-L2 containing sympathetic preganglionic neurons. The Rexed laminae system subdivides gray matter into ten layers based on cytoarchitecture and function.

White matter columns surround the gray matter, organized into dorsal, lateral, and ventral funiculi containing ascending and descending tracts. The dorsal columns carry discriminative touch and proprioception ipsilaterally before synapsing in the medulla. Lateral columns contain the corticospinal tract descending from motor cortex and spinothalamic tract ascending with pain and temperature information. Understanding these tract locations allows clinicians to predict deficit patterns from spinal cord lesions and localize pathology.

<image>Panel A: The full spinal cord within the vertebral canal in posterior view, with cervical enlargement (C4-T1) and lumbar enlargement (L1-S2) indicated by widening, conus medullaris ending at L1-L2, cauda equina (nerve roots fanning out below conus), and filum terminale extending to coccyx. Panel B: A cross-section with gray matter central H-shape labeled showing dorsal horn (sensory, laminae I-VI), ventral horn (motor, laminae VIII-IX), lateral horn (autonomic at thoracic levels, lamina VII), and central canal (lamina X), with white matter columns indicated including dorsal columns (touch/proprioception), lateral columns (corticospinal and spinothalamic tracts), and ventral columns. Panel C: Rexed laminae numbered I-X on a cross-section with corresponding functions. Panel D: The relationship of cord segments to vertebral levels, explaining why lumbar puncture at L3-L4 is safe (below conus, accessing cauda equina).</image>

10. Clinical Correlations

Neural tube defects represent the most common CNS malformations, resulting from failure of neural tube closure during the fourth gestational week. Spina bifida occulta, the mildest form, involves incomplete vertebral arch fusion without meningeal or neural herniation, often discovered incidentally by a overlying hair tuft or dimple. Meningocele involves protrusion of meninges through the vertebral defect without spinal cord involvement. Myelomeningocele, the most severe survivable form, includes spinal cord and nerve roots within the herniated sac, causing paralysis and sensory loss below the lesion level. Anencephaly results from failed rostral closure, producing absence of the forebrain and skull vault, incompatible with life. Folic acid supplementation before and during early pregnancy reduces neural tube defect risk by up to 70%.

Herniation syndromes occur when expanding intracranial masses displace brain tissue across dural reflections or through skull foramina. Subfalcine herniation pushes the cingulate gyrus under the falx cerebri, potentially compressing the anterior cerebral artery and causing leg weakness. Transtentorial or uncal herniation forces the medial temporal lobe through the tentorial notch, compressing the ipsilateral oculomotor nerve (causing pupil dilation and ptosis), the cerebral peduncle (causing contralateral hemiparesis), and the posterior cerebral artery (causing visual field cuts). Progressive herniation leads to brainstem compression and death. Tonsillar herniation pushes the cerebellar tonsils through the foramen magnum, compressing the medulla and causing respiratory arrest, representing a neurosurgical emergency.

Lesion localization represents a fundamental neurological skill, applying anatomical knowledge to predict deficit patterns. Cortical lesions produce contralateral deficits with variable patterns depending on specific location, often sparing face or leg due to the homunculus organization. Internal capsule lesions cause contralateral hemiparesis or hemisensory loss with face, arm, and leg equally affected due to fiber compaction. Brainstem lesions produce characteristic crossed findings with ipsilateral cranial nerve deficits and contralateral long tract signs, enabling precise localization. Spinal cord lesions cause bilateral deficits below the lesion level, with specific patterns (Brown-Séquard, central cord, anterior cord syndromes) providing additional localization information.

<image>Panel A: Neural tube defects in order of severity showing spina bifida occulta (incomplete vertebral arch only, with overlying skin dimple), meningocele (meninges protruding through defect as fluid-filled sac), myelomeningocele (cord and nerve roots within herniated sac, with infant showing back lesion), and anencephaly (absent forebrain and calvarium, shown schematically). Panel B: Herniation syndromes on coronal brain sections showing subfalcine (cingulate under falx, ACA compression), transtentorial/uncal (temporal lobe through tentorial notch, CN III and PCA compression, Kernohan notch phenomenon), and tonsillar (cerebellar tonsils through foramen magnum compressing medulla). Panel C: Lesion localization patterns showing cortex (face-sparing or leg-sparing deficit patterns based on homunculus) and internal capsule (complete hemiparesis affecting face/arm/leg equally). Panel D: Lesion localization patterns showing brainstem (crossed findings diagram with ipsilateral CN palsy and contralateral hemiparesis) and spinal cord (below-level bilateral deficits with Brown-Sequard pattern illustrated).</image>

Summary

The nervous system divides into central (brain and spinal cord) and peripheral (nerves and ganglia) components, with functional classification into somatic, autonomic, and sensory versus motor divisions. Gray matter contains neuronal cell bodies for processing while white matter consists of myelinated axons for transmission.

Neural tube formation during week three to four of development gives rise to the CNS, with primary and secondary brain vesicles differentiating into telencephalon (cerebral hemispheres), diencephalon (thalamus, hypothalamus), mesencephalon (midbrain), metencephalon (pons, cerebellum), and myelencephalon (medulla). Neural crest cells migrate to form PNS structures, adrenal medulla, melanocytes, and other derivatives.

The cerebrum contains four lobes (frontal, parietal, temporal, occipital) with distinct functions. The diencephalon includes thalamus (relay), hypothalamus (homeostasis), and related structures. The brainstem (midbrain, pons, medulla) contains vital centers and cranial nerve nuclei. The cerebellum coordinates movement and balance.

Neurons exist in multipolar, bipolar, and pseudounipolar forms. Glial cells include astrocytes (support, BBB), oligodendrocytes (CNS myelin), microglia (immune), ependymal cells (CSF), Schwann cells (PNS myelin), and satellite cells (ganglia support).

The meninges comprise dura mater (tough outer layer with reflections forming falx and tentorium), arachnoid mater (middle layer with granulations), and pia mater (adherent inner layer). Hemorrhage patterns are diagnostic: epidural (lens-shaped, arterial), subdural (crescent, venous), subarachnoid (fills sulci, aneurysmal).

The spinal cord extends to L1-L2 with cervical and lumbar enlargements. Central gray matter (H-shaped) contains sensory (dorsal horn), motor (ventral horn), and autonomic (lateral horn) regions. Surrounding white matter carries ascending and descending tracts.

Key Terms

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