Neuroanatomy

The spinal cord is organized segmentally and contains central gray matter surrounded by white matter tracts. Dorsal roots bring sensory information toward the cord, while ventral roots carry motor output away; they unite to form mixed spinal nerves. Gray matter contains neuronal cell bodies and local circuits, while white matter contains ascending and descending axons connecting spinal levels and the brain.

A spinal cord segment is not the same thing as the vertebra bearing the same number, especially in the lower spine because the vertebral column grows longer than the cord during development. This distinction matters for imaging, lumbar puncture and lesion localization. Damage to a tract produces a pattern determined by its function, position and crossing behavior.

The brainstem consists of midbrain, pons and medulla and serves as both a conduit and an integrative center. Ascending sensory and descending motor tracts pass through it, while nuclei associated with many cranial nerves occupy characteristic levels. Reticular and autonomic centers contribute to consciousness, respiration and cardiovascular regulation.

Brainstem localization becomes manageable when cross-sectional anatomy is combined with longitudinal pathways. A focal lesion can affect a cranial nerve or nucleus on one side and a long tract carrying information to or from the opposite side of the body, producing 'crossed' patterns. This is why simply memorizing external brainstem landmarks is insufficient.

The cerebellum compares intended movement with sensory feedback and contributes to coordination, timing, balance and motor learning. It connects extensively with brainstem and cerebral motor systems through cerebellar peduncles. Its cortex has a highly regular layered organization, while deep nuclei provide major output pathways.

Cerebellar dysfunction typically produces incoordination rather than primary paralysis. Findings can include dysmetria, intention tremor, gait or truncal ataxia and impaired rapid alternating movements, depending on the region affected. The key distinction is that the muscles may retain strength but lose smooth, accurately timed control.

The cerebral hemispheres contain cortex, subcortical white matter and deep nuclei. Cortical regions participate in motor, sensory, language, memory and higher cognitive functions, but real behavior emerges from networks rather than isolated dots on a map. White matter pathways connect cortical areas with each other and with deeper structures. Basal nuclei participate in motor selection and other loops; the thalamus is a major relay and integration station for many pathways.

A focal lesion may produce a recognizable syndrome because certain functions are concentrated within particular networks, yet simplistic one-center explanations should be avoided. Internal capsule anatomy is especially important because many densely packed projection fibers pass through a small region, so a small lesion can produce extensive deficits.

Descending motor pathways carry commands from higher centers toward brainstem and spinal motor circuits. The corticospinal system is especially important for skilled voluntary movement and crosses substantially in the lower medulla. Ascending systems carry different sensory modalities: dorsal-column pathways are important for fine touch, vibration and conscious proprioception, while anterolateral pathways carry pain, temperature and related sensations.

The side of a deficit depends on where a pathway crosses. This is why memorizing only the tract name is not enough. For each pathway, know the modality, first major neurons, route, crossing and destination. Then lesion questions become predictions rather than guesses.

The brain and spinal cord are covered by dura, arachnoid and pia. Spaces associated with these coverings have distinct anatomical meanings. Cerebrospinal fluid is produced mainly by choroid plexus within ventricles, circulates through the ventricular system and subarachnoid space, and is returned to venous circulation. Obstruction of flow can enlarge ventricles and raise intracranial pressure.

Cerebral blood supply is provided mainly through internal carotid and vertebrobasilar systems that communicate through arterial connections at the brain base. Different vascular territories supply different functional regions, so ischemia can create recognizable deficits. Epidural, subdural and subarachnoid hemorrhages differ in vessel source, location and imaging pattern because they occupy different anatomical relationships.