The Brain: Structure and Function

The human brain is arguably the most complex structure in the known universe — a 1.3–1.4 kg organ containing approximately 86 billion neurons and a roughly equal number of glial cells, connected by an estimated 100 trillion synapses. It consumes roughly 20% of the body's resting oxygen supply despite representing only 2% of body weight. This guide is for educational purposes only.

## Cerebral Cortex and Lobes

The cerebral cortex — the highly folded outer layer of the cerebrum — is 2–4 mm thick and contains about 16 billion neurons arranged in six layers (the neocortex). Its surface area is approximately 2,500 cm², three-quarters of which is hidden within sulci (grooves); the exposed ridges are called gyri. Each cerebral hemisphere is divided into four lobes by major sulci.

The frontal lobe lies anterior to the central sulcus and above the lateral (Sylvian) fissure. It houses the primary motor cortex (precentral gyrus), premotor and supplementary motor areas, Broca's area (left hemisphere, typically; speech production), and the prefrontal cortex (executive function, personality, working memory). Damage to Broca's area produces expressive (non-fluent) aphasia.

The parietal lobe lies posterior to the central sulcus and contains the primary somatosensory cortex (postcentral gyrus), which receives contralateral body sensation in a somatotopic map (sensory homunculus). The superior parietal lobule integrates spatial information; the inferior parietal lobule (supramarginal and angular gyri) is involved in language comprehension. Wernicke's area is classically located in the posterior superior temporal gyrus (temporal lobe) but its network extends into the angular gyrus; damage produces receptive (fluent) aphasia.

The temporal lobe lies below the lateral fissure and contains the primary auditory cortex (Heschl's gyri), Wernicke's area, and, on its medial surface, the hippocampal formation and amygdala. It plays roles in hearing, language comprehension, memory, and emotion. The occipital lobe at the posterior pole contains the primary visual cortex (V1, striate cortex) surrounding the calcarine sulcus and the visual association areas. Lesions here cause contralateral homonymous hemianopia.

## Limbic System

The limbic system is a set of interconnected structures on the medial aspect of each hemisphere that regulates emotion, memory, and motivation. Key components include the hippocampus (essential for forming new declarative memories; bilateral hippocampal damage produces anterograde amnesia, as seen in patient H.M. following bilateral temporal lobectomy), the amygdala (emotion processing, fear conditioning, emotional modulation of memory), the cingulate cortex (attention, emotion-motor integration), the parahippocampal gyrus, the fornix (the major output tract of the hippocampus, projecting to the mammillary bodies), and the mammillary bodies (relay station in the Papez circuit, involved in memory; damaged in Wernicke's encephalopathy from thiamine deficiency).

## Brainstem

The brainstem connects the cerebrum to the spinal cord and consists of three regions: the midbrain (mesencephalon), pons, and medulla oblongata. It contains the nuclei of cranial nerves III–XII, the reticular formation (arousal and consciousness), and ascending and descending tracts.

The midbrain contains the cerebral peduncles (major descending motor pathways), the substantia nigra (dopaminergic neurons depleted in Parkinson's disease), the red nucleus (motor coordination), and the superior and inferior colliculi (visual and auditory reflexes). The pons contains the pontine nuclei (relay station in the cortico-ponto-cerebellar pathway), cranial nerve nuclei V–VIII, and the pneumotaxic and apneustic centres (breathing control). The medulla contains the cardiac, respiratory, and vasomotor centres (vital centres) — making medullary lesions immediately life-threatening — and cranial nerve nuclei IX–XII. The pyramidal decussation, where the corticospinal tracts cross the midline, occurs in the medulla; this is why left brain lesions produce right body weakness.

## Cerebellum

The cerebellum occupies the posterior cranial fossa beneath the tentorium cerebelli. It contains more neurons than the entire cerebral cortex, primarily granule cells. Anatomically it consists of two cerebellar hemispheres and a central vermis. The cerebellar cortex has three layers: molecular, Purkinje cell, and granule cell layers. Purkinje cells provide the sole output of the cerebellar cortex (inhibitory, via GABA) to the deep cerebellar nuclei.

Functionally the cerebellum is divided into three zones: the vestibulocerebellum (flocculonodular lobe; balance and eye movements), the spinocerebellum (vermis and paravermis; limb and trunk coordination), and the cerebrocerebellum (lateral hemispheres; planning of voluntary movement, motor learning). Cerebellar lesions produce ipsilateral signs: dysmetria (past-pointing), intention tremor, dysdiadochokinesia, ataxia, and nystagmus. The cerebellum does not initiate movement; it modulates it.

## Ventricular System and CSF

The brain is bathed in cerebrospinal fluid (CSF), which is produced by the choroid plexus at a rate of approximately 500 mL/day (total CSF volume ~150 mL, so it is replaced about 3–4 times daily). CSF flows from the lateral ventricles through the interventricular foramina (of Monro) into the third ventricle, through the cerebral aqueduct (of Sylvius) into the fourth ventricle, and out through the median foramen (of Magendie) and lateral foramina (of Luschka) into the subarachnoid space, where it is reabsorbed at the arachnoid granulations into the dural venous sinuses.

Hydrocephalus — abnormal accumulation of CSF — may be obstructive (non-communicating; blockage within the ventricular system) or communicating (impaired reabsorption). Increased intracranial pressure (ICP) may cause the Cushing triad: hypertension, bradycardia, and irregular respirations — a sign of impending brainstem herniation.

## Blood-Brain Barrier

The blood-brain barrier (BBB) is formed by tight junctions between brain capillary endothelial cells, supported by astrocyte endfeet and pericytes. It selectively restricts the passage of substances from the blood into the CNS, protecting against pathogens and toxins while allowing glucose (via GLUT1), oxygen, and lipid-soluble molecules to cross freely. Several regions lack a complete BBB — the circumventricular organs (area postrema, subfornical organ, median eminence) — allowing these areas to sense blood-borne signals.

The BBB breaks down in many conditions including bacterial meningitis, stroke, and brain tumours, enabling contrast enhancement on MRI. Many drugs fail to treat CNS disease because they cannot penetrate the BBB; strategies to overcome this include lipid-soluble prodrugs, nanoparticle carriers, and focused ultrasound-mediated transient opening.