The brain: what each area does

1. Four lobes and the central sulcus

The lateral (Sylvian) fissure separates the temporal lobe below from the frontal and parietal lobes above; the central sulcus separates frontal from parietal. Behind the parieto-occipital sulcus, best seen on the medial surface, lies the occipital lobe. The left hemisphere is dominant for language in about 95% of right-handed and most left-handed people.

Surgeon’s corner: On axial MRI the central sulcus is found by the "hand knob", an omega-shaped bulge of the precentral gyrus; the precentral gyrus is thicker than the postcentral.

2. Primary motor cortex: the homunculus

The precentral gyrus holds the primary motor cortex. Its pyramidal cells send the corticospinal tract down through the internal capsule, the cerebral peduncle and the pyramid, crossing in the lower medulla. The body is mapped upside down: leg and foot on the medial surface (paracentral lobule), then trunk, arm, a huge hand area, and the face just above the Sylvian fissure. Tap a body part in the panel.

Surgeon’s corner: A lesion here weakens the opposite side in the part represented: a small "hand knob" infarct can mimic a peripheral nerve palsy of the hand. Upper motor neurone signs follow: spasticity, brisk reflexes, upgoing plantar.

3. Sensation and the parietal lobe

Behind the central sulcus, the postcentral gyrus is the primary somatosensory cortex, mapped like the motor strip. The superior parietal lobule builds a map of the body in space; the inferior parietal lobule (supramarginal and angular gyri) joins vision, hearing and touch — reading, writing and calculation.

Surgeon’s corner: Dominant (left) angular gyrus: Gerstmann syndrome — agraphia, acalculia, finger agnosia, left–right confusion. Non-dominant (right) parietal lesion: neglect of the left side of space and of the body, often with anosognosia.

4. Language: Broca and Wernicke

In the dominant hemisphere, Broca’s area (pars opercularis and triangularis of the inferior frontal gyrus) plans speech; Wernicke’s area, in the posterior superior temporal gyrus behind the primary auditory cortex (Heschl’s gyri), understands it. White-matter tracts such as the arcuate fasciculus join them.

Surgeon’s corner: Broca (expressive) aphasia: halting, effortful speech with relatively spared comprehension, often with right face and arm weakness. Wernicke (receptive) aphasia: fluent speech without meaning and poor comprehension, often without weakness. Conduction aphasia: fluent, understands, cannot repeat.

5. Vision: the occipital lobe

The primary visual cortex lines the calcarine sulcus on the medial surface: the lower bank (lingual gyrus) receives the upper visual field, the upper bank (cuneus) the lower field, and the macula is represented at the occipital pole. Ventral areas such as the fusiform gyrus recognise faces and colours; dorsal areas follow movement and position.

Surgeon’s corner: One occipital lobe lost: opposite homonymous hemianopia, often with macular sparing. Both: cortical blindness with normal pupils, sometimes denied by the patient (Anton syndrome). Bilateral fusiform lesions: prosopagnosia.

6. The prefrontal cortex

In front of the motor areas, the prefrontal cortex plans, decides, holds information in working memory and restrains behaviour. The orbitofrontal cortex, above the orbits, links decisions to emotion and reward. The frontal eye field, in the middle frontal gyrus, turns both eyes to the opposite side.

Surgeon’s corner: Frontal lobe damage (trauma, tumour, frontotemporal dementia): apathy or disinhibition, poor judgement, perseveration, primitive reflexes. An acute lesion of the frontal eye field deviates the eyes towards the side of the lesion.

7. Memory and emotion: the limbic system

On the medial surface, a ring of cortex around the corpus callosum and the brainstem forms the limbic lobe. The hippocampus, in the medial temporal lobe, lays down new memories; its output runs through the fornix to the mamillary bodies and on to the thalamus and the cingulate gyrus (the Papez circuit). The amygdala, just in front of it, tags experience with fear and emotion.

Surgeon’s corner: Early Alzheimer disease: hippocampal atrophy and loss of new memories. Bilateral hippocampal (herpes encephalitis, hypoxia) or mamillary damage (thiamine deficiency, Korsakoff) gives dense anterograde amnesia. Mesial temporal sclerosis is the commonest cause of drug-resistant epilepsy in adults.

8. Deep inside: basal ganglia and thalamus

The basal ganglia — caudate, putamen and globus pallidus — tune movement through loops with the cortex and thalamus; the substantia nigra feeds them dopamine. The thalamus relays almost every pathway to the cortex: sensation, vision (lateral geniculate body), hearing (medial geniculate body) and the motor loops. Between them runs the internal capsule.

Surgeon’s corner: Parkinson disease: loss of nigral dopamine → slowness, rigidity, rest tremor. Huntington disease: caudate atrophy. Hypertensive haemorrhage favours the putamen and the thalamus. A small infarct in the internal capsule gives a pure motor stroke of face, arm and leg.

9. Brainstem and cerebellum

The brainstem carries every tract between brain and spinal cord and the nuclei of cranial nerves III–XII: midbrain (III, IV), pons (V–VIII), medulla (IX–XII), with the centres for breathing, wakefulness and the circulation. The cerebellum coordinates: the vermis controls the trunk and gait, each hemisphere the limbs on the same side.

Surgeon’s corner: Brainstem lesions give crossed signs: a cranial nerve palsy on the side of the lesion with weakness of the opposite body. Cerebellar hemisphere: ipsilateral ataxia, intention tremor, dysdiadochokinesia; vermis: truncal ataxia and a broad-based gait. Raised pressure can push the tonsils into the foramen magnum.