Cerebral Cortex
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Direct answer
Two hemispheres of grey matter, folded into gyri and sulci and divided into frontal, parietal, temporal and occipital lobes, carry a functional map that clinical neurology uses daily. The precentral gyrus is the primary motor area, the postcentral gyrus the primary somesthetic area, the calcarine cortex of the occipital lobe is visual, the transverse temporal gyri on Heschl are auditory, and the inferior frontal and superior temporal gyri of the dominant hemisphere hold Broca's and Wernicke's language areas. Each primary area is fringed by association areas, and the whole map is compressed and distorted in the motor and sensory homunculi, where the hand, lips and tongue monopolise territory far beyond their bodily size.
What you must remember
- Central sulcus identification: begins on the superomedial border a little behind the midpoint between the frontal and occipital poles and runs downwards and forwards; the precentral gyrus in front is motor, the postcentral behind is sensory.
- Primary motor area (area 4): precentral gyrus and paracentral lobule; agranular cortex with giant Betz pyramidal cells; body inverted — leg medially in the paracentral lobule, face lowest, hand and lips largest.
- Premotor area (area 6) and frontal eye field (area 8): motor planning and voluntary gaze; a frontal eye field lesion deviates the eyes towards the lesioned side.
- Broca's area (areas 44, 45): opercular and triangular parts of the inferior frontal gyrus of the dominant (usually left) hemisphere — non-fluent, effortful aphasia with comprehension preserved.
- Primary somesthetic area (areas 3, 1, 2): postcentral gyrus, granular cortex, inverted somatotopy like the motor strip; two-point discrimination and proprioception localise here.
- Wernicke's area (area 22, dominant): posterior part of the superior temporal gyrus — fluent but meaningless speech with poor comprehension and repeating.
- Visual and auditory: primary visual cortex (area 17) on the two lips of the calcarine sulcus; primary auditory cortex (area 41) on the anterior transverse temporal gyrus, with unilateral lesions easily overlooked because the pathway is bilateral.
- Dominance and the non-dominant lobe: language and praxis sit in the left hemisphere in nearly all right-handers; the right parietal lobe owns visuospatial function, so right parietal lesions cause neglect of the opposite side and dressing apraxia.
A short case that walks the lobe map
A retired teacher reaches casualty after sudden left-sided weakness; an hour of examination turns the cortex into a map. She cannot lift the right arm or leg, the right face sags, but speech is fluent and she follows commands — pointing to a right hemisphere (non-dominant) lesion of the left motor strip. A left middle cerebral occlusion instead gives cortical right-sided weakness, face and arm worse than leg, plus Broca's aphasia when the inferior frontal gyrus is involved — she would understand but struggle to speak. Ask her to copy a figure and comb her hair, and the right parietal contribution emerges: right-hemisphere lesions classically add left-sided neglect and dressing apraxia. A pure occipital stroke, by contrast, is sudden homonymous hemianopia with macular sparing from area 17 territory, no weakness at all; a temporal-lobe seizure begins with formed auditory or olfactory aura from the superior temporal gyrus and uncus. Each lobe announces itself through its specialism, which is why the cortical examination is really a lobe-by-lobe inventory.
Where students slip
The homunculus gets drawn upright: on the motor and sensory strips the leg and foot lie on the medial surface in the paracentral lobule and the face at the lateral end, an arrangement many sketches reverse. Students also mislabel Wernicke's area as frontal — it is posterior superior temporal; the quick clinical discriminator, fluent-but-empty versus effortful-but-meaningful speech, is the viva answer that scores. A third recurring error: attributing hearing loss to a unilateral auditory cortex lesion, forgetting that each ear projects bilaterally so unilateral area 41 lesions merely dull sound localisation.
Frequently asked questions
How is the central sulcus identified on the brain surface?
It starts on the superomedial border slightly behind the midpoint between the frontal and occipital poles and slopes downwards and forwards, usually intersecting the superolateral surface without reaching the lateral fissure. The precentral gyrus in front is motor; the postcentral gyrus behind is sensory.
Which cortical areas form the language network, and what does each do?
Broca's area (44, 45) in the dominant inferior frontal gyrus programmes speech output; Wernicke's area (22) in the dominant posterior superior temporal gyrus decodes speech; the arcuate fasciculus links them, and the dominant angular gyrus (39) handles written language. Lesions produce non-fluent, fluent and conduction aphasias respectively.
What is the somatotopic arrangement of the motor cortex?
Inverted: lower limb on the medial surface in the paracentral lobule, then trunk, upper limb and face progressively downwards and forwards on the lateral surface, with the hand and lips occupying disproportionately large areas. This distortion is the motor homunculus.
Why does a unilateral auditory cortex lesion not cause deafness in one ear?
Because fibres from each cochlear nucleus relay to the auditory cortex of both hemispheres via the medial geniculate bodies. A unilateral area 41 lesion therefore only mildly reduces hearing and impairs sound localisation.
Which functions concentrate in the non-dominant parietal lobe?
Visuospatial perception, attention to the opposite half of space, and constructional ability. Right parietal lesions classically produce contralateral hemineglect, dressing apraxia and poor copying of figures.