Anatomy chapters
Cerebral cortex
The lobes, sulci and gyri of the cerebral hemispheres, the main functional areas (motor, sensory, speech, vision, hearing), the white matter tracts, and what happens when each area is damaged.
- The central sulcus separates the motor (precentral, area 4) from the sensory (postcentral, areas 3, 1, 2) cortex.
- Broca's area (44, 45) in the frontal lobe makes speech; Wernicke's area (22) in the temporal lobe understands it.
- Vision is in the occipital lobe (area 17) around the calcarine sulcus; hearing in the temporal lobe (areas 41, 42).
On this page
Overview
The cerebral cortex is the thin outer layer of grey matter covering each cerebral hemisphere. It is folded into ridges (gyri) and grooves (sulci), which greatly increase its area.
Different areas do different jobs: movement, sensation, vision, hearing, speech, and planning.
Lobes and landmarks
Each hemisphere has four main lobes, named after the skull bones over them: frontal, parietal, temporal and occipital. The insula lies hidden deep in the lateral sulcus.
Key landmarks:
- Central sulcus: separates the frontal and parietal lobes.
- Lateral sulcus (Sylvian fissure): separates the temporal lobe below from the frontal and parietal lobes above.
- Parieto-occipital sulcus: on the medial surface, separates the parietal and occipital lobes.
- Calcarine sulcus: on the medial surface of the occipital lobe; the visual cortex lies around it.
Layers of the cortex
Most of the cortex (the neocortex) has six layers. Layer IV receives sensory input and is thick in sensory areas; layer V contains the large pyramidal (Betz) cells whose axons form the corticospinal tract, and is thick in the motor cortex.
Functional areas
| Area | Location | Brodmann | Function |
|---|---|---|---|
| Primary motor | Precentral gyrus | 4 | Voluntary movement of the opposite side |
| Premotor | In front of area 4 | 6 | Plans and organises movement |
| Frontal eye field | Middle frontal gyrus | 8 | Moves both eyes to the opposite side |
| Broca's (motor speech) | Inferior frontal gyrus, dominant side | 44, 45 | Produces speech |
| Prefrontal | Front of the frontal lobe | 9–12 | Personality, judgment, planning |
| Primary somatosensory | Postcentral gyrus | 3, 1, 2 | Touch, pain, temperature and position sense from the opposite side |
| Primary visual | Around the calcarine sulcus | 17 | Vision from the opposite half of the visual field |
| Visual association | Around area 17 | 18, 19 | Recognises what is seen |
| Primary auditory | Transverse temporal (Heschl's) gyri | 41, 42 | Hearing (from both ears) |
| Wernicke's (sensory speech) | Back of the superior temporal gyrus, dominant side | 22 | Understands spoken and written language |
Broca's area is in the frontal lobe and produces speech. Wernicke's area is in the temporal lobe and understands speech. They are joined by the arcuate fasciculus.
"Broca = Broken speech; Wernicke = Wordy but makes no sense."
The homunculus
The motor and sensory cortex map the opposite side of the body upside down.
- The leg and foot are on the medial surface, in the paracentral lobule.
- The trunk and arm are higher on the lateral surface.
- The face, lips and tongue are lowest, near the lateral sulcus.
- The hand, face and lips have very large areas because of their fine control and sensitivity.
The leg area is on the medial surface, supplied by the anterior cerebral artery. The face and arm areas are on the lateral surface, supplied by the middle cerebral artery.
Dominance
In most people, including most left-handed people, the left hemisphere is dominant for language. The non-dominant (usually right) hemisphere is more important for spatial awareness and recognising faces.
White matter of the hemispheres
The white matter contains three types of fibres:
- Association fibres: link areas in the same hemisphere (for example the arcuate fasciculus).
- Commissural fibres: cross between the two hemispheres; the largest is the corpus callosum.
- Projection fibres: link the cortex with lower centres; they pass through the internal capsule.
The internal capsule
A compact band of white matter between the thalamus and caudate nucleus (medially) and the lentiform nucleus (laterally). It has an anterior limb, a genu and a posterior limb.
- Genu: corticobulbar fibres (to cranial nerve nuclei).
- Posterior limb: corticospinal fibres (arm, then leg further back) and the sensory fibres from the thalamus to the cortex.
A small lesion in the posterior limb of the internal capsule (a lacunar stroke) can weaken the whole opposite side of the body, because the fibres are packed tightly together.
Clinical relevance
Broca's (expressive) aphasia
How it happens: a stroke in the inferior frontal gyrus of the dominant hemisphere (upper MCA branches).
What you see: slow, effortful, broken speech with few words; understanding is largely preserved. Often with right arm and face weakness.
Wernicke's (receptive) aphasia
How it happens: a stroke in the back of the superior temporal gyrus of the dominant hemisphere (lower MCA branches).
What you see: fluent speech full of wrong or made-up words; poor understanding. The patient often does not realise there is a problem.
Frontal lobe damage
How it happens: tumour, head injury or dementia affecting the prefrontal cortex.
What you see: personality change, poor judgment, loss of social control, apathy; primitive reflexes may return.
Non-dominant parietal lobe damage
How it happens: a stroke in the right parietal lobe.
What you see: neglect of the left side of the body and space; the patient may ignore the left arm or shave only the right side of the face.
Dominant parietal lobe damage
What you see: Gerstmann syndrome: finger agnosia, left-right confusion, difficulty writing and difficulty with calculation.
Occipital lobe damage
How it happens: a posterior cerebral artery stroke.
What you see: loss of the opposite half of the visual field in both eyes (homonymous hemianopia), often with macular sparing.
Expect questions on the Brodmann numbers of the primary areas, the location and type of aphasia for Broca's and Wernicke's areas, the arrangement of the homunculus (leg on the medial surface), the parts of the internal capsule, and the effects of frontal, parietal and occipital lesions.
Quick revision
- Central sulcus: motor (area 4) in front, sensory (3, 1, 2) behind.
- Broca's: areas 44, 45; frontal; expressive aphasia.
- Wernicke's: area 22; temporal; receptive aphasia.
- Visual: area 17 around the calcarine sulcus. Auditory: 41, 42.
- Homunculus: leg on the medial surface; face lowest laterally.
- Internal capsule posterior limb: corticospinal and sensory fibres.
- Right parietal lesion: left-sided neglect.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
Where is the primary motor cortex?
In the precentral gyrus of the frontal lobe, Brodmann area 4.
What is the difference between Broca's and Wernicke's aphasia?
Broca's aphasia is slow, effortful speech with good understanding. Wernicke's aphasia is fluent but meaningless speech with poor understanding.
Where is the primary visual cortex?
Around the calcarine sulcus on the medial surface of the occipital lobe, area 17.
What happens after damage to the right parietal lobe?
The patient often neglects the left side of the body and the space around it.