Structure: lobes and sulci
Each hemisphere has four main lobes, named after the bones over them, and a hidden insula. Three sulci mark the boundaries.
- Central sulcus: separates the frontal and parietal lobes. It begins on the medial surface, notches the upper border of the hemisphere just behind its midpoint, then runs down and forward across the lateral surface. It usually stops just short of the lateral sulcus.
- Lateral sulcus (Sylvian fissure): separates the temporal lobe below from the frontal and parietal lobes above. The insula lies buried in its floor, covered by the opercula.
- Parieto-occipital sulcus: on the medial surface, separating the parietal and occipital lobes. On the lateral surface the boundary is an imaginary line from the top of this sulcus to the pre-occipital notch.
Most of the cortex is neocortex with six cellular layers. Brodmann numbered its areas by their cell structure, and those numbers are still used for the main functional areas.
Motor and sensory gyri
The precentral and postcentral gyri run parallel on either side of the central sulcus and together form the sensorimotor strip.
- Precentral gyrus: the primary motor cortex, Brodmann area 4, the most posterior gyrus of the frontal lobe. Its large pyramidal (Betz) cells in layer V contribute to the corticospinal and corticobulbar tracts, which control the opposite side of the body.
- Postcentral gyrus: the primary somatosensory cortex, areas 3, 1 and 2, the most anterior gyrus of the parietal lobe. It receives touch, proprioception, pain and temperature from the opposite side through the thalamus.
- Homunculus: both strips map the body upside down. The face and tongue lie lowest, near the lateral sulcus; the hand is large and above them; the trunk is near the upper border; and the leg and foot lie on the medial surface in the paracentral lobule.
In front of the motor strip, the premotor cortex (area 6) plans movement and the frontal eye field (area 8) turns the eyes to the opposite side.
Medial surface and functional areas
The medial surface is seen when the hemispheres are separated along the longitudinal fissure.
- Corpus callosum: the arched band of white matter joining the hemispheres. From front to back it has a rostrum, a genu (the curved front end), a body and a thick splenium. Fibres from the genu arch forward into both frontal lobes as the forceps minor.
- Cingulate gyrus: curves over the corpus callosum, separated from it by the callosal sulcus and from the gyri above by the cingulate sulcus. It belongs to the limbic system and is involved in emotion, motivation and the emotional response to pain.
- Paracentral lobule: the continuation of the precentral and postcentral gyri on the medial surface, carrying the leg and involved in voluntary control of the bladder.
- Calcarine sulcus: in the occipital lobe; the primary visual cortex (area 17) lines its banks.
| Area | Location | Effect of a lesion |
|---|---|---|
| Broca's area (44, 45) | Inferior frontal gyrus, dominant hemisphere | Expressive (non-fluent) aphasia |
| Wernicke's area (22) | Posterior superior temporal gyrus, dominant hemisphere | Receptive (fluent) aphasia |
| Primary auditory cortex (41, 42) | Transverse temporal gyri in the lateral sulcus | Little deficit if one side only |
| Primary visual cortex (17) | Banks of the calcarine sulcus | Contralateral homonymous hemianopia |
Blood supply and venous drainage
Three cerebral arteries supply the cortex, and the boundaries between their territories predict stroke patterns.
- Anterior cerebral artery: runs over the corpus callosum and supplies the medial surface back to the parieto-occipital sulcus, plus a strip over the upper border. This includes the leg area and the cingulate gyrus.
- Middle cerebral artery: runs through the lateral sulcus and supplies most of the lateral surface, including the face and arm areas, and Broca's and Wernicke's areas.
- Posterior cerebral artery: supplies the occipital lobe, including the visual cortex, and the inferior and medial temporal lobe.
Superficial cerebral veins drain upwards into the superior sagittal sinus and downwards into the sinuses of the skull base. The upper veins cross the subdural space as bridging veins, which tear in acceleration injuries to cause a subdural haematoma.
Clinical relevance
Cortical lesions produce deficits that match the map of the cortex.
- Anterior cerebral stroke: weakness and sensory loss of the opposite leg more than the arm, sometimes with urinary incontinence.
- Middle cerebral stroke: weakness and sensory loss of the opposite face and arm; aphasia if the dominant side is affected, neglect if the non-dominant side.
- Posterior cerebral stroke: contralateral homonymous hemianopia, often sparing the macula.
- Focal motor seizures: a lesion irritating the precentral gyrus can cause jerking that spreads along the homunculus, from hand to arm to face (a Jacksonian march).
- Parasagittal meningioma: pressing on both paracentral lobules can weaken both legs, mimicking a spinal cord lesion.
- Diffuse axonal injury: shearing lesions commonly affect the corpus callosum.
On the specimen
The specimen shows a hemisphere from the lateral side and in midline section.
- Finding the central sulcus: it is the long, continuous sulcus running from the upper border almost to the lateral sulcus, with two parallel gyri either side. On the medial surface, the marginal part of the cingulate sulcus turns up to the upper border just behind the top of the central sulcus.
- Precentral or postcentral gyrus: the precentral gyrus is in front, facing the frontal pole; the postcentral is behind.
- Cingulate gyrus or corpus callosum: the cingulate gyrus is grey cortex arching above; the corpus callosum is the white band below it, in the depth of the longitudinal fissure.
- Genu or splenium: the genu is the anterior bend, curving down into the thin rostrum; the splenium is the thick posterior end.