Structure
Each cerebral hemisphere is divided into lobes named after the overlying bones of the skull vault, bounded by a few constant sulci.
- Central sulcus separates the frontal lobe from the parietal lobe. It runs downwards and forwards from just behind the midpoint of the upper border of the hemisphere.
- Lateral sulcus separates the temporal lobe below from the frontal and parietal lobes above. Opening it reveals the insula.
- Parieto-occipital sulcus, seen on the medial surface, separates the parietal and occipital lobes.
- Calcarine sulcus runs forwards on the medial surface of the occipital lobe, lined by the primary visual cortex.
| Lobe | Key areas | Main functions |
|---|---|---|
| Frontal | Precentral gyrus (primary motor cortex); Broca area in the inferior frontal gyrus; prefrontal cortex | Voluntary movement, expressive language, planning, personality |
| Parietal | Postcentral gyrus (primary somatosensory cortex); superior and inferior parietal lobules | Touch and position sense, spatial awareness, attention |
| Temporal | Transverse temporal gyri (primary auditory cortex); Wernicke area; hippocampus and amygdala on the medial side | Hearing, receptive language, memory, emotion |
| Occipital | Primary visual cortex around the calcarine sulcus | Vision |
| Insula | Hidden in the depth of the lateral sulcus | Taste, visceral sensation, interoception |
The motor and sensory maps are arranged upside down: the leg and foot are on the medial surface in the paracentral lobule, and the face lies lowest on the lateral surface, near the lateral sulcus. In most people the left hemisphere is dominant for language.
White matter and deep structures
The white matter of the cerebrum consists of three types of fibre.
- Association fibres link areas within one hemisphere, for example the arcuate fasciculus joining Wernicke and Broca areas.
- Commissural fibres cross between hemispheres. The corpus callosum is the largest; from front to back it has a rostrum, genu, body and splenium. The anterior commissure links the temporal lobes.
- Projection fibres connect the cortex with the thalamus, brainstem and spinal cord. They fan out as the corona radiata and converge into the internal capsule.
Internal capsule
The internal capsule is a compact band of projection fibres between the thalamus and caudate nucleus medially and the lentiform nucleus laterally. On a horizontal section it is V-shaped, with an anterior limb, a genu and a posterior limb. The posterior limb carries the corticospinal fibres and the thalamocortical sensory radiation, so a small lesion here can cause a dense contralateral hemiplegia and sensory loss. The optic and auditory radiations pass behind and below the lentiform nucleus.
Basal nuclei and ventricles
The basal nuclei are deep grey masses: the caudate nucleus, putamen and globus pallidus (the last two forming the lentiform nucleus), with the amygdala in the temporal lobe. They work with the subthalamic nucleus and substantia nigra to regulate movement. Each hemisphere contains a C-shaped lateral ventricle with an anterior horn in the frontal lobe, a body, a posterior horn in the occipital lobe and an inferior horn in the temporal lobe. Each drains through an interventricular foramen into the third ventricle.
Blood supply and innervation
Three paired arteries supply the cerebrum, each with its own cortical territory.
| Artery | Origin | Territory |
|---|---|---|
| Anterior cerebral | Internal carotid artery | Medial surface of the frontal and parietal lobes, including the leg area; most of the corpus callosum |
| Middle cerebral | Internal carotid artery | Most of the lateral surface, including face and arm areas, Broca and Wernicke areas; lenticulostriate branches to the basal nuclei and internal capsule |
| Posterior cerebral | Basilar artery | Occipital lobe, including visual cortex; inferior temporal lobe; much of the thalamus |
The circle of Willis joins the carotid and vertebrobasilar systems at the base of the brain through the anterior and posterior communicating arteries. Superficial cerebral veins drain into the superior sagittal and other dural sinuses; bridging veins cross the subdural space to reach them. Deep structures drain to the internal cerebral veins, which join to form the great cerebral vein and enter the straight sinus. Brain tissue itself has no pain receptors; headache arises from the meninges and vessels.
Clinical relevance
Stroke produces deficits that map to arterial territories:
- Middle cerebral artery: contralateral weakness and sensory loss of face and arm more than leg, homonymous hemianopia, and aphasia if the dominant hemisphere is affected or neglect if the non-dominant side is.
- Anterior cerebral artery: contralateral leg weakness and sensory loss, sometimes with behavioural change and urinary incontinence.
- Posterior cerebral artery: contralateral homonymous hemianopia, often with sparing of the macula.
- Lacunar infarct of the internal capsule: pure motor hemiparesis of face, arm and leg.
Raised intracranial pressure pushes parts of the cerebrum through the dural openings. Subfalcine herniation drives the cingulate gyrus under the falx. Uncal herniation forces the medial temporal lobe through the tentorial notch, compressing the oculomotor nerve and producing an ipsilateral fixed, dilated pupil. Bridging veins tear with acceleration and deceleration injury, causing subdural haematoma, particularly in older adults with brain atrophy.