The cerebral cortex
Basic anatomy
Hemispheres
The brain is divided into two halves or hemispheres, which are divided into four lobes (frontal, parietal, temporal and occipital) (Fig. 7.1). The left and right hemispheres are connected by the corpus callosum (Fig. 7.2), which facilitates communication between the two sides of the brain. The limbic system, sometimes referred to as a 5th lobe, forms the inner border of the cortex and is made up of several different structures scattered throughout the four lobes.
Grey/white matter
The outermost layer of the cerebral cortex is termed the ‘grey matter’ (Fig. 7.2) and is primarily composed of the cell bodies of neurons (S2.6). The grey matter is an expansive sheet, approximately 2–4 mm (0.08–0.16 inches) thick, which is intricately folded to form grooves termed ‘sulci’ and raised areas termed ‘gyri’ (Fig. 7.2). The compact nature of the grey matter allows closer contact between neurons and consequently faster communication. The white matter (Fig. 7.2) below is formed predominantly by the myelinated axons of the neurons (S2.6), which connect to different regions of the central nervous system.
The internal capsule
The internal capsule lies superiorly to the thalamus (S2.9) and consists of neurons passing from the thalamus to the cerebral cortex and vice versa. Between the cortex and the internal capsule, the ascending and descending neurons form the corona radiata. All afferent and efferent information is routed through the internal capsule and most sensory information enters the cerebral cortex via the thalamus.
Connections
The left and right hemispheres of the cerebral cortex are connected by corpus callosum with each hemisphere being connected, in the main, to the contralateral side of the body. Thereby the left hemisphere receives information from the right side of the body and has motor control of the right side of the body. This is the result of the decussation or crossing of the ascending and descending tracts (S2.14, 15). However, there are a minority of neurons which remain ipsilateral, e.g. in the corticospinal tract (S2.14). That is, some neurons within the tracts remain on the same side of the body. This phenomenon is thought to have important implications during rehabilitation of neurologically impaired patients, especially those following stroke.
