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.

The hemispheres are richly connected to various subcortical structures such as the thalamus (Fig. 7.2) and basal ganglia, however the vast majority of connections (99%) are from one area of the cortex to another.

Cells of the cerebral cortex

Overview

The areas responsible for encoding sensory information and commanding movement account for about one-fifth of the cerebral cortex. These areas that function to integrate information and produce a meaningful experience of the world are termed the ‘association areas’.

The integration of sensory information takes place in the parietal, temporal and occipital lobes, typically in the right hemisphere. The frontal lobe or pre-frontal association area is involved in planning actions and movement, as well as abstract thought. Our language abilities are localized to the association areas of the parietal-temporal-occipital lobes, typically in the left hemisphere.

Sensory

The sensory areas are located in both hemispheres of the cortex in the parietal, temporal and occipital lobes and receive and process information from the sensory receptors. The parts of the cortex that receive sensory input from the thalamus are called the primary sensory areas. The senses of vision, audition and touch are served by the primary visual cortex (occipital lobe), primary auditory cortex (temporal lobe) and primary somatosensory cortex (parietal lobe). The latter receives information about the contralateral side of the body.

There is a regional specialization of function within the sensory areas. In simple terms, this means that neurons performing similar roles and therefore needing to communicate efficiently will tend to be found together. This organization gives rise to a cortical map of the sensory area known as a ‘topographic map’. The somatosensory topographic map is represented by a deformed human shape and is termed the ‘homunculus’ (Fig. 7.3). In the homunculus, the sizes of the different body parts reflect their relative importance in terms of the density of innervations. For example, areas with lots of sensory receptors, such as the fingertips and the lips have a larger area of representation because the number of sensory neurons entering the sensory cortex is greater.

Motor

The motor areas are located in both hemispheres of the frontal lobe and control the contralateral side of the body; they are also arranged topographically forming a motor homunculus (Fig. 7.3). Three main areas of the cortex have been found to be important in voluntary movement:

The dorsolateral pre-frontal cortex decides on the voluntary movements required according to higher-order instructions, rules, and self-generated thoughts.

The supplementary motor area and pre-motor areas select voluntary movements from learned stored memory or motor programmes.

The primary motor cortex executes voluntary movement and is integrally linked with the primary somatosensory cortex.

Anatomical areas and their function linked to assessment

Based upon past and present research, Tables 7.2–7.6 represent a summary of the basic functions of the four main lobes and the limbic lobe. Each table also includes the terminology used to describe a dysfunction of the relevant function and the section/chapter code linking it to an assessment tool, where appropriate. It may be useful to read these tables in conjunction with Figure 7.1, which shows the anatomical position of these areas within the lobes of the cerebral cortex.

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