Inferior of the cerebrum (Fig. 7.15)

The surface of the cerebral cortex is composed of grey matter (nerve cell bodies). Within the cerebrum the lobes are connected by masses of nerve fibres, or tracts, which

make up the white matter of the brain. The afferent and efferent fibres linking the different parts of the brain and spinal cord are as follows.

• Association (arcuate) tracts connect different parts of a cerebral hemisphere by extending from one gyrus to another, some of which are adjacent and some distant.

• Commissural tracts connect corresponding areas of the two cerebral hemispheres; the largest and most important commissure is the corpus callosum.

• Projection tracts connect the cerebral cortex with grey matter of lower parts of the brain and with the spinal cord, e.g. the internal capsule.

The internal capsule is an important projection tract that lies deep within the brain between the basal ganglia (nuclei) and the thalamus. Many nerve impulses passing to and from the cerebral cortex are carried by fibres that form the internal capsule. Motor fibres within the internal capsule form the pyramidal tracts (corticospinal tracts) that cross over (decussate) at the medulla oblongata.

FUNCTIONS OF THE CEREBRAL CORTEX

There are three main varieties of activity associated with the cerebral cortex:

• mental activities involved in memory, intelligence, sense of responsibility, thinking, reasoning, moral sense and learning are attributed to the higher centres

• sensory perception, including the perception of pain, temperature, touch, sight, hearing, taste and smell

• Initiation and control of skeletal (voluntary) muscle contraction.

FUNCTIONAL AREAS OF THE CEREBRAL CORTEX

(Fig. 7.16)

The main functional areas of the cerebral cortex have been identified but it is unlikely that any area is associated exclusively with only one function. Except where specially mentioned, the different areas are active in both hemispheres; however, there is some variation between individuals. There are different types of functional areas:

• motor, which direct skeletal (voluntary) muscle movements

• sensory, which receive and decode sensory impulses enabling sensory perception

• association, which are concerned with integration and processing of complex mental functions such as intelligence, memory, reasoning, judgement and emotions.

In general, motor impulses leave from the anterior part of each cerebral hemisphere while sensory impulses arrive at the posterior part, I.E. Areas behind the central sulcus.

Other areas of the cerebrum

Deep within the cerebral hemispheres there are groups of cell bodies called nuclei, the exception being those that form the basal ganglia, which act as relay stations where impulses are passed from one neurone to the next in a chain. Important masses of grey matter include the:

• basal ganglia

• thalamus

• hypothalamus.

Basal ganglia. These are areas of grey matter, lying deep within the cerebral hemispheres, with connections to the cerebral cortex and thalamus. The basal ganglia form part of the extra pyramidal tracts and are involved in initiation and fine control of complex movement, and learned coordinated activities. If control is inadequate or absent, movements are jerky, clumsy and uncoordinated.

Thalamus. The thalamus consists of two masses of nerve cells and fibres situated within the cerebral hemispheres just below the corpus callosum, one on each side of the third ventricle. Sensory input from the skin, viscera and special sense organs is relayed to the thalamus before redistribution to the cerebrum.

Hypothalamus. The hypothalamus is composed of a number of groups of nerve cells. It is situated below and in front of the thalamus, immediately above the pituitary gland. The hypothalamus is linked to the posterior lobe of the pituitary gland by nerve fibres and to the anterior lobe by a complex system of blood vessels. Through these connections, the hypothalamus controls the output of hormones from both lobes of the gland (see p. 213).

Other functions of the hypothalamus include control of:

• the autonomic nervous system (p. 170)

• appetite and satiety

• thirst and water balance

• body temperature (p. 361)

• emotional reactions, e.g. pleasure, fear, rage

• sexual behaviour including mating and child rearing

• biological clocks or circadian rhythms, e.g. sleeping and waking cycles, body temperature and secretion of some hormones.

BRAIN STEM

MIDBRAIN

The midbrain is the area of the brain situated around the cerebral aqueduct between the cerebrum above and the pons below. It consists of nuclei and nerve fibres (tracts), which connect the cerebrum with lower parts of the brain and with the spinal cord. The nuclei act as relay stations for the ascending and descending nerve fibres.

PONS

The pons is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists mainly of nerve fibres (white matter) that form a bridge between the two hemispheres of the cerebellum, and of fibres passing between the higher levels of the brain and the spinal cord. There are nuclei within the pons that act as relay stations and some of these are associated with the cranial nerves. Others form the pneumotaxic and apnoustic centres that operate in conjunction with the respiratory centre in the medulla oblongata.

The anatomical structure of the pons differs from that of the cerebrum in that the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface.