Forebrain

The forebrain is the largest and most elaborate division of the brain. It is built up of three parts: the cerebrum, the thalamus and the hypothalamus. Together these structures handle conscious thought, the coordination of sensory and motor information, and the control of many basic drives and emotions.

Cerebrum

The cerebrum forms the major part of the human brain. A deep, lengthwise cleft runs along the middle and divides the cerebrum into two halves, the left and right cerebral hemispheres. These two hemispheres do not work in isolation; they are joined by a thick band of nerve fibres called the corpus callosum, which allows the two sides to communicate.

Each cerebral hemisphere is covered by an outer layer of cells known as the cerebral cortex. This layer is thrown into prominent folds, which greatly increase its surface area. The cortex is described as grey matter because of its greyish appearance, a colour that comes from the large number of neuron cell bodies packed into it.

The cerebral cortex is not uniform in its work. It contains different functional regions:

• Motor areas, which direct voluntary movements.
• Sensory areas, which receive incoming sensations.
• Association areas, large regions that are neither clearly motor nor clearly sensory. These areas carry out complex functions such as linking together information from different senses (intersensory associations), memory and communication.

Beneath the cortex lies the inner part of each cerebral hemisphere, made up of nerve-fibre tracts whose fibres are wrapped in a myelin sheath. This covering gives the region an opaque, whitish look, and so it is called the white matter.

Thalamus

The cerebrum wraps around a structure called the thalamus. The thalamus serves as a major coordinating centre for sensory and motor signalling, relaying and integrating these signals as they pass to and from the higher centres of the brain.

Hypothalamus

Lying at the base of the thalamus is another very important region, the hypothalamus. Despite its small size, it controls several essential functions. It contains a number of centres that regulate:

• Body temperature.
• The urge for eating.
• The urge for drinking.

The hypothalamus also holds several groups of neurosecretory cells. These cells secrete chemical messengers called hypothalamic hormones, linking the nervous system to the hormonal control of the body.

Limbic system …