Forebrain

The forebrain consists of the cerebrum, the thalamus and the hypothalamus. The cerebrum forms the major part of the human brain.

A deep cleft divides the cerebrum lengthwise into the left and right cerebral hemispheres, which are joined by a tract of nerve fibres, the corpus callosum.

The cell layer covering each hemisphere is the cerebral cortex. It is thrown into prominent folds and is called grey matter because neuron cell bodies are concentrated there.

The cortex has motor areas, sensory areas, and large association areas that are neither clearly sensory nor motor; these handle complex functions such as intersensory associations, memory and communication.

Myelin-covered tract fibres form the inner part of each hemisphere; their opaque white look gives this layer the name white matter.

The cerebrum wraps round the thalamus, a major coordinating centre for sensory and motor signalling.

The hypothalamus lies at the base of the thalamus. Its centres control body temperature and the urge to eat and drink, and its neurosecretory cells secrete hypothalamic hormones.

The inner parts of the hemispheres with deep structures such as the amygdala and hippocampus form the limbic lobe or limbic system. With the hypothalamus it regulates sexual behaviour, the expression of emotions (excitement, pleasure, rage, fear) and motivation. The chapter summary also links the limbic system with smell (olfaction) and autonomic responses.