The Forebrain

The forebrain is the largest, most anterior and most highly developed part of the human brain, and it comprises the cerebrum together with the thalamus, the hypothalamus and the limbic system.

The cerebrum is by far the largest part of the forebrain, divided into left and right cerebral hemispheres that are connected to each other by a thick band of nerve fibres called the corpus callosum. Its outer layer, the cerebral cortex, is composed of grey matter (densely packed neuron cell bodies) and is thrown into folds — the raised ridges being called gyri and the grooves between them sulci — which greatly increases the surface area of cortex that can be packed inside the skull. Functionally, the cerebral cortex can be mapped into three broad kinds of area. Motor areas contain the neurons that initiate and control voluntary movement, sending commands down to the muscles. Sensory areas receive and process specific kinds of incoming sensory information — for instance, the visual area at the back of the cortex processes signals from the eyes, and the auditory area processes signals from the ears. Association areas, which occupy a large proportion of the human cortex, do not deal directly with a single sense or a single muscle group but instead integrate information arriving from several sensory and motor areas, and are involved in complex higher functions such as reasoning, planning, language, learning and personality.

Beneath the cerebrum lies the thalamus, which acts as the principal relay and coordinating centre of the forebrain: nearly all sensory information travelling towards the cerebral cortex, and much motor information travelling from the cortex onward, is relayed through the thalamus, which also plays a role in filtering and prioritising which signals reach conscious awareness.

Just below the thalamus lies the hypothalamus, a comparatively small structure with a very wide range of regulatory functions. It contains centres that regulate body temperature, control the sensations of hunger and thirst and the associated feeding and drinking behaviour, regulate sexual behaviour, and are linked to the experience of pleasure and pain (reward and punishment centres). The hypothalamus also produces hormones that control the pituitary gland, linking the nervous system to the endocrine system, and it houses the body's principal biological clock, governing circadian (roughly 24-hour) rhythms such as the sleep-wake cycle. …