The brain: functional divisions

The incompletely separated cerebral hemispheres consist of a thin outer folded cortex of grey matter containing organized neuronal cell bodies and interneurons. Some of the surface convolutions subserve particular sensory or motor functions. Incoming afferent and projected efferent fibres constitute the underlying white matter, which connects different parts of each hemisphere, the hemispheres to each other and (as the corona radiata) to subcortical nuclei, especially components of the deeply-embedded diencephalon and the basal ganglia, and continuing between the latter as the internal capsule to and from the cerebellum and brainstem. Divisions of the diencephalon, the deeper part of the embryonic forebrain, include the epithalamus (pineal gland), the thalamus (connected extensively with the cortex), the subthalamus and the hypothalamus (involved in the autonomic nervous system, limbic system and neuro-endocrine system). The brainstem, consisting of the midbrain, pons and medulla, allows passage of many ascending and descending nerve fibre tracts between the brain and spinal cord, carrying sensory information from and allowing movement of the limbs and trunk. It is also the site of many of the cranial nerve nuclei, through which the brain innervates the head region. It houses the centres controlling vital aspects related to respiration, cardiovascular function and consciousness levels. The cerebellum also has a cortex of grey matter, tightly convoluted into folia, and containing layered neuronal cell bodies projecting laterally and, as underlying white matter, to a collection of deep nuclei. Fibres run to and from the cerebellar nuclei in a series of peduncles to the midbrain, pons and medulla, and allow the cerebellum to coordinate movement at an unconscious level.