The brain is composed of a great many cell types, organised in a highly structured manner. It has a high rate of blood, and therefore nutrient, supply (Section 4.2). Amongst the many cell types found in the brain, neurons themselves are outnumbered approximately nine times by other cells, generally referred to as glial cells (from the Greek for glue). These glial cells perform many functions of mechanical support and electrical 'insulation', protection against infection and repair of damage. The most abundant type, the astrocytes (so-called because of their star shape, with multiple radiating projections), probably act as intermediaries between the capillaries and the neurons, regulating the supply of nutrients and also the extracellular ionic environment. In recent years it has been realised that glial cells can themselves transmit signals, in the form
Fig. 7.3 The human brain and its main components.
of calcium waves that pass between glial cells and can also communicate with neurons. It is not yet understood what part glial cells may play in information processing within the CNS.
The brain contains both complete neurons, and the cell bodies of neurons that extend into the spinal cord and beyond. It is organised into a number of relatively discrete structural parts (Fig. 7.3). The two cerebral hemispheres are the most prominent part. In fact they are roughly quarter spheres, together making up about one hemisphere, but the terminology is unlikely to change. The outer layer, a few mm thick, contains many cell bodies and is referred to as grey matter because of its appearance when fixed with alcohol for microscopy. It forms the cerebral cortex and is responsible for many higher functions: receipt of information from the special senses, and motor control (control of muscles). The cerebral cortex, although only 2-4 mm thick, occupies a surprisingly large proportion of brain volume (around 40%) because of the many convolutions of the brain surface, and hence large surface area. Underlying the cortex is the cerebral white matter (again from its appearance when fixed with alcohol), largely composed of myelinated fibres grouped into large bundles, responsible for transmission within the brain. Amongst the white matter are found a number of local regions of grey matter, known as nuclei, where there are further groups of cell bodies. These nuclei have names and their functions are becoming clear, but more detailed description is beyond the scope of this book. Within the cerebral hemispheres is the central core of the brain, the dien-cephalon, a structure itself composed of three parts - the thalamus, underneath which is the hypothalamus with the epithalamus behind.
The hypothalamus is the region of the brain of most interest with respect to metabolic regulation. It is an integrating centre: it receives information and also sends it out. It receives information from other brain areas, but in addition the hypothalamus itself contains important sensors. It monitors the concentration of glucose in the blood, and initiates appropriate responses to maintain this close to a constant level of around 4 -5 mmol/l. (This includes both autonomic responses, e.g. initiation of glycogen breakdown in response to a fall in blood glucose concentration, and regulation of dietary intake by control of appetite.) The hypothalamus also senses fluid balance by monitoring the osmolarity of the blood, and initiates appropriate measures to maintain an optimal level (both via regulation of thirst, and control of water excretion by the kidneys). It has a temperature-sensitive region, monitoring the temperature of blood flowing through it, and responding as appropriate to maintain the required body temperature. This includes elevation of the body temperature when this is seen as appropriate during infection.
The hypothalamus controls drives such as thirst and appetite by signalling to other brain areas. It regulates other bodily functions in two main ways. It is responsible for most of the output of the sympathetic nervous system. Signals from the hypothalamus are transmitted via other brain centres to the sympathetic tracts within the spinal cord, and thus to tissues and organs within the body. It also regulates the secretion of hormones by the pituitary gland. Connections between the hypothalamus and pituitary were discussed in Section 5.3. Thus, the hypothalamus is a very important part of the brain in terms of the role of the nervous system in metabolic regulation. The term neuroendocrine system is often used to describe the combination of nervous and hormonal systems of regulation, and the hypothalamus is at the centre of this combination.
Other parts of the brain act as further regulatory centres, and as 'relay stations'. The cerebellum has important functions in coordinating movement; disorders of cerebellar function can lead to uncoordinated movements, trembling, etc. The brainstem is the connection between higher centres of the brain and the spinal cord. In some ways it is analogous to a primitive brain, and regulates very basic functions such as heart rate, breathing and blood pressure in a 'preprogrammed', automatic manner. Thus, if the spinal cord is severed from the brainstem, these vital functions cease. On the other hand, if the brainstem remains intact after severe injury to other parts of the brain, the victim can enter a state of primitive existence in which consciousness is absent but life can be maintained so long as food is provided - the state sometimes called 'vegetative existence'.
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