The central nervous system (CNS) consists of the brain and the spinal cord, immersed in the cerebrospinal fluid (CSF).
Weighing about 3 pounds (1.4 kilograms), the brain consists of three main structures: the cerebrum, the cerebellum and the brainstem.
Cerebrum - divided into two hemispheres (left and right), each consists of four lobes (frontal, parietal, occipital and temporal). The outer layer of the brain is known as the cerebral cortex or the ‘grey matter’. It covers the nuclei deep within the cerebral hemisphere e.g. the basal ganglia; the structure called the thalamus, and the ‘white matter’, which consists mostly of myelinated axons.
Cerebellum – responsible for psychomotor function, the cerebellum co-ordinates sensory input from the inner ear and the muscles to provide accurate control of position and movement.
Brainstem – found at the base of the brain, it forms the link between the cerebral cortex, white matter and the spinal cord. The brainstem contributes to the control of breathing, sleep and circulation.
Other important areas in the brain include the basal ganglia, thalamus, hypothalamus, ventricles, limbic system, and the reticular activating system.
The brain and the different areas of the brain can be illustrated using images of the brain in different orientations or ‘sections’. The most commonly used sections are the mid-sagittal (simply, from front to back) and coronal (simply fom one side to the opposite side or cross section) sections.
Collectively the caudate nucleus, putamen and globus pallidus form the basal ganglia, and are involved in movement control. These highly specialised clusters of cells/nuclei are found within the white matter, beneath the cerebral cortex.
Thalamus and Hypothalamus
The thalamus and hypothalamus are prominent internal structures. The thalamus has wide-ranging connections with the cortex and many other parts of the brain, such as the basal ganglia, hypothalamus and brainstem. It is capable of perceiving pain but not of accurately locating it. The hypothalamus has several important functions, including control of appetite, sleep patterns, sexual drive and response to anxiety.
Within the brain there are a number of cavities called ventricles. Ventricles are filled with CSF, which is produced within the ventricle wall. The CSF also surrounds the outer surfaces of the brain and ‘cushions’ the brain against trauma, maintains and controls the extracellular environment, and circulates endocrine hormones. during a lumbar puncture (LP), the CSF is collected from the spinal canal of the patient. Laboratory analyses of the CSF (e.g. glucose and electrolyte concentrations) can show whether there is an infection in or around the brain.
The limbic system is not a structure, but a series of nerve pathways incorporating structures deep within the temporal lobes, such as the hippocampus and the amygdala. Forming connections with the cerebral cortex, white matter and brainstem, the limbic system is involved in the control and expression of mood and emotion, in the processing and storage of recent memory, and in the control of appetite and emotional responses to food. All these functions are frequently affected in depression and the limbic system has been implicated in the pathogenesis of depression. The limbic system is also linked with parts of the neuroendocrine and autonomic nervous systems, and some psychiatric disorders, such as anxiety, are associated with both hormonal and autonomic changes.
Reticular Activating System
At the core of the brainstem is a collection of nuclei called the reticular formation.These nuclei receive input from most of the body’s sensory systems (e.g. sight, smell, taste, etc) and other parts of the brain, such as the cerebellum and cerebral hemispheres.
Some neurons from the reticular formation project to meet motor neurons of the spinal cord and influence functions such as cardiovascular and respiratory control. In addition, there are also neurons projecting into most of the rest of the brain. The ascending fibres of the reticular formation form a network called the reticular activating system, which influence wakefulness, overall degree of arousal and consciousness – all factors which may be disturbed in depressed patients.
Although extremely complex, the brain is largely made up of only two principal cell types: neurons and glial cells. There are over 100 000 million neurons in the brain and an even greater number of glial cells. It is estimated that there are more than 10 000 million cells in the cerebral cortex alone.
Neurons are involved in information transmission – receiving, processing and transmitting information through their highly specialised structure. Neurons consist of a cell body and two types of projections – the dendrites and an axon. Most neurons have many dendrites, but only one axon.
The majority of neurons are unable to undergo cell division or repair. This limitation results in irreversible damage to the nervous system after trauma, intoxication, oxygen deficiency or stroke.
Neurons use their highly specialised structure to both send and receive signals. Individual neurons receive information from thousands of other neurons, and in turn send information to thousands more. Information is passed from one neuron to another via neurotransmission. This is an indirect process that takes place in the area between the nerve ending (nerve terminal) and the next cell body. This area is called the synaptic cleft or synapse.
Glial cells are major constituents of the central nervous system, and while they do not have a direct role in neurotransmission, glial cells play a supporting role that helps define synaptic contacts and maintain the signalling abilities of neurons. Various types of glial cells can be found in the brain (or CNS); including astrocytes, oligodendroglia and microglia. The total number of glial cells exceeds that of neurons by approximately three-fold.
Glial cells are smaller than neurons and lack axons and dendrites. The well-defined roles of the glia include: modulating the rate of nerve impulse propagation; controlling the uptake of neurotransmitters; and playing a pivotal role during development and adulthood. Some evidence also suggests that glial cells aid (or, in some cases, prevent) recovery from neuronal injury and that they are involved in a number of diseases, such as Alzheimer’s disease, multiple sclerosis and other central and peripheral neuropathies.
Last updated: 20.12.2011