The basic unit of the nervous system is the nerve cell, also known as a neuron. These cells contain fibres called axons which send electrical impulses, and dendrites that branch out from the cell to receive these impulses.
The nervous system has two distinct parts: the central nervous system (CNS) consisting of the brain and spinal cord, and the peripheral nervous system describing the nerves outside the brain and spinal cord.
Neuropathies describe disorders involving the peripheral nerves. If motor nerves (neurons transmit signals from the CNS to stimulate effector cells such as muscles) are damaged, muscles may weaken or become paralysed, and if sensory nerves (neurons that convey information, in the opposite direction, from tissues to the CNS) are damaged, sensation may be lost or abnormal sensations may be felt. The most common hereditary motor and sensory neuropathy is Charcot-Marie-Tooth disease.
Many diseases result from the degradation of myelin sheaths. These are the membranes surrounding neuronal axons which enables nerve impulses to travel quickly. Composed of fatty substances known as lipids, myelin can be likened to the insulation around an electrical wire, acting as an insulator to electrical signals. In demyelinating diseases myelin sheaths become degraded impeding the transport of impulses. The most common demyelinating disease is multiple sclerosis where the myelin is destroyed by the body’s own immune system possibly in response to some environmental trigger, and possibly in combination some level of genetic susceptibility. One of the most common inherited demyelination diseases is adrenoleukodystrophy.
A degeneration of neurons in the cerebellum and nerve tissue in the spinal cord, important for controlling muscle movement in the arms and legs can result in a progressive loss in coordination called ataxia, while another major group of diseases resulting from degeneration of neurons in the spine are hereditary spastic paraplegia.
Some neuropathies affect the autonomic system which regulates functions not under conscious control, such as heart rate or breathing. For instance the heart rate can increase for no apparent reason, or the kidneys suddenly fail to properly retain water. Several disorders associate with dysautonomia such as chronic fatigue system, the autoimmune multiple system atrophy and the inherited familial dysautonomia. One of the most severe dysautonomic diseases is known as Ondine’s Curse in which there is no autonomic control of breathing.
Degeneration of neurons or areas in the brain can cause characteristic clinical syndromes. Amyloidlateral sclerosis (also known as Motor neuron disease) causes paralysis, due to death of motor neurons in the motor cortex, brain stem and spinal cord while Huntington’s disease damages regions of the brain that control movement, emotion, and cognitive ability.
Parkinson's disease results from loss of neurons in regions of the brain controlling movement and is characterised by unusual clumps of protein, known as Lewy bodies, building up in brain cells. Further types of young onset Parkinson's disease include Parkin disease. A further disease characterised by these aggregations is Lewy body dementia, that shows degradation in cognition as well as motor control.
The most common neurodegenerative disease and the most common cause of dementia (Lat. de; away; mentis; mind) is Alzheimer's disease. This disease also associates with the accumulation of clumps of amyloid protein, called plaques or tau protein as tangles. The aggregation of tau in different regions of the brain such as in the frontal and temporal lobes, causes frontotemporal dementia leading behavioural abnormalities rather than memory loss while progressive supranuclear palsy results from such aggregations in the midbrain.
Prion degenerative diseases result from an infectious agent that is neither bacterial, fungal, viral and in fact contains no genetic material at all but an altered protein.
Mental retardation is present in around 2 - 3% percent of the population; defined as cognitive ability that is markedly below average. Many environmental, genetic or multiple factors can cause mental retardation. A number of single-gene disorders include fragile X syndrome, neurofibromatosis, tuberous sclerosis, Noonan's syndrome and as many as 25% of persons with mental retardation have a detectable chromosome abnormality such as Down syndrome, DiGeorge, Prader-Willi, Angelman and Williams syndromes.