Somatic nervous system

The somatic nervous system is the other branch of the peripheral nervous system in the efferent portion of the nervous system. Unlike the autonomic nervous system, it only controls one effector organ and that is skeletal muscle. Therefore the main function of the somatic nervous system is to control the movement of the body. The system is under voluntary control as the individual is aware of moving a muscle.

Anatomy
There is a single motor neuron that runs from the central nervous system to the skeletal muscle. The motor neurons of the somatic nervous system originate in the ventral horn of the spinal cord and receive a number of different inputs from the afferents, brain stem extrapyrimidal tracts and the cerebral cortex pyrimidal tract. One neuron will affect and innervate many different muscle fibres and this makes up a motor unit. When stimulated, the neuron will cause all the muscle fibres in one unit to contract.

As there is only one neuron rather than a pair of neurons like the autonomic nervous system, any stimulation is excitatory and a cease of stimulation is the only way to decrease the contraction of the muscles.

Neuromusclar Junctions
The neuromuscular junctions are the synapses of the motor neurons and only respond to acetylcholine. The terminal boutons release the acetylcholine across the synapse to the motor end plate which contains a series of invaginations. Within these invaginations are the receptors for acetylcholine. An enzyme called acetylcholinesterase can be found in the synaptic space and it breaks down excess acetylcholine to terminate the signal. Acetylcholine is released in response to depolarisation of the motor neuron.

Problems with the Somatic Nervous System
There are a number of different toxins that can affect the somatic nervous system by acting on the neuromuscular junctions. Black widow spider venom causes constant release of acetylcholine so that the muscles go into spasm and rigidity. This can cause respiratory failure as the respiratory muscles are skeletal muscles and under this control. There are a number of other toxins that can also affect this system such as Curare which paralyses a person by preventing the binding of acetylcholine to the receptor and the venom of a rattlesnake which causes flaccid paralysis.

Diseases can also affect the neuromuscular junctions such as myasthenia gravis. This causes progressive weakening of the muscles and patients have trouble speaking and the breathing can be affected. It is believed that the immune system attacks the acetylcholine receptors and therefore the muscles cannot respond to the release of neurotransmitter.