Nervous System

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The Peripheral Nervous System

2.4     The 31 pairs of spinal nerves as well as all the other nerves and nerve cells outside of the central nervous system make up the third part of the nervous system. This system carries nerve impulses to and from the body’s sensory cells and the central nervous system. The central nervous system analyses the information it receives from the peripheral nervous system and responds to it. The motor neurons of the peripheral nervous system carry out the instructions received from the central nervous system. It acts as a switchboard that controls and coordinates the activities of the entire nervous system.

 The Nervous Tissue

2.5     The tissue of the nervous system is composed of two basic types of cells, neurons and glia (glial cells). A neuron consists of a cell body; dendrites, branched structures that conduct impulses to the cell body; and an axon, a long fibre that transmits impulses away from the body. There are three types of neurons. Sensory neurons send signals from the sense organs to the spinal chord and the brain. Motor neurons send signals from the brain and the spinal chord to the body’s muscles. Association neurons link sensory neurons to motor neurons. Glial cells help support and nourish neurons and account for approximately one-half of the volume of the nervous tissue. There are four types of glial cells, each with a different function.

 The Neurotransmitters

2.6     Neurons are separated from each other and from other body cells by a tiny gap called a synapse. When a nerve impulse reaches a synapse, the axon releases a chemical called a neurotransmitter. The neurotransmitter transfers the impulse across the synapse to the target cell. Then, the synapse is cleared so that another impulse can cross. Over 60 different chemicals have been identified either as neurotransmitters or as substances that modify the activity of neurotransmitters.

2.7     Neurotransmitters and Drugs – The use of psychoactive drugs alters the transmission of nerve impulses. For example, caffeine stimulates the nervous system by facilitating synaptic transmission. Cocaine, amphetamines, and some antidepressant drugs cause substances to remain in the synapse longer. The result is prolonged stimulation of the nervous system. Some psychoactive drugs are addictive. The body adjusts to the changes these drugs cause. If the drug is withdrawn, the body will be unable to function as it had previously. The addicted drug user will then experience withdrawal symptoms.