Neuron

Technical term for nerve cell.

Neurons are the basic working unit of the nervous system, sending, receiving, and storing signals through a unique blend of electricity and chemistry. The human brain has more than 100 billion neurons.

Neurons that receive information and transmit it to the spinal cord or brain are classified as afferent or sensory; those that carry information from the brain or spinal cord to the muscles or glands are classified as efferent or motor. The third type of neuron connects the vast network of neurons and may be referred to as interneuron, association neuron, internuncial neuron, connector neuron, and adjustor neuron.

Although neurons come in many sizes and shapes, they all have certain features in common. Each neuron has a cell body where the components necessary to keep the neuron alive are centered. Additionally, each neuron has two types of fiber. The axon is a large tentacle and is often quite long. (For example, the axons connecting the toes with the spinal cord are more than a meter in length.) The function of the axon is to conduct nerve impulses to other neurons or to muscles and glands. The signals transmitted by the axon are received by other neurons through the second type of fiber, the dendrites. The dendrites are usually relatively short and have many branches to receive stimulation from other neurons. In many cases, the axon (but not the cell body or the dendrites) has a white, fatty covering called the myelin sheath. This covering is believed to increase the speed with which nerve impulses are sent down the axon.

An unstimulated neuron has a negative electrical charge. The introduction of a stimulus makes the charge a little less negative until a critical point—the threshold—is reached. Then the membrane surrounding the neuron changes, opening channels briefly to allowing positively charged sodium ions to enter the cell. Thus, the inside of the neuron becomes positive in charge for a millisecond (thousandth of a second) or so. This brief change in electrical charge is the nerve impulse, or spike, after which the neuron is restored to its original resting charge.

This weak electrical impulse travels down the axon to the synapse. The synapse or synaptic gap forms the connection between neurons, and is actually a place where the neurons almost touch, but are separated by a gap no wider than a few billionths of an inch. At the synapses, information is passed from one neuron to another by chemicals known as neurotransmitters. The neurotransmitter then combines with specialized receptor molecules of the receiving cell.

Neurotransmitters either excite the receiving cell (that is, increase its tendency to fire nerve impulses) or inhibit it (decrease its tendency to fire impulses), and often both actions are required to accomplish the desired response. For example, the neurons controlling the muscles that pull your arm down (the triceps) must be inhibited when you are trying to reach up to your nose (biceps excited); if they are not, you will have difficulty bending your arm.

Physiological psychologists are interested in the involvement of the nervous system in behavior and experience. The chemistry and operation of the nervous system is a key component in the complex human puzzle. A number of chemical substances act as neurotransmitters at synapses in the nervous system and at the junction between nerves and muscles. These include acetylcholine, dopamine, epinephrine (adrenalin), and neuropeptides (enkephalins, endorphins, etc.). A decrease in acetylocholine has been noted in Alzheimer's disease which causes deterioration of the thought processes; shortage of dopamine has been linked to Parkinson's disease, whereas elevated dopamine has been observed in schizophrenics.

Drugs that affect behavior and experience—the psychoactive drugs —generally work on the nervous system by influencing the flow of information across synapses. For instance, they may interfere with one or several of the stages in synaptic transmission, or they may have actions like the natural neurotransmitters and excite or inhibit receiving cells. This is also true of the drugs which are used in the treatment of certain psychological disorders.