The basic unit of the nervous system is a specialized cell called the neuron (Murre, 1992). It is important to understand neurons because they undoubtedly hold the secrets of how the brain works and in turn the nature of human consciousness.
Neurons differ markedly in size and appearance but they have certain common characteristics. Projecting from the cell body is a number of short branches called dendrites. The dendrites and cell body receive neural impulses from adjacent neurons. These messages are transmitted to other neurons or to muscles and glands by a slender tube like extension of the cell called an axon. At its end the axon branches into a number of fine projections and end in small swellings called synaptic terminals (Peters, Palay, Webster, 1991).
The synaptic terminal does not actually touch the neuron that it will stimulate. Rather, there is a slight gap between the synaptic terminal and the cell body or dendrites of the receiving neuron. This junction is called synapse and the gap itself is called the synaptic gap. When a neural impulse travels down the axon and arrives at the synaptic terminals, it triggers the secretion of a chemical called the neurotransmitter. The neurotransmitter diffuses across the synaptic gap and stimulates the next neuron, thereby carrying the impulse from one neuron to the next. The axons from many neurons synapse on the dendrites and cell body of a single neuron.
Stimulation of the dendrites and cell body leads to a neural impulse that travels down the length of the axon. Sensory neurons transmit signals from sense organs to the brain and spinal cord; motor neurons transmit signals from the brain and spinal cord to muscles and glands. A nerve is a bundle of elongated axons belonging to hundreds or thousands of neurons.
A stimulus moves along a neuron as an electrochemical impulse that travels from the dendrites to the end of the axon. This traveling impulse or action potential is caused by a self-propagating mechanism called depolarization that changes the permeability of the cell membrane to different types of ions that float in and about the cell. An action potential once started travels down the axon to many small swellings at the end of the axon called synaptic terminals (Peters, Palay, Webster, 1991).
These terminals release chemical substances called neurotransmitters that are responsible for transferring the signal from one neuron to an adjacent one. The neurotransmitters diffuse across a small gap between the juncture of the two neurons and bind to neuroreceptors in the cell membrane of the receiving neuron. Some neurotransmitter-receptor bindings cause the cell membrane to depolarize while other causes it to polarize.
If depolarization reaches a threshold level an action potential is fired down the length of the receiving neuron. The occurrence of the action potential is an all or none event (Reed & Marks, 1999). There are many different kinds of neurotransmitters-receptor interactions and they help explain a range of psychological phenomena.
Since the neurons are responsible for sending the impulses that communicate to the brain and the body what it is supposed to do, that any disruption in the ability of the axon and the dendrites to receive and send electric impulses will also cause problems with basic functioning.
Recent research has found that when neurons fail to efficiently process the electrical impulses that send information to the brain the person also suffers lapses in almost every physical behavior necessary for normal functioning (Reed & Marks, 1999). For example, if there is a lag in the sending and receiving of impulses from the eye to the brain, then the person would not be able to adequately process the visual information in the environment.
Another example is when the neurons in the brain is unable to send the signal to the part of the brain that controls sleep patterns, then the person may suffer from sleep disorders such as apnea. Although not necessarily problematic, the feeling of déjà vu has been traced to the neural activity of the brain wherein signals to new experience is waylaid to the area of long term memory.
Murre, J. (1992). Learning and categorization in modular neural networks. New York: Harvester.
Peters, A., Palay, S. & Webster, H. (1991) The Fine Structure of the Nervous System, 3rd ed.,
New York: Oxford University Press.
Reed, R. & Marks II, R.(1999). Neural Smithing. Supervised Learning in Neural Networks. Cambridge: The MIT Press.