The function of a neuron is to transmit information within the nervous system. Neural transmission occurs when a neuron is activated, or fired (sends out an electrical impulse). Activation (firing) of the neuron takes place when the neuron is stimulated by pressure, heat, light, or chemical information from other cells. (The type of stimulation necessary to produce firing depends on the type of neuron.) The fluid inside a neuron is separated from that outside by a polarized cell membrane that contains electrically charged particles known as ions. When a neuron is sufficiently stimulated to reach the neural threshold (a level of stimulation below which the cell does not fire), depolarization, or a change in cell potential, occurs.
Potentials. The term potential refers to a difference in electrical charges. Neurons have two types of potentials, a resting potential and an action potential. The neural threshold must be reached before a change from resting to action potential occurs (Figure 1).
Transmission of a Nerve Impulse
Resting potential is the potential maintained by the inactive neuron. When unstimulated, a neuron is like a small battery and has a measurable negative electrical charge (about 70 millivolts) called the resting potential.
Action potential is the potential produced when appropriate stimulation is high enough to reach the neural threshold and causes the neuron to fire, that is, alters the membrane permeability. Alteration of membrane permeability (polarization) allows a change of electrical charges (negative to positive) that runs along the entire cell membrane. The neuron then returns to its resting electrical state, the resting potential, until stimulated again. The rate of neural transmission, however, is independent of the level of stimulation. That is, if the neural threshold is not reached, the neuron will not fire. If the threshold is reached or exceeded, the amplitude of the action potential is the same regardless of the level of stimulation.
The relationship between the level of stimulation and the production of a neural impulse is called the all or none principle. Once triggered, the action potential continues the length of the axon without diminishing because the action potential depends upon cell membrane permeability, a cell characteristic, and not upon the strength of the triggering stimulus.
After the action potential occurs, however, there is a short period of refractoriness, which affects neuron firing. During the first part of the refractory period (the absolute refractory period), the neuron will not fire again no matter how great the stimulation. The absolute refractory period lasts for only a short time. It is followed by the relative refractory period, during which a stronger than usual stimulus is required to trigger the action potential before the neuron returns to resting state. After the refractory period, the neuron will fire when the neural threshold is reached.
Synaptic transmission. The synapse is the name given the junction between neurons where information is exchanged. The action potential causes information to be transmitted from the axon of the first neuron (presynaptic neuron) to the dendrites or cell body of the second neuron (postsynaptic neuron) by secretion of chemicals called neurotransmitters. Neurotransmitters are stored in small containers (vesicles) located in knoblike structures (terminal buttons) on the axon tips. The axon of the presynaptic neuron does not actually touch the dendrites of the postsynaptic neuron and is separated from them by a space called the synaptic cleft. Stimulation of the presynaptic neuron to produce an action potential causes the release of neurotransmitters into the synaptic cleft. Most of the released neurotransmitters bind with molecules at special sites, receptors, on the dendrites of the postsynaptic neuron. (Molecules of the neurotransmitter that do not bind to receptors in the postsynaptic neuron are taken up again by the presynaptic neuron, a process called reuptake).
The combination of the neurotransmitter molecules to receptor cell molecules in the postsynaptic cell membrane produces a change of potential in the postsynaptic cell membrane called the postsynaptic potential (PSP). The PSP allows ions to enter or leave the cell membrane of the postsynaptic neuron. The ionic movements increase or decrease the probability of a neural impulse occurring in the postsynaptic neuron. There are two types of PSPs, excitatory (EPSPs) and inhibitory (IPSPs). EPSPs increase and IPSPs decrease the likelihood that the postsynaptic neuron will fire a neural impulse. The rate of firing of a neuron at a particular time depends upon the relative number of EPSPs and IPSPs.
Neurotransmitters. Many drugs, both therapeutic and recreational, work by affecting the level of neurotransmitters (the chemicals released at the axon terminal buttons of the presynaptic neuron), and some disorders are associated with neurotransmitter deficiencies or excesses. Neurotransmitters are of several types:
Acetylcholine occurs throughout the nervous system and is the only neurotransmitter found in synapses between motor neurons and voluntary muscle cells. (Degeneration of cells producing acetylcholine is associated with Alzheimer's disease.)
Biogenic amines include three neurotransmitters: norepinephrine, dopamine, and serotonin. Parkinson's disease is believed to be related to a deficiency of dopamine; certain types of depression are associated with low levels of norepinephrine; levels of serotonin increase with the use of the recreational drug LSD (lysergic acid diethylamide).
GABA (gamma aminobutyric acid) appears to produce only inhibitory PSPs. Many tranquilizers work by increasing the inhibitory actions of GABA.
Glycine is an inhibitory neurotransmitter found in the lower brainstem, spinal cord, and retina.
Endorphins modulate the activity of other neurotransmitters and are called neuromodulators. They seem to function in the same way as opiates such as morphine; “runner's high” is produced by an increase in endorphins.
Substance P is a neurotransmitter in many neural circuits involving pain.