There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous system, perform four functions:
The other cells, neurons, act as the communicators of the nervous system. Neurons receive information, integrate it, and pass it along. They communicate with one another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
In 1952, physiologists Alan Hodgkin and Andrew Huxley made some important discoveries about how neurons transmit information. They studied giant squid, whose neurons have giant axons. By putting tiny electrodes inside these axons, Hodgkin and Huxley found that nerve impulses are really electrochemical reactions.
Nerves are specially built to transmit electrochemical signals. Fluids exist both inside and outside neurons. These fluids contain positively and negatively charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions constantly cross into and out of neurons, across cell membranes. An inactive neuron is in the resting state. In the resting state, the inside of a neuron has a slightly higher concentration of negatively charged ions than the outside does. This situation creates a slight negative charge inside the neuron, which acts as a store of potential energy called the resting potential. The resting potential of a neuron is about –70 millivolts.
When something stimulates a neuron, gates, or channels, in the cell membrane open up, letting in positively charged sodium ions. For a limited time, there are more positively charged ions inside than in the resting state. This creates an action potential, which is a short-lived change in electric charge inside the neuron. The action potential zooms quickly down an axon. Channels in the membrane close, and no more sodium ions can enter. After they open and close, the channels remain closed for a while. During the period when the channels remain closed, the neuron can’t send impulses. This short period of time is called the absolute refractory period, and it lasts about 1–2 milliseconds. The absolute refractory period is the period during which a neuron lies dormant after an action potential has been completed.
Neural impulses conform to the all-or-none law, which means that a neuron either fires and generates an action potential, or it doesn’t. Neural impulses are always the same strength—weak stimuli don’t produce weak impulses. If stimulation reaches a certain threshold, or minimum level, the neuron fires and sends an impulse. If stimulation doesn’t reach that threshold, the neuron simply doesn’t fire. Stronger stimuli do not send stronger impulses, but they do send impulses at a faster rate.
The gap between two cells at a synapse is called the synaptic cleft. The signal-sending cell is called the presynaptic neuron, and the signal-receiving cell is called the postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the terminal buttons. When an action potential reaches the terminal buttons, which are at the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft. When they reach the postsynaptic cell, neurotransmitter molecules attach to matching receptor sites. Neurotransmitters work in much the same way as keys. They attach only to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites on the postsynaptic cell can be excitatory or inhibitory:
Unlike an action potential, a PSP doesn’t conform to the all-or-none law. At any one time, a single neuron can receive a huge number of excitatory PSPs and inhibitory PSPs because its dendrites are influenced by axons from many other neurons. Whether or not an action potential is generated in the neuron depends on the balance of excitation and inhibition. If, on balance, the voltage changes enough to reach the threshold level, the neuron will fire.
Neurotransmitter effects at a synapse do not last long. Neurotransmitter molecules soon detach from receptors and are usually returned to the presynaptic cell for reuse in a process called reuptake.