III. Neurons: Cells of the Nervous System
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:
Role of Myelin
People with multiple sclerosis have difficulty with muscle control because the myelin around their axons has disintegrated. Another disease, poliomyelitis, commonly called “polio,” also damages myelin and can lead to paralysis. Communication Between Neurons 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. The Resting Potential 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. The Action Potential 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. The All-or-None Law 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 Synapse 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 When electrical impulses reach the axon terminal, they stimulate the release of chemical messengers called neurotransmitters that cross the synaptic cleft, also called the synaptic gap. After these molecules traverse the tiny synaptic gap (cleft) between neurons, they bind to receptor sites on neighboring neurons, thus passing on their excitatory or inhibitory messages. The sending neuron, in a process called reuptake, normally absorbs the excess neurotransmitter molecules in the synaptic gap. |
Vocabulary to Learn Today
Glial cells Neurons Soma Dendrite Axon Nerves Myelin Sheath Terminal Buttons Neurotransmitters Synapse Ions Resting State Resting Potential Action Potential Absolute Refractory Period All-or-None Law Synaptic Cleft/ Gap Presynaptic Neuron Postsynaptic Neuron Neurotransmitters Reuptake |
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