Nervous coordination enables an organism's rapid response to an external or internal stimulus. Characteristic of animals only, nervous coordination is the function of the nervous system. The receptors for nervous coordination are generally located in the sense organs at the body surface, while the response in nervous coordination generally involves a gland or muscle. The function of coordination is accomplished by means of a set of signals conducted along a series of nerve cells.
Animal nerve cells are specialized cells called neurons. Depending upon function, these cells can be divided into sensory neurons, interneurons, and motor neurons. These three types of nerve cells coordinate with each other to receive external stimuli and to transmit the impulse to muscles or glands of the body for an appropriate response to the stimulus.
The neuron is the nerve cell. (Approximately 12 billion neurons exist in the human body, the great majority of them in the brain and spinal cord.) The main portion of the neuron is the cell body. Protruding from the cell body are one or more short extensions called dendrites and one long extension called the axon. Axons are covered by a fatty layer of material called the myelin sheath. Bundles of axons bound together are referred to as a nerve.
There are three types of neurons in animals: sensory neurons, interneurons, and motor neurons. Sensory neurons receive stimuli from the external environment;interneurons (or association neurons) connect sensory and motor neurons and carry stimuli in the brain and spinal cord; motor neurons transmit impulses from the brain and spinal cord to the muscle or gland that will respond to the stimulus. The neurons are supported, protected, and nourished by cells of the nervous system known as glial cells. Together with extracellular tissue, the glial cells make up the neuroglia.
The nerve impulse is an electrochemical event that occurs within the neuron. In an inactive neuron, the cytoplasm is negatively charged with respect to the outside of the cell. This difference in electrical charge is maintained by the active transport of sodium ions out of the cytoplasm. A cell in this state is said to have a resting potential, and it is polarized.
A nerve impulse is generated when the difference in electrical charge disappears. This occurs when a stimulus contacts the tip of a dendrite and increases the permeability of the cell membrane to sodium ions. The ions rush back into the cytoplasm, and the difference in electrical charges disappears. This creates a pulse of electrochemical activity called the nerve impulse. A neuron displaying a nerve impulse is said to have an action potential. The cell is depolarized.
More specifically, the influx of sodium ions into the neuron cytoplasm activates the adjacent portion of the cell membrane to admit sodium ions also. Successively, the adjacent areas of the neuron lose their differences of electrical charge, and a wave of depolarization is generated in the neuron. This wave of depolarization is the nerve impulse. After the wave of depolarization has passed, the neuron reestablishes the difference in charges by pumping potassium ions out of the cytoplasm, and then pumping sodium ions in.
The nerve impulse passes down the dendrite, through the cell body, and down the axon. At the end of the axon, the impulse encounters a fluid-filled space separating the end of the axon from the dendrite of the next neuron or from a muscle cell. This space is the synapse. A synapse located at the junction of a neuron and muscle fiber is a neuromuscular junction.
As the impulse reaches the end of the axon, it induces changes in the cell membrane and the release of chemical substances called neurotransmitters, for example, acetylcholine. Molecules of neurotransmitter accumulate in the synapse and increase the membrane permeability of the next dendrite. This causes an influx of sodium ions, and a new nerve impulse is generated. After the nerve impulse has swept down the next dendrite, the neurotransmitters in the synapse are destroyed.
The reflex arc, the simplest unit of nervous activity, involves the detection of a stimulus in the environment by sensory nerve endings, followed by impulses that travel via the sensory neurons to the spinal cord. Here the impulses synapse with interneurons, and the interneurons generate impulses to respond to the stimulus. The impulses travel along the motor neurons to muscles or glands that respond appropriately.
In some cases, a reflex arc involves an interpretation. For this activity, interneurons transmit impulses up the spinal cord to the conscious area of the brain, where an analysis occurs.