Nerve Cells

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I. Neurons/nerve cells A neuron is a cell specialized to conduct electrochemical impulses called nerve impulses or action potentials. Neuron is the main cellular component of the nervous system, a specialized type of cell that integrates electrochemical activity of the other neurons that are connected to it and that propagates that integrated activity to other neurons. They are the basic information processing structures in the CNS. There are as many as 10,000 specific types of neurons in the human brain, A. Types of Neurons a. Motor neurons >These transmit impulses from the central nervous system to the * muscles and * glands that carry out the response. >Most motor neurons are stimulated by interneurons, although some are stimulated directly by sensory neurons. b. Sensory neurons These run from the various types of stimulus receptors, e.g., to the central nervous system (CNS), the brain and spinal cord. * touch * odor * taste * sound * vision c. Interneurons Interneurons >are found exclusively within the spinal cord and brain. They are stimulated by signals reaching them from * sensory neurons * other interneurons or * both. > also called association neurons. B. Structure of Neurons a. Processes 1. Dendrite Dendritic connections are the basic receiving stations by which neurons form the signaling networks that constitute the brain's circuitry. 2. AxonsAll neurons outside the central nervous system (and many within it) conduct impulses along hairlike cytoplasmic extensions, the nerve fibers or axons. The axons connecting your spinal cord to your foot can be as much as 1 m long C. Development of Neurons A. life span D. Kinds of Neurons II. Nerve impulse ConductionFirst, an action potential is generated near the cell body portion of the axon III. Nerve impulse transmission A. Neurotransmitters a. Excitatory AcH b. Inhibitory AcH B. Synaptic transmission A. Synapse A synapse is where two neurons communicate electrically or chemically. A synapse is the junction point between two neurons. |

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Nerve Cells

Nerve cells consist of a body, with branches at one end. The branches are called axons. The axons are positioned near an adjacent nerve or a muscle. Nerve impulses pass from the axons of one nerve to the next nerve or muscle. The impulse transmission speed can be reduced in damaged nerves.

Surrounding a nerve is a tough protective coat of a material called myelin. Nerve damage can involve damage or loss of myelin, damage to the nerve body, or damage to the axon region. The nerve conduction study, which was devised in the 1960s, can detect the loss of nerve function due to these injuries, and, from the nature of the nerve signal pattern that is produced, offer clues as to the nature of the problem.

Depending on the nature of the nerve damage, the pattern of signal transmission can be different. For example, in a normal nerve cell, sensors placed at either end of the cell will register the same signal pattern. But, in a nerve cell that is blocked somewhere along its length, these sensors will register different signal patterns. In another example, in a nerve cell in which transmission is not completely blocked, the signal pattern at the axon may be similar in shape, but reduced in intensity, to that of the originating signal, because not as much of the signal is completing the journey down the nerve cell.

Diseases of the nerve itself mainly affect the size of the responses (amplitudes); diseases of the myelin mainly affect the speed of the responses.

Nerve conduction studies are now routine, and can be done in virtually any hospital equipped with the appropriate machine and staffed with a qualified examiner. The nerve conduction study utilizes a computer, computer monitor, amplifier, loudspeaker, electrical stimulator, and filters. These filters are mathematical filters that can distinguish random, background electrical signals from the signal produced by an activated nerve. When the study is done, small electrodes are placed on the skin over the muscles being tested. Generally, these muscles are located in the arms or legs. Some of the electrodes are designed to record the electrical signal that passes by them. Other electrodes (reference electrodes) are designed to monitor the quality of the signals to make sure that the test is operating properly. If monitoring of the test is not done, then the results obtained are meaningless.

After the electrodes are in place, a small electrical current can be applied to the skin. The electrical stimulation is usually done at several points along the nerve, not just at a single point. This is done because conduction of an impulse through a nerve is not uniform. Some regions of a nerve conduct more slowly than other regions. By positioning the stimulating electrodes at several sites, a more accurate overall measurement of conduction velocity is obtained.

The electrical current activates nerves in the vicinity, including those associated with the particular muscle. The nerves are stimulated to produce a signal. This is known as the "firing" of the nerve. The nerve signal, which it also electric, can be detected by some of the electrodes and conveyed to the computer for analysis.

The analysis of the nerve signal involves the study of the movement of the signal through the nerve and from the nerve to the adjacent muscle. Using characteristics such as the speed of the impulse, and the shape, wavelength, and height of the signal wave, an examiner can assess whether the nerve is functional or defective.

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Mechanism of impulse transmission

A nerve impulse travels through a nerve in a long, slender cellular structure called an axon, and it eventually reaches a structure called the presynaptic membrane, which contains neurotransmitters to be released in a free space called the synaptic cleft. Freely flowing neurotransmitter molecules are picked up by receptors (structures that appear on cellular surfaces that pick up molecules that fit into them like a "lock