Nerve Impulse
A nerve impulse is a message which is transmitted along a nerve fiber or axon. It originates in the neuron's soma, or cell body, passes into the axon hillock, and then speeds down along the axon until it reaches the axon terminal or synaptic knobs. The impulse must then make its way across a gap called the synapse, to stimulate another neuron's or muscle cell's soma.
The nerve impulse occurs due to changes in the electrical charge along the axon. Normally, at rest, the chemical makeup of the axon's membrane allows the inside of the membrane to be slightly more electrically negative compared to the slightly more positive outside of the membrane. This condition is maintained by the presence of various channels or pumps within the membrane which work to maintain a particular proportion of potassium and sodium molecules on the inside and the outside of the neuronal membrane.
When a nerve is stimulated, however, this potassium-to-sodium proportion is changed, so that the inside of the axon's membrane becomes more positive than the outside. When this positive charge reaches a particular level, called the threshold, the charge is conducted down the axon. This is called the action potential.
A nerve impulse traveling down an axon moves at a rate of about 0.5 meters per second (slightly greater than one mile per hour). Some axons, however, are wrapped with layers of specialized cells. These cells have fatty membranes which line the outside of the axons, creating what is called a myelin sheath. Small interruption occurring about every millimeter along the myelin sheath are called nodes of Ranvier. These myelinated axons can transmit nerve impulses much more quickly, at a rate of about 130 meters per second (nearly 300 miles per hour). This is accomplished because the action potential skips over the myelinated area of the axon, jumping rapidly from node to node. This type of nerve impulse is called saltatory conduction or saltatory propagation.
The nerve impulse continues down the axon until it reaches the axon terminal, where the synaptic knobs are located. The electrical charge reaching this location causes the synaptic knobs to unload little packets of chemicals (neurotransmitters) into a gap (synapse) which exists between the axon terminal and the next neuron's soma. These neurotransmitters cross the synapse, heading toward the new neuron. The arrival of these chemicals at the next neuron's soma thus stimulates an action potential which runs down its axon.
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