Neuron Structure

A neuron is a nerve cell. The nucleus of a neuron and most of its organelles are located in the cell body. Extending from the cell body in different directions are membrane-covered extensions of the cell called dendrites. Dendrites receive information from other neurons or other cells and carry the information toward the cell body. Another cell extension, the axon, is a long, membrane-bound projection that transmits information away from the cell body in the form of electrical signals called action potentials. A neuron may have a single axon or branching axons that contact several other cells. The end of an axon is called the axon terminal. It may contact and communicate with a muscle cell, a gland cell, or another neuron.

The axons of many neurons are covered with a lipid layer known as the myelin (MIE-uh-lin) sheath. The myelin sheath insulates the axon much as a rubber coating insulates an electrical cord. The myelin sheath speeds up transmission of action potentials along the axon. In neurons that are not part of the brain or spinal cord, cells called Schwann cells surround the axon and produce myelin. Gaps in the myelin sheath along the length of the axon are called nodes of Ranvier (RAHN-vee-ay).

Neurons communicate with other neurons and other cells at special junctions called synapses. Neurons usually do not touch each other or other cells. Instead, a small gap, called a synaptic cleft, is present between the axon terminal and the receiving cell. At a synapse, the transmitting neuron is called a presynaptic neuron. The receiving cell is called a postsynaptic cell.

Electrical activity in the neuron usually causes the release of chemicals called neurotransmitters into the synaptic cleft. Often, neurotransmitters cause electrical activity in another neuron. Thus, the signaling activity of the nervous system is made up of electrical activity within neurons and chemical flow between neurons.

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