Neurons

A neuron is a cell that transmits nerve impulses. It consists of the following parts, shown in Figure 1:

  • The cell body (soma or perikaryon) contains the nucleus and other cell organelles.

  • There are clusters of rough endoplasmic reticulum (not shown in Figure 1) that are called Nissl bodies or are sometimes referred to as chromatophilic substances.

  • The dendrite is typically a short, abundantly branched, slender process (extension) of the cell body that receives stimuli.

  • The axon is typically a long, slender process of the cell body that sends nerve impulses. It emerges from the cell body at the cone‐shaped axon hillock. Nerve impulses arise in the trigger zone, generally located in the initial segment, an area just outside the axon hillock. The cytoplasm of the axon, the axoplasm, is surrounded by its plasma membrane, the axolemma. A few axons branch along their lengths to form axon collaterals, and these branches may return to merge with the main axon. At its end, each axon or axon collateral usually forms numerous branches ( telodendria), with most branches terminating in bulb‐shaped structures called synaptic knobs (synaptic end bulbs, also called terminal boutons). The synaptic knobs contain neurotransmitters, chemicals that transmit nerve impulses to a muscle or another neuron.

figure 1. Parts of a neuron.

figure

Neurons can be classified by function or by structure. Functionally, they fall into three groups:

  • Sensory neurons ( afferent neurons) transmit sensory impulses from the skin and other sensory organs or from various places within the body toward the central nervous system (CNS), which consists of the brain and spinal cord.

  • Motor neurons ( efferent neurons) transmit nerve impulses from the CNS toward effectors, target cells that produce some kind of response. Effectors include muscles, sweat glands, and many other organs.

  • Association neurons ( interneurons) are located in the CNS and transmit impulses from sensory neurons to motor neurons. More than 90 percent of the neurons of the body are association neurons.

Neurons are structurally classified into three groups, as shown in Figure 1:

Multipolar neurons have one axon and several to numerous dendrites. Most neurons are of this type.

  • Bipolar neurons have one axon and one dendrite. They emerge from opposite sides of the cell body. Bipolar neurons are found only as specialized sensory neurons in the eye, ear, or olfactory organs.

  • Unipolar neurons have one process of emerging from the cell body that branches, T‐fashion, into two processes. Both processes function together as a single axon. Dendrites emerge from one of the terminal ends of the axon. The trigger zone in a unipolar neuron is located at the junction of the axon and dendrites. Unipolar neurons are mostly sensory neurons.

The following terms apply to neurons and groups of neurons:

  • A nerve fiber is an axon.

  • A nerve is a bundle of nerve fibers in the peripheral nervous system (PNS). Most nerves contain both sensory and motor fibers. Cell bodies are usually grouped into separate bundles called ganglia.

  • A peripheral nerve consists of three layers:

  • The epineurium is the outer layer that surrounds the entire nerve.

  • The perineurium surrounds bundles of axons. Bundles of axons are called fascicles. There could be 10 or more fascicles per nerve.

  • Surrounding each individual axon is the endoneurium.

  • A nerve tract is a bundle of nerve fibers in the CNS.