Neurons.

Neuroglia.

Neuron

A neuron consists of a cell body which gives off a variable number of processes. The cell body is also called the soma or perikaryon. The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes (one axon and several dendrites) and the cell body is, therefore, multipolar. Some neurons have only one axon and one dendrite and are bipolar. Another type of neuron has a single process (axon), this neuron is unipolar. Some neurons have a single process which after a very short course divides into two (axon and dendrite), those neurons are pseudounipolar.

Like a typical cell neuron consists of a mass of cytoplasm surrounded by a cell membrane. The cytoplasm contains a large central nucleus (usually with prominent nucleolus), numerous mitochondria, lysosomes and a Golgi complex. Centrioles are also present.

The cytoplasm of a neuron has some distinctive charactertistics not seen in other cells:

1. The cytoplasm shows the presence of a granular material that stains intensely with basic dyes; this material is the Nissl substance (also called Nissl bodies or granules). These bodies are composed of rough endoplasmic reticulum. The presence of abundant granular endoplasmic reticulum is an indication of the high level of protein synthesis in neurons. The proteins are needed for maintenance and repair, and for production of neurotransmitters and enzymes.

2. The cytoplasm is permeated by a network of fibrils (neurofibrils). The neurofibrils consist of microfilaments and microtubules.

The processes arising from the cell body of a neuron are called neurites. These are of two kinds. Most neurons give off a number of short branching processes called dendrites and one longer process called an axon.

The dendrites are characterized by the fact that they terminate near the cell body. They are irregular in thickness, and Nissl granules extend into them. They bear numerous small spines which are of variable shape.

The axon may extend for a considerable distance away from the cell body. The longest axons may be as much as a metre long. Each axon has a uniform diameter, and is devoid of Nissl substance.

In a dendrite, the nerve impulse travels towards the cell body whereas in an axon the impulse travels away from the cell body.

Myelinated fibre

During its formation each axon comes to be associated with certain cells that provide a sheath for it. The cells providing this sheath for axons lying outside the central nervous system are called Schwann cells. Axons lying within the central nervous system are provided a similar covering by a kind of neuroglial cell called an oligodendrocyte. An axon lying near a Schwann cell invaginates into the cytoplasm of the Schwann cell. In this process the axon comes to be suspended by a fold of the cell membrane of the

Schwann cell: this fold is called the mesaxon. In some situations the mesaxon becomes greatly elongated and comes to be spirally wound around the axon, which is thus surrounded by several layers of cell membrane. These layers of the mesaxon, along with the lipids, form the myelin sheath. Outside the myelin sheath a thin layer of Shwann cell cytoplasm persists to form an additional sheath which is called the neurilemma.

Axons having a myelin sheath are called myelinated axons. The presence of a myelin sheath increases the velocity of conduction It also reduces the energy expended in the process of conduction.

An axon is related to a large number of Schwann cells over its length. Each Schwann cell provides the myelin sheath for a short segment of the axon. At the junction of any two such segments there is a short gap in the myelin sheath. These gaps are called the nodes of Ranvier.

Nonmyelinated fibre

There are some axons that are devoid of myelin sheaths. These unmyelinated axons invaginate into the cytoplasm of Schwann cells, but the mesaxon does not spiral around them. Several such axons may invaginate into the cytoplasm of a single Schwann cell.

Neurons vary considerably in the size and shape of their cell bodies and in the length an manner of branching of their processes. The cell body varies in diameter from about five micrometers, in the smallest meurons, to as much as one hundred and twenty micrometers in the largest ones. The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes and the cell body is,therefore, multipolar. Some neurons have only one axon and one dendrite and are bipolar.

Another type of neuron has a single process (which is highly convoluted). After a very short course this process divides into two. One of the divisions represents the axon; the other is functionally a dendrite, but its structure is indistinguishable from that of an axon. This neuron is pseudounipolar.

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