Elementary Structure of a Typical Neuron
Neurons vary considerably in size, shape and other features. However, most of them have some major features in common and these are described below (Figs. 1.1 to 1.4).
A neuron consists of a cell body that gives off a number of processes. The cell body is also called the soma or perikaryon. Like a typical cell it consists of a mass of cytoplasm surrounded by a cell membrane. The cytoplasm contains a large central nucleus (usually with a prominent nucleolus), numerous mitochondria, lysosomes and a Golgi complex (Fig. 1.2). In the past it has often been stated that centrioles are not present in neurons, but studies with the electron microscope (usually abbreviated to EM) have shown that centrioles are present. In addition to these features, the cytoplasm of a neuron has some distinctive characteristics not seen in other cells. 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) (Fig. 1.3). When examined with the EM, these bodies are seen to be composed of rough surfaced endoplasmic reticulum (Fig 1.2). 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.
Another distinctive feature of neurons is the presence of a network of fibrils permeating the cytoplasm (Fig. 1.4). Theseneurofibrils are seen, with the EM, to consist of microfilaments and microtubules. (The centrioles present in neurons may be concerned with the production and maintenance of microtubules).
Some neurons contain pigment granules (e.g., neuromelanin in neurons of the substantia nigra). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes).
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 characterised 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 that 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 addition to these differences in structure, there is a fundamental functional difference between dendrites and axons. In a dendrite, the nerve impulse travels towards the cellbody whereas in an axon the impulse travels away from the cell body.
We have seen above that the axon is free of Nissl granules. The Nissl-free zone extends for a short distance into the cell body: this part of the cell body is called the axon hillock. The part of the axon just beyond the axon hillock is called the initial segment(Fig. 1.2).
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. The nature of this sheath isbest understood by considering the mode of its formation (Fig. 1.5). An axon lying near a Schwann cell (1) invaginates into the cytoplasm of the Schwann cell (2,3). 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 (3). 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 (4,5). Lipids are deposited between adjacent layers of the membrane. These layers of the mesaxon, along with the lipids, form the myelin sheath. Outside the myelin sheath athin layer of Schwann cell cytoplasm persists to form an additional sheath which is called the neurilemma (also called the neurilemmalsheath or Schwann cell sheath). Axons having
a myelin sheath are called myelinated axons. The presence of a myelin sheath increases the velocity of conduction (for a nerve fibre of the same diameter). 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 (Fig. 1.6). 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.
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 (Fig. 1.7). Another difference is that several such axons may invaginate into the cytoplasm of a single Schwann cell.
An axon may give off a variable number of branches (Fig. 1.1). Some branches, that arise near the cell body and lie at right angles to the axon are called collaterals. At its termination the axon breaks up into a number of fine branches called telodendria that may end in small swellings (terminalboutons or bouton terminaux). An axon (or its branches) can terminate in two ways. Within thecentral nervous system, it always terminates by coming in intimate relationship with another neuron, the junction between the two neurons being called a synapse. Outside the central nervous system, the axon may end in relation to an effector organ (e.g., muscle or gland), or may end by synapsing with neurons in a peripheral ganglion.
Axons (and some dendrites that resemble axons in structure:) constitute what are commonly called nerve fibres.