Nerves

 
Nerves take many forms. They may be short fibres within the brain, for instance, or they may extend for the length of the spine. What are usually thought of as nerves are in fact bundles of fibres called axons extending from the cell bodies of specialised cells called neurons.

A sketch of a hypothetical neuron is shown above. From the cell body can be seen a long strand, the axon, which with some nerves, like those in the spine, may be very long. The axon ends in a multitude of synapses, or synaptic terminals, which form junctions with the dendrites, or the cell bodies, of other neurons. In effect, axons are transmitters, dendrites are receivers. Collectively, axons, dendrites and synapses are often called neurites, just to confuse matters further.

All cells carry an electrical charge of -70 millivolts relative to their surrounding fluid, because of the difference in chemical concentrations inside and outside of the cell membrane. Neurons have the unique property that, if some part of an axon is given a positive charge, a voltage pulse travels along it towards the synaptic terminals. This is how nerves communicate.

Where the axon leaves the cell body of a neuron is an area called the axon hillock. If a positive voltage develops at this point, it generates a pulse which travels along the axon towards the synapse. If the voltage remains at the hillock, it generates a train of pulses, thus the magnitude of the signal is expressed in the length of the burst of several pulses. At most synapses, there is a gap. Succeeding electrical pulses cause the release of discrete quantities of chemical molecules called neurotransmitters, which 'key in' to the membrane on the opposite side. This causes an electrical charge to be transferred to the next neuron. Thus the signal is graded in discrete steps. Some synaptic inputs are excitatory, some are inhibitory. If there are a number of inputs to the neuron they are summed algebraically.

This is where the present-day tendency to think of the brain as a computer breaks down. Individual elements in the nervous system can process graded inputs and outputs, in contrast to the on/off states in digital computers. Adding to the complexity is that there are a number of different neurotransmitters. Different synapses on various neurons may be sensitive to different neurotransmitters, or individual synapses may be responsive to more than one neurotransmitter.

The nervous system is a complex and beautiful network of connections, which makes the Internet look like a pocket calculator. Yet this immensely powerful device uses only about 150 watts of power.

At one time it was thought that the growth of the nervous system was fixed, presumably by the genes. It became apparent however that there is not enough code to programme the intricate multitude of interconnections.

As the fetus grows, neurons appear and migrate towards their final positions and multiply many times. Both before and after birth, many neurons grow and may die off again. There comes a time when they stop dividing and it is reasonable to say that the neurons one has a baby are what one will have for a lifetime. The only known exception are the neurons that form the olfactory sensors in the nose, that are prone to damage and can regenerate.

Axons and dendrites appear and gradually form their final connections. Many thousands may form and disappear again, extending to their target organs, not by some predetermined code, but by subtle guidance signals produced by the target and at intermediate stages on their path. The result is that the nervous system of each individual is precisely optimised, such that no two people will be alike.

Development.

Within a few hours from conception, the embryo has multiplied to some 10,000 cells. At about this time, a complex movement occurs whereby it rearranges itself into three concentric layers. At the centre is the endoderm which will become the internal organs and, around this, is the mesoderm, which will form the skeleton and muscles. The outer layer is the ectoderm which will mainly form the skin.

At one side, a depression appears in the ectoderm, which deepens, then closes to form the neural tube, which will later contain the spine of the developing embryo. Adjacent to the tube where it meets the outer surface a group of cells appear, the neural crest.

These divide and proliferate and then begin to migrate in very precise ways to their allotted position. They continue dividing for a time, aggregating and forming tissues that will become the structures of the nervous system. At some point, each individual structure reaches a point of maturation after which no more neurons are likely to be produced during the life of the animal.

While there is little, initially, to distinguish the individual cells, they develop in terms of the function of the structure that they will be part of, whether it is the motor unit of a muscle, a sensory neuron in the skin, or a structure in the brain.

What happens next is an intricate process that we know a great deal about in general, yet very little at the level of specific cells. Each neuron begins to extend axons, much like an amoeba extending pseudopodia. Somehow the axon detects its particular target cell, towards which it extends, aided by chemical guidance cues along the way.

Each neuron may extend many axons, and each axon branches many ways. The many axons, in effect, compete for synaptic space on their target cells. Once an axon makes a firm connection it stabilises. The way this happens is not fully understood, but it is believed that the activity in the synapse causes a chemical messenger to travel back up the axon to the cell body.

The remaining axons may retract, or they may sometimes become what are known as 'silent synapses.' The bald statement in many popular works that "many axons grow and then die away" sounds dreadfully wasteful, but it is not. It means that each individual's nervous system is precisely optimised to take account of physical differences in the body's geometry - the distance apart of the eyes, for instance.

As the axons mature, cells appear which wrap a coating of myelin around them. An electrical insulator, this has the effect of making them more efficient.

Growth in the mature nervous system.

There is a general assumption that myelinisation indicates maturity in a nerve, and probably signals the end of axon and dendritic development.

It also used to be thought that nerves severed in an injury could never repair themselves. More recently, it has been realised that if the myelin sheath is intact, the severed part remote from the cell body disappears, while the part of the axon connected to the cell body will extend through the myelin sheath and reconnect to its target.

The problem arises if the myelin sheath is cut. The axon tries to grow out but has nothing to guide it. Some success has been achieved in microsurgery, in developing artificial materials to guide the axon as it grows across the gap, but a large nerve fibre may have tens or hundreds of axons as small as 0.1 mm in diameter.

We know virtually nothing of the day-to-day dynamics of the nervous system. Investigation of the human brain was generally through post-mortem tissue samples, using light or electron microscopes, electroencephalograms, or ad hoc investigations during brain surgery. Even though the modern PET and MRI scanners have extended our knowledge enormously, their resolving power is too low to distinguish individual neurons, let alone axons.

There have been hints of growth and decay of neurites throughout life, especially in some of the brain diseases, the dementias. One study has also tracked the cyclic growth and decay of neurites in the brain of female rats through their reproductive cycle.

Thus while the neurons we are born with may be all we will have, the same cannot be said of their connections.

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