The brain is the most intriguing part of the body and the part about which least is known. In explanations of how it works, the brain is frequently compared to a telephone exchange or – in the modern age – a computer. This, however, is a mistake. The human memory has a storage capacity which is almost two million times greater than a sizeable home computer. Although a fair amount is now known about the structure of the brain, very little is known about how it works. For example, no one has ever been able to demonstrate that the brains of intelligent people are different from those of the less intelligent. Researchers cut Einstein’s brain into slices in search of a larger number or different brainthan in the brains of ordinary people, but it has not yet been possible to demonstrate a convincing difference. It is known that the size of the brain is not related to intelligence. It has also been found that we can do without a fairly large part of our brain, sometimes even without adverse phenomena occurring.
Nerve, the cells which make up the greater part of our brain, have a very specialized function. They form an extremely complicated network in which they pass on endless quantities of information to each other.
How specialized these cells are is apparent from the fact that they can not be replaced; once all the cells have been formed after birth they stop multiplying permanently.
For the human body to work as a complete unit there must be communication between individual cells. This communication can be fast or slow and over short or long distances. Adjacent cells can pass on information slowly by mechanisms that rely on direct contact between them. Another relatively slow form of communication occurs whencirculating in the stream carry chemical messages to remote cells. Rapid communication in the human body depends on the special properties of excitable cells. These cells carry out specialized tasks when they receive the appropriate stimulus. The nervous system is made up of excitable cells called neurons, and it is this communications system that detects and transmits information concerning changes both within the body and in the outside world, enabling the body to make suitable responses.
All cells in the body – including neurons – have a cell membrane which separates the fluid outside the cell from its inside. This membrane has the property of maintaining certain ions within the cell in different concentrations from those outside the cell. Ions, such as sodium, potassium and bicarbonate, carry electrical charges, so the different concentrations of ions within and without the cell lead to a potential gradient (voltage difference) across the membrane. The inside of the cell is negatively charged relative to the outside. The membrane is said to be polarized, and the potential gradient across the cell membrane is known as the ‘resting membrane potential’. To maintain this state the cell expends energy. In excitable cells, certain stimuli alter the ability of the cell to maintain the different concentrations of ions on each side of the membrane. Such a stimulus causes ions to flow across the membrane, leading to a change in the resting potential.
When this occurs at one site on the membrane surface a potential gradient is created between that site, which is said to have been depolarized, and adjacent sites on the membrane. The membrane restores the electrical balance by allowing ions to pass through it and restoring the unequal concentrations of ions on each side. Depolarization lasts for a short time. If a sufficiently great stimulus is applied to the membrane, however, the process of depolarization spreads farther afield and the resulting change in the electrical charge on the membrane can produce a specialized response from the cell. In muscle cells, for example, the spread of depolarization over the surface of the cell membrane results in contraction of the cell. In the excitable cells of the nervous system, the neurons, an adequate stimulus causes a self-perpetuating spread of depolarization known as an action potential. This leads in turn to an electrical impulse travelling along the length of the nerve cell. This is the basis of communication within the nervous system.
The sodium pump
The key to the production of an electrical nerve impulse is the movement of ions through the cell membrane that forms the outer ‘wall’ of a neuron. Many years of research were devoted to finding out how this happens, culminating in the award of the 1963 Nobel Prize in physiology and medicine to two British scientists, Alan Hodgkin and Andrew Huxley, who finally unravelled the puzzle. Working with micro-electrodes implanted into nerve cells from the giant squid, they discovered the basic mechanism that has become known as the sodium pump. At rest, the potential across the cell membrane (the resting potential) derives mainly from the presence of potassium ions inside the cell and sodium ions outside. Then as a nerve impulse passes, the membrane’s permeability to sodium ions increases so that these ions flood into the cell, pushing potassium ions out. Once the impulse has passed, the pumping mechanism restores the ions to their former positions and concentrations, ready for the next impulse. This takes about a two-thousandth of a second, placing a maximum limit on the speed of nerve transmission of about 500 pulses per second. The ‘holes’ or channels through which the ions pass through the membrane are thought to be controlled by ‘gates’ which open and close at the appropriate times. The ‘door’ of the gates consists of a protein molecule which, during the resting phase, blocks the entrance. But a small electric current which just precedes the arrival of a nerve impulse makes the protein change shape, thus opening the door and letting ions through. Another protein, this time an enzyme, is thought to act as a carrier for the ions. It ‘picks up’ a sodium ion outside the cell and transports it through the gate; it then carries a potassium ion on the inside back out through the gate. It is interesting to contemplate that, as you read this web page, millions of ions are passing in and out of the cells along your optic nerve, carrying visual messages to your brain.
Although different neurons are required to perform specialized tasks within the nervous system, and differ accordingly in size and shape, they all have many features in common. The neuron is different from other body cells in having long, branch-like extensions of the cell body. These extend for varying distances from the cell body to make contact with other cells. Each neuron has a single axon which conducts electrical impulses from the cell body and makes contact with other cells. Axons can range in length from one hundredth part of a millimetre to more than a metre. Each neuron also has one or more branch-like extensions called dendrites which conduct electrical impulses towards the cell body.
Neurons are subdivided into three groups. A unipolar neuron has a single filament giving rise to an axon and a dendrite; a bipolar neuron has a single axon and dendrite arising from opposite sides of the cell body; and the multi-polar neuron has a single axon arising on the opposite side of the cell body from two or more dendrites. Unipolar and bipolar neurons are specially adapted to carry sensory information from the periphery of the body, the reason why they are called receptor neurons, and their axons make contact with other neurons within the nervous system. While the unipolar and bipolar receptor neurons bring information into the nervous system, multi-polar neurons are responsible for the onward transmission of information within the nervous system, and for carrying instructions back to the periphery of the body. Neurons that relay information within the nervous system are known as interneurons, whereas neurons which carry instructions to the muscles and glands are called effector neurons. The greater the number of dendrites arising from a neuron, the greater surface area it presents for axons of other neurons to make contact with it. Axons may branch along their course, and each branch finally divides into a number of finer branches which make contact with other cells. This arrangement of interconnecting neurons provides the basis for a communications system of vast complexity.
Electrical impulses are not conducted directly from neuron to neuron. The area where one axon makes contact with the membrane of another neuron is known as a synapse. There is a small gap between the two membranes and, when an electrical impulse arrives at the end of the axon, small quantities of chemical substances known as neurotransmitters are released.
Neurotransmitters alter the resting potential of the receiving membrane. This can result in the depolarization of the membrane, and the synapse is then said to be an excitatory synapse. Alternatively, the effect of the neurotransmitter may be to increase the polarization of the membrane, making it resistant to depolarization (it is then known as an inhibitory synapse). Whether or not the cell membrane of a neuron becomes sufficiently depolarized to cause the action potential to pass down the axon depends on the sum of all the excitatory and inhibitory stimuli applied to it. A single neuron releases a specific neurotransmitter and a growing number of these substances such as acetylcholine, adrenaline, dopamine and serotonin are being recognized and chemically analysed. Some debilitating nervous disorders, such as Parkinsonism, are thought to result from an inbalance of neurotransmitters in the patient’s brain. A similar mechanism operates at the point of contact between a motor neuron and the muscle that it stimulates. The release of neurotransmitters at the neuromuscular junction causes depolarization of the membrane of the muscle cell. This in turn causes specialized protein filaments to contract, with the result that the muscle cell shortens.
Normally the neurotransmitter that is released in the synaptic gap is taken away by enzymes, for example cholinesterase. It is possible to influence the action of those enzymes – and thereby the transmission of impulses – using various drugs. Many modern anaesthetics work in this way: injection of such chemicals affects the nerves so that they are not able to transmit impulses any more. Thus muscles become paralyzed, which is necessary for many surgical techniques. Insect sprays may also contain chemicals that block the transmission of nerve impulses. In large doses the sprays may be harmful to humans too.
Sensory and motor neurons
We recognize changes in our bodies, and in our external environment, through specialized cells known as sensory receptors. The structure of these cells allows them to react to specific stimuli, such as changes in temperature, touch, injury to the skin, sound waves, light waves, vibration or the stretching of muscles. They respond by signalling this information to the rest of the nervous system, which usually passes it on to the brain for interpretation or action. Many receptors are neurons whose dendrites are distributed to the skin or other, but some are cells from outside the nervous system which make contact with the various sensory neurons.
Receptors are divided into three broad groups, named according to where they function. Those that recognize changes taking place in the external environment such as temperature, light, sound and touch are called exteroceptors; those that recognize mechanical changes such as the stretching of muscles or the position of joints are proprioceptors; and those responding to chemical and pressure changes within the body, such as the acidity or pressure of thein arteries, are interoceptors.
The peripheral endings of sensory neurons are sometimes associated with organs that transform specific stimuli in such a way that the stimulus is recognized by the sensory neurons. The skin contains such organs, called corpuscles, which aid the recognition of pressure and vibration. Muscles contain spindle-shaped organs, known as stretch receptors, that help to alert the nervous system when the muscle is stretched. If a sensory neuron is sufficiently stimulated, an electric current passes along the dendrite towards the cell body and then, by the axon, to its contacts within the nervous system.
Motor neurons arise within the brain and spinal cord and carry instructions to the muscles and glands. Their axons sometimes extend for long distances before they reach their target cells. For example, some axons run for more than a metre, from the lower end of the spinal cord to the foot.