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Language of the Brin
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encoded in the nervous system by having a different nerve-cell and nerve-fibre for each quality. That is why we bave such a vast number of nerve-cells. The brain is csscatially a multichannel system. Its method of coding is to make up messages by putting each item of information into a separate channel.. 4. The Synapses and Neuro-transmitters
Each cell of the brain or spinal cord called neuron has a number of receptive branches, the dendrites, and a single outgoing fibre, the axon. The dendrites are spread through a limited volume around the cell body and they serve to initiate activity in the cell so that it sends signals along its axon, reaching to a greater or lesser distance. Each fibre entering the nervous system from a sense-organ branches many times and makes connections with many cells in the spinal cord or brain. Conversely, each central cell receives the endings of many incoming fibres and of the fibres arising from cells ekcwbere within the brain. The points where the endings of the aXon of onc cell meet the dendrites of another are known as synapses. At the synapse, the membranes of the two nerve-cells are pressed against each other, but the contents of the fibres are not continuous. There is, therefore, a barrier between the inside of incoming (pre-synaptic) fibre and that of the post-synaptic cells. This barrier makes it impossible (usually) for the electrical nerve-impulse to spread from one to the other. The transmission is effected by the release from the pre-synaptic fibre of a chemical, such as acetylcholine, to which the post-synptic surface is especially sensitive. The response to the chemical serves to amplify the effect of impulses arriving, so that they influence the post-synaptic cell.
These chemical transmitters have become well-known only in recent years. They are found in the knobs (known as boutons) at the ends of the pre-syaptic fibres. There may be up to 50,000 of these little knobs attached to the surface of single cell of the cerebral cortex. They will include branches of several different incoming nerve-fibres. The currents produced under the different boutons will summate. If they are sufficiently close together in space and in time, then they will set up an action potential, tavelling away down the axon of the post-synaptic cell
