A reflex arc

Much of human behaviour is highly complex, difficult to predict or even to explain after it has taken place, but some elements in our behaviour are surprisingly stable and it is these which we shall now consider. As an example let us take the simple withdrawal reflex. Take a spoon from a hot cup of tea and place it unexpectedly on somebody's hand. The response takes place before the victim has time to think of what is happening to him and is quite involuntary. Responses such as these are dependent upon pre-formed'neurological connections and many of them function independently of the brain. If the spinal cord of the dog is cut just below the brain so that no nerve impulses can reach the brain it will still make a withdrawal response if its paw is pinched.

The net of neurological connections responsible for a simple response of this kind is known as a reflex arc. It is usual to think of a reflex arc as consisting of three nerve units or neurons: a sensory neuron which brings in a nerve impulse (e. g. from the skin), a connecting neuron in the spinal cord, and a motor neuron which conducts the nerve impulse out to

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a muscle (i.e. 'reflects' the impulse). This, however, is an over-simplifi­cation, because the more complex reflexes are concerned. Before going further into this let us pause for a moment to consider what a neuron is and how the nerve conducts impulses.

A nerve consists of a single nerve cell and may take many forms, but all consist of a central cell body which lengthens out into a long fibre (as much as several feet in some cases) and ends in fine branches. The other end of the cell body branches immediately into a fine network known as dendrites or dendrons. A nerve consists of a bundle of such neurons enclosed within a protective sheath except when they enter within the brain. The axon of one neuron branches within the dendrons of others so that it may pass on its impulse to any one of perhaps hundreds of other neurons with whose dendrons it has contact.

The point of contact between an axon and a dendron is known as a synapse and it is the relative amount of resistance at the various synap­ses which determines the route of the nerve impulse. Conduction through the synapse is relatively slow and may require as much time as the traversing of the complete neuron. Hence responses which are mediated through several synapses may be considerably delayed. The simple reflex is thus a very rapid response. In fact some of the reflex responses are just about ten times as fast as the quickest voluntary action, e.g. pressing a telegraph key on a given signal.

(C.J. Adcock. «Fundamentals of Psychology». Penguin Books, pp. 26 —27)

Read after Lesson III.