Structure and function of the nervous system

A reflex is the body’s response to an external or internal stimulus involving the central nervous system. The path of nerve excitation is the reflex arc; it has five parts: receptor, sensory neuron, nerve centre in the central nervous system, motor neuron and working organ. The receptor converts a stimulus (heat, pressure, light, sound) into a nerve impulse, which goes by the sensory neuron to the centre and from there by the motor neuron to the working organ. For example, we pull a hand back from a hot object without thinking; sneezing, coughing and blinking are simple reflexes too. A nerve impulse travels at speeds from a few metres up to over a hundred metres per second. In the central nervous system excitation and inhibition alternate: inhibition weakens or stops a reaction. Their coordination lets, for example, flexor and extensor muscles work in turn so movement is smooth.