The nervous system has millions of receptors, which are the distal ends of the dendrites of the sensory nerves. These receptors detect changes in the external and internal environment. All sensations are felt through the stimulation of these receptors. In response to these stimuli, nerve impulses are initiated and transmitted along the nerve to the spinal cord and brain, where they are interpreted and the appropriate responses selected. For example, if the receptors register cold, these stimuli are conducted to the brain. Here they are interpreted and the brain sends impulses to stimulate muscles to contract rapidly: this produces body heat, which we know as shivering.
The main groups of receptors are:
Exteroceptors: there are many types, which tend to lie on the surface of the body: they detect changes in the external environment. They are found in skin, mucous membranes, and register cold, heat, touch, pressure, pain etc. Highly specialised ones are found in the eye and ear for sight and sound. Those in the skin include Meissner's corpuscles, which sense light touch, Merkel's discs sense touch and stretch, and Pacinian corpuscles sense deep pressure.
Interoceptors or visceroceptors: these lie internally and detect changes in the internal environment. They are found in the internal organs such as the intestine, stomach, liver, kidneys etc and in the walls of blood vessels. These register changes in the internal organs.
Proprioceptors: these are located in muscles, tendons and joints and register the degree of stretch or tension in a muscle while others provide information of joint position and the spatial location of body parts.
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