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|These last functions are carried out by the inner ear, which is discussed on a separate page. Link to it. The other functions are discussed here.|
Light touch is detected by receptors in the skin. Many of these are found next to hair follicles so even if the skin is not touched directly, movement of the hair is detected.
Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm2; the back of the hand fewer than 10 per cm2.
This can be demonstrated with the two-point discrimination test. With a pair of dividers like those used in mechanical drawing, determine (in a blindfolded subject) the minimum separation of the points that produces two separate touch sensations. The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back.The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body.
|Link to illustrated discussion of the somatosensory cortex of the human brain.|
Because of its relatively large size, a single Pacinian corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency is applied to the corpuscle by the stylus. The electrical activity is detected by electrodes attached to the preparation.
Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron.
Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing.
Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated. This is why Pacinian corpuscles respond especially well to vibrations.
The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception — such as spindle fibers — adapt slowly if at all.
Meissner corpuscles, like Pacinian corpuscles, adapt quickly to a sustained stimulus but are activated again when the stimulus is removed. Thus they are especially sensitive to movement across the skin.
They are situated closer to the surface of the skin than Pacinian corpuscles where they respond to the gentlest of touches. However, they have poor two-point discrimination.
They form synapses with Aβ sensory neurons leading back to the CNS.
Merkel cells are also transducers of light touch, responding to the texture and shape of objects indenting the skin. Unlike Pacinian and Meissner corpuscles, they do not adapt rapidly to a sustained stimulus; that is, they continue to generate nerve impulses so long as the stimulus remains.Merkel cells are found in the skin often close to hairs. They mediate excellent two-point discrimination In the rat, light movement of a hair triggers a generator potential in a Merkel cell. If this reaches threshold, an influx of Ca++ ions through voltage-gated calcium channels generate action potentials in the Merkel cell. These cause the release of neurotransmitters at the synapse with its Aβ sensory neuron. (This neuron may also have its own mechanically-gated ion channels able to directly generate action potentials more rapidly than Merkel cells can.)
The skin of mice contains axon terminals of C fibers that extend from the dermis up into the epidermis. C fibers are unmyelinated but are ensheathed by a specialized type of Schwann cell. Mechanical stimulus of the Schwann cell that is strong enough to signal pain generates action potentials in the enclosed C fibers. These nerve impulses travel back to the brain where they are interpreted as pain. (Pain receptors are called nociceptors.)