Page 99 - 84 human physiolofy part-1

Senses

type and include Mechanoreceptors (sensitive to pressure, vibration and slip), Thermoreceptors
(sensitive to changes in temperature), and Nocioreceptors (responsible for pain).

Pacinian Corpuscles

Pacinian corpuscles detect gross pressure changes and vibrations. They are the largest of the
receptors. Any deformation in the corpuscle causes action potentials to be generated, by opening
pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx in,
creating a receptor potential. Pacinian corpuscles cause action potentials when the skin is rapidly
indented but not when the pressure is steady, due to the layers of connective tissue that cover the nerve
ending (Kandel et al., 2000). It is thought that they respond to high velocity changes in joint position.

Meissner's Corpuscle

Meissner's corpuscles are distributed throughout the skin, but concentrated in areas especially
sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face, nipples and the external
skin of the male and female genitals. They are primarily located just beneath the epidermis within the
dermal papillae. Any physical deformation in the Meissner’s corpuscle will cause an action potential in
the nerve. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease
and eventually cease. If the stimulus is removed, the corpuscle regains its shape and while doing so (ie:
while physically reforming) causes another volley of action potentials to be generated. (This is the
reason one stops "feeling" one's clothes.) This process is called sensory adaption. Because of their
superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations, but
for the same reasons, they are limited in their detection because they can only signal that something is
touching the skin. Meissner's corpuscles do not detect pain; this is signaled exclusively by free nerve
endings.

Merkel’s Discs

Merkel’s Discs are Mechanoreceptors, making them sensitive to pressure and vibration. In
humans, Merkel cells occur in the superficial skin layers, and are found clustered beneath the ridges of
the fingertips that make up fingerprints. They’re somewhat rigid in structure, and the fact that they are
not encapsulated, causes them to have a sustained response (in the form of action potentials or spikes)
to mechanical deflection of the tissue. Merkel nerve endings are extremely sensitive to tissue
displacement, and may respond to displacements of less than 1 _m. Several studies indicate that they
mediate high-resolution tactile discrimination, and are responsible for the ability of our fingertips to
feel fine detailed surface patterns (e.g. for reading Braille).

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