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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Understand how the skin elaborates a variety of receptors that sense various kinds of touch, temperature, and pain.

  • See how receptors for various skin sensations have different anatomic morphologies and physiologic transduction mechanisms.

  • See the relationship between different mechanoreceptor types and the ensuing perceptions, such as vibration or pressure, associated with stimulation of that receptor type.

  • Understand how the spatial resolution of touch is related to the density of receptors in any skin area and inversely related to receptive field (RF) size.

  • Follow the pathway for transmitting touch, temperature, and pain perception from sensory neurons in the dorsal root ganglia of the spinal cord, up through spinal cord pathways to the brainstem and thalamus, and then to somatosensory cortex in the most anterior portion of the parietal lobe.

  • See how transient receptor potential (TRP) channels mediate temperature sensation.

  • Understand psychological aspects of pain perception and the role of endorphins.

  • Learn what regions of the brain process somatosensory information.

  • See how proprioception and kinesthesis (joint position, movement, and force) are mediated by receptors similar to mechanoreceptors.

OVERVIEW

Embedded in the skin are receptors for touch, temperature, and pain. These receptors are at the axon endings of neurons whose soma are in the dorsal root ganglia outside the spinal cord, or other ganglia outside the brain for cranial nerves mediating somatosensation in the head. Action potentials produced by somatosensory receptors travel retrogradely past the axonal bifurcation in the dorsal root ganglion to synapses in the spinal gray area. Somatosensory synapses in the spinal gray area participate in local circuits that mediate reflexes and project to the thalamus and other brain areas.

CUTANEOUS & SUBCUTANEOUS SOMATIC SENSORY RECEPTORS

The skin is the boundary between the body and the world, enclosing the body’s tissue, retaining water, excluding bacteria and dirt, and providing thermal insulation. The skin detects the world through the sense of touch, which is called cutaneous or somatosensory perception. The kinds of touch that we can perceive include various kinds of mechanical sensations (pressure, movement, and flutter), as well as temperature and pain. These different kinds of perceptions arise from the activation of different kinds of receptors in the skin.

Detecting something as a skin sensation without or before perceiving what has made this contact is called passive touch. Touch has another, more active function, called haptic perception, which enables us to perform complicated manipulations of objects whose shape and orientation are perceived through touch. This type of perception (active touch) is particularly important for tool use and dexterity skills involving the fingers and fingertips. It has been shown, for example, that people blind from birth who have no visual input to the occipital visual cortex experience activation in the visual cortex during their finger manipulations when reading Braille. Both active and passive touch depend on ...

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