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Chapter 8: Somatosensory Neurotransmission: Touch, Pain, & Temperature

OBJECTIVES

OBJECTIVES

After studying this chapter, you should be able to:

  • Describe the cutaneous receptors that mediate the sensations of touch, pressure, pain, and temperature.

  • Explain the four attributes of a stimulus to sensory receptors.

  • Explain acute and chronic pain, hyperalgesia, and allodynia.

  • Describe referred pain.

  • Compare the pathway that mediates sensory input from touch, proprioceptive, and vibratory senses to that mediating information from nociceptors and thermoreceptors.

  • Describe processes involved in modulation of transmission in pain pathways.

  • Identify drugs used for relief of pain, their mechanism of action, and the rationale for their use.

INTRODUCTION

Sensory receptors are transducers that change a particular form of energy in the environment into electrical signals, which are then relayed to the central nervous system (CNS) which can then interpret the information received. Table 8–1 is a list of different types of receptors in the skin, muscles, and specialized organs like the eye and ear and the specific energy (stimulus) to which they respond. The emphasis in this chapter is on the characteristics of cutaneous mechanoreceptors, thermoreceptors, and nociceptors; the way they generate impulses in afferent neurons; and the central pathways that mediate or modulate information from these receptors. Subsequent chapters cover sensory receptors in the eye, ear, tongue and nose, and skeletal muscles and joints.

TABLE 8–1Principle sensory modalities.
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TABLE 8–1 Principle sensory modalities.
Sensory Receptor Receptor Class Modality Stimulus
Meissner corpuscles Cutaneous mechanoreceptor Touch Tap, flutter 5–40 Hz
Hair follicle receptors Cutaneous mechanoreceptor Touch Motion
Pacinian corpuscles Cutaneous mechanoreceptor Touch Deep pressure, vibration 60–300 Hz
Merkel cells Cutaneous mechanoreceptor Touch Touch, pressure
Ruffini corpuscles Cutaneous mechanoreceptor Touch Sustained pressure
Muscle spindles Mechanoreceptor Proprioception Stretch
Golgi tendon organ Mechanoreceptor Proprioception Tension
Cold and warmth receptors Thermoreceptor Temperature Thermal
Chemical, thermal, and mechanical nociceptors (or polymodal) Chemoreceptor, thermoreceptor, and mechanoreceptor Pain Chemical, thermal, and mechanical
Rods, cones Photoreceptor Vision Light
Hair cells (cochlea) Mechanoreceptor Hearing Sound
Hair cells (semicircular canals) Mechanoreceptor Balance Angular acceleration
Hair cells (otolith organs) Mechanoreceptor Balance Linear acceleration, gravity
Olfactory sensory neuron Chemoreceptor Smell Chemical
Taste buds Chemoreceptor Taste Chemical

TOUCH, PRESSURE, PAIN, & TEMPERATURE RECEPTORS

Touch and pressure are sensed by four types of mechanoreceptors that are specialized dendritic endings of Aα and Aβ afferent nerve fibers (Figure 8–1). Meissner corpuscles respond to changes in texture and slow vibrations; Merkel cells respond to sustained pressure and touch; Ruffini corpuscles respond to sustained pressure; and Pacinian corpuscles respond to deep pressure and fast vibration.

FIGURE 8–1

Sensory systems encode four elementary attributes of stimuli: modality, location (receptive field), intensity, and duration (timing). (A) The human hand has four types of mechanoreceptors; their combined activation produces the sensation of contact with an object. Selective activation of Merkel cells and Ruffini endings causes sensation of steady pressure; selective activation of ...

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