Peripheral Nervous System - Sensory Receptors and Pain

The Peripheral Nervous System - Part 2

13.1 Sensory Receptors

• Sensory receptors are activated by changes in the internal or external environment.
• Survival depends upon:
  • Sensation: awareness of changes in the internal and external environment.
  • Perception: conscious interpretation of those stimuli.

Sensory Receptors and Sensation

• Sensory receptors: specialized to respond to changes in environment (stimuli).
• Activation results in graded potentials that trigger nerve impulses.
• Three ways to classify receptors:
  • Type of stimulus
  • Body location
  • Structural complexity

Classification by Stimulus Type

• Mechanoreceptors: respond to touch, pressure, vibration, and stretch.
• Thermoreceptors: sensitive to changes in temperature.
• Photoreceptors: respond to light energy.
• Chemoreceptors: respond to chemicals.
• Nociceptors: sensitive to pain-causing stimuli.

Classification by Location

• Classified by location of receptor or location of activating stimulus.
  • Exteroceptors
    • Respond to stimuli arising outside the body.
    • Receptors in skin for touch, pressure, pain, and temperature.
    • Most special sense organ receptors.
  • Interoceptors (Visceroceptors)
    • Respond to stimuli arising in internal viscera and blood vessels.
    • Sensitive to chemical changes, tissue stretch, and temperature changes.
    • Sometimes cause discomfort but usually person is unaware of their workings.
  • Proprioceptors
    • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles.
    • Inform brain of one's movements.

Classification by Receptor Structure

• Majority of sensory receptors belong to one of two categories:
  • Simple receptors of the general senses
    • Modified dendritic endings of sensory neurons.
    • Found throughout body and monitor most types of general sensory information.
    • Covered in Ch 13
  • Receptors for special senses
    • Vision, hearing, equilibrium, smell, and taste.
    • All are housed in complex sense organs.
    • Covered in Ch 15

Simple Receptors of General Senses

• General senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and “muscle sense.”
• No “one-receptor-one-function” relationship
  • One receptor type can respond to multiple stimuli
  • Different types of receptors can respond to similar stimuli

Simple Receptors: Nerve Endings

• General sensory receptors are nerve endings that are either:
  • Nonencapsulated (free) nerve endings
  • Encapsulated nerve endings

Nonencapsulated Nerve Endings

• Present nearly everywhere in the body.
• Abundant in epithelia and connective tissues.
• Most are nonmyelinated, small-diameter fibers.
• Free nerve endings: nonmyelinated fibers with knoblike swellings on their ends.
  • Respond to temperature and painful stimuli.
  • Some respond to tissue movement caused by pressure.
• Tactile (Merkel) discs: Free nerve endings associated with enlarged disc-shaped epidermal cells (tactile cells).
  • Function as light touch receptors.
  • Located in deeper layers of epidermis.
• Hair follicle receptors: free nerve endings that wrap around hair follicles.
  • Act as light touch receptors that detect bending of hairs.

Nonencapsulated Nerve Endings - Summary Table

Encapsulated Nerve Endings

• Consist of one or more fiber terminals of sensory neurons enclosed in a connective tissue capsule.
• Almost all are mechanoreceptors.
• Vary greatly in shape, size, and distribution in body.
  • Tactile (Meissner’s) corpuscles: small receptors involved in discriminative touch.
  • Lamellar (Pacinian) corpuscles: large receptors respond to deep pressure and vibration when first applied.
• Bulbous corpuscles (Ruffini endings): respond to deep and continuous pressure.
  • Muscle spindles: spindle-shaped proprioceptors that respond to muscle stretch.
  • Tendon organ: proprioceptors located in tendons that detect stretch.
  • Joint kinesthetic receptors: proprioceptors that monitor joint position and motion.

Encapsulated Nerve Endings - Summary Table

13.2 Sensory Information Processing

Somatosensory System

• Part of sensory system serving body wall and limbs.
• Receives inputs from exteroceptors, proprioceptors, and interoceptors.
• Input is relayed toward head, but processed along the way.

Levels of Neural Integration

• Three levels:
  • Receptor level: sensory receptors
  • Circuit level: processing in ascending pathways
  • Perceptual level: processing in cortical sensory areas

Pain

• Warns of actual or impending tissue damage so protective action can be taken.
• Stimuli include extreme pressure and temperature, histamine, $K^+$, ATP, acids, and bradykinin.
• Impulses travel on fibers that release neurotransmitters glutamate and substance P.
• Some pain impulses are blocked by inhibitory endogenous opioids (example: endorphins).

Perception of Pain

• Visceral pain: results from stimulation of visceral organ receptors.
• Somatic pain: comes from stimulation of joints, bones, muscles, and soft tissues.

Pain Tolerance

• We all perceive pain at roughly the same stimulus intensity.
• Pain tolerance varies.
• “Sensitive to pain” means low pain tolerance, not low pain threshold.
• Genes help determine pain tolerance as well as response to pain medications.

Referred Pain

• Pain from one body region perceived as coming from different region.
• Visceral and somatic pain fibers travel along same nerves; brain assumes stimulus comes from common (somatic) region.
• Example: left arm pain during heart attack.

Long-Lasting Pain

• Long-lasting or intense pain, such as limb amputation, can disrupt body's pain system.
• Can lead to hyperalgesia (pain amplification), chronic pain, and phantom limb pain.
• Early pain management critical to prevent.
• Phantom limb pain: pain felt in limb that has been amputated.
• Now use epidural anesthesia during surgery to reduce phantom pain.