Chapter 13 - Peripheral Nervous System

Define Sensory Receptors

• Specialized to respond to changes in environment (stimuli)

  • Activation results in graded potentials that trigger nerve impulses

• Awareness of stimulus (sensation) and interpretation of meaning of stimulus (perception) occur in brain

List three ways to classify receptors

1. Type of stimulus

2. Body location

3. Structural complexity

Types of Sensory Receptors

1. Mechanoreceptors

   • Respond to touch, pressure, vibration, and stretch

2. Thermoreceptors

   • Sensitive to changes in temperature

3. Photoreceptors

   • Respond to light energy

     • EX: retina

4. Chemoreceptors

   • Respond to chemicals

     • EX: smell, taste, changes in blood chemistry

5. Nociceptors

   • Sensitive to pain-causing stimulus

     • EX: extreme heat or cold, excessive pressure, inflammatory chemicals

Location of Sensory Receptors

1. Exteroceptors

   • Respond to stimuli arising outside body

   • Receptors in skin for touch, pressure, pain, and temperature

   • Most special sense organ

2. Interoceptors (visceroceptors)

   • Respond to stimuli arising in internal viscera and blood vessels

   • Sensitive to chemical changes and temperature changes

   • Sometimes cause discomfort but usually person is unaware of their workings

3. Proprioceptors

   • Like interoceptors, but located in skeletal muscles, tendons, joints and muscles

Difference between Sensation and Perception

• Survival depends upon:

  1. Sensation

     • The awareness of changes in the internal and external environment

  2. Perception

     • The conscious interpretation of those stimuli

Perception of Pain

• Warns of actual or impending tissue damage so protective action can be taken

  • Stimuli include extreme pressure and temperature

• Impulses travel on fibers that release neurotransmitter glutamate

• Some pain impulses are blocked by inhibitory endogenous opioids

  • EX: endorphins

Pain tolerance

• All perceive pain at same stimulus intensity

• Pain tolerance varies

• “Sensitive to pain” means low pain tolerance, not low pain threshold

Structure & Function of Nerves

• STRUCTURE

  • Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

    • Two types of nerves: spinal or cranial, depending on where they originate

• FUNCTION

  • Cordlike organ of PNS

Connective Tissue Coverings

1. Epineurium

   • Tough fibrous sheath around all fascicles to from the nerve

2. Perineurium

   • Coarse connective tissue that bundles fibers into fascicles

3. Endoneurium

   • Loose connective tissue that encloses axons and their myelin sheaths (Schwann cells)

Classification of Nerves according to Direction

• Mixed nerves

  • Contain both sensory and motor fibers

  • Impulses travel BOTH to and from CNS

• Sensory (afferent) nerves

  • Impulses only TOWARD CNS

• Motor (efferent) nerves

  • Impulses only AWAY from CNS

Pure sensory (afferent) or pure motor (efferent) nerves are rare; most are mixed

Types of Fibers in Mixed Nerves

1. Somatic afferent

   • Sensory from muscle to brain

2. Somatic efferent

   • Motor from brain to muscle

3. Visceral afferent

   • Sensory from organs to brain

4. Visceral efferent

   • Motor from brain to organs

Define Ganglia

• Ganglia: contain neuron cell bodies associated with nerves in PNS

  1. Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons

     • Dorsal root ganglia (sensory, somatic)

  2. Ganglia associated with efferent nerve fibers contain autonomic motor neurons

     • Autonomic ganglia (motor, visceral)