16. Sensory Nerves (PNS and Sensory Receptors)

Learning Objectives

  • Define the PNS and list its components
  • Classify sensory receptors by stimulus, location, and structure
  • Outline events leading to sensation and perception including adaptation and pain perception
  • Describe the structure of a nerve
  • Name the 12 pairs of cranial nerves and their function
  • Describe the general structure of spinal nerves and plexuses
  • Describe a dermatome

Peripheral Nervous System (PNS)

  • Definition: The PNS consists of all neural structures outside the brain and spinal cord.
  • Components: Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings.
  • Function: Provides pathways for links to and from the external environment.
  • Divisions:
    • Sensory (afferent) division: Transmits sensory information to the CNS.
    • Motor (efferent) division: Transmits commands from the CNS to effectors.
    • Somatic Nervous System: Controls voluntary movements.
    • Autonomic Nervous System (ANS): Controls involuntary actions (subdivided into sympathetic and parasympathetic divisions).

Sensory Receptors

  • Definition: Specialized structures that respond to stimuli (changes in the environment).
  • Function: Activation results in depolarization, creating nerve impulses sent to the CNS.
  • Processes: Sensation (awareness of stimuli) and perception (interpretation) occur in the brain.
  • Classification: By type of stimulus, location, and structural complexity.

Classification by Type of Stimulus

  1. Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.
  2. Thermoreceptors: Sensitive to temperature changes.
  3. Photoreceptors: Respond to light energy (found in the retina).
  4. Chemoreceptors: Respond to chemicals in solutions (e.g., smell and taste).
  5. Nociceptors: Sensitive to pain-causing stimuli (extreme pressure or temperatures).

Classification by Location

  1. Exteroceptors: Respond to external stimuli; located near the body surface (e.g., skin).
  2. Interoceptors: Respond to internal stimuli; found in internal organs (e.g., blood vessels).
  3. Proprioceptors: Respond to body position and movement; found in skeletal muscles, joints, and connective tissues.

Classification by Structural Complexity

  • Simple Receptors: Can be encapsulated or non-encapsulated.
    • Nonencapsulated (free) nerve endings: Abundant in epithelia and connective tissues; sensitive to pain, temperature, and light touch.
    • Encapsulated nerve endings: Include specialized receptors like tactile corpuscles and muscle spindles.

Sensory Integration

  • Definition: The process of transforming sensory input into perceptual experience.
  • Levels of Neural Integration:
    1. Receptor Level: The sensory receptors activate.
    2. Circuit Level: Sensory information is conducted to the brain via ascending pathways.
    3. Perceptual Level: Interpretation of sensory input in the sensory cortex.

Processing at the Receptor Level

  • Requires receptor specificity, stimulation of receptive fields, and conversion of stimulus energy into graded potentials (transduction).
  • Adaptation: Sensory receptors become less responsive with constant stimulus (e.g., phasic receptors adapt quickly; tonic receptors adapt slowly).

Processing at the Circuit Level

  • Neural Chains: Composed of first-order, second-order, and third-order neurons that relay sensory impulses from the periphery to the brain.
    • First-order neurons: Conduct impulses from receptors to the spinal cord.
    • Second-order neurons: Transmit impulses to the thalamus.
    • Third-order neurons: Relay signals to the somatosensory cortex.

Processing at the Perceptual Level

  • Interpretation of sensory stimuli varies by location of target neurons in the sensory cortex.
  • Aspects of Sensory Perception:
    • Perceptual detection: Ability to detect stimulus presence.
    • Magnitude estimation: Intensity of stimulus reflected by impulse frequency.
    • Spatial discrimination: Determining the location of stimuli.
    • Feature abstraction: Identification of complex stimulus properties.
    • Quality discrimination: Differentiating between submodalities (e.g., tastes).
    • Pattern recognition: Identifying familiar patterns in stimuli.

Pain Perception

  • Function: Warns of potential or actual tissue damage, leading to protective actions.
  • Types of Stimuli: Includes extreme pressure, temperature, and various chemicals.
  • Pain Tolerance: Varies between individuals; influenced by genetic factors and experience.

Visceral and Referred Pain

  • Visceral Pain: Results from organs; manifests as vague discomfort.
  • Referred Pain: Pain felt in a different part of the body than the site of injury due to nerve interconnections (e.g., arm pain during heart attacks).

Structure of Nerves

  • Definition: Nerves are bundles of axons (both myelinated and nonmyelinated) enclosed in connective tissue.
  • Types of Nerves: Sensory (afferent), Motor (efferent), and Mixed nerves (both sensory and motor).
  • Connective Tissue Layers:
    • Endoneurium: Encloses individual axons.
    • Perineurium: Groups axons into fascicles.
    • Epineurium: Surrounds the entire nerve.

Cranial Nerves

  • Total: 12 pairs, each with specific sensory and motor functions.
  • General Functions: Include smell, vision, hearing, and movement of facial and neck muscles.
  • Mnemonic: Oh Oh Oh To Touch And Feel Very Green Vegetables A H.

Specific Cranial Nerves

  • I - Olfactory: Smell
  • II - Optic: Vision
  • III - Oculomotor: Eye movement, pupil size
  • V - Trigeminal: Facial sensations and chewing muscles
  • VIII - Vestibulocochlear: Hearing and balance
  • X - Vagus: Visceral functions beyond the head and neck

Spinal Nerves and Plexuses

  • Total Spinal Nerves: 31 pairs, named according to their region (Cervical, Thoracic, Lumbar, Sacral, Coccygeal).
  • Plexuses: Networks formed by spinal nerves (except T2-T12). Provide redundancy in innervation to muscles to prevent paralysis from nerve damage.

Dermatomes

  • Definition: Area of skin supplied by sensory fibers of a single spinal nerve.
  • Importance: Used to assess spinal cord injuries; overlap ensures minimal sensory loss from single nerve damage.
  • Spinal Nerve Participation: All nerves except C1 contribute to dermatomes, aiding in clinical diagnostics.