PNS

Chapter 13: The Peripheral Nervous System

Introduction

• Peripheral Nervous System (PNS)
    - All neural structures outside the brain.
    - Key components:
      - Sensory receptors
      - Peripheral nerves and associated ganglia
      - Efferent motor endings

Organization of the Nervous System

• Central Nervous System (CNS)
    - Comprises the brain and spinal cord.
• Peripheral Nervous System (PNS)
    - Divided into:
      - Sensory (afferent) division
      - Motor (efferent) division
        - Somatic nervous system
        - Autonomic nervous system (ANS)
          - Sympathetic division
          - Parasympathetic division
    - Visual representation reflects the structural organization of the entire nervous system.

Sensory Receptors

• Definition: Specialized to respond to changes in the environment (stimuli).
• Activation and Function:
    - Activation leads to graded potentials that trigger nerve impulses.
    - Sensation: Awareness of stimulus.
    - Perception: Interpretation of the meaning of the stimulus.
• Categories of Sensory Receptors:
    1. Type of stimulus they detect
    2. Location in the body
    3. Structural complexity

Classification of Sensory Receptors by Type of Stimulus

1. Mechanoreceptors
      - Respond to touch, pressure, vibration, and stretch.
2. Thermoreceptors
      - Sensitive to changes in temperature.
3. Photoreceptors
      - Respond to light energy (e.g., retina).
4. Chemoreceptors
      - Respond to chemicals (e.g., smell, taste, changes in blood chemistry).
5. Nociceptors
      - Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Classification of Sensory Receptors by Location

1. Exteroceptors
      - Respond to stimuli arising outside the body.
      - Present in skin for touch, pressure, pain, and temperature.
      - Includes most special sense organs.
2. Interoceptors
      - Respond to stimuli arising in internal viscera and blood vessels.
      - Sensitive to chemical changes, tissue stretch, and temperature.
3. Proprioceptors
      - Respond to stretch in skeletal muscles, tendons, joints, and ligaments.
      - Inform the brain of one’s movements.

Classification by Receptor Structure

• Simple Receptors
    - For general senses including tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.
    - Comprised of modified dendritic endings of sensory neurons.
• Types of Receptors:
    - Nonencapsulated:
      - Free nerve endings
      - Thermoreceptors
      - Nociceptors
      - Light touch receptors (Tactile (Merkel) discs, Hair follicle receptors).
    - Encapsulated:
      - Tactile (Meissner's) corpuscles
      - Lamellar (Pacinian) corpuscles
      - Bulbous corpuscles (Ruffini endings)
      - Muscle spindles
      - Tendon organs
      - Joint kinesthetic receptors.
• Receptors for Special Senses:
    - Vision, hearing, equilibrium, smell, and taste (detailed in Chapter 15).

Sensory Integration

• Somatosensory System: Part of the sensory system serving body wall and limbs.
• Receives Inputs from:
    - Exteroceptors
    - Proprioceptors
    - Interoceptors
• Levels of Neural Integration:
    1. Receptor Level—sensory receptors.
    2. Circuit Level—processing in ascending pathways.
    3. Perceptual Level—processing in cortical sensory areas.

Perceptual Processing Levels

• Processing at different levels involves the following:
    - Receptor Level:
      - Distinguishes between different stimuli using graded potentials called generator potentials.
      - Transduction occurs: a stimulus is changed to graded potential, which must reach threshold to generate action potentials.
    - Circuit Level:
      - Involves three neuron types:
        1. First-order sensory neurons: Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS.
        2. Second-order sensory neurons: Transmit impulses to third-order sensory neurons.
        3. Third-order sensory neurons: Conduct impulses from the thalamus to the somatosensory cortex.
    - Perceptual Level:
      - Interpretation depends on the specific location of target neurons in the sensory cortex.
      - Aspects of Sensory Perception:
        - Perceptual detection: The ability to detect a stimulus (requires summation of impulses).
        - Magnitude estimation: Intensity is coded in frequency of impulses.
        - Spatial discrimination: Identifies site or pattern of stimulus.
        - Quality discrimination: Identifies submodalities of a sensation (e.g., sweet or sour tastes).
        - Pattern recognition: Recognizes familiar or significant patterns in stimuli (e.g., melody in music).

Nerves and Associated Ganglia

• Definition of Nerve: Cordlike organ of the PNS, comprised of bundles of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.
• Types of Nerves:
    - Spinal Nerves or Cranial Nerves, depending on origin.
    - Most nerves are mixtures of afferent and efferent fibers, as well as somatic and autonomic (visceral) fibers.
    - Classification of Nerves by Direction of Impulse:
      - Mixed Nerves: Contain both sensory and motor fibers.
      - Sensory (afferent) Nerves: Transmit impulses only toward the CNS.
      - Motor (efferent) Nerves: Transmit impulses only away from the CNS.

Structure and Classification (cont.)

• Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed.
• Types of Fibers in Mixed Nerves:
    - Somatic afferent: Sensory fibers from skeletal muscle to the brain.
    - Visceral afferent: Sensory fibers from organs to the brain.
    - Somatic efferent: Motor fibers from the brain to skeletal muscle.
    - Visceral efferent: Motor fibers from the brain to organs.

Motor Endings and Motor Activity

• Motor Endings: PNS elements that activate effectors by releasing neurotransmitters.
    - These elements innervate skeletal muscle, visceral muscle, and glands.
    - Cerebellum and Basal Nuclei: Ultimate planners and coordinators of complex motor activities.

Hierarchy of Motor Control

• Levels of Motor Control:
    - Precommand Level (highest):
      - Involves the cerebellum and basal nuclei, programming and modifying instructions based on feedback.
    - Projection Level (middle):
      - Comprises motor cortex (via pyramidal pathways) and brainstem nuclei.
      - Conveys instructions to spinal cord motor neurons and sends copies to higher levels.
    - Segmental Level (lowest):
      - Exists in the spinal cord, containing central pattern generators (CPGs).

Functional Classification of Reflexes

• Inborn (Intrinsic) Reflex: Rapid, involuntary motor responses to stimuli.
    - Examples: Maintain posture, avoid pain, control visceral activities.
    - Can be modified by learning and conscious effort.
• Learned (Acquired) Reflexes: Results from practice or repetition.
    - Example: Driving skills.

Reflex Arc Components

• Components of a Reflex Arc:
    1. Receptor: Site of stimulus action.
    2. Sensory Neuron: Transmits afferent impulses to CNS.
    3. Integration Center: Either monosynaptic or polysynaptic region within CNS.
    4. Motor Neuron: Conducts efferent impulses from integration center to effector organ.
    5. Effector: Muscle fiber or gland cell that responds to efferent impulses by contracting or secreting.

Reflex Types

• Functional Classification:
    - Somatic Reflexes: Activate skeletal muscle.
    - Autonomic (Visceral) Reflexes: Activate visceral effectors (smooth or cardiac muscle or glands).
• Spinal Reflexes:
    - Integration center located in spinal cord (does not involve higher brain centers).
    - Effectors are skeletal muscles.
    - Testing of somatic reflexes is crucial clinically to assess the condition of the nervous system.
    - Exaggerated, distorted, or absent reflexes signal potential degeneration/pathology of specific nervous system regions.

Stretch and Tendon Reflexes

• Need for Proprioceptor Input: Nervous system must receive proprioceptor input regarding:
    1. Length of muscle (from muscle spindles).
    2. Amount of tension in muscle (from tendon organs).
• Stretch Reflex:
    - Maintains muscle tone in large postural muscles and adjusts it reflexively.
    - Activates muscle contraction in response to increased muscle length (stretch).
    - Operating mechanism:
      - Stretch activates muscle spindles.
      - Sensory neurons synapse directly with motor neurons in spinal cord to cause contraction of the stretched muscle (mediated by a-motor neurons).
      - All stretch reflexes are monosynaptic and ipsilateral.
      - Reciprocal Inhibition occurs: fibers synapse with interneurons that inhibit a-motor neurons of antagonistic muscles (e.g., in the patellar reflex: quadriceps contract while hamstrings relax).

The Tendon Reflex

• Purpose: Helps prevent damage due to excessive stretch.
• Response: Produces muscle relaxation (lengthening) in response to tension.
    - Mechanism of action involves tension activation leading to:
      - Afferent impulses transmitted to spinal cord.
      - Contracting muscle relaxes, antagonist contracts (reciprocal activation).
      - Additional information is relayed to cerebellum to adjust muscle tension accordingly.