Peripheral Nervous System

Human Anatomy and Physiology: Peripheral Nervous System

Chapter 13 - Overview of the Peripheral Nervous System

  • Purpose: The peripheral nervous system (PNS) links the body to the external world and assists in recognizing and treating nerve damage.

  • Components: The PNS consists of all neural structures outside the brain and spinal cord and is divided into four main parts:

    • Part 1: Sensory Receptors

    • Part 2: Transmission Lines (Nerves) and Their Structure and Repair

    • Part 3: Motor Endings and Motor Activity

    • Part 4: Reflex Activity

Part 1 - Sensory Receptors and Sensation

13.1 Sensory Receptors

  • Definition: Specialized receptors responding to environmental changes (stimuli).

  • Activation: Leads to graded potentials which trigger nerve impulses.

  • Sensation vs. Perception:

    • Sensation: Awareness of stimulus.

    • Perception: Interpretation of the meaning of the stimulus.

  • Classification of Receptors:

    • By Type of Stimulus:

    • Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.

    • Thermoreceptors: Sensitive to temperature changes.

    • Photoreceptors: Respond to light energy (e.g., retina).

    • Chemoreceptors: Respond to chemicals (e.g., smell, taste).

    • Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat, excessive pressure).

    • By Body Location:

    • Exteroceptors: Respond to external stimuli (e.g., skin receptors).

    • Interoceptors (Visceroceptors): Respond to internal stimuli (e.g., internal organs).

    • Proprioceptors: Monitor stretch in skeletal muscles, tendons, etc.

    • By Receptor Structure:

    • Simple Receptors of General Senses:

      • Modified dendritic endings of sensory neurons (e.g., pain).

      • Are found throughout the body.

    • Receptors for Special Senses:

      • Complex sense organs (e.g., vision, hearing).

13.2 Sensory Processing

  • Importance for Survival:

    • Sensation: Awareness of changes in environment.

    • Perception: Conscious interpretation of sensory input.

  • General Organization of the Somatosensory System:

    • Receives inputs from exteroceptors, proprioceptors, and interoceptors.

    • Input is relayed toward the head and processed along the way.

  • Levels of Neural Integration:

    1. Receptor Level: Sensory receptors.

    2. Circuit Level: Processing in ascending neural pathways.

    3. Perceptual Level: Processing in cortical sensory areas.

Processing at Different Levels

  • At the receptor level:

    • Stimulus must excite a receptor and generate action potentials.

    • Must match receptor specificity, applied within receptive field, and involve transduction.

  • At the circuit level:

    • Pathways consist of three neurons that conduct sensory impulses.

  • At the perceptual level:

    • Interpretation affected by target neuron location.

    • Aspects of sensory perception:

    • Perceptual Detection: Ability to recognize a stimulus.

    • Magnitude Estimation: Intensity coded in frequency of impulses.

    • Spatial Discrimination: Identifying site or pattern of stimulus.

    • Feature Abstraction: Identification of complex aspects of stimuli.

    • Quality Discrimination: Recognizing submodalities of sensations (e.g., tastes).

    • Pattern Recognition: Recognition of familiar stimuli patterns.

Perception of Pain

  • Purpose: Warns of impending tissue damage.

  • Stimuli include extreme pressure, temperature, histamine, etc.

  • Pain impulses affected by endogenous opioids (e.g., endorphins).

  • Pain Tolerance: Varies among individuals; influenced by genetics.

  • Examples of pain types:

    • Visceral Pain: Vague, aching pain from internal organs.

    • Referred Pain: Pain perceived at a site other than the source (e.g., heart attack pain felt in the arm).

Clinical Implications of Pain Perception

  • Phantom Limb Pain: Pain felt in an amputated limb.

  • Hyperalgesia: Pain amplification due to long-lasting pain.

Part 2 - Transmission Lines: Nerves and Their Structure

13.3 Nerves and Associated Ganglia

  • Definition: Nerves are cordlike organs of the PNS formed by a bundle of myelinated and nonmyelinated peripheral axons enclosed in connective tissue.

  • Types of Nerves:

    • Spinal Nerves: Originate from the spinal cord.

    • Cranial Nerves: Originate from the brain.

  • Connective Tissue Coverings:

    • Endoneurium: Encloses individual axons and myelin sheaths.

    • Perineurium: Bundles fibers into fascicles.

    • Epineurium: All fascicles are encased to form the nerve.

Structure and Classification of Nerves

  • Mixed Nerves: Contain both sensory and motor fibers.

  • Classification by Function:

    • Afferent (Sensory) Nerves: Impulses towards the CNS.

    • Efferent (Motor) Nerves: Impulses away from the CNS.

    • Pure sensory or motor nerves are rare.

Regeneration of Nerve Fibers

  • Mature Neurons: Are usually amitotic, yet PNS axons may regenerate if the cell body is intact.

  • CNS Axons: Do not typically regenerate due to inhibitory proteins and scar tissue.

  • PNS Axon Regeneration Steps:

    1. Degeneration of axon fragments and myelin sheaths distal to injury (Wallerian degeneration).

    2. Macrophages clean out debris, Schwann cells stimulate growth.

    3. Axon filaments grow through regeneration tubes formed by Schwann cells.

    4. Axon regenerates and forms new myelin sheaths.

Part 3 - Motor Endings and Motor Activity

13.6 Peripheral Motor Endings

  • Motor Endings: Activate effectors (muscles and glands) through neurotransmitter release.

Innervation of Skeletal Muscle

  • At Neuromuscular Junction:

    • Acetylcholine (ACh) is released when an action potential reaches the axon terminal.

    • Binds to receptors on the muscle cell membrane leading to sodium (Na+) influx and subsequent muscle contraction.

    • Termination: ACh effects are terminated by acetylcholinesterase breaking it down.

Innervation of Visceral Muscle and Glands

  • Autonomic endings and effectors are simpler with diffuse junctions for neurotransmitters like ACh and norepinephrine affecting visceral responses.

Levels of Motor Control

  • Three Control Levels:

    1. Segmental Level: Reflexes and basic movements.

    2. Projection Level: Voluntary movements initiated by upper motor neurons.

    3. Precommand Level: Regulates motor activity, starts/stops movements and coordinates them with posture.

Part 4 - Reflex Activity

13.9 Components of a Reflex Arc

  • Reflex Arc Components:

    1. Receptor: Stimulus site.

    2. Sensory Neuron: Sends impulses to CNS.

    3. Integration Center: In CNS (monosynaptic or polysynaptic).

    4. Motor Neuron: Conducts impulses to effector organ.

    5. Effector: Responds to the impulse (muscle or gland).

  • Types of Reflexes:

    • Somatic Reflexes: Activate skeletal muscles.

    • Autonomic Reflexes: Activate visceral effectors.

Stretch and Tendon Reflexes

  • Functional Anatomy:

    • Muscle spindles consist of intrafusal fibers and send sensory input regarding muscle length.

    • Stretch Reflex: Maintains muscle tone and keeps posture (e.g., knee-jerk reflex).

    • Tendon Reflex: Protects muscles by preventing excessive tension.

The Flexor and Crossed-Extensor Reflexes

  • Flexor Reflex: Withdraws body part from harmful stimuli (polysynaptic).

  • Crossed-Extensor Reflex: Maintains balance, typically happens concurrently with flexor reflexes.

Superficial Reflexes

  • Elicited by cutaneous stimulation, reflect integrity of motor pathways.

    • Examples: Plantar reflex (involves assessment from L4–S2) and abdominal reflex (assesses T8–T12).

Homeostatic Imbalance in Reflexes

  • Damaged motor pathways can alter reflex responses which are clinically significant (e.g., hyperactive or hypoactive reflexes).

Developmental Aspects of the Peripheral Nervous System

  • Spinal nerves emerge from spinal cord during development and correlate with body segmentation; age-related changes in reflexes and sensory receptor sensitivity can occur.