Marieb Human Anatomy & Physiology - Peripheral Nervous System and Reflex Activity

Introduction to the Peripheral Nervous System (PNS)

  • The PNS provides links from and to the external environment.
  • It consists of all neural structures outside the brain and spinal cord.
  • The PNS can be divided into four parts:
    • Part 1: Sensory Receptors and Sensations
    • Part 2: Transmission Lines: Nerves and Their Structure and Repair
    • Part 3: Motor Endings and Motor Activity
    • Part 4: Reflex Activity

Importance of Understanding the PNS

  • Knowledge of the PNS is crucial for recognizing and treating nerve damage.

Sensory Receptors and Sensation

13.1 Sensory Receptors

  • Definition: Sensory receptors are specialized to respond to environmental changes (stimuli).
  • Activation triggers graded potentials that lead to nerve impulses.
  • Awareness of stimuli (sensation) and interpretation (perception) occur in the brain.

Classification of Sensory Receptors

  • Receptors can be classified in three ways:
    • By type of stimulus
    • By body location
    • By structural complexity

Classification by Stimulus Type

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

Classification by Location

  1. Exteroceptors: Respond to external stimuli (e.g., skin receptors for touch).
  2. Interceptors (Visceroceptors): Respond to stimuli from internal organs.
  3. Proprioceptors: Respond to stretch in muscles and joints; inform the brain of body position.

Classification by Receptor Structure

  • Majority fall into two categories:
  1. Simple receptors of the general senses: Modified dendritic endings of sensory neurons, found throughout the body.
  2. Receptors for special senses: Vision, hearing, smell, etc., housed in complex organs.

General Sensory Receptors (Structural Classification)

  • Nonencapsulated (free) nerve endings:
    • Abundant in epithelia and connective tissues.
    • Mostly nonmyelinated, small-diameter group C fibers; respond to temperature, pain, light touch.
  • Tactile (Merkel) discs: Light touch receptors found in the epidermis.
  • Hair follicle receptors: Detect bending of hairs.
  • Encapsulated dendritic endings:
    • Almost all are mechanoreceptors encased in connective tissue
    • Includes tactile (Meissner’s) corpuscles, lamellar (Pacinian) corpuscles, bulbous corpuscles (Ruffini endings), muscle spindles, tendon organs, and joint kinesthetic receptors.

Sensory Processing

13.2 Sensory Processing

  • Depend on sensation (awareness of changes) and perception (interpretation of stimuli).

General Organization of the Somatosensory System

  • Part of sensory system serving the body wall and limbs; receives inputs from the following:
    • Exteroceptors, proprioceptors, interoceptors.

Levels of Neural Integration in Sensory Systems

  1. Receptor level: Involves sensory receptors.
  2. Circuit level: Involves ascending pathways processing sensory input.
  3. Perceptual level: Involves processing in cortical sensory areas.

Processing at the Receptor Level

  • Generator potential and receptor potential methods of transduction.
  • Important for sensory signal adaptation (phasic and tonic receptors).

Perception of Pain

13.3 Pain Mechanisms

  • Pain warns about tissue damage; involves extreme pressure, temperature, histamines, ATP, acids.
  • Pain impulses travel through glutamate and substance P neurotransmitters.
  • Pain tolerance differs among individuals due to genetic influences.
  • Visceral pain: Arises from organs; can be vague.
  • Referred pain: Pain perceived at different locations than the stimulus.

Transmission Lines: Nerves and Their Structure and Repair

13.4 Nerves

  • Definition: Nerve is a cordlike organ of the PNS containing myelinated and nonmyelinated axons
  • Enclosed by connective tissue (endoneurium, perineurium, epineurium).

Classification of Nerves

  • Cut into three types based on impulse direction:
    • Mixed nerves: Both sensory and motor fibers.
    • Sensory nerves: Sensory impulses only toward CNS.
    • Motor nerves: Motor impulses only away from CNS.

Structure of Nerves

  • Connective tissue coverings reinforce nerve structure.

Regeneration of Nerve Fibers

  • Unlike the CNS, the PNS can regenerate under certain conditions.
  • Steps include Wallerian degeneration, macrophage cleanup, growth through regeneration tubes, new myelin sheath formation.

Cranial Nerves

13.4 Cranial Nerves Overview

  • 12 pairs of cranial nerves, primarily attached to brain.
  • Mixed nerves, except for two purely sensory.

Detailed Overview of Cranial Nerves

  1. Olfactory Nerves (CN I): Sensory for smell; damage can result in anosmia.
  2. Optic Nerves (CN II): Sensory for vision; damage can result in anopsia.
  3. Oculomotor Nerves (CN III): Motor for eye movement and pupil constriction.
  4. Trochlear Nerves (CN IV): Motor for superior oblique muscle movement.
  5. Trigeminal Nerves (CN V): Mixed; sensory from face and motor for chewing.
  6. Abducens Nerves (CN VI): Motor for lateral rectus muscle movement.
  7. Facial Nerves (CN VII): Mixed; control facial expressions and taste from the anterior tongue; affected in Bell's palsy.
  8. Vestibulocochlear Nerves (CN VIII): Mostly sensory for hearing and balance; lesions can cause dizziness.
  9. Glossopharyngeal Nerves (CN IX): Mixed; taste from posterior tongue, motor for swallowing.
  10. Vagus Nerves (CN X): Mixed; regulate heart, lungs, abdominal viscera.
  11. Accessory Nerves (CN XI): Motor for trapezius and sternocleidomastoid muscles.
  12. Hypoglossal Nerves (CN XII): Motor for tongue movements; damage results in difficulty speaking.

Spinal Nerves

13.5 Overview of Spinal Nerves

  • 31 pairs of spinal nerves classified into cervical, thoracic, lumbar, sacral, and coccygeal.

Structure of Spinal Nerves

  • Each spinal nerve has ventral and dorsal roots containing motor and sensory fibers, respectively.

Rami Distribution

  • After exiting the vertebral column, spinal nerves divide into three branches:
    • Dorsal ramus: supplies posterior body trunk.
    • Ventral ramus: supplies the rest of the trunk and limbs.
    • Meningeal branch: re-enters the vertebral canal to innervate meninges.

Plexuses and Innervation

  • Cervical Plexus: Innervates neck and includes the phrenic nerve (diaphragm).
  • Brachial Plexus: Innervates the upper limb; includes major branches like the axillary, musculocutaneous, median, ulnar, and radial nerves.
  • Lumbosacral Plexus: Innervates the lower limb; includes femoral and sciatic nerves.

Motor Endings and Motor Activity

13.6 Peripheral Motor Endings

  • Motor endings activate effectors by releasing neurotransmitters that innervate skeletal muscles, visceral muscle, and glands.

Neuromuscular Junction

  • ACh released at neuromuscular junction leading to muscle contraction.

Levels of Motor Control

  1. Segmental level: Consists of reflexes and automatic movements.
  2. Projection level: Involves upper motor neurons and oversees indirect control.
  3. Precommand level: Coordinates movements with posture and planning.

Reflex Activity

13.8 Reflex Arc

  • Definition: An inborn reflex is rapid and involuntary, while learned reflexes result from practice.

Components of a Reflex Arc

  1. Receptor: detects stimulus.
  2. Sensory neuron: transmits impulses to CNS.
  3. Integration center: processes information.
  4. Motor neuron: conveys impulses to the effector.
  5. Effector: responds.

Somatic and Autonomic Reflexes

  • Somatic reflexes activate skeletal muscle while autonomic reflexes activate visceral effectors.

Spinal Reflexes

  • Spinal reflexes include stretch reflexes that help maintain posture and require sensory input from muscle spindles.

Examples of Reflexes

  1. Stretch Reflex: E.g., knee-jerk reflex keeps knee stable when standing.
  2. Tendon Reflex: Prevents muscle damage due to excessive stretch.
  3. Flexor Reflex: Initiated by painful stimuli; protective in nature.
  4. Crossed-Extensor Reflex: Balances weight during withdrawal of the injured limb.
  5. Superficial Reflexes: Tested clinically; includes plantar and abdominal reflexes.

Developmental Aspects of the PNS

  • Spinal nerves develop from spinal cord and neural crest cells.
  • Distribution correlates with body plan; sensory receptors decrease with age.
  • Peripheral nerves can remain functional throughout life unless damaged significantly.