The Peripheral Nervous System and Reflex Activity

THE PERIPHERAL NERVOUS SYSTEM

  • Comprises all neural structures outside the brain and spinal cord.

  • Contains sensory and motor neurons transmitting information to/from the CNS.

SENSORY RECEPTORS

  • Specialized neurons responding to environmental changes.

  • Types:

    1. Mechanoreceptors

    2. Thermoreceptors

    3. Photoreceptors

    4. Chemoreceptors

    5. Nociceptors

NERVES

  • Cordlike organs of the PNS, composed of nerve fibers in fascicles.

  • Connective coverings:

    • Epineurium: surrounds the entire nerve.

    • Perineurium: surrounds bundles of fibers.

    • Endoneurium: surrounds individual fibers.

  • Ganglia: collections of neuron cell bodies in the PNS.

  • Nerves can be mixed, sensory, or motor.

CRANIAL NERVES

  • 12 pairs, associated with the brain.

  • Key cranial nerves:

    1. Olfactory (I): smell.

    2. Optic (II): vision.

    3. Oculomotor (III): eye movement, pupil size.

    4. Trochlear (IV): eye muscle.

    5. Trigeminal (V): mixed; face sensation, mastication.

    6. Abducens (VI): eye position.

    7. Facial (VII): face sensation, muscle control, lacrimal glands.

    8. Vestibulocochlear (VIII): hearing and balance.

    9. Glossopharyngeal (IX): taste, swallowing.

  1. Vagus (X): internal organ impulses.

  2. Accessory (XI): neck and larynx muscles.

  3. Hypoglossal (XII): tongue movement.

SPINAL NERVES

  • 31 pairs, arise from the spinal cord (all mixed nerves).

  • Attach via dorsal (sensory) and ventral (motor) roots.

  • Important nerves include:

    • Intercostal

    • Phrenic

    • Ulnar and Radial

    • Sciatic

REFLEXES

  • Rapid, involuntary motor responses to stimuli.

  • Regulated by the spinal cord, types:

    1. Somatic Reflexes: skeletal muscle.

    2. Autonomic Reflexes: smooth and cardiac muscles, glands.

  • Reflex Arc Components:

    1. Receptor

    2. Sensory Neuron

    3. Integration Center (brain/spinal cord)

    4. Motor Neuron

    5. Effector (muscle/gland)

  • Important reflexes:

    1. Patellar Reflex

    2. Achilles Reflex

    3. Babinski Sign

CLINICAL TERMS

  • Analgesia: reduced pain sensation.

  • Neuralgia: nerve-related pain.

  • Neuritis: nerve inflammation.

SENSATION

  • Awareness of stimuli, types include temperature, hearing, balance, smell, pain.

  • Components:

    1. Stimulation: stimuli activating sensory neurons.

    2. Transduction: conversion of stimuli into impulses.

    3. Conduction: impulse movement to CNS.

    4. Integration: impulse conversion into sensation (in brain).

SENSORY NEURONS

  • Selective: respond to one stimulus type.

  • Classified by location:

    1. Exteroceptors

    2. Interoceptors

  • Classified by stimulus type:

    1. Mechanoreceptors

    2. Thermoreceptors

    3. Nociceptors

    4. Photoreceptors

    5. Chemoreceptors

CATEGORIES OF SENSES

  • General (Somatic): skin/body sensations.

  • Special Senses: taste, smell, sight, hearing, balance.