The Peripheral Nervous System

Peripheral Nervous System Overview

• Definition: Comprises all neural structures outside the Central Nervous System (CNS), including nerves, ganglia, sensory receptors, and motor endings.
• Ganglia: Clusters of neuron cell bodies located in the PNS.
• Nerve: A cordlike organ composed of bundles of peripheral nerve fibers surrounded by connective tissue.

Sensory Function & Receptors

• Sensation vs. Perception: Sensation is the detection of environmental stimuli; perception is the brain's interpretation of those stimuli.
• Sensory Adaptation: Process where sensory receptors become less responsive over time to constant, unchanging stimuli.
• Receptor Types:
  • Mechanoreceptors: Detect physical deformation from touch, pressure, or stretch.
  • Thermoreceptors: Respond to changes in temperature.
  • Photoreceptors: Respond to light energy.
  • Chemoreceptors: Detect changes in chemical concentration (e.g., blood glucose, oxygen).
  • Nociceptors (Pain Receptors): Respond to tissue damage.
• Pain Classification:
  • Visceral Pain: Results from stimulation of internal organ receptors.
  • Referred Pain: Visceral pain perceived as originating from a somatic (surface) area.

Structure & Classification of Nerves

• Connective Tissue Coverings:
  • Endoneurium: Surrounds individual axons and myelin sheaths.
  • Perineurium: Wraps groups of fibers into fascicles.
  • Epineurium: Outer fibrous sheath enclosing the whole nerve.
• Functional Types: Sensory (afferent), motor (efferent), or mixed nerves.

Cranial Nerves

• Total: 12 pairs connected to the brain.
• Functions:
  • I Olfactory: Sensory for smell.
  • II Optic: Sensory for vision.
  • III Oculomotor: Motor for eye movements, iris constriction, lens shape, and raising the eyelid.
  • IV Trochlear: Motor for superior oblique eye muscle.
  • V Trigeminal: Sensory for face; motor for chewing (Ophthalmic, Maxillary, Mandibular divisions).
  • VI Abducens: Motor for lateral rectus eye muscle.
  • VII Facial: Sensory for taste; motor for facial expression, lacrimal, and salivary glands.
  • VIII Vestibulocochlear: Sensory for hearing and balance.
  • IX Glossopharyngeal: Sensory for tongue taste; motor for pharynx and throat.
  • X Vagus: Sensory and motor for pharynx, larynx, and thoracic/abdominal viscera.
  • XI Accessory: Motor for neck and upper back muscles.
  • XII Hypoglossal: Motor for tongue muscles in speech and swallowing.

Spinal Nerves, Plexuses, & Dermatomes

• Total Pairs: 31 pairs connected to spinal cord via ventral (motor) and dorsal (sensory) roots, exiting intervertebral foramina:
  • 8 pairs Cervical nerves
  • 12 pairs Thoracic nerves
  • 5 pairs Lumbar nerves
  • 5 pairs Sacral nerves
  • 1 pair Coccygeal nerves
• Nerve Plexus: Interwoven network of nerves from different spinal levels serving a common body region, reducing risk of total muscle paralysis from single segment damage.
• Hilton's Law: Any nerve supplying a joint also innervates the muscles moving that joint and the overlying skin.
• Dermatome: Area of skin innervated by cutaneous branches of a single spinal nerve (all except C1).

Somatic vs. Autonomic Nervous System

• Somatic Nervous System:
  • Control: Voluntary.
  • Effectors: Skeletal muscles.
  • Pathway: Single myelinated neuron.
  • Neurotransmitter: Acetylcholine (ACh); always stimulatory.
• Autonomic Nervous System (ANS):
  • Control: Involuntary.
  • Effectors: Cardiac muscle, smooth muscle, glands.
  • Pathway: Two-neuron chain (preganglionic and postganglionic).
  • Neurotransmitters: Acetylcholine (ACh), epinephrine, norepinephrine; stimulatory or inhibitory.

Autonomic Nervous System Divisions

• **Sympathetic Division (