Peripheral Nervous System & General Senses Lecture

Peripheral Nervous System (PNS)

• Definition: all neural tissue outside the CNS (brain & spinal cord)
  • Includes cranial nerves, spinal nerves, peripheral ganglia, sensory receptors, special-sense organs (eye, ear, nose, tongue)
• Functional role: bidirectional highway for information
  • Afferent → sensory data from body to CNS
  • Efferent → motor commands from CNS to effectors (muscles, glands)
• PNS + CNS cooperate in neural integration (incoming analysis & outgoing response)

Sensation vs. Perception

• Sensation: conscious OR subconscious awareness of internal/external change
  • Each distinct sensation = "sensory modality"
• Perception: meaningful interpretation, mainly in cerebral cortex

Categories of Sensory Modalities

• General (somatic + visceral) senses
  • Tactile: touch, pressure, vibration, itch, tickle
  • Thermal: warm, cold, temperature change
  • Pain: nociception
  • Proprioception: body position & movement, joint/muscle tension
  • Visceral: stretch, chemical, pressure inside organs (blood pressure, bladder fullness, GI contents, etc.)
• Special senses (handled in Ch. 17): smell, taste, vision, hearing, equilibrium

Sensory Receptor Types (by stimulus)

• Chemoreceptors – detect chemicals (odorants, tastants, blood \text{pH}, \text{CO}2, \text{O}2, \text{Na}^+, hunger signals)
• Photoreceptors – respond to light (retina)
• Mechanoreceptors – plasma-membrane distortion; include:
  • Touch/pressure/vibration corpuscles
  • Hair cells (hearing & equilibrium)
  • Proprioceptors (muscle spindles, tendon organs, joint kinesthetic)
• Thermoreceptors – warm & cold; sensitive mainly to ΔT, not absolute T
• Nociceptors – pain (physical/chemical damage); neurotransmitter: Substance P; no peripheral adaptation
• Osmoreceptors – monitor osmotic pressure (water vs. solute concentration)

Sensory Receptor Types (by structure)

• Free nerve endings – bare dendrites (pain, temperature, itch, tickle, some light touch)
• Encapsulated nerve endings – dendrites enclosed in connective tissue (deep pressure, vibration)
• Specialized receptor cells (NOT neurons)
  • Gustatory cells (taste buds)
  • Photoreceptors (rods & cones)
  • Hair cells (inner ear)
• Olfactory receptor – is a bipolar neuron (unique among special senses)

Sensory Receptor Types (by location)

• Exteroceptors – monitor external environment (touch, vision, smell, hearing, etc.)
• Interoceptors – monitor internal milieu (visceral stretch, blood chemistries, internal temperature)
• Proprioceptors – monitor body position (muscles, tendons, joints, inner ear)

Adaptation

• Peripheral adaptation: receptor’s generator potential ↓ during sustained stimulus (clothing touch, perfume smell)
• Central adaptation: CNS inhibition of sensory pathway despite ongoing receptor firing (e.g., pain suppression via endorphins during fight-or-flight)
• Adaptation rates
  • Rapid: thermoreceptors, olfactory receptors, tactile corpuscles
  • Slow/none: nociceptors, proprioceptors

Somatic Senses in Detail

Tactile

• Light touch (Meissner corpuscles), deep touch (Pacinian / lamellated corpuscles), pressure, low-/high-frequency vibration, itch, tickle

Thermal

• Cold receptors (10–40 °C), warm receptors (32–48 °C), respond best to change

Pain

• Fast (acute, sharp, pricking) – <0.1 s to cortex; myelinated fibers
• Slow (chronic, burning, throbbing) – >1 s; unmyelinated fibers
• Somatic (superficial & deep) vs. Visceral pain
• Referred pain: visceral pain perceived in superficial region sharing spinal segment (e.g., heart → left arm/jaw or female back/shoulder, gallbladder → right shoulder)

Proprioception

• Receptors: muscle spindles, Golgi tendon organs, joint kinesthetic receptors
• Provide data on muscle length, tendon tension, joint angle & movement

Cranial Nerves (12 pairs)

• Memory mnemonics
  • Names: "On Old Olympus’ Towering Tops A Fin And German Viewed Some Hops"
  • Function (Sensory/Motor/Both): "Some Say Marry Money But My Brother Says Big Brains Matter More"

Spinal Nerves & Associated Structures

• 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
• Roots
  • Posterior (dorsal) root: sensory; has dorsal root ganglion
  • Anterior (ventral) root: motor
• Connective-tissue wrappings
  • Endoneurium (around axon + myelin)
  • Perineurium (around fascicle)
  • Epineurium (around whole nerve)
• Branches = rami (sing. ramus)
• Dermatome: skin segment supplied by single spinal nerve—clinically important for diagnosis (e.g., shingles) & anesthesia

Nerve Plexuses & Key Peripheral Nerves

Cervical Plexus (C1–C5)

• Supplies head, neck, superior shoulder, diaphragm
• Key nerve: Phrenic (C3–C5) → diaphragm; lesion → respiratory arrest

Brachial Plexus (C5–T1)

• Shoulder & upper limb
  • Axillary – deltoid, teres minor, skin over shoulder
  • Musculocutaneous – biceps brachii, brachialis
  • Radial – posterior arm/forearm, triceps; wrist/finger extensors
  • Median – anterior forearm flexors, lateral palm (carpal tunnel syndrome)
  • Ulnar – intrinsic hand mm., medial hand (“funny bone”)
• Injuries → palsies (wrist drop = radial; claw hand = ulnar; ape hand = median)

Lumbar Plexus (L1–L4)

• Anterolateral abdominal wall, external genitals, part of lower limb
  • Femoral nerve – anterior thigh (quads, sartorius); hip flexors; skin of ant./med. thigh & leg
  • Obturator, iliohypogastric, ilioinguinal, genitofemoral (not tested in lab)

Sacral Plexus (L4–S4)

• Buttocks, perineum, posterior thigh & leg, foot
  • Sciatic nerve (largest in body) → splits to
  • Tibial nerve – posterior leg & plantar foot
  • Common fibular (peroneal) nerve – anterior/lateral leg & dorsum of foot
  • Injury → sciatica (pain), foot drop

Reflexes

• Reflex arc components: receptor → sensory neuron → integrating center (gray matter) → motor neuron → effector
• Terminology
  • Ipsilateral / contralateral
  • Monosynaptic (1 synapse, e.g., stretch reflex) vs. polysynaptic (≥2 synapses)
  • Reciprocal innervation – simultaneous activation of agonist & inhibition of antagonist
• Examples
  • Stretch (patellar) reflex – knee jerk; monosynaptic; maintains muscle tone
  • Tendon reflex – causes muscle relaxation when tendon tension too high
  • Withdrawal (flexor) reflex – ipsilateral, protective
  • Crossed-extensor reflex – contralateral support during withdrawal (maintains balance)

Neurological Disorders Highlighted

• ALS (Amyotrophic Lateral Sclerosis) – progressive motor neuron degeneration; no muscle nourishment → paralysis; e.g., Stephen Hawking
• Poliomyelitis – poliovirus infection; destroys spinal motor neurons; preventable via vaccine; post-polio syndrome possible decades later
• Shingles (Herpes zoster) – reactivation of varicella-zoster virus in a dermatome; painful vesicular rash; vaccine recommended ≥50 yrs

Somatic Sensory Pathways to Cortex

• Typically three-neuron chains
  1. First-order: receptor → spinal cord/brainstem
  2. Second-order: SC/BS → thalamus (decussates)
  3. Third-order: thalamus → primary somatosensory cortex (postcentral gyrus)
• Somatotopic map (sensory homunculus)
  • Large cortical areas: hands, face, lips

Somatic Motor Pathways from Cortex

• Upper motor neurons (precentral gyrus) → decussate → lower motor neurons in SC/brainstem → skeletal muscles
• Direct (pyramidal) vs. indirect (extrapyramidal) tracts
• Motor homunculus mirrors sensory map (hands & facial muscles dominate)

Cerebellar Integration

• Compares intended movement (cortical commands) with actual proprioceptive feedback
• Sends corrective signals to cortex & spinal cord → smooth, coordinated, balanced actions

Wakefulness, Sleep, Memory

• Reticular Activating System (RAS) – maintains consciousness; inhibited for sleep
• REM sleep (dreaming, rapid eye movements) vs. NREM (non-dreaming)
• Coma – profound RAS depression; unarousable
• Memory
  • Immediate (seconds), Short-term (minutes–hours), Long-term (days–lifetime)
  • Long-term potentiation via repetition or single strong emotional event (limbic system + catecholamines)

Key Numerical Facts & Data

• Stomach normal pH ≈ 2 (high [H^+] detected by chemoreceptors)
• Cranial nerves: 12 pairs
• Spinal nerves: 31 pairs (C8 despite 7 cervical vertebrae)
• Brachial plexus roots: C5 – T1
• Phrenic nerve origins: C3 – C5 (“C3,4,5 keep the diaphragm alive”)

Ethical, Clinical & Practical Notes

• Accurate dermatome mapping critical for diagnosing spinal lesions & administering regional anesthesia
• Vaccinations (Polio, Varicella-Zoster) illustrate preventive neurology
• Sports/fight-or-flight: central adaptation overrides pain—risking hidden injury
• Laser-focused chemoreception regulates respiration (CO₂/O₂ sensors) & acid-base homeostasis