Chapter 13: The Peripheral Nervous System (PNS)

Overview of the PNS

The Peripheral Nervous System (PNS) serves as the communication link between the Central Nervous System (CNS) and the rest of the body, detecting sensory stimuli and delivering them to the CNS, and transmitting motor commands from the CNS to effectors such as muscles and glands.

• Divisions of the PNS:

  • Sensory (Afferent) Division: Detects sensory stimuli and brings them to the CNS.

  • Motor (Efferent) Division: Transmits impulses from the CNS to effectors.

• Sensory Division Subdivisions:

  • Somatic Sensory Division: Detects stimuli from skin, musculoskeletal system, and special senses (e.g., touch, temperature, pain, vision, hearing).

  • Visceral Sensory Division: Relays sensory stimuli from internal organs (e.g., blood pressure, organ stretch).

• Motor Division Subdivisions:

  • Somatic Motor Division: Controls voluntary movements via lower motor neurons that innervate skeletal muscle fibers.

  • Visceral Motor Division (Autonomic Nervous System, ANS): Controls involuntary functions, innervating cardiac muscle, smooth muscle, and glands. Subdivided into:

    • Sympathetic Nervous System: "Fight or flight" responses, maintains homeostasis during physical activity and emotional responses.

    • Parasympathetic Nervous System: "Rest and digest" functions.

Peripheral Nerves and Ganglia

The main organs of the PNS are peripheral nerves, which are bundles of axons bound by connective tissue sheaths. Most are mixed nerves (containing both sensory and motor axons), but some are primarily sensory or motor.

• Types of Nerves:

  • Spinal Nerves: Originate from the spinal cord; all are mixed nerves (31 pairs).

  • Cranial Nerves: Attach to the brain; may be sensory, motor, or mixed (12 pairs).

• Connective Tissue Sheaths:

  • Epineurium: Surrounds the entire nerve.

  • Perineurium: Surrounds each fascicle (bundle of axons).

  • Endoneurium: Surrounds individual axons.

• Ganglion: A collection of cell bodies in the PNS (e.g., posterior root ganglion).

Cranial Nerves

Classification and Functions

Cranial nerves transmit impulses to and from the brain. Each is identified by a Roman numeral and a name reflecting its function.

• Sensory Cranial Nerves: Olfactory (I), Optic (II), Vestibulocochlear (VIII)

• Motor Cranial Nerves: Oculomotor (III), Trochlear (IV), Abducens (VI), Accessory (XI), Hypoglossal (XII)

• Mixed Cranial Nerves: Trigeminal (V), Facial (VII), Glossopharyngeal (IX), Vagus (X)

Mixed nerves contain somatic sensory, somatic motor, and parasympathetic axons, leading to diverse functions and vulnerability to multiple dysfunctions if damaged.

Spinal Nerves and Plexuses

Structure and Branches

There are 31 pairs of spinal nerves, each formed by the fusion of an anterior (motor) and posterior (sensory) root. After exiting the vertebral cavity, each spinal nerve splits into:

• Posterior Ramus: Supplies the posterior body.

• Anterior Ramus: Supplies the anterior body and limbs; forms plexuses.

• Rami Communicantes: Contain autonomic axons (not mixed nerves).

Spinal nerves are grouped as follows: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Major Nerve Plexuses

• Cervical Plexus (C1–C4): Supplies neck, head, chest, shoulders; major branch is the phrenic nerve (diaphragm control).

• Brachial Plexus (C5–T1): Supplies upper limb; major nerves:

  • Axillary: Deltoid, teres minor, skin over deltoid.

  • Radial: Triceps brachii, forearm extensors, posterior hand skin.

  • Musculocutaneous: Forearm flexors, biceps brachii, lateral forearm skin.

  • Median: Forearm/hand flexors, anterior hand skin; involved in carpal tunnel syndrome.

  • Ulnar: Forearm/hand flexors not supplied by median, intrinsic hand muscles, medial hand skin.

• Thoracic Nerves (T1–T12): Do not form plexuses (except T1); anterior rami become intercostal nerves, supplying intercostal muscles, chest wall, and abdominal wall.

• Lumbar Plexus (L1–L4): Supplies pelvis and lower extremity; major nerves:

  • Obturator: Thigh adductors, hip joint, medial thigh skin.

  • Femoral: Anterior thigh extensors (quadriceps), anterior/medial thigh and leg skin.

• Sacral Plexus (L4–S4): Supplies pelvis, gluteal region, lower extremity; major nerve:

  • Sciatic Nerve: Largest nerve in the body; splits into:

    • Tibial Nerve: Posterior thigh/leg muscles, plantar foot.

    • Common Fibular (Peroneal) Nerve: Lateral/anterior leg muscles, dorsum of foot.

Sensation: Role of the PNS

Sensory Receptors and Transduction

Sensory receptors detect changes in the internal and external environment, converting stimuli into electrical signals (sensory transduction).

• Types of Sensory Receptors:

  • Encapsulated Nerve Endings: Surrounded by supportive cells.

  • Free Nerve Endings: Lack supportive cells; "naked" endings.

• Steps in Sensory Transduction:

  1. Stimulus opens gated ion channels in the axolemma.

  2. Ion flow generates a receptor potential (depolarization).

  3. If threshold is reached, an action potential (AP) is triggered and propagated to the CNS.

• Adaptation: Rapidly adapting receptors respond quickly but stop firing; slowly adapting receptors maintain constant APs.

Classification of Sensory Receptors

• By Location:

  • Exteroceptors: Near body surface; detect external stimuli.

  • Interoceptors: Within body; detect internal stimuli (e.g., BP, organ stretch).

• By Stimulus Type:

  • Mechanoreceptors: Respond to mechanical deformation (touch, pressure, stretch).

  • Thermoreceptors: Respond to temperature changes.

  • Chemoreceptors: Respond to chemical changes (e.g., pH, O2, CO2).

  • Photoreceptors: Respond to light (only in the eye).

  • Nociceptors: Respond to noxious/painful stimuli.

Types of Mechanoreceptors (Superficial to Deep)

• Merkel (Tactile) Endings: Discriminative touch (form, texture); high spatial resolution.

• Meissner (Tactile) Corpuscles: Discriminative touch; less fine resolution.

• Ruffini (Bulbous) Corpuscles: Stretch and movement; slowly adapting.

• Pacinian (Lamellated) Corpuscles: Deep pressure, high-frequency vibration; rapidly adapting.

• Hair Follicle Receptors: Detect hair movement.

• Proprioceptors: Detect joint/muscle position and movement.

Thermoreceptors

• Cold Receptors: 50–104°F (10–40°C); superficial dermis.

• Hot Receptors: 90–118°F (32–48°C); deeper dermis.

• Temperatures outside these ranges are detected by nociceptors.

Sensory Neurons

• Structure: Pseudounipolar neurons with cell body in posterior root ganglion, peripheral process (to receptor), and central process (to CNS).

• Classification by Conduction Speed:

  • Large diameter, thick myelin: Fast conduction (discriminative touch).

  • Small diameter, thin/no myelin: Slow conduction (pain, temperature).

• Receptive Field: Area served by a neuron; measured by two-point discrimination threshold.

Dermatomes and Referred Pain

• Dermatome: Skin segment supplied by a single spinal nerve; mapped for clinical testing.

• Referred Pain: Visceral pain perceived as cutaneous pain along a dermatome (e.g., heart attack pain in left arm).

Movement: Role of the PNS

Motor Output Pathway

• Upper Motor Neurons: Originate in CNS, synapse on lower motor neurons.

• Lower Motor Neurons: Cell bodies in CNS, axons in PNS; innervate skeletal muscle fibers.

• Excitation-Contraction: Lower motor neurons release acetylcholine (ACh) to trigger muscle contraction.

• Damage to Lower Motor Neurons: Results in muscle paralysis.

Reflex Arcs: Integration of Sensory and Motor Functions

Reflexes and Reflex Arcs

Reflexes are programmed, automatic responses to stimuli, often protective in nature. The sequence of a reflex arc:

1. PNS detects and delivers stimulus to CNS.

2. CNS integrates the stimulus.

3. PNS delivers motor response to effectors.

Most reflexes are negative feedback loops.

Stretch Receptors in Skeletal Muscle

• Muscle Spindles: Detect muscle stretch; contain intrafusal fibers innervated by sensory and motor neurons.

• Golgi Tendon Organs: Detect muscle tension; located near muscle-tendon junction.

Types of Reflexes

• Monosynaptic Reflex: Single synapse (e.g., simple stretch reflex).

• Polysynaptic Reflex: Multiple synapses (e.g., withdrawal reflex).

• Visceral Reflex: Involves internal organs, often via ANS.

Examples of Reflexes

• Simple Stretch Reflex: Maintains optimal muscle length (e.g., patellar/knee-jerk reflex).

• Flexion (Withdrawal) Reflex: Rapid withdrawal from painful stimulus; often accompanied by crossed-extension reflex to maintain balance.

• Golgi Tendon Reflex: Causes muscle relaxation in response to excessive tension; protects muscles/tendons.

• Cranial Nerve Reflexes: Involve both afferent and efferent cranial nerves (e.g., gag reflex, corneal blink reflex).

Clinical Relevance: Sensory and Motor Neuron Disorders

Peripheral Neuropathies

Disorders affecting PNS sensory and motor neurons, caused by trauma, systemic illness (e.g., diabetes), or toxins.

• Sensory Neuron Disorders: May cause loss of sensation, pain, or proprioceptive deficits.

• Lower Motor Neuron Disorders: Result in paralysis, loss of muscle tone, and reduced reflexes.

• Upper Motor Neuron Disorders: Damage to CNS pathways; initial paralysis (spinal shock), followed by spasticity, increased reflexes, and abnormal signs (e.g., Babinski sign).

• Amyotrophic Lateral Sclerosis (ALS): Degeneration of upper and lower motor neurons, leading to progressive paralysis.

Table: Major Nerve Plexuses and Their Key Nerves

Example: Patellar Reflex (Knee-Jerk)

• Tap on patellar tendon stretches quadriceps muscle.

• Muscle spindle detects stretch, sends signal via sensory neuron to spinal cord.

• Motor neuron stimulates quadriceps to contract, producing knee extension.

Additional info:

• Some details about the specific cranial nerve functions and clinical testing (e.g., Babinski sign) were expanded for clarity.

• Table entries and some anatomical context were inferred for completeness.