Peripheral Nervous System (PNS) Overview

Function and Pathways

The Peripheral Nervous System (PNS) connects the Central Nervous System (CNS)—the brain and spinal cord—to the rest of the body and the external environment. It is essential for transmitting sensory information to the CNS and carrying out motor commands from the CNS to effectors.

• Function: Links CNS to body and environment.

• Pathways:

  1. Sensory input: Detects stimuli and sends signals to CNS.

  2. CNS integration: Processes input.

  3. Motor output: Signals muscles and glands to respond.

• Main organs: Peripheral nerves (bundles of neuron axons wrapped in connective tissue).

Divisions of the PNS

Sensory (Afferent) Division

The sensory division carries information from sensory receptors to the CNS for interpretation.

• Somatic Sensory Division:

  • Detects stimuli from skin, muscles, and joints.

  • Senses: touch, temperature, pain, muscle stretch, and chemical changes.

  • Includes special senses: vision, hearing, equilibrium, taste, smell.

• Visceral Sensory Division:

  • Senses internal changes (e.g., blood pressure, organ stretch).

Motor (Efferent) Division

The motor division carries commands from the CNS to effectors such as muscles and glands.

• Somatic Motor Division: Controls skeletal muscles (voluntary).

• Autonomic Nervous System (ANS): Controls smooth & cardiac muscle, glands (involuntary).

Peripheral Nerves

Types of Peripheral Nerves

• Mixed nerves: Carry both sensory and motor axons.

• Sensory nerves: Only sensory axons.

• Motor nerves: Only motor axons (may retain a few sensory fibers).

Spinal and Cranial Nerves

• Spinal nerves: Emerge from spinal cord, innervate body below the head and neck.

  • Anterior root: Motor axons (exit CNS).

  • Posterior root: Sensory axons (enter CNS).

  • Posterior root ganglion: Houses sensory neuron cell bodies.

  • Spinal nerve: Mixed (sensory + motor).

• Cranial nerves: Attach to brain; mainly serve head & neck. Can be sensory, motor, or mixed.

Cranial Nerves

Mnemonic for Order

"Oh Once One Takes The Anatomy Final Very Good Vacations Are Happening" (I–XII order)

Mnemonic for Type (Sensory/Motor/Both): "Some Say Money Matters But My Brother Says Big Brains Matter More"

Important Cranial Nerve Conditions

• Trigeminal Neuralgia: Chronic pain due to trigeminal nerve compression. Symptoms: sharp, electric facial pain, triggered by chewing or touch. Treated with anticonvulsants.

• Bell's Palsy: Damage to facial nerve (CN VII). Symptoms: one-sided facial paralysis, loss of taste (anterior tongue), blinking issues. Often resolves in 3–8 weeks.

Spinal Nerves and Plexuses

Structure

• Spinal nerves are mixed (sensory + motor).

• Split into:

  • Posterior rami: Back skin & muscles.

  • Anterior rami: Front of body & limbs.

  • Rami communicantes: Autonomic fibers (visceral motor).

Plexuses (Nerve Networks)

Mnemonic for Phrenic Nerve: "C3, C4, C5 keep the diaphragm alive"

Key Clinical Notes

• Hiccups: Diaphragm spasm; can be stopped by pressing on phrenic nerve area in neck.

• "Funny bone" pain: Ulnar nerve contusion near elbow.

Sensory Receptors & Sensation

Sensory Transduction

Sensory transduction converts stimulus into electrical signals. The process involves:

• Stimulus opens ion channels → sodium influx

• Receptor potential → action potential

Types of Receptors

Example: Capsaicin (in chili peppers) activates nociceptors → heat/pain signal.

Sensory Neurons and Pathways

• Different fiber diameter: Larger diameter = faster conduction.

• Cell bodies: In dorsal root ganglia.

• Speed: Larger diameter + more myelination = faster conduction.

Sensory Pathways

• Stimulus: Sensory receptor → PNS sensory neuron → CNS for interpretation.

Motor Pathways

• CNS motor areas → Upper motor neurons → Lower motor neurons (PNS) → Muscles/glands

Reflexes

Reflex Arc

Reflexes are automatic response pathways:

• Stimulus → Receptor → Sensory neuron → CNS integration → Motor neuron → Effector

Types of Reflexes

Sensory and Motor Neuron Disorders

Peripheral Neuropathies

• Damage to peripheral nerves (e.g., from diabetes, trauma, toxins).

Sensory Disorders

• Loss of sensation or exaggerated pain.

• Can affect proprioception—poor balance/movement.

Motor Disorders

• Loss of motor neuron function; loss of reflexes, flaccid tone.

• Upper Motor Neuron Disorders:

  • In CNS; cause spasticity, clonus, and Babinski sign (toes fan out).

• Amyotrophic Lateral Sclerosis (ALS):

  • Degeneration of motor neurons (upper & lower).

  • Symptoms: muscle weakness, paralysis, death within a few years.

  • No cure yet.

Quick Review Mnemonics