PNS Powerpoint

Peripheral Nervous System Overview

  • The Peripheral Nervous System (PNS) consists of nervous structures that are located outside of the brain and spinal cord.

Components of the PNS

  • Nerves: These are bundles of axons.

  • Ganglia: Collections of cell bodies located in the PNS.

  • Sensory Receptors: Specialized neurons responsible for detecting specific stimuli.

Divisions of the PNS

Flow of Information

  • Sensory information flows toward the Central Nervous System (CNS).

  • Motor commands flow away from the CNS.

Functions of the Peripheral Nervous System

  • The PNS is essential for:

    • Sending sensory information to the CNS for integration.

    • Receiving motor commands from the CNS.

Anatomy of Nerves

Composition of Nerves

  • Nerves are made up of hundreds of nerve fibers.

Connective Tissue Layers in Nerves

  • Endoneurium: This delicate loose connective tissue surrounds individual axons.

  • Perineurium: This connective tissue wraps around nerve fascicles (bundles of axons).

  • Epineurium: A tough fibrous sheath that surrounds the entire nerve.

Organization of Axons in a Nerve

  • Axons within a nerve are organized into bundles known as fascicles.

Blood Supply

  • Nerves are also formed with blood vessels, which supply necessary nutrients and oxygen.

Functional Classification of Nerves

Direction of Action Potentials

  • Nerves can be classified based on the direction of action potentials in relation to the CNS:

    • Sensory Nerves: Carry information towards the CNS.

    • Motor Nerves: Carry information away from the CNS.

    • Mixed Nerves: Carry information in both directions (both towards and away from the CNS).

Classification of Nerves by Location

  • Nerves are also categorized based on where they attach to the CNS:

    • Cranial Nerves: Nerves that attach directly to the brain.

    • Spinal Nerves: Nerves that attach to the spinal cord.

Cranial Nerves vs Spinal Nerves

Nerve Classes and Locations

  • The PNS consists of:

    • Cervical Nerves: C₁ - C₈

    • Thoracic Nerves: T₁ - T₁₂

    • Lumbar Nerves: L₁ - L₅

    • Sacral Nerves: S₁ - S₅

    • Coccygeal Nerve: Co₁

Overview of Cranial Nerves

  • There are 12 paired cranial nerves, which are described by:

    • Name

    • Number (Roman numeral)

    • Location of attachment to the brain

    • Function

    • Functional classification (sensory, motor, or mixed).

List of Cranial Nerves

  1. Olfactory Nerve (I)

    • Function: Olfaction (smell)

    • Functional Classification: Sensory

    • Location of Attachment: Cerebrum (Frontal lobe, olfactory bulb, olfactory tract, cribriform plate of ethmoid bone, nasal mucosa)

  2. Optic Nerve (II)

    • Function: Sight

    • Functional Classification: Sensory

    • Location of Attachment: Thalamus (Eyeball, retina, optic chiasma, lateral geniculate nucleus, optic radiation, visual cortex)

  3. Oculomotor Nerve (III)

    • Function: Innervates four extrinsic eye muscles, raises eyelid, constricts pupil

    • Functional Classification: Motor

    • Location of Attachment: Midbrain

  4. Trochlear Nerve (IV)

    • Function: Movement of one extrinsic eye muscle (superior oblique)

    • Functional Classification: Motor

    • Location of Attachment: Midbrain

  5. Trigeminal Nerve (V)

    • Function: Movement of chewing muscles (motor) & facial sensation (sensory)

    • Functional Classification: Mixed

    • Location of Attachment: Pons

  6. Abducens Nerve (VI)

    • Function: Movement of one extrinsic eye muscle (lateral rectus)

    • Functional Classification: Motor

    • Location of Attachment: Pons

  7. Facial Nerve (VII)

    • Function: Movement of facial expression muscles (motor) & taste sensation from anterior two-thirds of tongue (sensory)

    • Functional Classification: Mixed

    • Location of Attachment: Pons

  8. Vestibulocochlear Nerve (VIII)

    • Function: Hearing and equilibrium (balance)

    • Functional Classification: Sensory

    • Location of Attachment: Medulla oblongata

  9. Glossopharyngeal Nerve (IX)

    • Function: Movement of pharyngeal muscles (motor) & taste sensation from posterior one-third of tongue (sensory)

    • Functional Classification: Mixed

    • Location of Attachment: Medulla oblongata

  10. Vagus Nerve (X)

    • Function: Sensory and motor nerve for pharynx, larynx, and viscera

    • Functional Classification: Mixed

    • Location of Attachment: Medulla oblongata

  11. Accessory Nerve (XI)

    • Function: Movement of neck muscles

    • Functional Classification: Motor

    • Location of Attachment: Medulla oblongata

  12. Hypoglossal Nerve (XII)

    • Function: Movement of the tongue

    • Functional Classification: Motor

    • Location of Attachment: Medulla oblongata

Mnemonics for Cranial Nerves

  • For remembering the names of the cranial nerves: "Oh, Oh, Oh, To Touch And Feel Very Good Velvet, Ah Ha!"

  • Each first letter corresponds to the cranial nerve's name in order.

  • For remembering the functional classification: "Some Say Marry Money, But My Brother Says Big Brains Matter More"

    • S = Sensory Nerve

    • M = Motor Nerve

    • B = Both (mixed nerve)

Spinal Nerves

General Information

  • There are 31 pairs of spinal nerves.

  • All spinal nerves are classified as mixed nerves, carrying both sensory information towards the CNS and motor commands away from the CNS.

Anatomy of Spinal Nerves

Dorsal Roots

  • Each spinal nerve features a dorsal root, which carries sensory information to the dorsal horn of the spinal cord.

    • Dorsal root ganglion: Contains the cell bodies of sensory neurons.

Ventral Roots

  • Each spinal nerve includes a ventral root, which carries motor commands away from the spinal cord.

    • Most cell bodies of these neurons are located in the anterior ventral horn.

Dorsal and Ventral Roots Summary

  • Receptive Endings: Receive stimuli from effectors (muscles and glands).

  • Peripheral Process (Axon): Part of the neuron outside the CNS.

  • Central Process (Axon): Part of the neuron that enters the spinal cord.

  • Cell Body: Located in the ganglion.

Branching of Spinal Nerves

  • Each spinal nerve branches into a dorsal ramus and a ventral ramus.

  • Both rami contain sensory and motor nerve fibers.

  • Dorsal Ramus: Innervates the posterior trunk or neck, including muscles in the back.

  • Ventral Ramus: Innervates structures not part of the posterior trunk.

  • Ventral rami of thoracic spinal nerves follow the ribs laterally and anteriorly.

Spinal Nerve Plexuses

Overview of Spinal Plexuses

  • A spinal plexus is a network formed by the interweaving sensory and motor axons from ventral rami of spinal nerves.

  • Exceptions are the 2nd to 12th thoracic ventral rami which do not form plexuses.

  • Nerve plexuses give rise to peripheral nerves, creating redundant innervation; thus, if one spinal nerve is damaged, overall function may remain intact.

Types of Spinal Nerve Plexuses

  1. Cervical Plexus: Includes C₁ to C₄; the phrenic nerve innervates the diaphragm.

  2. Brachial Plexus: Includes C₅ to C₈ and T₁; one significant nerve is the ulnar nerve.

  3. Lumbar Plexus: Includes L₁ to L₄; the femoral nerve is a crucial nerve here.

  4. Sacral Plexus: Includes L₄ to S₄; the sciatic nerve is one of the most important nerves.

Dermatomes

Definition of Dermatomes

  • A dermatome is a skin area predominantly innervated by the somatic sensory fibers of a single spinal nerve.

Clinical Application

  • A dermatome map shows body regions innervated by specific nerves, which can be crucial for diagnosing sensory deficits or skin issues impacted by certain spinal nerves, like a rash due to shingles.