CHAPTER 14 - Peripheral Nervous System - Part 2

Long-lasting or intense pain, such as limb amputation, can lead to what?

Hyperalgesia (pain amplification), chronic pain, and phantom limb pain

What receptor is activated by long-lasting or intense pain and allows the spinal cord to “learn” hyperalgesia?

NMDA receptors

Why is early pain management critical? `

To prevent hyperalgesia and chronic pain

What is phantom limb pain?

Pain felt in a limb that has been amputated

What anesthesia is now used during surgery to reduce phantom limb pain?

Epidural anesthesia

What is a nerve?

A cordlike organ of the PNS

What does a nerve consist of?

Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

What are the two types of nerves based on origin?

Spinal or cranial nerves

What connective tissue surrounds an entire nerve?

Epineurium

What connective tissue surrounds fascicles within a nerve?

Perineurium

What connective tissue surrounds individual axons?

Endoneurium

Most nerves are mixtures of what types of fibers?

Afferent and efferent fibers, somatic and autonomic (visceral) fibers

What are mixed nerves?

Nerves containing both sensory and motor fibers; impulses travel both to and from CNS

What are sensory (afferent) nerves?

Nerves with impulses only toward CNS

What are motor (efferent) nerves?

Nerves with impulses only away from CNS

Are pure sensory or pure motor nerves common?

No, most nerves are mixed

What are the types of fibers in mixed nerves?

Somatic afferent, somatic efferent, visceral afferent, visceral efferent

What are ganglia?

Structures containing neuron cell bodies associated with PNS nerves

What do ganglia associated with afferent nerve fibers contain?

Cell bodies of sensory neurons (dorsal root ganglia)

What do ganglia associated with efferent nerve fibers contain?

Autonomic motor neurons (autonomic ganglia)

How many pairs of cranial nerves are there?

12 pairs

Where do cranial nerves attach?

Two to forebrain, the rest to brain stem

Are cranial nerves usually mixed or purely sensory?

Most are mixed; two pairs are purely sensory

How are cranial nerves numbered?

I through XII, rostral to caudal

Mnemonic for cranial nerves in order

“On occasion, our trusty truck acts funny—very good vehicle anyhow,” or “Oh, once one takes the anatomy final, very good vacations are heavenly.”

What is the trigeminal nerve (V)?

Largest cranial nerve; mixed nerve; fibers extend from pons to face

What are the three divisions of the trigeminal nerve?

Ophthalmic (V1), Maxillary (V2), Mandibular (V3)

Which trigeminal divisions convey sensory impulses?

V1 and V2

Which trigeminal division supplies motor fibers for mastication?

V3

Which foramen does V1 pass through?

Superior orbital fissure

Which foramen does V2 pass through?

Foramen rotundum

Which foramen does V3 pass through?

Foramen ovale

What is the primary function of Cranial Nerve I?

Olfactory – Special sense (smell)

What is the primary function of Cranial Nerve II?

Optic – Special sense (vision)

What is the primary function of Cranial Nerve III?

Oculomotor – Motor (eye muscles)

What is the primary function of Cranial Nerve IV?

Trochlear – Motor (eye muscles)

What is the primary function of Cranial Nerve V?

Trigeminal – Sensory (face) & motor (jaw muscles)

What is the primary function of Cranial Nerve VI?

Abducens – Motor (eye muscles)

What is the primary function of Cranial Nerve VII?

Facial – Special sense (taste) & motor (facial expressions)

What is the primary function of Cranial Nerve VIII?

Vestibulocochlear – Special sense (hearing, equilibrium)

What is the primary function of Cranial Nerve IX?

Glossopharyngeal – Special sense (taste) & motor (swallowing & gag reflex)

What is the primary function of Cranial Nerve X?

Vagus – Sensory (baroreception) & motor (autonomic)

What is the primary function of Cranial Nerve XI?

Spinal Accessory – Motor (head and neck)

What is the primary function of Cranial Nerve XII?

Hypoglossal – Motor (tongue movement)

Mnemonic for cranial nerve functions (S=sensory, M=motor, B=both)

“Some say marry money, but my brother believes (it’s) bad business (to) marry money”

How many pairs of spinal nerves are there?

31 pairs

Are spinal nerves mixed or purely sensory/motor?

All are mixed nerves

Which parts of the body do spinal nerves supply?

All body parts except head and part of neck

How many pairs of cervical, thoracic, lumbar, sacral, and coccygeal nerves? 

Cervical 8, Thoracic 12, Lumbar 5, Sacral 5, Coccygeal 1

Why are there 8 cervical spinal nerves but only 7 cervical vertebrae?

First 7 pairs exit above vertebra; C8 exits below C7

How are spinal nerves connected to the spinal cord?

Via ventral roots (motor) and dorsal roots (sensory)

What do ventral roots contain?

Motor (efferent) fibers from ventral horn motor neurons

What do dorsal roots contain?

Sensory (afferent) fibers from sensory neurons in dorsal root ganglia

What is the cauda equina?

Long lumbar and sacral roots extending through lower vertebral canal

What are the three branches of a spinal nerve?

Dorsal ramus, ventral ramus, meningeal branch

What do dorsal rami supply?

Posterior body trunk

What do ventral rami supply?

Rest of trunk and limbs

What are rami communicantes?

Autonomic nerve fibers that join ventral rami in thoracic region

Difference between roots and rami?

Roots are medial and purely sensory or motor; rami are lateral and mixed

What is a spinal nerve plexus?

Interlacing nerve network formed by ventral rami (except T2–T12

Where are spinal nerve plexuses found?

Cervical, brachial, lumbar, and sacral regions

What is the significance of fibers crisscrossing in a plexus?

Each branch contains fibers from several spinal nerves; limb muscles are innervated by multiple spinal nerves so damage to one does not cause paralysis

Cervical plexus location and formation

Formed by first four ventral rami (C1–C4) in the neck

Branches of cervical plexus

Most form cutaneous nerves innervating skin of neck, ear, back of head, shoulders; others innervate neck muscles

Phrenic nerve function

Major motor and sensory nerve of diaphragm; major muscle for breathing

Phrenic nerve spinal origin

Receives fibers from C3 to C5

Phrenic nerve clinical significance

Irritation causes hiccups; severed phrenic nerves or C3–C5 spinal damage paralyzes diaphragm, causing respiratory arrest; mechanical respirator required

Brachial plexus formation

Ventral rami of C5–C8 and T1 (often C4/T2) unite to form plexus

Four major branches of brachial plexus

Roots (C5–T1) → Trunks (upper, middle, lower) → Divisions (anterior/posterior) → Cords (lateral, medial, posterior)

Axillary nerve function

Innervates deltoid, teres minor, and skin/joint capsule of shoulder

Musculocutaneous nerve function

Innervates biceps brachii, brachialis, coracobrachialis, skin of lateral forearm

Median nerve function

Innervates skin, most flexors, forearm pronators, wrist/finger flexors, thumb opposition muscles

Ulnar nerve function

Supplies flexor carpi ulnaris, part of flexor digitorum profundus, most intrinsic hand muscles, medial hand skin, wrist/finger flexion

Radial nerve function

Innervates all extensor muscles, supinators, posterior skin of limb

Brachial plexus injury causes

Stretching upper limb (e.g., football tackle), blows to top of shoulder forcing humerus inferiorly

Median nerve injury symptoms

Difficulty using pincer grasp, seen in carpal tunnel syndrome, wrist

Ulnar nerve injury symptoms

Sensory loss, paralysis, muscle atrophy; little/ring fingers hyperextended at knuckles, flexed at distal interphalangeal joints; "clawhand"

Radial nerve injury symptoms

Wrist drop, inability to extend hand at wrist; "Saturday night paralysis" from crutch misuse or arm compression

Lumbar plexus origin and function

Arises from L1–L4; innervates thigh, abdominal wall, psoas muscle

Femoral nerve function

Innervates quadriceps, skin of anterior thigh and medial leg; damage affects hip flexion and knee extension

Obturator nerve function

Passes through obturator foramen, innervates adductor muscles; damage causes medial thigh pain

Sacral plexus origin and function

Arises from L4–S4; serves buttock, lower limb, pelvic structures, perineum

Sciatic nerve function

Longest/thickest nerve; innervates hamstrings, adductor magnus, most leg/foot muscles; composed of tibial and common fibular nerves

Lumbar plexus injury symptoms

Compressed roots cause gait problems, anterior thigh pain/numbness; femoral nerve damage impairs hip flexion/knee extension; obturator nerve impairment causes medial thigh pain

Sciatica definition

Stabbing pain along sciatic nerve; caused by fall, disc herniation, or poorly placed buttock injection

Dermatome definition

Area of skin innervated by cutaneous branches of single spinal nerve

C1 spinal nerve and dermatomes

All spinal nerves except C1 participate in dermatomes

Dermatome overlap significance

Destruction of single spinal nerve usually does not cause complete numbness

Hilton's Law

Any nerve serving a muscle that moves a joint also innervates that joint and the overlying skin