Chapter 14: The Peripheral Nervous System

A set of flashcards summarizing key terms and concepts related to the Peripheral Nervous System from the lecture notes.

Peripheral Nervous System (PNS)

• The part of the nervous system that provides links from and to the world outside our body, consisting of all neural structures outside the brain and spinal cord.

  • Sensory receptors

  • Transmission Lines (Nerves)

  • Motor Endings and motor activity

  • Reflex Activity

Nerve

• A cordlike organ of the peripheral nervous system made up of a bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.

• Two types of nerves: spinal or cranial,

  depending on where they originate

Endoneurium

Loose connective tissue that encloses axons and their myelin sheaths (Schwann cells) within a nerve.

Perineurium

Coarse connective tissue that bundles fibers into fascicles within a nerve.

Epineurium

A tough fibrous sheath surrounding all fascicles to form the nerve.

Structure and Classification of nerves:

• Most nerves are mixtures of afferent and efferent fibers and somatic

and autonomic (visceral) fibers

• Nerves are classified according to the direction they transmit impulses

• Mixed nerves: contain both sensory and motor fibers

• Impulses travel both to and from CNS

• Sensory (afferent) nerves: impulses only toward CNS

• Motor (efferent) nerves: impulses only away from CNS

• Pure sensory (afferent) or pure motor (efferent) nerves are rare; most

nerves are mixed

Types of fibers in mixed nerves:

• • Somatic afferent (sensory from muscle to brain)

  • Somatic efferent (motor from brain to muscle)

  • Visceral afferent (sensory from organs to brain)

  • Visceral efferent (motor from brain to organs)

Afferent nerves

Nerves that carry impulses only toward the central nervous system.

Efferent nerves

Nerves that carry impulses only away from the central nervous system.

Ganglia

• Structures that contain neuron cell bodies associated with nerves in the peripheral nervous system.

Ganglia associated with afferent nerve fibers:

• contain cell bodies of

  sensory neurons

  • Dorsal root ganglia (sensory, somatic)

Ganglia associated with efferent nerve fibers:

• contain autonomic

  motor neurons

  • Autonomic ganglia (motor, visceral) (Chapter 14 - ANS)

Cranial Nerves

• 12 pairs of nerves that arise directly from the brain, mostly mixed in function, but some are purely sensory.

• Most are mixed nerves, but two pairs purely sensory

  • Each numbered (I through XII) and named from rostral to

  caudal

Cranial Nerve Mnemonics:

• “On occasion, our trusty truck acts funny—very good vehicle anyhow”

  “Oh once one takes the anatomy final, very good vacations are heavenly”

Olfactory Nerve (I)

The nerve responsible for the sense of smell; originates in the olfactory epithelium.

Optic Nerve (II)

The nerve responsible for vision; originates from the retina and forms the optic chiasma.

Oculomotor Nerve (III)

A primarily motor nerve that controls most eye movements and regulates pupil size.

Trigeminal Nerve (V)

The largest cranial nerve responsible for facial sensation and motor functions such as chewing.

Facial Nerve (VII)

A mixed nerve that functions mainly for facial expressions and also conveys taste sensations from the anterior two-thirds of the tongue.

Vestibulocochlear Nerve (VIII)

Primarily a sensory nerve responsible for hearing and balance.

Glossopharyngeal Nerve (IX)

A mixed nerve that provides sensory and motor functions to the throat and tongue.

Vagus Nerve (X)

A mixed nerve that extends beyond the head and neck, providing motor and sensory function to thoracic and abdominal organs.

Accessory Nerve (XI)

Primarily a motor nerve that innervates neck muscles, specifically the sternocleidomastoid and trapezius.

Hypoglossal Nerve (XII)

Primarily a motor nerve that controls tongue movements crucial for speech and swallowing.

Spinal Nerves

• 31 pairs of mixed nerves that emerge from the spinal cord and innervate the body, except for the head and part of the neck.

• All are mixed nerves named for point

  of issue from spinal cord

  • Supply all body parts except head

  and part of neck

  • 8 pairs of cervical nerves (C1–C8)

  • 12 pairs of thoracic nerves (T1–T12)

  • 5 pairs of lumbar nerves (L1–L5)

  • 5 pairs of sacral nerves (S1–S5)

  • 1 pair of tiny coccygeal nerves (Co1)

•  cervical vertebrae give rise

  to 8 pairs of cervical spinal

  nerves because:

  • Each of the first 7 pairs (C1

  to C7) exits the vertebral

  canal superior to vertebra for

  which it is named

  • Last spinal nerve (C8) exits

  canal inferior to C7

  • So vertebra C7 has a nerve

  that leaves above it and

  one that leaves below it

  • Each of the other spinal

  nerves exits inferior to

  vertebra for which it is named

Spinal Nerves: Ventral Roots:

• Contain motor

(efferent) fibers from

ventral horn motor

neurons

Spinal Nerves: Dorsal Roots:

• • Contain sensory

  (afferent) fibers from

  sensory neurons from

  peripheral receptors

Interbertebral foramina:

• Spinal nerves emerge

  from vertebral column via

  their respective

Cauda Equina

Collection of spinal nerves that extend beyond the end of the spinal cord.

Spinal Nerve Branches:

• • Spinal nerves are quite short

  (~1–2 cm)

  • Almost immediately after

  exiting the intervertebral

  foramen, spinal nerves divide

  into branches:

  • Dorsal ramus: smaller branch

  • Ventral ramus: larger branch

  • Rami communicantes contain

  autonomic nerve fibers that join

  ventral rami in thoracic region

  (Chapter 14 – ANS)

Dorsal ramus

The smaller branch of a spinal nerve that supplies the posterior body trunk.

Ventral ramus

The larger branch of a spinal nerve that supplies the rest of the trunk and limbs.

Dermatome

An area of skin innervated by cutaneous branches of a single spinal nerve.

Innervation of Specific Body Regions:

• Spinal nerve rami and their main branches supply entire somatic

  region of body from neck down

  • Dorsal rami supply posterior body trunk

  • Ventral rami supply rest of trunk and limbs

  • All ventral rami (except T2–T12) form interlacing nerve networks called

  nerve plexuses

  • Found in cervical, brachial, lumbar, and sacral areas

  • Only ventral rami form plexuses

Within plexus, fibers crisscross so that:

1. 2. Each branch contains fibers from several different spinal nerves

Fibers from ventral ramus go to body periphery via several routes

• Means each limb muscle is innervated by more than one spinal nerve, so damage to

one does not cause paralysis

Cervical plexus and the neck:

• First four ventral rami (C1–

  C4) form looping cervical

  plexus

  • Most branches of this form

  cutaneous nerves

Phrenic nerve:

• • Major motor and sensory

  nerve of diaphragm, major

  muscles for breathing

  • Phrenic nerve receives

  fibers from C3-C5

Clinical Notes: Phrenic Nerve:

• Irritation causes spasms of the diaphragm, also

  called hiccups

  • If both nerves are severed, or if C3–C5 region of spinal cord is

  destroyed, diaphragm becomes paralyzed

  • Respiratory arrest occurs

  • Victim requires mechanical respirators to stay alive

  • Air is mechanically forced into the lungs—literally breathing for them

Brachial plexus and upper limb:

• • Formed by ventral rami of C5–C8

  and T1 (and often C4 and/or T2)

  • Gives rise to nerves that

  innervate upper limb

  • Four major branches of this

  plexus:

  • Roots—five ventral rami

  (C5–T1) unite to form…

  • Trunks—upper, middle,

  and lower, which unite to

  form…

  • Divisions—anterior and

  posterior, which unite to

  form …

  • Cords—lateral, medial,

  and posterior

Brachial plexus: Cords give rise to nerves of the upper limb:

• Axillary

• Musculocutaneous

• Median

• Ulnar

• Radial

Clinical Notes: Median Nerve:

• Seen in carpal tunnel syndrome

• • Injury to median nerve makes it difficult to use pincer grasp

  (opposed thumb and index finger) to pick up small objects

  • Ulnar nerve is very vulnerable to injury

  • Affected individuals have trouble making a fist and gripping objects

  • Causes hand to contort into a clawhand

  • Striking the “funny bone”, the spot where this nerve rests against

  medial epicondyle, can make the little finger tingle.

Lumbar plexus:

• • Arises from L1 to L4

  • Innervates thigh,

  abdominal wall, and

  psoas muscle

Femoral nerve:

• innervates quadriceps

  and skin of anterior

  thigh and medial

  surface of leg

Obturator nerve:

• passes through

  obturator foramen to

  innervate adductor

  muscles

Sacral plexus:

•  Arises from L4 to S4

  • Serves the buttock, lower

  limb, pelvic structures, and

  perineum

  • Sciatic nerve

  • Longest and thickest

  nerve of body

  • Innervates hamstring

  muscles, adductor

  magnus, and most

  muscles in lower leg

  and foot

  • Composed of two

  nerves: tibial and

  common fibular

Clinical Notes: Sciatic nerve:

• Cause of Sciatica, characterized by stabbing pain radiating over course of this nerve

  • • Injury could be caused by a fall, disc herniation, or badly placed injection into

    the buttock

• • If the nerve is transected, leg is nearly useless and cannot be flexed

  because hamstrings are paralyzed

  • Recovery from sciatic nerve injury is usually slow and incomplete

  • For lesions below knee, thigh muscles are spared

Anterolateral thorax and abdominal wall:

• Ventral rami of T2–T12 are

  intercostal nerves that

  supply muscles of ribs,

  anterolateral thorax, and

  abdominal wall

  • Give off cutaneous

  branches to skin

Back:

• innervated by dorsal

  rami via several branches

  • Each branch innervates a

  strip of muscle and skin in

  line with where it emerges

  from spinal cord

Innervation of skin: Dermatomes:

• • Dermatome: area of skin innervated by cutaneous branches of single

  spinal nerve

  • All spinal nerves (except C1) participate in dermatomes

  • Extent of spinal cord injuries ascertained by affected dermatomes

  • Most dermatomes overlap, so destruction of a single spinal nerve will not

  cause complete numbness