Nurs 1002-Obj #13

The Peripheral Nervous System (PNS)

Overview

The Peripheral Nervous System (PNS) provides communication between the central nervous system (CNS) and the rest of the body. It consists of all neural structures outside the brain and spinal cord, including sensory receptors, nerves, ganglia, and motor endings. The PNS is divided into four functional parts:

• Sensory Receptors and Sensations

• Transmission Lines: Nerves and Their Structure and Repair

• Motor Endings and Motor Activity

• Reflex Activity

Somatic Sensory Receptors: Location and Function

Types of Sensory Receptors

Sensory receptors are specialized to respond to changes in the environment (stimuli) and trigger nerve impulses. Awareness of stimulus (sensation) and interpretation (perception) occur in the brain. Receptors are classified by:

• Type of stimulus

• Body location

• Structural complexity

Receptors by Type of Stimulus

• Mechanoreceptors: Respond to touch, pressure, vibration, and stretch

• Thermoreceptors: Sensitive to temperature changes

• Photoreceptors: Respond to light energy (e.g., retina)

• Chemoreceptors: Respond to chemicals (e.g., smell, taste, blood chemistry)

• Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat/cold, pressure, inflammatory chemicals)

Receptors by Location

• Exteroceptors: Respond to stimuli outside the body (skin for touch, pressure, pain, temperature; most special sense organs)

• Interceptors: Respond to stimuli within internal viscera and blood vessels (chemical changes, tissue stretch, temperature changes)

• Proprioceptors: Respond to stretch in skeletal muscles, tendons, joints, ligaments, connective tissue (inform brain of body movements)

Somatosensory System

• Sensation: Awareness of changes in the internal and external environment

• Perception: Conscious interpretation of those stimuli

• Somatosensory system: Part of sensory system serving body wall and limbs; receives input from exteroceptors, proprioceptors, and interceptors

Levels of Neural Integration in Sensory Systems

• Receptor level: Sensory receptors generate signals; stimulus must excite receptor and action potential (AP) must reach CNS

• Circuit level: Processing in ascending pathways; three-neuron pathways conduct impulses to cortical regions

  • First-order: Receptor to spinal reflexes or second-order neurons in CNS

  • Second-order: To third-order sensory neurons

  • Third-order: From thalamus to somatosensory cortex

• Perceptual level: Processing in cortical sensory areas; interpretation depends on location of target neurons

Aspects of Sensory Perception

• Perceptual detection: Ability to detect a stimulus (requires summation of impulses)

• Magnitude estimation: Intensity coded in frequency of impulses

• Spatial discrimination: Identifying site or pattern of stimulus (e.g., two-point discrimination test)

• Feature abstraction: Identification of complex aspects and several stimulus properties

• Quality discrimination: Ability to identify sub modalities (e.g., sweet vs. sour taste)

• Pattern recognition: Recognition of familiar or significant patterns (e.g., melody in music)

Perceptions of Pain

• Warns of actual or impending tissue damage; protective action

• Stimuli: extreme pressure, temperature, histamine, $K^+$, ATP, acids, bradykinin

• Impulses travel on fibers that release neurotransmitters

Visceral and Referred Pain

• Visceral pain: Results from stimulation of visceral organ receptors (e.g., stretching, ischemia, chemicals, muscle spasms)

• Referred pain: Pain from one body region perceived as coming from another (e.g., left arm pain during heart attack)

Generalized Structure of a Spinal Nerve

Spinal Nerve Anatomy

• Nerve: Cordlike organ of PNS; bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

• Two types: spinal or cranial (based on origin)

Spinal Nerve Classification

• 31 pairs of spinal nerves (all mixed nerves):

  • 8 pairs cervical (C1–C8)

  • 12 pairs thoracic (T1–T12)

  • 5 pairs lumbar (L1–L5)

  • 5 pairs sacral (S1–S5)

  • 1 pair coccygeal

• Named for point of issue from spinal cord; supply all body parts except head and part of neck

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

Roots and Rami

• Ventral roots: Motor (efferent) fibers from ventral horn motor neurons

• Dorsal roots: Sensory (afferent) fibers from sensory neurons in dorsal root ganglia

• Both roots branch medially as rootlets and join laterally to form spinal nerve

• After exiting foramen, spinal nerves divide into:

  • Dorsal ramus: Supplies posterior body trunk

  • Ventral ramus: Supplies rest of trunk and limbs

  • Meningeal branch: Reenters vertebral canal to innervate meninges and blood vessels

  • Rami communicants: Autonomic fibers joining ventral rami in thoracic region

  • Roots are medial and form spinal nerves (purely sensory or motor); rami are lateral branches (can carry both sensory and motor)

Ganglia

• Contain neuron cell bodies associated with nerves in PNS

• Dorsal root ganglia: Sensory neurons

• Autonomic ganglia: Motor neurons (visceral)

• PNS axons can regenerate if damage is not severe; CNS fibers rarely regenerate

Major Nerve Plexuses

Overview

• 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

• Each branch contains fibers from several different spinal nerves; fibers from ventral ramus go to body periphery via several routes

• Each limb muscle innervated by more than one spinal nerve; damage to one does not cause paralysis

Cervical Plexus and the Neck

• First four ventral rami (C1–C4)

• Most branches form cutaneous nerves (innervate skin of neck, ear, back of head, shoulders)

• Other branches innervate neck muscles

• Phrenic nerve: Major motor and sensory nerve of diaphragm (breathing); receives fibers from C3–C5

Brachial Plexus and Upper Limb

• Gives rise to nerves that innervate upper limb

• Major nerves: axillary, musculocutaneous, median, radial, ulnar

• Severe injuries can weaken or paralyze entire upper limb

Lumbosacral Plexus and Lower Limb

• Lumbar and sacral plexuses have significant overlap

• Fibers of lumbar plexus contribute to sacral plexus via lumbosacral trunk

• Lumbar plexus: Serves mostly lower limb, also abdomen, pelvis, buttocks (femoral, obturator nerves)

• Sacral plexus: Serves lower limb (sciatic nerve)

Table: Major Plexuses, Vertebral Rami, and Major Nerves

Plexus	Vertebral Rami	Major Nerves
Cervical	C1–C4	Phrenic
Brachial	C5–T1 (some C4, T2)	Axillary, musculocutaneous, median, radial, ulnar
Lumbar	L1–L4	Femoral, obturator
Sacral	L4–S4	Sciatic

Innervation of Skin: Dermatomes

• Dermatome: Area of skin innervated by cutaneous branches of a single spinal nerve

• All spinal nerves except C1 participate in dermatomes

• Extent of spinal cord injuries ascertained by affected dermatomes

• Most dermatomes overlap; destruction of a single spinal nerve will not cause complete numbness

Clinical Implications

• Irritation of phrenic nerve causes spasms of diaphragm (hiccups)

• If both phrenic nerves are severed or C3–C5 region spinal cord is destroyed, diaphragm becomes paralyzed (respiratory arrest)

• Victim requires mechanical respirators to stay alive

Cranial Nerves: Identification and Function

Overview

• 12 pairs of cranial nerves associated with brain

• Two attach to forebrain, rest with brain stem

• Most are mixed nerves; two pairs are purely sensory

• Numbered I–XII from rostral to caudal

• Mnemonic: "On occasion, our trusty truck acts funny—very good vehicle anyhow"

Table: Cranial Nerves and Functions

Cranial Nerve	Type	Function
I Olfactory	Sensory	Smell
II Optic	Sensory	Vision
III Oculomotor	Motor	Eye movement, pupil constriction
IV Trochlear	Motor	Eye movement (superior oblique muscle)
V Trigeminal	Mixed	Facial sensation, mastication
VI Abducens	Motor	Lateral eye movement
VII Facial	Mixed	Facial expression, taste (anterior 2/3 tongue)
VIII Vestibulocochlear	Sensory	Hearing, equilibrium
IX Glossopharyngeal	Mixed	Taste (posterior 1/3 tongue), swallowing
X Vagus	Mixed	Regulates heart, lungs, abdominal organs; taste, visceral sensation
XI Accessory	Motor	Head/neck movement (trapezius, sternocleidomastoid)
XII Hypoglossal	Motor	Tongue movement (swallowing, speech)

Somatic Reflexes

Reflex Arc

• Inborn (intrinsic) reflex: Rapid, involuntary, predictable motor response to stimulus (e.g., posture, visceral activities); can be modified by learning/conscious effort

• Learned (acquired) reflex: Result from practice or repetition (e.g., driving skills)

Components of a Reflex Arc

• Receptor: Site of stimulus action

• Sensory neuron: Transmits afferent impulses to CNS

• Integration center: Monosynaptic or polysynaptic region within CNS

• Motor neuron: Conducts efferent impulses from integration center to effector organ

• Effector: Muscle fiber or gland cell that responds to efferent impulses

Classification of Reflexes

• Somatic reflexes: Activate skeletal muscle

• Autonomic (visceral) reflexes: Activate visceral effectors (smooth/cardiac muscle or glands)

Spinal Reflexes

• Occur without direct involvement of higher brain centers

• Brain is advised of spinal reflex activity and may affect the reflex

• Testing somatic reflexes is important clinically to assess nervous system condition

• If exaggerated, distorted, or absent, may indicate degeneration or pathology of specific nervous system regions

• Most commonly assessed: stretch, flexor, superficial reflexes

Stretch and Tendon Reflexes

Stretch Reflex

• Brain sets muscle's length via stretch reflex

• Example: knee-jerk reflex keeps knees from buckling when standing upright

• Causes muscle contraction on side of spine in response to increased muscle length (stretch) on other side

Tendon Reflex

• Involves polysynaptic reflexes

• Prevents damage due to excessive stretch

• Important for smooth onset/termination of muscle contraction

• Produces muscle relaxation (lengthening) in response to tension

• Contraction or passive stretch activates tendon reflex

• Afferent impulses transmitted to spinal cord; contracting muscle relaxes, antagonist contracts (reciprocal activation)

• Information transmitted to cerebellum and used to adjust muscle tension

Example:

• Knee-jerk reflex: Stretch reflex that keeps knees from buckling when standing upright

• Reciprocal activation: When one muscle contracts, its antagonist relaxes