The Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS)

P A R T A

Peripheral Nervous System (PNS)

PNS – all neural structures outside the brain and spinal cord

Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings

Provides links to and from the external environment

PNS in the Nervous System

Figure 13.1

Sensory Receptors

Structures specialized to respond to stimuli

Activation of sensory receptors results in depolarizations that trigger impulses to the CNS

The realization of these stimuli, sensation and perception, occur in the brain

Receptor Classification by Stimulus Type Mechanoreceptors – respond to

touch, pressure, vibration, stretch, and itch

Thermoreceptors – sensitive to changes in temperature

Photoreceptors – respond to light energy (e.g., retina)

Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)

Nociceptors – sensitive to pain-causing stimuli

Receptor Class by Location: Exteroceptors

Respond to stimuli arising outside the body

Found near the body surface Sensitive to touch, pressure, pain,

and temperature Include the special sense organs

Receptor Class by Location: Interoceptors

Respond to stimuli arising within the body

Found in internal viscera and blood vessels

Sensitive to chemical changes, stretch, and temperature changes

Receptor Class by Location: Proprioceptors

Respond to degree of stretch of the organs they occupy

Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

Constantly “advise” the brain of one’s movements

Receptors are structurally classified as either simple or complex

Most receptors are simple and include encapsulated and unencapsulated varieties

Complex receptors are special sense organs

Receptor Classification by Structural Complexity

Simple Receptors: Unencapsulated

Free dendritic nerve endingsRespond chiefly to temperature

and pain Merkel (tactile) discs Hair follicle receptors

Simple Receptors: Encapsulated

Meissner’s corpuscles (tactile corpuscles)

Pacinian corpuscles (lamellated corpuscles)

Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles

Joint kinesthetic receptors

Unencapsulated Receptors

Table 13.1.1

Simple Receptors:Encapsulated

Table 13.1.2

From Sensation to Perception

Sensation is the awareness of changes in the internal and external environment

Perception is the conscious interpretation of those stimuli

Organization of the Somatosensory System

Input comes from exteroceptors, proprioceptors, and interoceptors

The three main levels of neural integration in the somatosensory system are:Receptor level – the sensor

receptorsCircuit level – ascending pathwaysPerceptual level – neuronal circuits

in the cerebral cortex

Figure 13.2

Processing at the Receptor Lever The receptor must have specificity

for the stimulus energy The receptor’s receptive field

must be stimulated Transduction

Conversion of the energy of a stimulus into the energy of a nerve signal

Processing at the Receptor Lever

Receptor potentialIt is a graded potential happening on

a receptorDepolarization or hyperpolarization

Generator potential It is a receptor potential strong

enough to cause an action potential in an afferent fiber

Adaptation of Sensory Receptors

Adaptation is a reduction in sensitivity in the presence of a stimulusReceptor membranes become

less responsiveReceptor potentials decline in

frequency or stop

Adaptation of Sensory Receptors

Tonic receptors Have little peripheral adaptation

Chemical interoceptors Pain receptorsMacula in the vestibular apparatus

Proprioceptors

Adaptation of Sensory Receptors

Phasic receptors Are fast adapting receptors

PressureTouch Smell

Processing at the Circuit Level

Chains of three neurons that conduct sensory impulses to the cerebral cortex

First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem

Processing at the Circuit Level

Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum

Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum

Processing at the Perceptual Level

The thalamus projects fibers to: The somatosensory cortexSensory association areas

The exact point in the cortex that is activated will refer to where in the body the stimulus is happening

The result is an internal, conscious image of the stimulus

Main Aspects of Sensory Perception

Perceptual detection – detecting that a stimulus has occurred and requires summation

Magnitude estimation =intensity of the stimulusFrequency of action potentials

Main Aspects of Sensory Perception

Spatial discrimination – identifies the location of the stimulus. It depends on the size of the receptor field. Two-point discrimination test –

smaller fields equals finer two-point discrimination test

Main Aspects of Sensory Perception

Feature abstraction – used to identify a specific feature of the stimulus (texture or shape)

Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes)

Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)

Structure of a Nerve Nerve – peripheral axons enclosed

by connective tissue Connective tissue coverings include:

Endoneurium – loose connective tissue that surrounds axons

Perineurium – coarse connective tissue that bundles fibers into fascicles

Epineurium – tough fibrous sheath around a nerve

Structure of a Nerve

Figure 13.3b

Classification of Nerves

Sensory (afferent) – carry impulse to the CNS

Motor (efferent) – carry impulses from CNS

Mixed nerves – carry somatic and autonomic (visceral) impulsesMost common type

Peripheral Nerves

The four types of mixed nerves are:Somatic

SensoryMotor

Visceral SensoryMotor

Peripheral nerves can be cranial or spinal

Regeneration of Nerve Fibers Mature neurons are amitotic If the soma remains intact, damage

can be repaired Steps

Separated ends seal themselvesWallerian degeneration of the

distal axon by macrophagesFormation of a regeneration tube

by the Schwann cellGuide the axon growth distally

Regeneration of Nerve Fibers

Figure 13.4

Regeneration of Nerve Fibers

Figure 13.4

Cranial Nerves

Twelve pairs of cranial nerves arise from the brain

They have sensory, motor, or both sensory and motor (mixed nerves) functions

Each nerve is identified by a number (I through XII) and a name

Cranial Nerves

Figure 13.5a

Summary of Function of Cranial Nerves

Figure 13.5b

Cranial Nerve I: Olfactory

Arises from the olfactory epithelium Passes through the cribriform plate

of the ethmoid bone Fibers run through the olfactory bulb

and terminate in the primary olfactory cortex

Function is the sense of smell

Cranial Nerve I: Olfactory

Figure I from Table 13.2

Cranial Nerve II: Optic

Arises from the retina of the eye Optic nerves pass through the optic

canals and converge at the optic chiasm

They continue to the thalamus where they synapse

From there, the optic radiation fibers run to the visual cortex

Functions carry impulses for vision

Cranial Nerve II: Optic

Figure II from Table 13.2

Cranial Nerve III: Oculomotor

Motor for movements of the eyes Parasympathetic fibers innervate

the intrinsic muscles of the eyeConstricting the iris, and

controlling lens shape

Cranial Nerve III: Oculomotor

Figure III from Table 13.2

Cranial Nerve IV: Trochlear

Figure IV from Table 13.2

Cranial Nerve V: Trigeminal

Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)

Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

Cranial Nerve V: Trigeminal

Figure V from Table 13.2

Cranial Nerve VI: Abducens• Primarily a somatic motor nerve

Figure VI from Table 13.2

Cranial Nerve VII: Facial

Somatic Motor to the muscles of facial expression, and the transmittal of

Visceral motor to lacrimal and salivary glands

Sensory function is taste from the anterior two-thirds of the tongue

Cranial Nerve VII: Facial

Figure VII from Table 13.2

Cranial Nerve VIII: Vestibulocochlear

Fibers arise from the hearing and equilibrium apparatus of the inner ear,

Two divisions – cochlear (hearing) and vestibular (balance)

A sensory nerve

Cranial Nerve VIII: Vestibulocochlear

Figure VIII from Table 13.2

Cranial Nerve IX: Glossopharyngeal Nerve IX is a mixed nerve with

motor and sensory functions Somatic Motor – innervates part of

the tongue and pharynx, and Visceral Motor fibers to the

parotid salivary gland Visceral Sensory –taste and

general sensory impulses from the tongue and pharynx

Cranial Nerve IX: Glossopharyngeal

Figure IX from Table 13.2

Cranial Nerve X: Vagus

The only cranial nerve that extends beyond the head and neck

The vagus is a mixed nerve Most visceral motor fibers are

parasympathetic fibers to the heart, lungs, and visceral organs

Its visceral sensory function is in taste

Cranial Nerve X: Vagus

Figure X from Table 13.2

Cranial Nerve XI: Accessory

Primarily a somatic motor nerve Supplies fibers to the larynx,

pharynx, and soft palateInnervates the trapezius and

sternocleidomastoid, which move the head and neck

Cranial Nerve XI: Accessory

Figure XI from Table 13.2

Cranial Nerve XII: Hypoglossal

Somatic motor innervates the muscles of the tongue, which contribute to swallowing and speech

Cranial Nerve XII: Hypoglossal

Figure XII from Table 13.2

The Peripheral Nervous System (PNS)

P A R T B

Spinal Nerves Thirty-one pairs of mixed nerves arise

from the spinal cord and supply all parts of the body except the head

They are named according to their point of issue8 cervical (C1-C8)12 thoracic (T1-T12)5 Lumbar (L1-L5)5 Sacral (S1-S5)1 Coccygeal (C0)

Spinal Nerves

Figure 13.6

Spinal Nerves: Roots

Each spinal nerve connects to the spinal cord via two medial roots

Each root forms a series of rootlets that attach to the spinal cord

Ventral roots arise from the anterior horn and contain motor (efferent) fibers

Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves: Roots

Figure 13.7a

Spinal Nerves: Rami

The short spinal nerves branch into three or four mixed, distal ramiSmall dorsal ramusLarger ventral ramusRami communicantes at the

base of the ventral rami in the thoracic region visceral nerve fibers

Nerve Plexuses

All ventral rami except T2-T12 form interlacing nerve networks called plexuses

Plexuses are found in the cervical, brachial, lumbar, and sacral regions

Each resulting branch of a plexus contains fibers from several spinal nerves

Nerve Plexuses

Each muscle receives a nerve supply from more than one spinal nerve

Damage to one spinal segment cannot completely paralyze a muscle

The back is innervated by dorsal rami via several branches

The thorax is innervated by ventral rami T1-T12 as intercostal nerves

Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall

Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall

Figure 13.7b

Cervical Plexus Most branches are cutaneous

nerves of the neck, ear, back of head, and shoulders

The most important nerve of this plexus is the phrenic nerveMotor and sensory nerve of the

diaphragm

Cervical Plexus

Figure 13.8

Brachial Plexus

It gives rise to the nerves that innervate the upper limb

There are four major branches of this plexus Roots TrunksDivisions Cords

Brachial Plexus

Figure 13.9a

Brachial Plexus: Nerves Axillary Musculocutaneous Median Ulnar Radial

Brachial Plexus: Distribution of Nerves

Figure 13.9c

Brachial Plexus: Nerves

Figure 13.9b

Lumbar Plexus

Innervates the thigh, abdominal wall, and psoas muscle

The major nerves are the Femoral

For anterior thigh muscles Obturator

Adductors muscles

Lumbar Plexus

Figure 13.10

Sacral Plexus Serves the buttock, lower limb,

pelvic structures, and the perineum (pudendal nerve)

The major nerve is the sciatic, the longest and thickest nerve of the bodyLower limb (except anteromedial

thigh muscles) Branches into two nerves: the tibial

and the common fibular (peroneus)

Sacral Plexus

Figure 13.11

Dermatomes

A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve

All spinal nerves except C1 participate in dermatomes

Dermatomes

Figure 13.12

Innervation of Joints

Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint itself and the skin over the joint

Motor Endings

PNS elements that activate effectors by releasing neurotransmitters at:Skeletal muscles Smooth muscle and glands

Levels of Motor Control

The three levels of motor control areSegmental level

Spinal cord circuitProjection level

Pyramidal and extrapyramidal systems

Precommand levelCerebellum and basal nuclei

Hierarchy of Motor Control

Figure 13.13

Segmental Level

The segmental level is the lowest level of motor hierarchy

It consists of segmental circuits of the spinal cord

Its circuits control locomotion and specific, oft-repeated motor activity

Projection Level

Controls the spinal cord Consists of:

Cortical motor areas that produce the direct (pyramidal) system

Brain stem motor areas that oversee the indirect (multineuronal) system

Send information to lower motor neurons and also to higher center

Precommand Level

Cerebellar and basal nuclei systems that:Regulate motor activityPrecisely start or stop movementsCoordinate movements with

postureBlock unwanted movementsMonitor muscle toneControl the output of the cortex

and brain stem motor centers

Reflexes

A reflex is a rapid, predictable motor response to a stimulus

Reflexes may: Be inborn (intrinsic) or learned

(acquired)Involve, peripheral nerves, brain

stem and spinal cord Somatic and visceral reflexes

Reflex Arc

There are five components of a reflex arcReceptorSensory neuron Integration center Motor neuron Effector

Reflex Arc

Figure 13.14

Somatic Reflexes

Spinal: Stretch reflex Golgi tendon reflex Withdrawal reflex Crossed-extensor reflex Superficial: Plantar

Babinski’s Abdominal

Stretch and Deep Tendon Reflexes

For skeletal muscles to perform normally: The Golgi tendon organs

(proprioceptors) must constantly inform the brain as to the state of the muscle

Stretch reflexes initiated by muscle spindles must maintain healthy muscle tone

Stretch reflex - monosynaptic Muscle Spindle Are composed of intrafusal muscle

fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces

Afferent fibers Motor fibers:

Extrafusal fibersIntrafusal fibers

Muscle Spindles

Figure 13.15

Operation of the Muscle Spindles

Stretching the muscles activates the muscle spindleThere is an increased rate of action

potential on sensory fibers Contracting the muscle reduces

tension on the muscle spindleThere is a decreased rate of action

potential on sensory fibers

Operation of the Muscle Spindle

Figure 13.17

Stretch Reflex - monosynaptic Stretching the muscle activates the

muscle spindle Excited motor neurons causes the

muscle to contract Afferent impulses from the spindle result

in inhibition of the antagonist Example: patellar reflex

Tapping the patellar tendon stretches the quadriceps and starts the reflex action

The quadriceps contract and the antagonistic hamstrings relax

Stretch Reflex

Figure 13.16

Golgi Tendon Reflex - polysynaptic

The opposite of the stretch reflex Contracting the muscle activates

the Golgi tendon organs Afferent Golgi tendon neurons are

stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated

As a result, the contracting muscle relaxes and the antagonist contracts

It moderates the muscle contraction

Golgi Tendon Reflex

Figure 13.18

Flexor ( Withdrawal) Reflexes

The flexor reflex happens on the limb receiving the painful stimulus Withdrawal reflex by contraction

of the flexor musclesReciprocal inhibition of the

extensorsPolysynaptic reflex

Crossed Extensor Reflex

The crossed extensor reflexHappens on the opposite limbContraction of the extensor

musclesRelaxation of the flexor musclesPolysynaptic

Crossed Extensor Reflex

106

Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Right arm(site of stimulus)

Left arm (site ofreciprocal activation)

Arm movements

Interneurons

Key:+ Excitatory synapse– Inhibitory synapse

Efferentfibers

FlexorinhibitedExtensorstimulated

+

–+

–

+

+

Flexes

Extends

Figure 13.19

Superficial Reflexes

Initiated by gentle cutaneous stimulation Example:

Plantar reflex is initiated by stimulating the lateral aspect of the sole of the foot

The response is downward flexion of the toes

Superficial Reflexes

Indirectly tests for proper corticospinal tract functioning

Babinski’s sign: abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally

The Babinski Reflexes

Figure 13.23

Developmental Aspects of the PNS

Spinal nerves branch from the developing spinal cord and neural crest cellsSupply motor and sensory

function to developing muscles Cranial nerves innervate muscles of

the head

Developmental Aspects of the PNS

Distribution and growth of spinal nerves correlate with the segmented body plan

Sensory receptors atrophy with age and muscle tone lessens

Peripheral nerves remain viable throughout life unless subjected to trauma