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5-2 PNS Part 1. Objectives Chapter 13. 1. Define peripheral nervous system and list its components. 2. Classify general sensory receptors by structure, stimulus detected, and body location. 3. Outline the events that lead to sensation and perception.

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objectives chapter 13
Objectives Chapter 13

1. Define peripheral nervous system and list

its components.

2. Classify general sensory receptors by

structure, stimulus detected, and body location.

3. Outline the events that lead to sensation

and perception.

4. Describe the general structure of a nerve.

5 Describe the formation of a spinal nerve

and the general distribution of its rami.

6. Define plexus.Name the major plexuses

and describe the distribution and function

of the peripheral nerves arising from each plexus.

7. Compare and contrast the motor

endings of somatic and autonomic nerve fibers.

peripheral nervous system pns
Peripheral Nervous System (PNS)

All neural structures outside the brain

Sensory receptors

Peripheral nerves and associated ganglia

Motor endings

slide4

Central nervous system (CNS)

Peripheral nervous system (PNS)

Sensory (afferent)

division

Motor (efferent) division

Somatic nervous

system

Autonomic nervous

system (ANS)

Sympathetic

division

Parasympathetic

division

Figure 13.1

sensory receptors
Sensory Receptors

Specialized to respond to changes in their environment (stimuli)

Activation results in graded potentials that trigger nerve impulses

Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

classification of receptors
Classification of Receptors

Based on:

Stimulus type

Location

Structural complexity

classification by stimulus type
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 (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

classification by location
Classification by Location

Exteroceptors

Respond to stimuli arising outside the body

Receptors in the skin for touch, pressure, pain, and temperature

Most special sense organs

classification by location1
Classification by Location

Interoceptors (visceroceptors)

Respond to stimuli arising in internal viscera and blood vessels

Sensitive to chemical changes, tissue stretch, and temperature changes

classification by location2
Classification by Location

Proprioceptors

Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

Inform the brain of one’s movements

classification by structural complexity
Classification by Structural Complexity

Complex receptors (special sense organs)

Vision, hearing, equilibrium, smell, and taste (Chapter 15)

Simple receptors for general senses:

Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense

Unencapsulated (free) or encapsulated dendritic endings

unencapsulated dendritic endings
Unencapsulated Dendritic Endings

Thermoreceptors

Cold receptors (10–40ºC); in superficial dermis

Heat receptors (32–48ºC); in deeper dermis

unencapsulated dendritic endings1
Unencapsulated Dendritic Endings

Nociceptors

Respond to:

Pinching

Chemicals from damaged tissue

Temperatures outside the range of thermoreceptors

Capsaicin

unencapsulated dendritic endings2
Unencapsulated Dendritic Endings

Light touch receptors

Tactile (Merkel) discs

Hair follicle receptors

encapsulated dendritic endings
Encapsulated Dendritic Endings

All are mechanoreceptors

Meissner’s (tactile) corpuscles—discriminative touch

Pacinian (lamellated) corpuscles—deep pressure and vibration

Ruffini endings—deep continuous pressure

Muscle spindles—muscle stretch

Golgi tendon organs—stretch in tendons

Joint kinesthetic receptors—stretch in articular capsules

from sensation to perception
From Sensation to Perception

Survival depends upon sensation and perception

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

Perception: the conscious interpretation of those stimuli

sensory integration
Sensory Integration

Input comes from exteroceptors, proprioceptors, and interoceptors

Input is relayed toward the head, but is processed along the way

sensory integration1
Sensory Integration

Levels of neural integration in sensory systems:

Receptor level—the sensor receptors

Circuit level—ascending pathways

Perceptual level—neuronal circuits in the cerebral cortex

slide21

Perceptual level(processing in

cortical sensory centers)

3

Motor

cortex

Somatosensory

cortex

Thalamus

Reticular

formation

Cerebellum

Pons

Medulla

Circuit level

(processing in

ascending pathways)

2

Spinal

cord

Free nerve

endings (pain,

cold, warmth)

Muscle

spindle

Receptor level

(sensory reception

and transmission

to CNS)

1

Joint

kinesthetic

receptor

Figure 13.2

processing at the receptor level
Processing at the Receptor Level

Receptors have specificity for stimulus energy

Stimulus must be applied in a receptive field

Transduction occurs

Stimulus energy is converted into a graded potential called a receptor potential

processing at the receptor level1
Processing at the Receptor Level

In general sense receptors, the receptor potential and generator potential are the same thing

stimulus

receptor/generator potential in afferent neuron

action potential at first node of Ranvier

processing at the receptor level2
Processing at the Receptor Level

In special sense organs:

stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)

adaptation of sensory receptors
Adaptation of Sensory Receptors

Adaptation is a change in sensitivity in the presence of a constant stimulus

Receptor membranes become less responsive

Receptor potentials decline in frequency or stop

adaptation of sensory receptors1
Adaptation of Sensory Receptors

Phasic (fast-adapting) receptors signal the beginning or end of a stimulus

Examples: receptors for pressure, touch, and smell

Tonic receptors adapt slowly or not at all

Examples: nociceptors and most proprioceptors

processing at the circuit level
Processing at the Circuit Level

Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions

First-order neurons

Conduct impulses from the receptor level to the second-order neurons in the CNS

Second-order neurons

Transmit impulses to the thalamus or cerebellum

Third-order neurons

Conduct impulses from the thalamus to the somatosensory cortex (perceptual level)

processing at the perceptual level
Processing at the Perceptual Level

Identification of the sensation depends on the specific location of the target neurons in the sensory cortex

Aspects of sensory perception:

Perceptual detection—ability to detect a stimulus (requires summation of impulses)

Magnitude estimation—intensity is coded in the frequency of impulses

Spatial discrimination—identifying the site or pattern of the stimulus (studied by the two-point discrimination test)

main aspects of sensory perception
Main Aspects of Sensory Perception

Feature abstraction—identification of more complex aspects and several stimulus properties

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

Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)

slide30

Perceptual level(processing in

cortical sensory centers)

3

Motor

cortex

Somatosensory

cortex

Thalamus

Reticular

formation

Cerebellum

Pons

Medulla

Circuit level

(processing in

ascending pathways)

2

Spinal

cord

Free nerve

endings (pain,

cold, warmth)

Muscle

spindle

Receptor level

(sensory reception

and transmission

to CNS)

1

Joint

kinesthetic

receptor

Figure 13.2

perception of pain
Perception of Pain

Warns of actual or impending tissue damage

Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin

Impulses travel on fibers that release neurotransmitters glutamate and substance P

Some pain impulses are blocked by inhibitory endogenous opioids

structure of a nerve
Structure of a Nerve

Cordlike organ of the PNS

Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

structure of a nerve1
Structure of a Nerve

Connective tissue coverings include:

Endoneurium—loose connective tissue that encloses axons and their myelin sheaths

Perineurium—coarse connective tissue that bundles fibers into fascicles

Epineurium—tough fibrous sheath around a nerve

slide34

Axon

Myelin sheath

Endoneurium

Perineurium

Epineurium

Fascicle

Blood

vessels

(b)

Figure 13.3b

classification of nerves
Classification of Nerves

Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

Pure sensory (afferent) or motor (efferent) nerves are rare

Types of fibers in mixed nerves:

Somatic afferent and somatic efferent

Visceral afferent and visceral efferent

Peripheral nerves classified as cranial or spinal nerves

ganglia
Ganglia

Contain neuron cell bodies associated with nerves

Dorsal root ganglia (sensory, somatic) (Chapter 12)

Autonomic ganglia (motor, visceral) (Chapter 14)

regeneration of nerve fibers
Regeneration of Nerve Fibers

Mature neurons are amitotic

If the soma of a damaged nerve is intact, axon will regenerate

Involves coordinated activity among:

Macrophages—remove debris

Schwann cells—form regeneration tube and secrete growth factors

Axons—regenerate damaged part

CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration

slide38

Endoneurium

Schwann cells

1

The axon

becomes

fragmented at

the injury site.

Droplets

of myelin

Fragmented

axon

Site of nerve damage

Figure 13.4 (1 of 4)

slide39

Macrophages

clean out the

dead axon distal

to the injury.

2

Schwann cell

Macrophage

Figure 13.4 (2 of 4)

slide40

Axon sprouts,

or filaments,

grow through a

regeneration tube

formed by

Schwann cells.

3

Aligning Schwann cells

form regeneration tube

Fine axon sprouts

or filaments

Figure 13.4 (3 of 4)

slide41

The axon

regenerates and

a new myelin

sheath forms.

4

Site of new

myelin sheath

formation

Schwann cell

Single enlarging

axon filament

Figure 13.4 (4 of 4)

cranial nerves
Cranial Nerves

Twelve pairs of nerves associated with the brain

Most are mixed in function; two pairs are purely sensory

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

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

slide43

Filaments of

olfactory

nerve (I)

Frontal lobe

Olfactory bulb

Olfactory tract

Optic nerve

(II)

Temporal lobe

Optic chiasma

Infundibulum

Optic tract

Facial

nerve (VII)

Oculomotor

nerve (III)

Trochlear

nerve (IV)

Vestibulo-

cochlear

nerve (VIII)

Trigeminal

nerve (V)

Glossopharyngeal

nerve (IX)

Abducens

nerve (VI)

Vagus nerve (X)

Cerebellum

Accessory nerve (XI)

Medulla

oblongata

Hypoglossal nerve (XII)

(a)

Figure 13.5 (a)

slide44

Cranial nerves

I – VI

Sensory

function

Motor

function

PS*

fibers

I

Olfactory

Yes (smell)

No

No

II

Optic

Yes (vision)

No

No

III

Oculomotor

No

Yes

Yes

IV

Trochlear

No

Yes

No

V

Trigeminal

Yes (general

sensation)

Yes

No

VI

Abducens

No

Yes

No

Cranial nerves

VII – XII

Sensory

function

Motor

function

PS*

fibers

VII

Facial

Yes (taste)

Yes

Yes

VIII

Vestibulocochlear

Yes (hearing

and balance)

Some

No

IX

Glossopharyngeal

Yes (taste)

Yes

Yes

X

Vagus

Yes (taste)

Yes

Yes

XI

Accessory

No

Yes

No

XII

Hypoglossal

No

Yes

No

*PS = parasympathetic

(b)

Figure 13.5 (b)

i the olfactory nerves
I: The Olfactory Nerves

Arise from the olfactory receptor cells of nasal cavity

Pass through the cribriform plate of the ethmoid bone

Fibers synapse in the olfactory bulbs

Pathway terminates in the primary olfactory cortex

Purely sensory (olfactory) function

ii the optic nerves
II: The Optic Nerves

Arise from the retinas

Pass through the optic canals, converge and partially cross over at the optic chiasma

Optic tracts continue to the thalamus, where they synapse

Optic radiation fibers run to the occipital (visual) cortex

Purely sensory (visual) function

iii the oculomotor nerves
III: The Oculomotor Nerves

Fibers extend from the ventral midbrain through the superior orbital fissures to the extrinsic eye muscles

Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape

iv the trochlear nerves
IV: The Trochlear Nerves

Fibers from the dorsal midbrain enter the orbits via the superior orbital fissures to innervate the superior oblique muscle

Primarily a motor nerve that directs the eyeball

v the trigeminal nerves
V: The Trigeminal Nerves

Largest cranial nerves; fibers extend from pons to face

Three divisions

Ophthalmic (V1) passes through the superior orbital fissure

Maxillary (V2) passes through the foramen rotundum

Mandibular (V3) passes through the foramen ovale

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

vi the abducens nerves
VI: The Abducens Nerves

Fibers from the inferior pons enter the orbits via the superior orbital fissures

Primarily a motor, innervating the lateral rectus muscle

vii the facial nerves
VII: The Facial Nerves

Fibers from the pons travel through the internal acoustic meatuses, and emerge through the stylomastoid foramina to the lateral aspect of the face

Chief motor nerves of the face with 5 major branches

Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands

Sensory function (taste) from the anterior two-thirds of the tongue

viii the vestibulocochlear nerves
VIII: The Vestibulocochlear Nerves

Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from the inner ear through the internal acoustic meatuses, and enter the brain stem at the pons-medulla border

Mostly sensory function; small motor component for adjustment of sensitivity of receptors

ix the glossopharyngeal nerves
IX: The Glossopharyngeal Nerves

Fibers from the medulla leave the skull via the jugular foramen and run to the throat

Motor functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands

Sensory functions: fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors

x the vagus nerves
X: The Vagus Nerves

The only cranial nerves that extend beyond the head and neck region

Fibers from the medulla exit the skull via the jugular foramen

Most motor fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera

Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx

xi the accessory nerves
XI: The Accessory Nerves

Formed from ventral rootlets from the C1–C5 region of the spinal cord (not the brain)

Rootlets pass into the cranium via each foramen magnum

Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscles

xii the hypoglossal nerves
XII: The Hypoglossal Nerves

Fibers from the medulla exit the skull via the hypoglossal canal

Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech