5-2 PNS Part 1

Objectives Chapter 131. 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.5Describe 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)

• All neural structures outside the brain– Sensory receptors– Peripheral nerves and associated ganglia– Motor endings

Figure 13.1

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

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

• Based on:– Stimulus type– Location– Structural complexity

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

1. Exteroceptors– Respond to stimuli arising outside the body– Receptors in the skin for touch, pressure,

pain, and temperature– Most special sense organs

Classification by Location

2. Interoceptors (visceroceptors)– Respond to stimuli arising in internal viscera

and blood vessels– Sensitive to chemical changes, tissue

stretch, and temperature changes

Classification by Location

3. 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

1. Complex receptors (special sense organs)

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

2. 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

• Thermoreceptors– Cold receptors (10–40ºC); in superficial dermis – Heat receptors (32–48ºC); in deeper dermis

Unencapsulated Dendritic Endings

• Nociceptors– Respond to:

• Pinching• Chemicals from damaged tissue• Temperatures outside the range of thermoreceptors• Capsaicin

Unencapsulated Dendritic Endings

• Light touch receptors– Tactile (Merkel) discs– Hair follicle receptors

Table 13.1

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

Table 13.1

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

• Input comes from exteroceptors, proprioceptors, and interoceptors

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

Sensory Integration

• Levels of neural integration in sensory systems:

1. Receptor level—the sensor receptors

2. Circuit level—ascending pathways

3. Perceptual level—neuronal circuits in the cerebral cortex

Figure 13.2

1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus

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 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 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 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 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

• 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

• 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

• 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)

Figure 13.2

1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus

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

• Cordlike organ of the PNS

• Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

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

Figure 13.3b

Bloodvessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

(b)

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

• Contain neuron cell bodies associated with nerves– Dorsal root ganglia (sensory, somatic)

(Chapter 12)– Autonomic ganglia (motor, visceral)

(Chapter 14)

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

Figure 13.4 (1 of 4)

Endoneurium

Dropletsof myelin

Fragmentedaxon

Schwann cells

Site of nerve damage

The axonbecomesfragmented atthe injury site.

1

Figure 13.4 (2 of 4)

Schwann cell Macrophage Macrophagesclean out thedead axon distalto the injury.

2

Figure 13.4 (3 of 4)

Fine axon sproutsor filaments

Aligning Schwann cellsform regeneration tube

3 Axon sprouts,or filaments,grow through aregeneration tubeformed bySchwann cells.

Figure 13.4 (4 of 4)

Schwann cell Site of newmyelin sheathformation

4 The axonregenerates anda new myelinsheath forms.

Single enlargingaxon filament

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”

Figure 13.5 (a)

Frontal lobe

Temporal lobe

InfundibulumFacialnerve (VII)Vestibulo-cochlearnerve (VIII)Glossopharyngealnerve (IX)Vagus nerve (X)Accessory nerve (XI)

Hypoglossal nerve (XII)

(a)

Filaments ofolfactory nerve (I)

Olfactory bulb

Olfactory tract

Optic chiasma

Optic nerve(II)

Optic tractOculomotornerve (III)Trochlearnerve (IV) Trigeminalnerve (V) Abducensnerve (VI)CerebellumMedullaoblongata

Figure 13.5 (b)

*PS = parasympathetic(b)

Cranial nervesI – VI

I

II

III

IV

V

VI

Olfactory

Optic

Oculomotor

Trochlear

Trigeminal

Abducens

Yes (smell)

Yes (vision)

No

No

Yes (generalsensation)

No

No

No

Yes

Yes

Yes

Yes

No

No

Yes

No

No

No

Cranial nervesVII – XII

Sensoryfunction

Motorfunction

PS*fibers

Sensoryfunction

Motorfunction

PS*fibers

VII

VIII

IX

X

XI

XII

Facial

Vestibulocochlear

Glossopharyngeal

Vagus

Accessory

Hypoglossal

Yes (taste)

Yes (hearingand balance)

Yes (taste)

Yes (taste)

No

No

Yes

Some

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

No

No

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

Table 13.2

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

Table 13.2

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

Table 13.2

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2