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Peripheral Nervous System

Learn and Understand:

• Peripheral nerves connect the edges of the

body and outside world to the CNS.

• Most nerves carry specific types of

information to/from specific locations.

• PNS includes nervous-like tissue: sensitive

and excitable.

• Humans possess both instinctual and

learned reflexes. Many subconscious and

involuntary, many protective, many

postural/positional, most homeostatic.

Figure 13.1 Place of the PNS in the structural organization of the nervous

system.

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

Provides neuronal link to and from body and outside world

Includes all neural structures outside brain

Sensory receptors

Peripheral nerves and associated ganglia, plexuses

Efferent motor endings

Figure 13.4b Structure of a nerve. Axon

Myelin sheath

Endoneurium

Perineurium

Epineurium

Fascicle

Blood

vessels

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Peripheral Nervous Tissue:

Classification of Nerves

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

• Classified according to direction of impulses

– Mixed nerves – both sensory and motor fibers; impulses both to and from CNS

– Sensory (afferent) nerves – impulses only toward CNS

– Motor (efferent) nerves – impulses only away from CNS

• Peripheral nerves classified as cranial or spinal nerves

Spinal Nerves

• 31 pairs of mixed nerves named for point

of issue from spinal cord

– Supply all body parts but head and part of

neck

– 8 cervical (C1–C8)

– 12 thoracic (T1–T12)

– 5 Lumbar (L1–L5)

– 5 Sacral (S1–S5)

– 1 Coccygeal (C0)

Figure 13.8a Formation of spinal nerves and rami distribution .

Dorsal root

Dorsal and ventral rootlets of spinal nerve

Dorsal root

ganglion

Dorsal ramus of spinal nerve

Ventral ramus

of spinal nerve

Spinal nerve

Sympathetic trunk ganglion

Anterior view showing spinal cord, associated nerves, and vertebrae.

The dorsal and ventral roots arise medially as rootlets and join laterally to

form the spinal nerve.

Rami communicantes

Ventral root

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Dorsal ramus

Ventral ramus

Spinal nerve

Rami communicantes

Sympathetic trunk ganglion Dorsal root ganglion

Dorsal root

Ventral root

Branches of intercostal nerve

Lateral cutaneous

Anterior cutaneous

Sternum

Intercostal nerve

Cross section of thorax showing the main roots and branches of a spinal nerve.

Figure 13.8b Formation of spinal nerves and rami distribution.

Figure 13.13 Map of dermatomes – Specificity of sensory information carried by individual spinal

nerves

C2

C3

C4

C5

T5 T4

T3 T2 T1

T6

T7

T8

T9

T10

T2

T11

T2

C5

C6

C6

C7 C8

T12 L1 L1

L2 L2

S2 S3 C8

C5

C6

C6

C7

L3

L4

L5 L5

L4

L3

S1 S1

C2

C3

C4

C5

C6

C7 C8

T1

T2

T3 T4

T5 T6 T7 T8 T9 T10

C5

C6

C7

C8 L2

L4

S1

T11

T12

L1

L3 L5

C8

C6

C7

S2 S3

S4

S5

S1 S2 S2 S1

L1

L2 L5 L5

L3

L4

L4

L5 L5

L4

S1

Posterior view Anterior view

Cranial Nerves

• Twelve pairs of nerves associated with brain

– Ten attach to brain stem

• Most are mixed nerves; two pairs purely sensory

• Each numbered (I through XII) and named from

rostral to caudal

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good vehicle anyhow"

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good vacations are heavenly"

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Filaments of

olfactory nerve (I)

Olfactory bulb

Olfactory tract

Optic nerve (II)

Optic chiasma

Optic tract

Oculomotor

nerve (III)

Trochlear

nerve (IV)

Trigeminal

nerve (V)

Abducens

nerve (VI)

Cerebellum

Medulla oblongata

Frontal lobe

Temporal lobe

Infundibulum

Facial nerve (VII)

Vestibulocochlear

nerve (VIII)

Glossopharyngeal

nerve (IX)

Vagus nerve (X)

Accessory nerve (XI)

Hypoglossal nerve (XII)

Figure 13.6a Location and function of cranial nerves.

Primary Functions of Cranial Nerves

Nerve Sensory Function Somatic Motor

Function

Parasympathetic Motor

Function

I – olfactory Smell None None

II – optic Vision None None

III – oculomotor None Extrinsic eye muscles

(rectus and inf. oblique);

elevates eyelid

Intrinsic eye muscles – control

pupil and lens

IV – trochlear None Superior oblique None

V – trigeminal General Senses: anterior scalp, nasal

cavity, nasopharynx, face, most of oral

cavity, anterior tongue, part of auricle,

meninges

Muscles of mastication None

VI – abducens None Lateral rectus None

VII – facial Taste: anterior tongue Muscles of facial expression Secretion of tears, saliva

VIII –

vestibulocochlear

Hearing, equilibrium None None

IX –

glossopharyngeal

General Sensory and taste: posterior

tongue; General Sensory: part of

pharynx; Visceral Sensory: carotid

bodies

One pharyngeal muscle Secretion of parotid salivary

glands

X – vagus Visceral Sensory: heart, lungs,

abdominal organs; General Sensory:

ear canal, pharynx, larynx

Most pharyngeal muscles

all laryngeal muscles

Smooth muscle and glands:

heart, lungs, larynx, trachea,

abdominal organs

XI – accessory None Trapezius,

sternocleidomastoid None

XII - hypoglossal none Intrisic and extrinsic tongue

muscles None

Peripheral Nervous Tissue:

Sensory Receptors

• Specialized to respond to changes in

environment (stimuli)

• Activation results in graded potentials that

trigger nerve impulses

• Sensation (awareness of stimulus) and

perception (interpretation of meaning of

stimulus) occur in brain

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Classification by Stimulus Type

• Mechanoreceptors—respond to touch, pressure, vibration, and stretch

• 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

• Exteroceptors respond to

– stimuli arising outside body

– Receptors in skin for touch, pressure, pain, and

temperature and most special sense organs

• Interoceptors (visceroceptors)respond to

– stimuli arising in internal viscera and blood vessels

– Sensitive to chemical changes, tissue stretch, and

temperature changes

– May cause discomfort but usually unaware of their

workings

• Proprioceptors respond to

– stretch in skeletal muscles, tendons, joints, ligaments, and

connective tissue coverings of bones and muscles

– Inform brain of movements, body part position/location

Table 13.1 General Sensory Receptors Classified by Structure and Function

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Table 13.1 General Sensory Receptors Classified by Structure and Function (2 of 3)

Processing at the Receptor Level

• In general sense receptors, graded

potential called generator potential

Stimulus

Generator potential in afferent neuron

Action potential

Processing at the Receptor Level

• In special sense receptors:

Stimulus

Graded potential in receptor cell called

receptor potential

Affects amount of neurotransmitter released

Neurotransmitters generate graded potentials in

sensory neuron

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Figure 13.2 Three basic levels of neural integration in sensory systems.

Perceptual level (processing in cortical sensory centers)

Motor cortex

Somatosensory cortex

Thalamus

Reticular formation

Cerebellum Pons

Medulla

Spinal cord

Circuit level (processing in ascending pathways)

Free nerve endings (pain, cold, warmth)

Muscle spindle

Receptor level

(sensory reception and transmission to CNS)

Joint kinesthetic receptor

3

2

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Peripheral Nervous Function:

Basic Reflexes

• Basic functional unit of nervous system and simplest portion capable of receiving a stimulus and producing a response

• Automatic response to a stimulus that occurs without conscious thought.

• Quick, protective, homeostatic.

• Some integrated within spinal cord; some within brain

(those related to head) using cranial nerves

• Some involve excitatory neurons yielding a response;

some involve inhibitory neurons that prevent an action

• Higher brain centers can influence, suppress, or

exaggerate reflex responses

Variety of Reflexes

Functional classification

– Somatic reflexes

• Activate skeletal muscle

– Autonomic (visceral) reflexes

• Activate visceral effectors (smooth or cardiac muscle

or glands)

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Figure 13.15 The five basic components of all reflex arcs.

Stimulus

Skin

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

1

2

3

4

5

Interneuron

Spinal cord (in cross scetion)

Receptor—site of stimulus action

Sensory neuron—transmits afferent impulses to CNS

Integration center—either 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 by contracting or secreting

Stretch and Tendon Reflexes

• To smoothly coordinate skeletal muscle

nervous system must receive

proprioceptor input regarding

– Length of muscle

• From muscle spindles

– Amount of tension in muscle

• From tendon organs

Figure 13.16 Anatomy of the muscle spindle and tendon organ.

Flower spray endings

(secondary sensory

endings)

Efferent (motor)

fiber to muscle spindle

Efferent (motor) fiber to extrafusal muscle fibers

Extrafusal muscle fiber

Intrafusal muscle fibers

Sensory fiber

Tendon Tendon organ

Anulo-

spiral

endings

(primary

sensory

endings)

Capsule

(connective

tissue)

Muscle

spindle

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• Excited in two ways

1. External stretch of muscle and muscle spindle

2. Internal stretch of muscle spindle

• Stretch causes increased rate of impulses

to spinal cord

• Adjustment for moving/contracting muscles:

– Contracting muscle reduces tension on muscle

spindle

– Sensitivity lost unless muscle spindle shortened

by impulses in motor neurons

– – coactivation maintains tension and sensitivity

of spindle during muscle contraction

Figure 13.17a Operation of the muscle spindle.

How muscle stretch is detected

Muscle spindle

Intrafusal muscle fiber

Sensory fiber

Extrafusal muscle fiber

Time

Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory fiber.

How muscle stretch is detected

Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs.

Time

Stretch Reflexes

• How stretch reflex works

– Stretch activates muscle spindle

– Sensory neurons synapse directly with

motor neurons in spinal cord

– motor neurons cause stretched muscle to

contract

• All stretch reflexes are monosynaptic and

ipsilateral

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Stretch Reflexes

• Reciprocal inhibition also occurs—IIa

fibers synapse with interneurons that

inhibit motor neurons of antagonistic

muscles

– Example: In patellar reflex, stretched muscle

(quadriceps) contracts and antagonists

(hamstrings) relax

Slide 6 Figure 13.18 Stretch Reflex (2 of 2)

Patellar ligament

Patella

Hamstrings (flexors)

Muscle spindle

Quadriceps (extensors)

Spinal cord (L2–L4)

Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons.

The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee.

The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps.

The patellar (knee-jerk) reflex—an example of a stretch reflex

1

2

3a 3b 3b

1

+

+

–

2

3a

3b

– Inhibitory synapse

+ Excitatory synapse

The Tendon Reflex

• Polysynaptic reflexes

• Helps prevent damage due to excessive

stretch

• Important for smooth onset and

termination of muscle contraction

• Produces muscle relaxation (lengthening)

in response to tension

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Slide 5 Figure 13.19 The tendon reflex.

Quadriceps strongly contracts.

Tendon organs are activated.

Afferent fibers synapse with

interneurons in the spinal cord.

Interneurons

Spinal cord

Quadriceps

(extensors)

Tendon organ

Hamstrings

(flexors)

Efferent

impulses to muscle

with stretched

tendon are damped.

Muscle relaxes,

reducing tension.

Efferent impulses

to antagonist muscle

cause it to contract.

3b

+ +

+ –

+ Excitatory synapse

– Inhibitory synapse

3a

2 1

The Flexor and Crossed-Extensor Reflexes

• Flexor (withdrawal) reflex

– Initiated by painful stimulus

– Causes automatic withdrawal of threatened

body part

– Ipsilateral and polysynaptic

– Protective; important

– Brain can override

• E.g., finger stick for blood test

Flexor and Crossed-Extensor Reflexes

• Crossed extensor reflex

– Occurs with flexor reflexes in weight-bearing

limbs to maintain balance

– Consists of ipsilateral withdrawal reflex and

contralateral extensor reflex

• Stimulated side withdrawn (flexed)

• Contralateral side extended

• e.g., step barefoot on broken glass

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Figure 13.20 The crossed-extensor reflex.

+ Excitatory synapse

– Inhibitory synapse

Afferent fiber

Efferent fibers

Arm movements

Extensor inhibited

Flexor stimulated

Interneurons

Efferent fibers

Flexor inhibited

Extensor stimulated

Site of reciprocal

activation: At the same time, the extensor muscles on the opposite side are activated.

Site of stimulus:

A noxious stimulus causes a flexor

reflex on the same side, withdrawing that limb.

+

+

+

–

–

+