Chapter 13: The Peripheral Nervous System and Reflex Activity

Chapter 13: The Peripheral Nervous System and Reflex Activity

Peripheral Nervous System (PNS)

• Links outside world and CNS• Includes all neural structures outside the brain and spinal

cord– Sensory receptors– Peripheral nerves and their ganglia– Motor endings

• Sensory receptors – respond to changes in environment – stimuli– Activated graded potential nerve impulse

• Sensation – awareness of stimuli• Perception – interpretation of meaning

– Both occur in brain

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

• Classified according to 1. Type of stimulus they detect2. Body location3. Structural complexity

Stimulus Type

1. Mechanoreceptors – mechanical force– Touch pressure (including BP), vibration, and stretch

2. Thermoreceptors – temperature changes3. Photoreceptors – light energy – retina of eye4. Chemoreceptors – chemicals in solution

– Molecules tasted or smelled, changes in blood or intestinal chemistry

5. Nocieptors – potentially damaging stimuli that result in pain – Searing heat, extreme cold, pressure, inflammatory chemicals

Location

1. Exteroceptors – sensitive to stimuli arising outside of the body– Body surface– Touch, pressure, pain, temperature– Senses – vision, hearing, equilibrium, taste, smell

2. Interoceptors – visceroceptors – stimuli with in the body– Internal viscera and blood vessels– Chemical changes, tissue stretch, temp

3. Proprioceptors – internal stimuli – skeletal muscles, tendons, joints, ligaments, and CT coverings– Advise the brain of body movements

Structural Complexity

• Simple Receptors of General Senses – – Receptors respond to several stimuli

• 2 types1. Unencapsualted Dendritic Endings – free or naked

nerve endings– Present nearly everywhere – Abundant in CT and epithelia– Unmyelinated, small diameter C fibers– Distal endings – small knoblike swellings– Respond to temperature and painful stimuli

Simple Receptors

1. Unencapsualted Dendritic Endings (cont)• Temperature outside range – cold – 10-40C and hot –

32-48C – perceived as painful• Also respond to pinch and chemicals released by

damaged tissue• Itch• Tactile (Merkle discs) – free nerve endings associated

with enlarged disc shaped epidermyal cells• Also wrap around hair follicles

Table 13.1

Simple Receptors

2. Encapsulated Dendritic Endings – consist of one or more fiber terminals of sensory neurons enclosed in a CT capsule– Most are mechanoreceptors – vary in size, shape, and

distribution• Meissner’s corpuscles – small receptors surrounded by

Schwann cells and thin CT capsule – touch receptors• Pacinian Corpuscles – lamellated corpuscles –

scattered deep in epidermis – pressure• Ruffini Endings – lie in dermis – flattened capsule –

deep and continuous pressure

Simple Receptors

• 2. Encapsulated Dendritic Endings (cont) – • Muscle spindles – fusiform proprioceptors –

perimysium of skeletal muscle – muscle stretch and reflex that resists stretch

• Golgi tendon organs – proprioceptors in tendons – tendon fibers stretched – nerve endings are activated

• Joint Kinesthetic Receptors – proprioceptors – monitor articular capsules of synovial joints – info on joint position and motion

Table 13.1

Complex Receptors

• Sense organs• Localized collections of cells associated with

the special senses

Sensory Integration

• Sensation – awareness of changes in internal and external environment

• Perception – conscious interpretation of these stimuli

• We depend on both to survive

Somatosensory System

• Part of the sensory system serving the body wall and limbs

• Receives input from exteroceptors, proprioceptors, and interoceptors

• 3 main levels of neural integration – – 1. receptor level – sensory receptors– 2. Circuit level – ascending pathways– 3. Perceptual level – neuronal circuits in 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

1. Receptor Level

• Sensation – stimulus must excite a receptor and APs must reach the CNS

• For this to happen – stimulus – • energy must match specifically to receptor• must be applied within the receptive field• Energy must be converted into a graded

potential (receptor potential) by transduction• Generator potential in the associated neuron

must reach a threshold

1. Receptor Level

• Adaptation– sensory receptors can change sensitivity in presence of a constant stimulus

• Phasic receptors – fast adapting – bursts of impulses at the begging and end of stimulus

• Tonic Receptors – sustained response – little or no adaptation

2. Circuit Level

• Delivers impulses to the cerebral cortex for stimulus localization and perception

3. Perceptual Level

• Interpretation of Sensory input in cerebral cortex

• Projection – exact point in cortex that is activated is always the same “where” regardless of how it is activated

3. Perceptual Level

• Sensory Perception – • Perceptual detection – ability to detect that a stimulus has

occurred• Magnitude Estimation – ability to detect how intense the

stimulus is• Spatial discrimination – identify the site or pattern of

stimulation• Feature Abstraction – mechanism by which one neuron or

circuit is turned to one feature in the presence of another• Quality discrimination – ability to differentiate submodalities

(qualities) of a sensation• Pattern recognition – ability to take in the scene around us and

recognize a familiar pattern

Perception of Pain

• Receptors activated by extremes of pressure and temperature, as well as, chemicals release by damaged tissue

• Sharp pain – small myelinated A delta fibers• Burning pain – small unmyelinated C fibers• Both release glutamate and substance P

activate 2nd order neurons • Hyperalgesia – pain amplification• Phantom Limb pain – pain in tissue that is no

longer present

Transmission Lines – Nerves & Their Ganglia

• Structure and Classification – • Nerve – cordlike organ • Vary in size• Consists of parallel bundles of peripheral axons

enclosed by CT• Axon – surrounded by endoneurium – CT layer• Groups of fibers (fascicles) bound together by

perineurium• Finally fascicles are enclosed by - epineurium

Figure 13.3b

Bloodvessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

(b)

Nerves & Their Ganglia

• Classified according to the direction which they transmit impulses

• Mixed nerves – both ways• Sensory (afferent) nerves – carry impulses

towards the CNS• Motor (efferent) nerves – carry impulses away

from CNS• Ganglia – collections of neuron cell bodies

associated with nerves in the PNS

Regeneration of Nerve Fibers

• Real mature neurons do not divide• Damage severe or close to cell body – entire

neuron may die• Other neurons attached to that neuron may

also die• Cell body intact – cut or compressed nerves

can regenerate successfully

Regeneration of Nerve Fibers

1. Axon becomes fragmented at the injury site2. Macrophages clean out the dead axon distal

to injury3. Axon sprouts, or filaments, grow through a

regeneration tube formed by Schwann cells4. The axon regenerated and a new myelin

sheath forms

Figure 13.4 (1 of 4)

Endoneurium

Dropletsof myelin

Fragmentedaxon

Schwann cells

Site of nerve damage

The axonbecomesfragmented atthe injury site.

1


Schwann cell Macrophage Macrophagesclean out thedead axon distalto the injury.

2


Fine axon sproutsor filaments

Aligning Schwann cellsform regeneration tube

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


Schwann cell Site of newmyelin sheathformation

4 The axonregenerates anda new myelinsheath forms.

Single enlargingaxon filament

Cranial Nerves

• 12 pairs associated with brain• 1st – forebrain• Rest – brain stem• Only head and neck structures

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

Cranial Nerves

I. Olfactory – tiny sensory nerves of smell• Run from nasal mucosa to synapse with the

olfactory bulbII. Optic – sensory nerve of vision – brain tractIII. Oculomotor – “eye mover” – 6 extrinsic

muscles that move the eyeIV. Trochlear – “pulley” innervates extrinsic eye

muscle through a pully shaped ligament

Table 13.2

Table 13.2

Table 13.2

Table 13.2

Cranial Nerves

V. Trigeminal – 3 branches, sensory fibers to the face and motor fibers to the chewing muscles

VI. Abducens – controls extrinsic eye muscle that abducts the eyeball

VII. Facial – large nerve – innervates muscles of facial expression

VIII. Vestibulocochlear – auditory nerve – hearing and balance

Table 13.2

Table 13.2

Table 13.2

Table 13.2

Table 13.2

Table 13.2

Cranial Nerves

IX. Glossopharyngeal – tongue and PharynxX. Vagus – only cranial nerve that extends

beyond the head into the thorax and abdomen

XI. Accessory – accessory part of the vagus nerve

XII. Hypoglossal – under the tongue, innervates the tongue muscles

Table 13.2

Table 13.2

Table 13.2

Table 13.2

Cranial Nerves

• Mixed nerves• Cell bodies located in cranial sensory ganglia

except – olfactory and optic• Somatic and autonomic motor fibers• Serve skeletal muscle and visceral organs• Primary functions: Sensory, Motor or both

Spinal Nerves

• 31 pairs • Each 1000s of nerve fibers• Named according to their point of issue • 8 pairs cranial spinal nerves – C1 –C8 only 7

vertebrae – C8 emerges inferior to 7th vertebrae• 12 pairs of thoracic – T1- T12• 5 pairs of lumbar – L1-L5• 5 pairs of sacral – S1-S5• 1 pair of coccygeal - Co1

Figure 13.6

CervicalnervesC1 – C8

ThoracicnervesT1 – T12

LumbarnervesL1 – L5

Sacral nervesS1 – S5

Coccygeal nerve Co1

Cervical plexus

Intercostalnerves

Cervicalenlargement

Lumbarenlargement

Cauda equina

Brachial plexus

Lumbar plexus

Sacral plexus

Spinal Nerves

• Connect to spinal cord by a dorsal root and a ventral root

• Each root forms a series of rootlets that attach along the length of spinal cord segment

• Ventral Roots – motor (efferent) fibers – arise from ventral horn motor neurons – impulses to CNS

• Dorsal Roots – sensory (afferents) fibers – arise from the sensory neurons in dorsal root ganglia – impulse to spinal cord

Figure 13.7 (a)

Dorsal rootganglion

Gray matterWhite matterVentral rootDorsal root

Dorsal andventral rootlets of spinal nerve

Dorsal ramusof spinal nerveVentral ramusof spinal nerve

Sympathetic trunkganglion

Spinal nerve

Rami communicantes

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.

Spinal Nerves

• Short• Immediately after emerging from spinal cord –

divides into dorsal ramus, ventral ramus, and a meningeal branch

• Meningral branch reenters the canal to innervate the meninges

• Based to the ventral rami – special rami communicantes – autonomic (visceral) nerve fibers

Figure 13.7 (b)

Dorsal ramus

Ventral ramus

Intercostal nerve

Spinal nerve

Rami communicantes

Dorsal rootganglion Dorsal rootVentral root

Sympathetic trunkganglion

Sternum

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

Branches of intercostalnerve

• Lateral cutaneous• Anterior cutaneous

Innervation of Body Regions

• Supply entire somatic region of body• Dorsal rami – posterior trunk• Ventral rami – rest of trunk and limbs• Nerve Plexuses – complicated interlacing networks

formed by ventral rami• Fibers from the various rami crisscross one another

and become redistributed so that – each branch contains fibers from several spinal nerves and fibers travel via several roots – damage to one segment cannot completely paralyze any muscle limb


• Back – segmented plan• Each dorsal ramus innervates a narrow strip of

muscle and skin in line with which it emerges from the spinal column

• Anterolateral thorax and Abdominal Wall – T1 – T12 – course anteriorly – deep to each rib – intercostal nerves

• T12 – subcostal nerve


• Cervical Plexus and Neck – cervical plexus – formed by ventral rami of the 1st 4 cervical nerves

• Cutaneous nerves supply the skin• Phernic nerve - diaphragm

Figure 13.8

Hypoglossalnerve (XII)

C1

C2

C3

C4

C5

Segmentalbranches

Lesser occipitalnerveGreater auricularnerve

Ansa cervicalis

Phrenic nerve

Supraclavicularnerves

Accessory nerve (XI)

Transversecervical nerve

Ventralrami:

Ventral rami


• Brachial Plexus and Upper Limb – situated partially in the next – gives rise to all nerves in upper limb

• 4 major branches –1. ventral rami2. Trunks3. Divisions4. Cords

Figure 13.9 (a)

Upper

Middle Trunks

Lower

Roots (ventral rami):

Upper subscapular

Lower subscapular

Thoracodorsal

Medial cutaneousnerves of the armand forearm

Long thoracic

Medial pectoral

Lateral pectoral

Nerve tosubclaviusSuprascapular

Dorsal scapular

Posteriordivisions

Anteriordivisions

Lateral

PosteriorCords

Medial

Axillary

Musculo-cutaneousRadial

Median

Ulnar

Posteriordivisions

Trunks Roots

C4

C5

C6

C7

C8

T1

(a) Roots (rami C5 – T1), trunks, divisions, and cords

Innervation of Body Regions- Auxiliary nerve – innervates deltoid, teres minor, skin

and joint capsule- Musculocutaneous nerve – biceps brachial, brachialis

muscle -lateral forearm- Median nerve – anterior forearm – skin and flexor

muscles- Ulnar nerve – flexors not supported by median nerve- Radial nerve – continuation of posterior cord, posterior

skin of limb, extensor muscles – elbow extension, forearm supination. Wrist and finger extension, and thumb abduction

Figure 13.9 (c)

Median nerve

Musculocutaneous nerve

Radial nerveHumerus

Ulna

Ulnar nerveMedian nerve

Radius

Radial nerve (superficial branch)

Superficial branch of ulnar nerveDorsal branch of ulnar nerve

Digital branch of ulnar nerveMuscular branchDigital branch

(c) The major nerves of the upper limb

Axillarynerve

Anteriordivisions

Posteriordivisions

Trunks Roots

Table 13.4


• Lumbosacral and Lower Limb – lumbosacral plexus

• Lumbar plexus – L1-L4 – anterior and medial thigh –

• femoral nerve – quads, thigh flexors and knee extensors

• Obturator nerve – adductor muscles


• Lumbosacral and Lower Limb –• Sacral Plexus – L4 –S4 – buttock and lower

limb• Sciatic nerve – entire lower limb• Tibial nerve – posterior compartments of leg• Superior and inferior gluteal nerves – buttock

and tensor fascia lata

Figure 13.10

(a) Ventral rami and major branches of the lumbar plexus

Iliohypogastric

L1

L2

L3

L4

L5

Ilioinguinal

Genitofemoral

Lateral femoralcutaneous

Obturator

Femoral

Lumbosacraltrunk

Lateral femoralcutaneous

Anterior femoralcutaneousSaphenous

Obturator

IliohypogastricIlioinguinalFemoral

Ventral rami Ventralrami:

(b) Distribution of the major nerves from the lumbar plexus to the lower limb

Table 13.5

Innervation of Skin: Dermatomes

• Dermatome – area of skin innervated by cutaneous branches of single spinal cord

• Uniform in width, almost horizontal, and in a direct line with their spinal nerves

Figure 13.12

C2C3

C4

C5T1

T2

T2T3T4T5

C6

C8C7 C7

C6

T6T7T8T9

T10

T11

T12L1

S2S3

L1

L2

L3

L4

L5

L2

L3

L4

L5

S1

C5

C6

C8

T2

C5

C6

S1

Anterior view

C2

C3

C4C5C6C7C8

C8 C8

C7 C7

T1T2T3T4T5T6T7T8T9

T10

T11T12

L1L2 L3

S1(b) Posterior view

L5S2

S1

S1

S3

S2 S1S2

S4S5

L5L5

L4L5L5

L4

C6 C6

C5

L4

L3

L2

L1

L4

Innervation of Joints

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

Motor Endings and Motor Activity

• Motor Endings – PNS elements that activate effectors by releasing neurotransmitters

Innervations of Skeletal Muscle

• Neuromuscular junction- axon reaches target – single muscle fiber

• Ending splits into axon terminals that branch over folds of sarcolemma

• Terminal – acetylcholine – diffuses across synaptic cleft

• ACh – binds – opens ion channels propagation of AP

Figure 9.8

Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal of neuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

Mitochondrion

Synaptic cleft

Junctionalfolds of sarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

AChNa+

K+

Degraded ACh

Acetylcholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

Action potential arrives at axon terminal of motor neuron.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.

Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.

ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

1

2

3

4

5

6

Innervation of Visceral Muscle and Glands

• Autonomic motor axons – branch – forming synapse en passant

• Series of varicosities – knoblike swellings – mitochondria and synaptic vessels

• ACh or norepinephrine

Figure 9.27

Smoothmusclecell

Varicosities releasetheir neurotransmittersinto a wide synaptic cleft (a diffuse junction).

Synapticvesicles

Mitochondrion

Autonomicnerve fibersinnervatemost smoothmuscle fibers.

Varicosities

Levels of Motor Control

• Segmental level• Projection level• Precommand level

Figure 13.13a

Feedback

Reflex activity Motoroutput

Sensoryinput

(a) Levels of motor control and their interactions

Precommand Level(highest)• Cerebellum and basal nuclei• Programs and instructions (modified by feedback)

Projection Level (middle)

• Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.)• Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels

Segmental Level (lowest)• Spinal cord• Contains central pattern generators (CPGs)

Internalfeedback

Segmental Level

• The lowest level of the motor hierarchy• Central pattern generators (CPGs): segmental

circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles

• Controls locomotion and specific, oft-repeated motor activity

Projection Level

• Consists of:– Upper motor neurons that direct the direct

(pyramidal) system to produce voluntary skeletal muscle movements

– Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions

• Projection motor pathways keep higher command levels informed of what is happening

Precommand Level

• Neurons in the cerebellum and basal nuclei– Regulate motor activity– Precisely start or stop movements– Coordinate movements with posture– Block unwanted movements– Monitor muscle tone– Perform unconscious planning and discharge in

advance of willed movements

Precommand Level

• Cerebellum– Acts on motor pathways through projection areas

of the brain stem– Acts on the motor cortex via the thalamus

• Basal nuclei– Inhibit various motor centers under resting

conditions

Figure 13.13a

Feedback

Reflex activity Motoroutput

Sensoryinput

(a) Levels of motor control and their interactions

Precommand Level(highest)• Cerebellum and basal nuclei• Programs and instructions (modified by feedback)

Projection Level (middle)

• Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.)• Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels

Segmental Level (lowest)• Spinal cord• Contains central pattern generators (CPGs)

Internalfeedback

Figure 13.13b

(b) Structures involved

Precommand level • Cerebellum• Basal nuclei

Projection level • Primary motor cortex• Brain stem nuclei

Segmental level • Spinal cord

Reflexes

• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

• Learned (acquired) reflexes result from practice or repetition, – Example: driving skills

Reflex Arc

• Components of a reflex arc (neural path)1. Receptor—site of stimulus action2. Sensory neuron—transmits afferent impulses to the CNS3. Integration center—either monosynaptic or polysynaptic

region within the CNS4. Motor neuron—conducts efferent impulses from the

integration center to an effector organ5. Effector—muscle fiber or gland cell that responds to the

efferent impulses by contracting or secreting

Figure 13.14

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

Spinal Reflexes

• Spinal somatic reflexes– Integration center is in the spinal cord– Effectors are skeletal muscle

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

Stretch and Golgi Tendon Reflexes

• For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary – Muscle spindles inform the nervous system of the

length of the muscle– Golgi tendon organs inform the brain as to the

amount of tension in the muscle and tendons

Muscle Spindles

• Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule

• Intrafusal fibers– Noncontractile in their central regions (lack

myofilaments) – Wrapped with two types of afferent endings:

primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

Muscle Spindles

• Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity

• Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibers

Figure 13.15

Secondary sensoryendings (type II fiber)

Efferent (motor)fiber to muscle spindle

Primary sensoryendings (type Iafiber)

Connectivetissue capsule

Muscle spindle

Tendon

Sensory fiber

Golgi tendonorgan

Efferent (motor)fiber to extrafusalmuscle fibers

Extrafusal musclefiber

Intrafusal musclefibers

Muscle Spindles

• Excited in two ways:1. External stretch of muscle and muscle spindle2. Internal stretch of muscle spindle:

• Activating the motor neurons stimulates the ends to contract, thereby stretching the spindle

• Stretch causes an increased rate of impulses in Ia fibers

Figure 13.16a, b

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

Musclespindle

Intrafusalmuscle fiber

Primarysensory (la)nerve fiberExtrafusalmuscle fiber

Time

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

Time

Muscle Spindles

• Contracting the muscle reduces tension on the muscle spindle

• Sensitivity would be lost unless the muscle spindle is shortened by impulses in the motor neurons

• – coactivation maintains the tension and sensitivity of the spindle during muscle contraction

Figure 13.16c, d

(d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length.

Time

(c) Only motor neurons activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes.

Time

Stretch Reflexes

• Maintain muscle tone in large postural muscles

• Cause muscle contraction in response to increased muscle length (stretch)

Stretch Reflexes

• How a stretch reflex works:– Stretch activates the muscle spindle– IIa sensory neurons synapse directly with motor

neurons in the spinal cord– motor neurons cause the stretched muscle to

contract• All stretch reflexes are monosynaptic and

ipsilateral

Stretch Reflexes

• Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles

• Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax

Figure 13.17 (1 of 2)

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

12

3a 3b

Figure 13.17 (1 of 2), step1






Sensoryneuron


Spinal cord

1

Figure 13.17 (1 of 2), step 2







Sensoryneuron


Spinal cord

12

Figure 13.17 (1 of 2), step 3a








Sensoryneuron


Spinal cord

12

3a

Figure 13.17 (1 of 2), step 3b






Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).



Sensoryneuron


Spinal cord

12

3a 3b

Figure 13.17 (2 of 2)

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

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 3b



Musclespindle


Hamstrings(flexors)

Patella

Patellarligament




+

–

1

1



Musclespindle


Hamstrings(flexors)

Patella

Patellarligament





+

–

1

2

1

2

Figure 13.17 (2 of 2), step 3a


Musclespindle


Hamstrings(flexors)

Patella

Patellarligament






+

–

1

2

3a

1

2

3a

Figure 13.17 (2 of 2), step 3b


Musclespindle


Hamstrings(flexors)

Patella

Patellarligament





The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.


+

–

1

2

3a

3b

1

2

3a3b 3b

Golgi Tendon Reflexes

• Polysynaptic reflexes• Help to prevent damage due to excessive

stretch • Important for smooth onset and termination

of muscle contraction

Golgi Tendon Reflexes

• Produce muscle relaxation (lengthening) in response to tension– Contraction or passive stretch activates Golgi tendon

organs – Afferent impulses are transmitted to spinal cord – Contracting muscle relaxes and the antagonist contracts

(reciprocal activation)– Information transmitted simultaneously to the cerebellum

is used to adjust muscle tension

Figure 13.18

+ Excitatory synapse– Inhibitory synapse

Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1 2

3a 3b

Figure 13.18, step 1



Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1

Figure 13.18, step 2




Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1 2

Figure 13.18, step 3a





Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1 2

3a

Figure 13.18, step 3b





Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord


Golgitendon

organHamstrings

(flexors)

1 2

3a 3b

Flexor and Crossed-Extensor Reflexes

• Flexor (withdrawal) reflex– Initiated by a painful stimulus– Causes automatic withdrawal of the threatened

body part– Ipsilateral and polysynaptic

Flexor and Crossed-Extensor Reflexes

• Crossed extensor reflex– Occurs with flexor reflexes in weight-bearing limbs

to maintain balance– Consists of an ipsilateral flexor reflex and a

contralateral extensor reflex• The stimulated side is withdrawn (flexed)• The contralateral side is extended

Figure 13.19

Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated


Superficial Reflexes

• Elicited by gentle cutaneous stimulation• Depend on upper motor pathways and cord-

level reflex arcs


• Plantar reflex– Stimulus: stroking lateral aspect of the sole of the

foot– Response: downward flexion of the toes– Tests for function of corticospinal tracts


• Babinski’s sign – Stimulus: as above– Response: dorsiflexion of hallux and fanning of

toes– Present in infants due to incomplete myelination– In adults, indicates corticospinal or motor cortex

damage


• Abdominal reflexes– Cause contraction of abdominal muscles and

movement of the umbilicus in response to stroking of the skin

– Vary in intensity from one person to another– Absent when corticospinal tract lesions are

present

Developmental Aspects of the PNS

• Spinal nerves branch from the developing spinal cord and neural crest cells– Supply both motor and sensory fibers to

developing muscles to help direct their maturation– 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 due to loss of neurons, decreased numbers of synapses per neuron, and slower central processing

• Peripheral nerves remain viable throughout life unless subjected to trauma

Documents

Chapter 13: The Peripheral Nervous System and Reflex Activity