copyright © 2010 pearson education, inc. the peripheral nervous system & reflex activity...

Copyright © 2010 Pearson Education, Inc.

THE PERIPHERAL NERVOUS SYSTEM &

REFLEX ACTIVITY

CHAPTER # 13(d)


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 action

2. Sensory neuron—transmits afferent impulses to the CNS

3. Integration center—either monosynaptic or polysynaptic region within the CNS

4. Motor neuron—conducts efferent impulses from the integration center to an effector organ

5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Copyright © 2010 Pearson Education, Inc. 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


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 spindle

2. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step1






Sensoryneuron


Spinal cord

1

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 2







Sensoryneuron


Spinal cord

12

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 3a








Sensoryneuron


Spinal cord

12

3a

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 3a


Musclespindle


Hamstrings(flexors)

Patella

Patellarligament






+

–

1

2

3a

1

2

3a

Copyright © 2010 Pearson Education, Inc. 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


+ 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

Copyright © 2010 Pearson Education, Inc. 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


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

copyright © 2010 pearson education, inc. the peripheral nervous system & reflex activity...

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