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Copyright © 2010 Pearson Education, Inc. THE PERIPHERAL NERVOUS SYSTEM & REFLEX ACTIVITY CHAPTER # 13(d)

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Presentation on theme: "Copyright © 2010 Pearson Education, Inc. THE PERIPHERAL NERVOUS SYSTEM & REFLEX ACTIVITY CHAPTER # 13(d)"— Presentation transcript:

1 Copyright © 2010 Pearson Education, Inc. THE PERIPHERAL NERVOUS SYSTEM & REFLEX ACTIVITY CHAPTER # 13(d)

2 Copyright © 2010 Pearson Education, Inc. Reflexes Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, Example: driving skills

3 Copyright © 2010 Pearson Education, Inc. 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

4 Copyright © 2010 Pearson Education, Inc. Figure Receptor Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross section) Interneuron Stimulus Skin

5 Copyright © 2010 Pearson Education, Inc. 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

6 Copyright © 2010 Pearson Education, Inc. 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

7 Copyright © 2010 Pearson Education, Inc. 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

8 Copyright © 2010 Pearson Education, Inc. 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

9 Copyright © 2010 Pearson Education, Inc. Figure Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle Primary sensory endings (type Ia fiber) Connective tissue capsule Muscle spindle Tendon Sensory fiber Golgi tendon organ  Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers

10 Copyright © 2010 Pearson Education, Inc. 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

11 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. Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Time

12 Copyright © 2010 Pearson Education, Inc. 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

13 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

14 Copyright © 2010 Pearson Education, Inc. Stretch Reflexes Maintain muscle tone in large postural muscles Cause muscle contraction in response to increased muscle length (stretch)

15 Copyright © 2010 Pearson Education, Inc. 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

16 Copyright © 2010 Pearson Education, Inc. 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

17 Copyright © 2010 Pearson Education, Inc. Figure (1 of 2) Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a3b

18 Copyright © 2010 Pearson Education, Inc. Figure (1 of 2), step1 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The events by which muscle stretch is damped Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1

19 Copyright © 2010 Pearson Education, Inc. Figure (1 of 2), step 2 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2

20 Copyright © 2010 Pearson Education, Inc. Figure (1 of 2), step 3a Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a

21 Copyright © 2010 Pearson Education, Inc. Figure (1 of 2), step 3b Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a3b

22 Copyright © 2010 Pearson Education, Inc. Figure (2 of 2) The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b

23 Copyright © 2010 Pearson Education, Inc. Figure (2 of 2), step 1 The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 1

24 Copyright © 2010 Pearson Education, Inc. Figure (2 of 2), step 2 The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. Excitatory synapse Inhibitory synapse +–+–

25 Copyright © 2010 Pearson Education, Inc. Figure (2 of 2), step 3a The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 1 2

26 Copyright © 2010 Pearson Education, Inc. Figure (2 of 2), step 3b The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b

27 Copyright © 2010 Pearson Education, Inc. Golgi Tendon Reflexes Polysynaptic reflexes Help to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction

28 Copyright © 2010 Pearson Education, Inc. 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

29 Copyright © 2010 Pearson Education, Inc. Figure 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 Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

30 Copyright © 2010 Pearson Education, Inc. Figure 13.18, step 1 + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1

31 Copyright © 2010 Pearson Education, Inc. Figure 13.18, step 2 + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2

32 Copyright © 2010 Pearson Education, Inc. Figure 13.18, step 3a + 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. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a

33 Copyright © 2010 Pearson Education, Inc. Figure 13.18, step 3b + 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 Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

34 Copyright © 2010 Pearson Education, Inc. Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex Initiated by a painful stimulus Causes automatic withdrawal of the threatened body part Ipsilateral and polysynaptic

35 Copyright © 2010 Pearson Education, Inc. 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

36 Copyright © 2010 Pearson Education, Inc. Figure Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Arm movements Interneurons Efferent fibers Flexor inhibited Extensor stimulated + Excitatory synapse – Inhibitory synapse

37 Copyright © 2010 Pearson Education, Inc. Superficial Reflexes Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord- level reflex arcs

38 Copyright © 2010 Pearson Education, Inc. Superficial Reflexes Plantar reflex Stimulus: stroking lateral aspect of the sole of the foot Response: downward flexion of the toes Tests for function of corticospinal tracts

39 Copyright © 2010 Pearson Education, Inc. Superficial Reflexes 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

40 Copyright © 2010 Pearson Education, Inc. Superficial Reflexes 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


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