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Transcript of Copyright © 2010 Pearson Education, Inc. THE PERIPHERAL NERVOUS SYSTEM & REFLEX ACTIVITY CHAPTER #...
Copyright © 2010 Pearson Education, Inc.
THE PERIPHERAL NERVOUS SYSTEM &
REFLEX ACTIVITY
CHAPTER # 13(d)
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
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
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
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
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
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
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
Copyright © 2010 Pearson Education, Inc. 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
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
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
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
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
Copyright © 2010 Pearson Education, Inc.
Stretch Reflexes
• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased muscle length (stretch)
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
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
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
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 events by which muscle stretch is damped
Initial stimulus(muscle stretch)
Cell body ofsensory neuron
Sensoryneuron
Muscle spindleAntagonist muscle
Spinal cord
1
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 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
Initial stimulus(muscle stretch)
Cell body ofsensory neuron
Sensoryneuron
Muscle spindleAntagonist muscle
Spinal cord
12
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 3a
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.
Initial stimulus(muscle stretch)
Cell body ofsensory neuron
Sensoryneuron
Muscle spindleAntagonist muscle
Spinal cord
12
3a
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (1 of 2), step 3b
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 (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
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 1
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.
Excitatory synapseInhibitory synapse
+
–
1
1
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 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.
Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.
Excitatory synapseInhibitory synapse
+
–
1
2
1
2
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 3a
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.
Excitatory synapseInhibitory synapse
+
–
1
2
3a
1
2
3a
Copyright © 2010 Pearson Education, Inc. Figure 13.17 (2 of 2), step 3b
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
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
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
Copyright © 2010 Pearson Education, Inc. 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
Quadriceps(extensors)
Golgitendon
organHamstrings
(flexors)
1 2
3a 3b
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)
Golgitendon
organHamstrings
(flexors)
1
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)
Golgitendon
organHamstrings
(flexors)
1 2
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)
Golgitendon
organHamstrings
(flexors)
1 2
3a
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)
Golgitendon
organHamstrings
(flexors)
1 2
3a 3b
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
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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
Copyright © 2010 Pearson Education, Inc. 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
+ Excitatory synapse– Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level reflex arcs
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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
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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
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