Download Peripheral Nervous System Part Two

The Nervous System
SPINAL REFLEXES
Reflexes
 Rapid, involuntary, predictable motor response to a
stimulus
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
 Identical stimulus should always elicit the same
responsestereotyped reflex
Reflex Arc
Components of a reflex arc

1.
2.
3.
4.
5.
Receptor—site of stimulus action
Sensory neuron—transmits afferent impulses to the CNS
Synapses in gray matter—either monosynaptic or
polysynaptic region within the CNS
Motor neuron—conducts efferent impulses away from cord
Effector—muscle fiber or gland cell that responds to the
efferent impulses by contracting or secreting
Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
Figure 13.14
The Stretch Reflex
 Muscle spindles
 Monosynaptic reflex
 Maintenance of posture
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
activating impulses to the quadriceps
causing it to contract, extending the
knee.
Figure 13.17 (2 of 2)
Secondary sensory
endings (type II fiber)
Primary sensory
endings (type Ia
fiber)
Muscle spindle
Connective
tissue capsule
Efferent (motor)
fiber to muscle spindle
 Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
Intrafusal muscle
fibers
Sensory fiber
Golgi tendon
organ
Copyright © 2010 Pearson Education, Inc.
Tendon
Figure 13.15
Reciprocal Inhibition
 Involves antagonistic flexor muscle
 Polysynaptic
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Figure 13.17 (2 of 2)
Withdrawal Reflex
 Polysynaptic
 Polysegmental


Association neurons
ascend and descend spinal
cord
Synapse with motor
neurons in other spinal
nerves
 Quickly withdraws
(flexes) threatened limb
Crossed-Extensor Reflex
 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

+ Excitatory synapse
– Inhibitory synapse
Interneurons
Efferent
fibers
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.
Arm
movements
Flexor
inhibited
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Figure 13.19
Questions?
Quiz 1 Results
Average = 73.7%
Form A
Form B
1. B
2. E
3. E
4. C
5. A
6. A
7. E
8. A
9. E
10. D
11. C
12. B
13. D
14. B
15. A,B
16. E
17. C
18. E
19. C
20. D
21. A
22. B
23. D
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2.
3.
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E
D
C
A
A
A
E
D
C
E
C
E
B
D
E
E
B
A,B
C
A
B
D
B