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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 responsestereotyped 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 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. E D C A A A E D C E C E B D E E B A,B C A B D B