The Nervous System S P I N A L R E F L E X E S

The Nervous System S P I N A L R E F L E X E S

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. Receptor site of stimulus action 2. Sensory neuron transmits afferent impulses to the CNS 3. Synapses in gray matter either monosynaptic or polysynaptic region within the CNS 4. Motor neuron conducts efferent impulses away from cord 5. Effector muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Stimulus Skin 1 2 3 4 5 Receptor Sensory neuron Integration center Motor neuron Effector Interneuron 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) 3a 3b 3b Muscle spindle 1 Patella Spinal cord (L 2 L 4 ) Hamstrings (flexors) + Excitatory synapse Inhibitory synapse 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 activating impulses to the quadriceps causing it to contract, extending the knee. Copyright 2010 Pearson Education, Inc. Figure 13.17 (2 of 2)

Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle Primary sensory endings (type Ia fiber) Muscle spindle Connective tissue capsule Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers Sensory fiber Golgi tendon organ Tendon Copyright 2010 Pearson Education, Inc. 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) 3a 3b 3b Muscle spindle 1 Patella Spinal cord (L 2 L 4 ) Hamstrings (flexors) + Excitatory synapse Inhibitory synapse 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 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 Afferent fiber Efferent fibers Efferent fibers Extensor inhibited Flexor stimulated Arm movements Flexor inhibited Extensor 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. Figure 13.19

Questions?

Quiz 1 Results Average = 73.7% Form A 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 Form B 1. E 2. D 3. C 4. A 5. A 6. A 7. E 8. D 9. C 10. E 11. C 12. E 13. B 14. D 15. E 16. E 17. B 18. A,B 19. C 20. A 21. B 22. D 23. B