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The Nervous System Reflexes Spinal Reflexes Reflex Arc the Stretch

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The Nervous System Reflexes Spinal Reflexes Reflex Arc the Stretch 1/17/2016 Reflexes • Rapid, involuntary, predictable motor response to a stimulus The Nervous System Spinal Reflexes Spinal Reflexes Reflex Arc • Spinal somatic reflexes • Components of a reflex arc – Integration center is in the spinal cord 1. Receptor—site of stimulus action – Effectors are skeletal muscle 2. Sensory neuron—transmits afferent impulses to the CNS • Testing of somatic reflexes is important clinically 3. Synapses in gray matter—either monosynaptic or to assess the condition of the nervous system polysynaptic region within the CNS 4. Motor neuron—conducts efferent impulses away from cord • Identical stimulus should always elicit the same 5. Effector—muscle fiber or gland cell that responds to response stereotyped reflex the efferent impulses by contracting or secreting Stimulus The Stretch Reflex Skin • Monosynaptic reflex – 2 neurons (sensory and motor), 1 synapse 1 Receptor Interneuron • Muscle spindles 2 Sensory neuron – Sensory receptors in belly of muscle 3 Integration center – Detects changes in length of muscle 4 Motor neuron • Muscle is stretched, reflex reverses the stretch 5 Effector • Important for coordination, maintenance of posture, keeps muscles from over stretching Spinal cord (in cross section) Figure 13.14 1 1/17/2016 Secondary sensory The patellar (knee-jerk) reflex—a specific example of a stretch reflex Efferent (motor) endings (type II fiber – fiber to muscle spindle senses when muscle 2 is still) Quadriceps 3a (extensors) 3b 3b ααα Efferent (motor) 1 Primary sensory fiber to extrafusal Patella endings (type Ia Muscle Spinal cord muscle fibers spindle Fiber – senses (L 2–L4) stretching) Extrafusal muscle 1 Tapping the patellar ligament excites fiber Hamstrings Patellar muscle spindles in the quadriceps. Muscle spindle (flexors) ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with Intrafusal muscle motor neurons and interneurons. Connective fibers 3a The motor neurons (red) send tissue capsule activating impulses to the quadriceps causing it to contract, extending the + Excitatory synapse knee. Sensory fiber – Inhibitory synapse Golgi tendon Tendon Organ – senses changes in Figure 13.17 (2 of 2) muscle tension Figure 13.15 Reciprocal Inhibition The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 3a 3b • When a muscle is stretched, its antagonist is 3b 1 Patella flexing Muscle Spinal cord spindle (L 2–L4) • To avoid damage, it must be turned off in the 1 Tapping the patellar ligament excites Hamstrings Patellar muscle spindles in the quadriceps. (flexors) ligament reflex 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with • Additional connection sends inhibitory signal to motor neurons and interneurons. 3a The motor neurons (red) send activating impulses to the quadriceps the antagonist of the stretched muscle causing it to contract, extending the + Excitatory synapse knee. – Inhibitory synapse – Polysynaptic 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 Crossed-Extensor Reflex • Polysynaptic • Crossed extensor reflex • Polysegmental – Occurs with flexor reflexes in weight-bearing limbs – Association neurons to maintain balance ascend and descend spinal cord – Consists of an ipsilateral flexor reflex and a – Synapse with motor contralateral extensor reflex neurons in other spinal • The stimulated side is withdrawn (flexed) nerves • The contralateral side is extended • Quickly withdraws (flexes) threatened limb 2 1/17/2016 + Excitatory synapse – Inhibitory synapse Interneurons Efferent Afferent fibers fiber Efferent fibers Extensor Flexor inhibited Arm inhibited movements Flexor Extensor stimulated stimulated Site of reciprocal activation: At the Site of stimulus: a noxious same time, the stimulus causes a flexor extensor muscles reflex on the same side, on the opposite withdrawing that limb. side are activated. Figure 13.19 3.
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