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Automatic patterned response to a stimulus
–Stretch reflex
–Withdrawal and crossed-extensor reflexes
Reflexes can be categorized into four groups, each of which has two categories |
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Neural pathway for a reflex
•Components
1.Sensory receptor
2.Afferent neuron
3.Integration center
4.Efferent neuron
5.Effector organ |
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| The highest level of integration occurs in the spinal cord |
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| The highest level of integration requires participation by the brain |
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| Involve signals sent via somatic neurons to skeletal muscle |
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Also called visceral reflexes
–Involve signals sent via autonomic neurons to smooth muscle, cardiac muscle, or glands |
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–All people with normal neurological function have the same innate reflexes
–We are born with them |
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–Differ according to each person’s experience s in life
–Example: After a person eats too much of a favorite food such that he becomes ill, future exposures to the smell of the food may cause him to develop nausea |
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| The neural pathway consists of only two neurons and a single synapse |
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| The neural pathway contains more than two neurons and multiple synapses. |
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Example: Knee-jerk reflex
–The stretch reflex is the only know monosynaptic reflex in the human body.
–The receptor is a muscle spindle which is a specialized structure found in skeletal muscle that detects lengthening of the muscle
•Tapping the patellar tendon below the kneecap stretches the quadriceps muscle in the upper thigh.
•This stretch excites muscle spindles in that muscle triggering an action potential that travel in afferent neurons to the spinal cord.•In the spinal cord the afferent neurons make a direct excitatory synaptic connections with efferent neurons that innervate the quadriceps muscle. (1)
•This stimulates the quadriceps to contract and the leg to extend.
•To avoid resistance to the leg extension by the hamstring muscles, the stretch reflex inhibits hamstring contraction.
•The afferent neurons from the muscle spindle also synapse on inhibitory interneurons innervating the motor neurons to the hamstrings. |
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Definition
–When a portion of the body is subjected to painful stimuli, it withdraws from the stimulus automatically.
•Stepping on a tack is perceived as painful because it activates special sensory receptors called nociceptors
–Special receptors that respond to intense damaging or potentially damaging stimuli.•Afferent neurons from the nociceptors transmit information to the spinal cord where they have excitatory synapses on interneurons.
•The interneurons then excite efferent neurons:
1.The neuron that innervate the skeletal muscles that cause withdrawal of the limb or other body part. The hamstring muscle contracts to withdraw the leg.
•For the withdrawal reflex to work effectively, the muscle that oppose the withdrawal should be inhibited.
2.Branches of the afferent neurons activate inhibitory interneurons to the efferent innervating the quadriceps. Therefore the withdrawal reflex simultaneously triggers contraction of the hamstrings and relaxation of the quadriceps. This allows flexion of the leg.
•But there is another problem.
•If you withdraw your leg while it is supporting your weight you may lose your balance and fall over.
•Cross-Extensor Reflex
–Fortunately, when the withdrawal reflex is initiated, another reflex is initiated simultaneously.
•The afferent neurons from the nociceptors have branches that send signals via interneurons to efferent neurons controlling muscle on the opposite leg.
•These signals trigger contraction of the extensor muscle (3, excitatory interneurons) and relaxation of the flexor muscles (4, inhibitory interneuron) in that leg.
•Thus when the first leg is withdrawn in response to painful stimuli the other leg is extended to support the body.
•Finally, a person needs to be aware that they stepped on a tack.
5.The afferent neuron also communicates with the ascending pathways that send information to the brain that a stimulus has been applied to the body. |
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| stimulates other leg to extend to hold body wieght after stepping on a tack with other leg |
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