Question 1. Draw a diagram to show different parts of reflex arc. Describe properties of reflex.
Answer:
Reflex arc:
- The anatomical nervous pathway for a reflex action is called reflex arc.
Parts:
1. Receptor:
- It is the end organ that receives the stimulus.
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2. Afferent nerve:
- It transmits sensory impulses from receptor to the center.
3. Center:
- Receives sensory impulses via afferent nerve fibers.
- It generates appropriate motor impulse
- It is located in the brain or spinal cord.
4. Efferent nerve:
- Transmits motor impulses from center to the effector organ.
5. Efferent organ:
- It shows response to the stimulus.
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Properties of reflex:
1. One way conduction [Bell – Megendie Law]:
- Impulses are transmitted in only one direction via reflex arc.
- They are transmitted from receptors to center then to effector organ.
2. Reaction Time:
- It is the time interval between application of stimulus and the onset of reflex.
- It depends upon length of the nerve fibers.
3. Summation:
- Repeated or simultaneous stimulation of motor neurons with sub minimal stimuli exhibit the phenomenon of summation.
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Types:
Spatial summation:
- In this the stimuli is simultaneously applied to different presynaptic neurons.
Temporal summation:
- Here a single nerve fiber is stimulated repeatedly with sub-threshold stimuli which are summed up to give response.
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4. Occlusion:
- When muscle with two nerve fibers are stimulated simultaneously, the tension developed is less than the sum of the tension developed when each nerve is stimulated separately.
- This phenomenon is called occlusion.
- It occurs due to overlapping of the nerve fibers during the distribution.
5. Subliminal fringe:
- When some muscle with two nerve fibers are stimulated simultaneously, the tension developed is greater than the sum of the tension developed when each nerve is stimulated separately.
- This phenomenon is called subliminal fringe.
- It is due to spatial summation.
6. Recruitment:
- When an excitatory nerve is stimulated with a stimulus of constant strength for a long time, there is a progressive increase in the response of reflex.
- This phenomenon is called recruitment.
7. After discharge:
- Even after the cessation of the stimulus the reflex response continues for some more time.
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8. Rebound phenomenon:
- A reflex response that is diminished by stimulation of an afferent nerve will attain full power when the stimulation ceases is known as rebound phenomenon.
9. Fatigue:
- The reflex response obtained gradually gets lessened.
- It is said to be in fatigue.
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10. Habituation:
- If the stimulus is benign and repeated at frequent intervals, the response declines and disappears, it is called habituation.
11. Sensitization:
- It is opposite to habituation.
12. Reciprocal innervations:
- Stimulation of an afferent fibers results in contraction of the agonist and relaxation of the antagonists.
- This called reciprocal innervation.
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Question 2. What is lower motor neuron? What are the effects of lower motor neuron lesion?
Answer:
Lower motor neuron:
- Lower motor neuron are the anterior gray horn cells in the spinal cord and the motor neuron of the cranial nerve nuclei situated in brainstem which innervate the muscles directly.
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