Central Nervous System Question And Answers

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.

Read And Learn More: BDS Previous Examination Question And Answers

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.

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.

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.

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.

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.

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.

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.