Process Of Synaptic Inhibition

PROCESS OF SYNAPTIC INHIBITION

Q. 1 All of the following statements regarding activity at the neuromuscular junction are true except:

 A Action potentials on the alpha motor neuron release acetylcholine (Ach) from the axon terminal
 B Ach stimulates nicotinic cholinergic receptors on the muscle endplate
 C Cholinergic receptors on the muscle endplate are cation channels
 D Alpha motor neurons excite, and Renshaw cells inhibit, the muscle endplate
 
Q. 1 All of the following statements regarding activity at the neuromuscular junction are true except:

 A Action potentials on the alpha motor neuron release acetylcholine (Ach) from the axon terminal
 B Ach stimulates nicotinic cholinergic receptors on the muscle endplate
 C Cholinergic receptors on the muscle endplate are cation channels
 D Alpha motor neurons excite, and Renshaw cells inhibit, the muscle endplate
 
Ans. B

Explanation:

There are no Renshaw cells at the neuromuscular junction. Renshaw cells inhibit motor neurons in the anterior gray horn of the spinal cord. Action potentials on the alpha motor neuron release acetylcholine (ACh) from the axon terminal. The ACh diffuses across the synaptic cleft and stimulates nicotinic receptors on the muscle endplate, generating an action potential that causes contraction of the muscle fiber. Cholinergic receptors on the muscle endplate are cation channels.


Q. 2

Which of the following statements is true about presynaptic inhibition?

 A

It results due to failure of the action potential to reach the synapse.

 B It occurs due to hyperpolarisation of presynaptic membrane
 C

It occurs due to inhibition of release of neurotransmitter from presynaptic terminal.

 D

It occurs due to blockade of neurotransmitter receptors.

Q. 2

Which of the following statements is true about presynaptic inhibition?

 A

It results due to failure of the action potential to reach the synapse.

 B It occurs due to hyperpolarisation of presynaptic membrane
 C

It occurs due to inhibition of release of neurotransmitter from presynaptic terminal.

 D

It occurs due to blockade of neurotransmitter receptors.

Ans. C

Explanation:

It occurs due to inhibition of release of neurotransmitter from presynaptic terminal [Ref: Ganong 22/e p92; Guyton I 1/e p.567]

  • Presynaptic inhibition occurs where the terminal of one neuron makes synaptic contact with the presynaptic ending of a second neuron which in turn excites a third neuron. The first neuron controls the amount of excitatory transmitter released from the second neuron and hence the level of stimulation of the third neuron. It appears that more than one process may be involved in the effect of the first neuron on the second. Control is exerted by the release of an inhibitory neurotransmitter.
  • Mostly the inhibitory transmitter substance is GABA (gamma amino butyric acid).
  • The inhibitory neurotransmitter increases the Cl- conductance of the presynaptic nerve ending. The increase in Cl­conductance causes a partial depolarization of the presynaptic nerve ending and a decrease in the magnitude of the action potential in the presynaptic nerve ending. This in turn reduces Ca++ entry which consequently reduces the amount of excitatory neurotransmitter released by the nerve terminal into the synaptic cleft (the amount of neurotransmitter released from the presynaptic neuron is directly proportional to the magnitude of calcium influx). Thus the magnitude of the postsynaptic potential is reduced. Reducing the magnitude of the postsynaptic potential decreases the probability that an action potential will be generated by the postsynaptic cell.

Alternatively, the transmitter may modulate the conductance of Ca++ channels via G proteins and second messengers.

There is also evidence for direct inhibition of excitatory neurotransmitter release independent of Ca++ influx.

  • Presynaptic inhibition involves axoaxonal synapses.

Presynaptic inhibition occurs in many of the sensory pathways in the nervous system. In fact, adjacent sensory nerve
fibers often mutually inhibit one another, which minimize sideways spread and mixing of signals in 


Q. 3

Regarding presynaptic inhibition which of the following combination is true:          

a)          Affected by axo-axonic transmission.

b)          Not affected by pharmacologic agent

c)          prolonged by anaesthetic agents

d)          not prolonged by strychnine.

e)          Inhibited by picrotoxin

f)           it will increase EPSP

 A

a, c and f are true

 B

a, d, e are true.

 C

d, c and b are true.

 D

a, d and b are true.

Q. 3

Regarding presynaptic inhibition which of the following combination is true:          

a)          Affected by axo-axonic transmission.

b)          Not affected by pharmacologic agent

c)          prolonged by anaesthetic agents

d)          not prolonged by strychnine.

e)          Inhibited by picrotoxin

f)           it will increase EPSP

 A

a, c and f are true

 B

a, d, e are true.

 C

d, c and b are true.

 D

a, d and b are true.

Ans. D

Explanation:

D i.e. With catelectronic current, some of voltage activated Na+ channels become active & when the firing level is reached the voltage activated Na+ channels overwhelm the K+ & other channels and a spike potential (AP) results. Repolarization is produced by opening of voltage gated K+ channels, this opening is slower & more prolonged than Na+ channels. This means much of the increase in K+ conductance comes after the increase in Na+ conductance.

If depolarization occurs rapidly (potential as in graph ‘A’ & ‘C’ ) , the opening of Na+ channels overwhelms the repolarizing force and action potential result —> nerve impulse  generates.

If the induced depolarization is produced gradually slowly (as in graph ‘D’), the opening of K+ channels balances the gradual opening of Na+ channels and an action potential does not occur


Q. 4

TRUE about Renshaw cell inhibition is:

 A

Add on collateral sensation

 B

Increases by local anaesthetics

 C

Has memory for spinal cord.

 D

Inhibition of feed back propogation

Q. 4

TRUE about Renshaw cell inhibition is:

 A

Add on collateral sensation

 B

Increases by local anaesthetics

 C

Has memory for spinal cord.

 D

Inhibition of feed back propogation

Ans. D

Explanation:D i.e. Inhibition of feed back propogation

Q. 5

Feed forward inhibition synapse seen in ‑

 A

Medulla

 B

Cerebellum

 C

Basal ganglia

 D

Hypothalamus

Q. 5

Feed forward inhibition synapse seen in ‑

 A

Medulla

 B

Cerebellum

 C

Basal ganglia

 D

Hypothalamus

Ans. B

Explanation:

Ans. is `b’ i.e., Cerebellum

In feed-forward inhibition, a neuron is connected through two pathways one excitatory and one inhibitory.

For example, in cerebellum the stimulation of Basket cells produces IPSPs (inhibitory postsynaptic potentials) in Purkinje cells.

However, the basket cells and the Purkinje cells are excited by the same parallel-fiber excitatory input.

This arrangement is called feed-forward inhibition and helps to prevent the duration of the excitation produced by any given afferent impulse.



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