Synapse: Structure & Function

Question 1

Define a Synapse

Answer 1

A synapse is a functional articulation (gap) between two successive neurons or between a neuron and an effector organ

Question 2

Types of synapse according to location

Answer 2

1) Neuro- neuronic synapse: it is between two successive neurons
2) neuro-muscular synapse: between a neuron and a muscle
3) neuro-glandular synapse: between a neuron and a galnd

Question 3

Types of neuro- neuronic syanpse

Answer 3

1) Axo-dendritic synapse: between the terminal arborization of the 1st neuron called presynaptic neuron anf the dendrite of the 2nd neuron called postsynaptic neuron
2) axo-somatic synapse: between the terminal arborization of the presynaptic neuron and the cell body of the post synaptic neuron

3) axo-axonal synapse:
between the terminal arborization of the presynaptic neuron and the axon of the postsynaptic neuron

Question 4

Steps of propagation of the message along the synapse

Answer 4

1) Arrival of the nervous message to the terminal knob of the presynaptic neuron
2) Entrance of calcium ions to the terminal knob of presynaptic neuron
3) Release of neurotransmitters of synaptic vesicles to the synaptic cleft by exocytosis
4) Fixation of neurotransmitters on the receptors of post synaptic membrane
5) Creation of new potential difference called post synaptic potential
6) Degradation of neurotransmitters by special enzyme or reabsorption of neurotransmitters by the presynaptic membrane

Question 5

After stimulating the presynaptic neuron, AP is recorded at presynaptic neuron

Answer 5

This indicates that the stimulation is effective and is = or > threshold.

Question 6

After effectively stimulating the presynaptic neuron, AP is recorded at the presynaptic neuron after a certain distance

Answer 6

This indicates that the nervous message is able to propagate

Question 7

1- After stimulating effectively the presynaptic neuron, AP is recorded at the pro and at the post synaptic neuron.
2- After stimulating effectively the post synaptic neuron, AP is recorded at the post synaptic neuron only
Derive indication for both and a general conclusion

Answer 7

1- The nervous message propagates from pre synaptic to the post synaptic membrane.
2- The nervous message cannot propagate from post to pre synaptic neuron

Conclusion:
The nervous message is unidirectional (has one direction) from pre to post synaptic membrane

Question 8

1- After stimulating effectively the pre which is placed in a container deprived from Ca 2+ , AP is recorded at the presynaptic neuron only
2- Injecting Ca 2+ inside the pre terminal without any stimulation, AP is recorded at the post-synaptic neuron only

Derive an indication for both and a general conclusion

Answer 8

1- Ca 2+ ions are responsible for the propagation of the nervous message from pre to pro
2- Ca 2+ ions inside the pre terminal play a role in the propagation of the message to the post synaptic cell

Conclusion: The inflow of Ca 2+ ions to the pre-synaptic neuron plays a role in the propagation of nervous message to the post- synaptic neuron

Question 9

1- After injecting [ACH] into the cleft in the absence of any stimulation , AP is recorded at the post synaptic neuron only
2- After injecting [ACH] into the post- synaptic neuron directly in the absence of any stimulation, no recordings are obtained.

Derive an indication for both and a general conclusion

Answer 9

1- [ACH] is responsible to conduct the nervous message to the post synaptic neuron
2- [ACH] doesn’t act inside the post-synaptic membrane

Conclusion: [ACH] acts on the level of the post synaptic membrane.

Question 10

After injecting radioactive ACH into the cleft, it gave 2 results:
1- The post synaptic membrane became radioactive
2- The concentration of Na+ increases inside the post synaptic neuron

Derive an indication for both results.

Answer 10

1- ACH binds to the post synaptic neuron

2- ACH provokes the diffusion of high amount of Na+ to the neuron

Question 11

After injecting [ACH] of increasing doses into the cleft in the absence of any stimulation, P.D is recorded at the post-synaptic neuron with increasing amplitude

Derive an indication and a conclusion

Answer 11

• Increasing dose of ACH causes an increase in P.D

Conclusion:
The modulation is coded by the concentration of neurotransmitters

Question 12

Explain the coding along the synapse

Answer 12

The intensity of stimulation determines the frequency of APs at the presynaptic neuron. This controls the numbers of opened Ca 2+ channels at the presynaptic membrane, which controls the amount of Ca 2+ ions entering into the presynaptic neuron. The amount of Ca 2+ ions entering into the presynaptic neuron regulates the amount of the released neurotransmitter by exocytosis which will control the number of opened post synaptic receptors. This determines the amount of type of ions entering or leaving the post synaptic neuron which will determine the type and amplitude of the post synaptic potential