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Physiology

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Nerve Muscle Physiology

Question
16 out of 24
 

Which of the following statements is true for excitatory postsynaptic potentials (EPSP)



A Are self propagating

B Show all or none response

C Are proportional to the amount of transmitter released by the presynaptic neuron

D Are inhibitory at presynaptic terminal.

Ans. C Are proportional to the amount of transmitter released by the presynaptic neuron
  1. EPSP and IPSP

i. EPSP makes the post synaptic membrane hypopolarised and if sufficiently strong causes an AP to develop in the post synaptic membrane.

ii. On the other hand IPSP causes hyperpolarisation of the post synaptic membrane that is it makes the post synaptic membrane resistant to stimulus.

An EPSP is the fore runner of an AP.,

  1. Ionic basis of EPSP development is as follows-

The AP comes synaptic transmitter (ST) released ST combines with the receptors of the post synaptic

membrane and this combining causes opening of ligand gated Na+ and other channels in the post synaptic membrane

entry of Na+ and other cations inside the cell from the ECF hypopolarisation and EPSP in the post synaptic

membrane].

  1. Presynaptic inhibition -

i. Another type of inhibition occuring in the CNS is presynaptic inhibition a process mediated by neurons that end on excitatary ending forming axo-axonal synapses..

ii. Three mechanisms of presynaytic inhibition have been described -

a.First activation of the presynaptic receptors in Cl- conductance and this has been shown to decrease the size of the action potentials reaching the excitatory ending.

b.This in turn reduces Ca2+ entry and consequently the amount of the excitatory transmitter released. Voltage gated K+ channels are also opened, and the resulting K efflux also decreases the Ca2+ influx.

c.Finally there is evidence for direct inhibition of transmitter release independent of Ca2+ influx into the excitatory ending.

iii. First transmitter to be shown to produce presynaptic inhibition was GABA. Q

iv. Thus mechanism of presynaptic inhibition is different. Thus option (4) is also excluded.

v. IPSP Sometimes receptors and the transmitter combination leads to hyperpolarisation. If the value of the ‘rmp’

vi. was —70mv, as a result of said combination, it now becomes say —80mv. This difference (- l0mv in this example) is called IPSP. IPSP is caused by influx of Cl- from ECF to ICF and K+ efflux. Q

Nerve Muscle Physiology Flashcard List

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