Synapse A on a motoneuron results in an EPSP with a peak depolarization of 2 mV.

Question

Synapse A on a motoneuron results in an EPSP with a peak depolarization of 2 mV. Synapse B also results in an EPSP with a peak depolarization of 2 mV. Synapse C results in an IPSP peak hyper polarization of -2.5 mV (note, hyper polarization results a more negative action potential). The resting potential is -70 mV.

A) Simultaneous firing of A and B results in what peak potential? What is this phenomenon called? Do you think it is enough to reach threshold?

B) Simultaneous firing of A and C results in what peak potential? What is this phenomenon called? Do you think it is enough to reach threshold?

C) Simultaneous firing of A, B and C results in what peak potential?

Explanation / Answer

Excitatory neurotransmitters open ligand-gated sodium ion channels, allowing sodium ions to flow in, Hence, the cell becomes less negative on the inside. These neurotransmitters cause a local depolarization, which is called Excitatory Postsynaptic Potential (EPSP) as the post-synaptic cell is excited. On the other hand, inhibitory neurotransmitters open ligand gated potassium channels, to allow potassium out. Hence, the interior of the cell become more negative. A local hyperpolarization results that is known as Inhibitory Postsynaptic Potential (IPSP)

A. The summation of the two EPSPs by one motor neuron causes an effect known as Temporal summation. If the two synapses do not occur simultaneously, then there is no summation. However, as the two synapses occur simultaneously, the second synapse occurs before the falling phase of the second one. Hence, the two EPSPs will be added. The peak potential will be 2+2= 4 mV. The membrane potential needs to be around -50 to-55 mV for it to reach the threshold. As this stage, the potential is less (-70+4=-66 mV), the membrane potential will not reach the threshold. EPSPs result in making the cell interior less negative. If the two EPSPs are produced in two separate sensory neurons, then it is known as spatial summation.

B) Cancellation occurs when an EPSP and an IPSP is fired simultaneously. The summation of the two= 2-2.5= -0.5 mV. The resting potential will only increase by 0.5mV. Hence, this potential will not reach the threshold, which is around -50 to -55 mV.

C) IPSP counteract the effects of EPSPs. A, B and C when fired simultaneously, will lead to summation of A and B and then cancellation of C. Hence, the peak potential will be 2+2-2.5= 4-2.5= 1.5 mV.

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