Question

1. Consider a typical cell at its resting membrane potential (rest Vm). The membrane temporarily becomes more permeable to Na+. Ena = +55mV, resting Vm = -60mV How would the change in membrane be represented in a graph of Vm vs. time?

a. no change b. Graph B c. Graph C

Please explain why the right answer is correct and why the wrong answers are incorrect.

Answer 1

Reason:

Permeability properties of nerve membrane:

Normally at resting stage, nerve membrane possess negative charge (therefore, resting Vm = -60mV), the dendrites of the neurons receives the information from the sensory neurons or others and passes the excitation through the neuron axonal membrane. This process involves that the excitatory post synaptic potentials are delivered from the presynaptic neurons in the form of action potentials (graph-b), finally result in the release of neurotransmitter from the nerve terminals to act on the postsynaptic neuronal membrane receptors. This receptor activation result in the opening of ion channels for sodium influx there by result in the alterations in the permeability of cell membrane leading to membrane potential. This synaptic binding is considered to be excitatory (elevated voltage) upon increased depolarization observed in graph -b. This synaptic binding considered to be inhibitory upon (reduced voltage) hyperpolarization.

The all-or-none principle: This principle explains the strength of stimulus through which a neuronal cell & explained in the above graph-b & graph -c or myofibril (muscle fiber) responds to stimulus intensity and it is completely independent of the total strength of the stimulus intensity. For instance, s spike train of action potential in neuronal or muscle cell is generated along with time & with a complete response (all) if the applied stimulus (short stimulus observed in graph -b with Ena = +55mV) predominates the threshold potential becuase membrane temporarily becomes more permeable to Na+; if it is less than the threshold there is no spike response that was observed in graph c.

Neuronal voltage gated ion channels upon depolarization allows input of sodium ions (observed in graph-b) into the neuron making the neuronal charge positive inside the membrane as membrane temporarily becomes more permeable to Na+. At resting stage, these voltage-gated channels are at rest thereby resting potential and membrane potential of the nerve membrane is near as observed on graph-c. Due to electrochemical gradient, potassium channels get activated result in outward flow of potassium ions (repolarisation) and finally bring the electrochemical gradient to normal. Here once action potential happened, afterrepolarization produced (negative shift) this is also called refractory period.   Typically, multicellular animals possess two types of action potentials generated by both sodium voltage gated channels and voltage gated calcium channels. The following figure represents the permeability and effects of action potential on the cell membranes (typical nerve membrane potential at resting stage is -60mv).

Spatial summation has potentially used to raise the total amplitude of a graded potential whereas temporal summation is potentially used to elevate the amplitude of an action potential. Both of these principles are based on all-or-none principle.