Graded potential

Graded Potentials - occur in dendrites, cell bodies or axon terminals. Graded potential refers to the postsynaptic electrical impulse. Called ‘graded’ because their size or amplitude is directly proportional to the strength of the triggering event. i.e. a large stimulus leads to the generation of a strong graded response and a small stimulus leads to the generation of a weak graded response. A graded potential will lose its strength and consequently die out within 1-2 mm from the origin. Summation of graded potentials may generate an action potential.

A graded potential may be depolarising or hyperpolarising. A depolarising local response will cause the membrane potential to be less negative, bringing it closer to the threshold value to generate an action potential. This response increases the excitability of the cell. Meanwhile, a hyperpolarising local response has a vice versa effect towards the membrane potential value. It will make the inside of the membrane become more negative, reducing its excitability.

The depolarising event may occur due to the inwards movement of positively-charged ions, such as sodium ions (Na⁺) to the cell, or the outwards movement of negatively-charged ions from the cell. The hyperpolarising local potential can arise due to either the negative anions, such as chloride ions (Cl^-) entering the cell or the positive cations exiting the cell.

A depolarising graded potential is known as an excitatory postsynaptic potential (EPSP).

A hyperpolarising graded potential is known as an inhibitory postsynaptic potential (IPSP).

If graded potentials reach the axon hillock and depolarise the membrane to the threshold voltage (-55 mV), an action potential is initiated.