balance '' is an increase in a cell's membrane potential.
balance '' is an increase in a cell's membrane potential.
what is an increase in a cell's membrane potential?
`` balance ''
bullet::::- panel 1 of the diagram shows a diagrammatic representation of a simple cell where a concentration gradient has already been established. this panel is drawn as if the membrane has no permeability to any ion. there is no membrane potential because despite there being a concentration gradient for potassium, there is no net charge imbalance across the membrane. if the membrane were to become permeable to a type of ion that is more concentrated on one side of the membrane, then that ion would contribute to membrane voltage because the permeant ions would move across the membrane with net movement of that ion type down the concentration gradient. there would be net movement from the side of the membrane with a higher concentration of the ion to the side with lower concentration. such a movement of one ion across the membrane would result in a net imbalance of charge across the membrane and a membrane potential. this is a common mechanism by which many cells establish a membrane potential.
resting potential
this phase is also known as the pacemaker potential. immediately following an action potential, when the membrane potential is very negative (it is hyperpolarised) the voltage slowly begins to increase. this is initially due to the closing of potassium channels, which reduces the flow of potassium ions (i) out of the cell (see phase 3, below). alongside the deactivation of the potassium channels, cause activation of hyperpolarisation-activated cyclic nucleotide–gated (hcn) channels. activation of these channels at very negative membrane potential is an unusual property for ion channels, therefore the flow of sodium (na) and some potassium (k) through the activated hcn channel is referred to as a "funny current" (i). this funny current causes the membrane potential of the cell to gradually increase, as the positive charge (na and k) is flowing into the cell. another mechanism involved in pacemaker potential is known as the calcium clock. here, calcium is released spontaneously from the sarcoplasmic reticulum (a calcium store) into the cell, this is known as a spontaneous ca spark. this increase in calcium within the cell then activates a sodium-calcium exchanger (ncx), which removes one ca from the cell, and exchanges it for 3 na into the cell (therefore removing a charge of +2 from the cell, but allowing a charge of +3 to enter the cell) therefore in-creasing the membrane potential. the calcium is later pumped back into the cell via calcium channels located on the cell membrane and sr membrane. the increase in membrane potential produced by these mechanisms, activates t-type calcium channels and then l-type calcium channels (which open very slowly). these channels allow a flow of calcium ions (ca) into the cell, making the membrane potential more positive.
sinoatrial node