Action Potentials

Understanding Electrical Communication in Human Cells

Electrical communication in human cells is fundamental to various biological processes, including muscle contraction. Neurons function as conduits that transmit electrical signals to muscles, using a system of charged atomic particles known as ions, rather than electrons. While we commonly associate electrical currents with the movement of electrons in our daily lives, in cellular communication, ions play a pivotal role.

Primarily, we focus on four ions essential for this communication: cations—specifically calcium (Ca²⁺), sodium (Na⁺), and potassium (K⁺)—and the anion, chlorine (Cl⁻). Calcium and sodium are present in higher concentrations outside the cell compared to the inside, where potassium is found at lower concentrations. It’s essential to recognize these ions by their symbols to grasp the cellular activities discussed throughout the semester.

The charge imbalance created by these ion distributions results in a polarization of the cell's membrane: the outside is slightly positive, while the inside is slightly negative. This polarization is quantified as the resting membrane potential, typically ranging from -70 to -60 millivolts. The term 'resting' denotes the state prior to any communication event, while 'potential' refers to the energy stored due to the charge difference across the membrane.

To facilitate communication, the cell employs channels that allow sodium ions to move across the membrane. Under normal circumstances, the lipid bilayer of the plasma membrane acts as a barrier to charged ions. However, through facilitated diffusion, sodium ions can transition from a region of high concentration outside the cell to a region of lower concentration inside, striving to reach equilibrium. If sodium were the only ion present, this movement would equalize the concentrations on both sides of the membrane and eliminate the charge imbalance, resulting in a voltage measurement of zero.

The process where the charge imbalance is adjusted by the inflow of sodium ions is called an action potential. As sodium enters the cell through the opened channels, the membrane potential shifts from -60 to -70 millivolts toward zero, a process termed depolarization. This action potential is the initial key concept when discussing how muscular communication occurs.