Table of Contents |
Most cells in the body make use of charged particles, ions, to build up a charge across the cell membrane.
For skeletal muscles to contract, based on excitation–contraction coupling, input from a neuron is required. Both of the cells make use of the cell membrane to regulate ion movement between the extracellular fluid and cytosol.
The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided. Charged particles, which are hydrophilic by definition, cannot pass through the cell membrane without assistance. Transmembrane proteins, specifically channel proteins, make this possible. Several passive transport channels, as well as active transport pumps, are necessary to generate a transmembrane potential and an electrochemical signal. Of special interest is the carrier protein referred to as the sodium-potassium pump that moves sodium ions (Na⁺) out of a cell and potassium ions (K⁺) into a cell, thus regulating ion concentration on both sides of the cell membrane.
Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing concentration gradient. Each ion channel is a protein formed by a unique sequence of amino acids. The amino acids that are present along the pore and the size of the pore will determine the type of ion that channel allows through. Channels for cations (positive ions) will have negatively charged side chains in the pore. Channels for anions (negative ions) will have positively charged side chains in the pore. Smaller pores cannot allow larger ions through while larger pores do not allow smaller ions (associated with water) through. Some ion channels are selective for charge but not necessarily for size, and thus are called a nonspecific ion channel.
Ion channels do not always freely allow ions to diffuse across the membrane. Some are opened by certain events, meaning the channels are gated. So another way that channels can be categorized is on the basis of how they are gated. Although these classes of ion channels are found primarily in the cells of nervous or muscular tissue, they also can be found in the cells of epithelial and connective tissues.
A ligand-gated channel, also known as a chemically-gated channel, opens because a signaling molecule, a ligand (chemical), binds to the extracellular region of the channel.
A mechanically gated channel opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch are mechanically gated. For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell.
A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane.
A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states.
The electrical state of the cell membrane can have several variations. These are all variations in the membrane potential. Recall that a membrane potential is a difference in electrical charge across a cell membrane. This charge is based on a differential distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking. As shown in the image below, there is an overall positive charge on the outside of the cell membrane and an overall negative charge on the inside. Therefore, the membrane potential (inside relative to outside) is negative. If the indicated charges were swapped (outside was negative and inside was positive), then the membrane potential would be positive.
The concentration of ions in extracellular and intracellular fluids is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals.
Source: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX “ANATOMY AND PHYSIOLOGY 2E.” ACCESS FOR FREE AT HTTPS://OPENSTAX.ORG/DETAILS/BOOKS/ANATOMY-AND-PHYSIOLOGY-2E. LICENSE: CC ATTRIBUTION 4.0 INTERNATIONAL.
