Anatomy & Physiology

Anatomy & Physiology Study Guide movement of the exchange pump precisely stabilizes the passive forces of diffusion, and the resting potential remains stable because the ionic concentration gradients are maintained. Changes in the Transmembrane Potential As noted previously, the resting potential is the transmembrane potential of an “undisturbed” cell. Yet, cells are dynamic structures that continuallymodify their activities, either in response to external stimuli or to perform specific functions. The transmembrane potential is dynamic and rises or falls in response to temporary changes in membrane permeability. Those changes occur from the opening or closing of specific membrane channels. Passive channels, or leak channels, are always open. Their permeability can vary from minute to minute as the proteins that make up the channel alter shape in response to local conditions. Plasma membranes also include active channels, often referred to as gated channels, which open or close in response to specific stimuli. Each gated channel can be in one of three states: closed but capable of opening, open (activated), or closed and incapable of opening (inactivated). Three classes of gated channels exist: chemically gated channels, voltage-gated channels, and mechanically gated channels. Chemically-gated channels open or close when they bind specific chemicals. The receptors that bind acetylcholine (ACh) at the neuromuscular junction are chemically gated channels. Chemically gated channels are most plentiful on the dendrites and cell body of a neuron, the areas where most synaptic activity occurs. Voltage-gated channels are characteristic of areas of excitable membrane , a membrane capable of generating and conducting an action potential. Voltage-gated channels open or close in response to changes in the transmembrane potential. The greatest significant voltage-gated channels are voltage- gated sodium channels, potassium channels, and calcium channels. Sodium channels have two gates that act alone: an activation gate that opens on stimulation, letting sodium ions enter into the cell, and an inactivation gate that stops the entry of sodium ions. Mechanic lly-gated channels open or close in response to physical distortion of the membrane surface. Such channels are important in sensory receptors that respond to touch, pressure, or vibration. At the resting potential, most gated channels are closed. The opening of gated channels increases the rate of ion movement across the plasma membrane and thus changes the transmembrane potential. Graded Potentials Graded potentials, or local potentials, are changes in the transmembrane potential that cannot spread far from the site of stimulation. Any stimulus that opens a gated channel will produce a graded potential. Sodium ions move into the cell and are drawn to the negative charges along the inner surface of the membrane. The arrival and spreading out of additional positive charges shift the transmembrane potential toward 0mV. Any change from the resting potential toward a more positive potential is called a depolarization , a definition that applies to changes in potential from –70 mV to smaller negative values (–65 mV, –45 mV, –10 mV) as well as to membrane potentials above 0 mV (+10 mV, +30 mV). At the resting potential, sodium ions are attracted to the outer surface of the plasma membrane, drawn by the excess of negative ions on the inside of the membrane. As the plasma Achieve Page 150 of 368 ©2018