Anatomy & Physiology

Anatomy & Physiology Study Guide • The electrochemical gradient is the sum of all chemical and electrical forces acting across the plasma membrane. • The sodium–potassium exchange pump calms the resting potential at approximately –70 mV. • The plasma membrane contains passive (leak) channels, which are always open, and active (gated) channels, which open or close in response to specific stimuli. • The three types of gated channels are chemically gated channels, voltage-gated channels, and mechanically gated channels. • A localized depolarization or hyperpolarization is a graded potential (a change in potential that decreases with distance). An action potential is a nerve impulse: • An action potential occurs when a region of excitable membrane depolarizes to its threshold. The processes involved, in order, are membrane depolarization to threshold, activation of sodium channels and rapid depolarization, inactivation of sodium channels and activation of potassium channels, and the return to normal permeability. • The generation of an action potential follows the all-or-none principle. The refractory period lasts from the time an action potential begins until the normal resting potential has returned. • In continuous propagation, an action potential extends across the entire excitable membrane surface in a series of small steps. • In saltatory propagation, an action potential appears to leap from node to node, skipping the intervening membrane surface. Saltatory propagation carries nerve impulses many times more rapidly than does continuous propagation. Axon diameter, in addition to myelin, affects propagation speed: • Axons are classified as Type A fibers, Type B fibers, or Type C fibers on the basis of their diameter, myelination, and propagation speed. • Compared with action potentials in neural tissue, those in muscle tissue have larger resting potentials, longer-lasting action potentials, and slower propagation of action potentials. At synapses, communication occurs among neurons or between neurons and other cells: • An action potential traveling along an axon is a nerve impulse. At a synapse between two neurons, information passes from the presynaptic neuron to the postsynaptic neuron. • A synapse is either electrical (with direct physical contact between cells) or chemical (involving a neurotransmitter). • Electrical synapses occur in the CNS and PNS, but they are rare. At an electrical synapse, the presynaptic and postsynaptic plasma membranes are bound by interlocking membrane proteins at a gap junction. Pores within these proteins permit the passage of local currents, and the two neurons act as if they share a common plasma membrane. • Chemical synapses are far more common than electrical synapses. Excitatory neurotransmitters cause depolarization and promote the generation of action potentials, Achieve Page 157 of 368 ©2018