University:
Arizona State University
Course:
BIO 201 | Human Anatomy & Physiology I
Academic year:

2024
Views:

142

Pages:

8
Author:

Maverick Martin

Absolute and Relative Refractory Periods in an AP Membrane potential (mV) Absolute refractory period Figure 11.13 Absolute and relative refractory periods in an AP. +30 Relative refractory period Depolarization (Na+ enters) 0 Repolarization (K+ leaves) Hyperpolarization -70 Stimulus 0 1 2 Time (ms) 3 4 5 Conduction Velocity • APs occur only in axons, not other cell areas • AP conduction velocities in axons vary widely • Rate of AP propagation depends on two factors: 1. Axon diameter ▪ Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction ▪ Think of moving through a narrow vs. wide hallway 2. Degree of myelination ▪ Two types of conduction depending on presence or absence of myelin – Continuous conduction – Saltatory conduction Conduction Velocity – Continuous conduction: slow conduction that occurs in nonmyelinated axons – Saltatory conduction: occurs only in myelinated axons and is about 30 times faster ▪ Myelin sheaths insulate and prevent leakage of charge ▪ Voltage-gated Na+ channels are located at myelin sheath gaps ▪ APs generated only at gaps ▪ Electrical signal appears to jump rapidly from gap to gap Action Potential Propagation in Nonmyelinated and Myelinated Axons Stimulus Size of voltage (a) In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite, voltage decays because current leaks across the membrane. Figure 11.14a Action potential propagation in nonmyelinated and myelinated axons. Action Potential Propagation in Nonmyelinated and Myelinated Axons Stimulus Voltage-gated ion channel (b) In nonmyelinated axons, conduction is slow (continuous conduction). Voltage-gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because it takes time for ions and for gates of channel proteins to move, and this must occur before voltage can be regenerated. Figure 11.14b Action potential propagation in nonmyelinated and myelinated axons. Action Potential Propagation in Nonmyelinated and Myelinated Axons Stimulus Myelin sheath Myelin sheath Myelin sheath gap 1 mm (c) In myelinated axons, conduction is fast (saltatory conduction). Myelin keeps current in axons (voltage doesn’t decay much). Action potentials are generated only in the myelin sheath gaps and appear to jump rapidly from gap to gap. Figure 11.14c Action potential propagation in nonmyelinated and myelinated axons. Clinical – Homeostatic Imbalance 11.2 • Clinical – Homeostatic Imbalance 11.2 • Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence • Treatment: drugs that modify immune system activity