11.6 Action Potentials

11.6 Action Potentials • Principal way neurons send signals – Means of long-distance neural communication • Occur only in muscle cells and axons of neurons • Brief reversal of membrane potential with a change in voltage of ~100 mV • Action potentials (APs) do not decay over distance as graded potentials do • In neurons, also referred to as a nerve impulse • Involves opening of specific voltage-gated channels Action Potential (1 of 7) Focus Figure 11.2 Action Potential. Generating an Action Potential (1 of 5) • Four main steps 1. Resting state: All gated Na+and K+ channels are closed  Only leakage channels for Na+ and K+ are open – Maintains the resting membrane potential  Each Na+ channel has two voltage-sensitive gates – Activation gates: closed at rest; open with depolarization, allowing Na+ to enter cell – Inactivation gates: open at rest; block channel once it is open to prevent more Na+ from entering cell  Each K+ channel has one voltage-sensitive gate – Closed at rest – Opens slowly with depolarization Action Potential (2 of 7) Focus Figure 11.2 Action Potential. Generating an Action Potential (2 of 5) 2. Depolarization: Na+ channels open  Depolarizing local currents open voltage-gated Na+channels, and Na+ rushes into cell  Na+activation and inactivation gates open  Na+influx causes more depolarization, which opens more Na+channels – As a result, ICF becomes less negative  At threshold (–55 to –50 mV), positive feedback causes opening of all Na+channels – Results in large action potential spike – Membrane polarity jumps to +30 mV Action Potential (3 of 7) Focus Figure 11.2 Action Potential. Generating an Action Potential (3 of 5) 3. Repolarization: Na+channels are inactivating, and K+ channels open  Na+ channel inactivation gates close – Membrane permeability to Na+declines to resting state – AP spike stops rising  Voltage-gated K+ channels open – K+ exits cell down its electrochemical gradient  Repolarization: membrane returns to resting membrane potential Action Potential (4 of 7) Focus Figure 11.2 Action Potential. Generating an Action Potential (4 of 5) 4. Hyperpolarization: Some K+ channels remain open, and Na+channels reset  Some K+ channels remain open, allowing excessive K+ efflux – Inside of membrane becomes more negative than in resting state  This causes hyperpolarization of the membrane (slight dip below resting voltage)  Na+ channels also begin to reset Action Potential (5 of 7) Focus Figure 11.2 Action Potential. Action Potential (6 of 7) Focus Figure 11.2 Action Potential. Action Potential (7 of 7) Focus Figure 11.2 Action Potential. A&P Flix: Generation of an Action Potential Click here to view ADA compliant Animation: A&P Flix: Generation of an Action Potential Generating an Action Potential (5 of 5) • Repolarization resets electrical conditions, not ionic conditions • After repolarization, Na+/K+ pumps (thousands of them in an axon) restore ionic conditions Resting Membrane Potential (2 of 2) Focus Figure 11.1-1 Resting Membrane Potential. Threshold and the All-or-None Phenomenon • Not all depolarization events produce APs • For an axon to “fire,” depolarization must reach threshold voltage to trigger AP • At threshold: – Membrane is depolarized by 15 to 20 mV – Na+ permeability increases – Na+ influx exceeds K+ efflux – The positive feedback cycle begins • All-or-None: An AP either happens completely, or does not happen at all Propagation of an Action Potential (1 of 2) • Propagation allows AP to be transmitted from origin down entire axon length toward terminals • Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas – Leads to depolarization of that area, which in turn causes depolarization in next area Propagation of an Action Potential (2 of 2) • Once initiated, an AP is self-propagating – In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes – Propagation in myelinated axons differs • Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there – AP occurs only in a forward direction Propagation of an Action Potential (AP) (1 of 3) Figure 11.11a Propagation of an action potential (AP). Propagation of an Action Potential (AP) (2 of 3) Figure 11.11b Propagation of an action potential (AP). Propagation of an Action Potential (AP) (3 of 3) Figure 11.11 Propagation of an action potential (AP). A&P Flix: Propagation of an Action Potential Click here to view ADA compliant Animation: A&P Flix: Propagation of an Action Potential Coding for Stimulus Intensity • All action potentials are alike and are independent of stimulus intensity • CNS tells difference between a weak stimulus and a strong one by frequency of impulses – Frequency is number of impulses (APs) received per second – Higher frequencies mean stronger stimulus Relationship Between Stimulus Strength and Action Potential Frequency Figure 11.12 Relationship between stimulus strength and action potential frequency. Refractory Periods (1 of 2) • Refractory period: time in which neuron cannot trigger another AP – Voltage-gated Na+ channels are open, so neuron cannot respond to another stimulus • Two types – Absolute refractory period  Time from opening of Na+channels until resetting of the channels  Ensures that each AP is an all-or-none event  Enforces one-way transmission of nerve impulses Refractory Periods (2 of 2) – Relative refractory period  Follows absolute refractory period – Most Na+channels have returned to their resting state – Some K+ channels still open – Repolarization is occurring  Threshold for AP generation is elevated  Only exceptionally strong stimulus could stimulate an AP – Think of a disobedient (refractory) dog – if he is absolutely refractory he will never come when called, but if he is relatively refractory, he may come but only if you call loud enough Absolute and Relative Refractory Periods in an AP Figure 11.13 Absolute and relative refractory periods in an AP. Conduction Velocity (1 of 4) • 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 2. Degree of myelination  Two types of conduction depending on presence or absence of myelin – Continuous conduction – Saltatory conduction Conduction Velocity (2 of 4) – 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 (1 of 3) Figure 11.14a Action potential propagation in nonmyelinated and myelinated axons. Action Potential Propagation in Nonmyelinated and Myelinated Axons (2 of 3) Figure 11.14b Action potential propagation in nonmyelinated and myelinated axons. Action Potential Propagation in Nonmyelinated and Myelinated Axons (3 of 3) Figure 11.14c Action potential propagation in nonmyelinated and myelinated axons. Clinical–Homeostatic Imbalance 11.2 (1 of 2) • Multiple sclerosis (MS) is an autoimmune disease that affects primarily young adults • Myelin sheaths in CNS are destroyed when immune system attacks myelin – Turns myelin into hardened lesions called scleroses – Impulse conduction slows and eventually ceases – Demyelinated axons increase Na+channels, causing cycles of relapse and remission Clinical–Homeostatic Imbalance 11.2 (2 of 2) • Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence • Treatment: drugs that modify immune system activity • May not be able to prevent, but maintaining high blood levels of vitamin D may reduce risk of development Conduction Velocity (3 of 4) • Nerve fibers are classified according to diameter, degree of myelination, and speed of conduction • Fall into three groups: – Group A fibers  Largest diameter  Myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints  Transmit at 150 m/s (~300 mph) Conduction Velocity (4 of 4) – Group B fibers  Intermediate diameter  Lightly myelinated fibers  Transmit at 15 m/s (~30 mph) – Group C fibers  Smallest diameter  Unmyelinated  Transmit at 1 m/s (~2 mph) – B and C groups include ANS visceral motor and sensory fibers that serve visceral organs Clinical–Homeostatic Imbalance 11.3 • Impaired AP impulse propagation can be caused by a number of chemical and physical factors. • Local anesthetics act by blocking voltage-gated Na+channels. • Cold temperatures or continuous pressure interrupt blood circulation and delivery of oxygen to neurons – Cold fingers get numb, or foot “goes to sleep”