Chapter 5: Membrane Potentials & Action Potentials

Membrane Potentials

• All cells have electrical potentials across their membranes.
• Nerve and muscle cells generate action potentials to transmit electrochemical signals.
• Membrane potentials result from ion concentration gradients and charge differences.
• Nernst potential calculates the equilibrium potential for specific ions, balancing diffusion forces.
• Sodium (Na+) tends to enter the cell, with an equilibrium potential of +61 mV, while potassium (K+) tends to leave, with an equilibrium potential of approximately -90 mV.
• Goldman equation considers ion polarity, membrane permeability, and concentration gradients to determine overall membrane potential.

Resting Membrane Potential

• Typical resting membrane potential is negative: around -90 mV in nerve fibers and -70 mV in muscle fibers.
• Na+/K+ pump establishes concentration gradients, using primary active transport, pumping 3 Na+ out and 2 K+ in, with a resultant electronegative effect of about -4 mV.
• K+ channels create the majority of resting membrane potential due to higher permeability, contributing approximately -86 mV.

Action Potentials

• Polarized: resting state (-70 mV to -90 mV).
• Depolarization: membrane potential becomes less negative.
• Overshoot: reversal in polarity, where the inside of the cell becomes more positive at peak.
• Repolarization: membrane potential decreases but remains above resting level.
• Hyperpolarization (undershoot): potential becomes more negative than resting level.
• Action potentials are initiated when Na+ diffuses into the cell, surpassing threshold potentials, typically around +15 mV from resting potential.
• Temporal summation involves rapid successive inputs; spatial summation involves simultaneous inputs from multiple sources.

Action Potential Sequence

• Initiation begins with polarized state: Na+ activation gates are closed, Na+ inactivation gates are open or closing, and K+ activation gates are closed.
• In depolarization, rapid Na+ influx causes the membrane to become positive as Na+ channels open.
• In repolarization, sodium channels inactivate and potassium channels open, resulting in K+ efflux and membrane negativity.
• Hyperpolarization follows as K+ activation gates close slowly, making the membrane more negative before returning to resting potential.

Refractory Periods

• Absolute refractory period: No action potential can occur due to closed Na+ inactivation gates.
• Relative refractory period: An action potential can be triggered with a stronger stimulus, coinciding with hyperpolarization.

Accommodation

• Membrane remains hypopolarized and does not return to resting potential, inhibiting Na+ inactivation gate opening.

Action Potential Propagation

• Propagation begins at the axon hillock, moving currents to adjacent areas to activate Na+ channels.
• Conduction velocity is affected by myelination (saltatory conduction) and the diameter of the nerve fiber – larger diameters increase speed.

Skeletal Muscle

• Skeletal muscle is voluntary, striated, long, cylindrical, and multinucleated.
• Composed of myofibrils made from organized myofilaments (actin and myosin) arranged in sarcomeres.
• Thin (actin) filaments contain regulatory proteins (troponin and tropomyosin) and are anchored by nebulin.
• Thick (myosin) filaments possess ATPase enzymes with structural protein titin anchoring them.

Sarcomere Structure

• Sarcomere: functional unit of muscle bounded by Z discs, composed of dark (A band) and light (I band) regions.
• M line (middle of A band) helps anchor myosin; the H zone contains only myosin filaments.

Skeletal Muscle Contraction

• Action potentials stimulate the sarcolemma and travel down T-tubules.
• Triggers calcium release from the sarcoplasmic reticulum, exposing actin binding sites.
• Cross bridge cycle involves myosin binding to actin, resulting in sarcomere shortening.

Energy Sources for Contraction

• Creatinine phosphate breakdown provides immediate energy.
• Glycogenolysis from glycogen stores supports energy needs anaerobically.
• Aerobic metabolism utilizes lipids, carbohydrates, and proteins for sustained energy.

Muscle Fiber Types

• Type I (slow, red muscle fibers) are smaller, less fatigable, and have higher aerobic capacity due to extensive blood supply and mitochondria.
• Type II (fast twitch, white muscle fibers) are larger, more powerful, but easily fatigued, relying on glycolysis for energy.

Types of Muscle Contraction

• Isometric contraction: muscle length remains unchanged, but tension increases.
• Isotonic contraction: muscle length changes while maintaining tension, further categorized into concentric (shortening) and eccentric (lengthening) contractions.

Neuromuscular Transmission

• Synapses: communication points between cells, can be electrical (gap junctions) or chemical (synaptic clefts).
• Motor unit: consists of a single motor neuron and the muscle fibers it innervates, varying in size for fine and gross motor control.
• Neuromuscular junction: a chemical synapse connecting motor neurons with skeletal muscle fibers.### Neuromuscular Junction Overview
• Bouton refers to the bulb-like terminal end of a motoneuron, crucial for neurotransmitter release.
• Voltage-gated calcium channels are activated by action potentials from the nervous system, facilitating neurotransmitter release.
• Acetylcholine (ACh) vesicles exit the neuronal membrane through exocytosis into the synaptic cleft.

Postsynaptic Muscle Fiber Interaction

• The motor end plate is the portion of the muscle fiber that interfaces with the neuromuscular junction.
• Nicotinic acetylcholine receptors on the motor end plate bind to the released ACh, acting as ligand-gated Na+ and K+ channels.
• End plate potentials can summate, opening enough Na+ channels for depolarization to reach the threshold, generating an action potential.
• Resulting currents cause depolarization and action potentials in adjacent muscle tissues, triggering muscle contraction.

Acetylcholine Synthesis and Release

• Vesicles containing ACh are produced in the motoneuron and transported to the neuromuscular junction.
• ACh is synthesized in the cytosol and rapidly stored in vesicles.
• Calcium channels facilitate ACh exocytosis into the synaptic cleft.
• ACh is degraded by acetylcholinesterase into acetate and choline, with choline reabsorbed for new ACh synthesis.
• New vesicles are formed via invagination of the nerve cell membrane, a process assisted by the contractile protein clathrin.

Mechanism of Skeletal Muscle Contraction

• Action potentials (AP) from the nervous system stimulate the sarcolemma and propagate through transverse tubules.
• AP triggers the release of calcium ions from the sarcoplasmic reticulum.
• Calcium binds to troponin, altering tropomyosin's configuration and revealing myosin binding sites on actin.
• The calcium pump reabsorbs calcium into the sarcoplasmic reticulum, allowing muscle relaxation.

Key Concepts in Muscle Physiology

• Action potentials have distinct phases where voltage-gated channels exhibit varying states of opening and closing.
• The threshold is the membrane potential level that must be reached for an AP to initiate.
• Polarized refers to a state where a membrane has a difference in charge across its surface.
• Goldman equation helps determine the equilibrium potential across a membrane considering multiple ion permeabilities, while Nernst equation applies to single ions.
• Refractory periods are critical for regulating AP propagation and preventing continuous stimulation.
• Summation allows for graded responses based on multiple stimuli affecting muscle fibers.
• Accommodation describes the ability of a neuron to gradually respond to increased stimulus intensity.
• Structural differences in muscle fibers (type 1 vs. type 2) influence their functional roles, with type 1 fibers being slow-twitch and fatigue-resistant, and type 2 fibers being fast-twitch and capable of rapid power generation.

Cross Bridge Cycle

• The power stroke is the movement of myosin heads pivoting to pull actin filaments during contraction.
• Rigor mortis exemplifies muscular rigidity post-mortem due to a lack of ATP, preventing detachment of myosin from actin.
• Distinctions between electrical and chemical synapses impact synaptic transmission speed and plasticity.
• Sodium-potassium pumps and potassium leak channels maintain resting membrane potential yet operate differently in terms of transport mechanisms and ion dependencies.

Membrane Potentials

• All cells possess electrical potentials across their membranes.
• Nerve and muscle cells generate action potentials, which are rapidly changing electrochemical impulses that transmit signals.
• Membrane potentials arise from ion concentration gradients and electrical charge differences.
• Nernst potential calculates the equilibrium potential of an ion, balancing its diffusion across the membrane.
  • Sodium (Na+) has a tendency to enter the cell, characterized by a potential of +61mV.
  • Potassium (K+) tends to leave the cell, with a potential of approximately -90mV.
• The Goldman Equation calculates the membrane potential based on ion polarity, membrane permeability, and concentration gradients of different ions.

Resting Membrane Potential

• The resting membrane potential is typically negative:
  • -90mV in nerve fibers
  • -70mV in muscle fibers
• The Na+/K+ pump establishes the concentration gradient necessary for resting membrane potential:
  • This pump is electrogenic, generating about -4mV and moving 3 Na+ out and 2 K+ in.
• Potassium leak channels contribute significantly to resting membrane potential:
  • Membrane is 100 times more permeable to K+ than Na+ at rest, resulting in a contribution of about -86mV.

Action Potentials

• Action potentials represent changes in membrane potential:
  • Polarized: Membrane at resting potential.
  • Depolarization: Membrane potential becomes less negative.
  • Overshoot: Membrane potential briefly becomes more positive.
  • Peak: The most positive point of the action potential.
  • Repolarization: Membrane returns to a more negative potential.
  • Hyperpolarization (undershoot): Membrane potential becomes more negative than resting.

Initiation of an Action Potential

• Diffusion of Na+ into a cell can trigger an action potential (AP), activated by mechanical, chemical, or electrical factors.
• Membrane potential must reach a defined threshold, typically about 15mV more positive than resting potential:
  • Resting membrane potential (RMP) of -70mV for muscle, -90mV for nerve cells.
• Summation can lead to threshold achievement:
  • Temporal Summation: Rapid succession of signals from a single source.
  • Spatial Summation: Signals from multiple sources arriving simultaneously.

Action Potential Sequence

• Polarized state initialized when:
  • Na+ activation gate is closed.
  • Na+ inactivation gate is open.
  • K+ activation gate is closed.
• Depolarization occurs when:
  • Na+ activation gates quickly open; inactivation gates close at the peak.
  • K+ activation gates close slowly.
• During repolarization:
  • Na+ influx halts and K+ rushes out, causing membrane potential to become more negative.
• Hyperpolarization:
  • Occurs as K+ activation gates are slow to close, resulting in a more negative potential until they close and restore RMP.

Refractory Periods

• Absolute Refractory Period: No action potential can be generated due to closed Na+ inactivation gates.
• Relative Refractory Period: An action potential can occur with a stronger stimulus during hyperpolarization phase.

Accommodation

• Membrane is held in a hypo-polarized state preventing return to RMP, leading to inhibited Na+ gate opening, utilized by the nervous system to manage pathways.

Action Potential Propagation

• Initiated in the axon hillock, depolarization spreads, activating adjacent Na+ channels, allowing signal conduction down the axon.
• Saltatory conduction, enabled by myelination, increases conduction velocity as action potentials jump between nodes of Ranvier.

Types of Muscle Fibers

• Fast Twitch Fibers (Type II, White Muscle):
  • Large size for strength.
  • Extensive sarcoplasmic reticulum for rapid calcium ion release.
  • High glycolytic enzyme content for quick energy release.
  • Less blood supply and fewer mitochondria compared to slow twitch.
  • Characterized by low myoglobin content, resulting in the white muscle appearance.

Types of Muscle Contraction

• Isometric: Muscle length remains unchanged while tension varies.
• Isotonic: Muscle length changes, and tension remains consistent:
  • Concentric: Muscle shortens.
  • Eccentric: Muscle lengthens.

Synapse Types

• Electrical Synapse: Gap junctions allowing bidirectional and fast communication, mainly in cardiac and some smooth muscle.
• Chemical Synapse: Involves a synaptic cleft regulated by neurotransmitters and allows unidirectional signal transmission with a synaptic delay.

Motor Unit

• Defined as a single motor neuron and the muscle fibers it innervates.
• Smaller motor units are utilized for fine motor control.

This quiz focuses on Chapter 5, exploring the concepts of membrane potentials and action potentials. It covers how nerve and muscle cells generate rapid electrochemical impulses essential for signal transmission. Test your understanding of these fundamental concepts in cellular physiology.