Chapter 5: Membrane Potentials & Action Potentials

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Questions and Answers

Isometric contraction is when muscle length ____________ and tension will change.

does not change

Isotonic contraction involves muscle length changes, and tension ____________.

does change

Concentric contraction is when the muscle ____________.

shortens

Eccentric contraction is when the muscle ____________.

lengthens Signup and view all the answers

What is a synapse?

Area of communication in between cells Signup and view all the answers

Electrical synapses are characterized by ____________ gap junctions.

bidirectional Signup and view all the answers

What are the characteristics of a chemical synapse?

Regulated by neurotransmitter Signup and view all the answers

What is a motor unit?

A single motor neuron and the muscle fibers it innervates Signup and view all the answers

A _________ motor unit is typically responsible for fine motor control.

small Signup and view all the answers

The neuromuscular junction is the chemical synapse between ____________ and skeletal muscle.

motor neurons Signup and view all the answers

What triggers the acetylcholine vesicles to leave the membrane?

Calcium Signup and view all the answers

Acetylcholine is released into the synaptic cleft during ____________.

exocytosis Signup and view all the answers

Nicotinic receptors act as ____________ gated Na+ and K+ channels.

ligand Signup and view all the answers

What causes depolarization in the muscle cell membrane?

Multiple end plate potentials summate Signup and view all the answers

Where are acetylcholine vesicles formed?

In the motoneuron Signup and view all the answers

Calcium ions bind to ____________, changing the configuration of tropomyosin.

troponin Signup and view all the answers

What is the role of the calcium pump in muscle relaxation?

It pumps calcium back into the sarcoplasmic reticulum. Signup and view all the answers

What does rigor mortis demonstrate?

The lack of ATP prevents muscle relaxation. Signup and view all the answers

All cells have electrical potentials across their __________.

membranes Signup and view all the answers

Nerve and muscle ___________ generate action potentials.

cells Signup and view all the answers

Membrane potentials are caused by ion concentration and .

charge Signup and view all the answers

The __________ potential calculates the equilibrium potential of an ion.

Nernst Signup and view all the answers

Sodium wants to __________ the cell.

enter Signup and view all the answers

Potassium wants to __________ the cell.

leave Signup and view all the answers

The __________ Equation considers permeability and concentrations of various ions.

Goldman Signup and view all the answers

The resting membrane potential is typically __________ mV in nerve fibers.

-90 Signup and view all the answers

The Na+/K+ pump pumps _________ ions out for every 2 K+ ions it brings in.

3 Na+ Signup and view all the answers

At rest, the membrane is 100x more permeable to K+ than to Na+, contributing about __________ mV.

-86 Signup and view all the answers

The __________ phase is when the inside of the cell becomes more positive.

overshoot Signup and view all the answers

The membrane potential must reach __________ to initiate an action potential.

threshold Signup and view all the answers

Temporal summation involves two or more presynaptic inputs arriving at a postsynaptic cell in __________ succession.

rapid Signup and view all the answers

What happens during the depolarization stage?

Na+ rushes in, and membrane potential becomes more positive. Signup and view all the answers

An action potential can be generated during the absolute refractory period.

False Signup and view all the answers

In accommodation, the membrane is held in a hypo-polarized state and Na+ inactivation gates cannot __________.

open Signup and view all the answers

What triggers the release of calcium ions during muscle contraction?

An action potential traveling down the T-tubules. Signup and view all the answers

The functional unit of muscle contraction is called a __________.

sarcomere Signup and view all the answers

Fast twitch fibers, also known as Type II or __________ muscle fibers, are adapted for strength.

white Signup and view all the answers

Slow fibers are known as __________ fibers and are less prone to fatigue.

red Signup and view all the answers

Calcium ions released from the __________ triggers the interaction of actin and myosin.

sarcoplasmic reticulum Signup and view all the answers

What is the significance of the Nernst potential in relation to ion diffusion?

It calculates the voltage at which there is no net movement of a specific ion across the membrane. Signup and view all the answers

Which of the following factors contributes to the resting membrane potential?

The activity of the Na+/K+ pump and the permeability of the membrane to different ions. Signup and view all the answers

What does the Goldman Equation primarily measure?

The combined effect of multiple ions on the membrane potential. Signup and view all the answers

When sodium ions diffuse into a cell, what effect does it typically have on the membrane potential?

It depolarizes the cell, making the membrane potential more positive. Signup and view all the answers

What is the typical resting membrane potential value for nerve fibers?

-90 mV Signup and view all the answers

Which ion is primarily responsible for establishing the concentration gradient necessary for the resting membrane potential?

Potassium Signup and view all the answers

During which phase of action potential does the inside of the cell become predominantly positive?

Depolarization phase Signup and view all the answers

What effect does the Na+/K+ pump have on intracellular ion concentrations?

It pumps sodium out and potassium into the cell, maintaining their respective gradients. Signup and view all the answers

Which ions are pumped out of the cell by the Na+/K+ pump?

3 Na+ Signup and view all the answers

What primarily determines the resting membrane potential?

Potassium permeability Signup and view all the answers

What happens during depolarization of a membrane?

The inside becomes less negative. Signup and view all the answers

What is the threshold for initiating an action potential relative to the resting membrane potential?

It is 15 mV more positive. Signup and view all the answers

What occurs during hyperpolarization?

Membrane potential is more negative than resting potential. Signup and view all the answers

During which phase does the action potential reach its peak positive value?

Overshoot Signup and view all the answers

What initiates the process of an action potential?

Any factor causing sodium to diffuse in Signup and view all the answers

What is the electrical charge of the resting membrane potential in nerve cells?

-70 mV Signup and view all the answers

What happens to the membrane during the absolute refractory period?

No action potentials can be generated. Signup and view all the answers

What characteristic defines saltatory conduction?

It facilitates AP jumping from node to node. Signup and view all the answers

Which phase corresponds with the ability to elicit an action potential during the relative refractory period?

Hyperpolarization phase. Signup and view all the answers

What is the primary consequence of accommodation in neuronal activity?

A failure to return to resting membrane potential. Signup and view all the answers

What initiates action potential propagation in neurons?

Depolarization at the axon hillock. Signup and view all the answers

What type of summation involves multiple presynaptic inputs arriving at a postsynaptic cell simultaneously?

Spatial Summation Signup and view all the answers

During which phase of the action potential is the sodium inactivation gate fully closed?

Depolarization Signup and view all the answers

How does myelination affect conduction velocity in nerve fibers?

It increases conduction velocity. Signup and view all the answers

What occurs when Na+ inactivation gates are closed?

No action potentials can be generated. Signup and view all the answers

What occurs during the repolarization phase of the action potential?

K+ channels open and K+ rushes out Signup and view all the answers

What is the primary function of sodium (Na+) in generating action potentials?

To facilitate rapid depolarization of the membrane. Signup and view all the answers

What is the resting membrane potential of a neuron typically between?

-70 to -90 mV Signup and view all the answers

What characterizes the hyperpolarization phase?

K+ activation gates are slow to close Signup and view all the answers

Which event initiates the depolarization stage of an action potential?

Quick opening of Na+ activation gates Signup and view all the answers

What is the state of the cell membrane during the polarized state?

Cell is at resting membrane potential Signup and view all the answers

What happens during the depolarization stage of an action potential?

Na+ rushes in, membrane potential becomes more positive Signup and view all the answers

What type of muscle fibers are known for having large amounts of glycolytic enzymes for rapid energy release?

Type II Fibers Signup and view all the answers

Which characteristic is NOT associated with fast twitch fibers?

Extensive blood supply Signup and view all the answers

In which type of muscle contraction does muscle length remain unchanged while tension changes?

Isometric Signup and view all the answers

What distinguishes a chemical synapse from an electrical synapse?

It is unidirectional as opposed to bidirectional Signup and view all the answers

What is the primary function of the extensive sarcoplasmic reticulum in fast twitch fibers?

To store and release calcium ions rapidly Signup and view all the answers

What type of motor unit is primarily responsible for fine motor control?

Small motor unit Signup and view all the answers

What process characterizes isotonic contractions?

Muscle length shortens or lengthens while tension remains constant Signup and view all the answers

In the context of synaptic transmission, what is the primary role of neurotransmitters?

To transmit signals across the synaptic cleft Signup and view all the answers

Study Notes

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.