Action Potentials Overview

Study Notes

Action Potentials

Action potentials reverse resting membrane potential; inside of the cell becomes positive and outside negative.
Triggered when gated ion channels open due to cell stimulation.
Ion diffusion across the membrane produces action potentials lasting 1 to 3 milliseconds.
Depolarization occurs with the influx of Na+, making the inside of the cell positively charged.
If depolarization reaches threshold, an action potential is initiated.
Action potential represents a rapid shift in ionic charges across the cell membrane.
Positive charge closure of Na+ channels and opening of K+ channels mark the end of depolarization.
Repolarization occurs as K+ exits, restoring resting membrane conditions.

Neuromuscular Junction Function

Each muscle fiber is innervated by a branch of a motor neuron at the neuromuscular junction.
Contact between axon terminal and sarcolemma elicits an action potential in the muscle fiber, prompting contraction.
Acetylcholine, released from the motor neuron, initiates the action potential in the muscle fiber.

Muscle Contraction Process

An action potential travels down the motor neuron to the presynaptic terminal.
Opening of Ca2+ channels leads to calcium influx into the terminal.
Calcium stimulates synaptic vesicles to release acetylcholine into the synaptic cleft.
Acetylcholine binds to Na+ channels in the sarcolemma, generating an action potential.
The generated action potential propagates along the sarcolemma and down T tubules.
Gated Ca2+ channels in the sarcoplasmic reticulum open, releasing stored calcium.
Ca2+ binds to troponin, resulting in tropomyosin repositioning and exposing myosin attachment sites on actin.
Cross-bridges form as myosin heads bind to actin, leading to muscle contraction through sliding filament movement.

Cross-Bridge Cycling

Mechanical aspect of muscle contraction is cross-bridge cycling.
One ATP molecule is consumed for each cross-bridge cycle.
ATP breakdown releases energy that is stored in myosin heads.
Cross-bridge release requires a new ATP binding; lack of ATP results in rigor mortis post-mortem.

Muscle Relaxation

Muscle relaxation occurs when acetylcholine release ceases at the neuromuscular junction.
Action potentials to the sarcoplasmic reticulum stop, halting calcium release.
Calcium is actively transported back into the sarcoplasmic reticulum using ATP.
Calcium dissociates from troponin, allowing tropomyosin to block actin attachment sites.
The cross-bridge cycle halts, resulting in muscle relaxation.

Muscle Twitch

A muscle twitch is a singular contraction of a muscle fiber triggered by a stimulus.
Three phases of a muscle twitch:
- Latent phase: Time between stimulus application and start of contraction.
- Contraction phase: Duration of muscle contraction.
- Relaxation phase: Time during which the muscle returns to resting state.

Types of Contractions

Isometric contractions: Increase muscle tension without changing muscle length.
Isotonic contractions: Increase muscle tension while decreasing muscle length.

Summation and Recruitment

Strength of muscle contraction determined by:
- Summation: Force generated by individual muscle fibers.
- Recruitment: Force generated by the entire muscle through activation of motor units.

Motor Unit

A motor unit comprises a motor neuron and all muscle fibers it controls.
Action potential in a motor neuron results in simultaneous contraction of all muscle fibers in its unit.
Smaller, delicate muscles have fewer fibers per motor unit; larger, powerful muscles have fewer but larger motor units.

Force of Contraction in Individual Muscle Fibers

The force produced by muscle fibers is correlated with the number of cross-bridges formed between actin and myosin.
Increased frequency of stimulation leads to more cross-bridges and a rise in force generated.
Incomplete Tetanus: Occurs when stimulation allows for partial relaxation.
Tetanus: Sustained contraction with no relaxation due to rapid stimulation.
Recruitment: Involves activation of multiple motor units for stronger contractions.

Muscle Tone

Muscle tone is the continual tension maintained in muscles over extended periods.
It stabilizes posture by providing tension to the back, legs, and abdomen.
Muscle tone arises from a small percentage of motor units being stimulated at any time, leading to a non-synchronous contraction of muscle fibers.

Types of Isotonic Contractions

Concentric contractions: Muscle tension increases while muscle shortens.
Eccentric contractions: Muscle tension is maintained but lengthens due to opposing resistance.

Skeletal Muscle Fiber Types

Slow twitch fibers:
- Contract slowly and resist fatigue.
- High myoglobin content; utilize aerobic respiration.
- Dark in color; suited for endurance activities, such as long-distance running.
Fast twitch fibers:
- Contract quickly and fatigue rapidly.
- Utilize anaerobic respiration, deriving energy from glycogen.
- Light in color; suited for high-intensity activities, such as sprinting.
Human muscles typically possess a mix of both fiber types, with distribution varying by muscle:
- Large postural muscles in the back and lower limbs contain more slow-twitch fibers.
- Muscles in the upper limbs contain a higher proportion of fast-twitch fibers.

Description

This quiz explores the concept of action potentials, detailing how they reverse resting membrane potentials through the opening of gated ion channels. You'll learn about the role of ion diffusion in generating action potentials and the significance of sodium ions in cellular activity. Test your understanding of this critical concept in neurobiology.

Action Potentials Overview

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Questions and Answers

What is the primary role of Ca2+ in muscle contraction?

How does muscle relaxation occur?

What initiates the release of stored calcium from the sarcoplasmic reticulum?

What is required for the myosin heads to release from actin during the cross-bridge cycle?

What leads to rigor mortis after death?

What is the primary characteristic of incomplete tetanus?

Which type of muscle contraction occurs when the muscle shortens while increasing tension?

Which muscle fiber type is primarily involved in long-distance running?

How does muscle tone maintain posture?

What energy source do fast twitch muscle fibers primarily use?