Action Potentials and Muscle Contraction

Study Notes

Action potentials transmit signals along neurons and muscle fibers
They are rapid, short-lasting changes in membrane potential
Depolarization is the process of the membrane potential becoming less negative, often leading to an action potential
Repolarization is the process of the membrane potential becoming more negative, restoring the membrane potential to its resting value

The site where a motor neuron communicates with a muscle fiber
Motor neurons release acetylcholine, a neurotransmitter, into the synaptic cleft
Acetylcholine binds to receptors on the muscle fiber, causing depolarization
Depolarization triggers an action potential in the muscle fiber

The process that links the action potential to the contraction of a muscle fiber
Occurs in three phases:
- Excitation phase: action potential travels along the sarcolemma and into the T-tubules
- Coupling phase: depolarization of the T-tubules triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum
- Contraction phase: calcium ions bind to troponin, allowing the thin filaments to move and interact with the thick filaments, triggering the contraction

Calcium ions are stored in the sarcoplasmic reticulum
The sarcoplasmic reticulum is a network of intracellular membranes that plays a critical role in the regulation of muscle contraction
When an action potential arrives at the neuromuscular junction, it triggers a series of events that ultimately lead to the release of calcium ions from the sarcoplasmic reticulum - causing muscle contraction

The process of repeated attachment and detachment of myosin head groups to the thin filaments
Each cycle requires energy, specifically ATP
The muscle fiber contracts as the myosin heads pull on the thin filaments
The cycle continues as long as calcium ions are bound to troponin

When the action potential ceases, calcium ions are pumped back into the sarcoplasmic reticulum
The myosin heads detach from the thin filaments
The muscle fiber relaxes

Muscle activity requires a substantial amount of energy
ATP is the immediate energy source for muscle contraction
ATP is generated through various metabolic pathways, including: oxidative phosphorylation, glycolysis, and creatine phosphate

Oxidative phosphorylation is the primary source of ATP during sustained muscle activity
Glycolysis is a quick method of ATP generation but produces lactic acid as a byproduct
Creatine phosphate provides a burst of energy at the start of muscle activity

Action Potentials are rapid changes in membrane potential that allow communication between neurons, muscles, and other excitable cells.

Neuromuscular Junctions are specialized synapses that allow for communication between motor neurons and muscle fibers.

Excitation-Contraction Coupling is the process by which action potentials initiate muscle contraction. It involves a series of events that link electrical signals from motor neurons to the initiation of muscle contraction.
The process is initiated by the release of acetylcholine (ACh) at the neuromuscular junction, leading to depolarization of the muscle fiber membrane.
This depolarization propagates along the sarcolemma and into the T-tubules, eventually reaching the sarcoplasmic reticulum.
The sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm, which then bind to troponin, triggering a conformational change in the troponin-tropomyosin complex.
This change exposes the myosin binding sites on actin filaments, enabling the formation of cross-bridges between actin and myosin.

Preparation for contraction includes the release of neurotransmitters by motor neurons and depolarization of the muscle fiber membrane.

The crossbridge cycle is the process by which myosin heads bind to actin filaments, pull the actin filaments past the myosin filaments, and then detach.

Muscle relaxation occurs when the acetylcholine signal is terminated and calcium ions are removed from the sarcoplasm, causing the troponin-tropomyosin complex to cover the binding sites on actin, preventing further crossbridge formation.

Muscle contraction requires energy from ATP.
Energy is required for the crossbridge cycle, the calcium pump that removes calcium from the sarcoplasm, and the sodium-potassium pump that maintains the membrane potential.