Excitation-Contraction Coupling

Excitation-contraction (E-C) coupling

the events that link a muscle fiber’s electrical signal (action potential) to the release of Ca2+ that triggers muscle contraction

step 1 - an action potential travels along the sarcolemma

the muscle fiber receives a signal and generates an action potential across its membrane

step 2 - the action potential travels down the T-tubules

T-tubules carry the electrical signal deep into the muscle fiber, reaching every myofibril

step 3 - voltage-sensitive proteins in the T-tubules change shape

this action potential causes T-tubule proteins (“voltage sensors”) to activate

this triggers linked proteins in the sarcoplasmic reticulum (SR) to open

step 4 - Sarcoplasmic reticulum releases Ca2+

The SR dumps calcium ions into the sarcoplasm

Step 5 - Ca2+ binds to troponin

troponin changes shape and moves tropomyosin, exposing binding sites on actin

this allows cross-bridge formation → starts sliding filament contraction

Step 6 - When stimulation ends, Ca2+ is pumped back into the SR

Ca2+ levels fall, tropomyosin covers actin again, and the muscle relaxes

Neuromuscular Junction

the synapse between a motor neuron and a skeletal muscle fiber

it includes:

• axon terminal of the motor neuron

• synaptic cleft (space)

• motor end plate on the muscle fiber

Role of the NMJ in muscle contraction

converts the neuron’s electrical signal into a chemical signal (ACh) that initiates a muscle action potential

Action potential

an electrical impulse that can travel along the sarcolemma

role of action potential in E-C coupling

travels along the muscle fiber surface and down T-tubules

triggers the release of Ca2+ from the SR, which leads to contraction

Acetylecholine (ACh)

the neurotransmitter that is released from the motor neuron’s axon terminal

Role of acetylcholine (ACh) in E-C coupling

• released into the synaptic cleft when a motor neuron fires

• binds to ligand-gated ion channels on the motor end plate

• causes Na+ to enter the muscle fiber → local depolarization

• this depolarization triggers a muscle action potential

• ACh is then broken down by acetylcholinesterase, stopping stimulation