Crossbridges go through the cycle simultaneously but

Excitation-Contraction Coupling: How Muscle Contractions Are Turned On
and Of
 Muscle cells are excitable, meaning that they are capable of generating action potentials
if their plasma membranes are depolarized to a sufficient degree
 When a muscle cell receives input from a motor neuron, the cell depolarizes and fires an
action potential that then stimulates contraction
 The sequence of events that links the action potential to the contraction is referred to as
excitation-contraction coupling
The Role of the Neuromuscular Junction in Excitation-Contraction Coupling
 Connection between a motor neuron and a muscle cell, referred to specifically as a
neuromuscular junction, is fundamentally similar to an “ordinary” synapse between two
neurons in the nervous system
 Motor neuron (the presynaptic cell) transmits an action potential and secretes the
neurotransmitter acetylcholine upon its arrival at the axon terminal
 A specialized region of the sarcolemma called the motor end plate, which is highly
folded and contains a high density of acetylcholine receptors
 The resulting depolarization (called an end-plate potential) is much larger than an
ordinary postsynaptic potential—so large, in fact, that it is always above threshold and
triggers an action potential in the muscle cell
 As the action potential travels through the T tubules, it triggers the release of calcium
from the lateral sacs of the sarcoplasmic reticulum
The Roles of Calcium, Troponin, and Tropomyosin in Excitation-Contraction
Coupling
 When a muscle cell is relaxed, the concentration of calcium in the cytosol is very low,
and little binding occurs between calcium and troponin