sarcolemma
Summary
The sarcolemma is the plasma membrane of a muscle fiber. It carries the action potential across the cell surface and into the T-tubules, which leads the sarcoplasmic reticulum to release Ca²⁺ and the fiber to contract.
Detail
Meaning: The sarcolemma is the plasma membrane of a muscle fiber, and it maintains a resting membrane potential. At the neuromuscular junction, acetylcholine from the motor neuron binds receptors on the sarcolemma, which depolarizes it and fires an action potential. The action potential spreads along the surface and down the T-tubules, which are inward folds of the sarcolemma that carry the signal deep into the fiber. There it leads the sarcoplasmic reticulum to release Ca²⁺, which binds troponin and allows contraction.
On the MCAT: Passages ask for the order of events in excitation-contraction coupling, or for the effect of a defect that stops the sarcolemma from depolarizing: no action potential, so no Ca²⁺ release and no contraction. Vocabulary matching is also common: sarcolemma is the plasma membrane, sarcoplasm is the cytoplasm, and the sarcoplasmic reticulum is the modified smooth ER.
Common confusion: Students mix up the sarcolemma with the sarcoplasmic reticulum. The sarcolemma is the outer cell membrane that conducts the action potential. The sarcoplasmic reticulum is an internal membrane network that stores and releases Ca²⁺. Neither is the sarcomere, which is the contractile unit of a myofibril.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.