Muscle Physiology: Motor Units, Contraction, and Calcium Regulation

Muscle fibers in a motor unit

They all contract at the same time.

Where does the sarcoplasmic reticulum release Ca²⁺?

Into the sarcoplasm.

Connective tissue around a fascicle

Perimysium.

Thin filament protein blocking actin's binding site

Tropomyosin.

Main function of the sarcoplasmic reticulum

Stores and releases calcium ions (Ca²⁺).

What follows ADP and phosphate release from myosin?

The power stroke (myosin pulls actin).

What does Ca²⁺ bind to in the sarcomere?

Troponin.

Define a motor unit

One motor neuron and all the muscle fibers it controls.

Which structure shortens during contraction?

Sarcomeres (and thus myofibrils).

What triggers cardiac muscle action potentials?

Ions passing through gap junctions between cells.

What activates ligand-gated Na⁺ channels at the NMJ?

Binding of acetylcholine (ACh).

Protein that prevents actin-myosin binding without Ca²⁺

Tropomyosin.

Effect of no ATP during contraction

Myosin heads can't detach from actin (rigor mortis).

Layers blood vessels pass to reach muscle fibers

Epimysium, perimysium, and endomysium.

Energy systems during moderate activity

Both glycolysis (anaerobic) and citric acid cycle (aerobic).

Definition of an action potential

Electrical signal generated by the nervous system and transmitted along axons.

Structure absent in cardiac muscle

Triads.

Location of nebulin

Along thin filaments, stabilizing actin.

Connective tissue surrounding a muscle (organ)

Epimysium.

Ca²⁺ concentration: T-tubule vs. sarcoplasm

2 mM in T-tubule / 0.1 µM in sarcoplasm.

What is produced during intense, frequent contractions?

Pyruvate and lactate.

Another thin filament protein besides tropomyosin

Troponin.

Status of myosin heads after Ca²⁺ removal

Remain energized (ADP + Pi bound).

Structure present in smooth muscle

Thin filaments (no sarcomeres or T-tubules).

What shortens during contraction—myofibrils or myofilaments?

Myofibrils shorten; myofilaments just slide.

ACh function at neuromuscular junction

Opens Na⁺ channels → depolarization of motor end plate.

Structure present in all cardiac muscle cells

T-tubules.

What controls muscle fibers in a motor unit?

The same motor neuron.

During contraction, which structure shortens overall?

Myofibrils.