Microscopic Anatomy of Skeletal Muscle: Sarcomeres, Neuromuscular Junctions, and Contraction Types

Sarcolemma

Plasma membrane of the muscle fiber

Sarcoplasm

Cytoplasm of a muscle fiber

Myofibrils

Rod-like structures that run the entire length of the muscle fiber

Mitochondria

Perform aerobic respiration to produce energy for muscle activity

Myoglobin

Protein that binds and stores oxygen, located in the sarcoplasm

Inclusions

Glycosomes - granules of stored glycogen

Sarcoplasmic reticulum (SR)

Specialized smooth ER; stores and releases Ca2+ into the sarcoplasm

Terminal cisternae

Expanded ends of the sarcoplasmic reticulum

Transverse tubules (T tubules)

Involutions of the sarcolemma into the sarcoplasm

Triad

Composed of one T tubule, sandwiched between two terminal cisternae

Sarcomere

Bounded by two Z discs, the structures of the sarcomere underlie muscle contraction

Myofilaments

Thick (myosin) and thin (actin) filaments of the sarcomere

A band

Region of thick filaments

H zone

Middle region of the A band (doesn't overlap with the thin filaments)

M line

Anchors the thick filaments

I band

Region of thin filaments (doesn't overlap with the A band)

Z disc

Anchors the thin filaments

Neuromuscular junction

Where a motor neuron meets a muscle fiber

Motor neurons

Induce movement by sending impulses to skeletal muscles that stimulate contraction

Axon terminals

Terminals of the motor neuron that make contact with the skeletal muscle

Motor unit

The motor neuron and all of the skeletal muscle fibers it innervates via its axon terminals

Excitation-contraction coupling

Action potential reaches axon terminal, inducing Ca2+ channels to open

Acetylcholine (ACh)

A neurotransmitter contained in synaptic vesicles

ACh

ACh is released into the synaptic cleft and binds to receptors on the sarcolemma.

Ion channels

Ion channels in the sarcolemma open, allowing an influx of ions that depolarizes the sarcolemma.

Triads

Triads propagate rapid depolarization throughout the muscle fiber.

Ca2+

Ca2+ is released from the SR, enabling the contraction of sarcomeres (and of the muscle fiber).

Isometric contraction

Isometric: force generated by the muscle increases at a constant muscle length.

Isometric contraction example

Plank: isometric contraction.

Isotonic contraction

Isotonic: force generated by skeletal muscle is constant and the muscle length changes.

Isotonic contraction example

Bicep curl: concentric isotonic contraction.

Concentric contraction

Concentric: muscle shortens.

Eccentric contraction

Eccentric: muscle lengthens (but doesn't relax).

Aerobic exercise

Aerobic or endurance exercise increases the capillaries, mitochondria, and myoglobin content in muscle fibers.

Aerobic exercise examples

Examples: jogging, biking, swimming.

Resistance exercise

Resistance exercise or strength training increases the number of myofibrils in muscle fibers to increase the size of skeletal muscles.

Resistance exercise examples

Examples: weight lifting or other isometric exercises where muscles are pitted against immovable objects.

Muscle fiber composition

Muscle fibers contain many myofibrils (long series of contractile sarcomeres).

Sarcomere contraction

Thick and thin filaments of the sarcomere slide against each other during contraction.

Critical elements in contraction

ACh and Ca2+ are critical to several steps of the contraction process.

Isotonic vs Isometric

Isotonic: force increases, muscle length is steady; Isometric: force is steady, muscle length changes.

Exercise effects on muscles

Different kinds of exercise increase muscle stamina or the size of muscles.