Skeletal Muscle & Sliding Filament Theory

What is the endomysium?

Connective tissue surrounding a single muscle fiber

What is the perimysium?

Connective tissue surrounding a bundle of muscle fibers (fascicle)

What is the epimysium?

Connective tissue layer that surrounds the entire muscle

What is a fasciculus (fascicle)?

A bundle of muscle fibers surrounded by perimysium

What is a muscle fiber?

A single muscle cell, long and multinucleated, covered by endomysium

Where is the sarcolemma?

The muscle cell membrane that surrounds the muscle fiber

What is the sarcoplasm?

The cytoplasm inside a muscle fiber

What is the I-band?

The light band containing only thin filaments (actin); spans two sarcomeres

What is the A-band?

The dark band where thick filaments (myosin) exist, including overlap with actin

What is the Z-line (Z-disc)?

The boundary between sarcomeres; anchors actin

What is the H-zone?

The central area of A-band with only myosin (no actin overlap)

What is a sarcomere?

The basic contractile unit of muscle, running from Z-line to Z-line

What is the sarcoplasmic reticulum (SR)?

A specialized endoplasmic reticulum in muscle that stores and releases calcium

What are T-tubules?

Extensions of the sarcolemma that carry action potentials deep into the muscle fiber

What is stored in the sarcoplasmic reticulum?

Calcium ions (Ca²⁺)

What is actin?

Filament protein that contains binding sites for myosin

What is myosin?

Filament protein with heads that form cross-bridges and generate force

What is tropomyosin?

A protein that wraps around actin and blocks myosin binding sites at rest

What is troponin?

A regulatory protein that binds calcium and moves tropomyosin to allow contraction

What is a crossbridge?

A connection formed when the myosin head binds to an actin active site during muscle contraction.

Where does the energy for muscle contraction and relaxation come from?

ATP (adenosine triphosphate)

On what protein is the ATPase enzyme located?

On the myosin head

Where is ATP located in muscle?

Bound to the myosin head and stored in the cytoplasm

What is the role of acetylcholine (ACh)?

Neurotransmitter that triggers an action potential in the muscle fiber

Where does the excitation signal come from?

A motor neuron in the nervous system

How is the excitation signal sent to the muscle fiber?

Nerve impulse → ACh release → binds to receptors on sarcolemma → action potential travels through T-tubules

What is the role of Ca²⁺ in muscle contraction?

Binds to troponin, allowing actin-myosin interaction

What does troponin do?

Binds calcium and moves tropomyosin off actin binding sites

What does tropomyosin do?

Blocks myosin-binding sites on actin at rest

How does muscle contraction occur?

Myosin heads attach to actin and pull thin filaments toward the center of the sarcomere

What swivels during contraction?

The myosin head (power stroke)

What slides during contraction?

Actin filaments slide over myosin toward the M-line

How is the myosin crossbridge recharged?

ATP binds myosin → myosin detaches → ATP is split by ATPase → myosin resets for another stroke

How does the muscle relax?

Nerve impulse stops, Ca²⁺ is pumped back into the SR, tropomyosin blocks actin binding sites again

What happens to Ca²⁺ during relaxation?

It is actively pumped back into the sarcoplasmic reticulum

Characteristics of Type I fibers?

Slow-twitch, red, high endurance, oxidative metabolism

Characteristics of Type IIa fibers?

Fast-twitch, intermediate fatigue resistance, both oxidative & glycolytic

Characteristics of Type IIx fibers?

Fast-twitch, white, high force, quick fatigue, glycolytic metabolism