BIO 203 E2 Spring 2026 Module 5

85+

Excitation

Synaptic transmission at the neuromuscular junction & sarcolemma
depolarization

Coupling

Release of intracellular calcium

Contraction

Sliding Filament Theory

muscle fiber

muscle cell

Muscle Type 1 - Skeletal

Location: Attached to bones (and a few tubes)
Function: Moves the skeleton (and a few sphincters)
NS Control: Somatic
Initiation of contraction: Membrane Depolarization
Cell structure: Large, multinucleated fibers/cells
Coordination of muscle contraction: One neuron depolarizes many fibers (motor unit)
Contractile Filaments: Actin & Myosin
Appearance: Striated

Muscle Type 2 - Cardiac

Location: Heart
Function: Generates heart contraction
NS Control: Autonomic
Initiation of contraction: Membrane Depolarization
Cell structure: Smaller, uninucleated cells; branched fibers
Coordination of muscle contraction: Gap junctions
Contractile Filaments: Actin & Myosin
Appearance: Striated

Muscle Type 3 - Smooth

Location: Internal organs & tubes
Function: Moves internal organs & tubes
NS Control: Autonomic
Initiation of contraction: Membrane Depolarization
Cell structure: Small, uninucleated cells
Coordination of muscle contraction: Gap junctions and motor units
Contractile Filaments: Actin & Myosin
Appearance: Not Striated

Organize from biggest on top to smallest on bottom (with relative quantity increasing the further they are down the pyramid)

Organelle, Tissue, Organ , Cell , Protein

1. Organ (Muscle)

2. Tissue (Fascicle)

3. Cell (Muscle Fiber)

4. Organelle (Myofibril)

5. Protein (Actin and Myosin)

Function of the M line?

The M line is responsible for anchoring thick filaments (myosin) in the center of the sarcomere and providing structural stability to the muscle fiber during contraction

Function of Z disk?

The Z disk serves as the boundary of the sarcomere, anchoring the thin filaments (actin) and helping to maintain the structural organization of the muscle fiber during contraction (also shortens)

Function of A band?

The A band is the dark region of the sarcomere that contains the entire length of thick filaments (myosin) and overlaps with thin filaments (actin). It is crucial for muscle contraction as it corresponds to the region where thick and thin filaments interact.

Function of H zone?

The H zone is the area within the A band of a sarcomere that appears lighter due to the absence of thin filaments (actin). It represents the region where only thick filaments (myosin) are present, and during muscle contraction, the H zone decreases in width as the thin filaments slide over the thick filaments

Function of I band?

The I band is the light region of the sarcomere that consists of thin filaments (actin) only and is located on either side of the A band. It shortens during muscle contraction as the thick and thin filaments slide past each other.

Heads of Myosin are _____ and have binding sites for _____

ATPases and G-Actin

What binds to troponin which moves the tropomyosin to reveal the myosin binding sites (which is on ACTIN)?

Calcium ions

How do muscles “turn on” and then how do they contract?

An axon potential travels onto a muscle fiber via a motor neuron, then acetylcholine goes onto ligan gated channels, Na+ rushes in and K+ leaks out causing a depolarization (action potential), this action potential travels down the sarcolemma and down T-tubules, this triggers Ca+ ions to flood into cell and then contract via revealing the Myosin sites on actin after Ca+ binds to Troponin

What does the sarcoplasmic reticulum inside the sarcolemma contain?

Ca+ ions

What do T-tubules (Transverse Tubules) mostly do and serve as their purpose?

Brings in the action potential to then allow Ca+ release and thus cause contraction

How does the cross-bridge cycle function?

Myosin heads start in the inactive state where there is ADP and Pi (phosphates), then the Myosin heads release the phosphates and bind to the sites on actin, ADP alone moves the actin (pull motion) and then drops off, after which ATP is reintroduced and then Hydrolyzes into ADP and phosphates, ADP and the new phosphates reconfigure and thus stop.

What causes Ca+ to go back into SR (Sarcolemma Reticulum)?

Ca2+-ATPase

Which sarcomere length would produce maximum tension?

C

What is a muscle twitch?

a single contraction of a single muscle fiber in response to stimulation from a motor neuron (twitch = unit of contraction)

What function does myoglobin serve on muscle fibers?

stores oxygen in the muscle fibers

Size of motor units determines _____?

precision of movements (smaller units = more precise movements)

Analyze the table and rewrite important info/differences

done