Skeletal Muscle Contraction and Fiber Types

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Questions and Answers

What happens to the Z lines during the contraction of a sarcomere?

They disappear completely.

They only move in the presence of oxygen.

They move further apart.

They come closer to the A band. (correct)

Which skeletal muscle fiber type is characterized by rapid contraction and fatigue due to anaerobic metabolism?

Intermediate glycolytic fibers

White or fast glycolytic fibers (correct)

Slow oxidative fibers

Intermediate fast oxidative fibers

How does the H zone change during muscle contraction?

It decreases in length. (correct)

It remains unchanged.

It increases in length.

It disappears entirely.

What is a key characteristic of slow oxidative fibers?

They are resistant to fatigue. Signup and view all the answers

Which type of skeletal muscle fiber is primarily used for activities requiring long duration and endurance?

Slow oxidative fibers Signup and view all the answers

What primarily controls skeletal muscle contraction?

Nerve impulses transmitted by motor neurons Signup and view all the answers

What differentiates fast oxidative fibers from slow oxidative fibers?

Fast oxidative fibers contract more powerfully. Signup and view all the answers

What relationship does the motor unit have with muscle contraction?

It controls multiple muscle fibers for fine movements. Signup and view all the answers

What is a unique feature of the T tubules in cardiac muscle compared to those in skeletal muscle?

Cardiac muscle T tubules are much larger. Signup and view all the answers

Which structure serves as an attachment site for thin filaments in smooth muscle?

Dense bodies Signup and view all the answers

Which statement accurately describes the organization of myofilaments in smooth muscle?

They do not achieve the same degree of order. Signup and view all the answers

What initiates muscle contraction in smooth muscle cells?

Myosin light chain phosphorylation Signup and view all the answers

Which of the following muscles do NOT consist of smooth muscle?

Skeletal muscles of the limbs Signup and view all the answers

How are smooth muscle cells interconnected?

By gap junctions Signup and view all the answers

What type of muscle control is associated with smooth muscle?

Involuntary control Signup and view all the answers

Which of the following structures is NOT found in smooth muscle tissues?

Sarcomeres Signup and view all the answers

Which of the following best describes the neuromuscular junction?

The synapse between a motor nerve and a muscle fiber. Signup and view all the answers

What is the role of acetylcholine in muscle contraction?

It acts as a neurotransmitter to stimulate muscle fibers. Signup and view all the answers

What structure connects cardiac muscle cells?

Intercalated discs. Signup and view all the answers

Which characteristic is unique to cardiac muscle compared to skeletal and smooth muscle?

Involuntary control and intercalated discs. Signup and view all the answers

What is the primary function of smooth muscle tissue?

To contract and relax to regulate blood pressure and digestive processes. Signup and view all the answers

How are muscle contractions graded?

By the frequency of action potentials and number of responding fibers. Signup and view all the answers

Which of the following statements is true regarding the smooth endoplasmic reticulum (sER) in cardiac muscle?

It is less organized than that of skeletal muscle. Signup and view all the answers

Which type of muscle fiber is innervated by a single motor neuron?

Skeletal muscle fibers. Signup and view all the answers

Study Notes

Sarcomere Contraction

During contraction, the sarcomere decreases in length.
Z lines come closer to the A band.
The length of the A band does not change, but the I band shortens in length.
Thin filaments are pushed towards the center of the sarcomere by myosin heads, pulling Z lines together.
The H band gradually decreases in length, and thin filaments penetrate deeper into the sarcomere.

Skeletal Muscle Fiber Types

Three types of skeletal muscle fibers: slow oxidative, fast oxidative, and fast glycolytic.
Slow oxidative fibers are dark red due to abundant myoglobin, have many mitochondria, and contract slowly but are resistant to fatigue.
Fast oxidative fibers stain less darkly than slow oxidative fibers, produce ATP via aerobic metabolism, contract faster and more powerfully than slow oxidative fibers, and are abundant in lower limbs.
Fast glycolytic fibers stain pale due to little myoglobin, are the largest of the three fiber types, depend on anaerobic glycogenolysis for ATP, contract rapidly but fatigue easily, and are found in extraocular muscles and finger muscles.

Neuromuscular Control

Skeletal muscle contraction is controlled by a nerve impulse transmitted by a motor nerve from the brain or spinal cord.
A motor unit consists of all the muscle fibers controlled by a single motor neuron.
Fine control muscles (e.g., eyelid muscles) have fewer muscle fibers per nerve (2:1).
A contraction is initiated by an action potential (nerve impulse), followed by the release of a chemical neurotransmitter at the neuromuscular junction (NMJ).
The neurotransmitter for skeletal muscle is acetylcholine.
Every muscle fiber is innervated by a single motor neuron.
Contractions may be graded or full depending on the number of muscle fibers that respond to the stimulus: the more fibers, the greater the muscle contraction.
Synapse: functional connection between a nerve fiber and its target cell.
Neuromuscular junction: synapse between a motor nerve and a muscle fiber.

Neuromuscular Junction

Synaptic knob, terminal, or bouton: bulbous swelling at the end of a motor nerve above the motor end plate on the muscle fiber.
Synaptic cleft: gap between the synaptic knob and the motor end plate.
Synaptic vesicles: small packets of neurotransmitter chemical (e.g., acetylcholine, norepinephrine).

Cardiac Muscle

Basic morphologic unit: cardiac muscle cell (cardiomyocyte) with cross striations and often branched.
Nuclei (1-2) are situated in the center of the cell.
Intercalated discs (ID) are connections between cardiac muscle cells.
On the transverse site of IDs are: fascia adherentes (α - actinin) for actin filament attachment and desmosomes for strong connection between cells.
On the lateral site of IDs are: gap junctions (nexus) for ion transport, impulse spreading, and metabolism.

Functions of Cardiac Muscle Tissue

Plays the most important role in the contraction of the atria and ventricles of the heart.
Causes the rhythmic beating of the heart, circulating blood throughout the body.
Influenced involuntarily by endocrine (hormones) and autonomic nervous systems.

Smooth Muscle

Generally occurs as bundles or sheets of elongated fusiform cells, with a centrally located single nucleus and finely tapered ends.
Controls slow, involuntary movements of the walls of hollow organs (e.g., stomach, gastrointestinal tract, uterus, glands, blood vessels).
Does not exhibit cross striations because myofilaments are not arranged in the same ordered way as in striated muscle.
Found in the walls of the digestive system (middle esophagus to anus), arteries, veins, large lymphatics, urinary and genital duct walls, respiratory tract, arrector pili muscle, dartos muscle, and iris and corpus ciliare (eye).

Smooth Muscle Tissue

Contains both thick and thin filaments and a cytoskeleton of desmin and vimentin intermediate filaments.
Not arranged in orderly sarcomeres.
Gap junctions interconnect smooth muscle cells.
Caveolae in the sarcolemma instead of T tubules.
Dense bodies provide an attachment site for thin filaments and intermediate filaments, serving as intracellular analogs of the striated muscle Z lines.
Dense bodies are important for transmitting contractile forces generated inside the cell to the cell surface, altering cell shape.
During contraction, filaments pull on the dense bodies, causing shortening of the muscle fiber.
Dense bodies are distributed throughout the sarcoplasm in a network of intermediate filaments containing the proteins desmin and vimentin.

Proteins Essential for Contraction

Myosin light chain kinase (MLCK) is important in smooth muscle contraction.

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Description

This quiz covers the mechanisms of sarcomere contraction and the different types of skeletal muscle fibers. Learn how the structure of the sarcomere changes during contraction, as well as the characteristics and functions of slow oxidative, fast oxidative, and fast glycolytic fibers. Test your understanding of muscle physiology and fiber composition!