Week 9 - Skeletal Muscle Structure and Function

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Questions and Answers

Which of the following is NOT a function of skeletal muscle?

Move bones

Produce heat

Support internal organs

Regulate blood pressure (correct)

What characteristic of skeletal muscle tissue allows it to respond to stimuli?

Contractility

Extensibility

Excitability (correct)

Elasticity

Which characteristic distinguishes contractility from extensibility in skeletal muscle?

Ability to generate force and movement (correct)

Ability to return to original length

Ability to respond to electrical stimuli

Ability to be stretched

What is the correct order of muscle structure from the smallest to largest component?

Muscle fiber, fascicle, whole muscle Signup and view all the answers

Which of the following describes a fascicle in skeletal muscle anatomy?

Bundle of muscle fibers Signup and view all the answers

Which of the following is a role of connective tissue in skeletal muscle?

Provide structural support and organization Signup and view all the answers

What is the primary function of muscle attachments?

To connect muscles to bones enabling movement Signup and view all the answers

Which function of skeletal muscle is most important for maintaining body temperature?

Produce heat through contractions Signup and view all the answers

What is the primary role of myoglobin in muscle fibers?

To store oxygen for aerobic ATP production Signup and view all the answers

Which component can quickly replenish ATP supply in muscle fibers?

Creatinine phosphate Signup and view all the answers

What defines a small motor unit?

Consists of less than five muscle fibers Signup and view all the answers

Where is the neuromuscular junction typically located?

At the mid-region of the muscle fiber Signup and view all the answers

What is the structure of a motor unit?

A motor neuron and all the muscle fibers it controls Signup and view all the answers

Which of the following describes the arrangement of fibers within a motor unit?

Fibers are dispersed throughout the muscle Signup and view all the answers

What type of energy production is primarily associated with skeletal muscle fibers?

Aerobic ATP production predominantly Signup and view all the answers

What is stored in muscle fibers for quick energy supply?

Glycogen Signup and view all the answers

What is the primary function of the epimysium in skeletal muscle?

To wrap the entire muscle and provide structural support Signup and view all the answers

What distinguishes a tendon from an aponeurosis?

A tendon is cordlike and dense regular connective tissue, while an aponeurosis is a thin, flattened sheet of dense irregular tissue. Signup and view all the answers

Which connective tissue layer functions to wrap individual muscle fibers?

Endomysium Signup and view all the answers

What is the role of deep fascia in skeletal muscle anatomy?

It separates individual muscles and binds those with similar functions. Signup and view all the answers

What type of connective tissue wraps around a fascicle?

Perimysium Signup and view all the answers

How is skeletal muscle innervated?

By somatic motor neurons at neuromuscular junctions Signup and view all the answers

What is the primary characteristic of skeletal muscle tissue?

It is striated and considered voluntary muscle. Signup and view all the answers

What is the function of blood vessels in skeletal muscle?

To deliver oxygen, nutrients and remove waste products Signup and view all the answers

What role does myoglobin serve in muscle fibers?

It stores oxygen for aerobic ATP production. Signup and view all the answers

What is a major difference between small and large motor units?

Large motor units innervate thousands of muscle fibers. Signup and view all the answers

Which component of a muscle fiber is most closely associated with the replenishment of ATP?

Creatine phosphate Signup and view all the answers

What is the primary function of the neuromuscular junction?

To facilitate communication between motor neurons and muscle fibers. Signup and view all the answers

What feature is unique to motor units in skeletal muscle?

A single motor neuron controls multiple muscle fibers. Signup and view all the answers

Which characteristic best describes the distribution of fibers in a motor unit?

Fibers are evenly distributed to allow for coordinated contractions. Signup and view all the answers

What type of cellular structure is abundant in muscle fibers to support sustained energy needs?

Mitochondria Signup and view all the answers

How does glycogen function in muscle fibers?

It stores energy that can be quickly mobilized when needed. Signup and view all the answers

What initiates the contraction process in smooth muscle?

Opening of voltage-gated Ca2+ channels Signup and view all the answers

What role does calmodulin play in smooth muscle contraction?

It forms a complex with calcium to activate MLCK Signup and view all the answers

What is a key characteristic of smooth muscle relaxation?

Inhibition of myosin light-chain kinase activity Signup and view all the answers

Why does smooth muscle contraction have a long duration?

Slow ATPase activity and need for dephosphorylation Signup and view all the answers

What mechanism slows down the relaxation of smooth muscle?

The latchbridge mechanism Signup and view all the answers

How do fast-twitch fibers differ from slow-twitch fibers in terms of their primary functions?

Slow-twitch fibers rely more on oxidative energy pathways than fast-twitch fibers. Signup and view all the answers

Which skeletal muscle fiber type is known for being slow and fatigue-resistant, and what is a key advantage of this type?

Type I fibers, which support endurance activities. Signup and view all the answers

Muscles that primarily maintain posture are mostly composed of which type of skeletal muscle fibers?

Slow-twitch oxidative fibers. Signup and view all the answers

What characterizes the latent period of a muscle twitch?

The time after stimulus before contraction begins. Signup and view all the answers

During which phase of a muscle twitch does the tension start to increase as power strokes pull thin filaments?

Contraction period. Signup and view all the answers

What differentiates oxidative fibers from glycolytic fibers?

Oxidative fibers predominantly use fats for energy, while glycolytic use sugars. Signup and view all the answers

In the context of muscle tension, what is defined as the minimum voltage that triggers a twitch?

Threshold stimulus. Signup and view all the answers

Which period of a muscle twitch generally lasts longer than the contraction period?

Relaxation period. Signup and view all the answers

What is the role of creatine phosphate in skeletal muscle metabolism?

It contains a high-energy bond for quick ATP regeneration. Signup and view all the answers

What is the net ATP yield from glycolysis per glucose molecule?

2 ATP Signup and view all the answers

Which metabolic process occurs in the mitochondria?

Aerobic cellular respiration Signup and view all the answers

Under which conditions does lactate formation from pyruvate occur?

Low oxygen availability Signup and view all the answers

How is ATP generated during aerobic cellular respiration?

By transferring chemical bond energy to NADH and FADH2 Signup and view all the answers

What type of fuel can be used during aerobic cellular respiration besides glucose?

Triglycerides Signup and view all the answers

What is the fate of lactate once formed in muscle tissue?

It can be converted to glucose in the liver and cycled back to muscle. Signup and view all the answers

What is the primary mechanism through which ATP is generated in aerobic respiration?

Oxidative phosphorylation Signup and view all the answers

What occurs during muscle tone?

Motor units are stimulated sporadically. Signup and view all the answers

What characterizes isometric contractions?

There is no movement despite increased tension. Signup and view all the answers

Which statement describes an eccentric contraction?

The muscle lengthens while it's contracting. Signup and view all the answers

What is a potential consequence of sustained isometric contractions?

Higher blood pressure. Signup and view all the answers

During an isotonic contraction, what happens to the muscle tone?

Muscle tone remains constant while length changes. Signup and view all the answers

What effect does deep sleep have on muscle tone?

Decreases muscle tone significantly. Signup and view all the answers

How is muscle tone generated?

Through random stimulation of various motor units. Signup and view all the answers

Which of the following scenarios best illustrates isometric contraction?

Holding a heavy weight in a static position. Signup and view all the answers

Study Notes

Skeletal Muscle as an Organ

Skeletal muscles are organs composed of various tissues: muscle fibers, connective tissue, blood vessels, and nerves
Muscle fibers are bundled within fascicles
A whole muscle contains multiple fascicles, and each fascicle contains numerous muscle fibers
A muscle fiber is a muscle cell

Connective Tissue Components

Three concentric layers of connective tissue wrap skeletal muscles:
- Epimysium: Dense irregular connective tissue surrounding the entire muscle
- Perimysium: Dense irregular connective tissue wrapping fascicles, containing blood vessels and nerves
- Endomysium: Areolar connective tissue surrounding individual muscle fibers, providing electrical insulation, capillary support, and binding between cells

Muscle Attachments

Muscles attach to bone (or skin or other muscles) via tendons or aponeuroses
- Tendon: Cordlike structure of dense regular connective tissue
- Aponeurosis: Thin, flattened sheet of dense irregular connective tissue

Fascia

Deep Fascia: Dense irregular connective tissue superficial to epimysium, separating individual muscles and binding muscles with similar functions
Superficial Fascia: Areolar and adipose connective tissue superficial to deep fascia, separating muscles from skin

Blood Vessels and Nerves

Skeletal muscles are vascularized with extensive blood vessels for oxygen and nutrient delivery, as well as waste removal
Skeletal muscles are innervated by somatic motor neurons
- Axons of these neurons branch and terminate at neuromuscular junctions
- Skeletal muscles are considered voluntary muscles, as their contraction is under voluntary control

Supplying Energy for Skeletal Muscle Metabolism

Creatine phosphate provides a high-energy phosphate bond for ATP production.
Creatine kinase catalyzes the transfer of phosphate from creatine phosphate to ADP.
Glycolysis is an anaerobic process that occurs in the cytosol.
- Glucose is converted to pyruvate, yielding 2 ATP per glucose molecule.
Aerobic cellular respiration requires oxygen and occurs within mitochondria.
- Pyruvate is oxidized to carbon dioxide, generating NADH and FADH2.
- Oxidative phosphorylation uses energy from NADH and FADH2 to produce ATP, yielding approximately 30 ATP per glucose molecule.
Triglycerides are an additional fuel source for aerobic respiration, providing more ATP per molecule than glucose.

Lactate formation and fate

Lactate is formed from pyruvate in low oxygen conditions.
Lactate dehydrogenase catalyzes the conversion of pyruvate to lactate.
Lactate can be used as fuel by skeletal muscle fibers, or enter the blood and be taken up by the heart or liver.
The lactic acid cycle involves the conversion of lactate to glucose in the liver and its transport back to muscles.

Microscopic Anatomy

Skeletal muscle fibers contain abundant mitochondria for aerobic ATP production.
Myoglobin in muscle fibers stores oxygen for aerobic ATP production.
Glycogen is stored as a readily available fuel source.
Creatine phosphate provides immediate ATP replenishment.

Motor Unit

Motor unit consists of a motor neuron and all the muscle fibers it controls.
Motor neurons innervate multiple muscle fibers ranging in size.
- Small motor units (less than five muscle fibers) enable precise control of force.
- Large motor units (thousands of muscle fibers) generate large amounts of force.
Fibers of a motor unit are dispersed throughout the muscle, facilitating smooth and coordinated contractions.

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
It consists of the synaptic knob, synaptic cleft, and motor end plate.

Comparison of Fiber Types

Fast-twitch fibers generate high force and fatigue quickly.
Slow-twitch fibers generate lower force but are fatigue resistant.
Oxidative fibers rely on aerobic respiration for energy.
Glycolytic fibers rely on anaerobic glycolysis for energy.

Muscle Tension

Muscle tension is the force generated during muscle contraction.
Myograms are graphs that measure muscle tension.
Muscle twitch is a brief contraction caused by a single stimulus.

Muscle Twitch Components

Latent period occurs between stimulus and the beginning of contraction.
Contraction period is marked by increasing tension as power strokes pull on thin filaments.
Relaxation period sees decreasing tension as cross-bridges detach and tension returns to baseline.

Muscle Tone

Muscle tone is the resting tension in a muscle due to involuntary nervous stimulation.
A small amount of tension is generated by randomly stimulated motor units.
Muscle tone prevents muscle atrophy and helps maintain posture.

Isometric Contraction

Isometric contraction increases muscle tension without change in muscle length.
Muscle tension does not overcome resistance.
An example is holding a weight without movement.

Isotonic Contraction

Isotonic contraction results in movement as muscle tension overcomes resistance.
Concentric contraction shortens the muscle while contracting (for example, lifting a weight).
Eccentric contraction lengthens the muscle while contracting (for example, lowering a weight).

Clinical View: Isometric Contraction and Increase in Blood Pressure

Sustained isometric contractions can cause a significant increase in blood pressure.
This is especially problematic for individuals with high blood pressure.

Smooth Muscle Contraction

Smooth muscle contraction is controlled by calcium influx.
Calcium binds to calmodulin, activating myosin light-chain kinase (MLCK).
MLCK phosphorylates myosin heads, enabling cross-bridge formation and contraction.
Contraction involves pulling on dense bodies and intermediate filaments, resulting in shortening of the entire cell.

Relaxation of Smooth Muscle

Cessation of stimulation and removal of calcium from the cytoplasm leads to relaxation.
Myosin light-chain phosphatase dephosphorylates myosin heads, preventing cross-bridge formation.
Latchbridge mechanism can contribute to the slow relaxation of smooth muscle.

Characteristics of Smooth Muscle Contraction

Smooth muscle contraction has a long latent period due to the time required for myosin phosphorylation and slow ATPase activity.
Long duration of contraction is caused by slow calcium pumps, dephosphorylation of myosin, and the latchbridge mechanism.
Smooth muscle's slow contractions are suited for its function, such as maintaining continuous tone in blood vessels.

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Description

This quiz covers the anatomy and function of skeletal muscles as organs, including their connective tissue components and muscle attachments. Explore the layers of connective tissue surrounding skeletal muscles and understand how they connect to bones or skin. Test your knowledge on the complex structure of skeletal muscles.