Muscle Contraction Mechanisms

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Questions and Answers

What is a unique characteristic of smooth muscle contraction compared to skeletal muscle contraction?

It requires a higher concentration of intracellular calcium.

It uses troponin to bind calcium.

It produces faster contractions than skeletal muscle.

It utilizes signal transduction and second messenger systems. (correct)

What role does calmodulin play in smooth muscle contraction?

It activates thick filaments by binding to calcium. (correct)

It facilitates the breakdown of ATP.

It directly binds actin to initiate contraction.

It serves as a neurotransmitter.

In which type of muscle is a pacemaker potential typically seen?

Skeletal muscle.

Multi-unit smooth muscle.

Single unit smooth muscle. (correct)

Cardiac muscle.

What is a distinguishing feature of multi-unit smooth muscle?

Cells are randomly organized and act independently. Signup and view all the answers

Which structure is NOT present in smooth muscle compared to skeletal muscle?

Sarcomeres. Signup and view all the answers

What describes the condition characterized by a reduction of muscle tonus?

Flaccidity Signup and view all the answers

Which of the following disorders is associated with excessive muscle tone leading to rigidity?

Spasticity Signup and view all the answers

Which term refers to the sudden involuntary twitch of a muscle?

Spasm Signup and view all the answers

Which condition is known for muscle pain and may include symptoms such as fatigue and depression?

Myalgia Signup and view all the answers

What might cause a tic that involves spasmodic contraction of a muscle?

Psychological factors Signup and view all the answers

What characterizes a cramp?

Painful spasmodic contraction without relaxation Signup and view all the answers

Which condition is associated with visible muscle contractions under the skin?

Fasciculation Signup and view all the answers

What is the main cause of paralysis in botulism?

Toxin preventing ACh release and action Signup and view all the answers

Which of the following describes tendinitis?

Inflammation of a tendon Signup and view all the answers

What condition is defined as a fibromyositis of the quadriceps femoris?

Charley Horse Signup and view all the answers

What is the primary characteristic of Amyotrophic Lateral Sclerosis (ALS)?

Loss of motor function due to motor neuron destruction Signup and view all the answers

Which form of Muscular Dystrophy is characterized by early onset and often leads to being wheelchair-bound by age 12?

Duchenne MD Signup and view all the answers

What causes the muscle weakness seen in Myasthenia Gravis?

Auto-antibodies targeting the ACh receptor Signup and view all the answers

What is a significant effect of endurance exercise on muscle physiology?

Increases muscle vascularization Signup and view all the answers

What replaces lost muscle mass due to aging after the age of 80?

Dense connective tissue Signup and view all the answers

What is the latent period in muscle contraction?

The time lag between stimulation and the initiation of contraction. Signup and view all the answers

What leads to increased contraction strength during wave summation?

Increased availability of Ca+2 in the muscle fibers. Signup and view all the answers

During which phase can muscle fibers begin to return to their original position?

Relaxation period Signup and view all the answers

What characterizes complete tetany?

Sustained contraction without any partial relaxation. Signup and view all the answers

What is the effect observed during the treppe phenomenon?

Contraction strength increases with successive stimulations. Signup and view all the answers

Why is contraction force greatest when actin and myosin overlap maximally?

Greatest number of cross-linkages between actin and myosin. Signup and view all the answers

What impact does Clostridium tetani have on muscle function?

It results in continuous stimulation leading to spastic paralysis. Signup and view all the answers

What occurs during incomplete tetany?

Partial relaxation occurs between successive contractions. Signup and view all the answers

Study Notes

Twitch

A rapid single muscle contraction due to a single stimulus
The latent period is the time between the stimulus and contraction initiation
The reason for this delay is that the impulse is propagated from the plasma membrane (PM) to the T-tubules to the sarcoplasmic reticulum (SR)
The refractory period is the time when the muscle cannot respond to a second stimulus because sodium ions are still entering the cell
The contraction period is the time from the initial contraction to maximum contraction force. The relaxation period is the time when muscle fibers return to their original position.
Muscle fiber type affects the duration of the contraction period.

Wave Summation

This occurs when a second stimulus is applied before the complete relaxation of the first stimulus.
Leads to increased contraction strength.
This appears to contradict the "all or none" principle, however, it is explained by increased calcium ion availability.

Treppe / Staircase Effect

An increase in the force of muscle contraction with successive stimulations (with relaxation)
Responsible for the "warm up" effect.
Due to increased calcium ion availability, just like wave summation.

Tetany

Sustained contraction resulting from successive stimulations
Leads to increased contraction strength

Incomplete / Unfused Tetany

There is partial relaxation before the next contraction

Complete / Fused Tetany

Lacks partial relaxation

Clostridium Tetani

An anaerobic bacterium that secretes a neurotoxin
Leads to continuous muscle stimulation
Result = tetanus (lockjaw)
The toxin blocks GABA (inhibitory neurotransmitter), causing continuous firing of motor nerves and spastic paralysis.

Muscle Stretch

Contraction force is strongest when actin and myosin overlap is maximized (greatest number of cross-bridges)
Maximum overlap and highest tension occur when the muscle is slightly stretched.

Tension is lowest when:

Sarcomeres are excessively stretched

Smooth Muscle

Short cells with a single nucleus
Increased mitotic capability compared to skeletal or cardiac muscle
Actin and myosin are randomly arranged (smooth) – sarcomeres are not present.
Lacks T-tubules and has a small SR
Most of the calcium ions are extracellular and found in caveolae
Contraction is similar to skeletal muscle: requires calcium ions, ATP, and the sliding filament mechanism.
Contraction differs from skeletal muscle: uses signal transduction / second messenger systems / hormones and neurotransmitters.
Lacks troponin – uses calmodulin to bind calcium ions.
Calmodulin activates myosin ATPase / kinase, which activates the thick filament and leads to contraction.
Slow, sustained contractions that last several seconds.

Single & Multiunit Smooth Muscle

Single unit / Unitary / Visceral Smooth Muscle
- Most common type
- Located in the walls of hollow organs (e.g., GI tract, blood vessels)
- Gap junctions allow cell to cell impulse transmission and the organ contracts as a unit.
- Uses a pacemaker potential to initiate the impulse.
- Controlled by the autonomic nervous system and endocrine system.
- Arrangements in the GI tract: outer longitudinal layer and inner circular layer.
- Peristalsis: rhythmic wave-like contraction of smooth muscle to propel "food" along the GI tract.
Multi-Unit Smooth Muscle
- Cells are more randomly organized.
- Location: large airways, large arteries, eye, arrector pili, etc.
- Lacks pacemaker cells and gap junctions.
- Contraction is mainly mediated by the autonomic nervous system, hormones, and reflexes.

Disorders & Myopathies

Flaccidity: Reduction of muscle tone, usually neurological (e.g., spinal cord damage). Muscles do not contract properly, leading to flaccid paralysis and atrophy.
Spasticity: Excessive muscle tone leading to rigidity (spastic paralysis). Example: late-stage Parkinson's disease.
Myalgia: Muscle pain due to any muscle disorder. Examples: fibromyalgia (tendons, ligaments, and fascia exhibit great pain), accompanied by fatigue, depression, etc.
Spasm: Sudden involuntary muscle twitch, cause unclear, may be mild or severe.
Tic: Spasmodic contraction of a muscle, often affecting the eye or face, may be psychological.
Cramp: Painful, spasmodic muscle contraction without relaxation.
Fasciculation: Involuntary and uncoordinated muscle contraction visible under the skin (e.g., ALS)
Fibrillation: Uncoordinated muscle contraction not visible under the skin, often used to refer to cardiac muscle, can accompany a heart attack.
Tendinitis: Inflammation of a tendon.
Strain: "Pulled muscle," excessive stretching or tearing of a muscle fiber, causing inflammation and pain.
Fibromyositis: Inflammation of muscle fibers, connective tissue coverings, tendons, and joint capsules, associated with pain, stiffness, and soreness.
Charley horse: Fibromyositis of the quadriceps femoris due to a contusion.
Botulism: Food poisoning caused by Clostridium botulinum. The toxin prevents acetylcholine release and action, causing paralysis.
Convulsions: Violent involuntary muscle contractions, many causes.
Myofascial Trigger Points / Muscle Knots: Irritable area of fascia surrounding taught muscles. Idiopathic, possibly due to overload, stress, or homeostatic imbalance.
Amyotrophic Lateral Sclerosis (ALS): Aka Lou Gehrig's Disease. Progressive loss of motor function due to destruction of motor neurons. Does not affect mental capacity. 30,000 cases in the US. Some forms are genetic, most are "sporadic" and linked to free radical damage.
Muscular Dystrophy: A group of genetic disorders characterized by progressive muscle weakness and degeneration.
Dystrophin: The mutant gene / protein that stabilizes the sarcolemma during contraction.
Duchenne Muscular Dystrophy: Sex / X-linked disorder characterized by weakness and shortening of muscles. Onset by age 3, wheelchair bound by age 12.
Myotonic Muscular Dystrophy: Autosomal, slower progression, and affects fewer muscles.
Myasthenia Gravis: Autoimmune disease affecting women aged 30-50, characterized by weak, fatigued muscles. Cause: auto-antibodies to the acetylcholine receptor.
Aging: Muscle may atrophy and strength may decrease up to 50% by age 80. Muscle is lost and replaced by dense connective tissue (fibrosis) = sarcopenia.

Exercise Effects on Muscle

Endurance Exercise: E.g., cycling or running
- Increases muscle vascularization (probably all fiber types)
- Increase in the number of mitochondria
- Increased levels of enzymes for the Krebs cycle and electron transport system
- Also, increases respiratory and circulatory system capacity.
Strength Exercise: E.g., weightlifting
- Results in hypertrophy: increased fiber diameter. Note: not due to increased mitosis of myocytes!

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Description

Explore the intricate processes of muscle contraction, including twitch, wave summation, and the treppe effect. Understand key concepts such as the latent period, refractory period, and the impact of muscle fiber type on contraction duration. This quiz delves into the physiological mechanisms that control muscle fiber responses.