Muscle Physiology Quiz

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Questions and Answers

What is the process called when extra oxygen is consumed after exercise to remove excess lactate and replenish energy stores?

Anaerobic discharge

Aerobic threshold

Recovery oxygenation

Oxygen debt (correct)

Which of the following accurately describes muscular hypertrophy?

Increase in the thickness of muscle fibers and total myofibrils (correct)

Increase in the number of muscle fibers in response to exercise

Decrease in muscle size and replacement by fibrous tissue

Loss of muscle function due to denervation

What occurs immediately after motor nerve damage that is observable under the skin?

Muscle hypertrophy

Muscle fasciculation (correct)

Muscle fibrillation

Muscle atrophy

Which method is used to record electrical activity in muscles?

Electromyography Signup and view all the answers

What results from complete degeneration of motor nerve fibers leading to spontaneous contractions that cannot be seen under the skin?

Muscle fibrillation Signup and view all the answers

What are the two major types of muscles in the human body?

Striated and smooth muscles Signup and view all the answers

Which function is NOT performed by skeletal muscles?

Regulating heart rate Signup and view all the answers

What is the primary role of titin in skeletal muscle fibers?

To help keep thick filaments centered Signup and view all the answers

What is the structure that divides myofibrils into functional units?

Z line Signup and view all the answers

Which statement about skeletal muscle fibers is accurate?

They are parallelly arranged and bundled together. Signup and view all the answers

What percentage of the human body is composed of smooth and cardiac muscles combined?

10% Signup and view all the answers

Where are the thick filaments primarily composed of?

Myosin Signup and view all the answers

What occurs when the strength of the stimulus is increased?

Increased recruitment of motor units Signup and view all the answers

What characterizes complete tetanus during muscle contraction?

No relaxation phase between contractions Signup and view all the answers

Which phenomenon describes the progressive increase of muscle tension during repetitive stimulation after a rest period?

Treppe Signup and view all the answers

How does increasing the frequency of muscle stimulation affect contraction strength?

It increases the force of contraction Signup and view all the answers

What describes the contractions that occur with incomplete relaxation between stimuli?

Clonic contractions Signup and view all the answers

What is the result of a maximal stimulus in terms of motor unit activation?

Activation of all motor units Signup and view all the answers

During moderate intensity voluntary movements, what primarily contributes to increased force?

Higher discharge frequency of impulses Signup and view all the answers

What is the primary mode of muscle contraction seen in complete tetanus?

Sustained contraction with no relaxation Signup and view all the answers

What happens to calcium ion levels during complete tetanus?

Calcium levels remain high Signup and view all the answers

What defines the all-or-none law in muscle fibers?

Muscle fibers contract fully or not at all Signup and view all the answers

What is the primary characteristic of an isometric contraction?

The muscle develops tension without altering its length. Signup and view all the answers

During isotonic contraction, what occurs after the muscle generates enough tension?

The muscle shortens while lifting the load. Signup and view all the answers

What happens to the series elastic elements during isometric contraction?

They stretch while the muscle maintains its length. Signup and view all the answers

How does the load affect isometric contraction duration?

Heavier loads increase the contraction duration. Signup and view all the answers

What initiates the contraction phase in isotonic contraction?

Sufficient tension generated to overcome the load. Signup and view all the answers

What is the first phase of muscle contraction in isotonic conditions?

Isometric contraction before shortening occurs. Signup and view all the answers

Which component is primarily activated during muscle contraction regardless of the type?

Sacromeres. Signup and view all the answers

When an isolated muscle begins to contract isotonic against a lighter load, what is true of the tension?

Tension rises and remains constant during shortening. Signup and view all the answers

What causes the muscle to relax after a brief contraction?

The absence of an action potential. Signup and view all the answers

What is a characteristic of type I muscle fibers?

They have a high resistance to fatigue. Signup and view all the answers

Which type of muscle fibers are primarily involved in rapid and powerful movements?

Type II b fibers Signup and view all the answers

What adaptation is associated with muscles that maintain long posture?

Higher proportion of slow fibers Signup and view all the answers

Which statement about muscle fibers is accurate?

Aging leads to a loss of fast fibers. Signup and view all the answers

What type of muscle fibers are found in muscles specialized for fine skilled movements?

Predominantly fast fibers Signup and view all the answers

What is one function of the extensive sarcoplasmic reticulum in fast fibers?

To facilitate rapid release of calcium ions Signup and view all the answers

Which motor unit characteristic is associated with muscles that perform fine movements?

Motor units have very few muscle fibers. Signup and view all the answers

How do muscle groups with a high percentage of fast fibers differ in performance?

They exert more force and greater velocity. Signup and view all the answers

What is a common feature of type II b muscle fibers?

Rapid energy release through glycolysis Signup and view all the answers

Which of the following statements about muscle aging is true?

Aging correlates with more slow fibers relative to fast fibers. Signup and view all the answers

Study Notes

Physiology of Muscle

Muscles are tissues that shorten and develop tension, leading to movement.
Muscles are categorized into:
- Striated muscles (skeletal and cardiac), characterized by alternating light and dark bands.
- Smooth muscles, lacking distinguishing surface features.
Skeletal muscles make up approximately 40% of the body, smooth muscles 10%.

Muscle Cells

Skeletal, cardiac, and smooth muscles are depicted in Fig 35.

Skeletal Muscles

Skeletal muscles are connected to bones, over 400 in the human body.
Their contraction is controlled by nerve supply.
Major functions of skeletal muscles:
- Force production for locomotion and breathing.
- Force generation for posture maintenance and stabilizing joints.
- Heat production
- Assisting in venous return.

Morphology of Skeletal Muscle

Skeletal muscle is made of muscle fibers bundled by connective tissues, arranged in parallel.
Each muscle fiber (a single cell) is encased by a sarcolemma.
Myofibrils, composed of thick myosin and thin actin filaments, comprise muscle fibers.
The arrangement of these protein filaments creates the striated appearance.
Sarcomeres are functional units within the myofibrils. They are divided by transverse protein sheets called Z lines.

Muscle Protein: Myosin

Myosin is a protein comprised of two heavy chains and four light chains.
The terminal portions of the heavy chains, along with light chains, form cross-bridges.
Cross-bridges have hinge points for flexibility.

Muscle Protein: Actin

Actin is a protein formed by two coiled chains.
Tropomyosin molecules cover the active sites on actin at rest.
Troponin molecules are globular proteins that attach tropomyosin to actin.
Troponin is comprised of three components: Troponin I (affiinity for actin), Troponin T (affinity for tropomyosin) and Troponin C (affinity for Ca²⁺).
Ca²⁺ binding to troponin C initiates the contraction process.

Neuromuscular Transmission

Definition: Transmission of nerve impulses from alpha motor neurons to skeletal muscle fibers.
Physiologic anatomy of neuromuscular junction: the alpha motor neuron branches as it approaches the muscle, sending axon terminals to several skeletal muscle fibers.
Acetylcholine (Ach): transmits impulses across the synaptic cleft, the space between axon terminals & the muscle membrane.
Motor-end plate (MEP): the muscle membrane under the nerve terminals is thickened with numerous receptors.
Synaptic cleft: the extracellular space between the nerve terminals and the muscle membrane, containing basal lamina with acetylcholinesterase.

Sequence of Events during Neuromuscular Transmission

Arrival of nerve impulses at nerve endings opens voltage-gated Ca²⁺ channels.
Ca²⁺ entering the nerve endings triggers the release of acetylcholine into the synaptic cleft.
Acetylcholine binds to its receptors on the motor end plate (MEP), initiating sodium influx and depolarization, creating the end-plate potential (EPP).
EPP triggers action potentials in the muscle fiber, leading to muscle contraction.
Acetylcholinesterase in the synaptic cleft hydrolyzes acetylcholine, preventing multiple muscle contractions.
New vesicles are formed from presynaptic membrane invaginations to be refilled with acetylcholine for future use.

Properties of Neuromuscular Transmission

Unidirectional: it travels only from nerve to muscle.
Delay of about 0.5 milliseconds (msec): represents time for acetylcholine release, membrane permeability change, sodium influx and depolarization to the firing level.
Fatigued easily: due to repeated stimulation and exhaustion of acetylcholine vesicles.
Effect of ions (Ca²⁺ and Mg²⁺): Ca²⁺ entry into end feet triggers acetylcholine release, while Mg²⁺ competes with Ca²⁺ reducing acetylcholine release.

Effect of Drugs on Neuromuscular Transmission

Some drugs stimulate neuromuscular transmission like methacholine, carbachol, and nicotine (in small doses).
Others (neostigmine, physostigmine, di-isopropyl fluorophosphate) block acetylcholinesterase, allowing acetylcholine to accumulate and excessively stimulate the muscle.
Curare-like drugs block neuromuscular transmission by competing with acetylcholine for receptor sites.

Myasthenia Gravis

Autoimmune disease causing skeletal muscle weakness and fatigue.
Antibodies against acetylcholine receptors impair neuromuscular transmission.
Treatment involves anticholinesterase drugs to increase acetylcholine levels.

Miniature End-Plate Potential

At rest, spontaneous rupture of few vesicles containing acetylcholine occurs at the motor end plate, producing a minute depolarization.

Changes Following Skeletal Muscle Stimulation

Electrical events in skeletal muscle are similar to nerve but with slight differences. The resting membrane potential is −90 mV, and action potentials last 2-4 milliseconds.
Action potentials precede contraction by about 2 milliseconds.
Excitability changes during action potentials temporarily make the muscle refractory to restimulation until excitability has been regained.

Mechanical Changes Following Skeletal Muscle Stimulation:

Molecular mechanism of muscle contraction (excitation-contraction coupling): Action potentials initiate the contractile process.

Release of Calcium

Action potential propagation into T tubules opens Ca²⁺ channels in terminal cisternae.
Ca²⁺ flows into the cytoplasm.

Activation of Muscle Proteins

Ca²⁺ binds to troponin C on actin, which induces a conformational change in tropomyosin, exposing myosin-binding sites on actin.
Myosin cross-bridges attach to actin, leading to sliding filaments and contraction.

Generation of Tension

Muscle contraction generates tension.
Binding of actin and myosin is followed by bending of cross-bridges and sliding of actin filament past myosin.
ATP hydrolysis provides energy for this process.
- Detachment of cross-bridges from actin happens when new ATP molecules bind to myosin.

Relaxation

Removal of Ca²⁺ from the cytoplasm by the Ca²⁺ pump in the sarcoplasmic reticulum.
Troponin returns to its normal shape.
Tropomyosin resumes blocking the myosin-binding sites on actin and cross-bridge cyclin stops as a result, the muscle relaxes.

All-or-none and Twitch Relationship

The all-or-none law states that a single muscle fiber contracts maximally or not at all upon stimulation.
A twitch is a brief contraction followed by relaxation resulting from a single action potential.
The twitch begins about 2 milliseconds following membrane depolarization.

Types of Skeletal Muscle Contractions

Isometric contractions: generate tension but there is no change in muscle length (e.g., holding a heavy weight).
Isotonic contractions: cause a change in muscle length while the tension stays constant (e.g., lifting a weight).

Factors Affecting Skeletal Muscle Contraction

Muscle types: Type I (slow-twitch) and type II (fast-twitch) fibers have varied properties which affect how they perform.
Stimulus factors(grading of muscle contraction): the frequency and strength of stimulus affects the intensity of contraction.

Smooth Muscles

Smooth muscles function involuntary in organs like blood vessels, viscera, and other internal passageways regulating material movement.
There are two types of smooth muscle: single-unit smooth muscle and multi-unit smooth muscle.
The characteristics of smooth muscle differ from skeletal muscle structure. For example, smooth muscle lacks striations (or sarcomeres).

Structure of Smooth Muscle

Filaments are arranged without sarcomeres and lack striations (or sarcomeres).
The sarcoplasmic reticulum is poorly developed (compared to skeletal muscle) Smooth muscle cells have relatively few voltage-gated calcium channels.
No troponin; instead, calmodulin protein is present.

Electrical Activity of Smooth Muscle

Membrane potentials in smooth muscle are unstable, averaging -50 to -60 mV with fluctuations (slow waves).
Action potentials can occur in two forms: spike potentials (similar to the form in skeletal muscle) and action potentials with plateaus (prolonged depolarization phase used for prolonged muscle contraction).
Depolarization primarily results from changes in intracellular Ca²⁺ concentrations, which is slower compared to skeletal/cardiac muscle activity.

Role of Calcium in smooth muscle Action Potentials

Ca⁺² influx is usually responsible for the smooth muscle action potential.
Ca²⁺ binds to calmodulin, which activates myosin light-chain kinase (MLCK).
MLCK phosphorylates myosin and this results in ATP hydrolysis and cross-bridge cycling, triggering muscle contraction.
Relaxation occurs when intracellular Ca²⁺ concentration decreases, allowing for dephosphorylation of myosin and ending the cross-bridge cycle.

Role of Nerve Supply in Smooth Muscle

Smooth muscle has a dual autonomic nerve supply.
The nerve supply does not initiate smooth muscle activity but rather modulates it through influences on spontaneous activity and sensitivity to chemical agents.

Factors Affecting Contraction of Smooth Muscles

Stretch
Local factors (e.g., acids, alkalis, pH fluctuations, oxygen availability)
Temperature influences contraction
Chemical mediators.

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Test your knowledge on muscle physiology and functions with this quiz. Explore various aspects such as muscular hypertrophy, types of muscles, and electrical activity in muscles. Challenge yourself with specific questions related to muscle fibers and their components.