Muscle Physiology Quiz

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Questions and Answers

What is the functional unit of a muscle fiber?

Myosin

Myofiber

Fascicle

Sarcomere (correct)

Which of the following protein is responsible for contracting muscles?

Myosin (correct)

Troponin

Actin

Tropomyosin

What is the role of the regulatory proteins in the sarcomere?

Provide structure and support to the sarcomere.

Act as a catalyst for the breakdown of ATP.

Control the interaction between actin and myosin. (correct)

Transport calcium ions within the sarcomere.

What is the H zone in the sarcomere?

The central region of the A band where only thick filaments are present. (A) Signup and view all the answers

Which of the following is NOT a characteristic of the light band (I band) of the sarcomere?

Contains both thick and thin filaments. (A) Signup and view all the answers

Why is the A band of the sarcomere considered "dark" when viewed under a light microscope?

It contains both thick and thin filaments, which overlap. (D) Signup and view all the answers

Which of the following statements BEST describes the relationship between the sarcomere and the muscle fiber?

A muscle fiber is composed of many sarcomeres arranged in series. (A) Signup and view all the answers

What is the function of the Z line in the sarcomere?

Anchors the thin filaments to the ends of the sarcomere. (A) Signup and view all the answers

What happens to the H zone during muscle contraction?

It shrinks. (B) Signup and view all the answers

During muscle relaxation, which ion is actively pumped into the sarcoplasmic reticulum?

Calcium (Ca2+) (A) Signup and view all the answers

What is the role of troponin and tropomyosin in the resting state of muscle contraction?

They block the binding sites on actin, preventing myosin from attaching. (A) Signup and view all the answers

What is the direct consequence of a muscle cell being excited by a nerve impulse?

Calcium release from the sarcoplasmic reticulum. (C) Signup and view all the answers

What is the role of the T-tubules in muscle contraction?

They conduct the action potential deep into the muscle fiber. (D) Signup and view all the answers

Which of the following is NOT a characteristic of the dihydropyridine receptor (DHPR)?

It directly binds to actin filaments. (C) Signup and view all the answers

What is the primary function of the ryanodine receptor (RyR) in muscle contraction?

To release calcium from the sarcoplasmic reticulum. (C) Signup and view all the answers

What is the role of ATP in the cross-bridge cycling process?

All of the above. (D) Signup and view all the answers

Which of the following events occurs directly after calcium binds to troponin?

Tropomyosin is moved away from the actin binding sites. (A) Signup and view all the answers

What is the primary reason for the loss of tension during muscle relaxation?

The removal of calcium from the sarcoplasm. (C) Signup and view all the answers

Choose the event that directly triggers the power stroke in muscle contraction.

The binding of the energized myosin head to actin. (D) Signup and view all the answers

What is the primary function of smooth muscle?

Propulsion of contents through hollow organs (D) Signup and view all the answers

Which type of muscle is responsible for the movement of food through the digestive system?

Smooth muscle (C) Signup and view all the answers

What is the main characteristic of skeletal muscle that makes it suitable for voluntary movement?

It's attached to bones and controlled by the nervous system (D) Signup and view all the answers

What feature makes smooth muscle different from skeletal muscle?

Both A and C are correct. (A) Signup and view all the answers

Which type of muscle is responsible for the beating of the heart?

Cardiac muscle (B) Signup and view all the answers

Which of the following agents directly affects the release of acetylcholine?

Clostridium botulinum toxin (A) Signup and view all the answers

The toxin produced by Clostridium botulinum causes which of the following effects?

Prevention of acetylcholine release (A) Signup and view all the answers

Which of the following agents irreversibly inhibits acetylcholinesterase?

Organophosphates (D) Signup and view all the answers

What is the primary cause of respiratory failure in individuals affected by organophosphate poisoning?

Inability of the diaphragm to relax (C) Signup and view all the answers

Which of the following processes is NOT involved in muscle relaxation?

Continuous ATP production (B) Signup and view all the answers

What is the mechanism by which curare causes muscle paralysis?

Binding to acetylcholine receptors (D) Signup and view all the answers

Which of the following statements about myasthenia gravis is TRUE?

It involves antibodies that inactivate acetylcholine receptors (A) Signup and view all the answers

What is the role of acetylcholinesterase in neuromuscular transmission?

Breaking down acetylcholine in the synaptic cleft (D) Signup and view all the answers

What are the similarities between the neuromuscular junction (NMJ) and a synapse?

Both rely on the movement of ions through channels to create a graded potential. (B), Both are activated by calcium ions, which trigger the release of neurotransmitters. (C), Both involve the release of neurotransmitters that bind to receptors on the postsynaptic cell. (E) Signup and view all the answers

Which of the following conditions would be most likely to result in muscle spasms and paralysis?

Organophosphate poisoning (A) Signup and view all the answers

What is the difference between an excitatory postsynaptic potential (EPSP) and an excitatory postsynaptic potential (EPP)?

An EPSP is a graded potential that can lead to an action potential, while an EPP is always an action potential. (D) Signup and view all the answers

What is the primary function of the dihydropyridine (DHP) receptors in muscle contraction?

Acting as voltage sensors (C) Signup and view all the answers

Which of the following is NOT a similarity between the NMJ and a synapse?

Both are always excitatory. (B) Signup and view all the answers

Which of the following statements BEST describes the mechanism of action of Botox®?

It blocks the release of acetylcholine (D) Signup and view all the answers

Which of these describes the immediate energy source for muscle contraction?

Breakdown of creatine phosphate. (A) Signup and view all the answers

Which energy system is responsible for a fraction of the energy needed for sustained muscle activity?

Nonoxidative energy system (C) Signup and view all the answers

Which of the following events occurs FIRST in the process of muscle relaxation?

Acetylcholine is broken down by acetylcholinesterase (B) Signup and view all the answers

What is the main difference in the energy production between the immediate and the nonoxidative energy systems?

Nonoxidative energy system produces lactic acid, while the immediate energy system does not. (D) Signup and view all the answers

Which of the following conditions is characterized by an inability to relax muscles after contraction?

Rigor mortis (B) Signup and view all the answers

Which of the following statements is TRUE regarding the nonoxidative energy system?

It is less efficient than oxidative metabolism. (B) Signup and view all the answers

What is the primary function of the sarcolemma in muscle contraction?

Transmitting the action potential (A) Signup and view all the answers

Identify the enzyme responsible for creatine phosphate's conversion to ATP.

Creatine Kinase (F) Signup and view all the answers

How does the release of calcium from the sarcoplasmic reticulum lead to muscle contraction?

It binds to actin, exposing the myosin binding sites (D) Signup and view all the answers

Which of the following statements about the neuromuscular junction is TRUE?

All of the above (D) Signup and view all the answers

Which type of muscle fiber is primarily responsible for powerful, coarse movements?

Type IIx (C) Signup and view all the answers

What is the primary factor responsible for the initial gains in strength during a strength training program?

Improved neural recruitment patterns (C) Signup and view all the answers

What is the relationship between the size of a motor unit and the force it generates?

Smaller motor units generate less force (A) Signup and view all the answers

Which of the following statements correctly describes the size principle of motor neuron recruitment?

The CNS recruits smaller motor units first, followed by larger units as intensity increases. (A) Signup and view all the answers

Which of the following muscle fiber types has the highest oxidative capacity?

Type I (D) Signup and view all the answers

Which of the following best describes the function of myoglobin in muscle tissue?

Myoglobin aids in the transport of oxygen from the lungs to the muscles. (A) Signup and view all the answers

Which of the following is NOT a characteristic of Type II fibers?

High mitochondrial density (A) Signup and view all the answers

What is the primary event that triggers muscle hypertrophy?

Increased protein synthesis within muscle fibers (B) Signup and view all the answers

Which of the following scenarios would illustrate the concept of motor unit recruitment?

All of the above (D) Signup and view all the answers

Which of the following is the most accurate description of a motor unit?

A single motor neuron and all of the muscle fibers it innervates (C) Signup and view all the answers

Study Notes

Skeletal Muscle Thought Questions Answers

1. h. Actin
1. e. Myosin
1. b. T-tubule
1. b. T-tubule
1. d. Sarcoplasmic reticulum
1. f. Troponin
1. g. Tropomyosin
1. c. ATP

Muscle

Almost all cells have intracellular machinery for movement
The contraction specialists of the body are muscle cells
Skeletal
Cardiac
Smooth Muscle Cells
Skeletal muscle fibers have a highly developed ability to contract, develop tension, and do work.

Contraction of Muscle Allows

Purposeful movement of the body in relation to the environment
Manipulation of external objects
Propulsion of contents through hollow organs
Emptying the contents of organs to the environment

Muscle Composition

Muscle comprises the largest group of tissues in the body
~ 40% body weight in men, ~32% in women
Skeletal muscle: 660 in the adult human
Smooth and Heart = 10%

Muscle Classification

Functional vs Structural
Skeletal Muscle - Voluntary
Cardiac Muscle - Involuntary
Smooth Muscle - Involuntary

Organization of the Nervous System

The nervous system is comprised of the central nervous system (CNS) and the peripheral nervous system (PNS)
The CNS includes the brain and spinal cord.
The PNS consists of all the nerve fibers branching out from the CNS, connecting it to the rest of the body, transmitting sensory stimuli.
Different divisions branch out from the CNS and PNS to control different functions, and effector organs will carry out orders.

Levels of Organization in Skeletal Muscle

Whole muscle = an organ
Muscle fiber = a cell
Relatively large (up to 2.5 ft)
Multinucleated to maintain high protein production
Myofibril = intracellular structure
Thick and thin filaments = myofilaments
Myosin and actin = contractile proteins

Contractile Proteins

Thick filaments: several hundred myosin (heads and tails)

Thin Filaments

Mostly actin (helix)
Regulatory proteins: tropomyosin and troponin

Changes in Banding Pattern During Shortening

Light microscope: light and dark bands
Relaxed: H zone, I band, A band
Contracted: H zone shorter, I band shorter, A band same width
Sarcomere shorter

Medical Mnemonic

Muscle sarcomere: Only one vowel in dark and light (dark = A band, light = I band)
Muscle sarcomere bands: "Zee Intelligent Animal Has Muscle" (ZIAHM) --> From the Z line working inward (ZIAHM)

Questions

Which of the following is the light band of the sarcomere?
I band
Which of the following remains the same width during contraction?
A band
Which of the following describes a sarcomere?
All of the above

Molecular Basis of Muscle Contraction

Resting State: Ca2+ absent in sarcoplasm; SERCA actively pumps Ca2+ into the SR; no attachment between actin & myosin filaments; muscle cell is relaxed

Excitation-Contraction Coupling

Excitation of Muscle Cell: Propagated action potential; Acetylcholine is released in the cleft; Acetylcholine binds to receptors on the motor end plate; cation channels open; Na+ moves in EPP
The Result of Excitation: Specialized membranes take AP from the surface to the center of the cell; SR = modified ER, consists of interconnecting tubules surrounding each myofibril. T-tubule runs perpendicular to surface.

Excitation-Contraction Coupling: Spread of AP down T-tubules, activates receptors, DHP receptor, voltage-gated Ca2+ channel.

Excitation-Contraction Coupling: Steric Block Model

Ca2+ binds to troponin
Troponin changes its shape
Troponin-tropomyosin complex is physically pulled aside
Actin's binding sites uncovered

Excitation-Contraction Coupling: Cross-bridge Cycling

Before X-bridge ever links: ATP hydrolyzed; ATP and P remain attached to myosin; Energy stored in X-bridge (“cocked” and ready to be fired). Steps: Energized, Resting, Binding, Bending, Detachment

Excitation-Contraction Coupling – Cross-bridge Cycling (Continued)

Binding of fresh ATP breaks linkage between actin and myosin

Which is responsible for removing the steric inhibition for the act of contraction?

Ca2+

Which is responsible for detachment?

Excitation-Contraction Coupling – Return to Resting State

Neural excitation stops: - Acetylcholine released by axon of motor neuron crosses cleft and binds to receptors on motor end plate
Previously released ACh is broken down by AChase
SERCA pumps Ca2+ back
Dihydropyridine channels close; diffusion of Ca2+ out of SR stops
Actin's binding sites are covered; actin slides back to relaxed position away from center of sarcomere

All of the following result in muscle relaxation EXCEPT:

No more ATP or Rigor Mortis

Agents & Diseases that Affect the NMJ

Alters the Release of ACh: Black widow spider venom, Clostridium botulinum toxin
Blocks ACh Receptor: Curare, Myasthenia Gravis
Prevents Inactivation of ACh: Organophosphates (certain pesticides and nerve gases)

Synapse vs NMJ

Similarities: 2 excitable cells separated by a narrow cleft; axon terminals store NT released by Ca2+ induced exocytosis; NT binds to receptors on the membrane, which open channels
Differences: Synapse – junction between 2 neurons, NMJ – junction between a motor neuron and a skeletal muscle fiber; NMJ is always excitatory (EPP); Synapse is excitatory (EPSP) or inhibitory (IPSP)

Skeletal Muscle has 3 Energy Systems

1. Immediate: ATP and CP, available immediately to drive muscle contraction
1. Nonoxidative: breakdown of glucose and glycogen
1. Oxidative: breakdown of carbohydrates, fats, & certain amino acids

Muscle Fiber Types

Based on: Speed of contraction (myosin ATPase activity), Type of metabolic pathway
Type I: slow twitch, slow oxidative (ST), SO
Type IIa: fast twitch, fast oxidative/glycolytic (FOG), FTa
Type IIx: fast twitch, fast glycolytic (FG), FTx

Type I vs Type II

Type I: Slow twitch, oxidative metabolism, ~50% of fibers, used for endurance-type activities, Soleus muscle
Type II: Fast twitch, Type IIa: moderate high ox capacity, high glycolytic capacity, ~25% fibers, used for higher intensity movements
Type IIx: low ox capacity, highest glycolytic capacity ~25% fibers, used for explosive movements

Motor Units

Motor unit = 1 motor neuron + all the muscle fibers it innervates
When a motor neuron is activated, all fibers it supplies contract simultaneously
For stronger contractions, more motor units are recruited

Motor Unit Recruitment

Method for altering force production: Less force production - smaller motor units (type I); More force production - larger motor units (type II)

Size Principle of Motor Neuron Recruitment

The CNS increases muscle force by activating additional motor units in the order of their increasing size, starting with the smallest. Recruitment order: type I, type Ila, type IIx

Increased use: strength training

Neural adaptations must take place before changes in muscle size
Early gains in strength due to neural factors. Later, increasing cross-sectional area (hypertrophy) becomes more important

Skeletal Muscle Mechanics: contraction of whole muscles

The force exerted by the same muscle can vary depending on the load.
Examples: piece of paper, book, 50lb weight

Two major factors determine gradation of whole muscle tension

The # of muscle fibers contracting within a muscle
- of motor units recruited; Size of the motor unit
The tension developed by each contracting fiber
- Frequency of stimulation; Length-tension relationship

Frequency of Stimulation

Twitch summation: fiber stimulated a second time before it relaxes = greater tension
Tetanus: fiber stimulated so rapidly no relaxation

Length of Muscle fiber

Length-tension relationship: The relationship between initial length and tension can be explained by the # of cross-bridges that can be formed during the contraction
Tension is greatest when the muscle is at a certain length; too short or too long cause less force, as the availability of overlapping myofilaments is less.

Muscle Contraction Types

Isometric: Muscle produces force but does not change length; Joint angle does not change
Isotonic: Muscle produces force and changes length; Joint movement produced

Isotonic Contraction Subtypes:

Concentric: Muscle shortens while producing force; Sarcomere shortens
Eccentric: Muscle lengthens while producing force; Cross-bridges form, but sarcomere lengthens

Smooth Muscle

Location: Walls of hollow organs (gallbladder, uterus, bladder); Tubes (GI tract, blood vessels)
Characteristics: tone (basal tension); contractions superimposed on tone; maintains shape and pushes contents along.
Contraction: Slower & longer; Generates comparable force using less ATP; Responds to variety of stimuli like nerves, hormones, stretch ; latch state is possible allowing prolonged contraction w/o input of ATP
Structure compared to skeletal muscle: lacks sarcomeres (no striations), troponin, and t-tubules; has dense bodies (analogous to Z lines), dense bodies anchor actin, held in place by intermediate filaments; tropomyosin, but role unclear; caveolae: indentations in sarcolemma, may act like T tubules, SR but not well developed.
Excitation-Contraction Coupling: RMP is relatively low (-50 to -60 mV), long AP = 10-50 ms (vs 2-3 ms in sk muscle); No voltage-gated Na+ channels
Ca2+ triggers contraction: Role of Ca2+: The state of the thick filaments(not thin filaments) are affected by Ca2+. Where the Ca2+ comes from: Depolarization leads to voltage-gated Ca2+ channel opening. NTs, hormones can also open Ca2+ channels; some SR release

Smooth Muscle Contraction Mechanism

Extracellular Ca2+ enters: Voltage-gated and ligand-gated channels; Stretch-activated channel
Chemical activation: Ca2+ binds to calmodulin, leading to activation of myosin light chain kinase (MLCK); MLCK phosphorylates the light chain of myosin; when myosin is phosphorylated, X-bridges can form and break repeatedly. Smooth: myosin is “off”; Skeletal: myosin always “on”

Smooth Muscle Relaxation Mechanism

Relaxation of smooth muscle is a result of removal of the contractile stimulus (↓ed [Ca2+]) or direct action of a substance that inhibits the contractile mechanism (↑ed myosin phosphatase activity)

Pharmacological Relaxation of Smooth Muscle

L-type Ca2+ channel blocker ↑ in cAMP or cGMP; cAMP (EPI via β₂) inhibits MLCK; cGMP (nitroglycerin) stimulates myosin phosphatase

Tonic Contraction

How can vascular smooth muscle (think aorta) endure 60 “insults” per minute & sustain BP w/o expending a lot of ATP?
Smooth muscle has a way for cross bridges to remain attached, cycle slowly, & consume less ATP

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Test your knowledge on muscle fiber functions and the sarcomere structure with this quiz. Explore key concepts like the role of proteins, the H zone, and the mechanisms of muscle contraction and relaxation. Perfect for biology students or anyone interested in human physiology.