Biol 224.3: Animal Locomotion - Lecture 13

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Questions and Answers

Which of the following is NOT a true statement about the sliding filament theory of muscle contraction?

  • Actin and myosin filaments shorten during muscle contraction. (correct)
  • The H zone shortens during muscle contraction.
  • The I band shortens during muscle contraction.
  • The A band remains the same width during muscle contraction.

According to the sliding filament theory, what is the direct consequence of the formation of a cross-bridge between actin and myosin?

  • Release of calcium ions from the sarcoplasmic reticulum.
  • Relaxation of the muscle fiber.
  • Shortening of the sarcomere. (correct)
  • Hydrolysis of ATP.

Which of the following is TRUE regarding the role of ATP in muscle contraction?

  • ATP is required for the synthesis of actin and myosin filaments.
  • ATP is not directly involved in muscle contraction.
  • ATP is used to detach myosin from actin after the power stroke. (correct)
  • ATP is required for the formation of the cross-bridge between actin and myosin.

What is the primary role of calcium ions (Ca++) in muscle contraction?

<p>Binding to troponin, which moves tropomyosin away from the myosin binding sites on actin. (B)</p> Signup and view all the answers

Which of the following describes the state of a muscle fiber when it lacks ATP?

<p>Muscle fiber is in a state of rigor mortis. (B)</p> Signup and view all the answers

What is the role of the dihydropyridine receptor (DHPR) in the process of muscle contraction?

<p>It binds to and activates the ryanodine receptor, causing calcium release from the sarcoplasmic reticulum. (B)</p> Signup and view all the answers

What is the function of the sarcoplasmic reticulum (SR) in muscle contraction?

<p>The SR acts as a storage site for calcium ions, which are released upon muscle fiber stimulation. (C)</p> Signup and view all the answers

How does the muscle action potential (MAP) initiate the release of calcium from the sarcoplasmic reticulum?

<p>The MAP causes a conformational change in the dihydropyridine receptor (DHPR), which then unblocks the ryanodine receptor (RyR). (C)</p> Signup and view all the answers

What is the role of ATP in muscle contraction?

<p>ATP is hydrolyzed to provide energy for the cross-bridge cycling between actin and myosin filaments. (B)</p> Signup and view all the answers

Which of the following statements accurately describes the relationship between the t-tubules and the sarcoplasmic reticulum?

<p>The t-tubules and sarcoplasmic reticulum are separate structures but are closely associated, allowing for efficient communication between them. (D)</p> Signup and view all the answers

How is the calcium concentration in the sarcoplasm maintained at a low level during muscle relaxation?

<p>Calcium is actively transported into the sarcoplasmic reticulum by a calcium-ATPase pump. (A)</p> Signup and view all the answers

What happens to the ryanodine receptor (RyR) when a muscle action potential arrives at the t-tubules?

<p>The RyR is activated by the muscle action potential, allowing calcium ions to flow out of the sarcoplasmic reticulum. (A)</p> Signup and view all the answers

How does the neuromuscular junction (NMJ) ensure that a single motor neuron can activate multiple muscle fibers?

<p>The motor neuron branches out at the NMJ, forming multiple synapses with different muscle fibers. (A)</p> Signup and view all the answers

What is the function of acetylcholine at the neuromuscular junction?

<p>Acetylcholine acts as a neurotransmitter, binding to receptors on the muscle fiber membrane and triggering the release of calcium from the sarcoplasmic reticulum. (A)</p> Signup and view all the answers

Why is it important to maintain a low calcium concentration in the cytoplasm of resting muscle fibers?

<p>A high calcium concentration would trigger spontaneous muscle contractions, leading to muscle cramps. (D)</p> Signup and view all the answers

What is rigor mortis primarily caused by?

<p>Lack of ATP (D)</p> Signup and view all the answers

What happens to the H zone during muscle contraction?

<p>It disappears (B)</p> Signup and view all the answers

How does the contraction of 200,000 sarcomeres affect muscle length?

<p>Muscle will shorten by 5 cm (B)</p> Signup and view all the answers

What role do reflex arcs play in muscle control?

<p>They operate automatically to maintain posture (A)</p> Signup and view all the answers

What is involved in the neural regulation of skeletal muscles?

<p>Reflex arcs and motor unit recruitment (D)</p> Signup and view all the answers

How does muscle force adjust through motor unit recruitment?

<p>By recruiting small and large motor units (A)</p> Signup and view all the answers

Which process involves crossbridge cycling for muscle contraction?

<p>Shortening of I bands and H zone (D)</p> Signup and view all the answers

What is the approximate length of a typical sarcomere?

<p>2.5 µm (A)</p> Signup and view all the answers

What does the sliding filament theory explain?

<p>How crossbridge formation leads to muscle shortening (C)</p> Signup and view all the answers

What is the relationship between motor unit size and muscle force?

<p>Recruiting more motor units increases muscle force (B)</p> Signup and view all the answers

Flashcards

Sliding Filament Theory

Muscle contraction occurs as filaments slide past each other, shortening the sarcomere.

Sarcomere

The contractile unit of muscle fibers where actin and myosin filaments are found.

Cross Bridge Binding

The interaction between actin and myosin that initiates muscle contraction.

Role of ATP

ATP is needed for the detachment of myosin from actin during muscle contraction and relaxation.

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Calcium's Influence

High levels of sarcoplasmic Ca++ trigger continuous binding and unbinding in muscle contractions.

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Neuromuscular Junction (NMJ)

A synapse where motor neuron meets muscle fiber.

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Acetylcholine

A neurotransmitter released at the NMJ causing depolarization.

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Depolarization

Change in membrane potential, leading to muscle action potential.

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Muscle Action Potential (AP)

Electrical signal that spreads along the muscle fiber membrane.

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T-tubules

Infoldings of muscle cell membrane that transmit APs deep into muscle fibers.

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Sarcoplasmic Reticulum (SR)

Organelle that stores calcium ions for muscle contraction.

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Calcium Release

Ca++ ions are released from SR into sarcoplasm upon muscle AP.

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Ryanodine Receptor (RyR)

A Ca++ channel in the SR that opens in response to AP.

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Dihydropyridine Receptor (DHPR)

A voltage-gated channel in t-tubules that interacts with RyR.

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Cross Bridge Cycling

Process where myosin heads bind to actin filaments, causing contraction.

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Rigor Mortis

Stiffening of muscles after death due to ATP depletion.

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Sarcomere Length

The length of a sarcomere is approximately 2.5 micrometers.

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Muscle Shortening

The distance a sarcomere shortens is around 0.25 micrometers.

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Motor Unit

A motor unit consists of one neuron and all muscle fibers it innervates.

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Tetanus

A sustained muscle contraction resulting from rapid stimulation.

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Reflex Arc

Pathway from sensory input through the CNS to motor output establishing reflexes.

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Antagonistic Pairs

Muscles that work in opposition to each other, e.g., biceps and triceps.

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Neural Regulation

Control of muscle contraction via nervous stimulation.

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Stretch Activation

Refers to the activation of muscles by the stretching stimulus.

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Study Notes

Animal Locomotion: Skeletal Muscles

  • Biol 224.3 - Animal Body Systems
  • Lecture 13
  • Dr. Joan Forder
  • Textbook (5th Edition), Chapter 43, pages 1181-1189

Neuromuscular Junction (NMJ)

  • Acetylcholine causes muscle fiber depolarization
  • Depolarization results in muscle action potential (AP)
  • The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber
  • Motor neuron axon terminal (presynaptic)
  • Muscle fiber (postsynaptic)
  • Acetylcholine-gated cation channels in motor end plate
  • Acetylcholine released into synapse
  • Example - Drosophila larva

Muscle Action Potential (AP)

  • Muscle AP is conducted to the interior of the muscle fiber along the membrane of t-tubules
  • T-tubules - similar to endoplasmic reticulum
  • Sarcolemma - plasma membrane of muscle fiber (continuous with sarcolemma)
  • Sarcoplasmic reticulum (SR) stores Ca++, keeping cytoplasmic Ca++ low
  • SR Ca++ high
  • Ca-ATPase pumps Ca++ from cytoplasm (sarcoplasm)
  • Ryanodine receptor (RyR) channels in sarcolemma - Ca++ channels
  • Dihydropyridine receptor (DHPR) voltage gated channels in t-tubule membrane, plug RyR

MAP from NMJ causes Ca++ Release into Sarcoplasm

  • Action potential in the neuromuscular junction stimulates release of acetylcholine
  • Acetylcholine diffuses across synapse and triggers AP in muscle fiber
  • AP moves across surface membrane and into muscle fiber
  • At the end of a T-tubule, action triggers Ca2+ release from SR into cytosol

Sliding Filament Theory of Muscle Contraction

  • Filaments slide past each other, shortening the sarcomere
  • Myosin stays the same length, doesn't move
  • Actin filaments move
  • Relaxed muscle - H zone, I band
  • Contracting muscle - H zone, I band shorten
  • Fully contracted muscle - H zone, I band practically disappear

Cross Bridge Binding

  • Sliding due to cross bridge binding between filaments (actin and myosin)
  • Actin and myosin are protein polymers with respective binding sites
  • Change in myosin shape after cross bridge formation moves filaments past each other
  • Ca2+ binds to troponin on actin filaments

ATP Required for Detachment

  • In the presence of high sarcoplasmic Ca++, the cycle of binding and unbinding continues
  • ATP is required to detach actin/myosin
  • No ATP (e.g., death) – filaments remain bound - rigor mortis
  • ATP also needed for Ca++ pump on sarcolemma

Crossbridge Cycling

  • Overall muscle contraction due to continual crossbridge cycling, plus formation of many crossbridges per sarcomere

Generating Force & Movement

  • Small molecular movements translate into overall muscle shortening
  • Sarcomere length ~2.5 μm
  • Distance shortened per sarcomere ~0.25 μm
  • ~40,000 sarcomeres may shorten, a muscle by 1.0 cm

Neural Regulation of Skeletal Muscles

  • Reflex Arc
  • Motor unit recruitment & tetanus
  • Stretch Activation

Reflex Arcs

  • Stretch receptors and motor neurons connect in central nervous system (CNS)
  • Reflex arcs operate automatically
  • Important in posture, coordinating limb movements
  • Integrated with conscious motor control by CNS

Motor Unit Recruitment & Tetanus

  • Adjusts muscle force
  • Motor unit = one neuron plus all muscle fibers it contacts
  • More motor units = larger movement
  • Recruitment of small motor units first, then large motor units

Tetanus

  • Multiple action potentials lead to tetanus (a form of summation)
  • Produces much more force than a twitch

Some muscles are stretch-activated

  • Asynchronous flight muscle
  • Smaller SR - more sarcomeres
  • Indirectly attached to wing

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Biol 224.3 - Animal Body Systems Lecture 13: Animal Locomotion: Skeletal Muscles PDF

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