Muscle Anatomy and Physiology

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Questions and Answers

What happens to muscle tension at low stimulation frequencies?

Muscle tension only changes when nerve stimulation stops.

Muscle tension continuously increases with each stimulus.

Muscle tension decreases back to a resting level between stimulations. (correct)

Muscle tension remains constant without fluctuations.

How does increasing the frequency of action potentials affect Ca2+ levels?

Ca2+ levels remain unchanged regardless of frequency.

Ca2+ is completely eliminated from the sarcoplasm when frequency increases.

Ca2+ levels decrease, leading to weaker muscle contractions.

Ca2+ levels rise, allowing for more crossbridge formation. (correct)

What is the primary function of the sarcoplasmic reticulum (SR) during muscle relaxation?

To transport Ca2+ ions into the sarcoplasm for contraction.

To generate action potentials that sustain muscle contraction.

To release neurotransmitters into the muscle.

To pump Ca2+ back into the SR and clear it from the sarcoplasm. (correct)

What occurs when many action potentials are fired in quick succession?

The level of Ca2+ increases, enhancing contraction strength. Signup and view all the answers

What role does frequency of stimulation play in muscular contraction?

Higher frequency increases both the strength and duration of contraction. Signup and view all the answers

What do large motor units primarily consist of?

A single motor neuron innervating thousands of muscle fibers Signup and view all the answers

What is the space between the motor neuron and muscle fiber called?

Synaptic cleft Signup and view all the answers

During neuromuscular transmission, what is required to elicit a muscle response?

Signals from the motor neuron to cross the synaptic cleft Signup and view all the answers

What initiates an action potential in a muscle fiber?

Local depolarization of the motor endplate Signup and view all the answers

What structure stores calcium ions (Ca2+) necessary for muscle contraction?

Sarcoplasmic reticulum Signup and view all the answers

What role do T-tubules play in muscle fibers?

They facilitate the electrical impulse to reach deeper muscle fiber areas Signup and view all the answers

How do motor units in a muscle coordinate movement?

Several motor units can work together for movement synchronization Signup and view all the answers

What is a characteristic of small motor units?

They allow for fine motor control Signup and view all the answers

What immediate result follows muscle depolarization?

Release of stored Ca2+ Signup and view all the answers

Which molecule does Ca2+ bind to during muscle contraction?

Troponin Signup and view all the answers

What surrounds each individual muscle fiber in skeletal muscle?

Endomysium Signup and view all the answers

What is the purpose of sarcomeres in skeletal muscle fibers?

They are the structural and functional units of the muscle. Signup and view all the answers

What is the role of ATP in the muscle contraction cycle?

It breaks the link between actin and myosin. Signup and view all the answers

What happens during the powerstroke phase of muscle contraction?

The myosin head rotates, pulling actin toward the sarcomere center. Signup and view all the answers

What is a motor unit comprised of?

Multiple muscle fibers and one motor neuron Signup and view all the answers

What occurs after ADP dissociates from the myosin head?

Myosin awaits another ATP molecule while still attached to actin. Signup and view all the answers

Which characteristic describes skeletal muscle's ability to stretch?

Extensible Signup and view all the answers

What describes excitation-contraction coupling?

The transition from electrical stimulus to mechanical response. Signup and view all the answers

What best defines the structure surrounding an entire skeletal muscle?

Epimysium Signup and view all the answers

Which of the following is NOT a property of skeletal muscle cells?

Inflexible Signup and view all the answers

Which step follows the hydrolysis of ATP in the contraction cycle?

Myosin head rotates to a 'cocked' position. Signup and view all the answers

What is the function of Pi dissociating from myosin?

It allows the binding of myosin to actin with high affinity. Signup and view all the answers

What is the primary role of electrical stimulation in muscle contraction?

To stimulate motor neuron activation Signup and view all the answers

How long can an individual skeletal muscle fiber potentially be?

Up to 30 cm Signup and view all the answers

What is required to initiate another crossbridge cycle between actin and myosin?

ATP molecule Signup and view all the answers

What happens to muscle contraction when the troponin-tropomyosin complex is activated?

Actin and myosin can no longer interact. Signup and view all the answers

During recruitment of motor units, which type of motor units is activated at lower stimulus strengths?

Smaller motor units Signup and view all the answers

What occurs when stimulus strength is increased during electrical stimulation of motor nerves?

Motor neurons begin to depolarize. Signup and view all the answers

What is created when all muscle fibers in a motor unit are activated?

Twitch Signup and view all the answers

What happens as more motor nerve fibers are excited?

The force of the twitch increases. Signup and view all the answers

What is it called when the maximum response is reached in muscle contraction?

Maximum response Signup and view all the answers

What happens when calcium ions are reuptaken by the sarcoplasmic reticulum?

Muscle relaxation occurs. Signup and view all the answers

Study Notes

Connective Tissue Layers

Endomysium: Surrounds each individual muscle fiber.
Perimysium: Surrounds bundles of muscle fibers called fascicles.
Epimysium: Surrounds the entire muscle.

Muscle Fiber Structure

Muscle fibers are up to 30 cm long and striated.
Striations arise from repeating units called sarcomeres, which are the functional units of the muscle.
Sarcomeres are found in myofibrils.

Muscle Cell Properties

Contractile: Muscle fibers can shorten in response to electrical stimulation.
Elastic: Can recoil and regain their resting length after stretching.
Excitable: Can receive and respond to stimuli.
Extensible: Can be stretched when not contracted.

Motor Units

A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
A single motor neuron can innervate a few muscle fibers (small motor unit) or thousands of muscle fibers (large motor unit).
Muscles often contain motor units of varying sizes, working together to coordinate movement.

Neuromuscular Transmission

The motor neuron and muscle fibers are separated by the synaptic cleft.
Signals travel across the cleft to elicit a muscle response.
Neuromuscular junction (NMJ): The specialized synapse between a motor neuron and a muscle fiber.
- Comprises the axon terminal, synaptic cleft, and motor endplate.

Muscle Fiber and Myofibril Structure

Sarcolemma: Muscle membrane surrounding each muscle fiber.
T-tubules: Invaginations of the sarcolemma that extend deep into the muscle fiber.
Sarcoplasmic reticulum (SR): Membrane-bound structure that stores Ca2+ in close association with t-tubules.

Excitation-Contraction Coupling

Depolarization of the motor endplate travels along the sarcolemma and down t-tubules, triggering the release of Ca2+ from the SR.
Ca2+ binds to troponin, causing a conformational change and shifting the troponin-tropomyosin complex to expose actin binding sites.
This allows actin and myosin to bind, forming crossbridges and initiating contraction.

Crossbridge Cycle

1. ATP binds: An ATP molecule attaches to the myosin head, detaching it from actin.
2. ATP hydrolysis: ATP is broken down into ADP and Pi, which remain attached to the myosin head. The myosin head rotates to a "cocked" position, aligning with a new actin binding site.
3. Pi dissociates: Pi leaves the myosin head, allowing it to bind with high affinity to actin and form a new crossbridge.
4. Powerstroke: The myosin neck rotates around the myosin head, pulling actin closer to the center of the sarcomere. The muscle shortens, generating force.
5. ADP dissociates: ADP leaves the myosin head, leaving it waiting for another ATP molecule to start the cycle again.

Sliding Filament Theory

The formation and breakdown of crossbridges allows actin filaments to slide over myosin, resulting in muscle shortening.
When nerve impulses stop, Ca2+ is reabsorbed by the SR, preventing actin and myosin interaction. Crossbridges detach, and the muscle relaxes.

Recruitment

Recruitment: The progressive activation of motor units during muscle contraction.
- Smaller motor units are recruited first with weaker stimuli.
- Larger motor units are recruited with stronger stimuli, increasing the strength of muscle contraction.

Frequency

Frequency: The number of action potentials delivered to a muscle within a set time period.
At low frequencies, the muscle relaxes between stimulations. Ca2+ is released with each stimulation.
Increasing frequency:
- More action potentials are fired during a muscle contraction.
- The level of Ca2+ in the sarcoplasm increases.
- More crossbridges form, leading to increased force and duration of muscle contraction.

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Description

Explore the structure of muscle tissue and its various connective layers such as endomysium, perimysium, and epimysium. Understand the properties of muscle fibers, including their contractile, elastic, excitable, and extensible characteristics. Learn about motor units and their role in muscle function.