Skeletal Muscle Structure and Function

Skeletal Muscle Structure

• A single skeletal muscle consists of many muscle fibers surrounded by layers of connective tissue: endomysium, perimysium, and epimysium.
• Each muscle fiber is enveloped by endomysium, grouped into bundles called fascicles surrounded by perimysium, and the entire muscle is enclosed by epimysium.
• Muscle fibers are long and striated due to the repetition of sarcomeres, the functional units of the muscle, found within myofibrils.

Muscle Fiber Properties

• Contractile: Muscle fibers can shorten in response to electrical stimulation.
• Elastic: They can recoil and return to their resting length after stretching.
• Excitable: They can receive and respond to stimuli.
• Extensible: They can stretch when not contracted.

Motor Units

• Motor neurons innervate groups of muscle fibers, forming a motor unit.
• An action potential in a single motor neuron activates all the fibers it innervates.
• Motor unit size varies: a single motor neuron can innervate a few (small motor unit) or thousands (large motor unit) of muscle fibers.
• Muscles often contain motor units of varying sizes, working together for coordinated movement.

Neuromuscular Transmission

• The motor neuron and muscle fiber are separated by a synaptic cleft.
• Signals cross the cleft at the neuromuscular junction (NMJ).
• The NMJ comprises the axon terminal, synaptic cleft, and motor endplate of the muscle fiber.

Muscle Fibers and Myofibrils

• Muscle fibers are enveloped by a sarcolemma (muscle membrane).
• The sarcolemma forms t-tubules, extending into the fiber's center.
• Depolarization of the motor endplate triggers an action potential that propagates along the sarcolemma and into the t-tubules.

Sarcomeres

• Sarcomeres, joined at their ends by Z lines, house actin (thin filaments) and myosin (thick filaments), which are tethered to the Z lines.
• Myosin overlaps with actin within the sarcomere.

Recruitment

• Progressive activation of motor units is called recruitment.
• Muscle contraction often involves hundreds of motor axons firing.
• Motor units are recruited based on their size: smaller units at lower stimulus strengths, larger units at higher strengths.
• Increasing recruitment enhances muscle contraction strength.

Frequency

• Frequency refers to the number of action potentials delivered to a muscle per unit time.
• At low frequencies, tension decreases to resting levels between stimulations due to Ca2+ release and uptake.
• Ca2+ is released with each stimulation, and the muscle relaxes when nerve stimulation ceases and Ca2+ is pumped back into the sarcoplasmic reticulum.
• The duration of an action potential is short compared to Ca2+ release and uptake, allowing for multiple action potentials during a contraction.
• Increasing frequency increases the muscle's tension, leading to tetanus.

Tetanus

• Tetanus is a state of sustained muscle contraction, caused by high stimulation frequency.
• When the frequency of stimulation is high enough, the tension in the muscle remains at its maximum level and the muscle does not relax.
• This is because the action potentials arrive so quickly that the muscle fibers do not have enough time to fully relax between contractions.
• Tetanus is important in sustaining muscle contractions needed for activities like holding a heavy object.

Muscle Fatigue

• Muscle fatigue is a decline in muscle force or ability to maintain force during sustained activity.
• The exact cause of fatigue is not fully understood, but it is likely due to a combination of factors, including:
  • Depletion of energy stores (e.g., glycogen)
  • Accumulation of metabolic byproducts (e.g., lactic acid)
  • Alterations in nerve conduction or muscle fiber excitability
• Fatigue can lead to a decrease in force production, reduced speed of contraction, and a slower rate of relaxation.
• It can also lead to pain and discomfort.

Muscle Adaptation

• Muscle fibers can adapt to changes in workload and activity levels.
• Regular exercise can lead to:
  • Increased muscle size (hypertrophy)
  • Improved muscle strength
  • Greater endurance
• Conversely, inactivity or reduced workload can lead to muscle atrophy, a decrease in muscle size and strength.
• Adaptations are due to changes in muscle protein synthesis, muscle fiber types, and energy generation capacity.

Skeletal Muscle Structure

• A single skeletal muscle contains many muscle fibers.
• Each muscle fiber is surrounded by endomysium (a layer of connective tissue).
• Muscle fibers are grouped into bundles called fascicles, each surrounded by perimysium.
• The entire muscle is surrounded by epimysium.
• Each muscle fiber is striated due to the presence of sarcomeres, which are the functional units of the muscle.
• Sarcomeres are found in myofibrils.

Muscle Fiber Properties

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

Motor Units

• Muscle fibers are innervated by motor neurons.
• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
• An action potential in a motor neuron activates all muscle fibers in the motor unit.
• Motor units vary in size, with some innervating only a few fibers (small motor unit) and others innervating thousands (large motor unit).

Neuromuscular Transmission

• The synapse between a motor neuron and a muscle fiber is called the neuromuscular junction (NMJ).
• The NMJ consists of the axon terminal, synaptic cleft, and motor endplate of the muscle fiber.
• Signals from the motor neuron are transmitted across the synaptic cleft to the muscle fiber, causing muscle contraction.

Muscle Fibers and Myofibrils

• Muscle fibers are surrounded by a sarcolemma (muscle membrane).
• T-tubules are invaginations of the sarcolemma that extend deep into the muscle fiber.
• Depolarization of the motor endplate initiates an action potential that travels along the sarcolemma and down the t-tubules.

Sarcomeres

• Sarcomeres are joined at their ends by Z lines, to which actin (thin filaments) and myosin (thick filaments) are attached.
• Myosin filaments overlap with actin filaments within the sarcomere.