Skeletal Muscle Functions and Anatomy

Functions of Skeletal Muscles

• Production of movement occurs when the muscle pulls tendons to move the skeleton.
• Maintenance of posture and body position is achieved through continuous muscle contraction.
• Soft tissues are supported by bearing the weight of visceral organs.
• Guarding entrances and exits is done by encircling openings to the digestive and urinary tracts, which controls swallowing, defecation, and urination.
• Body temperature is maintained because energy from contraction is converted to heat.

Gross Anatomy of Skeletal Muscles

• Epimysium is a deep fascia that surrounds muscles
• Bundles of muscle fibers are separated into fascicles
• Endomysium surrounds individual muscle fibers and binds adjacent fibers together
• All three layers converge to attach the muscle to bone via a tendon.
• Perimysium divides muscles into compartments, each containing a bundle of muscle fibers called a fascicle.

Blood Vessels and Nerves

• Contractions require energy which ATP provides
• Blood vessels provide oxygen and nutrients to produce ATP
• Muscle contractions under the stimulation of the central nervous system.
• This process is under voluntary control.
• Axons from the CNS connect to individual muscle fibers.

Sarcolemma and T-Tubules

• Cells are very large and contain hundreds of nuclei.
• The cell membrane has pores that are open to T-tubules.
• T-tubules form a network of narrow tubules with extracellular fluid that creates passageways through the muscle fiber.

Myofibrils

• These are cylindrical structures as long as the entire muscle fiber
• Each fiber contains hundreds to thousands of myofibrils
• Responsible for enabling muscular contraction
• When myofibrils contract, the entire cell contracts.
• Myofibrils consist of proteins, which include actin (thin filaments) and myosin (thick filaments).

Sarcoplasmic Reticulum

• Specialized form of smooth endoplasmic reticulum (SER).
• Tubular network surrounds each myofibril.
• It is in contact with T-tubules.
• Cisternae are expanded chambers of the SR and store calcium.

Sarcomere

• It is the smallest functional unit of a muscle fiber.
• Each myofibril contains approximately 10,000 sarcomeres arranged end to end.
• Interactions between thick and thin filaments cause contraction, resulting in a banded appearance.

Thick and Thin Filaments

• Thin filaments consist of twisted actin molecules, each with an active site that interacts with myosin.
• In a resting state, the active site is covered by tropomyosin, which is held in place by troponin.
• Thick filaments are made of myosin.
• The myosin head attaches to actin, facilitating contraction.
• This only occurs when troponin changes position, which moves tropomyosin to expose the active site on actin.

Sliding Filaments and Cross Bridges

• Sarcomere contraction follows the sliding filament theory.
• Thin filaments slide toward the center of the sarcomere.
• Thick filaments are stationary.
• The myosin head attaches to the active site on actin, forming a cross bridge.
• Actin is pulled toward the center and then detaches.

Control of Muscle Fiber Contraction

• Muscle fiber contraction is controlled by the nervous system.

Neuromuscular Junction

• The neuromuscular junction links the nervous system and muscles.
• Motor neurons control skeletal muscle fibers through the synaptic terminal.
• Acetylcholine (Ach), a chemical released by the neuron, communicates with other cells.
• Acetylcholine triggers changes in the sarcolemma, initiating muscle contraction.

Action Potential and Neurotransmission

• Action potential travels down the motor neuron to the axon.
• Acetylcholine(Ach) is released into the synaptic cleft.
• Ach diffuses across the synaptic cleft and binds to Ach receptors on the sarcolemma.
• This Ach binding changes the permeability of sarcolemma to sodium.
• A rush of sodium into the sarcolemma which causes an action potential.
• This action potential spreads over sarcolemma and down the t-tubules to cisternae.
• The cisternae release large amounts of calcium.
• An increase in calcium causes the sarcomeres to contract.
• Acetylcholine is then broken down by acetylcholinesterase (AchE).

Contraction Cycle

• Resting sarcomere has ADP + P attached to the myosin head, storing energy.
• Step 1: Calcium ions bind to troponin, exposing the active site on actin.
• Step 2: The myosin head attaches to actin.
• Step 3: The cross bridge pulls toward the center of the sarcomere, releasing ADP + P (energy is used).
• Step 4: The myosin head binds another ATP, causing detachment of the cross bridge.
• Step 5: ATP is converted to ADP + P, reactivating the myosin head.