Skeletal Muscle Structure and Function

• Pre-lecture material is designed to help prepare for the lecture and assist with note-taking, but it is not a substitute for the lecture.
• Efforts are made to ensure the pre-lecture material corresponds to the live lecture, but there may be differences or additions.
• Lecture 8 covers skeletal muscle structure and function.

Objectives

• Briefly describe the three main types of muscle in the body.
• Describe key elements of skeletal muscle structure at the tissue and cellular levels.
• Describe the sequence of events that occur during skeletal muscle excitation.
• The related reading material is Martini et al. Modules 9.2 (p. 358), 9.3 (p. 360), and 9.8 (p. 371).

Three Main Muscle Types

• Smooth muscles primarily line hollow organs and are not under voluntary control.
• Cardiac muscle is located only in the heart, generates force to pump blood, and is not under voluntary control.
• Skeletal muscle applies force to bones for posture and movement and is mostly under voluntary control.
• All three types are made up of cells called fibres and primarily generate force via contraction.
• The next two lectures are about skeletal muscle.

Skeletal Muscle

• Skeletal muscles are mostly under voluntary control.
• The primary function of skeletal muscle is to develop force.
• Muscles develop force by contracting (shortening).
• Skeletal Muscles are important for movement and posture.
• Skeletal Muscles support and protect soft internal organs, such as muscles of the abdominal wall.
• Skeletal Muscles provide voluntary control over major openings.
• Skeletal Muscles convert energy (in part) to heat, which is used to maintain core temperature (e.g., shivering).

Whole Muscle Structure

• Individual muscle cells are called muscle fibres.
• Muscle fibres gather into bundles called fascicles.
• Fascicles are further bundled into muscles.
• Fibres, fascicles, and muscles contain blood vessels and nerves, and are sheathed in connective tissue.
• Connective tissue gathers to form tendons, which connect muscles to bones.
• Muscle fibres also contain hundreds or thousands of nuclei.

Individual Fibres – Cellular Structure

• Muscle fibre comprises bundles of myofibrils, made of repeating units known as sarcomeres.
• Sarcomeres are made of contractile proteins or myofilaments.
  • Actin (thin filaments)
  • Myosin (thick filaments)
• Myofilaments organization gives muscle a striated (striped) appearance.

Cellular Structure

• Muscle fibres possess a system of structures organized to regulate the activity of the force-producing elements.
• The muscle fibre is lined by a cell membrane called the sarcolemma.
• Transverse tubules (T-tubules) are tube-like extensions (invaginations) of the sarcolemma.
• T-tubules conduct electrical signals (action potentials) deep into the fibre's core.
• Sarcoplasmic reticulum (SR) is an extensive membrane network associated with the T tubules at regular intervals.
• The SR takes up and stores calcium (Ca2+) while the muscle is relaxed and then releases calcium into the cytoplasm when the muscle contracts.

Excitation-Contraction Coupling (EC-Coupling)

• Pairing of a signaling event (excitation of the muscle cell) with a mechanical event (contraction of the muscle cell)
• Three key proteins participate in excitation:
  • Voltage-gated sensor (DHPR): Receives signals from the T-tubules and interacts with ryanodine receptors
  • Ryanodine receptor (RyR): A passive calcium channel on the SR that opens to allow Ca2+ out into the cell
  • SR Calcium-ATPase (SERCA): An active calcium pump that uses ATP to move Ca2+ back into the SR against its concentration gradient, ending the excitation.
• EC-Coupling is initiated when a signal is transmitted down the T-tubules.
• The signal is coming from the nerves running throughout the muscle.
• The voltage sensor (DHPR) is activated by the signal moving through the T-tubule.
• The active DHPR interacts with RyR, causing them to open.
• RyR is a passive calcium channel, and once it opens, calcium starts pouring out of the SR and into the cell.
• That calcium then diffuses throughout the cell, activating the myofilaments and causing a contraction.
• SERCA senses the rise in intracellular calcium.
• SERCA burns some ATP to produce energy and uses that energy to pump calcium back into the SR, against the concentration gradient.
• This removes the calcium from the cell, stopping the signaling event and allowing the muscle to relax.

Summary

• Muscle types are smooth, cardiac, and skeletal, with many similarities but a few key differences.
• Skeletal muscle fibres are organized into fascicles and muscles at the tissue level, associated with nerves, vessels, and connective tissue.
• Skeletal muscle fibres are organized into sarcomeres comprised of myofilaments at the cellular level, giving the muscle a striated appearance.
• They also have critical signaling components, including the T-tubules and the sarcoplasmic reticulum.
• Excitation-contraction coupling pairs a signalling event with a mechanical event.
• Three key proteins contribute to excitation signalling (DHPR, RyR, and SERCA).