Bio 17.1 Types of Muscles and Myocyte Structure

Types of Muscles

• Muscles are composed of cells specialized to produce force through the interaction of actin and myosin.
• Muscle cells are called myocytes.
• There are three types of muscles:
  • Skeletal muscle - attached to bones, responsible for movement.
  • Cardiac muscle - found in the heart, responsible for pumping blood.
  • Smooth muscle - found in the walls of internal organs, responsible for involuntary movements.

Myocyte Structure

• Myocytes are specialized for force production.
• Force production is dependent on the concentration of calcium ions (Ca2+).
• Myocytes are nucleated.
• The plasma membrane of myocytes is called the sarcolemma.
• Myocytes have a specialized type of endoplasmic reticulum called the sarcoplasmic reticulum.
• Myocyte cytoplasm is called sarcoplasm.

Actin and Myosin

• Actin and myosin interact to produce force.
• Actin molecules form thin filaments.
• Myosin molecules form thick filaments.
• Myosin filaments contain a rod-like central region with heads that extend outwards.
• During muscle contraction, myosin heads bind to and pull on actin filaments.
• This sliding movement shortens the muscle cell, but the length of the actin and myosin filaments do not change.

Striated Muscle

• Striated muscles have actin and myosin filaments arranged parallel to one another, creating visible striations under a microscope.
• Striated muscle cells are long cylindrical fibers.
• Force production is greatest when actin-myosin overlap is maximal.
• Striated muscles have repeated functional units along the length of myofibrils called sarcomeres.
• Each sarcomere contains:
  • I band: only actin filaments
  • H band: only myosin filaments
  • A band: includes both actin and myosin filaments
  • M line: center of the sarcomere
  • Z line: boundary between sarcomeres

Smooth Muscle

• Smooth muscles lack sarcomeres.
• Thick filaments in smooth muscles have myosin heads along their entire length, allowing optimal actin-myosin interaction over a greater length.
• Smooth muscle cells are spindle-shaped.
• Smooth muscles are specialized for slower, sustained contractions.

Muscle Contraction and Energy

• Muscle contraction requires energy from ATP hydrolysis.
• ATP is used to power the movement of myosin heads and to pump ions across cellular membranes.
• Striated muscles that are actively used have more capillaries, myoglobin, and mitochondria, indicating a greater reliance on aerobic energy production.
• Striated muscles with infrequent use rely on less efficient but faster methods, like glycolysis and creatine phosphate.

Muscle Activation and Regulation

• Skeletal muscles are innervated by the somatic nervous system.
• The interface between a somatic motor neuron and a skeletal muscle fiber is called a neuromuscular junction (NMJ).
• The NMJ converts motor neuron action potentials into muscle fiber action potentials.

Neuromuscular Junction

• Action potentials at the axon terminal of a motor neuron trigger the release of acetylcholine (ACh) into the synaptic cleft.
• ACh diffuses across the cleft and binds to receptors in the sarcolemma, opening ion channels.
• This opens the channels, allowing ions to flow across the membrane and trigger muscle contraction.

Glossary

• Contraction: An instance of muscle force production.
• Myocyte: Muscle cell.
• Sarcolemma: Plasma membrane of a muscle cell.
• Sarcoplasmic reticulum: Specialized type of endoplasmic reticulum in muscle cells.
• Sarcoplasm: Cytoplasm of a muscle cell.
• Actin: Protein that forms thin filaments in muscle.
• Myosin: Protein that forms thick filaments in muscle.
• Striations: Regularly spaced lighter and darker bands in striated muscle.
• Smooth muscle: Muscle lacking striations.
• Myofibril: Tiny cylindrical bundles of interacting thick and thin filaments within a myocyte.
• Sarcomere: Functional contractile unit of a striated muscle cell.
• Titin: Large, elastic protein that helps maintain the ordered arrangement of actin and myosin filaments in a sarcomere.
• Neuromuscular junction (NMJ): Specialized synapse between a motor neuron and a skeletal muscle fiber.
• Acetylcholine (ACh): Neurotransmitter released at the NMJ.
• Acetylcholinesterase: Enzyme that breaks down ACh.