Muscle structure

• Muscles are effectors - they contract in response to nervous impulses
• There are three types of muscle in the body:

  1. smooth muscle - contracts without conscious control, found in walls of internal organs (apart from the heart) e.g. stomach, intestine, and blood vessels
  2. skeletal muscle - (also called striated, striped or voluntary muscle) is the type of muscle you use to move e.g. biceps and triceps move the lower arm
  3. cardiac muscle - contracts without conscious control (like smooth muscle) but is only found in the heart
• Skeletal muscles are attached to bones by tendons. Ligaments attach bones to other bones, to hold them together
• Pairs of skeletal muscles contract and relax to move bones at a joint - bones of the skeleton are incompressible (rigid) so they act as levers, giving muscles something to pull against
• Antagonistic pairs are muscles that work together to move a bone. The contracting muscle is called the agonist and the relaxing muscle is called the antagonist.
• Skeletal muscle is made up of large bundles of long cells, called muscle fibres.
• The cell membrane of muscle fibres is called the sarcolemma. Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (a muscle cell's cytoplasm). These folds are called transverse (T) tubules and they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
• A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm.
• The sarcoplasmic reticulum stores and releases calcium ions that are needed for muscle contraction.
• Muscle fibres have lots of mitochondrion to provide the ATP needed for muscle contraction
• Muscle fibres are multinucleate, meaning they have many nuclei. and lots of long cylindrical organelles called myofibrils.
• Myofibrils are made up of proteins and are highly specialised for contraction
• Myofibrils contain bundles of thick and thin myofilaments that move past each other to make muscles contract.
• Thick myofilaments are made of the protein myosin and the thin myofibrils are made of the protein actin. If you look at a myofibril under an electron microscope - see pattern of alternating dark and light bands
• Dark bands contain the thick myosin filaments and some overlapping thin actin filaments - A bands
• Light bands contain thin actin filaments only - I bands
• A myofibril is made up of many short units called sarcomeres
• The end of each sarcomere are marked with a Z-line
• In the middle of each sarcomere is an M-line. The M-line is the middle of the myosin filaments.
• Around the M-line is the H-zone. The H-zone only contains myosin filaments.