7.2 Muscles and movement
Structure and function of muscles
Muscle tissue is made of specialised cells that use energy from the hydrolysis of ATP to become shorter by contraction.
Striated (skeletal/voluntary) muscle attaches to the bones by tendons and appears stripy under a microscope. Skeletal muscles contract and relax to move bones at a joint. The cells in striated muscle are highly specialised muscle fibres. Each fibre contains many nuclei (multinucleate), mitochondria (provides ATP for contraction) and sarcoplasmic reticulum (contains calcium ions). The cell membrane of a muscle fibre is the sarcolemma. Parts of the sarcolemma fold inwards across the fibres and stick into the sarcoplasm (cytoplasm containing organelles such as mitochondria). These folds are fibules and help spread electrical impulses through the sarcoplasm.
Muscle fibres are made of myofibrils, which are made of many short units called sarcomeres. The ends of sarcomeres have a Z line and the middle has an M line (attachment for myosin). They contain bundles of protein filaments (myofilaments) called actin and myosin which move past each other to make muscles contract.
They produce alternating patterns of light and dark bands:
- Dark A bands contain the thick myosin filaments and some overlapping thin actin filaments
- Light I bands contain thin actin filaments only
- Around the M line is the H zone which contains myosin filaments only
When the muscle contracts, the dark band overlaps the intermediate band, shortening the length of the muscle and the sarcomere.
| Skeletal | Cardiac | Smooth | |
| Function | Locomotion | Pumping blood through heart | Line blood vessels, digestive tract, uterus etc. |
| Cells | Striated | Specialised striated | Unstriated |
| Control | Voluntary | Involuntary | Involuntary |
| Arrangement | Regular – muscle contracts in one direction | Cells branch and interconnect – efficient transfer of impulses – simultaneous contraction | No regular arrangement – cells contract in different directions |
| Speed of contraction | Rapid | Intermediate | Slow |
| Length of time contracted | Short | Intermediate | Long |
Types of muscle fibres
Within striated muscle tissue, there are two types of muscle fibre. Fast twitch fibres are adapted for rapid contraction over a short period of time. Slow twitch fibres are adapted for less rapid contraction over longer periods of time (continuous activity).
| Slow twitch | Fast twitch |
| Contract slowly | Contract quickly |
| Muscles used for posture contain many slow fibres | Muscles used for fast movement |
| Good for endurance activities e.g. long-distance running | Good for short bursts of speed and power e.g. sprinting |
| Fatigue resistant | Tire/fatigue easily (lactic acid) |
| Energy released through aerobic respiration | Energy released through anaerobic respiration (mostly glycolysis) |
| Contains many mitochondria and capillaries to supply muscles with oxygen and for Krebs cycle and ETC | Contain fewer mitochondria and capillaries |
| Reddish because of richness of myoglobin (red, oxygen storing protein) | White due to lack of myoglobin (few reserves of oxygen) |
| Low glycogen content | High glycogen content |
| Low levels of creatine phosphate | High levels of creatine phosphate |
| Less sarcoplasmic reticulum | More sarcoplasmic reticulum |
| Relatively narrow so oxygen can diffuse rapidly | Relatively wide |
The sliding filament theory
Muscle contraction works as actin and myosin filaments slide between each other, causing the sarcomeres to become shorter, shortening the whole muscle fibre.
- A nerve impulse (action potential) arrives at the neuromuscular junction and depolarises the sarcolemma
- Calcium ions (Ca2+) are released from the sarcoplasmic reticulum and diffuse across the sarcoplasmic reticulum into the muscle fibre
- Calcium ions bind to tropopin, pulling the attached tropomyosin, exposing the myosin binding sites on the actin
- Myosin head binds with the actin filaments, forming cross-bridges
- ATP on the myosin head breaks down into ADP + Pi, providing energy needed for muscle contraction
- Myosin head nods forward, pulling the actin towards the centre of the sarcomere
- An ATP molecule binds to the myosin head, breaking the cross-bridge, the myosin head detaches from the actin filament
- An ATPase molecule on the myosin head hydrolyses ATP into ADP and Pi
- The myosin heads move back to their original upright position
- This process repeats as long as the action potentials continue to arrive
When the muscle is not longer being stimulated, calcium ions are moved back into the sarcoplasmic reticulum by active transport (using ATP). The troponin molecules move back to their original shape and the tropomyosin blocks the actin binding sites. This means the myofilaments cannot slide past each other because the myosin heads cannot bind to the actin. The actin filaments slide back to their relaxed position, lengthening the sarcomeres.
Tissues of the skeletal system
Striated muscles are attached to the skeleton by strong, inelastic tendons. Tendons are made of long fibres of the protein collagen, and small amounts of elastin. When a muscle contracts it pulls on the tendons which transmit a force.
Bones are connected at joints such as finger joints, elbow joints and synovial joints. Ligaments hold these bones in synovial joints together. The hip, knee and ankle are synovial joints. They are where the bones in the joint are separated by a cavity filled with synovial fluid, secreted by the synovial membrane. This acts as a lubricant and allows for free movement.
Ligaments also contain collagen and elastin but have a higher proportion of elastin, so they can stretch more than tendons. Ligaments control and restrict the amount of movement in the joint.
Cartilage is firm and elastic, it protects the bones within the joints. Cartilage absorbs synovial fluid and acts as a shock absorber.
Muscles produce a force when they contract. When they relax, they stay in the same position unless pulled to their lengthened state. For example the major muscle causing the arm to bend at the elbow is the biceps, known as the flexor muscle. The triceps are the extensor muscle, causing the arm to straighten when it contracts. As the arm bends, the biceps contract and the triceps relaxes, although it may contract slightly to control the movement. Muscles can only pull they cannot push, two work together for movement, they are antagonistic muscle pairs. When one muscle is contracting, the other is relaxing.
Keywords
- Flex: When muscles contract to bend joints
- Extend: When muscles relax to straighten joints
- Flexor: A muscle that contracts to flex a muscle e.g. hamstrings (knee)
- Extensor: A muscle that contracts to extend a muscle e.g. quadriceps (knee)
- Joint: Where muscles bring about movement
- Antagonistic: A pair of muscles that work together that pull to extend or flex a joint
- Synovial joints: Bones of a joint separated by a cavity of synovial fluid
- Synovial fluid: A fluid that enables joints to move freely, acting like a lubricant
- Ligaments: Joins bones to bones, they are strong and flexible
- Tendons: Joins muscle to bone
- Cartilage: Protects bones at joints, absorbs synovial fluid and acts as a shock absorber
- Muscle fibre: Bundles of muscle fibres make up muscles and each is a single muscle cell
- Multinucleate: Many nuclei to control the metabolism of the cell
- Myofibrils: Are made up of a series of contractible sarcomere units
- Sarcomeres: Made of 2 proteins called actin and myosin
- Actin: Thin filament in a sarcomere
- Myosin: Thicker filament in a sarcomere
- Troponin: Protein molecule on actin filament, calcium ions bind to it
- Tropomyosin: Protein molecule on actin, they move and expose myosin binding sites on the actin
- Sliding filament theory: Myosin and actin slide over each other causing muscle contraction and movement
- Sarcoplasmic reticulum: Membrane-bound sacs around the myofibrils which release calcium ions for muscle contraction.
- Sarcoplasm: Cytoplasm of a muscle cell