Topic 11.2: Movement
In the Movement unit we will learn
- The roles of the musculoskeletal system are movement, support and protection.
Nature of science:
- Developments in scientific research follow improvements in apparatus—fluorescent calcium ions have been used to study the cyclic interactions in muscle contraction. (1.8)
11.2 U 1 Bones and exoskeletons provide anchorage for muscles and act as levers.
- Bones act as levers so the body can move and provide structural support (skeleton).
- Ligaments are strong bands that connect bone to bone strengthening the joint during movement.
- Tendons have dense connective tissue that connects muscles to bones, allowing movement of the bone when a muscle contracts.
- Muscles provide the force for movement by contracting (shortens the muscle fibers)
- The joint acts as a pivot point or a fulcrum
- The force applied (when the muscle contracts) is called the effort
- The force or load needed to overcome for movement to take place is called the resistance
- Levers are classified by first, second, and third class, depending upon the positions among the fulcrum, the effort, and the resistance.
- First-class levers have the fulcrum in the middle, like a seesaw. An example of a first class lever is when a human nods their head (top of the spinal column is the fulcrum, the effort force is provided by the muscles in the back of the neck, and the resistance is weight of the head).
- Second-class levers have a resistance in the middle, like a load in a wheel-barrow. The body acts as second class lever when engaged in push up or calf raise. During a calf raise ball of foot is fulcrum, the body’s mass is the resistance and the effort is applied by calf muscle.
- Third-class levers have the effort from the muscle in the middle of the lever. The majority of the human body's musculoskeletal levers are third class. These levers are built for speed and range of motion. Muscle attachments are usually close to the fulcrum. In the example of the arm, the effort force is provided by the contraction of the biceps, the fulcrum is the elbow joint and the resistance would be provided by whatever weight is being lifted.
- Exoskeletons in insects and crustaceans can facilitate the movement by providing an anchorage for muscles; similarly to how bones provide anchorage for animals with internal skeletons.
11.2 U 2 Synovial joints allow certain movements but not others.
The type of joint determines the amount of movement that is possible. For ball and socket joints, such as the hip or the shoulder, movement through all three planes are possible. At the hip joint, the head of the femur is the ball the fits into the socket of the pelvis. The movements possible at the joint are flexion, extension, rotation, abduction and adduction.. For hinge joints, such as the knee, flexions (bending) and extensions (straightening) are the possible movements (movement in one plane); however, slight side to side movements are possible.
11.2 U 3 Movement of the body require1s muscles to work in antagonistic pairs.
Skeletal muscles occurs in pairs that are antagonistic. This means that when one contracts, the other relaxes. Antagonistic muscles produce opposite movements at a joint. For example, in the elbow, the triceps extends the forearm while the biceps flex the forearm.
11.2 U 4 Skeletal muscle fibres are multinucleate and contain specialized endoplasmic reticulum.
- Skeletal muscles are composed of bundles of muscle fibers and have a striped appearance because of areas of thick and thin filaments (myosin and actin)
- Muscle cells have many nuclei and are long because the embryonic muscle cells fuse together
- Muscle fibers are composed of many parallel elongated fibers called myofibrils
- A modified endoplasmic reticulum, called the sarcoplasmic reticulum (fluid-filled membranous sacs), extends throughout the muscle fibre, wrapping around each myofibril, sending a signal to the all parts of the muscle fibre to contract at the same time
- The sarcoplasmic reticulum stores calcium - between myofibrils are large numbers of mitochondria, which provide ATP needed for contractions.
11.2 U 5 Muscle fibres contain many myofibrils.
Within each muscle fibre there are many parallel, elongated structures called myofibrils. These have alternating light and dark bands, which give striated muscle its strips. In the centre of each light band is a disc-shaped structure, referred to as the Z-line.
11.2 U 6 Each myofibril is made up of contractile sarcomeres.
Myofibrils – rod-shaped parallel bodies consisting of actin and myosin filaments
- Sarcolemma – plasma membrane of the muscle cell
- Mitochondria – large numbers; found dispersed around individual myofibrils
- Lies between two Z-lines which are dense protein discs
- Contains the thick filament (myosin) and thin filament (actin)
- Myosin contains a head which binds to the binding site on the actin; interaction between myosin and actin (cross-bridge) is responsible for muscle contraction
- Myosin is seen as dark bands while actin is seen as light bands
- Actin filaments are attached to a Z-line at one end
- Myosin filaments are interdigitated with actin filaments at both ends and occupy the centre of the sarcomere
- Each mysosin filament is surrounded by six actin filaments
- A bands contain a full length of myosin and some of the actin filaments
- I bands contain only actin filaments
11.2 U 7 The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments.
- During a muscle contraction, myosin filaments pull actin filamentstowards the centre of the sarcomere
- This shortens the sarcomere and the overall length of the muscle fibre
- When this occurs, the myosin heads bind to sites on the actin filaments, creating cross-bridges, pulling (sliding) the actin filaments along the myosin filaments with energy from ATP
- This is called sliding filament theory and is explained further below
11.2 U 8 ATP hydrolysis and cross bridge formation are necessary for the filaments to slide.
- ATP attaches to the myosin heads breaking the cross-bridges between the myosin heads and actin binding sites
- The ATP undergoes a hydrolysis reaction forming ADP + Pi.
- This causes a positional change in the myosin head (cocked back).
- The myosin heads bind to actin filaments forming cross-bridges at a site one position further from the centre of the sarcomere
- When the ADP + Pi are released the myosin heads change conformational position, sliding the actin filaments towards the center of the Sarcomere.
- This is called the “power stroke”.
- After the power stroke ATP again binds to the myosin head, causing it to detach from the actin filament ready for another cycle.
11.2 U 9 Calcium ions and the proteins tropomyosin and troponin control muscle contractions.
- An action potential propagated along a motor neuron arrives at the neuromuscular junction.
- This causes the release of the neurotransmitter acetylcholine into the synapse between the terminal axon of the motor neuron and the sarcolemma of the skeletal muscle.
- The acetylcholine binds to receptors on the sarcolemma, causing voltage-gated channels to open and Na+ ions to flow into the muscle cells.
- This creates an action potential in the striated muscle.
- The action potential is further propagated along the sarcolemma of the skeletal muscle.
- The action potential moves into the interior of the muscle cell through folds called t tubules.
- The depolarization of the t tubules causes voltage-gated Ca+ channels on the sarcoplasmic reticulum to open, causing an influx of Ca+ ions into the sarcoplasm.
- Ca+ ions bind to troponin which causes tropomyosin to move exposing the myosin binding sites (troponin and tropomyosin are regulatory proteins blocking the myosin binding sites).
11.2 A 1 Antagonistic pairs of muscles in an insect leg
- The hind limbs of grasshoppers are specialized for jumping
- It has a jointed appendage with three parts
- Below the joint is the tibia, and at the base of the tibia is another joint called the tarsus
- Above the joint is the femur
- When the grasshopper jumps, the flexor muscles contract, and the femur and tibia are brought closer together (flexing)(extensor muscles are relaxed)
- As the grasshopper jumps the extensor muscles contract, extending the tibia, creating a powerful jump force
11.2 S 1 Annotation of a diagram of the human elbow. include cartilage, synovial fluid, joint capsule, named bones and named antagonistic muscles.
11.2 S 2 Drawing labelled diagrams of the structure of a sarcomere. Include Z lines, actin filaments, myosin filaments with heads, and the resultant light and dark bands.
11.2 S 3 Analysis of electron micrographs to find the state of contraction of muscle fibres.Measurement of the length of sarcomeres will require calibration of the eyepiece scale of the microscope.
Notice in the fully contracted sarcomere the actin filaments slide along the myosin causing the light bands to shorten, even though the dark bands stay the same length. The Z lines get closer together as the sarcomere contracts.. The muscle also can be in various states of partial contraction.
In The News
In The News
Alex Lee discusses Topic 11.2