B.3.3. Flashcards
(16 cards)
Sliding filament theory
- The muscle is at restThe motor neuron is not signaling the muscle to contract therefore the sarcomere is relaxed. Due to getting ATP from the mitochondria in the skeletal muscle cells and using it in the myosin heads they then become cocked and ATP turns into ADP + P. The myosin heads are not bound to actin and myosin binding sites on actin are blocked by tropomyosin.
- Arrival of action potential triggers release of acetylcholine at the neuromuscular junction.An action potential reaches the synaptic termina lof the motor neuron at a neuromuscular junction. The neurotransmitter acetylcholine is released into the synaptic cleft, and when it bind to its receptor on the muscle fibre sarcolemma, acetylcholine triggers the opening of ligand-gated Na+ channels which depolarize the membrane and trigger a wave of action potentials along the sarcolemma.
- Action potential travels along the sarcolemma membrane and down T-tubules.Acetylcholine triggers an action potential in the muscle fibre, this electrical impulse travels along the sarcolemma and continues down the T-tubules. The T-tubules being invaginations of the sarcolemma that carry the electrical impulse into the interior of the muscle fibre cell.
- Release of Ca+2 ions from the sarcoplasmic reticulumTriggered by the arrival of the electrical signals in the T-tubules, Ca +2 ions are released from the sarcoplasmic reticulum. Those calcium ions move through facilitated diffusion out of the sarcoplasmic reticulum and onto the myofibril. The calcium ions act as intracellular signaling molecules to trigger contraction.
- Ca +2 binds to troponin causing tropomyosin to move of the myosin binding sites on actin.The calcium ions released from the sarcoplasmic reticulum bind to troponin, when this happens it causes a conformational change that shifts the thin filament protein tropomyosin. This shift move tropomyosin off the myosin binding sites on actin exposing them.
- Myosin heads bind to actin forming a cross bridge.With the myosin binding sites on actin now exposed, the myosin heads bind to actin and form a cross-bridge.
- Myosin head flexes moving the actin filaments inwards, shortening the sarcomere.After binding to the actin, the myosin head flexes which pulls the actin filament slightly inwards towards the center of the sarcomere. This movement is called the “power stroke” where ADP and P are released.
- ATP attaches to the myosin heads, breaking the cross-bridge.The cross bridge is broken when ATP binds to the myosin head causing the myosin head to detach from the actin filament. The hydrolysis of ATP provides the energy needed for the myosin head to again extend itself in the cocked position ready to bind to a new actin binding site.
- Cross-bridge cycleAs long as calcium ions are present and bound to troponin, myosin will repeatedly bind and pull on actin in the cross bridge cycle.
- Contraction ends when Ca +2 is pumped back into the sarcoplasmic reticulum.Muscle contraction stops when he motor neuron stops releasing acetylcholine, which causes repolarization where the sarcolemma and T-tubules repolarize. It causes calcium channels to close, calcium then is pumped back into the sarcoplasmic reticulum, and myosin binding sites become blocked by tropomyosin again.
Sarcomere
The sarcomere is the basic unit of muscle contraction and it consists of the following:
- It has thick filaments that are in the middle of the sarcomere, they are made of a protein called myosin
- It has thin filaments parallel to the thick filaments that extend to the Z-line at either side of the sarcomere, they contain different proteins called, troponin, tropomyosin, and actin.
- Titin is a protein that anchors the thick filaments made of myosin to the Z-line, after contraction it recoils to aid in muscle relaxation.
- Lastly the Z-lines are protein structures that anchor the actin filaments and define the boundaries of the sarcomeres
When the muscle contracts, the sarcomeres get shorter because the myosin and actin bind and the myosin pushes the actin towards the middle, therefore when the muscle contracts the Z-lines are pulled closer together.
Types of skeletons
Skeletons are the framework that support and protect an animal’s body. Skeletons also help in locomotion by giving the muscles a place to anchor themselves, skeletons acting as levers.
There are 2 types of skeletons:
- Exoskeletons, that are hard and protective made of chitin and are on the outside of the body. These are found usually in arthropods and crustaceans
- Endoskeletons are skeletons made of bone and cartilage and are inside the body. These are usually found in vertebrate animals.
Antagonistic muscles in relaxation
Muscle relaxation happens when the sarcomere returns to a relaxed state after contracting, this relaxation is influenced by titin and antagonistic muscles.
The impact of antagonistic muscles is because skeletal muscles work in pairs, where one muscles contracts and the other relaxes to allow a movement to happen. Examples of this are with biceps and triceps working together to make the forearm move. Another example is the quadriceps and hamstrings working together to move the lower leg. And lastly the Internal and external intercostals work together to move the ribs.
Antagonists muscle pairs are needed because skeletal muscles cannot only exert enough energy to relax by themselves, therefore they need the energy exerted from the contraction of the antagonistic muscle.
Titin in relaxation
Titin is a big protein that has a spring-like structure, acting as a molecular spring and provides passive elasticity that helps muscles return to their resting length during relaxation.
Because of its spring properties is has elastic potential energy, storing potential energy when stretched and compressed.
When the muscles are contracted titin is compressed, acting line a spring the protein will recoil and contribute to returning the sarcomere to its relaxed state.
When titin is stretched it will recoil back to its relaxed state allowing for the sarcomere to do the same. Animals use the stretching of titin to store potential energy that can be use to rapidly power a motion.
Intercostal muscles
Intercostal muscles are group of muscles located between the ribs that assist in ventilation by expanding and contracting the ribcage. The external intercostals connect the outside of the ribs, while the internal intercostals are located between the ribs.
External intercostal muscles contract to pull the ribs up and out during inspirations, expanding the chest cavity and allowing air to flow into the lungs.
Internal intercostal muscle contract to pull the ribs down and in during forced exhalation, reducing the chest cavity volume and pushing air out of the lungs.
When one of the intercostal muscle layers contract it stretches the other, storing potential energy in titin in the sarcomere.
Reasons for locomotion
Locomotion is the movement of motile organisms from one place to another. For many organisms the ability to move is very important for survival and reproduction, there are many reasons for why an organism may locomote.
- For finding food. Herbivores move to find the plant foods they eat and predators move to catch and kill their prey.
- For escaping danger. Prey move to escape predators, and many organisms move to a roosting/nestling site at higher ground for safety
- For searching for a mate. To avoid inbreeding, maturing individuals may leave their original social groups to find mates in other social groups.
- For migration, some organisms will migrate between ecosystems to maximise opportunities for finding food and/or mates. Marine mammals have adaptations for swimming allowing locomotion through water.
Motile organisms
Motile organisms are organisms that use their own energy to move from place to place. They are locomotive.
- They are active feeders that search for food
- They require higher amounts of nutrients
- They have higher metabolic rates
- They must search for mating partners.
Examples are, some prokaryotes like E.coli. Single cells eukaryotes like amoeba or paramecium. And most animals.
Sessile organisms
Sessile organisms are ones that cannot direct their movement from place to place. These organisms are usually attached to a surface but they can be moved by external forces like water currents of wind.
- They are autotrophs, or passive feeders
- Require less nutrients
- Have slower metabolic rates
- May be easily attacked by predators.
Examples are, some prokaryotes like the ones in biofilms, some animals like sponges and corals, and most fungi and plants.
Motor units
The reason for skeletal muscles contracting is the stimulation coming from a motor neuron. Motor neurons branch at the end of the axon and each branch will stimulate a different muscle fibre.
A motor unit is the single motor neuron together with all the muscle fibres it stimulates.
When the nerve impulse reaches the end of all branches, the different muscle fibre cells will all be stimulated to contract simultaneously. This coordinates the contraction of the muscle.
Neuromuscular junctions
Neuromuscular junctions are the synapse between a motor neuron and a muscle. When an action potential reaches the synaptic terminal of the motor neuron, it causes the neurotransmitter acetylcholine to go into the synaptic cleft. When it binds to its receptor on the muscle fibre sarcolemma, acetylcholine triggers the opening of ion channels. Sodium ions flow into the muscle fibre and cause it to contract.
Human hip
The human hip is an example of a ball and socket synovial joint that connects the thigh bone (femur) to the pelvis. It is composed of:
- The hip capsule, its a connective tissue that surrounds and encloses the joint to provide stability and protection
- Bones of the pelvis form a socket into which the rounded head of femur fits.
- A ring of cartilage lines the bones of the pelvis and femur
- The synovial fluid fills the cavity in the joint
- The ligaments provide support and restrict movement
- There are 17 muscles of the hip and thigh that function and move and stabilise the pelvis
- Tendons are tough and fibrous band at the end of the muscles that insert into the bone and facilitate bone movement.
Types of synovial joints
There are 3 different types of joints:
- Fibrous joints, non-movable
- Cartilaginous, slightly movable
- Synovial joints, freely movable
Examples of synovial joints are:
- The hinge jointsA hinge joint has a convex surface fitting into a concave surface. Hinge joints provide a limited range of motion allowing for bending (flexion) and extension (straightening) in one place.Seen in the:
- Elbow joint
- Knee joint
- Finger joints
- Ball and socket jointsA ball and socket has a rounded “ball” fitting into a cup-like “socket”. Ball and socket joints provide a wide range of motion allowing for movement in multiple dimensions.Seen in:
- Hip joint
- Shoulder
Skeletons as levers
In the skeleton the bones act as levers to move fulcrum point, this being the joints. The effort put into moving the lever is the muscles that pull on the bones at the intersection point, while the load is the mass being pulled away.
There are 3 types of levers depending on the position of the fulcrum, effort, and load.
- First class levers have the fulcrum placed between the load and effort.
- Contraction of the muscle in the neck pulls on the skull and causes the altlanto-occipital joint to pivot making the face rise.
- Second class levers have the load between the effort and the fulcrum
- Contraction of the calm muscle, when the leg pulls on the heel it causes the metarsophalangeal joint to pivot making the foot rise.
- Third class levers have the effort placed between the lead and the fulcrum
- Contractions of the bicep muscle in the arm pull on the radius causing the elbow joint to pivot, making the hand rise.
Synovial joints components
Synovial joints are the most common type of joint, these feature a fluid-filled space between smooth cartilage pads at the end of the articulating bones, they are composed of:
- A joint capsule, a flexible a fibrous tissue that surround the joint and provides protection and stability
- Bones serve as levers and anchor muscles
- Cartilage is tough tissue that covers the bond at the joint, this prevents friction and absorbs shock
- Synovial fluid fills the cavity in the joint and lubricates it to reduce friction
- Ligaments are tough cords that connect bone to bone at the joint
- Muscles provide the effort force to move the bone at the joint
- Tendons attach the muscles to the bone.
Range of motion in synovial joints
Different joints have different degrees to which they can move in different directions, those directions of movements are:
- Flexion, which is bending a joint decreasing the angle of the bones at these joints
- Extension which is straightening a joint increasing the angle between the bones at these points
- Abduction is the movement of a limb away from the center of your body
- Adduction is the movement of a limb towards the center of your body
- Medial rotation is the rotating of a limb towards the center of the body.
- Lateral rotation is the rotating of a limb away from the center of the body.
The range of motion is how far a joint can move, and it is measured in degrees in a specific direction. This is a measure of flexibility, techniques for measuring the range of motion include:
- Using a goniometer, which is a tool with 2 arms that are hinged together and positioned at a joint to measure the angle
- Analysis of images using computer programs or phone applications that measure angles.
