Smooth muscle tissue, such as skeletal and cardiac muscle tissue, can undergo hypertrophy the increase in the volume of tissue due to the enlargement of its component cells.
Smooth muscle fibres are usually involuntary i.e. not under conscious control, and they are nonstriated meaning smooth.
In addition, certain smooth muscle fibres retain a capacity for division and can grow by a process known as hyperplasia, like those in the uterus of women.
Cardiac muscle tissue forms the bulk of the wall of the heart. Like skeletal muscle tissue, it is striated (the muscle fibers contain alternating light and dark bands (striations) that are perpendicular to the long axes of the fibers). Unlike skeletal muscle tissue, its contraction is usually not under conscious control (involuntary).
Skeletal muscle tissue is named for its location – attached to bones. It is striated; that is, the fibers (cells) contain alternating light and dark bands (striations) that are perpendicular to the long axes of the fibers. Skeletal muscle tissue can be made to contract or relax by conscious control (voluntary).
All skeletal muscle fibres are not alike in structure or function. For example, skeletal muscle fibres vary in colour depending on their content of myoglobin (myoglobin stores oxygen until needed by the mitochondria). Skeletal muscle fibres contract with different velocities, depending on their ability to split Adenosine Triphosphate (ATP). Faster contracting fibres have greater ability to split ATP. In addition, skeletal muscle fibres vary with respect to the metabolic processes they use to generate ATP. They also differ in terms of the onset of fatigue. Based on various structural and functional characteristics, skeletal muscle fibres are classified into three types: Type I fibres, Type II B fibres and type II A fibres
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The different types of muscle fibres & types of sports each is associated with
Type I Fibres
These fibres, also called slow twitch or slow oxidative fibres, contain large amounts of myoglobin, many mitochondria and many blood capillaries. Type I fibres are red, split ATP at a slow rate, have a slow contraction velocity, very resistant to fatigue and have a high capacity to generate ATP by oxidative metabolic processes. Such fibres are found in large numbers in the postural muscles of the neck. A sporting example of this could be a prop forward involved in a scrum in rugby.
Type II A Fibres
These fibres, also called fast twitch or fast oxidative fibres, contain very large amounts of myoglobin, very many mitochondria and very many blood capillaries. Type II A fibres are red, have a very high capacity for generating ATP by oxidative metabolic processes, split ATP at a very rapid rate, have a fast contraction velocity and are resistant to fatigue. Such fibres are infrequently found in humans. A sporting example of this is a sprinter such as Usain Bolt or a hurdler such as Colin Jackson.
Type II B Fibres
These fibres, also called fast twitch or fast glycolytic fibres, contain a low content of myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glycogen. Type II B fibres are white, geared to generate ATP by anaerobic metabolic processes, not able to supply skeletal muscle fibres continuously with sufficient ATP, fatigue easily, split ATP at a fast rate and have a fast contraction velocity. Such fibres are found in large numbers in the muscles of the arms. A sporting example could be an Olympic weightlifter.
How muscles produce movement in antagonistic pairs and the role of fixators and synergists
There are up to four functional groups of muscles acting on joints.
1. Agonist: actively contract to make a movement. Muscle length reduces.
2. Antagonist: resists the muscle on opposite side, thereby controls the speed of the agonist muscle contraction.
That’s why they say both agonist and antagonist muscles are working in pairs.
Furthermore when the movement is reversed the original agonist becomes the antagonist and the original antagonist becomes the agonist.
3. Stabilisers: some muscles will hold the joint area stable while other three types of muscles are making a movement.
4. Modifiers: some muscles can slightly change the direction of force exerted by agonists dynamically
Different types of muscle contractions
Muscle Contractions can be divided into:
All lifting exercises require isotonic contractions. This happens when the muscle shortens as it contracts. An example of isotonic contraction can be seen when we flex the bicep muscle. Stand with one arm straight and the palm of the hand facing up. Roughly measure the length from the start of the biceps muscle to the point where it meets the shoulder. Now curl the hand towards the shoulder, the biceps muscle shortens as it contracts. When you reach the end point take another rough measurement of the biceps again, it will be much shorter.
Another example is the triceps muscle (opposite of biceps). Do the same experiments again this time measure the triceps and start at the curled position. The triceps shortens as the arm straightens.
Other examples are
- lifting objects above the head – front shoulder (anterior deltoid) shortens
- lifting object up from lying position – chest muscle shortens
- lifting body up from squat position – quadriceps muscle shortens as legs extend
- doing a sit up
- throwing a ball
- swinging a bat
Eccentric contraction is the opposite of isotonic; the muscle lengthens as it gains tension. These are much less common and not as beneficial as the common Isotonic. An example is when someone manages to pull your arm straight while at the same time you are try to keep the arm locked in one position. In other words, the load is too great!
Other examples are
- running downhill
- walking downstairs
- landing on the ground from a jump
An Isometric contraction occurs when there is tension on a muscle but no movement is made causing the length of the muscle to remain the same. This type of contraction is also referred to as a static contraction. Some bodybuilders make up their own exercises using Isometric contraction in order to develop strength; an example is when someone attempts to curl one arm upwards but is held by using equal resistance from the other arm.
- attempting to lift an immoveable object
- holding a weight at arm’s length
- some wrestling movements
Similar to the isotonic contraction, the Isokinetic contraction causes the muscle to shorten as it gains tension. The difference is Isokinetic requires a constant speed over the entire range of motion, therefore this type of contraction require special equipment to exercise properly. An example is an arm stroke when swimming, the even resistance from the water offers a constant speed at maximal contractions.
Sliding Filament Theory
The sliding filament theory is the basic summary of the process of skeletal muscle contraction. Myosin moves along the filament by repeating a binding and releasing sequence that causes the thick filament to move over the thinner filament. This progresses in sequential stages. By progressing through this sequence the filaments slide and the skeletal muscles contract and release.
The first stage is when the impulse gets to the unit. The impulse travels along the axon and enters the muscle through the neuromuscular junction. This causes full two to regulate and calcium channels in the axon membrane to then open. Calcium ions come from extra cellular fluid and move into the axon terminal causing synaptic vessels to fuse with pre synaptic membranes. This causes the release of acetylcholine (a substance that works as a transmitter) within the synaptic cleft. As acetylcholine is released it defuses across the gap and attaches itself to the receptors along the sarcolemma and spreads along and across the muscle fibre.
The second stage is for the impulse spreads along the sarcolemma. The action potential spreads quickly along the sarcolemma once it has been generated. This action continues to move deep inside the muscle fibre down to the T tubules and the action potential triggers the release of calcium ions from the sarcoplasmic reticulum.
During the third stage calcium is released from the sarcoplasmic reticulum and actin sites are activated. Calcium ions once released begin binding to Troponin. Tropomyosin blocking the binding of actin is what causes the chain of events that lead to muscle contraction. As calcium ions bind to the Troponin it changes shape which removes the blocking action of Tropomyosin (thin strands of protein that are wrapped around the actin filaments). Actin active sites are then exposed and allow myosin heads to attach to the site.
The fourth stage then begins in which myosin heads attach to actin and form cross bridges, ATP is also broken down during this stage. Myosin binds at this point to the exposed binding sites and through the sliding filament mechanism the muscles contract.
During the fifth stage the myosin head pulls the Actin filament and ADP and inorganic Phosphates are released. ATP binding allows the myosin to detach and ATP hydrolysis occurs during this time. This recharges the myosin head and then the series starts over again.
Cross bridges detach while new ATP molecules are attaching to the myosin head while the myosin head is in the low-energy configuration. Cross bridge detachment occurs while new ATP attaches itself to the myosin head. New ATP attaches itself to the myosin head during this process.
During stage seven the ATP is broken down and used as energy for the other areas including new cross bridge formation. Then the final stage (stage 8) begins and a drop in stimulus causes the calcium concentrate and this decreases the muscle relaxation.
Below is an example of how sliding filament theory works
How the muscular system responds to exercise
How muscles work
Muscles fall in to two types:
Voluntary and involuntary.
Brain stimulation through a signal to voluntary muscles makes them work to do a task like pulling.
There is no brain stimulation for involuntary muscles. When people exercise their voluntary muscles, they more efficiently they function. When functioning efficiently, it is easier for people to do their work.
Muscles will function with greater efficiency and ease when they have regular exercise. This is known as the first lesson of exercise
Muscular exercise and the affects of exercise on the muscles
Inside the muscles nerves relay messages to and from the brain.
Food is bought to the muscles by blood vessels which do the work that the brain has ordered.
When muscles are exercised, they convert a substance known as glucose into energy. During exercise, heat is produced and carbon dioxide is given off as a waste product.
- Short term effects:
When we begin to exercise the body has to respond to the change in activity level in order to maintain a constant internal environment (homeostasis). Here are the changes which must take place to the muscles so that the exercise can be performed:
The higher rate of muscle contraction depletes energy stores and so stimulates a higher rate of energy metabolism.
The body’s energy stores are slowly depleted
Myoglobin releases its stored oxygen to use in aerobic respiration. O2 can now be diffused into the muscle from the capillaries more quickly due to the decreased O2 concentration in the muscle.
- Long term effects:
Increased numbers of mitochondria (the cells powerhouse) means an increase in the rate of energy production.
The muscles, bones and ligaments become stronger to cope with the additional stresses and impact put through them. with the additional stresses and impact put through them.
The amount of myoglobin within skeletal muscle increases, which allows more Oxygen to be stored within the muscle, and transported to the mitochondria.
Muscles are capable of storing a larger amount of glycogen for energy.
Enzymes involved in energy production become more concentrated and efficient to aid the speed of metabolism.
Benefits of exercise
Muscles are working hard during exercise, which is good for them. The harder they are worked over time, the more they can do. Muscles must have the proper intake of food (in the shape of protein, complex carbohydrates and fats) along with sufficient water to achieve the maximum amount of work possible.
Contraction of a muscle makes it a more efficient tool. Contraction with resistance aids the muscle growth and increases its capacity for future demands. Multiple contractions through exercise brings about the greatest efficiencies.
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