Molecular Mechanisms of Muscle Contraction Flashcards
What is a fascicle?
A bundle of structures, such as nerve or muscle fibres (contain numerous muscle cells or muscle fibres, contain 100s to 1000s of myofibrils)
What is a myofilament?
A filament composed of either multiple myosin or actin proteins that slide over each other to generate tension
What is a myofibril?
A fibre made up of several myofilaments that facilitates the generation of tension in a myocyte, found in striated muscle cells
What is myosin?
A motor protein which forms myofilaments that interact with actin filaments to generate tension
What is actin?
A protein which forms myofilaments that interact with myosin filaments to generate tension
What is striated?
The striped appearance of certain muscle types in which myofibrils are aligned to produce a constant directional tension
What is a myocyte?
A muscle cell (numerous make up muscle tissue)
Myocytes contain myofibrils comprised of actin and myosin filaments which slide past each other producing tension that changes the shape of the myocytes
Describe pennate muscles
- Feather like in arrangement of their fascicles
• Unipennate, bipennate, multipennate
Muscle with fascicles that attach obliquely (in a slanting position) to its tendon. When a muscle contracts and shortens, the pennation angle increases.
What can pennate muscle generate?
These types of muscles generally allow higher force production but smaller range of motion
Describe fusiform muscles
Spindle-shaped (wider in middle, narrower both ends) e.g. biceps
Describe parallel muscles
Fascicles lie parallel to long axis of muscle (flat muscles with parallel fibres often have aponeuroses)
What is aponeurosis?
A sheet of pearly white fibrous tissue that takes the place of a tendon in flat muscles having a wide area of attachment
Describe convergent muscles
Have a broad attachment from which the fascicles converge to a single tendon
Describe circular muscle
Surround a body opening or orifice, constricting it when contracted
Describe skeletal muscle
striated, multinucleated, voluntary, non-branching, attached to skeleton
Describe cardiac muscle
striated, single nucleus, involuntary, branched, heart muscle
Describe smooth muscle
non-striated, single nucleus, involuntary, tapered, forms walls of organs
Describe order of myocytes to sarcomeres
- Myocytes composed of myofibrils
- Myofibrils composed of myofilaments (actin and myosin) and numerous adjacent sacromeres
How are myocytes bound together?
By perimysium (a sheath of connective tissue) into fascicles which are bundled to form muscle tissue
Where are sarcomeres?
Segment between 2 Z lines
In electron micrographs of cross-striated muscle, what does the Z-line appear as?
A series of dark lines
What is the Z line?
The disc in between the I-bands
What surrounds the Z-line?
The region of the I-band
What is the I-band?
the zone of thin filaments that is not superimposed by thick filaments
What follows the I-band?
A-band
How are the bands named?
For the properties under a polarising microscope
What does an A band contain?
The entire length of a single thick filament
What is within the A-band?
H-zone –> a paler region (lighter appearance under polarising microscope)
What is the H-zone?
H-band is the zone of the thick filaments that is not superimposed by the thin filaments
What is inside the H-zone?
A thin M-line
What is the M-line?
The disc in the middle of the sarcomere formed of cross-connecting elements of the cytoskeleton
What are actin (thin) filaments a major component of?
I-band and extend into A-band
Where are myosin (thick) filaments found?
- Are bipolar and extend throughout the A-band
* They are cross-linked at the centre by the M-band
What is Titin? Where is it found?
A giant protein
• Extends from the Z-line of the sarcomere, where it binds to the thick filament (myosin) system, to the M-band, where it is thought to interact with the thick filaments
What is Nebulin? Where is it found?
A giant protein
• Hypothesised to extend along the thin filaments and the entire I-band
What is sarcolemma?
The fine transparent tubular sheath which envelops the fibres of skeletal muscles.
Describe processes that leads to muscle contraction
- AP arrives at NMJ
- ACh released (binds to receptors and opens sodium ion channels leading to an action potential in sarcolemma)
- AP travels along T-tubules
- Ca++ released from sarcoplasmic reticulum
- Ca++ binds to TnC region of Troponin
- Troponin changes shape, moving Tropomyosin, exposing binding site on actin filament
- Myosin head binds with ADP + Pi binds actin
- Myosin head bends, pulling along actin filament, ADP and Pi are released
What is formed when myosin head attaches to actin?
A cross bridge forms
What is the ‘power stroke’?
Myosin head bends, pulling along the actin filament. ADP is then released
What initiates power stroke?
Inorganic phosphate generated in the previous contraction cycle is released
What happens as the new ATP attaches to myosin?
The link weakens and the cross bridge detaches
As ATP splits into ADP and phosphate, the myosin head is energised
During contraction, what happens to the:
- A-band
- H-zone
- Distance between 2 Z-lines (sarcomere)
- A-band –> Stays constant
- H-zone –> Gets shorter
- Z-lines –> sarcomere shortens
What are isotonic contractions?
Isotonic contractions are those which cause the muscle to change length as it contracts and causes movement of a body part
What are the 2 types of isotonic contractions?
- Concentric
2. Eccentric
What are concentric contractions?
- Cause the muscle to shorten as it contracts
- An example is bending the elbow from straight to fully flexed, causing a concentric contraction of the Biceps Brachii muscle
- Most common type of muscle contraction
What are eccentric contractions?
- Opposite of concentric
- Occur when the muscle lengthens as it contracts
- Less common and usually involves the control or deceleration of a movement being initiated by the eccentric muscles agonist
- For example, when kicking a football, the Quadriceps muscle contracts concentrically to straighten the knee and the Hamstrings contract eccentrically to decelerate the motion of the lower limbs
Which type of contraction is normally involved in injuries and why?
Eccentric contractions –> puts a lot of strain through muscle
What are isometric contractions?
Occur when there is no change in the length of the contracting muscle
Give an example of isometric contraction
• This occurs when carrying an object in front of you
o The weight of the object is pulling your arms down, but your muscles are contracting to hold the object at the same level
• Another example is when you grip something, such as a tennis racket
o There is no movement in the joints of the hand, but the muscles are contracting to provide a force sufficient enough to keep a steady hold on the racket
What does the amount of force a muscle is able to produce during an isometric contraction depend on?
The length of the muscle at the point of contraction (each muscle has an optimum length at which the maximum isometric force can be produced)
What are the stages of a twitch?
- Latent period
- Contraction period
- Relaxation period
What is the latent period?
The delay of a few milliseconds between an action potential and the start of a contraction and reflects the time for excitation-contraction coupling
When is the contraction phase?
Starts at the end of the latent period and ends when the muscle tension peaks
What occurs during the contraction phase in regards to calcium?
cytosolic calcium levels are increasing as released calcium exceeds uptake
When is the relaxation phase?
the time between peak tension and the end of the contraction when the tension returns to zero
What occurs during the relaxation phase in regards to calcium?
cytosolic calcium is decreasing as reuptake exceeds release
What is a twitch?
A twitch is the mechanical response of an individual muscle fibre, an individual motor unit, or a whole muscle to a single action potential
Are muscle reproducible?
Yes - Repetitive stimulation produces twitches of the same magnitude and shape
What fibres are involved in a slow twitch?
Type 1 fibres (red in colour)
Why are type 1 fibres red in colour?
Due to high concentrations of myoglobin
Describe type 1 fibres
- Very resistant to fatigue
- Dense capillaries & myoglobin
- Contract slowly
- Produce low amount of power when contracted
- Contains large amounts of mitochondria
When are type 1 fibres used?
Used in aerobic activities e.g. long-distance running
What fibres are involved in a fast twitch?
- Type IIa fibres (red in colour) (also called fast twitch A fibres)
- Type IIb fibres (white in colour) (also called fast twitch B fibres)
Why are type IIa fibres red in colour?
Due to high concentrations of myoglobin
Describe type IIa fibres
- Resistant to fatigue (not as much as Type I fibres)
- Contains large amounts of mitochondria
- Contracts relatively quickly
- Produces moderate amount of power when contracted
When are type IIa fibres used?
Used in long-term anaerobic activities such as swimming (activities lasting less than 30 mins)
Why are type IIb fibres white in colour?
Due to low myoglobin concentrations
Describe type IIb fibres
- Fatigue very easily
- Contains low amounts of mitochondria
- Contracts very quickly
- Produces a high amount of power when contracted
When are type IIb fibres used?
Used in short-term anaerobic activities such as sprinting and lifting heavy weights (activities lasting less than a minute)
What does the force of muscle contraction depend on?
- The amount of overlap between thin and thick filaments
- Number of APs per second
- Number of motor units recruited
