Molecular Mechanism of Muscle Contraction Flashcards
What is a fascicle?
A bundle of structures
e.g. bundle of muscle fibres
What is a pennate muscle?
What is their motion like?
Feather-like arrangement of fascicles
Fascicles attach to their tendon in a slanting position
They produce a higher force but a smaller range of motion
What are the types of pennate muscle?
Unipennate
Bipennate has the tendon in the centre
Multipennate
What does a multipennate muscle allow?
e.g. deltoid in the shoulder
Allows movement in a multidirectional manner
What is a fusiform muscle and what is its motion like?
Spindle-shaped with a tendon on each end of the muscle belly
They are arranged to provide the greatest degree of shortening
They are not very powerful but can produce a quick and wide range of motion
What are parallel muscles?
They have fascicles that lie parallel to the long axis of the muscle
What are aponeuroses and where are they found?
Flat muscles with parallel fibres often have aponeuroses
It is a sheet of white fibrous tissue that takes the place of a tendon
Found when muscles have a wide area of attachment
What are convergent muscles and what is their motion like?
Broad attachment from which the fascicles converge to a tendon
Arrangement of fibres allows for maximum force production
Where are circular muscles found?
Around a body opening or orifice
The opening is constricted when the muscle contracts
What is the structure of skeletal muscle?
What contractions is it involved in?
It is striated and multinucleated
It does not have branches attached to its skeleton
Involved in voluntary contractions
What is the structure of cardiac muscle?
What contractions is it involved in?
It is striated but each cell has its own nucleus
It is branched
It is involved in involuntary contractions
What is the structure of smooth muscle?
What contractions is it involved in?
It is non-striated and each cell has its own nucleus
Cells are spindle-shaped and form the walls of organs
Involved in involuntary contractions
What is the role of the tendon?
It attaches the muscle to the bone
The tendon leads to the muscle belly
What surrounds a muscle?
A connective tissue sheath called the epimysium
How are “muscle compartments” created and what surrounds them?
Portions of the epimysium project inwards to divide the muscle into compartments
Each compartment contains a fasciculus surrounded by the perimysium
What is the arrangement of connective tissue within the fasciculus?
Each individual muscle cell (fibre) is surrounded by a layer of connective tissue called the endomysium
What is the role of connective tissue layers within the muscle?
They provide support and protection to delicate cells so they can withstand the forces of contraction
Coverings provide pathways for passage of blood vessels and nerves
What is found beneath the endomysium?
The plasma membrane of the muscle cell - the sarcolemma
What is the majority of the volume of the muscle cell filled with?
Numerous long myofibrils
These contain two types of filament - thick and thin
What do thin filaments consist of?
Two strands of actin arranged in a double helix
Troponin and tropomyosin molecules cover binding sites on actin
What do thick filaments consist of?
Groups of myosin
Each myosin filament forms a protruding head at one end
What is a sarcomere?
A contractile segment of muscle
It is the distance between 2 Z lines
How are actin and myosin filaments arranged?
They are arranged side by side and are parallel
They will overlap
Actin filaments are attached to the Z line
Myosin filaments are not attached to the Z line and float between the actin
Where is the I band found?
Why does it have this name?
The zone of thin filaments that is not superimposed by thick filaments
Isotropic as the structure is uniform
Where is the A band found?
Why does it have this name?
It contains the entire length of a single thick filament
Anisotropic as the structure is directional and not uniform
Where is the H-zone found?
It is a paler region within the A band
It is the zone of the thick filaments that is not superimposed by thin filaments
There is no overlap
What line is found within the H-zone?
What is it formed of?
The M-line is formed of cross-connecting elements of the cytoskeleton
It is the disc in the middle of the sarcomere
Where are the myosin filaments cross-linked?
Cross-linked at the centre of the A band by the M-line
What is the role of titin protein?
Titin extends from the X-line to bind the thick filament system to the Z band
What is the role of nebulin protein?
It is an actin-binding protein that extends along the thin filaments and the entire length of the I-band
It regulates the length of the thin filaments
How is muscle contraction initiated?
Action potential arrives at the neuromuscular junction causing release of ACh
How does the action potential reach the T-tubules?
ACh binds to nicotinic receptors
This causes opening of sodium ion channels
Na+ influx leads to an action potential in the sarcolemma that will travel along T-tubules
How is calcium released?
Calcium ions are released from the sarcoplasmic reticulum
What is the role of calcium in muscle contraction?
It binds to the TnC region of troponin
This causes troponin to change shape and move tropomyosin
This exposes the binding site on the actin filament
What happens once the binding site on actin is revealed?
The myosin head attaches to the binding site to form a cross-bridge
At this stage the myosin head is attached to ADP and Pi
How is the power stroke initiated?
Pi generated in the previous contraction cycle is released
The myosin head pivots and bends as it pulls on actin and slides it towards the M line
ADP is then released
What happens when ATP attaches to the myosin head?
It weakens the link between actin and myosin causing the cross-bridge to break
What happens when ATP splits into ADP and inorganic phosphate?
The myosin head is energised and the contraction process repeats
How do the lengths of the A- and I-bands change during contraction?
The width of the A-band stays the same as the myosin does not change position
The width of the I-band is reduced as the Z lines move closer together
What are isotonic contractions?
Contractions that cause the muscle to change length as it contracts
This causes the movement of a body part
How do isotonic contractions allow for the control of movement?
They occur in pairs - one eccentric and one concentric contraction
What is a concentric contraction?
They cause the muscle to shorten as it contracts
When do concentric contractions usually occur?
They are the most common type of muscle contraction and occur frequently in daily and sporting activity
What type of contraction occurs when the elbow bends from straight to fully flexed?
Concentric
Causes contraction of the Biceps Brachii muscle
What is an eccentric contraction?
They cause the muscle to lengthen as it contracts
When do eccentric contractions usually occur?
Usually involves the control or deceleration of a movement
What type of contraction usually is involved in muscle injury?
Eccentric contractions
This type of contraction puts a lot of strain on the muscle
What two muscles are involved when kicking a ball?
The quadriceps muscle contracts concentrically to straighten the knee
The hamstring muscle contracts eccentrically to decelerate the motion of the lower limb
What is an isometric contraction?
Isometric contraction occurs when there is no change in the length of the contracting muscle
Why is carrying an object in front of you an isometric contraction?
The weight of the object pulls the arms down
The muscles are contracting to hold the object at the same level
What type of contraction occurs when you grip something and why?
Isometric contraction
There is no movement in the joints of the hand
The muscles contract to provide a force that is sufficient enough to keep a steady hold
What is a muscle twitch?
The mechanical response of an individual muscle fibre, individual motor unit or a whole muscle, to a single action potential
What is a motor unit?
A motor neurone and all the muscle fibres that it innervates
What is the latent period of muscle twitch?
A delay of a few milliseconds between an action potential and the start of a contraction
It reflects the time for excitation-contraction coupling
What happens in the sarcomere during the latent period of muscle twitch?
The calcium ions have been released from the sarcoplasmic reticulum but have not yet bound to troponin
What is the contraction period of a muscle twitch?
It starts after the latent period and ends when muscle tension peaks
What is muscle tension?
The force of contraction, expressed in grams
What happens in the sarcomere during the contraction period of muscle twitch?
Cytosolic Ca2+ levels are increasing
The amount of released calcium exceeds the amount taken up extracellularly
Myosin head attaches to binding sites on actin as the tropomyosin has been moved aside
What is the relaxation phase of the muscle twitch?
The time between peak tension and the end of the contraction
What happens in the sarcomere during the relaxation period of the muscle twitch?
Cytosolic Ca2+ is decreasing
Reuptake of Ca2+ exceeds its release
Myosin is released from the actin as ATP binds to the myosin head
What is meant by a muscle twitch having reproducibility?
Repetitive stimulation produces twitches of the same magnitude and shape
Unless the twitches follow one another very closely
Why do twitches vary among muscles and muscle fibres?
Differences in size of the muscle fibre
Differences in the speed of contraction among fibres
What type of fibres are type I muscle fibres?
Slow twitch fibres
What colour are type I fibres?
How do they contract and when are they used?
Red due to high concentration of myoglobin and dense capillaries
Resistant to fatigue
Contract slowly and with little force
Large amounts of mitochondria
Used in aerobic activities such as long distance running
What type of fibres are type II muscle fibres?
fast twitch fibres
What are the properties of type IIa fibres?
Very similar to type I
Used in long term anaerobic activities like swimming
Activities lasting less than 30 minutes
What are the colour/properties of type IIb fibres?
White due to low concentration of myoglobin
Fatigue very easily
Low amounts of mitochondria
Contract quickly with a lot of power
Used in short-term anaerobic activities such as sprinting
Activities lasting less than 1 minute
What 3 factors influence the force of muscle contraction?
- number of action potentials per second
- amount of overlap between thin and thick filaments
- number of motor units recruited
How is tetanus reached?
When the frequency of stimulation is high enough, Ca2+ conc reaches peak level
Summation results in the level of tension reaching a plateau - tetanus
How is an unfused/incomplete tetanus reached?
the frequency of stimulation may be high enough to reach tetanus
the tension may oscillate around an average level
How is fused/complete tetanus reached?
at greater stimulation frequencies when Ca2+ reaches peak level, this causes the maximum number of cross-bridges to cycle
the tension plateau will smooth out
When does the muscle reach maximum tetanic tension?
When the muscle is at its maximum sustained tension
When is the maximum number of active cross-bridges achieved?
When the muscle is at the optimum length
What happens when the muscle is stretched beyond the optimum length and why?
The number of active cross-bridges decreases
The overlap between the actin and myosin fibres decreases
What happens as the muscle becomes shorter than the optimum length?
The filaments at opposite ends of the sarcomere begin to overlap one another and interfere with each other’s movements
This causes a slow decrease in tension as the sarcomeres get shorter
As the filaments become shorter, what makes the decrease in tension become steeper?
The thick filaments come into contact with the Z lines
What type of motor units are recruited when a larger force is required?
When larger forces are needed, larger motor units are recruited
How are fine movements controlled?
They are controlled by smaller increments of force generated by the smaller motor units
How does force of contraction increase?
Force of contraction increases as larger motor units with increasing numbers of fibres are recruited
This allows more action potentials to occur
When is an electromyography test used?
It is used in patients when you are unsure what exactly is occurring in the muscle
What does an EMG test measure?
It measures electrical activity in response to stimulation of a motor neurone
This assesses health of muscles and motor neurones that control them
How does an EMG test work?
A recording electrode is attached to the skin over the nerve
A stimulating electrode is placed a known distance away from the recording electrode
Nerve is stimulated with a brief electrical shock given through the stimulating electrode
Time taken for the signal to reach the recording electrode is calculated
What type of muscle disorder is muscular dystrophy?
Genetic muscle disorder
Name 3 neurological muscle disorders?
Myasthenia gravis
Multiple sclerosis
Parkinson’s disease
Name 2 muscle disorders caused by inflammation
myositis
polymyalgia rheumatica
What 4 muscle disorders are caused by injury or overuse?
strain
sprain
cramps
tendinitis
