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Nerve & Muscle > Molecular Mechanism of Muscle Contraction > Flashcards

Molecular Mechanism of Muscle Contraction Flashcards

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1
Q

What is a fascicle?

A

A bundle of structures

e.g. bundle of muscle fibres

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2
Q

What is a pennate muscle?

What is their motion like?

A

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

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3
Q

What are the types of pennate muscle?

A

Unipennate
Bipennate has the tendon in the centre
Multipennate

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4
Q

What does a multipennate muscle allow?

A

e.g. deltoid in the shoulder

Allows movement in a multidirectional manner

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5
Q

What is a fusiform muscle and what is its motion like?

A

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

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6
Q

What are parallel muscles?

A

They have fascicles that lie parallel to the long axis of the muscle

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7
Q

What are aponeuroses and where are they found?

A

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

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8
Q

What are convergent muscles and what is their motion like?

A

Broad attachment from which the fascicles converge to a tendon

Arrangement of fibres allows for maximum force production

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9
Q

Where are circular muscles found?

A

Around a body opening or orifice

The opening is constricted when the muscle contracts

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10
Q

What is the structure of skeletal muscle?

What contractions is it involved in?

A

It is striated and multinucleated

It does not have branches attached to its skeleton

Involved in voluntary contractions

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11
Q

What is the structure of cardiac muscle?

What contractions is it involved in?

A

It is striated but each cell has its own nucleus

It is branched

It is involved in involuntary contractions

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12
Q

What is the structure of smooth muscle?

What contractions is it involved in?

A

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

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13
Q

What is the role of the tendon?

A

It attaches the muscle to the bone

The tendon leads to the muscle belly

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14
Q

What surrounds a muscle?

A

A connective tissue sheath called the epimysium

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15
Q

How are “muscle compartments” created and what surrounds them?

A

Portions of the epimysium project inwards to divide the muscle into compartments

Each compartment contains a fasciculus surrounded by the perimysium

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16
Q

What is the arrangement of connective tissue within the fasciculus?

A

Each individual muscle cell (fibre) is surrounded by a layer of connective tissue called the endomysium

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17
Q

What is the role of connective tissue layers within the muscle?

A

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

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18
Q

What is found beneath the endomysium?

A

The plasma membrane of the muscle cell - the sarcolemma

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19
Q

What is the majority of the volume of the muscle cell filled with?

A

Numerous long myofibrils

These contain two types of filament - thick and thin

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20
Q

What do thin filaments consist of?

A

Two strands of actin arranged in a double helix

Troponin and tropomyosin molecules cover binding sites on actin

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21
Q

What do thick filaments consist of?

A

Groups of myosin

Each myosin filament forms a protruding head at one end

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22
Q

What is a sarcomere?

A

A contractile segment of muscle

It is the distance between 2 Z lines

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23
Q

How are actin and myosin filaments arranged?

A

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

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24
Q

Where is the I band found?

Why does it have this name?

A

The zone of thin filaments that is not superimposed by thick filaments

Isotropic as the structure is uniform

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25
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
26
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
27
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
28
Where are the myosin filaments cross-linked?
Cross-linked at the centre of the A band by the M-line
29
What is the role of titin protein?
Titin extends from the X-line to bind the thick filament system to the Z band
30
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
31
How is muscle contraction initiated?
Action potential arrives at the neuromuscular junction causing release of ACh
32
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
33
How is calcium released?
Calcium ions are released from the sarcoplasmic reticulum
34
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
35
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
36
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
37
What happens when ATP attaches to the myosin head?
It weakens the link between actin and myosin causing the cross-bridge to break
38
What happens when ATP splits into ADP and inorganic phosphate?
The myosin head is energised and the contraction process repeats
39
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
40
What are isotonic contractions?
Contractions that cause the muscle to change length as it contracts This causes the movement of a body part
41
How do isotonic contractions allow for the control of movement?
They occur in pairs - one eccentric and one concentric contraction
42
What is a concentric contraction?
They cause the muscle to shorten as it contracts
43
When do concentric contractions usually occur?
They are the most common type of muscle contraction and occur frequently in daily and sporting activity
44
What type of contraction occurs when the elbow bends from straight to fully flexed?
Concentric Causes contraction of the Biceps Brachii muscle
45
What is an eccentric contraction?
They cause the muscle to lengthen as it contracts
46
When do eccentric contractions usually occur?
Usually involves the control or deceleration of a movement
47
What type of contraction usually is involved in muscle injury?
Eccentric contractions This type of contraction puts a lot of strain on the muscle
48
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
49
What is an isometric contraction?
Isometric contraction occurs when there is no change in the length of the contracting muscle
50
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
51
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
52
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
53
What is a motor unit?
A motor neurone and all the muscle fibres that it innervates
54
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
55
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
56
What is the contraction period of a muscle twitch?
It starts after the latent period and ends when muscle tension peaks
57
What is muscle tension?
The force of contraction, expressed in grams
58
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
59
What is the relaxation phase of the muscle twitch?
The time between peak tension and the end of the contraction
60
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
61
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
62
Why do twitches vary among muscles and muscle fibres?
Differences in size of the muscle fibre Differences in the speed of contraction among fibres
63
What type of fibres are type I muscle fibres?
Slow twitch fibres
64
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
65
What type of fibres are type II muscle fibres?
fast twitch fibres
66
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
67
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
68
What 3 factors influence the force of muscle contraction?
1. number of action potentials per second 2. amount of overlap between thin and thick filaments 3. number of motor units recruited
69
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
70
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
71
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
72
When does the muscle reach maximum tetanic tension?
When the muscle is at its maximum sustained tension
73
When is the maximum number of active cross-bridges achieved?
When the muscle is at the optimum length
74
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
75
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
76
As the filaments become shorter, what makes the decrease in tension become steeper?
The thick filaments come into contact with the Z lines
77
What type of motor units are recruited when a larger force is required?
When larger forces are needed, larger motor units are recruited
78
How are fine movements controlled?
They are controlled by smaller increments of force generated by the smaller motor units
79
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
80
When is an electromyography test used?
It is used in patients when you are unsure what exactly is occurring in the muscle
81
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
82
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
83
What type of muscle disorder is muscular dystrophy?
Genetic muscle disorder
84
Name 3 neurological muscle disorders?
Myasthenia gravis Multiple sclerosis Parkinson's disease
85
Name 2 muscle disorders caused by inflammation
myositis | polymyalgia rheumatica
86
What 4 muscle disorders are caused by injury or overuse?
strain sprain cramps tendinitis

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