Activation and mechanical properties of skeletal muscle Flashcards
what are the types of muscle contraction that occurs during skeletal muscle contraction?
- isotonic contraction
- isometric contraction
what is isotonic contraction?
- isotonic contraction means with the same force, and is movement against a force
what is isometric contraction?
- isometric contraction means with the same length, and is contraction against an immovable force
- any load too large for the muscle to move is called an isometric contraction
what determines active force?
the amount of overlap between actin and myosin, as this changes the max. force produced will also change
what determines the active tension?
The amount of active tension that can be created depends upon the length of the sarcomere at the time.
what is passive force determined by?
- Passive force is due mostly to elastic proteins known as titin, which stretch like springs, so passive tension increases with sarcomere length.
- titin:
. It is key to 1) creating contractile force when a muscle is highly extended and, 2) ensuring that myosin (thick) filaments stay parallel to the actin (thin) filaments.
how does the length-tension relationship work?
- Passive tension results from muscle being stretched.
- Active tension represents force developed during cross-bridge cycling.
- Total tension is the sum of these.
Note that many joints are limited to keep your muscles in the middle range
what else can affect tension?
- Differences occur on the basis of
• Fibre length
• Fibre diameter
• Energetics of muscle contraction
These allow different muscle types to contract in a way that is suitable for each of their roles.
If the duration of depolarisation is the same for each of these muscles, they create force for different amounts of time. In the case shown here, we are looking at an isometric twitch of each muscle. A twitch is a response to a single, short stimulus. Note that this is NOT a proper power curve, which would measure force x distance / time. The difference is that, when looking at a twitch, we assume the distance to be negligible (almost zero).
The longer the fibre the more actin and myosin can interact
The wider the diameter the more actin and myosin are interacting
slow vs fast fibres
Histologically, we can see these differences:
Length can simply be measured.
Diameter can simply be measured.
Energetics can be implied by the colouration of the fibres, as darker fibres have more mitochondria and rely more on aerobic respiration.
slow (type I) fibres
Type I fibres
• endurance activity
• long periods of activity, low levels of tension
• fatigue resistant
Examples
• postural muscles
• soleus (used in walking, long distance running, functional activities)
There are many differences, but the overall effect is that slow (Type 1) fibres produce slow, but steady contractions that rely on oxidative phosphorylation (aerobic respiration) and don’t tire easily (these are prevalent in ‘dark meat’). Fast fibres rely on glycolysis (anaerobic resp.) and tire easily, but they are faster to act (these fibres are prevalent in ‘white meat’).
fast (type II) fibres
Type II fibres
burst activity
fast, powerful explosive contractions
fibres fatigue easily
Examples
• sprinting
• weight lifting
e.g. gastrocnemius
motor units
A motor unit is all fibres innervated by a single neuron.
The size of the motor unit depends upon muscle type and the level of detail and range of force that is required of the overall muscle.
Consider:
• Laryngeal muscle (2-3 fibres)
• Soleus muscle (100s of fibres)
force summation
A muscle can be instructed to contract with different amounts of force in two ways:
Recruit increasing number of motor units (spatial summation)
Increase frequency of contractions (resulting in frequency summation)
what is spatial summation
Notice that a single nerve has many neurons and that each neuron leads to multiple fibres (comprising its motor unit). If more than one neuron sends a signal, the muscle will contract more strongly because more fibres are being recruited.
what is temporal summation
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