2. Muscular Contractions - NS Flashcards
Events during a muscle twitch after single nerve activation:
a) Latent period
b) Contraction
c) Relaxation
a) Latent period
Motor end-plate depolarisation
Depolarisation (AP) transmitted down T tubules
Ca2+ channels open in SR
[Ca2+] in the sarcoplasm
Ca2+ binds to troponin revealing myosin binding site on actin
b) Contraction
Myosin binds to actin, moves (powerstroke, ADP ejected), releases (new ATP binds) and reforms many times causing sarcomeres to shorten.
c) Relaxation
Ca2+ actively transported back into SR
Troponin-tropomyosin complex blocks myosin binding
Muscle fibre lengthens passively (relaxation)
SR
Sarcoplasmic reticulum
MF
Muscle fibre
Motor UNIT
Motor unit = 1 motor neuron & its muscle fibres
1 motor neuron branches and contracts ….
several muscle fibres
(Number of muscle fibres depends on the muscle)
Fine motor control requires a smaller ratio of…
Muscle fibres to nerve fibres
- extraocular muscles (the eye) 1:10
- the gastrocnemius (calf) 1:2000
Recruitment of force depends on the number of active muscle fibres
Muscles are made up of many motor units
One nerve impulse on one nerve =
activation of 1 motor unit = small contraction over whole muscle
Activation of more motor neurons = more motor units =
more muscle fibres = more contractile force
Gradation of force depends on the recruitment of motor units
3 Basic principles
1) The all or nothing principle
2) Threshold
3) Recruitment
1) The all or nothing principle
The skeletal muscle fibre/motor unit either operates or it does not
2) Threshold
If the threshold stimulus for a nerve is reached and the threshold for muscle contraction is reached, the muscle fibre will contract, otherwise it will not
3) Recruitment
The greater the force of contraction needed, the more motor units (one nerve and its associated innervated muscle fibres) are required. Each motor unit operates in an all or none fashion
Recruiting motor units by increasing stimulus intensity:
> Controls the force of contraction (in absence of internal changes, such as fatigue, fibres will contract ‘fully’ each time)
> The more motor units the bigger the twitch
Is there Maximum for tension at stimulus?
YES
Stimulation frequency and contractile force
Consider:
Lowest frequencies
Low frequencies (slightly higher than above described)
High frequencies
Lowest frequencies
Muscle fibres relax fully before next AP arrives
>
Twitches (tension returns to baseline – not shown)
Low frequencies (slightly higher than LOWER frequencies)
Next AP arrives before fibres are fully relaxed
AP
Action Potential
High frequencies (other graph)
No time for the muscle fibres to relax before the next AP arrives
Tetanus
Calciums role in high frequencies -
Ca2+ continually available, enabling MAXIMUM contraction