Muscles Flashcards
What is a motor unit?
A group of muscle fibres innervated by a single neuron
Where do T tubules invaginate the sarcoplasm?
At the junction of the I and A bands
What is a triad?
A T tubule flanked by two terminal cisternae of the SR
What hydrolyses ATP in the actin/myosin structure?
The globular myosin head which is as ATPase
What are you seeing when striations appear?
The organisation of thick and thin filaments
What becomes narrower when the sarcomer contracts?
The H and I bands
Where is the A band
Extends length of thick filaments
Where is I band
Extends from the ends of each group of thick filaments between two sarcomeres. Includes the Z disc in the middle
Where is the H band
Extends the distance between the thin filaments in the middle of the sarcomere.
What promotes the myosin head to hydrolyse ATP?
Actin binding
Steps of cross bridge cycle
1) Cross bridge formation. Ca2+ release exposes actin binding sites on actin filaments. ATP hydolyses on the myosin heads and they become ‘cocked’ and bind to actin binding sites. Inorganic phosphate is released and the bond between actin and myosin becomes stronger
2) The power stroke. With the release of ADP the myosin head move sliding the thin filament towards the M line
3) Cross bridge detachment. Another ATP binds to myosin head releasing the bond between actin and myosin
4) reactivation of the myosin head. ATP is hydrolysed activating the myosin head.
What level is Ca2+ when muscle is relaxed and contracted?
Relaxed
What process removes Ca2+ from the cytoplasm?
Active transport. Ca2+ channels close (voltage gated), pumps return Ca2+ to stores (SR) and extracellular space
What is the optimal length of a sarcomere for contraction?
Between 2.0 and 2.2 micrometers
What is resting membrane potential of the muscle cell?
-90mV
Describe Ca2+ transient, stimulus, change in levels, return of Ca2+ to SR
AP in t-tubules causes SR to open Ca2+ channels.
Ca2+ levels change from 10-7 to 10-5
Ca2+ is then actively transported back to the SR via a Ca-ATPase pump.
What is the muscle contraction called that is a result of one AP?
A twitch
Three methods of ATP production for muscle energy
1) The enzyme creatinine phosphokinase transfers the phosphate group from Creatinine phosphate to ADP
2) Anaerobic- glucose is glycolysised to form pyretic acid which produced 2 ATP snd also lactic acid. Fast action but can’t be prolonged due to build up of metabolic products that inhibit cell reactions
3) Aerobic metabolism. Slow and steady oxidative form of ATP production via cellular respiration and the citric acid cycle.
Control of muscle tension can occur with two mechanism…?
Increase the frequency of the AP
Recruit additional motor units
Describe temporal summation
AP is shorter than Ca release, so there is still residual Ca if another AP occurs in quick succesion. If frequency of AP is continued the muscle reaches a higher steady state called a tetanus contraction.
Three muscle fibre types
Type 1, slow oxidative. Type 2B, fast glycolytic. Type 2A (fast oxidative)
Describe Type 1 muscle fibres
Slow ATPase rate
Have a low maximum force production
Oxidative in metabolism
Large amounts of myoglobin and mitochondria giving them red appearance
High oxygen demand offset by small diameter allowing rapid O2 diffusion
Describe type 2B muscle fibres
Glycolytic in metabolism
high ATPase rate
low amounts of myoglobin and mitochondria
white in colour
Large diameter
Fatigue rapidly but generate large forces for short periods of time.
Type 2A muscle fibres
Not found in humans
Glycolytic and oxidative
Large quantity of myoglobin and mitochondria
High ATP ase rate
How many types of muscle fibre can one nerve cell innervate?
One… So the motor unit is given the same classification as that type
Describe skeletal muscle fibre recruitment
Many more type 1 muscle fibres so these are recruited first in greater numbers, type 2b fibres and the last to be recruited.
Main types of fibres in cardiac muscle
Oxidative
Where do the t tubules invaginate the cardiac muscle fibres?
At the Z disc
Difference between skeletal SR and cardiac SR
Cardiac SR less developed
Describe the initiation of contraction in cardiac muscle
It is myogenic, in that it initiates in the muscle fibre. Contraction is initiated in the senatorial node where it spreads via specialised conduction fibres know as purkinje fibres.
Describe action potential in cardiac muscle fibres
10Depolarisation due to large Na+ influx
2) Plateau due to slow release Ca2+ channels opening and influx of Ca2+
3) Repolarisation due to Ca2+ channels inactivating and K+ channels opening, K+ efflux
What controls the regulation of the heart rate?
Symapthethic and parasympathetic stimulation. Sympathethic increases (noradrenaline) Parasympathetic decreases (vagus nerve)
What controls the force of the heart contraction?
Increasing the rate of firing
Increase the diameter of the ventricle (stretch)
Use neurotransmitters to alter rate and calcium handling
How long is an action potential in skeletal muscle
1-2mSec
how long is a muscle twitch in skeletal muscle
20-200msec
