Lecture 33: Muscle Cells

Question 1

Describe a muscle cell

Answer 1

• regarded as a excitable cells
• a muscle is made up of many muscles fibres (cells)
• each cells is surrounded by an excitable membrane called the sarcollema
• the nuclei are found just below the sarcollema
• skeletal muscles are multinucleated
• there is a network of membrane criss-crossing each cell

Question 2

What is the network of membrane criss-cross of each cell

Answer 2

the transverse system (T-system) is continuous with the sarcolemma

• the longitudinal system, the sarcoplasmic reticulum, is separate from the T system but often in close proximity to it. It’ contains a high concentration of Ca2+ due to s specific Ca2+ pump
• within the cell there are many parallel myofibrils immersed in the cytoplasm
• mitochondria are often found clustered in rows between myofibrils

Question 3

Each myofibril consists of 2 kinds of interacting protein filaments:

Answer 3

thick filaments- primarily contain the protein myosin
-thin filaments-contain actin, troponin and tropomyosin
The thick and thin filaments interact via cross-bridges, which emerge at regular intervals from the thick filaments
-these cross-bridges actually represent the site where the contractile force is generated

Question 4

Explain the sliding filament theory

Answer 4

When a muscle contracts, the filaments remain the same length but slide over each other decreasing the overall length of the myofibrils by as much as a third of its original length.

• achieved by the cross-bridges entering into a continuous association-dissociation reaction - a process requiring ATP hydrolysis
• the ATPase activity is associated with the myosin filaments

Question 5

What is the physiological regulator of muscle contraction?

If ATP and its hydrolysis causes an association-dissociation if the two filaments, what allows the muscle to be relaxed?

Answer 5

-it has been shown that the other 2 components of the thin filaments, troponin and tropomyosin, inhibit the interaction between actin and myosin,
-this inhibition is overcome by Ca2+
Therefore the association-dissociation cycle and accompanying ATP hydrolysis can only occur in the presence of Ca2+

Question 6

Excitation: contraction coupling

Answer 6

1. The arrival of a nerve impulse at the neuromuscular junction causes depolarisation of the sarcolemma via acetylcholine.
2. This depolarisation is transmitted to the interior if the muscle fibre by the T-system
3. The close proximity of the T-system to the sarcoplasmic reticulum causes a sudden increase in its permeability to Ca2+ and release of the previously accumulated Ca2+
4. The high Ca2+ in the sarcoplasmic activates the actomyosin association-dissociation cycle initiating a contraction
5. Subsequently, with no further stimulation, the permeability of the sarcoplasmic reticulum returns to normal and the Ca2+ pump re-accumulates the released Ca2+

Question 7

How to muscles sustain their activity when they use up ATP so rapidly?

Answer 7

For more sustained effort ATP has to be regenerated via catabolism.
-to allow for rapid contraction eg sprint events, skeletal muscles use lactic acid fermentation. Although this yields less ATP than aerobic metabolism, the rate of ATP supply is faster. Because the lactic acid builds up, this soon results in fatigue
-for periods beyond 1-2 minutes, like in triathlon, muscle contraction is dependant on the rate of supply of ATP by aerobic respiration
Refer to page 131 in lab book for picture (do it)