Skeletal Muscle physiology

Question 1

sarcomere

Answer 1

fundamental contractile unit

Question 2

Titin

Answer 2

passive elastic elements of the sarcomere that hold the structure together and provide resistance to stretching/elasticity

Question 3

Where is myosin in the sarcomere?

Answer 3

attached to the thick filaments

Question 4

What are the thin filaments primarily made up of?

Answer 4

actin

Question 5

What molecule drives the movement of myosin heads/how?

Answer 5

ATP - hydrolysis of ATP drive conformational changes of myosin heads that allow them to attach, move, and detach

Question 6

What is the cause of rigor mortis?

Answer 6

Myosin heads get stuck to actin filaments when there is no more ATP available to remove it.

SERCA pumps fail to remove calcium and sequester it back into the SR, leaving binding sites open

Question 7

What are the steps of E-C coupling in skeletal muscle?

Answer 7

1. axon terminal of somatic motor neuron releases ACh at motor end-plate.
2. action potential is created.
3. action potential is propagated along the muscle fiber.
4. action potential stimulates the Cav1.2 calcium channel, which opens RyR, allowing Ca2+ to flow from the cytoplasm into the sarcoplasmic reticulum
5. Calcium binds to Tropomyosin, which makes myosin binding sites available, allowing for contraction
6. Calcium reuptake is performed by SERCA back into the SR

Question 8

What is the purpose of T-tubules?

Answer 8

allows excitation to travel deep into the muscle fibers

Question 9

What is the purpose of the sarcoplasmic reticulum?

Answer 9

provides internal calcium stores

Question 10

Characteristics of type-I slow-twitch fibers

Answer 10

lower density RyR, Cav1.1, which means slower Ca2+ release - slower increase in force

lower density of SERCA, which means slower sequestering of Ca2+ - slower relaxation of force

Question 11

Characteristics of type-II fast-twitch fibers

Answer 11

Higher density of RyR and Cav1.1 - faster Ca2+ release and faster increase in force

higher density of SERCA - faster calcium sequestration and faster relaxation of force

Question 12

How do we have graded control over strength of contraction of muscles?

Answer 12

1. twitch isn’t an all-or-none process, can have some summation
2. smaller, weaker motor units (with smaller axons) are recruited first, followed by alpha motor neurons with larger axons

Question 13

What is the mechanism underlying size-ordered recruitment?

Answer 13

Resistance is higher in smaller motor neurons due to having fewer ion channels -> synaptic input will generate bigger EPSPs due to higher resistance -> smaller motoneurons innervate fewer muscle fibers so the elicit less force