MSK 20 - Muscle Physiology-I

Question 1

Answer 1

Question 2

What type of ACh receptors in the NMJ?

Answer 2

nicotinic

Question 3

What does the EPP depend upon? What occurs if the EPP does not reach threshold?

Answer 3

Magnitude of EPP depends upong the amount of Ach released (graded response)

If EPP does not reach threshold then it dissipates and no AP occurs

Question 4

What is a T-tubule and what is its purpose?

Answer 4

Transverse tubules (T-tubules) are extensions of the sarcolemma that penetrate into the centre of skeletal and cardiac muscle cells. The contain large amounts of ion channels that allow action potentials to enter rapidly into the cell, to initiate muscle contraction.

Question 5

Differentiate the L-type calcium channel from the ryanodyne receptor from the dihydropyridine receptor.

Answer 5

L-type calcium channel/dihydropyridine receptors are the same thing. They are voltage gated calcium channels they stay activaged for longer (L-type) than usual. They also take a long time to become activated. In skeletal muscle, their purpose is to sense a voltage change and undergo a conformational change that opens the ryanodyne receptor in the SR membrane. Very little to no Ca++ passes through the DHP receptors.

Question 6

Describe how an EPP leads to an increase in cytosoloic [Ca²⁺] in skeletal muscle cells.

Answer 6

Once an EPP reaches threshold the V-gated Na+ channels at the edge of the motor end plate open up triggerin an AP wave across the sarcolemma that continues into the T-tubules. The L-type V-gated Ca++ channel then undergoes a conformational change that then opens the ryanodyne receptor on the SR. This releases Ca++ from the SR into the cytosol.

Question 7

Why is the L-type sensor an important adaptation for skeletal muscle?

Answer 7

Question 8

Answer 8

Question 9

How does cytosolic {Ca++] decrease again during skeletal muscle relaxation?

Answer 9

The sarco/endoplasmic reticulum Ca++ ATPase or SERCA actively pumps Ca++ from the cytosol back into the SR.

Question 10

What is calsequestrin and why is it important?

Answer 10

Calsequestrin is a Ca++ binding protein found within the SR. It serves to decrease the free [Ca++] in the SR assisting SERCA in pumping Ca++ into the SR against a concentration gradient.

Question 11

How does an increase in cytosolic [Ca++] casuse muscle contraction?

Answer 11

Ca++ binds to tropinin-C (TnC) causing a conformational change that will move tropomyosin from the mysoin binding sites on actin. This will the allow crossbridge cycling to occur.

Question 12

Decribe the concept of all or none contraction.

Answer 12

Question 13

Answer 13

Question 14

Describe the entire myosin crossbridge cycle

Answer 14

Question 15

How controls the rate of crossbridge cycling? Can this be modifided in skeletal muscle?

Answer 15

Question 16

During a power stroke, how much does the myosin head move?

Answer 16

It moves from a 90 to a 45 degree angle

Question 17

How do the different sarcomere bands/lines change/move during muscle contraction?

Answer 17

Question 18

What is the optimal sarcomere length for reaching maximal force?

Answer 18

2.0-2.2um

Question 19

T/F - The shorter the sarcomere, the more filament overlap, and the stronger the contraction force.

Answer 19

False - once the sarcomere gets shorter than 2um the acting filaments begin to overlap as well and interfere with myosin binding. This lessens the maximal contractile force the sarcomere can produce

Question 20

What occurs if a sarcomere becomes extremely shortened?

Answer 20

The thick and thin filaments beging to contact the Z lines and the max force that the sarcomere can produce falls to zero.

Question 21

Describe the force to length relationship from shortest to longest possible sarcomere length.

Answer 21

From the shortest possible sarcomere length to the optimal length, force is directly proportional to length.

Within the optimal length range (2.0-2.2um), force and length are independent of each other

At lengths greater than the optimal length range, force is inversely proportional to length

Question 22

What occurs/causes muscle atrophy and hypertrophy?

Answer 22

Muscle Atrophy - wasting away of the muscle caused by disuse or denervation. If chronic, muscle tissue transitions into connective tissue and cannot be reversed

Muscle Hypertrophy - an increase in muscle cell contents (ie - myofilaments, SR, & mitrochondria) but not muscle cell number caused by forceful and repetitive muscular acrtivity

Question 23

What is botulism? Describe its cause, mechanism of action, and symptoms.

Answer 23

Question 24

What is Myasthenia Gravis? What are its symptoms?

Answer 24

Question 25

Facts to know about curare.

Answer 25