Muscle Physiology

Question 1

How is skeletal muscle contraction initiated?

Answer 1

Stimulation by the motor neuron

Question 2

Provide the steps of excitation-contraction coupling in skeletal muscles

Answer 2

1. An AP in the muscle membrane
2. Voltage-gates Ca+ channel opens and initiates the release of Ach to the synaptic cleft
3. Ach binds to a ligand gated receptor which results in depolarization
4. If threshold is reached, Na+ voltage gated channels open and the AP is propagated into the T-tubule
5. An L-type Ca+ channel couples with the ryanodine receptor and increases intracellular Ca+
6. Ca+ binds troponin-C and initiates the cross-bridge cycle
7. Ca+ is reabsorbed into the SR by SERCA and the cross-bridge cycle stops

Question 3

Describe the actin-myosin cross-bridge cycle steps

Answer 3

1. The active site on actin is exposed as Ca+ binds troponin-C
2. Myosin head forms a cross-bridge with actin
3. During the power stroke, the myosin head bends, releasing ADP + Pi
4. ATP attaches to the myosin head, causing detachment of the bridge
5. ATP is hydrolyzed which allows for myosin to reattach

Question 4

What is the relationship between calcium concentration and force generation?

Answer 4

An increases cytoplasmic calcium concentration will result in contraction

Question 5

What is a twitch?

Answer 5

A single action potential leading to a spike in calcium that initiates a small contractile force

Question 6

What is tetanus?

Answer 6

Released stimulation of muscle before it had time to relax, leading to sustained increases in calcium and sustained force generation

Question 7

What is temporal summation?

Answer 7

Occurs when two excitatory inputs arrive at a postsynaptic neuron in rapid succession and add in a stepwise fashion

Question 8

What is an isometric contraction?

Answer 8

A contraction that generates force with no shortening, the muscle length is held constant and the developed tension during contraction is measured

Question 9

What is passive tension in the length-tension relationship?

Answer 9

The tension developed by stretching the muscle to different lengths

Question 10

What is total tension in the length-tension relationship?

Answer 10

The tension developed when the muscle is stimulated to contract at different lengths

Question 11

What is active tension in the length-tension relationship?

Answer 11

The difference between total and passive tension and represents the active force developed from contraction of the muscle which is proportional to the number of cross-bridge cycles

Question 12

What does the length-tension relationship describe?

Answer 12

The amount of tension generated depends on the length of the muscle before it was stimulated

Question 13

Tension (force) generated under isomeric contraction is influenced by what?

Answer 13

The number of cells that are stimulated, the frequency of the stimulation, length of muscle before stimulation

Question 14

What is isotonic contraction?

Answer 14

shortening against a constant force, the load is held constant and when the muscle is stimulated to contract, shortening is measured

Question 15

What is the force-velocity relationship?

Answer 15

measuring the velocity of shortening of isotonic contractions when the muscle is challenged with different loads, the velocity of shortening decreases as the load increases

Question 16

What determines the difference in velocity in the force-velocity relationship?

Answer 16

velocity is determined by the number of cross-bridges taking part in the contraction

Question 17

Does Vmax change as muscle lenth changes in the force-velocity relationship?

Answer 17

no, Vmax is dependent on how fast a single cross-bridge can cycle unimpeded

Question 18

What is a major role in the contraction of smooth muscle?

Answer 18

regulating the diameter of the organ which is important for blood pressure and digestion

Question 19

Describe multi-unit smooth muscle

Answer 19

Composed of discrete, separate SM fibers, each operated independently of the others and is often innervated by a single nerve ending

Question 20

Define unitary smooth muscle

Answer 20

Also called syncytial or visceral SM, the fibers are arranged in sheets or bundles and their cell membranes adhere to one another at gap junctions. Ions can flow without APs.

Question 21

What are the steps in smooth muscle contraction?

Answer 21

1. depolarization of the cell membrane that then opens voltage-gated Ca+ channels
2. intracellular Ca+ concentration increases and more is released from the SR
3. Ca+ binds calmodulin then together they bind to an active myosin light chain kinase
4. An active myosin light chain kinase phosphorylates myosin and allows it to bind actin and initiate the cross-bridge cycle
5. a decrease in intracellular Ca+ produces relaxation

Question 22

What determines the amount of tension produced in smooth muscle contraction?

Answer 22

the amount of tension produced is proportional to the intracellular calcium concentration

Question 23

What is the length-tension relationship modulated by in smooth muscle?

Answer 23

alternating the length of the muscle, the Vmax can be shifted up or down through stimulation of the sympathetic and parasympathetic nervous systems