Muscle Physiology

Front 1

What is the theory that explains muscle contraction?

Back 1

Sliding filament theory. It proposes that changes in overall fiber length are directly associated with changes in the overlap between the two sets of filaments.

Front 2

How does the crossbridge cycle result in shortening of the muscle?

Back 2

1) At rest, ATP has been bound to the myosin head and hydrolyzed, but the energy of the reaction cannot be released until 2) the myosin head can interact with actin.
3) The release of the hydrolysis products is associated with 4) the power stroke.
5) The rotated and still-attached crossbridge is now in the rigor state.
6) Detachment is possible when a new ATP molecule binds to the myosin head and is 7) subsequently hydrolyzed.

Front 3

What is rigor mortis?

Back 3

The stiffening of skeletal muscle after death. There is cessation of metabolic energy production - and therefore no ATP is available to release actin from myosin - and so the muscles are fixed in a stiff contraction.

Front 4

What mechanisms are responsible for the effect of sarcomere length on force production?

Back 4

At longer sarcomere lengths (when the muscle is stretched beyond its normal resting length) fewer myosin heads are able to interact with the actin filaments and force is reduced. At shorter sarcomere lengths, physical factors interfere with further shortening (for example, as thin myofilaments penetrate the A band from opposite sides, they begin to meet in the middle and interfere with each other).

Front 5

Why is the troponin-tropomyosin complex said to have an inhibitory role?

Back 5

It prevents the interaction of myosin and actin, and therefore prevents contraction in the muscle's resting state.

Front 6

How does Ca2+ remove the inhibition caused by the troponin-tropomyosin complex?

Back 6

Ca2+ binds to a subunit of tropomyosin and causes it to changes its position slightly, uncovering the myosin binding sites on the actin filaments.

Front 7

How does Ca2+ release and troponin saturation differ between skeletal and cardiac muscle?

Back 7

In skeletal muscle, enough Ca2+ is released to saturate troponin so that a maximal force is produced. In contrast, in cardiac muscle, troponin is only partially saturated and the strength of the cardiac contraction can be adjusted by controlling the Ca2+ concentration.

Front 8

What is the difference between actin-linked regulation and myosin-linked regulation?

Back 8

Actin-linked regulation is found in skeletal and cardiac muscle; the switching action of the calcium-troponin-tropomyosin complex is extended by the structure of the thin filaments, which allows one troponin molecule (via its tropomyosin connection) to control seven actin monomers. Myosin-linked regulation is found in smooth muscle; in this case, the calcium switch's effect is exerted on thick (myosin) filaments.

Front 9

What causes muscle relaxation?

Back 9

Ca2+ pumps in the sarcoplasmic reticulum re-sequester calcium, reducing its concentration so that it no longer binds to troponin. Contractile activity ceases and relaxation begins.

Front 10

What is the role of creatine phosphate in muscle cells?

Back 10

When ATP is split, the resulting ADP is rephosphorylated by phosphocreatine. This keeps the ATP supply stead until ATP can restore the creatine phosphate pool.

Front 11

What is the major difference between glycolysis and oxidative phosphorylation?

Back 11

Oxidative phosphorylation is aerobic and generates 36 more ATPs than anaerobic pathways like glycolysis, which only produces 2 ATPs.

Front 12

What is a twitch?

Back 12

A single brief contraction caused by a single action potential.

Front 13

What is tetanus?

Back 13

A sustained contraction due to rapid, repeated motor signals.

Front 14

What is the tetanus-twitch ratio?

Back 14

The amount of force produced in a tetanus compared to the amount of force produced in a twitch; the force of tetanus is typically several times that of a twitch.

Front 15

Why is the force of tetanus several times that of a twitch?

Back 15

During a twitch, the relaxation processes begin to operate as soon as the muscle is activated; full activation lasts less time than is required for the muscle to reach its peak force. Also, internal dimensional changes take place upon activation of a muscle, and the deformable structures make up the series elastic component (SEC) of the muscle. The brief activation time of a twitch is not sufficient to extend the SEC fully; repeated activation in tetanus allows time for the internal "slack" to be more fully taken up.

Front 16

What is a motor unit?

Back 16

A single motor axon and all the fibers it innervates.

Front 17

What is motor unit summation?

Back 17

The change in force produced by a muscle as more motor units become active.

Front 18

What is isometric contraction? What is isotonic contraction?

Back 18

Isometric: no change in muscle length; it is a development of force.
Isotonic: changes of length but no change in force.

Front 19

Describe the different contraction and relaxation phases of a single isotonic twitch.

Back 19

1) Rise of isometric force, until sufficient force has developed to lift the weight.
2) Isotonic shortening and 3) isotonic relaxation as the force is constant.
4) Isometric relaxation because the muscle is no longer lifting the weight.

Front 20

How do changes in afterload affect the phases of a single isotonic twitch?

Back 20

The duration of the early isometric portion depends on the afterload. At low afterloads, the muscle requires little time to develop sufficient force to begin to shorten, and conditions will be isotonic for a longer time. At very high afterloads, the muscle cannot exert enough force to move the weight, and the contraction is isometric for its entire duration.

Front 21

Light loads can be moved faster than heavy loads. Describe an experimental set-up that can be used to analyze this.

Back 21

A muscle is arranged to move an afterload of varying masses. The muscle is then stimulated to contract. The distance that the mass moves and how fast it moves are measured.

Front 22

What does "Vmax" represent?

Back 22

It is the maximum velocity that a muscle can contract when maximally stimulated with a theoretical condition of no afterload.

Front 23

How does Vmax relate to ATPase activity?

Back 23

Vmax represents the maximal rates of ATPase activity; ATPase activity is the limiting factor to how quickly crossbridges can be formed, broken, and reformed between myosin and actin.

Front 24

What are the 3 structural arrangements of smooth muscle cells?

Back 24

Circular organization around the circumference of a hollow space
A combination of circular and longitudinal layers
Complex fiber arrangements, layers of varying alignments

Front 25

What provides the relengthening force for each of the 3 arrangements of smooth muscle?

Back 25

Circular: pressure of contents
Combo of circular and longitudinal layers: contents of lumen
Complex: gradual accumulation of contents

Front 26

What is a sphincter?

Back 26

A circular band of muscle whose contraction restricts flow or stops it completely

Front 27

What are the 2 sources of Ca2+ in smooth muscle?

Back 27

Extracellular fluid
Sarcoplasmic reticulum

Front 28

What must occur for smooth muscle to relax?

Back 28

Ca2+ must be pumped out of the cell and back into the SR.

Front 29

What are the major routes of Ca2+ into and out of the cytoplasm of smooth muscle cells?

Back 29

Calcium enters through voltage-gated channels, ligand-gated channels, "leak" channels, and from the SR.
Second messengers can also cause calcium release from the SR
Calcium exits the cell or enters the SR via a calcium ATPase pump; it also exits the cell via a Ca2+/Na+ exchanger

Front 30

Compare the characteristics of regulatory systems for skeletal muscle (actin-linked) and smooth muscle (myosin-linked).

Back 30

In smooth muscle, the actin filaments lack troponin and tropomyosin, which keeps skeletal muscle in a state of inhibited readiness. Instead, myosin must be phosphorylated by myosin light chain kinase which is activated by the binding of calcium to calmodulin.

Front 31

List the sequence of events starting with an increase in cytoplasmic Ca2+ concentration and leading to the activation of the crossbridge cycle in smooth muscle.

Back 31

Calcium enters the sarcoplasm and binds calmodulin, activating the myosin light-chain kinase (MLCK). MLCK phosphorylates the regulatory myosin light chain, changing it to an active form. The phosphorylated myosin can participate in the mechanical crossbridge cycle.
**It is important to remember that variations in the internal [Ca2+] can cause corresponding gradations in the contractile force in smooth muscle.

Front 32

What is the role of myosin light chain phosphatase in smooth muscle?

Back 32

It dephosphorylates the myosin, making it inactive. As a result, the cross-bridge cycle stops and the muscle relaxes.

Front 33

Compare smooth muscle to skeletal muscle in terms of their force-velocity curves.

Back 33

The maximum shortening velocity of smooth muscle is typically 100 times lower than that of skeletal muscle.

Front 34

Compare smooth muscle to skeletal muscle in terms of their length-tension curves.

Back 34

The length tension curves of smooth and skeletal muscle have similar shapes, but smooth muscle can function at much shorter lengths and a greater range of lengths than skeletal muscle.