What does excitation-coupling include?

1. AP

2. T-tubule

3. SR

4. Muscle fibers (thick and thin)

E-C coupling covers events from the action potential in muscle cell through  initiation and termination of crossbridge cycling

      1.   Action potential propagation

      2.   Transverse tubular depolarization

Transverse tubular membrane charge movement

      3.   Ca²⁺ release from terminal cisternae of sarcoplasmic reticulum (SR)

      4.   Ca²⁺ binding to troponin-tropomyosin complex

Ca²⁺ removal by Ca²⁺ ATPase of the sarcoplasmic reticulum

      Ca²⁺ storage in the terminal cisternae and relaxation of muscle

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1.  Sarcolemma is the surface plasma membrane

2. Transverse tubule (T-tubule) is an invagination of the sarcolemma

3. Sarcoplasmic reticulum is like a specialized ER.  It surrounds the fibers and controls calcium storage.

4. Triad consists of two terminal cisternae of the SR surrounding a T-tubule

5. Sarcomere is the functional contractile unit

Transverse tubular system (T-tubule)

1.  What's it continuous with?

2.  Are content intracellular or extracellular?

3.  Volume

4.  Relationship with other structures

5. Function

      1.   Forms extensive network within cell that is continuous with sarcolemma

      2.   Contents are extracellular

      3.   Volume is very small (ca. 0.2% of cell)

      4.   Intimately associated with sarcoplasmic reticulum surrounding each myofibril.

      5.  Allows action potential to reach the interior of the cell and decreases activation                               time.

Sarcoplasmic reticulum (SR)

1.  Location and volume

2. Components

3.  Function

4.  Pump components

5. Action

1.   Surrounds each myofibril and forms intracellular compartment of small volume (1-2% of cell).

      2.   Longitutinal reticulum and terminal cisternae.

      3.   Has high content of Ca²⁺ of which most is loosely bound and stored in the terminal cisternae.

      4.   Most of sarcoplasmic reticulum membrane consists of Ca²⁺ pumps.

      5.   Ca²⁺ release occurs from the terminal cisternae following depolarization of the transverse tubular membrane.

Describe the T-tubule function of the mechanism of excitation-contraction coupling (ECC)

Function = voltage

      1.   Action Potential (Na⁺ Channels, K⁺ Channels) and depolarization

      2.   Voltage-sensor for E-C Coupling (potential-dependent channel regulator)

      3.   Voltage-sensor for ECC – junctional tetrad – dihydropyridine receptor (DHPR)

      4.   Membrane charge movement occurs in T-system – initiation of release from SR

      5.   Ca²⁺ release from SR is dependent on the T-system membrane potential (-50mV to -20mV)

Describe the SR function of the mechanism of excitation-contraction coupling (ECC)

Function = calcium transport

      1.   Ca²⁺ ATPase (100,000 Da)

      2.   High affinity for Ca²⁺

      3.   Transports Ca²⁺ ions into SR for each ATP hydrolyzed

      4.   Maintains low cytoplasmic Ca²⁺ and relaxation of muscle

Describe the SR function of the mechanism of excitation-contraction coupling (ECC)

Function = calcium storage

1.   Terminal cisternae site of Ca²⁺ storage

      2.   Calsequestrin low affinity high capacity Ca²⁺ binding protein (40-50 moles Ca²⁺ binds 1 mole calsequestrin protein)

      3.   Lowers free Ca²⁺ in the terminal cisternae

Describe the SR function of the mechanism of excitation-contraction coupling (ECC)

Function = calcium release

      1.   SR feet couple junctional membrane to junctional T-tubule membrane

      2.   Ryanodine receptor – Ca²⁺ release channel (560,000 Da), does not require ATP

Describe the coupling mechanism of excitation-contraction coupling (ECC)

      1.   Depolarization of T-tubule membrane

      2.   Charge movement in the voltage sensor

      3.   Mechanical coupling between voltage sensor and Ca²⁺ release channel

      4.   Activation of Ca²⁺ release channel

      5.   Other mechanisms of Ca²⁺ release – caffeine-induced Ca²⁺ release

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1.  A tetrad of four L-type Ca channels (DHP receptor) on the T-tubules (voltage sensor)

2.  One Ca release channel (Ryanodine receptor) of the SR

*Each L-type Ca channel interacts with the foot of one of the four subunits of the Ca release channel. 

*Depolarization in one membrane causes a conformational change and opens the calcium channel to pour out Ca.

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1. Central t-tubule lumen

2. terminal cisternae of the SR flanking either side of the T-tubule

3. SR feet/junctional feet at the arrows spanning between the T-tubule and terminal cisternae

4.  Calsequestrin seen as electron dense dots

1.   The ATPase activity of isolated myofibrils

      2.   Stress development in skinned fibers is dependent on the [Ca²⁺]; (skinned fibers are muscle in which the plasma membrane is removed so that the cytoplasmic [Ca²⁺]; can be regulated by changing the bathing fluids)

      3.   Muscle cells maintain [Ca²⁺]i < 10 - 6 M at rest and rapidly increase [Ca²⁺];I to > 10 - 6 M on stimulation

Define:

1. Twitch

2. Tetanus

B.  Twitch – Single action potential

C.  Tetanus – Producing a complete contraction

                     – temporal summation of action potentials to produce maximal force/contraction

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During tetanus calcium levels are maintained at high levels and muscle maintains the contraction.

1.  The mouse phenotypically appears normal at birth but dies because it cannot breathe

2. There is an absence of exitation-contraction coupling (therefore, no contraction)

3.  The mouse lacks functional voltage-sensor for E-C coupling

4.  It has normal sarcoplasmic filaments, normal SR, and normal calcium release channels

5.  No equivalence found in humans

Describe malignant hyperthermia (found in humans and pigs)

A. Properties

B. Testing

C. Basic defect

D. Treatment

A.  Properties  

      1.   Rare dramatic complication of anesthesia: 1:15,000 children, 1:50,000 adults

      2.   Develops during induction of anesthesia with succinylcholine and volatile anesthetics, particularly halothane

      3.   Muscle stiffness as skeletal muscle contracts and rapid rise in body temperature (body uses up lots of ATP) increase plasma K⁺ and leads to cardiac failure

      4.   Death 30-50% cases

B.  Testing

      1.   In vitro muscle preparation more sensitive to caffeine and halothane.

      2.   Genetic testing

C. Basic Defect     

      1.   Genetically determined

      2.   Basic defect appears to be disorder of regulation of Ca²⁺ linked to abnormal RyR (Ca²⁺ release channel).  The channel is mutated but not missing

      3.   Also found in pigs where halothane and stress can induce response

D.  Treatment

      1.   Dantrolene – muscle relaxant that blocks Ca²⁺ release from SR

Define:

1. Calcium ATPase

2. DHPR - voltage sensor - junctional triad

3. SR feet - ryanodine receptor - Ca release channel

1.  Uses ATP energy (slow mechanism) to pump 2 calcium against the gradient.  Found at the SR membrane and pumps calcium into the SR

2.  When the membrane depolarizes, the channel (found on the T-tubule) opens to allow calcium entry. There is a mechanical connection to the calcium release channel.  Together this is ECC

3.  When activated by DHPR, the channel opens to allow calcium release from the SR.  The channel is found on the SR

The muscle sodium channel is a large integral membrane protein complex.  A number of toxins are known to interact with this channel and block its function.  In the presence of one of these toxins, activation of which membrane protein would initiate muscle contraction?

      A.   Neuronal sodium channel

      B.   Presynaptic calcium channel

      C.  Acetylcholine receptor

      D   Sarcoplasmic reticulum calcium ATPase

     E.   Transverse tubular voltage sensor

     E.   Transverse tubular voltage sensor

During skeletal muscle relaxation calcium increases in:

      A.   Synaptic cleft

      B.   Myofilament space

     C.  Sarcoplasmic reticulum

      D.  Transverse tubules

      E.   Mitochondria

     C.  Sarcoplasmic reticulum

A 38-year-old male is evaluated by a neurologist for muscle weakness.  As part of the evaluation, he undergoes a muscle biopsy.  The electron microscopy report returns to your office one week later and states "normal except for a marked reduction in Junctional Tetrads".  Which of the following would be a consequence of the reduction in Junctional Tetrads.

      A.   Prolonged muscle contraction

     B.   Reduced calcium release

      C.  Increased calcium release

      D.  Reduced endpoint potential

      E.   Increased tension

     B.   Reduced calcium release