Flashcards in Physiology of skeletal muscle contraction Deck (94):
What binds to troponin C in skeletal muscles?
What binds to troponin C in the heart muscle?
What happens to troponin after calcium binds?
What does a conformational change in troponin (TnC) lead to?
Shuts off TnI
tropomyosin-troponin leaves F-actin groove
Unmasks the myosin binding site on actin
What happens to the next myosin that binds?
Next myosin head makes cross bridges (cycling) to actin
Myosin breaks down ATP
Myosin pulls thin filaments
What is total TnI used as a marker of?
Total muscle breakdown
What is cardiac TnI used as a marker of?
What is cross bridge cycling?
Molecular cycle of actin-myosin interaction
Mechanism of contraction at a molecular level
What does contraction through cross bridge cycling depend on?
Binding of myosin heads to thin filaments (actin) at specific binding sites
Describe the resting state of a sarcomere
Myosin heads are blocked from binding to actin by tropomyosin, which occupies the specific binding sites ( F actin double helical groove)
What can increase the force generated?
Increased overlap of thick and thin filaments
When does the greatest force generation occur?
When sarcomere is at optimal length
Describe the process of cross bridge cycling in terms of te 4 reactions
Myosin releases actin
Myosin head cleaves ATP
Myosin binds actin
Where is creatine found?
What is creatine phosphorylated to?
How is energy stored in muscle?
Stored as creatine phosphate in muscle fibres
Describe what happens to ATP during cross bridge cycling
ATP is hydrolysed to ADP + Pi , creatine phosphate donates high energy phosphate to ADP restoring it to ATP
How are ATP levels in muscle kept stable?
Buffering and regeneration
Which enzyme catalyses the reaction of ATP during cross bridge cycling?
Creatine kinase or Creatine phosphokinase (CK, CPK) in both directions
What is creatine?
Small molecule that accepts high energy phosphate bonds from ATP
What is plasma creatine phosphokinase a marker of?
What is creatinine a diagnostic marker of?
What is creatinine?
Breakdown product of creatine
What are the two calcium gradients?
Extracellular vs cytosolic free calcium
SR vs cytosolic free calcium
What provides most of the calcium?
Efflux of calcium from the sarcoplasmic reticulum to cytoplasm
What does calcium entering the cell from the outside provide?
Only a small fraction of the calcium needed for muscle contraction
What does calcium trigger?
Which neurotransmitter leads to depolarisation?
What does depolarisation cause?
Increase in calcium
Name the receptor where there is a net inward current
Active nicotinic AChR
How does depolarisation spread?
Through T tubules
What do local action potentials trigger?
Ca2+ efflux from terminal cisternae across sarcoplasmic reticulum into the fibre cytoplasm
Where is the ryanodine receptor found?
In the sarcoplasmic reticulum
What does the ryanodine receptor do?
Releases Ca2+ from SR
What is the ryanodine receptor triggered by?
Voltage sensor on Ca2+ channel
Where is SERCA found?
In sarcoplasmic reticulum
What does SERCA stand for?
Smooth endoplasmic reticulum calcium ATPase
What is the function of SERCA?
Pumps Ca2+ back into SR
What does SERCA require?
What causes a twitch?
Single action potential leading to calcium release from sarcoplasmic reticulum
What causes a twitch to end?
Ca2+ ions are rapidly pumped back into the SR
Describe the molecular basis of tetany
Insufficient Ca2+ resequestration
Summation of contraction
Name the two types of muscle fibres
Describe the structure of slow twitch fibres
Give the size, colour and property of slow twitch fibres
Give the size, colour and property of fast twitch fibres
How do the two muscle fibre types differ?
Aerobic (slow fibres) vs anaerobic respiration
Faster calcium reuptake in fast fibres
Maximum tension produced in fast fibres
Fatigue resistance in slow fibres
Name the fibre types in order of slow to fast fibres
What do muscles contain?
A mixture of fibre types
What does muscle composition depend on?
Give the composition of soleus
80% Type 1 (slow)
20% Type IIA
Give the composition of vastus lateralis
Mixture of type 1, IIA, IIX
Name the different groups of physical fitness
What colour do type 1 fibres stain?
What colour do type II fibres stain?
List the 3 types of coordination of contraction
Fusion of myocytes into long myofibres
Define motor unit
A single alpha motor neuron and all muscle fibres that it innervates
What do motor units function as?
A single contractile unit of skeletal muscle
What type are all muscle fibres in a single motor unit?
Describe the synapses in large muscles responsible for powerful gross contractions
Single motor neuron may synapse on 1000 fibres
Describe the synapses in small muscles mediating precision movements
Single motor neuron may synapse with as few as 2 - 3 fibres
What determines the muscle fibre?
Type and function of lower motor neuron
Define isometric contraction
Generates a variable force while length of the muscle remains unchanged
Define isotonic contraction
Generates a constant force while the length of the muscle changes
Describe the types of force generation when picking up a glass
stage 1: isometric – force increases, joint does not move
Muscle Force < force of gravity –> force increases
biceps and brachioradialis generate force by isometric contraction as muscles have not yet shortened
stage 2: isotonic – force remains the same, arm moves
Glass moves upward in response to force
an isotonic contraction starts as the force generated by the muscles overcomes gravitational and inertial forces keeping glass on the table
glass starts to rise as the muscles shorten and the elbow bends and force generated by the muscle is constant as the glass is moving
What does muscle contraction not necessarily mean?
What is meant by concentric?
Force during contraction
Give an example of concentric force
Tossing a ball into the air
What is meant by eccentric?
Force during muscle elongation
Give an example of eccentric force
When the weight of the object is overwhelming - catching a ball
When can both types of force occur?
In one behaviour
What does proprioception control?
What is proprioception based on?
Length and strength
Describe the size principle
As the initial isometric contraction occurs more and more motor units are recruited starting with smaller ones and progressively adding larger ones
Allows fine gradation of force for small movements
What does lower motor neuron disease cause?
What does upper motor neuron disease cause?
Describe the stretch reflex
Controls muscle length
Increases muscle force
What is westphal's sign?
Lack of patellar reflex
What is the function of the patellar reflex
Posture and balance
Name the fibres for sensation in the stretch reflex
Name the fibres for contraction in the stretch reflex
What does the muscle spindle fibre do?
Give the location of the muscle spindle
Parallel to muscle fibres
What do the muscle spindles contain?
3-12 intrafusal fibres
What do gamma motor neurons do?
Drive contraction of edge of intrafusal fibres
What are sensors from muscle spindles called?
Type 1a and Type 2
Where are sensors from muscle spindles found?
Wrap around the intrafusal fibres
What do sensors from muscle spindles do?
Detect stretch of central non-contacting region using stretch receptors
What is the function of the muscle spindle?
Like a thermostat that regulates the relationship between the muscle length and muscle contractility
ie. the relationship between neural drive and force generation
What can absence of the muscle spindle reflex suggest?
Femoral nerve damage
Peripheral nerve disease eg. peripheral neuropathy
What can happen to the muscle spindle reflex in upper motor neuron disease
Can lead to hypertonia and spasticity
UMN inhibits normal descending inhibitory input to spinal interneurons
Spindle reflex can become over sensitive - can attempt to contract the muscle all the time
What is the function of the tendon reflex?
Protects from overloading
Decreases muscle force- dropping the load - sensor firing - decreased contraction
Describe the path of the tendon reflex
Sensor to spinal cord
Interneuron to motor neuron
Motor neuron inhibited
Motor neuron to muscle