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204- MSK > Physiology of skeletal muscle contraction > Flashcards

Physiology of skeletal muscle contraction Flashcards

1
Q

How does troponin work?

How is it used a a biomarker?

A
  • 4 Ca bind to troponin C (in heart TNC bind to only 3 Ca) resulting in TNC conformational change
  • This ‘shuts off’ TnI and tropmyosin-troponin leaves F-actin groove which unmasks myosin binding site on actin
  • Next myosin head makes cross bridges (cycling) to actin; myosin breaks down ATP and pulls actin

Total TnI= marker for total muscle breakdown
Cardiac TnI= marker for MI

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2
Q

What is cross-bridge cycling?

How is prevented at rest?

A

Molecular cycling of actin-myosin interaction

  • mechanism of contraction at a molecular level
  • contraction depends on binding of myosin heads to actin at specific binding sites
  • at rest, myosin heads are blocked from binding actin by tropomyosin occupying F-actin double helical groove
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3
Q

What is the correlation between force and filament overlap?

A

The more overlap in actin and myosin, the greater the force

- However the greatest force generation occur swhen sarcomeres is at optimal length

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4
Q

Briefly state what happen in cross bridge cycling

A
  1. Myosin releases actin
  2. Myosin head cleaves ATP
  3. Myosin bind actin giving off a phosphate and ADP
  4. Power stroke
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5
Q

How is energy stored?

Why is this important?

A

Energy is stored in muscle as creatine phosphate

  • This acts as a phosphate buffer: it is made during times of excess ATP and broke down during times of low ATP
  • A high energy phosphate from creatine phosphate restores ATP during hydrolysation in cross bridge cycling

It is important to keep ATP levels stable

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6
Q

Outline the equation/process of ATP production/use from creatine phosphate

A

Creatine —> creatine phosphate by mitochondrial CPK. This uses energy (creating ADP)

Creatine phosphate –> creatine by myofibrillar CPK. This yields ATP

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7
Q

What is the clinical relevance of CPK?

A

Plasma marker for muscle destruction

  • large
  • detected by antibodies
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8
Q

What is the difference between creatine and creatinine?

A

CREATINE
- small and can accept high energy phosphate from ATP

CREATININE
- diagnostic marker for kidney function. It is a breakdown product of creatine

Molecularly similar

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9
Q

Calcium acts as a trigger for muscle contraction on two gradients

What are they?

A
  • Extracellular vs cytosolic free Ca

- SR vs cytoplasmic fee Ca (efflux from SR provides most Ca)

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10
Q

What happens during DEPOLARISATION?

A

ACh causes depolarisation of cell membrane

  • Active nicotinic AChR causes a net inward current
  • Depolarisation spreads via T tubules
  • Local action potentials trigger calcium efflux from terminal cisternae across SR into fibre cytoplasm
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11
Q

Describe how the ryanodine receptor and SERCA facilitate excitation- contraction coupling

A

RyR
- In SR membrane, releases calcium triggered by voltage sensor on calcium channel

SERCA
- also in SR membrane, pumps calcium back into SR. This requires ATP via smooth endoplasmic reticulum calcium ATPase

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12
Q

What happens during tetany?

State the three stages of a twitch

A
  • A single action potential causes the release of Ca from SR –> twitch
  • Ca is then rapidly pumped back into SR –> end of twitch
    Therefore frequent APs cause insufficient calcium resequestration –> summation of contraction

Latent period, contraction (increased tension), relaxation (decreased tension)

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13
Q

Muscle fibres have varied contractile properties. Describe the characteristics of SLOW TWITCH fibres

A
  • “Type 1”, red
  • oxidative
  • small diameter
  • high myoglobin, many mitochondria
  • aerobic
  • fatigue resistant
  • fuel for storage= triglycerides
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14
Q

Muscle fibres have varied contractile properties. Describe the characteristics of FAST TWITCH fibres

A
  • “Type 2”, white
  • non oxxidative
  • wide diameter
  • lower myoglobin, high energy from glycolysis
  • anaerobic
  • faster Ca2+ reuptake
  • maximum tension produced
  • fuel for storage: glycogen, creatine phosphate
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15
Q

How do the various types of muscle fibres differ?

I, IIA, IIX, IIB

A

Spectrum
I- slow, sustained movement e.g stature (erector spinae)
IIA (have many capillaries)
IIX
IIB- fast, intermittent movement e.g hand and eye muscles

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16
Q

In reality what is the distribution of the different muscle fibres in muscles

Give examples

How can physical activity impact this?

A
  • Muscles contain mixtures of fibre types in varied compositions dependent on their action
  • Soleus: 80% type 1, 20% type IIA
  • Vastus lateralis: mixture of type I, IIA, IIX
  • Gastrocnemius = mixed
  • Inactivity, moderate activity, endurance athletes and anaerobic athletes will have varied muscle fibre proportions
    e. g. long distance runners have majority type 1, sprinters 2
17
Q

State 3 factors involved in coordination of muscle contraction

A
  1. Fusion of myocytes into long myofibrils
  2. Tetany
  3. Motor units
18
Q

What is a motor unit?

How does their size vary between large and small muscles?

A

A single a-motor neuron and all muscle fibres innervated by it
- functions as single contractile unit and all muscle fibres in 1 motor unit are of the same type

  • in large muscles (responsible for powerful gross contractions) 1 motor neuron may synapse on 1000 fibres
  • in small muscles (precision movement) 1:2-3 muscle fibres.
19
Q

What determines the type of muscle fibre in a motor unit?

A

Type and function of the LMN

20
Q

Define the following terms

  1. Isometric
  2. Isotonic

What is the relationship between muscle length and contraction?

A
  1. generates a variable force while length of muscle stays the same. Important for maintaining posture
  2. generates a constant force whilst length of muscle changes. Important in body movement

Muscle contraction is not always equal to shortening of muscle length.

21
Q

Using the terms isometric and isotonic contraction, describe the process of picking up a glass

A

STAGE 1: ISOMETRIC CONTRACTION (force increases, joint doesnt move)

  • Muscles force< force of gravity –> force increasing
  • force generated by IM contraction of biceps and brachioradialis

STAGE 2L ISOTONIC CONTRACTION (force same, arm moves)
- Glass moves up due to force of muscle contraction + muscle shortening + elbow bending. Force overcomes gravitational and inertial forces (which usually keep the glass on the table)

22
Q

Define the following terms

  1. Concentric
  2. Eccentric
    What is the relationship between these two events?
  3. Proprioception
A
  1. Force during contraction
  2. Force during muscle elongation
    - Both types of isotonic contraction
    * * These can both occur in one behaviour at different stages e.g. during long jump: concentric as you leap and eccentric as you land. Quadriceps shorten during launch and lengthen during land
  3. Controls force generation based on length and stretch
23
Q

What is recruitement?

A

It occurs during initial isometric contraction. More motor units are employed with progressively bigger size to allow fine gradation of force for small movements

24
Q

Describe UMN and LMN in terms of

  1. Trajectory
  2. Presentation of disease
A

UMN

  • Signal from brain and extend to spinal cord axons
  • Disease: weakness, muscle atrophy, muscle atrophy

LMN

  • Signal from spinal cord and extend to effector muscles in periphery
  • Disease: spasticity, hypertonia
25
Q

What is hypertonia?

A

muscle is overcontracted at rest

26
Q

What is the function of the stretch reflex?

How would a lack of patellar reflex present? Cause?

A
  • Controls muscle length and increases muscle force. It can shorten muscle if too long for current neural drive. Contains sensory fibres (intrafusal fibres) inside sheath, contraction via extrafusal fibres
27
Q

What is the patellar reflex?

How would a lack of patellar reflex present? Cause?

UMN cause presentation?

A
  • Muscle spindle fibres detect stretch (ie length) and proprioception. It is parallel to other spindle fibres and ipsilateral to spinal reflex
  • Only 1 synapse is needed for reflex (MONOSYNAPTIC)

WESTPHALS SIGN

  • normal function is posture and balance
  • pathologically indicates damage to femoral nerve, receptor damage, peripheral nerve disease or cerebellar disease
  • in UMN disease, can lead to hypertonia and spasticity as UMN inhibits normal descending inhibitory input to spinal interneurons. The spindle reflex becomes over sensitive and can attempt to contract muscle all the time
28
Q

What do muscle spindles consist of?

Function?

A
  • contains 3-12 intrafusal fibres
  • y-motor neurons increase sensitivity by driving contraction of edge of intrafusal fibres
  • Sensors are type IA and II and wrap around the intrafusal fibres
  • Detect stretch or central non-contracting region using stretch receptors
  • REGULATES RELATIONSHIP BETWEEN NEURAL DRIVE AND FORCE GENERATION
29
Q

What is the function of the tendon reflex?

What happens during it?

A
  • protects from overloading
  • Decreased muscle force –> dropping of load as sensor firing –> less contraction
  1. Muscle contracts: sensory to spinal cord
  2. inhibitory interneuron to motor neuon
  3. motor neuron inhibited
  4. muscle relaxes
  5. load is dropped
30
Q

What kind of spinal reflex is the tendon reflex?

How many synapses is it dependent on?

What is the sensor?

A

Ipsilateral spinal reflex

Disynaptic

Golgi tendon organ which detects tension in series with muscle located in tendon near border of muscle

204- MSK (16 decks)

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