T2 L5 Physiology of skeletal muscle contraction Flashcards
How many thin filaments are adjacent to 1 thick filament?
6
How many thick filaments are adjacent to 1 thin filament?
3
In the sarcomere, which bands become shorter during contraction?
H and I
What 3 proteins are on thin filaments?
F actin
Tropomyosin
Troponin
What ions travel across membrane during skeletal action potential?
Na+ goes in during depolarisation
k+ goes out during repolarisation
Ca2+ goes in during depolarisation
What process connects action potential to muscle contraction?
Excitation-contraction coupling
What microscopic structures anchor the thick and thin filaments?
Thick filaments are anchored by M band (minor proteins of thick filament)
Thin filaments are anchored to Z disc
What happens when calcium binds to troponin C?
Troponin C changes conformation which shuts off troponin I. Tropomyosin-troponin leaves F-actin groove which unmasks myosin binding site on actin
What is troponin C?
Calcium binding
What is troponin I?
Inhibitory
What is the marker for muscle breakdown?
Total troponin I
What is the marker for myocardial infarct?
Cardiac troponin I
What is cross-bridge cycling?
Molecular cycle of actin-myosin interaction
Mechanism of control at molecular level
What is F-actin?
Filamentous actin
Long polymer chain of actin
What is G-actin?
Globular actin
Single subunit
What are the 4 steps of cross bridge cycling?
1) Myosin releases actin
2) Myosin head cleaves ATP
3) Myosin binds actin
4) Power stroke
What is the effect of increased overlap of thin and thick filaments?
Increased force
Describe the creatine found in muscle fibres
Small, organic, nitrogen-containing molecule
What happens when creatine accepts high energy phosphate bonds from ATP?
Phosphorylated to creatine phosphate which stores energy in muscle
Why can’t energy be stored as excess ATP?
The ATP levels must be kept stable
What is creatine phosphokinase?
Also known as creatine kinase
Enzyme that adds a phosphate to creatine
Plasma marker of muscle destruction
Large molecule detected by antibodies
What is creatinine?
Breakdown of creatine - marker of kidney function
What are the 2 calcium gradients involved in contraction?
Extracellular vs cytosolic free Ca2+
Sarcoplasmic reticulum vs cytosolic free Ca2+
What is excitation contraction (EC) coupling?
Molecular mechanism for how depolarisation of plasma membrane leads to release of Ca2+ into cytoplasm following contraction
Describe the ryanodine receptor (RyR)
In sarcoplasmic reticulum membrane
Releases calcium from sarcoplasmic reticulum
RyR channels in terminal cistern of SR are in contact with L-type calcium channels in T-tubule membranes
What triggers the release of calcium through RyR?
Voltage sensor on Ca2+ channel
Describe SERCA
Smooth endoplasmic reticulum calcium ATPase
In SR membrane
Pumps Ca2+ back into sarcoplasmic reticulum using ATP
Describe L-type calcium channels
Most common calcium selective channel
Voltage gated so opens when cell is depolarised
In plasma membrane
Describe how tetany occurs
Single AP –> sufficient Ca2+ released from SR –> twitch
Ca2+ pumped back into SR –> end of twitch
Frequent APs –> time interval between APs is shortened –> insufficient Ca2+ re-sequestered –> summation of contraction
What are the 2 main types of muscle fibres?
Slow twitch - type 1
Fast twitch - type 2
Describe slow twitch fibres
Red, oxidative, small diameter
High myoglobin
Many mitochondria
Postural or weight bearing muscles
Describe fast twitch fibres
White, non oxidative, wide diameter
Lower myoglobin
Increased energy from glycolysis
What are the differences between the muscle fibre types?
Aerobic (slow), anaerobic (fast)
Faster calcium re-uptake in fast twitch
Maximum tension produced in fast twitch
Fatigue resistance in slow twitch
What does the fibre type distribution depend on?
Muscle action
Describe the fibre type distribution in soleus muscle
80% type I (slow)
20% type IIA
Describe the fibre type distribution in vastus lateralis
Mixture of type I, IIA, IIX
Describe the characteristics of type I fibres
Sub-name: slow-oxidative ATP generation: aerobic Contraction rate: slow Fatigue resistance: high Fibre diameter: small Myoglobin: high Mitochondria: many Capillaries: many Fuel for storage: triglycerides
Describe the characteristics of type IIA
Sub-name: fast oxidative-glyoclytic ATP generation: aerobic / anaerobic Contraction rate: fast Fatigue resistance: intermediate Fibre diameter: intermediate Myoglobin: high Mitochondria: many Capillaries: many Fuel for storage: creatine phosphate, glycogen
Describe the characteristics of type IIB (IIX) fibres
Sub-name: fast, glycolytic ATP generation: anaerobic Contraction rate: fastest Fatigue resistance: low Fibre diameter: high Myoglobin: low Mitochondria: few Capillaries: few Fuel for storage: creatine phosphate, glycogen
What are the 3 types of coordination?
Motor units
Tetany
Fusion of myocytes into long myofibres
What is a motor unit?
Single alpha motor neuron and all the muscle fibres it innervates
How many motor units in a peripheral muscle?
100-500
How many muscle fibres per motor unit in a biceps branch?
500
How many muscle fibres per motor unit in gastrocnemius?
2000
What is the percentage of slow twitch in gastrocnemius in sprinters?
25%
What is the percentage of slow twitch in gastrocnemius in marathoners?
93-99%
How many muscle fibres per motor unit in extrinsic eye muscles?
9
What determines the muscle fibre?
Type and function of lower motor neuron
What is isometric?
Generates variable force while length of muscle remains unchanged
What is isotonic?
Generates constant force while length of muscle changes
Describe the steps for picking up a glass
Stage 1 - isometric
- force increases, joint doesn’t move
- muscle force < force of gravity –> force increases
- biceps and brachioradialis generate force by isometric contractions as muscles haven’t yet shortened
Stage 2 - isotonic
- force remains same, arm moves
- glass moves upward in response to force
- isotonic contraction starts as force generated by muscle exceeds gravitational and inertial forces keeping glass on table
What is concentric?
Force during contraction e.g. tossing a ball into air
What is eccentric?
negatives
Force during muscle elongation
Eg when braking or when weight of object is overwhelming such as when catching a ball
Describe the recruitment size principle
As initial isometric contraction occurs:
- more motor units are recruited (start with smaller ones and progressively add larger ones)
- allows fine gradation of force for small movements
Describe lower motor neurons
Signal starts in spinal cord and extends to effector muscles in periphery
Describe the signs of lower motor neuron disease
Weakness
Muscle atrophy
Describe upper motor neurons
Signal starts in brain and extends to spinal cord to synapse with lower motor neuron
Describe upper motor neuron disease
Inhibits normal inhibitory input to spinal interneurons
Spindle becomes oversensitive and attempts to contract muscle all the time
What are the signs of upper motor neuron disease?
Spasticity
hypertonia
What does the stretch reflex do?
Controls muscle length
Increases muscle force
What is Westphal’s sign and what causes it?
Lack of patellar reflex Damage to: - femoral nerve - receptor damage - peripheral nerve disease e.g. peripheral neuropathy -cerebellar damage
Describe sensory intrafusal fibres
Skeletal muscle fibres that serve as specialised sensory organs (proprioceptors) that detect amount and rate of change in length of muscle
Constitute muscle spindle and don’t generate contractile force associated with muscles
Describe contraction extrafusal fibres
Skeletal muscle fibres innervated by alpha motor neurons
Generate tension by contracting which allows skeletal movement
Attached to bone by fibrous tissue extensions
What is the function of the patellar reflex?
Posture and balance
What spinal cord level does flexion for patellar reflex occur?
L4
Describe a muscle spindle
Consists of 3-12 intrafusal fibres
Gamma motor neurons to increase sensitivity
Describe gamma motor neurons
Drive contraction of edge of intrafusal fibres
Sensory from muscle spindle:
- type 1a and type 2
- wrap around intrafusal fibre
- detect stretch of central non-contracting region using stretch receptors
What is the function of the tendon reflex?
Prevents overloading
Decreases muscle force to drop load
- sensor fires to decrease contraction
Describe the steps of the Golgi tendon reflex
1) Neuron from Golgi tendon organ fires
2) Motor neuron is inhibited
3) Muscle relaxes
4) Load is dropped
