Muscle Microstructure Flashcards
Three types of muscle
Smooth muscle
Cardiac muscle
Skeletal muscle
Smooth muscle function
Smooth muscle is under involuntary control, from the autonomic nervous system
Cardiac muscle
Contract autonomously , under the influence of the autonomic nervous system and circulating chemicals
Skeletal muscles
Voluntary control , usually attached to bones and contract to bring about movement
Skeletal muscle function
Skeletal muscles are under voluntary control from the somatic nervous system
They are usually attached to the bones
They contract to bring about movement
Arrange,ent of muscle fibres
Parallel
Fusiform
Triangular
Pennate - unipennate, Bipennate and multipennate
What is a fascicles
Bundles of muscle fibres ( myofibres
Sarcolemma and T tubules
Covered by plasma membrane - sarcolemma
T tubule tunnels into centre and this has many ion channels and so the whole area of the muscle can be excited
Sarcoplasm
Myoglobin and mitochondria present
Sarcoplasmic reticulum
Network of fluid filled tubules
Structure of myofibrils
1-2 um in diameter
Extend along the entire length of the myofibres
Composed of two main types of protein - actin and myosin
What gives muscles the striated appearance
Light and dark bands
However the actin and myosin do not extend along length of myofibres
They overlap and are arranged in compartments called sarcolemma
Dark bands
A band and are composed of myosin
Light bands
I band and are composed of actin ( thin)
What separates the sarcomeres
Z discs
What moves to cause the muscle contraction
The binding of myosin filaments to the actin heads causes the muscle contraction and force generation
Structure of myosin
Two globular heads
Single tail formed by two alpha helices
Tails of several hundred molecules form one filament
Structure of actin
Actin molecules twist into helix
Each molecule has a myosin binding site
Filaments also contain troponin and tropomyosin
Sliding filament theory of muscle contraction
During contraction the I band becomes shorter
A band remains the same length
H zone narrows or disappears
Myasthenia Graves
Where the antibodies attack the ACh receptor , this residual ACh can be prolonged and so remains at the receptor
Initiation of muscle contraction
Action potential along surface membrane and T tubule
Dihydropridine ( DHP) receptor in T tubule membrane senses change in voltage and changes shape of the protein linked to the ryanodine receptor
This opens ryanodine receptor calcium channel into sarcoplasmic reticulum
Calcium released from ST into space around filaments
What happens in the initiation of muscle contraction after the calcium ions have been released from the SR into the space around the filaments
Calcium binds to troponin and tropomyosin moves which allows cross ridges to attach to actin
Calcium ions are actively transported into the SR continuously while AP continue .
ATP driven pump and so uptake rate =/< release rate
Excitation contraction coupling
In the presence of calcium ions movement of troponin from tropomyosin chain
Movement exposes myosin binding site on the surface of the actin chain
Charged myosin heads bind to exposed site on actin filament
This binding and discharge of ADP causes myosin head to pivot ( the power stroke) - pulls actin filament towards the centre of the sarcomeres
ATP binding - releases myosin head from actin chain
ATP hydrolysis - provides energy to recharge myosin head
What is a motor unit
Name given to a single motor neuron together with all the muscle fibres that it innervates . Stimulation of one motor unit causes contraction of all muscle fibres in that unit
How can fine control be maintained in terms of motor units
Having only one nerve innervate has
Three types of motor units
Slow, ( S type 1) Fast fatigue resistant( FR, type IIA) Fast fatiguable ( FF , type II B)
Which muscle contains the greates proportion of slow twitch type muscle fibres
Muscles with postural muscle control tend to have a greater proportion of slow twitch muscle fibres
What are motor unit types classified by
Amount of tension generates
Speed of contraction
Fatiguability of the motor unit
Type IIB
Fast twitch
High force
High fatigue
Type IIa
Fast twitch
Moderate force
Fatigue resistant
Type I
Slow twitch
Low force
Fatigue resistant
Regulation of muscle force
Recruitment - there is an order in which muscle fibres are recruited . ‘ size principle’ . As more force is required , more units are recruited. This allows fine control ( e.g. when writing ) where low force levels are required
Rate coding - a motor unit can fire at a range of frequencies , slow units fire at a lower frequency. As the firing rate increases, the force produced by the unit increased.
What are neurotrophic factors
Are types of growth factors
Prevent neuronal death
Promote growth of neurones after injury
Effect of neurotrophic factors
Motor units and fibre characteristics are dependent on the nerve which innervates them
If a fast and slow twitch muscle are cross innervates , the slow one becomes fast and vice versa
The motor neurone has some effect on the properties of the muscle fibres it innervates
Three types of muscle contraction
Isometric
Concentric
Eccentric
Isometric
Produces force but doesn’t change length
Concentric
Shortens muscle to produce movement
Eccentric
Contraction in which muscle is producing fforce but muscle is getting longer - dumbbell which is heavier than the person can handle and so the muscle is still contracting but overall the movement is outwards.
This is when MUSCLE DAMAGE can occur and the muscle can get torn as it’s too heavy
What is the most common change in fibre type following training
Change from type IIB ( fatiguable) to IIA ( less fatiguable)
So from fast fatiguable to fatigue resistant
Microgravity and changing of muscle fibre types
Type 1 to II possible in cases of severe de conditioning or spinal cord injury. Microgravity during space flight results in shift from slow to fast muscle fibre types
Ageing and change in muscle fibre types
Ageing associated with loss of type 1 and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle ( evidence from slower contraction times )
