Muscle Microstructure and Contraction Flashcards
What are the 3 main types of muscle?
What type of control are they under, where are they found… etc.?
- Smooth - involuntary control from ANS, usually found around tube structures e.g. airways, blood vessels etc.
- Cardiac - confined to the heart, under the control of the ANS and circulating chemicals, striated (striped) muscle
- Skeletal - voluntary control (SNS), usually attached to bones, contract to bring about movement; striated (striped) muscle
How are muscle fibres arranged? And why?
Arranged in different ways with respect to the tendons and bone to influence their function - different arrangements allow for different tensions to form in the muscle and pull in different directions
e.g. parallel = all fibres parallel to each other
fusiform = larger bulge in the middle
pennate = offset at an angle
What are fasicles in muscles?
What are muscle fibres / myofibres?
What are myofibrils?
What are myofilaments?
Can you illustrate these in a diagram?
Fasicles = bundles of muscle fibres / myofibres
Myofibres are made of bundles of myofibrils
Myofibrils are made up of muscle filaments
Individual units (myosin and actin)
What surrounds the muscles and what are it’s properties and roles?
What is compartment syndrome?
What is the name given to the layer that surrounds muscles?
What is the name given to the layer that surrounds muscle fascicles?
What is the name given to the layer that surrounds muscle fibres?
Collagen based connective tissue which creates compartments - tough / rigid, which doesn’t allow for expansion hence keeps the body in shape
When there is an increase in pressure in the muscle compartment e.g. from bleeding, causing increased pressure on the tissues, resulting in ischemia and necrosis of the tissue (extremely painful)
Epimysium
Perimysium
Endomysium
What is the name given to cells from which skeletal muscle fibres arise?
What surrounds the myofibres?
What is found under the sarcolemma?
Myoblasts
Sarcolemma - muscle plasma membrane
T-tubules
What are myofibrils? What are the 2 main proteins that are made up of?
What are sarcomeres?
What are the different bands / lines and which protein(s) are they made up of? Are they light or dark?
Thick (myosin) filaments and thin (actin) filaments
Sarcomere - distance between 2 Z lines
Dark = myosin = A band
Light = actin = I band
Why are there dark and light bands in the myofibrils?
What gives muscles it striated (striped) appearance?
The different proteins reflect light to different extents - under the light microscopic, they polarised light different
The light and dark bands give the muscle it’s striated appearance
How did the sliding filament theory come about?
Under a light microscope, when muscle contracted, certain light or dark bands disappeared
What is the structure of myosin?
Has 2 globular heads, and a tail formed by two alpha-helices
All the heads tend to point / face the same direction, and same with their tails
What is the structure of actin?
Made of actin, but also contains 2 filaments - troponin complex and tropomyosin
It has a myosin head binding site on the tropomyosin that is normally covered by the troponin complex
What is the sliding filament theory?
Which bands shorten, disappear and stay the same when the muscle contracts (AKA shortens)?
I band shortens
A band remains same length
H zone narrows or disappears
Sarcomere overall shortens
How does is the signal sent by the lower motor neuron to the the NMJ?
What is the simulation that occurs at the NMJ?
Once the action potentials stop, the acetylecholine is broken down by acetylcholine esterase to stop muscle contraction
What are the activation steps to muscle contraction / the sliding filament theory? (more detail)
- Action potential propagates along surface membrane into the T-tubules
- DHP (dihydropyridine) receptors in the t-tubule membranes sense the change in voltage and causes a conformational change to a protein linked to the ryanodine receptor
- The ryanodine receptor is coupled with a Ca2+ channel, hence the change of shape in the protein causes the Ca2+ store in the sarcoplasmic reticulum to travel down the concentration gradient into the sarcoplasm
- This allows for the rest of the sliding filament theory to take place
What is rigor mortis?
A state after death, where ATP stores are depleted, therefore due to lack of ATP, myosin heads are unable to detach from the binding sites on the actin filaments
What type of neuron attaches to skeletal muscle? Where does this neuron come from?
What type of neuron comes from the brain?
Lower motor neuron - comes from the brain stem / spinal cord, forms the final common pathway from the CNS to the muscle
Upper motor neuron - comes from the primary motor cortex (sits in front of the central gyrus)
How does innervation to the muscle fibres change as we grow from foetuses to adults?
How many muscle fibres are supplied by a single motor neuron?
Can muscle fibres be innervated by more than one motor neuron?
What is a motor unit?
During development - lots of muscle fibres are innervated by lots of nerves. One neuron wins out of them to innervate the collection of muscle fibres
600 muscle fibres
No - muscle fibres are only innervated by one motor neuron
The one motor neuron and all the muscle fibres it innervates = unit (smallest functional unit of muscle contraction)
Why is there motor unit variability?
Muscles that require greater dexterity control = innervation ratio of fewer muscle fibres by a single motor neuron (e.g. hands)
Muscles responsible for simple movements / power = innervation ratio of greater muscle fibres by a single motor neuron
What are the 3 types of motor units?
S (Type I) - Slow = smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity = usually for postural control
FR (Type IIA) - Fast, Fatigue resistant = larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduciton velocity = usually contract when called upon
FF (Type IIB) - Fast, Fatiguable = larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity = usually contract when called upon
Fill in table:
Muscle tissue looks different colours depending on type of motor unit:
Greater myoglobin content = redder appearance
Slowly contracting = use continuous supply of oxygen
Fast muscles (fast contracting) = require on reserves to carry out respiration anaerobically
Properties of motor units:
What are the 3 things to consider to classify the different motor unit types?
Amount of tension generated, speed of contraction, and fatiguability of the motor unit
Slow = takes a long time to generate its tension fully, low force, fatigue resistant
Fast = generates more power and more quickly but over more quickly
Fast a = moderate force, fatigue resistant, Fast b = high force, high fatigue
What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?
What is the size principle?
What is summation?
- Recruitment - gets more muscle fibres on board via more neurons = increases strength of contraction; uses the size principle so smaller units are recruited first = allows for fine control
- Rate coding - send more impulses / change frequency of impulses arriving at the muscles = force builds up; slow units fire at a lower frequency
Slow units recruited first, then fatigue resistant, then fatiguable ones
Summation occurs when the units fire at a frequency that allows no time for the muscles to relax between the arriving action potentials (which are causing the contractions)
This is a graph to show recuitment, explain how this supports the recruitment of muscle fibres?
Top graph read from left to right, bottom graph read from bottom to top
First motor unit is always firing, second motor unit begins to fire when the muscle reaches a certain percentage of contraction… etc.
The 5th motor unit comes on when the muscle has reached peak contraction, but also comes off quickly once the peak contraction has passed
This is a graph to show rate coding, explain how this graph supports the rate coding of muscles?
Y axis is measuring the firing rate, X axis is measuring the amount of force being produce
At low force, the slow units begin to fire (i.e. at 8 hz); as the voluntary force increases, the slow units begin to fire more frequently and the fast fatigue resistant units begin to fire; then when the force is at its greatest, the slow, fast fatigue resistant and fast fatiguable all fire at their maximum frequency
What happens if the nerve supply to the muscle fibres has been cut?
Neurons do more than just send impulses to the muscle fibres, what else does it do?
Muscular atrophy (muscle wastage)
The nerve must do more than just send signals to the muscle fibres if cutting off the nerve supply causes muscle wastage. So the nerve also has influence on the way the muscle behaves (i.e. slow, fast) - not just amount of myoglobin etc. as those things are changeable
What was the outline of the experiment conducted that showed nerve supply to muscles also have influence over the behaviour of the muscle (i.e. slow or fast)?
Surgical procedure involving soleus (slow), flexor digitorium longus (fast), and flexor hallucis longus (fast)
The nerves innervating the soleus and FDL were switched, and the nerve innervating the FHL was kept the same (control)
Noticed the FDL started acting more like a slow twitch fibre, and soleus started acting more like a fast twitch fibre
What are the 3 types of muscle contraction?
Concentric - active movement inwards
Eccentric - active movement backwards, lengthening contraction (muscle gets longer as force is generated)
Isometric - no movement, kept in the same place (muscle stays the same length but there is force generated)
Can muscle fibre types be changed from one type to another?
Ageing is associated with which muscle fibre loss?
Change between fast types (i.e. between Type IIa and Type IIb) - with training
But cannot change between slow and fast types (unless in space or something where there is no gravity = no postural muscle use)
Ageing associated with loss of both types of fibres, but preferentially with Type II fibres (which is why ageing people have slower contraction times)
