Skeletal muscle Flashcards
What defines skeletal muscle?
Under voluntary control <> striated <> single long cylindrical cells <> multiple peripheral nuclei
What do muscles connect to?
Tendon to tendon
What defines cardiac muscle?
Located only in the heart <> striated, branched cells with 1-3 central nuclei <> connected by intercalated discs <> involuntary control
What defines smooth muscle?
Found in the wall of internal organs (i.e. gut, blood vessels etc.) <> Involuntary control <> Spindle shaped <> Uninucleated <> Not striated
What makes the muscle striated?
The actin and myosin are in a very parallel structure within the sarcomeres
What is a muscle fibre?
An individual muscle cell
How long and wide can a muscle fibre be?
up to 35cm and 0.1mm
What are the muscle cells composed of? What are they?
Myofibrils <> Highly organised contractile filaments
When there is muscle growth, what is changing?
There are more myofibrils being packed into a muscle cell
What is the structure of a myofibril?
Repeating units of sarcomeres
What is the structure of a sarcomere? Describe each structure
Z disc - coin-shaped sheet of proteins that anchor the thin filament and connect sarcomeres to one another <> H zone - Just the thick filaments <> I band - just the thin filaments <> A band - whole length of thick filaments (includes thin filaments) <> M line - line of protein myosin that holds adjacent thick filaments together
What is the thick and thin filament?
Thick = myosin filament <> Thin = actin filament
What and where are T-tubules?
Deep invagination that are continuous with the sarcolemma in a ring surrounding the sarcomere
What do T-tubuels do?
Connects all the sarcomeres together allowing for the action potential to conduct throughout the entire cell
What is the sarcoplasmic reticulum (SR)?
A very specialised endoplasmic reticulum of the sarcomere that store calcium
What is the sarcolemma?
the fine tubular sheath which envelops the fibres of skeletal muscles.
Label the diagram
What is the thick filaments made of and structure?
Myosin which each have two subunits each with a globular head and tail, the tails are intertwined with each other
What do the myosin heads have on them?
Binding sites for actin
What are myosin heads always trying to do?
Bind on to actin (i.e. fin filament)
What is the head made of? What does this do?
ATPase <> Hydrolyses ATP
What action of the myosin causes muscle contraction?
The backwards and forwards movement of the myosin head on the hinge of the myosin filament
What does titin do?
Anchors the thick filament to the Z-discs
What is the primary protein of thin filaments? What alternative name does this give it?
Actin <> None, it can’t be called actin as there are lots of other protein involved
What is the filament structure?
A double stranded helical actin chain
What are troponin and tropomyosin?
Regulatory proteins
The binding sites on the thin filaments have what on them in normal conditions?
Tropomyosin
What does tropomyosin?
Stops myosin from binding onto actin filaments on the thin filament
What does troponin do?
When Ca2+ bonds onto it it cause the tropomyosin to detach from the actin filament exposing it for myosin attachment
During muscle contraction, what band changes and stays the same?
The A band says the same <> The I band is shortened
What causes the I band to get shorter?
The thin filament is pulled over the thick filament
What is the cross-bridge cycle?
The formation of cross-bridges between the myosin heads and actin filament
What are the 4 steps of the cross-bridge cycle?
1 - cross-bridge formation <> 2 - power stroke <> 3 - detachment <> 4 - energisation of myosin head
For cross-bridge formation what must there be?
A high amount of calcium available
When can cross-bridges only form?
When the myosin binding site on the actin is exposed
What happens during cross-bridge formation?
Myosin binds to the actin bindi site forming a cross bridge
Label the diagram
In the cross-bridge formation is in what position relative to the thick filament?
Perpendicular
What breaks down during cross-bridge formation? What is formed?
ATP, forms ADP and inorganic phosphate (Pi)
What does the hydrolysis of ATP do to the myosin head?
It energises the myosin head by stretching it
What is the power stroke?
Where the energy from the stretched myosin is released
What is released during the power stroke?
ADP is released
How does the myosin head change position? Why does it change position?
It pivots to 45º of the actin filament <> It is its low energy state
As the myosin goes into its low energy state, what does it do?
Pulls the thin filament with it therefore shortens the sarcomere
After the pulling of the thin filament what needs to happen?
The myosin head needs to detach from the actin filament
How does the myosin head detach from the actin?
A new ATP molecule binds to the myosin and weakens the bond between the actin
What is the energisation of the myosin head? What does this do?
Where the myosin head hydrolyses the ATP into ADP + Pi <> Causes the myosin head to move back to tis high energy confirmation
What happens if there is not ATP present after the power stroke? What is this called in a dead person?
Then the myosin head doesn’t detach from the actin <> Rigor mortis
What is the high energy position of the myosin head?
90º to the actin filament
Put these into the correct order and identify the step being undertaken
What is the importance of Ca2+ in muscle contraction?
provides the ‘on’ switch fro the cross-bridge cycle to begin
What is the critical threshold concentration of Ca2+ for muscle contraction?
10^-6 –> 10^-5 mol
Where is the calcium normally stored in muscle fibres?
Sarcoplasmic reticulum (SR)
Where is the Ca2+ always being transported to?
Always being actively transported into the SR
What kind of contraction occurs when there is muscle shortening? Give some examples of this
Isotonic <> Lifting stuff
What kind of contraction occurs when there is no muscle shortening? Give some examples of this
Isometric <> Holding stuff
What is the spread of tension inside the muscle tissue for isotonic and isometric contractions?
Isotonic = constant tension <> Isometric = variable
What does the length tension relationship describe?
The relationship between the sarcomere length and force generated
Why does a change in sarcomere length affect force generation?
More myosin heads that are able to attach onto the actin produces more force
When is the maximum force generation? Why?
At the resting length of 2-2.2 µm because there are the most number of myosin heads able to attach to the actin filament
How does stretching past the normal range of a sarcomere affect the force generation? Why?
It decreases force generation <> Less myosin heads are able to attach to the actin so less cross-bridges can form
How does stretching a sarcomere less than its normal range affect the force generation? Why?
It decreases force generation <> The thick filaments can begin to overlap each other and the myosin heads attach onto the wrong actin filament causing interference resulting in less muscle generation and eventually the filaments are unable to shorten any more therefore more force generation
What is the normal range of sarcomere extension? Can muscles extend past this point?
1.6-2.6µm <> Most can’t because of joint limitations (e.g. elbow, knee etc.) but some such as the hip and shoulder have greater mobility and can stretch past normal range
When a muscle is stretched longer than the normal range, what develops?
Passive tension/force
What causes passive tension?
Connective tissue holding the muscle together
What is the total tension equation that is developed in a muscle?
Total Tension = active (i.e. cross bridge cycling) + passive force
What is a motor unit?
One motor nerve and all the fibres that it innovates
What is the 10 step process of Excitation-Contraction Coupling
1 - ACh released into neuromuscular junction <> 2 - Activation of ACh receptors <> 3 - A Muscle Action Potential is triggered <> 4 - Calcium is released from the SR <> 5 - Ca2+ binds with troponin <> 6 - Cross-bridge cycle: <> 6a - Cross-bridge formation <> 6b - Power stroke <> 6c - Detachment <> 6d - Energization of myosin head <> 7 - Contraction ends when Ca2+ levels fall
What is the point where a neuron contacts a muscle cell called?
Neuromuscular junction
What are the steps of initiating a response from muscle fibres?
1 - an action potential propagates through the axon to an axon terminal <> 2 - voltage gated Ca2+ open and diffuse into the terminal <> 3 - Ca2+ causes vesicles with ACh to fuse with terminal membrane <> 4 - ACh released it into the synaptic cleft and diffuses across, binding to ACh receptors which open ligand gated cation channels <> 5 - Na+ enter and K+ exit muscle fibre causing depolarisation <> 6 - An action potential is produce across the sarcolemma
When is the action potential deactivated?
When ACh is removed from synaptic cleft by diffusion, acetylcholinesterase or recycling
What is the difference between action potential generation in neurons and muscles? What causes this?
RMP of muscle cell is slightly lower (-70mV vs -61.5mV) <> Due to slightly different ratios of leak channels
What is excitation-contraction coupling?
Excitation–contraction coupling is the process by which a muscular action potential in the muscle fiber causes the myofibrils to contract
When an action potential is generated in the muscle, how does the AP propagate through the entire muscle fibre?
By transverse tubules
How are the Ca2+ channels controlled in skeletal muscle? Where are these channels found? Why are they controlled in this way?
Voltage gated channels <> Found in the sarcoplasmic reticulum <> The sarcoplasmic reticulum is so close to the T-tubules that they can detect change in the voltage
When the Ca2+ channels are opened what does this allow?
It allows the Ca2+ to be released into the cytosol of the muscle fibre
What does the Ca2+ do in the myofibrils?
Binds to the troponin exposing the actin fibre allowing the cross-bridge cycle to occur
When does contraction of muscle fibres end?
When Ca2+ levels fall
How is Ca2+ removed from the cytosol?
It is actively pumped back into the sarcoplasmic reticulum by Ca2+ ATPase pump
How is phosphate stored in the muscle?
As creatine phosphate
What does creatine phosphate function as?
As a ATP store
How long can creatine phosphate be used as an energy source for?
<15sec
What does creatine phosphate and ADP produce? What kind of respiration is this?
creatine + ATP Anaerobic
MOVE TO 81 What are the two types of anaerobic respiration?
Creatine phosphate and Anaerobic glycolysis
What is the speed and efficiency of anaerobic glycolysis?
It is fast but inefficient
What are the limiting factors for anaerobic glycolysis? How long can humans perform this for?
Build up of lactic acid and H+ limits <> 30-40 sec
What is the energy source for anaerobic glycolysis?
Muscle glycogen or glucose from the blood
What kind of respiration is utilised for longer duration of activities?
Aerobic metabolism
What is the speed and efficiency of aerobic respiration?
Slow but efficient
What does aerobic respiration require that is different form anaerobic respiration? What is needed for this?
Requires oxygen <> A good supply of oxygen
What are the two types of muscle fibres?
Type I and IIA, IIB
What is a main difference between type I and II muscle fibres?
Type I has many more mitochondria
What are the characteristic of type 1 muscle fibres? -Metabolic mechanism -Max ATPase rate -SR pumping capacity -Diameter -Mitochondria concentration -Glycolytic capacity -Primary ATP pathway
-Slow oxidative <> -Slow ATPase rate <> -Moderate Ca2+ pumping ability <> -Small diameter <> -High number of mitochondria <> -Moderate glycolytic capacity <> -Aerobic pathway
What are the characteristic of type 2b muscle fibres? -Metabolic mechanism -Max ATPase rate -SR pumping capacity -Diameter -Mitochondria concentration -Glycolytic capacity -Primary ATP pathway
-Fast glycolytic <> -Fast ATPase rate <> -High Ca2+ pumping ability <> -Large diameter <> -Low number of mitochondria <> -High glycolytic capacity <> -Anaerobic glycolysis
How can force be regulated?
-Rate of stimulation of individual motor units <> -Number of motor units recruited
How does the rate of stimulation of individual motor units affect tension in muscle fibres? Why the differences between them?
If a single stimulation (i.e. action potential) is produced it creates a single twitch <> If there is low stimulation frequency it results in unfused/incomplete tetanus as the muscle fibre has some time to relax resulting in ‘jerky’ increase of tension generated <> If there is high stimulation frequency it results in fused/complete tetanus as the muscle fibres have been stimulated before they had time to relax resulting in a smooth and then peak increase of tension generated
What is a serious disease caused by over stimulation of the muscles? What is its death rate?
Tetanus <> 30%
How are more motor units recruited?
By increasing the stimulus voltage
What is the order of the type of fibres that are recruited in relation to increasing tension?
Small fibres recruited first for low tensions <> Small and Medium fibres are recruited for larger tension <> Small, medium and large fibres are recruited for maximum tension generation
