Muscle 1 Flashcards
Which muscles are striated?
Skeletal and cardiac
What is striated muscle?
A distinct striped pattern
What are the skeletal muscle body function?
- Movement
- Generation of heat – shivering - exothermic process, in shivering you are contracting and relaxing muscles quickly
How is skeletal muscle connected to bone?
Via a tendon
What is an individual muscle cell/ myocyte also called?
A muscle fibre
What is a muscle fibre covered with?
a layer of connective tissue called the Endomysium
What is the muscle cell membrane called?
The sarcolemma?
How many nuclei does each skeletal muscle fibre have and how are they formed?
Each of the fibres is multinucleated and is formed by the fusion of cells during development
What are muscle fibres grouped into?
Muscle fibres grouped into bundles called a Fascicle
What is a fascicle covered by?
a layer of connective tissue called Perimysium
Within the muscle fibre what do we have?
Bundles of filaments called myofibrils
Myofibrils are composed of bundles of filaments called myofilaments
What leads from the Sarcolemma into the interior of the muscle?
Tunnels called t tubules
What is wrapped around the myofibrils?
The sarcoplasmic reticulum
What interacts with the t tubules and what structure does this form?
Terminal cisterna (special parts of reticulum) interact with t tubules to form a structure called a triad
What is the structure of myofibrils?
A long string of repeated units called Sarcomeres
What gives muscle it’s striped appearance?
The Sarcomere
Describe the structure of a sarcomere unit
- There are bands in the sarcomere, this is what gives muscles the striped appearance
- The M line is the central line (middle)
- The H zone straddles the M line
- The next band is the A band
- The I band straddles across two Sarcomeres
- The Z line is the dark line which is the boundary between two sarcomeres
What are the two major structural features of sarcomeres?
- Thick filaments Myosin Titin – largest protein in human genome, string like function, enables muscle to return to resting state after being stretched (3rd most abundant protein in muscle) - Thin filaments Actin Nebulin - Both connected to z lines - Thick filaments are connected to M line
What does the mechanism of contracting depend on?
filaments sliding over each-other - this changes the banding pattern
Give features of the different bands and contraction in myofibrils
- A band – overlap between thick and thin filaments
- H zone – just thick filaments
- I band – just thin filaments
- The H zone and the I band shorten during contraction as they are the areas of the thick filament that do not overlap with thin filaments and the areas of the thin filaments that do not overlap with thick filaments
What is there an optimal overlap between and why is this?
- There is an optimal overlap between myosin heads and actin. If muscle is stretched so that actin and myosin heads can’t interact anymore our force of contraction goes down. The same thing happens if you compress the muscle
- Optimum length is between 2.1 and 2.2 um
- Frank Starling Law of heart is like this
- Muscle has a relationship between the sarcomere length and force of contraction
What is the structure of myosin?
- Hexamer
- Formed from two heavy chains which have large head groups and then tails that coil together to form an alpha helix
- Myosin is a member of a highly diverse family of MOTOR proteins
- Muscle myosin is a member of myosin class II
- 9 different kinds of myosin II in various muscle types
8 skeletal/cardiac
1 smooth - Myosin head has ATPase activity
- Motor powered by hydrolysis of ATP
- Four light chains associated with necks of heavy chains:
- Regulatory light chains regulate activity of myosin head group (important in smooth muscle)
- Essential light chains are mostly structural
What forms an interaction with actin in thin filaments?
Heavy chain heads of myosin
What can the conformation of the heavy chains be changed by?
binding and hydrolyses of ATP
What is the structure of actin?
- Major component of muscle and eukaryotic cytoskeleton
- Exists in two forms:
- Globular: G actin
- Filamentous form: F actin which is found in muscle
- In skeletal muscle it is associated with the proteins Troponin and Tropomyosin
- Tropomyosin runs along the actin chain and consists of two alpha helical proteins which are coiled together
- One Tropomyosin molecule for 7 actin monomers
Give features of troponin
- Troponin is associated with both Tropomyosin and actin
Complex of 3 separate proteins
Troponin T associates with Tropomyosin
Troponin I associated with actin and inhibits binding of myosin
Troponin C can bind calcium – allowing contraction of muscle to be regulated by myosin
What does it mean that muscle spindles are proprioceptors?
Muscle spindles are proprioceptors that respond to muscles being stretched. Their function is to help the body maintain posture, but they are most familiar in the knee-jerk reflex.
What do muscle spindles contain and what do these do?
The muscle spindles contain specialised forms of muscle fibres called intrafusal fibres, They send sensory signals to the spinal cord via type la fibres
How does contraction occur?
- Have myosin heads interacting with actin in a starting conformation
- Myosin head binds ATP – causes dissociation of bond between Myosin and Actin
- Myosin hydrolyses ATP –> ADP and phosphate stay bound to myosin head. This changes myosin conformation and head tilts and enters the cocked state
- Formation of weak bond between actin and myosin head – they are now interacting in a different position – myosin head has moved down the phosphate chain
- Dissociation of phosphate – much stronger bond between actin and myosin, myosin head returns from cocked state to starting position, this moves the filaments relative to each other
- ADP dissociates and we are now back in our starting state, but our myosin head has moved along the actin chain and the filaments have moved relative to each other – the muscle has contracted
How is contraction regulated?
- Troponin T binds to tropomyosin
- Troponin C won’t have calcium bound at rest when calcium levels are low
- Troponin I inhibits the formation of cross bridges between myosin and actin by binding to actin and holding the whole complex in a position that covers up the binding site for myosin
- When muscle contraction is stimulated the calcium concentration in cytoplasm rises and calcium can rise to troponin C – when this happens we see a conformational change in the troponin complex, it shifts on the actin, this change in conformation pulls the tropomyosin further in the actin groove and uncovers the myosin binding sites on the actin, cross bridges can form and our muscle can contract
What is Excitation-Contraction coupling?
Action potential needs to be converted into a calcium based signal that can trigger contraction in the NMJ
Give features of the T tubule and how it is involved in excitation-contraction coupling
- As it enters muscle fibre it becomes associated with Sarcoplasmic reticulum
- Where the sarcoplasmic reticulum associates with the t tubule it forms a bulbous structure called a terminal cisternae. We have two terminal cisternae associated with each t tubule, this structure is called a triad
- Our action potential spreads as a wave down the t tubule into the interior of the muscle
Give features of the distribution of Calcium in muscle cells
- About 100 nM calcium in the cytoplasm of cell
- In mM concentrations in the outside of cell and in the cytoplasmic reticulum
- Asymmetric distributions caused by activity of an active transport pump: SERCA: pumps calcium out of the cytoplasm and into the cytoplasmic reticulum using the power of ATP hydrolyses
- It is important for there to be a low concentration of calcium in the cytoplasm
Where does Calcium come from in skeletal muscle cells?
- We have a big pool of calcium inside the sarcoplasmic reticulum (terminal cisternae – also known as lateral sacs) – we have Ryanodine calcium receptors in the membrane
- When an action potential travels down the t tubule the DHP receptor (membrane calcium channel) changes it’s conformation. This change in conformation is transmitted through the physical linkage to the ryanodine receptor. The ryanodine receptor opens it’s channel and calcium floods out of the sarcoplasmic reticulum into the cytoplasm causing contraction
- Here we are using the calcium channel in the membrane as a remote voltage sensor for the ryanodine receptor
What is the DHP receptor tetrad?
- Four DHP receptors associating with each ryanodine receptor
- This arrangement is called a tetrad
How is Calcium balance restored in muscle cells?
- Role of SERCA calcium pump
- It pumps the calcium from cytoplasm back into the sarcoplasmic reticulum
Give a summary of nerve to muscle contraction
- Acetylcholine released by motor neurone
- Activates nACh receptors
- Sarcolemma depolarised, action potential triggered and spreads to T tubules
- DHP receptor activated. Triggers ryanodine receptor
- Calcium ions released from sarcoplasmic reticulum
- Troponin C binds Ca2+ and is activated
- Muscle contraction initiated
- Calcium ions pumped back into the sarcoplasmic reticulum
What is the neuromuscular junction/ end plate?
The neuromuscular junction, or end plate, is the synapse between motor neurones and skeletal muscle. Although it is a relatively specialised synapse, it is very well characterised and so has served as a model of how synapses work for many years
What is the motor unit?
When each motor neurone makes synapses with several muscle fibres
What is the end plate region?
When a major branch of a motor neurone axon meets a muscle fibre, it splits into a series of finely branched presynaptic nerve terminals or boutons (‘arborization’). The patch of the muscle fibre membrane where these terminals make contact, is the end plate region.
Where are acetylcholine receptors located?
Under each of the presynaptic terminals the muscle fibre membrane has a series of folds (junctional folds). It’s in these junctional folds that the nicotinic acetylcholine receptors are located
What is Myasthenia gravis (MG)?
Myasthenia gravis (MG) is a disorder in which there is an autoimmune attack on skeletal muscle. The damage that this attack causes leads to muscle weakness and fatigue. AS the muscles involved in respiration are skeletal muscles, MG can ultimately be fatal but effective treatments are available and most patients will have a normal life-span
Who is MG more common in and what age does it most commonly occur?
It is 2x more frequent in woman and occurs most commonly between ages 20 and 40
What may play a key role in the causes of MG?
the thymus gland may play a key role. Removal of the thymus gland can prove beneficial for many patients, and a substantial number of MG suffers turn out to have problems with the thymus gland such as tumours (thymomas)
What is the target of the autoimmune attack in MG?
The target of the autoimmune attack is the nicotinic receptor on the muscle membrane. This ligand-gated ion channel is the ‘molecular switch’ that triggers muscle contraction. If the function of this receptor is compromised then contraction will be too
What are some of the symptoms of MG?
You can get symptoms such as drooping eyes, inability to smile and pick up arms – often known as the rag doll illness
What is the structure of the nicotinic acetylcholine receptor?
- The nAChR is a pentameric transmembrane protein
- There are two alpha subunits, a beta, a delta and an epsilon. In the foetal receptor, there is a gamma subunit instead of epsilon
- All of the subunits are very similar in structure: they have four transmembrane domains and a large extracellular N terminus. Amino acids in the a subunits and in the delta and epsilon come together to form two binding sites for acetylcholine in the extracellular part of the receptor.
- The second transmembrane domains of each subunit come together to form the lining of the ion channel
Which part of the nAChR do the antibodies produced in MG target?
If you examine the antibodies produced in MG, it quickly becomes obvious that most of them target the same part of the nAChR. This region, known as the main immunogenic region (MIR), is in the extracellular part of the a subunits, specifically amino acids 67-76
What happens with MG?
antibodies bind to the MIR section in alpha subunits?
What are the three mechanisms of MG?
- Receptors become internalised
- Destruction and simplification of the end plate
- block of the acetylcholine binding sites
what happens with the receptor becoming internalised in MG?
- The cells recognise that something is wrong with membrane due to antibody binding and try to get rid of it
- Does this by endocytosis, forms a vesicle and internalises the receptor.
- Receptor broken down into amino acids
- Compromises function o f muscle as it depletes amount of nicotinic receptors
What happens with the destruction and simplification of the end plate with MG?
- End plate is portion of the muscle membrane under the nerve terminal
- Nicotinic receptors localised to top of folds
- Get complement attack on muscle which results in change in structure of end plate:
- Becomes simplified, folds are lost, lose a lot of nicotinic receptors
- Also get widening of synapse
What happens with the block of the acetylcholine binding sites with MG?
- Least important of the 3
- If antibodies bind to these sites they act as competitive antagonists and stop acetylcholine binding
- This doesn’t happens in the MIR section of alpha subunits
What are the four ways to diagnose myasthenia gravis?
- pattern of muscle weakness
- Presence of antibodies against the nAChR
- Electromyography
- Tensilon/ edrophonium test
How do you diagnose MG with a pattern of muscle weakness?
- The muscle weakness in MG is not usually accompanied by pain but sufferers may feel overwhelmingly fatigued
- The first muscles to be affected are usually in the face: drooping eyelids and difficulty smiling are early symptoms. The patient may also experience difficulty swallowing and have slurred speech. Later muscles in the limbs and trunk may be affected
How do you diagnose MG with the presence of antibodies against the nAChR?
Testing the patient’s serum will often reveal the presence of antibodies against the nicotinic receptor. Not all patients have high levels of antibodies, however, especially those whose symptoms are mainly limited to their eyes
How do you diagnose MG with an electromyography?
- EMG is a procedure where the electrical activity of skeletal muscles is measured. This can be done using surface electrodes or using needle electrodes inserted into the muscle
- The muscle is then stimulated and the resulting action potentials are recorded
- Patients with MG will show a decline in the size of the action potential with repeated stimulation
How do you diagnose MG with a Tensilon/ edrophonium test?
- Acetylcholinesterase inhibitors produce an improvement in the symptoms of MG and are one of the main-stays of treatment
- This therapeutic effect can also be used diagnostically. Patients are administered a dose of a short-acting acetylcholinesterase inhibitor called edrophonium. If this drug produced an improvement in their symptoms, it provides evidence for a diagnosis of MG
What is the main method of treatment for mild MG?
- For mild MG, the main treatment is acetylcholinesterase inhibitors such as pyridostigmine and neostigmine. These drugs inhibit the breakdown of acetylcholinesterase.
- Acetylcholinesterase is responsible for the termination of signalling at the neuromuscular junction
What is given if the symptoms of MG are more severe?
- If the symptoms are more severe, then steroids or drugs such as azathioprine can be given to try and suppress the immune response.
- However, immunosuppression increases the risk of acquiring a bacterial or viral infection and can also increase the risk of cancer
How may the patient be treated if their symptoms of MG are very severe?
If the symptoms are very severe, the patient may be treated using plasmapheresis or plasma exchange to remove the autoantibodies from their circulation
What are nerve gases/ nerve agents typified by and what is this?
- Nerve gases/ nerve agents are typified by the compound sarin
- Sarin was developed as an insecticide in Germany in 1938. Tests revealed it to be easily absorbed through the skin or via inhalation and that is was around 500x more deadly than cyanide
What does Sarin do?
- Sarin inhibits acetylcholinesterase not just at the NMJ but also at cholinergic synapses in the autonomic nervous system and in the brain. As a result, almost every body system will begin to malfunction
- At first victims will have difficulty breathing and start to droll uncontrollably. After they will experience paralysis and convulsions, enter a coma and eventually die from respiratory failure. These effects occur due to acetylcholine acting at both nicotinic and muscarinic acetylcholine receptors
Why does Sarin have it’s effect?
- nACHR also exists in a desensitized state. This is a kind of safety mechanism that the receptor switches on when it is exposed to acetylcholine for a long period of time – the receptor still binds ACh closely but the channel closes.
- If nAChRs are open continuously then the muscle will be continuously depolarised and thus will not be able to fire further action potentials because the voltage gated sodium channels will be largely inactivated
- Added together, these effects mean that if a large concentration of acetylcholine persists in the synapse, the muscle will not be able to contract after an initial burst of activity
- This is why nerve gases result in paralysis – the muscle will become limp (flaccid)
What is the molecular mechanism of Sarin?
- the enzymatic mechanism of acetylcholinesterase centres around a group of 3 amino acids: a serine, glutamate and histidine
- Of these serine is particularly important because it is the group that accepts the acetate group from acetylcholine to release free choline
- The acetyl-serine is then broken down to recycle the active site back to its starting form. It is this serine residue to which sarin attaches. However, the sarin-serine bond is too strong to be broken down and the enzyme cannot be recycled
What are antidotes to sarin?
- Injections of the muscarinic antagonist atropine can save the life of someone exposed to sarin as many of the worst effects of sarin are caused by actions at muscarinic acetylcholine receptors
- Another effective treatment for sarin is a drug called pralidoxime. Pralidoxime recycles acetylcholinesterase back to an active form, but is only effective if administered very quickly after exposure
What has been used to model MG and why?
- The electric organs of the electric ray and electric eel are embryologically derived from muscle and are controlled by the same molecular switches: nicotinic acetylcholine receptors.
- The electric organ consists of a stack of flat cells called electrocytes. The stack of cells acts like a set of miniature batteries and although each cell only produced a tiny current, there are so many electrocytes in the electric organs of torpedo electric rays that they can produce a discharge of 220 V at up to 30 A.
- The abundance of nAChRs in the Torpedo electric organs means that this tissue has been used extensively in studying the receptor. It is possible to obtain milligram quantities of nAChR from Torpedo
- It has been found that injecting animals with purified nAChR from Torpedo will provoke an immune response that culminates in autoimmunity against the animal’s own receptors: experimental myasthenia gravis
