Muscles - Cell Biology Semester 2
What are the 3 types of muscles
Skeletal - These are voluntary muscles so we are able to control them to govern movement, posture and regulate body temp
Smooth - These are involuntary and will line an organ sich as stomach, bladder as well as blood vessels
Cardiac- These are involuntary and only found in the heart
All of the muscles will convert ATP into mechanical energy to then cause contraction
What are Muscle Fasciculus and muscle fibres
Muscle Fasciculus are the term used to descrive a bundle of muscle fibres
Muscle fibres are also called myocytes and are cylindrecal multinucleated cells composed of many myofibrils
What are myofibrils and myofilaments
Myofibrils are the basic rod like unit of muscle cells
Myoflimanets are the are the types of filamenst in the myofibril being Thick myosin filaments, the thin F-actin filaments and the titin elastic fibres that run through the core of each myosin filanment and anchor them to the Z line
How are skeletal muscles studied
In the past to look at them we used light microscopes that showed the Light and Dark band as well as the Z line
In newer electron microscopes it shows that the nucleus in similar locations in the cells and many mitochondria and myofibrils
There are many glycogen granules due to large energy demand of muscles
There are many smooth membraneous sytsmes that are used for muscle conttractions as well as Dark bands that are bisected by lighter H bands that then have the M lines in them
What structures are shown under a microscope of a muscle
There are Tubular triads that contain a flattened T tubule and a pair of terminal cistae
There are also sarcoplasmic reticulum to connect mutliple tubular triads
The mitochondria are mostly located in the I bands close to the part of the actin and myosin that intercat during contraction
What are the different bands of the sarcomere
The 2 Z lines are at either end of the Sarcomere
The H zone is purely made of myosin and is the middle section of the sarcolemma
A zone is where the actin and myosin will overlap each other
The I band is purely just the actin filaments
What is the sarcomere
This is the basic repeating unit of the striated muscle tissue and is the repeating unit between the 2 Z lines
The plasma membrane of the muscle is called the sarcolemma and there are holes in the sarcolemma is where there are RT tubules triads that join togehter and run through the whole muscle tissue
What is the sliding filament theory
The filaments dont contract and will only change in size and the level that they will overlap with each other so then cause the zone size to change during contraction of the muscle
The myosin is held in place by elastin titin that runs whole lentght of the sarcomere and hold the myosin at 90 degree angles
During the contraction of the muscle the actin will move closer to the myosin and the M line so then the overall sarcomere get smaller
There will also be a large A band where there is more overlap of the mysosin and the actoin filaments and a smaller H zone as well as smaller I band
What is the structure of the myosin filament
The thick filaments are made of myosin that are able to form dimers with each other to then make the thick filaments
There are long tail regions that can interact with other tails and form a helix with then other dimers
The myosin heads have an actin binding site so the myosin can bind to the actin as well as Myosin ATPase that is the site for ATP hydrolysis to provide energy for the muscle contraction
What is the structure of the actin filaments
On the actin filamets there are 2 actin molecules that are wrapped round each other being stabilised by tropomyosin as wlel as troponin on the tropomyosin to regulate contraction process
The tropomin has Ca2+ binding sites on them
The tropomyosin lie on top of the actiin filaments and then during contraction is moved out of the way of the mysoin head binding site
This happens due to a conformational change in troponin when Ca2+binds to it to then reveal the mysoin head binding site
How is the neuromuscular junction used to send signals
It is a voluntary so there is a signal sent down neurone to the neuromuscular junction when the muscle wants to contract
This works due to net entry of Na+ that cause an action potential that is propagated throught the sarcolemma and down T tubules triads that will then cause voltage gated ion channels to opeso Ca2+ released from terminal cistae to the sarcoplamsic reticulum
The Ca2+ will the bind to troponon and cause conformational shape change and remove the tropomyosin from the mysoin head bindingn site on the actin
Myosin head then bind and detach to then pull the actin to the centre of the filament and there is also ATP hydrolysis for energy
Ca2+ then removed by active transport back to sarcoplasmic reticulum
What is the cross bridge cycle
There is ATP bind to the myosin ATPase and then there is the unbinding of the myosin and actin to allow the myosin head to cock in its high energy formation
The cocking is caused by the ATP hydrolysis and then allows the mysoin to bind to the actin and the binding released a phosphate group
The power stoke is driven by the loss of ADP and cause the actin being pulled together in the H zone so Z line get closer together
ADP is then released and myosin in low energy state and so then another ATP can bind and restart the cycle
What is the gradation of skeletal muscles
The strength of the skeletal muscle contraction can be seperated to:
Twitch - Single contraction cycle that is prodiuced by an action potential in the muscle fibre itself
Simmation - If anothber action potential is added before the complete relaxation of the twitch contraction then another contraction added to make an overall stronger contraction
Tetanus - When the frequency of action potentials in the muscle increase so that then the muscle contraction reach its peak foce and then will plateu
Each power stroke will contract 1% of the muscle so need to happen many times for effective ciontraction of whole muscle
What is the motor unit
It is made up of the neuron and then all the muscle fibres that are connected to to as there are only 1 neuromuscular junction per muscle fibre
When a neurone is stimulated the fibres will all contract fully due to all or nothing principle
The large the motor unit number the stronger the contraction and muscles that have finer movements have smaller motor units and large movement muscle have larger motor unit
What is the size principle
As a greater force is required from the nervous system will stimulate more motor units and motor units with larger fibres and more of them to achive the desired contraction strength
So if greater force needed then nervous systenm will stimulate more and larger motor units
What are smooth muscles
These are involuntary musles and line the organs like stomach and bladder as well as blood vessels and convert chemical energy to mechanical energy
It is under the autonomic and hormonal control and doesnt have cross striations and specialises in repeative low force contractions
Where are smooth muscles found
Walls of hollow organs, walls of passageways like arteries and veins and tracts, in the eyes where change shape of iris and lens shape and then in the skin to cause hair to stand on end due to cold temperatures
What is the structure of the smooth muscles
It is thicker in the middle and taper at either end of the muscle
There 1 nucleus per cell and no T tubules or triads
There are circular and longitudinal smooth muscle fibres that are attached to dense bodies on plasma membrane and the cytoplasm
The dense bodies act as Z disks and there are caveolae invaginations on the plasma membrane that are similar to T tubule
What are the unique features of the smooth muscle
They produce their own connective tissue called endomysium and they have myosin and actin but no sarcomere
But opposite to skeletal the thin filaments dont move asd they are anchored by dense bodies
Smooth muscles need extracellular matrix to secrete collagen and glycoproteins thatr anchor smooth muscle in functioanl units
Tension is generated by cintraction and it is transmitted via focal adhesion densities so suyrrounding connective tissue so group of cells act as 1
How do smooth muscles contract
Ca2+ are released from ER and enter via caveloae to bidn to calmodulin and change its shape
This then activate the myosin light chain kinase MLCK
MLCK will then phosphorylate and activate Myosin ATPase to then bind to actoin filament
ATP depenedent contraction keep going as long as myosin is phosphorylated
What is Calmodulin
An intracellular target for Ca2+ and is part of the calcium signal transduction pathway by interacting with Kianse and Phosphatse and act in same way as troponin
What will stop smooth muscle contractions
Contraction keep going till ATP dependent Ca2+ pumps pump Ca2+ to the ER but a small amount will stay to then keep muscle tone and keep them slightly contracted in certain blood vessels
How are smooth muscles stimulated
They are involuntary and contract without the nervous system but due to hormones, stretch, metabolic state and the autonomic nerve fibrss to stimukate many myocyte at diffuse junctions
What are the innervation of the smooth muscle
The contratile activity of smooth muscles is caused by neurotransmitters from autonomic neurons in vesicles
There are no motor end plate regions but there are swollen ends called varicosities that have many neurotransmitter vesicles released during action potential
Varicosities from single axon may be located along several muscles cells and a single muscle cell may be located near varicosities belogn to postganglionic fibres of neurons
1 neuron may influence many smooth muscles cells but 1 smooth muscle cell may be influcenced by many neurons
How are smooth muscle organsiued in single units
Has its muscle fibres joined by a gap junction so that the wall of the muscle will contract as a single unit
This is found in the walls of visceral organs bar the heart
The gap junction allow the movement of electrical impulse
between muscle fibrs in contact with each other
They have a stress relaxation response that means that the muscle of a hollow organ is stretched when it fills and the mechanical stress of the stretching will then trigger contraction that is then immediately followed by the relaxation period so the organ wont empty its content prematurely
Also the smooth muscle will maintain a muscle tone when the organ empties and shrinks so prevent flabbiness in the empty organ
It will in general produce a slow, steady contractions that allow substances to move through
How is smooth muscle organised in multi units
These rarely possess gap junctions so are not electrically couple and impulses cant spread between cells Due to electrical isolation it means there can be finer motor control using this system
This means the contraction will not spread from cell to cell but is confined to the original cell
The stimuli will come from the autonomic nervous system or hormones and there is no stretching that can start contraction as well
They are found around large blood vessels, in the respiratory airways and the eyes
What is the hormonal control of the smooth muscles
The flight, foght or freeze response controls a lot of smooth muscle
How are smooth muscles needed in lungs
There are smooth muscles existing in the bronchus to the bronchioles and they are important to regulate the diameter of the bronchioles
In asthma conditions there are greater stimulation and so there is greater smooth muscles and many more goblet cells making more mucus so then a smaller lumen
This can be treated by Therapeutic targeting of smooth muscles:
Beta 2 Receptor agonist from inhalers will target the Beta 2 receptor and bind to it
This can be caused by adrenaline in the FFF response
This then lead to increase in Cyclic AMP
Then there will be more phosphorylation of myosin light chain kinase and so will inactivate it
Leading to relaxing of the bronchial smooth muscles and so bronchodilation
How are smooth muscles needed in gastrointestinal tract
The longitudinal and cross sectional muscle coordinate the movement of peristaltic movement of food down the tract
The FFF will decrease peristalsis
The smooth muscles are stretch activated so when they fill it cause contraction and push the food through
How are smooth muscles needed in oesphagus
Is a strong muscular tube that convey food from mouth to stomach and is voluntary as is swallowing
But then then after voluntary swallowing the peristalsis stake over so then become involuntary
Both skeletal and smooth muscles are involved in the function
How are smooth muscles needed in uterus and myometrium
Main bulk of uterus made of smooth muscles in the myometrium
In pregnancy there are greater levels of oestrogen so myometrium increase in size
Then during birth there are strong contractions that are reinforced by the oxytocin levels rising that then expel the foetus
How can smooth muscles be therapeutic targeted
Can use intravenous salbutamol to relax smooth muscles to delay premature births
Conversely oxytocin can cause greater contractions due to
Receptors cause Ca2+ release from the ER that then bind to the calmodulin that activate the myosin light chain kinase and so then cause contractions