skeletal, smooth and cardiac muscle Flashcards
brief description of skeletal, smooth and cardiac muscle:
Skeletal muscle fibers
- large
- multinucleated cells ‘fused together’
- striated.
- attached to the bones of skeleton to enable control of movement
e.g. diaphragm & voluntary muscles
Cardiac muscle fibers
- small
- uninucleated cells
- striated, with intercalated disks.
- electrically conductive to move potentials through intercalated disks
- moves blood through the circulatory system
e.g. heart
Smooth muscle fibers
- small looking blobs
- unstriated.
- can generate a lot of tension and contraction to regulate flow of materials.
e.g. blood vessels, vas deferens, airways, uterus, GI tract, bladder… etc.
how do skeletal muscle multinucleated cells merge to form muscle fibres?
This process happens in utero from precursor cells called mononucleate myoblasts
This supply of myoblasts is not the same after we are born, they don’t replace damaged cells, instead the body uses satellite cells for repairing injuries, which are in limited supply.
Muscle fibres are small in diameter (10-100 microns) but large in length (20 cm)
what are satellite cells?
they’are commited stem cells that become active and differentiate into muscle when needed for muscle growth and repair.
When injured, fibres near the injured area undergo a process called hypertrophy, they increase in size, become stronger, and deposit more proteins.
tendons - connects muscle to bone
Skeletal muscle → Muscle fascicle → Muscle fibers → Myofibrils
what are myofibrils?
Myofibrils are composed of thick and thin filaments which form a structure called Sarcomere
Thick filaments → Myosin
Thin filaments → Actin
what is titin?
a spring-like protein that spans the sarcomere from one Z disk to the neighboring M line.
Functions:
1- Stabilizes the position of the contractile filaments
2- Provides elasticity to return stretched muscle to their resting length
describe the energy requiring process called sliding filament thoery:
When the sarcomere shortens i.e. the actin and myosin slide past one another, the muscle contracts.
I band is small during contraction
When the sarcomere expands i.e. the actin and myosin slide in the opposite direction, the muscle relaxes.
I band is large during relaxation
according to the sliding filament theory, what is tension generated in the muscle fiber directly proportional to?
the number of high force cross bridges between thick and thin filaments
what is troponin and tropomyosin? what do they do?
it’s a calcium-binding protein complex that controls the positioning of an elongated protein polymer called tropomyosin
In resting skeletal muscle - tropomyosin partially blocks the myosin-binding site on the actin filament
Before contraction occurs, Ca2+ binds to troponin and pulls tropomyosin away from the binding site.
describe what happens in the cross bridge cycle (4 steps):
[1] ATP binds and myosin detaches
- ATP binding decreases the actin-binding affinity of myosin
[2] ATP Hydrolysis provides energy for the myosin head to rotate and reattach to actin}}
- 45° to 90° → cocked position
- weak crossbridge because tropomyosin is partially blocking the binding site
- Products of hydrolysis: ADP + Pi
[3] The power stroke
- begins after Ca2+ binds to troponin and removes tropomyosin off the rest of the binding site
- head and hinge tilt from 90° to 45° (release of Pi) and begins power stroke
[4] Myosin releases ADP
- ADP is released and the cycle is ready to begin as a new ATP binds to myosin head
what is the excitation contraction coupling process?
the process in which muscle action potentials are translated into calcium signals. The calcium signals in turn initiate a contraction-relaxation cycle.
First phase is the inititation of Muscle Action Potential (at the NMJ)
Then this process begins:
1- Action potential in t-tubule alters conformation of DHP (dihydropyridine) receptor
2- DHP receptor opens RyR (ryanodine receptor) Ca2+ release channels in sarcoplasmic reticulum (SR), and Ca2+ enters cytoplasm.
This then causes the Contraction Phase
1- Ca2+ binds to troponin, allowing actin-myosin binding
2- Myosin executes power stroke
3- Actin filament slides toward center of sarcomere
what is stored in the sacroplasmic reticulum that is around the myofibrils?
Ca2+
what are Transverse tubules (T-tubules)?
an extension of the cell membrane (sarcolemma) that allow rapid conduction of the action potential which comes from the neuromuscular junction.
when does the relaxation phase happen?
- Sarcoplasmic Ca2+-ATPase pumps Ca2+ back into the sarcoplasmic reticulum.
2- Decrease in [Ca2+] causes Ca2+ to unbind from troponin
3- Tropomyosin partially re-covers binding site on the actin filament, so myosin detaches, and the filaments are pulled back into their relaxed position.
what are motor units composed of?
motor neurons + motor fibres.
There are hundreds of them in muscles, they allow graded twitches, from minimal all the way up to big, single, maximal, prolonged contractions depending on how many are utilised.
what does it mean when motor units are redundant? what does it allow?
- Instead of having one motor neuron innervating multiple muscle fibres
- You have multiple motor neurons innervating multiple muscle fibers
This allows:
- A decrease in the risk of being easily paralyzed.
i.e. if one motor neuron died, you still have others to take its place.
what is the tension and load?
tension is the force that a muscle exerts
Load is the force that is exerted on a muscle
what is isotonic, isometric and eccentric contraction?
Isometric contraction is a contraction with constant muscle length e.g. holding phone up in the air
Isotonic/Concentric contraction is a contraction with shortening muscle length e.g. running
Eccentric conctraction is a contraction with increasing muscle length e.g. sitting down
what is a twitch contraction?
a single contraction and relaxation cycle produced by a single action potential within the muscle fiber.
what does latent period and contraction time mean?
Latent period is the time before excitation contraction starts
Contraction time occurs between start of tension and peak tension
what is the latent period of isometric/ isotonic contraction?
isometric contraction - low latent period but longer contraction event
Isotonic contraction - high latent period but shorter contraction event
A typical muscle action potential lasts between 1 to 2 msec, while the muscle contraction may last up to 100 msec.
How does summation work in action potentials?
Because the interval between action potentials is shortened, the muscle fiber does not have time to relax completely between two stimuli, resulting in a more forceful contraction, this is called summation.
