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.
what happens to contraction velocity and distance shortened as load exerted on the muscle increases?
decreases
describe tetanus/tetanic contraction?
a type of contraction where action potentials continue to stimulate the muscle fiber repeatedly at short intervals (high frequency) and relaxation between contractions diminishes until we reach a state of maximal contraction.
what are the 2 types of tetanus / tetanic contraction?
Unfused - the stimulation rate is not at max value, so the fiber relaxes between stimuli
Fused - the stimulation rate is at max value, no relaxation and reaches maximum tension
Why is tetanic tension greater than twitch tension?
Because [Ca2+] never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites in tetanic tension, so we can keep prolonged contraction.
Movement around a limb requires 2 antagonistic groups of muscles called a flexor and an extensor
why are antagonistic groups of muscles arranged in lever systems?
It amplifies muscle shortening velocity producing increased maneuverability
what 3 ways is ATP used in skeletal muscles?
1- Hydrolysis of ATP energises X-bridges between myosin and actin (contraction)
- Myosin ATPase
2- ATP binding to myosin detaches it from the binding site on actin
3- ATP powers Ca2+-ATPase in sarcoplasmic reticulum to uptake Ca2+ (relaxation)
what is muscle fatigue?
a safety mechanism that prevents muscle from using up vast amounts of ATP, which would cause rigor (i.e. muscles would not be able to activate new cross-bridge cycles)
The muscle stops responding to motor neuron action potentials.
After this, the longer the resting period we have, the greater the ability we have to generate more tension.
what 3 things does the rate of muscle fatigue depends on?
1- Fibre type
2- Length of contraction
3- Fitness of individual
what are the factors causing fatigue during high intensity, short term exercise? e.g. sprinting
1- Conduction failure due to accumulation of [K+] out of the cell
2- Rise of lactic acid → acidifies proteins (nonoptimal pH levels for enzymes)
3- Rise of [ADP] and [Pi] inhibits X-bridge cycle, delaying myosin detachment from actin filaments.
what are the factors causing fatigue during low intensity, long term exercise? e.g. marathon running
1- ↓ muscle glycogen
2- ↓ blood glucose
3- Dehydration & electrolyte concentration
what is central command fatigue?
a type of psychological fatigue where the cerebral cortex cannot excite motor neurons anymore because of loss of ‘will to win’
e.g. depression / mental health → demotivation and tiredness
Oxidative fibers are a type of muscle fiber with:
1- ↑ mitochondria → ↑ oxidative phosphorylation
2- ↑ vascularisation to deliver more O2 and nutrients
3- myoglobin (darker color) → ↑ O2 delivery
4- red color with low diameters (for faster diffusion)
Glycolytic fibers are a type of muscle fiber with:
Glycolytic fibers are a type of muscle fiber with:
1- Few mitochondria
2- ↑ glycolytic enzymes (for glycolysis) and glycogen
3- Lower blood supply (less reliant on oxygen)
4- white/pinker color with large diameters
what are the 3 types of muscle fibers according to fatigue resistance?
[1] Slow oxidative (I) → resist fatigue
(reliant on oxygen - aerobic)
e.g. maintain posture
[2] Fast oxidative (IIa) → intermediate fatigue resistant
(either anaerobic/aerobic depending on training)
e.g. standing/walking
[3] Fast glycolytic (IIb) → quick fatigue
(less reliant on oxygen - anaerobic)
what is recruitment? how is this process done??
the process of increasing the number of active motor units.
The more load you carry, the more motor units are needed to be activated/recruited.
process is done in a sequence (activation of types of fibers):
→ Slow oxidative fibres → Fast oxidative fibres → Fast glycolytic
what 2 things does neural control of muscle tension depend on?
1- Frequency of action potentials sent to motor units
2- Recruitment of motor units (how many are activated?)
what is denervation atrophy?
when the nerves/NMJs supplying a muscle fiber are destroyed, which causes a gradual deterioration of the muscle.
Because the muscle is no longer receiving action potentials from the motor neuron.
what is disuse atrophy?
when the muscle stops contracting e.g. plaster cast or bedbound or long-term bed rest, it gradually becomes weaker, less protein.. etc.
what is hypertrophy?
when you do strength/resistance exercise which leads to microtears/microtrauma and leads to an increase in muscle mass by packing muscle fibres with more proteins.
There is no generation of new muscle cells.
what type of fibres does aerobic exercise lead to?
leads to more slow/fast oxidative fibres:
- ↑ mitochondria
- ↑ vascularisation
- ↓ fibre diameter
what type of fibres does anaerobic exercise lead to?
Anaerobic (strength) exercise e.g. sprinting/weight lifting, leads to more fast glycolytic fibres:
- ↑ fibre diameter
- ↑ glycolysis
Compare (Similarities/Differences) between smooth and skeletal muscle fibres
Similarities:
- Both have an X-bridge cycle & use Ca2+
- Both contain thick myosin and thin actin filaments
Differences:
- Smooth → involuntary & ANS, while Skeletal → voluntary & SNS
- Smooth muscles are anchored to membranes and cell structures by dense bodies, while Skeletal muscles are anchored by Z-lines
describe the smooth muscle x-bridge cycle:
[1] ↑ [Ca2+]
[2] Ca2+ binds to calmodulin forming a Ca2+-Calmodulin complex
[3] Ca2+-Calmodulin activates Myosin Light Chain Kinase (MLCK)
[4] MLCK phosphorylates myosin X-bridges with ATP (detached) at the neck region ‘RLC’
[5] ATP Hydrolysis through myosin ATPase activity → ‘cocked’
[6] Phosphorylated X-bridges at head region bind to actin filaments
[7] Release of Pi and performs powerstroke
[8] Release of ADP and bring in new ATP (detaches now)
[6] Contraction + Tension (cycle continues until MLCP removes Pi at neck region ‘RLC’)
describe the smooth muscle relaxation:
[1] Ca2+ ↓
[2] Less Ca2+-calmodulin → decrease MLCK activity
[3] Myosin Light Chain Phosphatase (MLCP) → dephosphorylates X-bridges at the neck region ‘RLC’, which decreases myosin ATPase activity
[4] Less myosin ATPase activity → decreases muscle tension
what does ca2+ bind to in skeletal muscles and in smooth muscles?
In skeletal muscles, Ca2+ binds to Troponin to initiate contraction.
In smooth muscles, Ca2+ binds to Calmodulin to initiate contraction
what controls contraction and relaxation in smooth muscle? what enzyme control this process
In smooth muscle, phosphorylation and dephosphorylation of the myosin light chain controls contraction and relaxation.
This process is controlled through the ratio of MLCK and MLCP activity
How do some types of smooth muscle maintain tension for a long time using low ATP consumption?
e.g. blood vessels
Phosphorylated X-bridges may be dephosphorylated when they’re still bound to actin (Latch state)
(normally it detaches then it gets dephosphorylated)
This leads to low rate of ATP splitting → slows down X-bridge cycle
So they maintain a state of base-line tone of contraction
i.e. persistent stimulation and a relatively high [Ca2+]
This helps maintain tension for a long time with low ATP consumption
Why do some types of smooth muscle, e.g. blood vessels, have a persistent stimulation and ↑ [Ca2+] all the time?
They have to be at a low level of contraction at all time to deliver blood to areas of the body.
Low-level contraction using relatively high levels of Ca2+ to maintain a base-line tone of contraction
In these types of smooth muscles:
- Phosphorylated X-bridges may be dephosphorylated when they’re still bound to actin (Latch state)
(normally it detaches then it gets dephosphorylated)
This leads to low rate of ATP splitting → slows down X-bridge cycle
This helps maintain tension for long time with low ATP consumption.
In Smooth muscles, Ca2+ comes from the sarcoplasmic reticulum and the ECF
- Contains less Ca2+ than Skeletal muscle, it also has no t-tubules.
- Voltage-gated Ca2+ channels in the ECF in Smooth muscle
- Ca2+ is pumped out of the cell or back into SR through Ca2+-ATPase in Smooth muscle.
- Slower process than skeletal muscle
In Skeletal muscle, Ca2+ only comes from the sarcoplasmic reticulum
what is the latch state?
Latch state is a significant factor in the ability of smooth muscle to sustain contraction without fatiguing.
When dephosphorylated myosin remains attached to actin in an isometric contraction, this allows maintenance of tension at low ATP consumption.
What are the factors affecting contractile activity in smooth muscles?
1– Spontaneous electrical activity in muscle membranes = pacemaker activity, e.g. gut
2– Autonomic neurotransmitters from varicosities
- Travels across sheets of long smooth muscles through buldges called varicosities, all along the sheet, releasing neurotransmitters at these sites.
3– Hormones (e.g. oxytocin)
- oxytocin for breastfeeding/childbirth - automatic lactation when hearing babies crying or high-pitched noises
4– Local factors (paracrine agents, pH, O2, osmolarity, ions, NO)
- because Smooth muscles are much thinner layered than the bundled Skeletal muscle fibers
5– Stretch
- Stretching muscles → contracts harder and faster
What’s the importance of Nitric Oxide in the cardiovascular system?
one of the main factors for regulation of blood pressure, because it maintains the relaxation and contraction of blood vessels
what are 2 types of smooth muscle? single/multiunit
Single units GIT, uterus, small blood vessels)
– Many cells linked by gap junctions (Signals travel between cells)
– Contract synchronously
– May contain pacemaker cells
– Stretch evokes contraction
Multiunit (airways, large arteries, hairs)
- Few or no gap junctions
- Richly innervated by ANS
- Not as responsive to stretch
