Skeletal, Smooth and Cardiac Muscle Flashcards
What do muscles do?
Generate force and movement
What are the three types of muscles?
Skeletal, smooth and cardiac
What type of tissue are muscles made up from?
Excitable tissue - made of cells that can alter their membrane potentials in response to a stimuli, and generate action potentials.
What muscles are striated?
Skeletal
Cardiac
What are examples of smooth muscle?
Blood vessels
Airways
Uterus
GI tract
Bladder
What causes striations in muscle tissue?
The striations are caused by the regular arrangement of contractile proteins (actin and myosin) aka sacromere
Discuss features of the skeletal muscle
Multinuculated
Increase in fibre size during growth
Myoblasts do not replace cells if damaged
How are skeletal muscles formed?
In utero from mononucleated myoblasts
How are muscles stored?
Bundles of fibres encased in connective tissue sheaths
How do skeletal muscles attach to bones?
Via tendons
What happens to skeletal muscle after injury?
Cells replaced after injury by satellite cells
Satellite cells differentiate to form new muscle fibres
Muscle will never completely recover
What is a sarcomere?
The smallest functional unit of striated muscle tissue.
Discuss the anatomy of a sarcomere
It is bordered by a Z-band on each end with adjacent I-bands, and there is a central M-line with adjacent H-bands and partially overlapping A-bands.
What does the sliding filament theory explain?
The mechanism of muscle contraction is based on muscle proteins that slide past each other to generate movement.
Describe the sliding filament theory.
The myosin (thick filaments) of muscle fibers slide past the actin (thin filaments) during muscle contraction, while the two groups of filaments remain at relatively constant length.
What are the thick and thin filaments of muscle?
Thick - myosin
Thin - actin
What are the stages of the myosin cross-bridge cycle?
- Cross-bridge binds to actin
- Cross-bridge moves
- ATP binds to myosin, causing cross-bridge death
- Hydrolysis of ATP energizes cross-bridges
What does tropomyosin cover?
Partially covers myosin binding site
Where is tropomyosin held?
In blocking positions by troponin
What binds to troponin?
Calcium
How does troponin alter the shape of the myosin binding site?
By pulling tropomyosin away, removing calcium and blocking the site again
What makes up a motor unit?
Motor neurons + muscle fibres
Where can muscles be found within a unit?
Scattered through the muscle
Define tension
Force exerted by muscle
Define load
Force exerted on muscle
Define isometric
Contraction on constant length eg weightlifitng
Define lengthening
Contraction on increasing length (eg sitting down)
Define isotonic (or concentric)
Contraction in shortening length eg running
What is a muscle twitch?
A muscle twitch is an involuntary contraction of the fibers that make up a muscle.
What is muscle tetanus?
Sustained contraction with no relaxation phase
What is muscle fatigue?
Muscle fatigue is defined as a decrease in maximal force or power production in response to contractile activity.
What is the latent period?
Time before excitation contraction starts
What does muscle contraction time depend on?
Calcium (Ca2+)
Discuss the latent period and contraction event in isometric muscles
Shorter latent period
Longer contraction event
What happens as the load increases?
As load increases, contraction velocity and distance shortened decreases
How long is the action potential and how long is the twitch?
AP= 1-2ms loong
Twitch = up to 100ms
Adding these up = summation
Is tenatic tension or twitch tension greater and why?
Tenatic tension is greater that twitch because calcium never gets low enough to allow troponin/tropomysin to re-block myosin binding sites
What does less overlap of filaments lead to?
Less tension
What does too much overlap of filaments lead to?
Filaments interfere with each other
What is the muscle length for greatest isometric tension?
Optimal length (I0)
What does movement around a limb require?
Two antagonistic groups of muscles
One flexes and the other extends
How much force do muscles exert?
Far more force than the load they support
What amplifies muscle shortening velocity and how?
Lever system
By producing increases maneuverability
What is needed for muscle contraction?
Energy from ATP
What energises cross bridges?
Hydrolysis of ATP
How does the hydrolysis of ATP energise cross bridges?
ATP binds to myosin
Dissociates bridges bound to actin
New cycle may begin
Other than the cross bridges, what else does ATP power?
Ca2+ -A ATPase in sarcoplasmic reticulum
How does ATP power the Ca2+ -A ATPase in sarcoplasmic reticulum?
Ca2+ pumped back into SR
Contraction ends
What causes muscle fatigue?
Repeated muscle stimulation
What does muscle fatigue depend on?
Length of contraction
Fibre type
Fitness of individual
When can a muscle contract again after fatigue?
After it relaxes
What does muscle fatigue prevent?
Muscles using up vast amounts of ATP which would cause rigor (ie muscles would not be able to activate new X bridges)
What happens in high intensity, short duration exercise that causes muscle fatigue?
Conduction failure due to increased K+ which leads to depolarisation
Increased lactic acid causing proteins to acidify
Increased ADP and Pi which inhibits X-bridge cycle, delaying myosin detachment from actin filaments
What happens in high low, long duration exercise that causes muscle fatigue?
Decrease in muscle glycogen
Decrease in blood glucose
Dehydration
What are the skeletal muscle fibre types characterised based on?
Fibres are fast or slow shortening
The oxidative or glycolytic ATP forming pathways are used
What happens in fast muscle fibres?
Mysosin has high ATPase activity
What happens in slow muscle fibres?
Mysosin has low ATPase activity
Discuss oxidative fibres
Increased mitochondria causes increase in oxidative phsophorylation
Increased vascularisation to deliver more o2 and nutrients
Contains myoglobin which increases O2 delivery
Fibres are red and have low diameters
Discuss glycolytic fibres
Few mitochondria
Increased glycolytic enzymes and glycogen
Lower blood supply
White fibres with large diameters
What are the three types of muscle fibres?
Slow oxidative (I)
Fast oxidative (IIa)
Fast glycolytic (IIb)
Discuss the relationship between slow oxidative (I) fibres and fatigue
Resists fatigue
Discuss the relationship between fast oxidative (IIb) fibres and fatigue
Intermediate resistance to fatigue
Discuss the relationship between slow glycolytic (IIb) fibres and fatigue
Fatigue quickly
Discuss slow twitch muscle fibres
Contract slowly
Fatigue resistant
Many mitochondria
Aerobic metabolism
Many myoglobin
Steady power/endurance (darker colour)
Discuss fast twitch muscle fibres
Contract quickly
Fatigue sensitive
Few mitochondria
Anaerobic respiration (glycolysis alone)
Sudden bursts of energy (lighter colour)
Give examples of exercises that slow twitch muscle fibres would be involved in
Endurance exercises:
Running
Cycling
Swimming
Give examples of exercises that fast twitch muscle fibres would be involved in
Sudden bursts of energy:
Sprinting
Jumping
Weightlifting
What does increased load require?
Increased need to activate more motor neurons
What does increased number of active motor units lead to?
Activation
Slow oxidative activate first
Fast oxidative second
Fast glycolytic third
What does neural control of muscle tension depend on?
Frequency of action potentials to motor units
Recruitment of motor units
What does destroying the nerve or NMJ lead to?
Denervation atrophy
What happens if a muscle is not used?
Disuse atrophy
What do denervation atrophy and disuse atrophy cause?
Decrease in muscle mass
What can cause hypertrophy?
Exercise
What does aerobic exercise lead to?
Increased mitochondria
Increased vascularisation
Increased fibre diameter
What does anaerobic (strength) exercise lead to?
Increased diameter
Increase in glycolysis
What feature of skeletal muscle do smooth muscles not have?
No striations
What are smooth muscles innervated by?
Autonomic nervous system
Do smooth muscles have cross bridges?
Has cross bridges and uses Ca2+
Where does smooth muscle exist?
In hollow organs
What shape are smooth muscles?
Spindle shaped
Discuss the nucleus of smooth muscle
Mononucleated
Divide through life
Discuss the myosin and actin filaments of smooth muscle?
Thick myosin
Thin actin
How are filaments arranged?
Diagonally across cells and are anchored to membranes and cell structures by dense bodies (like Z lines)
How does the cross bridge in smooth muscle activate?
- Increased calcium
2, Calcium binds calmodulin - Ca2+ - Calmodulin binds to myosin light chain kinase
- Kinases phosphorylates myosin cross bridges with ATP
- Phosphorylated cross bridges bind to actin filaments
- Contraction and tension
How do smooth muscles relax?
Via action of myosin light chain phosphatase which dephosphorylates cross bridges
What does persistent stimulation and increase in calcium in some smooth muscle cause?
Phosphorylated cross bridges may be dephosphorylated when still bound to actin
Decreased rate of ATP splitting
Slows cross bridge cycle
You can maintain tension for long times with low ATP consumption
Useful in blood vessel walls that have to stay open for long periods
What are sources of cytosolic Ca+ ?
Sarcoplasmic reticulum (SR)
- Less SR in smooth muscle that skeletal, no T-tubules and randomly arranged
Extracellular calcium
- Voltage-activated Ca2+ channels (VACC’s)
Calcium removed from cytosol by pumping back into SR and out of cell by Ca2-ATPase (slower process than in skeletal muscle)
How much calcium is released in skeletal muscle?
1 AP releases enough C2+ to saturate all troponin sites
What does it mean to say that smooth muscle has tone?
A basal level of calcium in cells causing a constant level of tension
What factors effect contractile activity?
Dynamic balance of:
Spontaneous electrical activity in muscle memnrane - pacemaker activity
Autonomic neurotransmitters from varicosites
Hormones (eg oxytocin)
Local factors (paracrine agents, pH, O2, osmolarity, ions, NO)
Stretch
What are the types of smooth muscle?
Single or multi unit smooth muscle
Most smooth muscles in organs are a mixture so organ has a mixture of properties in different areas
Discuss single unit smooth muscle
Many cells linked by gap junctions
Signal travels between cells
Contract synchronously
May contain pacemaker cells
Stretch evokes contraction
Discuss multi unit smooth muscle
Few or no gap junctions
Richly innervated by ANS
Don’t respond to stretch
Where can single unit smooth muscle be found?
GIT
Uterus
Small blood vessels
Where can multiunit smooth muscles be found?
Airways
Large arteries
Hairs
What are the stages of the contraction cycle?
- ATP binds to myosin, releasing actin
- Myosin hydrolises ATP.
Energy from ATP rotates the myosin head to the cocked position.
Myosin binds to actin weakly.
- Power stroke begins when tropomyosin moved off the binding site
- Myosin releases ADP at the end of theh power stroke
What initiates excitation-contraction coupling?
Acetylcholine
What are the stages in excitation contraction coupling?
- Initiation of muscle action potential
- Excitation-contraction coupling
- Relaxation phase
What happens in the initiation phase of excitation contraction coupling?
- Somatic motor neuron releases ACh at neuromuscular junction
- Net entry of Na+ through ACh receptor-channel initiates a muscle action potential
What happens in excitation contraction coupling?
- Action potential in t-tubule alters conformation of DHP receptor
- DHP receptor opens RyR Ca2+ releases channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm
- Ca2+ binds to troponin, allowing actin-myosin binding
- Myosin heads execute power stroke
- Actin filaments slides towards center of sarcomere
What happens in relaxation phase of excitation contraction coupling?
- Sarcoplasmic Ca2+-ATPase pumps Ca2+ back into SR
- Decrease in free cytosolic (Ca2+) back into SR
- Tropomyosin re-covers binding site. When myosin heads release, elastic elements pull filaments back to their relaxed position
What initiates skeletal muscle contraction?
Calcium signals
What is the contraction period?
Time during actual muscle contraction
What is the relaxation period?
Time during which Ca 2+ are returned to the sarcoplasmic reticulum by active transport.
What is the refractory period?
The time immediately following a stimulus.
What are actin and myosin?
Two protein molecules in muscles
Function by controlling the voluntary muscular movements
Regulatory proteins - troponin, tropomyosin and meromyosin.
Why is myosin thicker than actin?
It contains myosin heads
What causes muscle contraction?
The brain via an action potential
What does the action potential cause in skeletal muscle?
Calcium influx
Calcium binds to troponin molecules
Causes a change in actin that exposes myosin binding sites
What are the stages of the sliding filament theory?
- Detachment
- Hydrolysis
- Cross bridge
- Power stroke
What is detachment?
ATP binds to myosin head
Myosin detaches from actin and goes into resting state
What happens in hydorlysis?
ATP split into ADP and Pi
What happens in cross bridge?
Myosin binds to actin filament
What happens in power stroke?
Loses ADP + Pi
Myosin head performs power stroke
