Muscular contraction Flashcards
What are the three muscle types?
Smooth muscle
Cardiac muscle
Skeletal muscle
What type of control is smooth muscle under control by?
Involuntary control from the autonomic nervous system
Describe the nucleation of smooth muscle
Mononucleated
Describe the nucleation of cardiac muscle
Mononucleated
How are cardiac cells connected?
Physically and electrically connected, ensuring that contraction signals are transmitted through cells, and the entire heart contracts as a singe unit
What type of nervous control, controls cardiac muscles?
Autonomous (myogenic)
and under influence of autonomic nervous system (cardiac control centre)
What is the nucleation of skeletal muscles?
Multinucleate
Which type of control are skeletal muscles subjected to?
Voluntary control by the somatic nervous system
How are skeletal muscles attached to bone?
Attached via tendons, consequently facilitating movement of bone through contraction
What connective tissue sheath wraps muscle?
Epimysium
What is the purpose of the epimysium?
Enables muscle to contract and more powerfully while maintain structural integrity.
How are muscle fibres arranged?
Arranged in bundles, known as fascicles
Which connective tissue layer surrounds the fascicles?
Perimysium
How does the fascicular arrangement support muscular contraction?
Triggers specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle
What is contained within each fascicle?
Myofibres, encased in a thin layer of collagen and reticular fibres, the endomysium
Which layer surrounds the outer layer of each myofibre?
Endomysium
What does the endomysium contain?
Surrounds the extracellular matrix of cells, plays a role in transferring force produced by muscle fibres to tendons
What is the term used to describe the plasma membrane of myofibres?
Sarcolemma
What is the term used to define the cytoplasm of myofibres?
Sarcoplasm
Which proteins are spent in myofibrils?
Myosin and actin
What is the smallest functional unit of a skeletal muscle fibre?
Sarcomere
What are t tubules?
Extensions of the sarcolemma that penetrate into the centre of skeletal muscle cells, a conduit of action potentials
What is the role perfumed by the sarcoplasm?
Cytoplasm of myocyte consisting of greater proportion of glycogen granules, myoglobin and mitochondria
What is the sarcoplasmic reticulum?
Network of fluid-filled tubules, constitutes the main intracellular calcium store in striated muscle, cardinal role in the regulation of excitation-contraction coupling.
What is the line that defines the region of a micofibril contained between two cytoskeletal structures?
z lines
Why is there a strained appearance of skeletal muscle fibres?
This is due to the arrangements of the thick and thin myofilaments within each sarcomere.
What is the A band?
Composed of thick filaments, containing myosin, span the centre of the sarcomere, extending towards the Z-discs
Which protein comprises the thick filaments?
Myosin
What is the M-line?
Thick filaments are anchored at the middle of the sarcomere by myomesin
What is the I band?
Lighter regions contain thin actin filaments anchored at the z-discs by alpha-actinin. The thin filaments extend into the A band towards the M-line overlapping the regions of thick filaments.
Why is the A band darker?
Due to the thicker composition of myosin filaments, in addition to overlapping actin filaments
What is the H-zone?
Middle of the A band, thin filaments do not extend into this region, there is no actin present.
What does a single sarcomere contain?
Single dark A band, with half of the I band on either side
What effects does contraction have on the distance between z-discs?
Distance shortens
What effect does contraction have on the length of myofilaments?
The length of myofilametns are not altered, however slide across each other
How is the length of the A band affected during contraction?
Does not change (thick myosin filaments remain constant length)
Which zones and bands decreases during contraction?
H-zone
I band
Why does the H-zone and I band decrease?
These regions represent areas where filaments do not overlap, however during contraction area of filament overlap increases
Which protein comprises the thin filaments?
Actin
Which site does myosin bind upon on each globular actin monomer?
Myosin binding site
Which two regulatory proteins are associated with the myosin binding site?
Troponin
Tropomyosin
Which troponin binds to actin?
Tn1
Which troponin binds to tropomyosin?
TnT
Which troponin binds to calcium ions?
TnC
Describe the structure the thick myofilaments:
Myosin protein complexes: 6 polypeptides: Two myosin heavy chains, four light chain molecules.
Heavy chains consist of a tail region, flexible hinge region, and globular head which contains an actin binding site, and a binding site for ATP
Which chain interacts with actin?
Globular heavy chain head (comprises of actin binding site)
How are myosin protein filaments arranged in respect to M-lines and Z-discs? (Direction)
Tails towards M-line, head extending towards the Z-discs
What is the function of Titin?
Align thick filaments, adds elastic element to sarcomere, titan anchored at M-line
Briefly describe the sliding filament theory:
Arrangement and interactions between thin and thick filaments enable for shortening of the sarcomeres, subsequently generating force.
In which direction are actin filaments pulled during sarcomere contraction?
Pulled towards the M-lines, sliding passed the thick myosin filaments within the sarcomere
Which cation influence is responsible for sarcomere contraction?
Calcium ions
What does tropomyosin do?
Tropomyosin is wound around actin filaments, and encapsulates the myosin-binding sites, preventing actin binding to myosin.
Where are calcium ions released upon stimulation in muscle cells?
Sarcoplasmic reticulum
At a presynaptic membrane, action potentials cause which voltage gated ion channels to open?
Calcium ion voltage gated ion channels
The influx of calcium into the pre-synaptic knob results in what occurring?
Vesicles fusing with the presynaptic membrane, and releasing acetylcholine into the synaptic cleft by exocytosis
What is the fate of acetylcholine during a synapse within the neuromuscular junction?
ACh diffuses across synaptic cleft, binding to ACh receptors on the post-synaptic membrane within the motor end plate of the sarcolemma.
Inducing an action potentials
Voltage gated sodium channels open to depolarise membrane
Acetylcholinesterase regulate ACh activity
What type of coupling is induces by action potentials?
Excitation-contraction coupling
Which enzyme hydrolyses acetylcholine in the synaptic cleft?
Acetylcholinesterase
Which structure propagates action potentials along the sarcolemma into the muscle cell?
T-tubules
Which voltage-sensitive receptors are present on the sarcolemma respond to action potential propagation via t tubules?
Dihydropyridine receptors (DHPR)
Which receptors are mechanically linked with DHPR?
Ryanodine receptors
What are ryanodine receptors?
Calcium channel in sarcoplasmic reticulum membrane
What effect is induced by action potential stimulation of DHPR?
DHPR undergoes conformational change by detecting potential difference, triggers opening of RyR, enabling calcium ion efflux from SR into the sarcoplasm.
Which channels do calcium ions efflux from the SR into the sarcoplasm?
Ryanodine receptors
Upon calcium binding to TnC, what effect is induced?
Causes tropomyosin to exposure the myosin binding sites of actin filament
How do the myosin globular heads bind to the myosin binding site?
Actin binding site of myosin globular head forms cross bridge with myosin binding site
How are calcium ions removed from the sarcoplasm?
Actively transported into the sarcoplasmic reticulum continuously whilst action potential continue
ATP driven pump, uptake > release rate
When does muscle contact stop?
Signal termination from the motor neurone, depolarisation of sarcolemma and t-tubules
Ryanodine receptors close, and tropomyosin occupies the myosin binding sites on the actin filaments
What is the first stage of excitation-contraction coupling? (Calcium)
In the presence of calcium, movement of troponin from tropomyosin chain, movement exposed the myosin binding site on the surface of actin chain.
How does the myosin head bind to the myosin binding sites on actin filaments?
Charged myosin head is attracted to actin, binding actin at its respective actin-binding site forming the cross bridge.
Cross bridge formation occurs when myosin head attaches to actin, ADP and Pi bound to myosin.
What happens to ADP and Pi upon myosin head binding to actin?
Released, resulting in myosin to form a stronger attachment to the actin, myosin head retracts towards the M-line, consequently pulling the actin filament inwards tinpot the A band
Movement is defined as power stroke.
What is the power stroke?
Movement of thin actin filament
What molecule is required for myosin head detachment?
ATP
Where does ATP bind to the myosin head?
ATP binding site on myosin globular head
How is ATP hydrolysed upon binding to the ATP-binding site on myosin globular head?
Intrinsic ATPase activity on myosin into ADP and Pi
Why is ATP hydrolysed by intrinsic ATPase enzymes on the myosin globular head?
The energy released from ATP hydrolysis changes the angle of the myosin head into a cocked position, position for further movement.
What is the cocked position?
Myosin is in a high energy configuration.
Energy is expended during the power stroke, end of power stroke, myosin head is in a low energy stage. ADP is released during the power stroke, however the cross bridge is intact, actin and myosin are bound
ATP readily attaches to myosin globular head, detachment enables cross bridge cycle to recur.
Where are the upper motor neurones located?
The primary motor cortex, provides the descending corticospinal tract, decussating at the medullary pyramids within the medulla oblongata
What is a motor unit?
Term used to describe a lower motor neurone, and all the muscle fibres it innervates
What is an innervation ratio?
Defines the number of muscle fibres innervated by a single motor neurone,
What is the relationship between innervation ratio and level of control?
The innervation ratio is inversely proportional to the level of control.
What expected innervation ratios enable fine control and delicate movements?
Innervation ratios will tend to be small, enabling nuances of movement of the entire movement.
What effect does a high innervation ratio have on muscular movements?
Coarse movements
No necessity for individual muscle fibres to undergo highly coordinated, differentiated contractions to produce a fine movement.
Which type of motor fibres are fast, fatigue resistant?
FR type IIa
Which muscle fibre type is oxidative?
Type IIa
Why are type IIa fibres fast?
ATP hydrolysis is faster, resulting in a faster cross-bridge cycling
Actin filaments are pulled to the M-line of the sarcomere at a faster rate
Oxidative fibres are more resistant to fatigue considering that ATP is primarily produced through acerbic pathways
What is the structure of type IIA fibres?
Large diameter cell bodies, large dendritic tress, thicker axons and faster conduction velocity
Which type of fast fibres are glycolytic?
Type IIb
Which type of muscle fibres are fast and fatiguable?
Type IIb
Why are TypeIIb fibres fast and fatiguable?
Primarily generate ATP through anaerobic glycolysis, produce less ATP per cycle, thus fatigue at a quicker rate, permitting them to be used for short periods
Rapid forceful contractions associated with quick powerful movements
What is the structure of FF typeIIb fibres?
Large diameter, and volume of glycogen
Do not posses substantial numbers of mitochondria, limited capillary supply, insignificant amounts of myoglobin.
Which type of fibre is type 1?
Slow oxidative
Why are slow type 1 fibres oxidative?
Maximises ability to generate ATP through aerobic metabolism
High presence of mitochondria increases sites of oxidative phosphorylation to synthesis ATP
Enabling slow oxidation fibres to contract for longer periods due to ATP supply
How are type 1 fibres supplied in accordance to their respective ability?
Extensibly supplied with blood capillaries to continuously supply oxygen.
High presence of myoglobin, oxygen binding molecules (high affinity); stores oxygen within fibre, results in dark red pallor
What colour are slow type 1 fibres?
Red
Why are slow type 1 fibres red?
High proportion of myoglobin content
Describe the aerobic capacity of slow type 1 fibres?
Aerobic capacity is high
What functions are performed by slow type 1 fibres?
Posture, isometric contractions and stabilising bones & joints.
Do not produce high tension, thus not used for powerful fast movements that require high amounts of energy and rapid cross bridge cycling
Describe the structure of slow type 1 fibres:
Small soma diameter, small dendritic tress, thin axons, slowest conduction velocity.
Describe the proportion & distribution of slow and fast twitch muscles:
Random distribution and proportions
What colour are fast fatigue resistant type IIa fibres?
Pink
What colour are FF IIb fibres?
White
Describe the aerobic capacity of FF Type IIB fibres:
Low
Describe the anaerobic capacity of FF TypeIIA/B fibres:
High
Which fibres produce high tension?
Type IIb
Which fibre produce low tension?
Type I
What are the two methods of regulating muscle contraction and activation of motor units?
Recruitment and rate coding
What is recruitment of motor units?
Refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle.
Activation of motor neurone will subsequently result in the respective muscle fibres being stimulated.
Multiple activated motor neurones will result in further muscle fibre activation, thus stronger muscle contraction.
Higher recruitment = stronger contractile force
In which order premotor units recruited?
In order of smallest to largest (Slow to fast twitch), as contraction increased, governed by size principle
Under low levels of required force, how is fine control adopted?
Activation of low innervation ratio motor units.
What is rate coding?
Force produced by a single motor unit is determined by the number of muscle fibres it innervates
Determinant of force is the frequency in which the muscle fibres are stimulated by their inneverating axon.
Motor units fire range of frequencies = slow units fire at a lower frequency
How does motor unit firing rate influence force produced?
Increasing firing rate increases the force produced by the unit
Summation occurs when units fire at frequency too fast, allows mucle to relax between arriving action potentials.
What are neurotrophic factors?
Peptides that support the growth, survival and differentiation of developing and mature neurones
Prevents neuronal death and promotes growth post injury ]
Motor unit and fibre characteristics depends on the nerve that innervates them
What happens to slow muscles when cross innervated with fast twitch nerves?
Exhibits characteristics functions and properties of that to fast muscle
Muscle emulates the properties of the innervating nerve
What are the two main types muscular contractions?
Isometric and isotonic
What is an isometric contraction?
Tension in the muscle is constant
Tension is produced, however muscle length does not change (Sarcomere shortening)
increasing muscle tensioN Though the force produced cannot overcome resistance provided by load
What is a concentric contraction?
Involves the muscle shortening to move a load
What is an eccentric contraction?
Muscle tension diminishes, and the muscle lengthens.
What is muscle plasticity?
Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it.
During conditions of severe conditions, muscle fibres are converted into which type?
Type 1 fibres to II
What effect does microgravity have on muscle fibre types?
Promotes shift from slow to fast muscle fibre types
Which system senses gravity?
Vestibular system
How does microgravity cause deconditioning?
Results in diminished land on the msk system and hydrostatic pressure difference, contributes towards muscular atrophy
How does ageing influence the types of fibres present?
Associated with loss of type 1 and 2 fibres
Preferential loss of type II fibres
Results in a larger proportion of type 1 fibres in aged muscle
(Evidence = slower contraction time)
How does muscle training influence muscle fibre types?
Conversion from IIb to IIa
