Muscle structure and function Flashcards
What is a muscle?
A composition of cell types that provide the contractual force required to move body parts or maintain their position against an opposing force
What is skeletal muscle?
Large striated fibres that appear nucleated under a microscope
What is cardiac muscle?
Has striated fibres but are smaller than skeletal muscle and are branched and uninucleate
Cells are joined in series by junctions called intercalated disks
What is functional syncytium?
Each cardiomyocyte contracts in coordination with its neighbouring cells
What is smooth muscle?
Small unstriated fibres
Describe the composition of skeletal muscle
A skeletal muscle is made up of bundles of muscle fibres
Each muscle fibre is a multinucleate cell containing numerous microfibrils which are highly ordered assemblages of thick myosin and thin actin
What is a sarcomere?
A unit of contraction
Describe the structure of a sarcomere
Each thin filament is anchored by the Z disk (protein capz) and capped at the end by tropomodulin
Thick filaments are held in position by the elastic molecule Titin
Describe the sliding filament theory
The myosin heads pull the actin towards the centre of the myosin filament
During the cross-bridge cycle, the myosin heads interact with actin but this is prevented as tropomyosin covers the myosin-binding sites
The position of tropomyosin is controlled by troponin
When Ca2+ binds to troponin the position of the tropomyosin changes thus exposing the myosin-binding sites
Where is the Ca2+ store in skeletal muscle?
In the sarcoplasmic reticulum
During contraction does the A band change?
No
During contraction does the I band change?
Yes becomes narrower
What is the sarcoplasmic reticulum?
A tubular network within the muscle fibre
Describe excitation-contraction coupling
A vertebrate muscle fibre is innervated by one motorneuron
Muscle cells can generate action potentials
The action potentials travel along T-tubules and depolarisation leads to a structural change in the DHP receptor which is linked to the RyR receptor
The RyR receptor opens and releases Ca2+ from the sarcoplasmic reticulum
What is recruitment?
The more motor neuron units are active, the stronger the tension of the muscle
What is 1 motor neuron unit?
1 unit=1 motor neuron+fibres innervated by the motor neuron
How is strength controlled in the muscles?
By variations in the number of active motor units
What is tetanus?
At high frequencies, the muscles are maximally contracted
What are the 2 types of muscle fibres?
Fast and slow-twitch
What is the difference between fast and slow twitch genes?
They express different myosin genes with different rates of ATPase activity
Describe slow-twitch fibres
Alos called oxidative or red muscle
Contain large amounts of myoglobin, mitochondria and are well supplied with blood vessels
Describe fast-twitch fibers
Also called glycolytic or white muscle Few mitochondria Little to no myoglobin Few blood vessels Major energy source is glycolysis Can develop max. tension faster and fatigues rapidly
What is isometric contraction?
A muscle can build up tension without changing its length
What is isotonic contraction?
Where the muscle shortens
How are heart muscle cells controlled?
Not controlled by motor neurons
Contractions originate within the muscle and are myogenic
How do heart muscle cells have auto-rhythmicity?
A small percentage of muscle cells are specialised to generate and conduct action potentials and are located within the SA node
How are the heart muscle cells connected?
Electrically connected by gap junctions within intercalated disks
What is the function of smooth muscle?
Causes slow contractions of many internal organs
Why is smooth muscle called so?
The actin and myosin are not as regularly arranged as in skeletal or cardiac muscle
Describe the Ca2+ mediated change in smooth muscle
The change is on myosin instead of the actin-tropomyosin filament
Describe depolarisation in smooth muscle
When as smooth muscle cell is depolarised by a neurotransmitter, Ca2+ enters the cytoplasm and binds to calmodulin which then activated an enzyme that phosphorylates the myosin heads causing them to bind to actin
How is smooth muscle organised in muscles?
In sheets with individual cells in electrical contact with one another through gap junctions thus the contractions are coordinated
Describe peristasis
In the digestive tract, a coordinated spreading wave of smooth muscle contraction pushes the contents through the central lumen
