Muscle microstructure and contraction Flashcards
(26 cards)
What are the 3 muscle types and their properties?
Smooth muscle is under involuntary control from the autonomic nervous system. Cardiac muscle can contract autonomously, but is under the influence of the autonomic nervous system and circulating chemicals. Skeletal muscles are under voluntary control, usually attached to bones and contract to bring about movement.
What are the different ways muscle fibres can be arranged?
Can be arranged in a parallel formation, fusiform or triangular. Pennate refers to the direction of the fibres: can be unipennate, bipenate or multipennate.
Describe the structure of skeletal muscle
Muscle is joined to bone via a tendon. The muscle contains fascicles which are bundles of muscle fibres. Each myofibre is made up of myofibrils which are thereby made up of myofilaments. The muscle is covered by an epimysium. Muscle fascicles are bound by perimysium while each fibre is surrounded by endomysium.
Describe structure of myofibre
Covered by a plasma membrane known as sarcolemma. Composed of myofibrils. Capillaries run along the surface as well as multiple nuclei. T-tubules tunnel into centre. Contains a cytoplasm called sarcoplasm – myoglobin and mitochondria present. A network of fluid filled tubules known as sarcoplasmic reticulum present.
Describe structure of myofibrils
Are 1-2 micrometres in diameter and extend along the entire length of the myofibre. Are composed of 2 main types of proteins - actin and myosin.
Describe structure of myofilaments
Consist of dark and light band. Light band made of thin filament actin, centre of which known as Z-disc and dark band made of thick filament myosin, centre of which known as H-zone. Light and dark bands give a striated appearance to muscle. These overlap and are arranged in compartments known as sarcomeres. Dense protein Z-discs separare sarcomeres. M-line is midline of H-zone. Isotropic region is the light band while anisotropic region is the dark band.
Describe structure of myosin
Has 2 globular heads and a single tail formed by two α-helices. Tails of several hundred molecules form one filament.
Describe structure of actin
Actin molecules twisted into helix. Each molecule has a myosin binding site. Filaments also contain troponin and tropomyosin. Tropomyosin moves away when there is a calcium influx, revealing myosin binding site.
Why was the sliding filament theory proposed?
During contraction I band became shorter but A-band remained same length and H-zone narrowed or disappeared. Hence thought that actin filaments slide over.
How is muscle contraction initiated?
Action potential opens voltage gated Ca2+ channels in presynaptic terminal. Ca2+ enters pre-synaptic terminal and triggers exocytosis of vesicles. Acetylcholine diffuses across cleft and binds to acetylcholine receptors, inducing action potentials (AP) in muscle.
How is initiation of muscle contraction halted?
Local currents flow from depolarized region and adjacent region. AP spreads along surface of muscle fibre membrane. Acetylcholine broken down by acetylcholinesterase. Muscle fibre response to that molecule of acetylcholine ceases.
How is muscle activated by action potential?
Action potential propagates along surface membrane and into T-tubules. Dihydropyridine (DHP) receptor in T-tubule membrane senses change in voltage & changes shape of the protein linked to ryanodine receptor and opens the ryanodine receptor Ca2+ channel in the sarcoplasmic reticulum (SR). Ca2+ released from SR into space around the filaments. Ca2+ binds to troponin & tropomyosin moves allowing cross-bridges to attach to actin. Ca2+ is actively transported into the SR continuously while action potentials continue.
How do actin and myosin bind?
In the presence of Ca2+, troponin moves from tropomyosin chain. Movement exposes myosin binding site on surface of actin chain. Charged’ myosin heads bind to exposed site on actin filament. This binding & discharge of ADP causes myosin head to pivot, pulling actin filament towards centre of sarcomere. ATP binding releases myosin head from actin chain. ATP hydrolysis provides energy to ‘recharge’ the myosin head.
What is a motor unit?
Name given to a single motor neuron together with all the muscle fibres that it innervates. Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
What are the types of motor units?
Slow fibres (S, I)
Fast, fatigue resistant fibres (FR, IIA)
Fast, fatiguable (FF, IIB)
What are the features of slow twitch fibres?
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
Have high myoglobin content, appear red, high aerobic capacity and low anaerobic capacity.
What are the features of fast twitch, fatigue resistant fibres?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
Have high myoglobin content and appear pink. Moderate aerobic capacity but high anaerobic capacity.
What are the features of fast twitch, fatiguable fibres?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
Low myoglobin content, appear white. Low aerobic capacity but high anaerobic capacity.
How are motor unit types classified?
Motor unit types are classified by the amount of tension generated, speed of contraction and fatiguability of the motor unit. Type I is slow twitch, low force, fatigue resistant. Type IIa is fast twitch, moderate force, fatigue resistant. Type IIb is fast twitch, high force, high fatigue.
How is muscle force regulated?
Two mechanisms by which the brain regulates the force that a single muscle can produce: Recruitment and rate coding.
What is recruitment?
Motor units are not randomly recruited, there is an order. Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).As more force is required, more units are recruited. This allows fine control (e.g. when writing), under which low force levels are required. Muscle force can be regulated by the number of motor units recruited.
What is rate coding?
A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency. As the firing rate increases, the force produced by the unit increases. Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What are neurotrophic factors?
Are a type of growth factor. Prevent neuronal death and promote growth of neurons after injury.
What is the impact of neurotrophic factors?
Motor unit and fibre characteristics are dependent on the nerve which innervates them. If a fast and slow twitch muscle are cross innervated, the slow one becomes fast and vice versa. The motor neuron has some effect on the properties of the muscle fibres it innervates.
