Skeletal Muscle Flashcards
Describe skeletal muscle fibres
long, cylindrical multinucleated cells ranging in length from a few mm to 10cm or more.
Describe the layers of skeletal muscle organisation
Sarcomere > Myofibrils > Muscle Fibres > Endomysium > Fascicle > Perimysium > Muscle > Epimysium
Where does muscle connective tissue originate?
The connective tissue matrix of the muscle is secreted by fibroblasts that lie inbetween muscle fibres. It contains collagen and elastic fibres that merge with the connective tissue of the tendons
What’s the diameter of a myofibril?
About 1 micrometer
What’s the rough length of a sarcomere?
The fundamental contractile unit is about 2 micrometers long
What is the M line?
At the very centre of the sarcomere is the M line where links are formed between adjacent myosin filaments.
What is the H zone?
A pale region at the centre of the sarcomere where myosin filaments don’t overlap with actin (when relaxed)
What is the A band?
The anisotropic band, which refracts polarised light, includes the extent of the myosin filaments, both in the H zone and where they overlap with actin filaments. This band doesn’t change length during contraction.
What is the I band?
The isotropic band doesn’t refract polarised light. It is the the region straddling every Z line, and on either end of each sarcomere, consisting only of actin filaments. Changes length during contraction.
What is the Z line?
At either side of the sarcomere, filamentous actin is stabilised by binding to the actinin in the Z line, which aligns with adjacent Z lines by binding with desmin.
Describe the sarcolemma
The cell membrane of the muscle fibre. Folded at resting length with indentations called caveolae to allow stretch.
What are T-tubules?
Transverse tubules run from the sarcolemma across the junction of the A and I bands. These meet the sarcoplasmic reticulum at terminal cisternae.
What is a triad?
A triad describes the complex in which each T-tubule comes into contact with the cisternae of two regions of SR.
What is a motor unit?
Collectively describes one motor neuron and all the muscle fibres it innervates.
What is the Neuromuscular Junction?
The NMJ refers to the axon terminal, synaptic cleft, and motor endplate of the muscle fiber, where a motor neuron and muscle fibre meet at a synapse.
Describe how motor neurons depolarise the sarcolemma
An action potential invades and depolarises the axon terminal of the motor neurone, voltage-gated Ca2+ ion channels in the terminal membrane open and ions flow in, triggering the fusion of docked synaptic vesicles, releasing ACh into the cleft, these bind to nicotinic receptors on the post-synaptic membrane, cation channels open, depolarising the endplate region of the membrane - epp, when this exceeds threshold, the sarcolemma generates an action potential which propogates along the whole fibre.
What is a miniature endplate potential?
mepps are small, spontaneous depolarisations of the end-plate region of the sarcolemma. It’s thought that these correspond to the release of a single vesicle of ACh. Frequency increases when the nerve terminal is artificially depolarised.
Why does ACh only act for a few ms?
Acetylcholinesterase in the synaptic cleft breaks it down into actetate and choline which can be taken up by the nerve terminal and recycled.
What does eserine do?
It inhibits the action of Acetylcholinesterase, causing continuous depolarisation of the sarcolemma and blocking neuromuscular transmission
What is curare?
An arrow poison used for hunting by native South Americans, acts as a neuromuscular blocker that blocks nicotinic receptors
Why are nicotinic receptors suited for the NMJ?
The ACh binding site is part of the same molecular complex as the Cation channel, so binding directly opens the channel with no intervening step - quicker.
What is denervation supersensitivity?
When the motor nerve is cut, the terminal degenerates and the endplate loses sensitivity to Ach, while the muscle weakens and atrophies. Following this, the non-junctional sarcolemma becomes sensitive to Ach - trophic action of the nerve.
Describe myasthenia gravis
Patients make antibodies that bind their own nicotinic receptors, causing functional denervation of muscle. This causes (especially cranial) muscle weakness as no. active receptors declines and NM transmission progressively fails.
How can myasthenia gravis be treated?
Inhibitors of acetylcholinesterase such as neostigmine prolong activity of Ach
