Muscle structure and pathologies Flashcards
Describe the histological appearance of skeletal muscle
- cylindrical cells
- striated
- multinucleate
- cm in length
- limited regeneration (satellite cells)
- voluntary control
Describe the histological appearance of cardiac muscle
- Branched cells with intercalated disks
- striated
- mono/binucleate
- length: 100 μm
- no regeneration
- spontaneous contraction
Describe the histological appearance of smooth muscle
- Spindle shaped
- central nucleus
- length: 20-200μm
- regenerate
- found in vessels, hollow organs, glands
- regulated by ANS and endocrine system
Describe the organisation of skeletal muscle
A muscle is surrounded by epimysium (tough dense connective tissue sheath)
Groups of muscle fascicles form a muscle. Muscle fascicles are surrounded by perimysium. Blood vessels, lymphatics and nerves are contained in the perimysium
Muscle fascicles contain groups of muscle fibres
Muscle fibres are individual muscle cells which have fused their membranes together and created one multinucleated cell. Therefore, muscle fibres are sometimes referred to as “muscle cells”. The cell membrane of a muscle fibre is the sarcolemma. Muscle fibres are surrounded by endomysium (thin loose connective tissue). Capillaries and nerve fibres are found in the endomysium.
Myofibrils make up muscle fibres . Myofibrils are formed from arrangements of myofilaments (actin and myosin) which are the contractile element.
Describe Type 1 muscle fibres
(slow twitch)
- Aerobic respiration
- Slow oxidative respiration
- Lots of mitochondria
- Slow contraction, fatigue resistant
Describe Type 2A muscle fibres
- Aerobic and anaerobic respiration
- Fast oxidative respiration
- Lots of mitochondria
- Intermediate contraction
Describe Type 2B muscle fibres
(Fast twitch)
- Anaerobic respiration
- Few mitochondria
- Fast powerful contraction, rapidly fatiguable
Define motor unit
Motor unit = the motor neurone and the muscle fibres it innervates.
Describe the stages of transmission at the NMJ
1) Action potential arrives and depolarises the pre-synaptic membrane of the motor neurone.
2) This triggers opening of voltage-gated calcium channels. Calcium moves into the pre-synaptic terminal down its electrochemical gradient.
3) Vesicles containing acetylcholine (ACh) move to the pre-synaptic membrane, fuse with the membrane, and release the ACh into the synaptic cleft by exocytosis.
4) ACh diffuses across the synaptic cleft and binds to nicotinic ACh receptors on the post-synaptic membrane (also called the motor end plate) (don’t get nicotinic ACh receptors mixed up with muscarinic Ach receptors which are G-protein coupled receptors on smooth muscle).
5) Nicotinic receptors are ligand-gated ion channels. Binding of Ach induces a conformational change in the receptor which opens the channel, allowing Na+ to enter to the motor end plate, and K+ to leave. The motor end plate depolarises creating an end plate potential
6) The end plate potentiate is usually sufficient to stimulate opening of voltage-gated sodium channels in the adjacent membrane which initiates an action potential which propagates down the muscle fibre.
7) ACh only binds briefly to the nicotinic receptors and dissociates. It is broken down in the synaptic cleft by the enzyme acetylcholinesterase (AChE) into acetate and choline, which is taken up into the pre-synaptic terminal and recycled.
Describe the changes in myasthenia gravis
- Autoantibodies against nicotinic acetylcholine receptors on the post-synaptic membrane
- Commonly affects extraocular muscles, facial muscles, and bulbar muscles
- Muscle weakness and fatigability of voluntary muscles. Can reverse with ice test.
What is the treatment for myasthenia gravis?
acetylcholinesterase inhibitors (e.g. neostigmine)
Describe the effects of botulinum toxin
- Produced by Clostridium botulinum
- Degrades the SNARE protein complex
- Block acetylcholine release from pre-synaptic terminals -> total blockade at the NMJ
- Results in paralysis (flaccid – LMN)
- Botulinum toxin A (botox) has medical uses, dystonias, ocular conditions
What is a sarcomere?
The basic contractile unit / the functional unit of contraction.
The region between two Z-lines.
What is the A band?
located in the centre of the sarcomere. Mainly thick filament, but some overlapping thin filaments. Appears dark under a light microscope.
What is the I band?
x 2, located either side of the A band . Contain thin filaments only (not overlapping) and appear light under a light microscope
