muscle contraction Flashcards by Zaynah Hassan

what are the 4 general classes of tissue of the body

epithelia
connective tissue
muscles
nervous system

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skeletal muscles act as a model for …. …..

all muscles

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what are skeletal muscles composed of

fascicles -> muscle fibres -> myofibrils -> myofilaments

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what do myofilaments contains

actin
myosin

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define motor unit

somatic efferent (eg.motor) plus all the muscle fibres it supplies

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what is the minimal functional unit of the motor system

the motor unit

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what is the importance of motoneurones to skeletal muscles

-don’t have the ability to generate inherent electrical activity
-active only when stimulated = motor neurons
-muscle cells = maintained by neurotrophic factors -> released by motor nerves
-motor nerves communicate with musckes by neuromuscular junction:
for electrical activity + continuous supply of neurotrophic factors

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what does the motor unit comprise of

1 a motoneurone
all extrafusal muscle fibres it supplies
extra-ocular muscles - 10 fibres
quadriceps - 100 fibres

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what is the motor system built from

motor neurons - neuronal
skeletal muscles - muscular
neuromuscular junction - interface

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what is the interface

neuromuscular junction

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what is neuronal

motor neurons

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what is muscular

skeletal muscles

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what is the NMJ the interface between

motoneuron + the muscle fibres it supplies

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what is the NMJ

neuromuscular junction

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describe the NMJ

-interface between the motoneuron + muscle fibres it supplies
-specialised 1:! relationship at the centre of neurology
-known as motor end-plate
-30nm gap through which chemical communication between a-motoneurone + skeletal muscle fibre takes place
-acetylcholine released intermittently to bridge this gap only when muscle is recruited by its nerve

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what is the importance of the NMJ

site which final output of the brain is translated into motor output
examination of the nervous system relies on the integrity + normal function of this synapse
disorders of the synapse or it constituents lead to dire outcomes

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without = no life
prob the main synapse within the PNS

what are the major elements of the NMJ

major elements:
presynaptic membrane - terminal of the motoneurone
synaptic cleft
postsynaptic membrane - membrane of the muscle

anatomy of the NMJ

confined to specific are of muscle - ENTRY POINT OR NERUOVASCULAR HILUM
nerve entry point is the:
- geographical centre of any given muscle
- site of entry of a motoneurone into the substance of the muscle
- site of entry of arterial supply to a muscle
- site of exit of venous drainage of the muscle
- site of aggregation of nicotinic receptors of healthy muscles

what is the nerve entry point

geographical centre of any given muscle
site of entry of a motoneurone into the substance of the muscle
site of entry of arterial supply to a muscle
site of exit of venous drainage of the muscle
site of aggregation of nicotinic receptors of healthy muscles

what is the synaptic cleft

gap/interface between pre- & post- synaptic membranes of the NMJ

what is the synaptic cleft normally flooded with

acetylcholine-esterase

what does acetylcholine-esterase do

breaks down Ach
terminates the actions of each action potential

describe the presynaptic membrane of the motor-end plate

derived from terminal processes of the
a-motoneurones
- eg. lower motoneurone
releases:
- acetylcholine to trigger contractions of the muscle
- neurotrophic factors:
- sustain the muscle
- aggregate nicotinic receptors within endplate
- determine the strength of the synapse

describe the pre-synaptic membrane of the motor-end plate

- derived from membranes of skeletal muscle fibres - oval shape - approx size = 3000μm^-2 - usually thrown into folds to increase surface area for communication with pre-synaptic counterparts - expresses approx 10,000 nicotinic receptors/nm

what is the caricature of the actin filament

- actin filament forms a helix - tropomyosin molecules coil around the actin helix, reinforcing it - a troponin complex is attached to each tropomyosin molecule - in the centre of the sarcomere the thick filaments are devoid of myosin heads - myosin heads extend towards the actin filaments in regions of potential overlap

describe the sliding filament theory

1. myosin heads hydrolyse ATP and become reoriented and energised 2. myosin heads bind to actin forming cross bridges 3. myosin heads rotate toward the centre of the sarcomere (power stroke) 4. as myosin heads bind ATP, the cross bridges detach from actin

sliding filament theory - stretched muscle

- I bands + H zone elongated due to reduced overlapping of the myosin and actin filaments - would be reduced muscle strength as few cross bridges can form between the actin + myosin

sliding filament theory - fully contracted muscle

- lots of overlap between the actin and myosin - as thin actin filaments have overlapped there is reduced potential for cross bridges to form again - so there will be low force population from the muscle

what is the sarcomere distinguished by

possessing 2 protein assemblies overlapping each other horizontally

what are the 2 major proteins in assemblies in the sarcomere

actin myosin

what does myosin form

thick filament

what does actin form

thin filament

what are the overlapping assemblies responsible for in the sliding filament theory

for the dark patches of the myofibril

describe the sarcomere

- muscle unit between any 2 successive Z discs - basic unit of striated muscle tissue - = sarcomere - sarcomere is repeated throughout the muscle - distinguished by possessing 2 protein assemblies overlapping each other horizontally - these overlapping assemblies are responsible for the dark patches of the myofibril - 2 major proteins in assemblies are: - actin - myosin - myosin forms the thick filament - RED - actin forms the thin filament - BLUE

what does a muscle fibre posses - descriptive purposes

- A bands - I bands - H zones - Z discs

describe the relationship between NMJ + denervation of mysathenia gravis

- in myasthenia gravis receptor density + numbers falls - denervation leads to reactive migration of extra-synaptic nicotinic receptors into the endplate - then followed by fasciculation - and death of muscle and consequent fibrosis

what is the viability of the NMJ dependent on

electrical activity

motor-end plate + aggregation of nicotinic receptors

- derived from terminal processes of the - a- motoneurones - i.e. lower motoneurone - releases: - acetylcholine to trigger contractions of the muscle - neurotrophic factors that sustain the muscle

describe the distinction of the NMJ

- nicotinic receptors highly conc + restricted to endplate - receptor packing density of 10,000-20,000μm^-2 - receptor density drops abruptly in the rest of the plasm membrane (<100 particles μm^-2) - high conc of nicotinic receptors within endplate confers - functional efficacy of NMJ - high strength of NMJ - in myasthenia gravis, number of receptor molecules becomes insufficient to generate power