1a muscle microstructure and contraction Flashcards
what are the 3 muscle types?
smooth, cardiac and skeletal muscle
what controls smooth muscle?
the autonomic nervous system, so is under involuntary control.
hence we have no control over this
what controls cardiac muscle?
autonomic nervous stems and circulating chemicals e.g Ca
what controls skeletal muscle?
from somatic nervous system and is under voluntary control
where is smooth muscle found?
A, in muscles around the eyes
B. in the walls of airways
C. in biceps brachii
B
what are skeletal muscles usually attached to?
bones
what do you call this type of skeletal muscle arrangement?
Parallel
what do you call this type of skeletal muscle arrangement?
Fusiform
what do you call this type of skeletal muscle arrangement?
Triangular
what do you call this type of skeletal muscle arrangement?
Unipennate
what do you call this type of skeletal muscle arrangement?
Bipennate
what do you call this type of skeletal muscle arrangement?
Multipennate
describe th components of skeletal muscle from macroscropic to microscopic?
muscle -> consists of fascicles containing myofibres->each myofibre contains multiple myofibrils-> each myofibril contains multiple myofilaments
what do you call a bundle of muscle fibres?
fascicles
what connects the bone to muscle?
tendon
what are individual muscle fibres surrounded by?
endomysium
what are muscle fascicles surrounded by?
perimysium
what are muscle fibres called?
myofibres
what is the name of the plasma membrane that covers the myofibre?
sarcolemma
what is this?
t- tubules
what is this?
sarcoplasmic reticulum
what is the sarcoplasmic reticulum?
network of fluid filled tubules
what are myofibres composed of?
myofibrils
what does this image show?
myofibril
what is this region of a myofibril called?
H zone
what is this region of a myofibril called?
Z discs
Do myofibrils extend for the entire length of myofibres?
yes
what 2 proteins are myofibrils composed of?
myosin and actin
what gives muscle striated appearance?
light and dark bands of muscle
do MYOFILAMENTS extend for the entire length if the myofibres?
No
what are the compartments found within myofilaments called?
sarcomeres
what does the A band represent in these images?
myosin (thick)
do myosin and actin filaments overlap?
yes
what does the I band represent in these images?
actin (thin)
what are light bands of muscle composed of?
actin (thin)
what are the dark bands in the muscle composed of?
myosin (thick)
Describe the structure of myosin
Composed of 2 globular heads and single tail composed from 2 alpha helices
What shape is the actin molecule?
Twisted in a helix
what binding site does actin have?
myosin binding site
What 2 important molecules are also found in actin?
Troponin and tropomyosin
explain the sliding filament theory behind muscle contraction?
essentially, when the myosin heads binds to actin, it pulls the actin inwards.
Hence, the I band becomes shorter
A band remains same length
H zone seems to have narrowed
Explain the initiation of skeletal muscle contraction
- (normal AP thing) , acetylcholine binds to Ach receptors + induces AP in muscle
- AP spreads along surface of muscle fibre membrane
- Ach broken down by Achesterase
- Action potential propagates into T tubules
- Voltage sensitive receptor on T tubule known as dihydropyridine (DHP) causes conformational change in ryanodine receptor, causing it to open
- Ca is released from sarcoplasmic reticulum
- Ca binds to troponin causing conformational change in tropomysin
- This reveals binding site for myosin
- Actin and myosin form cross bridges
What is the name of the voltage sensitive receptor found on T tubule
Dihydropyridine (DHP)
What is the name of the receptor found on the sarcoplasmic reticulum?
Ryanodine receptor
explain the excitation contraction coupling?
- In the presence of Ca2+-> movement of troponin 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 (the ‘power stroke’) ->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 separates sarcomeres?
dense protein Z discs
Describe neuronal control/pathway of muscle contraction
- Upper motor neurones are in brain
- These synapse onto lower motor neurones in brainstem or spinal cord
- We have voluntary neural control from upper and lower motor neurones
What is a motor unit?
A single neurone and all of the muscle fibres it innervates (around 600 fibres per neurone on average)
What are the 3 types of motor unit?
- slow (s, type 1)
- fast, fatigue resistant ( type IIa)
- fast fatiguable (IIB)
What are the features of slow (type I) motor unit?
neurones have small diameters and thin axons
have slowest conduction velocity, low force + fatigue resistant
what are the features of fast, fatigue resistant (type IIa) motor units?
axons have larger diameters
faster conduction velocity, moderate force and fatigue resistant
what are the features of Fast, fatiguable (FF, type IIB motor units?
- larger diameter cell bodies
faster conduction velocity, high force, high fatigue (produce less force as time goes on)
how are muscle fibre types distributed around the muscle?
randomy distributed throughout the muscle
how are motor units classified? (3)
- by the amount of tension generated,
- speed of contraction
- fatiguability of motor unit
By what 2 ways does the brain regulate the force a single muscle can produce?
- recruitment
- rate coding
how does recruitment work?
- There is an order in how motor units are recruited
- smaller units are recruited first
- As more force is required, more units are recruited
what type of control does recruitment allow?
allows fine control e.g writing
Are motor units randomly recuited in recruitment?
No, there is an order
how does rate coding work?
increases the frequency of AP generated when needed.
more APS = more force produced
when does summation occur?
when motor units fire APs too fast to allow the muscle to relax between arriving APs
What are neurotrophic factors?
- Are a type of growth factor that prevent neuronal death
- They promote the growth of neurones after injury
- Even with a good blood supply, if a nerve to a muscle is severed, the muscle wastes away because of the lack of neurotrophic factors provided to it
What are the 3 types of muscle contraction?
- Isometric
- Concentric
- Eccentric
what is isometric muscle contraction?i
muscle produces force but doesn’t change in length
what is concentric muscle contraction? i
muscle shortens to produce movement
what is eccentric muscle contraction? i
muscle produces force but is getting longer e.g. holding something heavier than can be managed
What is the plasticity of motor units/muscle fibres and give examples?
Fibre types can change properties under many different conditions.
e.g if you do exercise you can gain muscle, if astronaut you lose muscle
Do myofibrils extend for the entire length of myofibres?
yes
what is this?
M line
Are motor units randomly recuited in recruitment?
No, there is an order
