Muscle microstructure and contraction Flashcards
What are the 3 main types of muscle?
briefly describe there nervous control?
Smooth-under INVOLUNTARY CONTROL from the AUTONOMIC NERVOUS SYSTEM
Cardiac- heart, controlled by AUTONOMIC NERVOUS SYSTEM and circulating chemicals
Skeletal-VOLUNTARY control from the SOMATIC nervous system, usually attached to bones
Where is smooth muscle found?
In the walls of airways
What is the difference between a lower and upper motor neuron?
Lower-goes to the muscles
Upper-comes from the brain
What are the different ways that muscle fibres can be arranged and what does this impact?
- Parallel
- Fusiform
- Triangular
- Multipennate
- Bipennate
- Unipennate
- Pennate the muscle fibres an be attached in different orientations with respect to other things they attach to e.g. ligaments and this impacts their function
What is the structure of skeletal muscles from macro to micro?
bone->tendon->muscle->FASICLES(bundles of muscle fibres)->MYOFIBRE(muscle fibre)->MYOFIBRIL->MYOFILAMENTS(myosin and actin)
What connective tissue is in skeletal muscle?
EPIMYSIUM-outer tissue surrounding muscle(group of fasicles)
PERIMYSIUM- surrounds individual muscle fascicles
ENDOMYSIUM-surrounds a single muscle fibre(myofibre)
What are muscle fibres formed from?
Lots of different MYOBLAST cells
What is a Sarcolemma?
PLASMA MEMBRANE that covers a MYOFIBRE(muscle fibre)
What are the 4 structures in a myofibre?
T-TUBULES-channels that tunnel into the centre of the muscle fibre to allow contraction of inner muscle
SARCOPLASMIC RETICULUM- network of fluid filled tubules
SARCOPLASM- cytoplasm containing MYOGLOBIN and MITOCHONDRIA
MYOFIBRILS-make it up
What is the structure of myofibrils?
SARCOMERE- repeated muscle units
Each muscle unit consists of:
H-ZONE-only myosin
Z-DISC-marks the end of a sarcomere
I-BAND-only actin
A-BAND-overlap of actin and myosin
M-LINE-Middle of sarcomere
Light and dark bands give the muscle STRIATED appearance
Dark bands=Thick bands = MYOSIN
Light bands =Thin bands=ACTIN
What is the structure of Myosin?
- TWO globular heads
- Single tail formed from 2 alpha helices
- tails of several hundred molecules form one filament
What is the structure of actin?
- molecule twisted into a HELIX
- each molecule has a MYOSIN binding site
- on the filament there are TROPOMYOSIN stands which have myosin binding sites on them but when the muscle is not contracting these binding sites are blocked by TROPONIN COMPLEXES
What is the sliding filament theory?
Theory that the myosin and actin filaments slide over eachother to cause muscle contraction
Describe what happens to zones when contraction occurs?
I-band-becomes shorter
A-band-SAME LENGTH
H-Zone- becomes narrowed or disappeared Z-bands get closer together
What initiates muscle contraction?
1-Action potential propagates along the surface membrane of the muscles fibre and into the T-TUBULES
2-DIHYDROPYRIDINE RECEPTOR in t-tubule membrane senses change in voltage and changes shape of the protein linked to the RYANODINE RECEPTOR
3-This causes the ryanodine receptor (calcium channel) in the SARCOPLASMIC RETICULUM to open
4-Ca2+ is released fro the SR into the space around the filaments
5-Calcium binds to TROPONIN and TROPOMYOSIN moves allowing CROSSBRIDGE formation between myosin and actin filaments
6-Calcium is actively transported (via an ATP driven pump) while action potentials continue
Describe excitation contraction coupling?
1-in the presence of Ca2+ troponin moves off of tropomyosin chain
2-this exposes the MYOSIN BINDING SITE on the surface of the actin chain
3- ‘charged’ myosin head binds
4-this binding and discharge of ADP causes myosin head to pivot ‘POWER STROKE’ =pulling actin filament towards the centre of sarcomere
5- New ATP binds causing myosin head to be released from the actin chain site
6- ATP hydrolysis provides energy for the charging of the myosin head
7-process repeats itself
which protein filament does the pulling during muscle contraction?
Myosin
Describe the neural control of muscle contraction?
Upper motor neuron found in the primary motor cortex(in front of the central sulcus) goes to spinal cord or brainstem
Lower motor neuron in brainstem or spinal cord goes to the muscle
What is a motor unit?
Name given to a SINGLE MOTOR NEURON together with all the MUSCLE FIBRES
How many motor neurons and skeletal muscle fibres do we have?
420000 motor neurons 250 million skeletal muscle fibres
What is meant by innervation ratio?
term to describe how many muscle fibres are innervated by a nerve in a motor unit Muscles that we have better control over generally have fewer muscle fibres innervated by a motor neuron=lower innervation ratio
What are the different types of neuron motor units and describe?
Type 1-slow contraction, smallest diameter cells bodies, small dendritic trees, thinnest axons, slowest conduction velocity
Type IIA-Fast contraction and fatigue resistant, large diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity
Type IIB-Fast contraction but fatiguable, large diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity
What are the characteristics of the muscle fibres in different motaor unit?(myoglobin, anaerbic/aerobic capacity, colour)
TYPE I: -myglobin content-high -colour-red -Aerobic capacity-high -Anaerobic capacity-Low
TYPE IIA: -myglobin content-High -colour-Pink -Aerobic capacity-Moderate -Anaerobic capacity-High
TYPE IIB: -myglobin content-Low -colour-White -Aerobic capacity-Low -Anaerobic capacity-High
which muscle has the greatest proportion of slow type muscle fibres?
Back muscles- as they have to be on all the time to keep posture and are non fatiguable
What helps classify different motor units?
What are the properties of different motor unit?
Classified based on: tension generated + Contraction + Fatiguability
Type I:
- Slow twitch
- Low force
- Fatigue resistant
Type IIa:
- Fast twitch
- Moderate force
- Fatigue resistant
Type IIb:
- Fast twitch
- High force
- High fatigue
What 2 mechanisms regulate muscle force?
2 Mechanisms by which the brain regulates force:
1.RECRUITMENT: -how many muscle fibres do i need turned on at any one time to get a certain force -Motor units are recruited in an order -Smaller units are recruited first ‘SIZE PRINCIPLE’ (these are generally the slow twitch units) -As more force is required, more units are recruited. This allows FINE CONTROL under which low force levels are required
2.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
In what order are motor units recruited?
1-Slow 2-Fast, fatigue resistant 3-Fast, fatiguable
The muscle fibres that are first recruited stay on as more are recruited and they are the last off
What happens as muscle force increase?
- Slow motor units start increasing the rate of firing
- more muscle fibres and units are being recruited
What happens if you put a nerve from a fast muscle into a slow muscle and vice versa?
The slow muscle starts to become fast and vice versa which shows that the motor unit and fibre characteristic are dependent on the NERVE that innervates them as muscle properties such as myoglobin, mitochondria etc are changable.
What are the 3 types of muscle contraction?
Concentric-muscle gets shorter(isotonic)
Eccentric-muscle get longer (isotonic)
Isometric-no movement but force generated
What is the consensus currently on the plasticity of muscle fibres?
Change from type IIB to IIA is most common following training
You cant change from slow to fast- only in severe deconditioning or spinal cord injury. Or the microgravity during spaceflight results in shift from slow to fast muscle fibre types(In microgravity you don’t need postural muscles)
Why do older people have slower contraction times?
Because aging is assosiated with a loss of all type of muscle fibres but especially fast ones(type II)
