Muscle Microstructure And Contraction

Question 1

What are the types of muscle?

Answer 1

Smooth - involuntary control from autonomic nervous system

Cardiac - can contract autonomously but is under the influence of the autonomic nervous system and circulating chemicals

Skeletal - under voluntary control. Usually attached to bones and contract to bring about movement

Question 2

What are some types of arrangement of muscle fibres?

Answer 2

Parallel

Fusiform

Triangular

Pennate: unipennate, bipennate, multipennate

Question 3

What is the structure of skeletal muscles?

Answer 3

Muscle (surrounded by epimysium)->

Fascicles (bundle of muscle fibres) (surrounded by perimysium) ->

Myofibre (surrounded by endomysium) (multi uncleared ass they are made from fused muscle cell) ->

Myofibril ->

Myofillament

Question 4

What is the structure of myofibres?

Answer 4

Covered by a plasma membrane - sarcolemma

T tubules tunnel into centre

Cytoplasm called sarcoplasm - myoglobin and mitochondria present

Network of fluid filled tubules called sarcoplasmic reticulum

Composed of many myofibrils

Question 5

What is the structure of myofibrils?

Answer 5

1-2 mu m in diameter

Extend along the entire length of myofibres

Composed of two main types of protein - actin (thin) and myosin (thick)

The light and dark bands gove muscle striated appearance - dark band is myosin, light is actin

These don’t extend along the length of myofibres

Overlap and are areanged in compartments called sarcomeres

Question 6

What is the structure of the myofillaments (with relation to bands)?

Answer 6

Sarcomeres are separated by dense protein bands - Z discs

Dark bands (myosin) - A band

Light bands (actin) - I band

Actin and myosin fillaments overlap

The M line is the centre line down the dark band of myosin

The H zone is the portion of the A band that does not overlap with the I band

Question 7

What is the structure of myosin?

Answer 7

Golf club

Two globular heads

Single tail formed by two alpha helices

Tails of several hundred molecules form one filament (all in same direction)

Question 8

What is the structure of actin?

Answer 8

Actin molecules twisted into a helix

Each molecule has a myosin binding site

Fillaments also contain troponin and myosin (these move and uncover binding sites)

Question 9

What happens (basically) in sliding filament theory?

Answer 9

Z discs get closer together

I Bands get shorter

A bands stay the same length

H zone disappeared or narrowed

Question 10

How does initiation of muscle contraction occur?

Answer 10

Occurs at neuromuscular junctions

1. Action potential opens voltage Ca2+ channels
2. Calcium enters presynaptic terminal
3. Calcium triggers exocytosis of vesicles
4. Acetylcholine diffuses across cleft
5. Binds to acetylcholine receptors and induces action potentials in muscles
6. Local currents flow from depolarised region and adjacent region. So action potential spreads along surface of muscle fibre membrane
7. Acetylcholine is broken down by acetylcholine esterase. Muscle fibres response to that molecule of ACH ceases

Question 11

How does activation of muscle contraction occur?

Answer 11

1. Action potential propagates along surface membrane and into T tubules
2. Dihydropyridine (DHP) receptor in T tubule membrane senses the change in voltage and changes shape of the protein linked to Ryan prime receptor. This opens the ryanodine receptor calcium channel in the sarcoplasmic reticulum.

This causes Ca2+ to be released from sarcoplasmic reticulum into the space around the fillaments

3. Ca2+ binds to troponin, so tropomyosin moves revealing the binding site on actin allowing…
4. Cross bridges to attach to actin
5. Calcium is actively transported to the sarcoplasmic reticulum continuously while action potentials contine. ATP driven pump

Question 12

What happens during excitation contraction coupling (interaction between actin and myosin)?

Answer 12

1. In the presence of calcium - movement of troponin from tropomyosin chain
2. Movement exposes myosin binding site on surface of actin chain
3. ‘Charged’ myosin heads bind to exposed site on actin filament
4. This binding and discharge of ADP causes myosin head to pivot (the power stroke) -> pulling actin filament towards centre of sarcomere
5. ATP binding -> releases myosin head from actin chain
6. ATP hydrolysis-> provides energy to ‘recharge’ the myosin head

Question 13

What is the neural control of muscle contraction?

Answer 13

Voluntary neural control from upper and lower motor neurones

Upper motor neurones in brain - synapses with Lower

Lower motor neurone in brainstem or spinal cord - this synapses with a muscle (NMJ)

Question 14

What is a motor unit?

Answer 14

The name given to a single motor neurone and all the muscle fibres that it innervates

On average each motor unit supplies about 600 muscle fibres

Question 15

What are the types of motor unit?

Answer 15

Slow (type I):

Smallest diameter cell bodies

Small dendritic trees

Thinnest axons

Slowest conduction velocity

Fast, fatigue resistant (FR, IIa):

Large diameter cell bodies

Large dendritic trees

Thicker axons

Faster conduction velocities

Fast, fatiguable (FF, IIb):

Same as above

Question 16

How are the muscle fibres types distributed?

Answer 16

Randomly throughout the muscle

Muscles have different proportions of slow and fast twitch muscles

They vary in myoglobin content, colour, aerobic capacity and anaerobic capacity

(Muscles involved in posture control have a larger proportion of slow twitch)

Question 17

What are the properties of motor units?

Answer 17

Classified on amount of tension generated, speed of contraction, and fatigueability of the motor unit

Type I: slow twitch. Moderate force. Fatigue resistant

Type IIA: fast twitch. Moderate force. Fatigue resistant

Type IIB: fast twitch. High force. High fatigue

Question 18

How is muscle force regulated?

Answer 18

Recruitment:

Motor units are not randomly recruited, there is an order

Governed by the “size principal “. Smaller units are recruited first (generally slow twitch)

As more force is required, more units are recruited. This allows fine control, under which low force levels are required

Rate coding:

The 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 fires at a frequency too fast to allow the muscle to relax between arriving action potentials

Question 19

What is the order of muscle fibres recruitment?

Answer 19

Slow

Fast, fatigue resistant

Fast fatiguable

The fibre recruited last is also the first fibre to be de recruited

Muscle force can be regulated by The number of motor units recruited

Question 20

What are neurotrophic factors?

Answer 20

A type of growth factor that prevents neuronal death and promotes growth of neurones after injury

Motor unit and fibre characteristics are dependant on the nerve which innervates the

If any fast and slow twitch muscle or cross innervated, The slow one becomes fast and vice versa

The motor neuron has some effect on the properties of the muscle fibres it innervates

Question 21

What are the types of muscle contraction?

Answer 21

Concentric - shortens muscle to produce movement (lifting the dumbbell)

Eccentric - muscle produces force but the muscle gets longer (the dumbell is too heavy to lift but you’re still trying)

Isometric - provides force but doesn’t change length

Question 22

What is the plasticity of motor units/muscle fibres?

Answer 22

Fibre types can change properties under many different conditions

Type IIB (fatiguable) to IIA (resistant) is the most common change during training

Type I to II is possible in cases of severe deconditioning or spinal cord injury. Microgravity during space flight results in shift from slow to fast muscle fibres

Ageing associated with loss of type I and II muscle fibres. But preferential loss of type II. So elederly people have slower contraction times