Muscle microstructure and contraction

Question 1

What are the 3 muscle types and their properties?

Answer 1

Smooth muscle is under involuntary control from the autonomic nervous system. Cardiac muscle can contract autonomously, but is under the influence of the autonomic nervous system and circulating chemicals. Skeletal muscles are under voluntary control, usually attached to bones and contract to bring about movement.

Question 2

What are the different ways muscle fibres can be arranged?

Answer 2

Can be arranged in a parallel formation, fusiform or triangular. Pennate refers to the direction of the fibres: can be unipennate, bipenate or multipennate.

Question 3

Describe the structure of skeletal muscle

Answer 3

Muscle is joined to bone via a tendon. The muscle contains fascicles which are bundles of muscle fibres. Each myofibre is made up of myofibrils which are thereby made up of myofilaments. The muscle is covered by an epimysium. Muscle fascicles are bound by perimysium while each fibre is surrounded by endomysium.

Question 4

Describe structure of myofibre

Answer 4

Covered by a plasma membrane known as sarcolemma. Composed of myofibrils. Capillaries run along the surface as well as multiple nuclei. T-tubules tunnel into centre. Contains a cytoplasm called sarcoplasm – myoglobin and mitochondria present. A network of fluid filled tubules known as sarcoplasmic reticulum present.

Question 5

Describe structure of myofibrils

Answer 5

Are 1-2 micrometres in diameter and extend along the entire length of the myofibre. Are composed of 2 main types of proteins - actin and myosin.

Question 6

Describe structure of myofilaments

Answer 6

Consist of dark and light band. Light band made of thin filament actin, centre of which known as Z-disc and dark band made of thick filament myosin, centre of which known as H-zone. Light and dark bands give a striated appearance to muscle. These overlap and are arranged in compartments known as sarcomeres. Dense protein Z-discs separare sarcomeres. M-line is midline of H-zone. Isotropic region is the light band while anisotropic region is the dark band.

Question 7

Describe structure of myosin

Answer 7

Has 2 globular heads and a single tail formed by two α-helices. Tails of several hundred molecules form one filament.

Question 8

Describe structure of actin

Answer 8

Actin molecules twisted into helix. Each molecule has a myosin binding site. Filaments also contain troponin and tropomyosin. Tropomyosin moves away when there is a calcium influx, revealing myosin binding site.

Question 9

Why was the sliding filament theory proposed?

Answer 9

During contraction I band became shorter but A-band remained same length and H-zone narrowed or disappeared. Hence thought that actin filaments slide over.

Question 10

How is muscle contraction initiated?

Answer 10

Action potential opens voltage gated Ca2+ channels in presynaptic terminal. Ca2+ enters pre-synaptic terminal and triggers exocytosis of vesicles. Acetylcholine diffuses across cleft and binds to acetylcholine receptors, inducing action potentials (AP) in muscle.

Question 11

How is initiation of muscle contraction halted?

Answer 11

Local currents flow from depolarized region and adjacent region. AP spreads along surface of muscle fibre membrane. Acetylcholine broken down by acetylcholinesterase. Muscle fibre response to that molecule of acetylcholine ceases.

Question 12

How is muscle activated by action potential?

Answer 12

Action potential propagates along surface membrane and into T-tubules. Dihydropyridine (DHP) receptor in T-tubule membrane senses change in voltage & changes shape of the protein linked to ryanodine receptor and opens the ryanodine receptor Ca2+ channel in the sarcoplasmic reticulum (SR). Ca2+ released from SR into space around the filaments. Ca2+ binds to troponin & tropomyosin moves allowing cross-bridges to attach to actin. Ca2+ is actively transported into the SR continuously while action potentials continue.

Question 13

How do actin and myosin bind?

Answer 13

In the presence of Ca2+, troponin moves 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, pulling actin filament towards centre of sarcomere. ATP binding releases myosin head from actin chain. ATP hydrolysis provides energy to ‘recharge’ the myosin head.

Question 14

What is a motor unit?

Answer 14

Name given to a single motor neuron together with all the muscle fibres that it innervates. Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.

Question 15

What are the types of motor units?

Answer 15

Slow fibres (S, I)
Fast, fatigue resistant fibres (FR, IIA)
Fast, fatiguable (FF, IIB)

Question 16

What are the features of slow twitch fibres?

Answer 16

1. smallest diameter cell bodies
2. small dendritic trees
3. thinnest axons
4. slowest conduction velocity
   Have high myoglobin content, appear red, high aerobic capacity and low anaerobic capacity.

Question 17

What are the features of fast twitch, fatigue resistant fibres?

Answer 17

1. larger diameter cell bodies
2. larger dendritic trees
3. thicker axons
4. faster conduction velocity
   Have high myoglobin content and appear pink. Moderate aerobic capacity but high anaerobic capacity.

Question 18

What are the features of fast twitch, fatiguable fibres?

Answer 18

1. larger diameter cell bodies
2. larger dendritic trees
3. thicker axons
4. faster conduction velocity
   Low myoglobin content, appear white. Low aerobic capacity but high anaerobic capacity.

Question 19

How are motor unit types classified?

Answer 19

Motor unit types are classified by the amount of tension generated, speed of contraction and fatiguability of the motor unit. Type I is slow twitch, low force, fatigue resistant. Type IIa is fast twitch, moderate force, fatigue resistant. Type IIb is fast twitch, high force, high fatigue.

Question 20

How is muscle force regulated?

Answer 20

Two mechanisms by which the brain regulates the force that a single muscle can produce: Recruitment and rate coding.

Question 21

What is recruitment?

Answer 21

Motor units are not randomly recruited, there is an order. Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).As more force is required, more units are recruited. This allows fine control (e.g. when writing), under which low force levels are required. Muscle force can be regulated by the number of motor units recruited.

Question 22

What is rate coding?

Answer 22

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.

Question 23

What are neurotrophic factors?

Answer 23

Are a type of growth factor. Prevent neuronal death and promote growth of neurons after injury.

Question 24

What is the impact of neurotrophic factors?

Answer 24

Motor unit and fibre characteristics are dependent on the nerve which innervates them. If a fast and slow twitch muscle are 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 25

What are the types of muscle contraction?

Answer 25

Isometric contraction is where the muscle does not move. Concentric contraction is where the muscle is contracted. Eccentric contraction is where the muscle is extended, which occurs when a load is too heavy.

Question 26

Explain plasticity of motor units

Answer 26

Fibre types can change properties under many different conditions. Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types. Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times). Type IIB to IIA most common following training.