Skeletal, Smooth And Cardiac Muscle

Question 1

Describe the structure and ultrastructure of skeletal muscle.

Answer 1

Muscle is an excitable tissue which generates force and movement. It is striated, arranged in lever systems. The regular repeating order provides strength and as it is multinucleate the nucleus bulges out the side. It is very long as there are multiple cells fused together packed with protein, enclosed in sheaths. They are attached to bones by tendons. They form in utero from mononucleate myoblasts. Separated into 2 groups, antagonists, with opposite actions.

Question 2

Excitation-contraction coupling

Answer 2

1. Acetylcholine is released by the axon terminal which diffuses to the muscle cell and attaches to the ACh receptors in the sarcolemma.
2. Depolarosation occurs and the AP is generated along the sarcolemma.
3. The AP, carried into the cell via Transverse tubules, causes the sarcoplasmic reticulum to release Ca2+ ions.
4. The Ca2+ concentration at myofilaments increases causing contraction.
5. ATP pumps remove Ca2+ and pump it back into storage and the muscle relaxes.

Question 3

Explain the mechanics of skeletal muscle contraction.

Answer 3

Motor neurones + muscle fibres = motor unit. Muscle fibres are scattered within a unit so even if one neurone is damaged the others maintain function.

• Tension is the force exerted by muscle.
• Load is the force exerted on muscle.
• Isometric is contraction with constant length.
• Lengthening is contraction with increasing length.
• Latent period; the time before contraction starts.
• Contraction time; occurs between the start of tension and peak tension. Varies based on different fibres and [Ca2+].

Question 4

Isometric twitches

Answer 4

Short latent period in which the muscle doesn’t change it’s length.

Question 5

Isotonic twitches

Answer 5

Longer latent period. The muscle changes in length without increasing tension.

Question 6

Twitch contraction

Answer 6

A single AP acts on the muscle fibre causing a twitch.

Question 7

Sliding-filament theory/X-bridge cycle

Answer 7

1. Surge of Ca2+ triggers binding of myosin x-bridges to actin. Tropomyosin partially covers the myosin binding sites which is held in place by troponin. When Ca2+ binds to troponin it alters the shape of troponin pulling tropomyosin away so actin can and myosin can bind.
2. X-bridge moves taking actin with it, releasing ADP and Pi, contracting the muscle.
3. ATP binds to myosin, causing x-bridge to detach.
4. Hydrolysis of ATP energises x-bridge ready for the next contraction.

Question 8

Motor units

Answer 8

Motor unit is the motor neurone plus the muscle fibres. Even if one motor neurone fails the others can maintain function as there are many muscle fibres in a unit which are scattered through out muscle. Making it difficult to paralyse someone.

Question 9

Tetanus

Answer 9

Unfused tetanus occurs when there are multiple APs spread out causing repetitive twitches.
Fused tetanus occurs when there are many APs at a high frequency so muscle contraction and tension is sustained.
Tetanic tension is greater than twitch tension since [Ca2+] never gets low enough to allow troponin/tropomyosin to re-block x-bridges.

Question 10

Fatigue

Answer 10

It’s a safety mechanism to stop using too much ATP which could cause rigor i.e. muscles won’t be able to activate new x-bridge cycles. Continuously high levels of Ca2+ will damage cells. Repeated muscle stimulation leads to muscle fatigue. The rate of onset depends on fibre type, length of contraction and fitness of an individual. If a muscle is rested it can contract again.

Question 11

Fibre types

Answer 11

Characterised based on whether the fibres are fast/slow shortening and the oxidative or glycolytic ATP forming pathways used.
Fast- myosin has high ATPase activity so splits ATP quickly.
Slow- low ATPase activity.
Most bundles are mixtures of both fibre types.
Slow oxidative (I) fibres resist fatigue due to a constant energy supply.
Fast oxidative (II) fibres have an intermediate resistance to fatigue.
Fast glycolytic (IIb) fibres fatigue quickly as don’t have a good O2 supply.

Question 12

Oxidative fibre types (aerobic)

Answer 12

Lots of mitochondria leads to an increase in oxidative phosphorylation . Good vascularisation is needed to deliver O2 and nutrients. They contain myoglobin so increased O2 delivery. Fibres are red and have small diameters.

Question 13

Glycolytic fibre types (anaerobic)

Answer 13

Lots of glycolytic enzymes and glycogen. Lower blood supply and no myoglobin. They are white fibres with larger diameters.

Question 14

Smooth muscle structure

Answer 14

No striations. Exists in hollow organs and is spindle shaped. Innervated by the ANS. They are mononucleate and divide through life. Thick myosin and thin actin filaments are arranged diagonally, anchored to membranes and cell structures by dense bodies (eg z lines)
Single unit; (GIT, uterus and small blood vessels) many cells linked by gap junctions. The signals travel between cells and contract synchronously. They may contain pacemaker cells. Stretch evokes contraction.
Multiunit; (Airways, large arteries, hairs)
Few/no gap junctions richly innervated by the ANS. Don’t respond to stretch.

Most smooth muscles are a mixture of single and multinucleate populations of cells. Meaning an organ can have a mixture of properties in different areas.

Question 15

Smooth muscle function

Answer 15

Found in the walls of hollow organs e.g. uterus to push baby out, bladder to urinate, blood vessels to dilate/constrict.

Question 16

Smooth muscle contraction

Answer 16

1. [Ca2+] increases in response to stimulus.
2. Ca2+ binds to calmodulin
3. Ca2+-calmodulin binds to myosin light chain kinase.
4. Kinase phosphorylates myosin x-bridges with ATP.
5. Phosphorylated x-bridges bind to actin filaments.
6. Contraction and tension.

Question 17

Snooth muscle relaxation

Answer 17

Myosin light chain phosphatase dephosphorylates x-bridges so they let go of actin and the muslce relaxes.