Muscular contraction

Question 1

Name three main types of muscle in the body

Answer 1

Cardiac Muscle Smooth Muscle Skeletal Muscle

Question 2

Name an antagonistic pair of muscles

Answer 2

Biceps and Triceps (or equivalent)

Question 3

What are muscle fibres called?

Answer 3

Myofibrils

Question 4

Describe 3 ways muscle fibres are adapted for efficient contraction?

Answer 4

Separate cells fuse together into muscle fibres as cell as the junction between adjacent cells would be a weak point. These fibres share nuclei and a cytoplasm called the sarcoplasm found around the circumference of the fibre. Within the sarcoplasm is a high concentration of mitochondria and endoplasmic reticulum.

Question 5

How are Actin and Myosin different from each other?

Answer 5

Actin- Thinner, globular protein whose molecules are arranged into 2 strands twisted around each other forming a helical strand. Myosin- Thicker and consists of long rod shaped fibres (Fibrous protein arranged into a filament) with bulbous heads projecting to the side. (Globular protein formed into two bulbous structures.

Question 6

What are the alternating light and dark bands called?

Answer 6

Light- Isotropic Bands (I-bands)- Actin and Myosin do not overlap=shade. Dark Anisotropic bands (A-bands) Actin and myosin overlap in this region = shade.

Question 7

What is at the centre of an anisotropic band?

Answer 7

Z-lines- The difference between adjacent z-lines is a sarcomere.

Question 8

What happens to sarcomeres during muscle contraction?

Answer 8

They shorten and the pattern of I and A bands changes.

Question 9

What’s at the centre of the isotropic band?

Answer 9

the H-zone.

Question 10

What are two other important proteins in muscle besides Myosin and Actin?

Answer 10

Tropomyosin- Fibrous protein around the actin filament. Troponin- Globular Protein involved in muscle contraction

Question 11

How do slow twitch muscle fibres contract, where are they located and what function are they adapted for?

Answer 11

Contract more slowly and less powerful contractions over a longer period of time. Common in muscles such as calf muscle. Adapted for endurance work.

Question 12

How do fast twitch muscle fibres contract, where are they located and what function are they adapted for?

Answer 12

Contract rapidly producing powerful contractions for a short period of time. Common in muscles e.g. bicep Built for strength (intense exercise) e.g. weightlifting

Question 13

How are slow twitch muscles adapted to their function?

Answer 13

Large myoglobin store (red molecule that stores oxygen accounts for red colour of slow twitch muscle) Supply of glycogen providing source of metabolic energy. Rich supply of blood vessels to deliver oxygen and glucose. Numerous mitochondria to produce ATP.

Question 14

How are fast twitch muscles adapted to their function?

Answer 14

Thicker and more numerous myosin filaments. High concentration of enzymes used in anaerobic respiration. Store of phosphocreatine a molecule that can rapidly generate ATP from ADP in anaerobic respiration.

Question 15

What is a neuromuscular junction?

Answer 15

The point where a motor neurone meets a skeletal muscle fibre.

Question 16

How many junctions are along the muscle? Why?

Answer 16

Many; if there was only one it would take too long for a wave of contraction to travel across the muscle, not all fibres would contract simultaneously and the movement would be slow. Rapid muscle contraction is essential for survival. therefore many neuromuscular junctions.Ensures muscular contraction is is rapid and powerful, simultaneously stimulated by action potentials.

Question 17

How do neuromuscular junctions transfer the nerve impulse to muscle fibres?

Answer 17

Nerve impulse received at neuromuscular junction. Synaptic vesicles fuse to presynaptic membrane and acetylcholine is released. Acetylcholine diffuses into postsynaptic membrane altering its permeability to sodium ions which rapidly enter depolarising the membrane. Acetylcholine broken down by acetylcholinesterase to ensure the muscle is not over-stimulated. Choline and ethanoic acid diffuse into presynaptic neurone where they are combined forming acetylcholine using energy provided by the mitochondria.

Question 18

What is the sliding filament mechanism?

Answer 18

A process involving actin and myosin filaments sliding past each other.

Question 19

What evidence is there for the sliding filament theory?

Answer 19

In the sarcomere during contraction: I-band becomes narrower Z-lines move closer together (sarcomere shortens) H-zone becomes narrower. A-band remains same wisth as it is governed by the length of myosin filaments therefore contraction not due to myosin filaments shortening.

Question 20

How is muscle stimulated?

Answer 20

The action potential reaches many neuromuscular junctions simultaneously. Ca²⁺ channels open and ions flood into synaptic knob. Ca²⁺ ions cause synaptic vesicles to fuse with presynaptic membrane and release acetylcholine into into the synaptic cleft. Acetylcholine diffuses across snyaptic cleft and binds with receptors on the postsynaptic membrane, depolarising it.

Question 21

Spread of the action potential across the muscle is…

Answer 21

The “Wave of Depolarisation”

Question 22

State the stages of muscular contraction.

Answer 22

1. Action Potential travels deep into the fibre through a system of tubules (T- tubules) branching throughout the cytoplasm of the muscle (sarcoplasm).
2. Tubules in contact with endolasmic rectilium of the muscle (sarcoplastic rectilium) which have actively transported calcium ions from the sarcoplasm.
3. The AP opens the voltage gated calcium ion channels and ions flood into cytoplasm of the muscle down a diffusion gradient.
4. Calcium ions cause tropomyosin molecules blocking the binding sites on the actin filament to puill away.
5. ADP attached t omyosin heads means they are in a state to binf to actin filament forming a cross-bridge.
6. Once attached to vthe filament, the myosin heads change their angle, pulling the actin filament along as they do so. Molecule of ADP released.
7. An ATP molecule attaches to each myosin head detaching it from the actin filament.
8. Calcium ions activate ATPase which hydrolises the ATP toADP+P_i. This hydrolysis prvides the energy for the myosin head to return to its original position.
9. Mysosin head, once more with an attached ADP reattaces itself further along the actin filament and the cycle repeats as long as stimulation of the muscle continues.

Question 23

How does muscle relaxation occur?

Answer 23

When stimulation ceases, Ca²⁺ actively transported back into ER using energy from ATP hydrolysis.

Reabsorption of these ions allows tropomyosin to block the actin filaament again.

Myosin heads now unable to bind to actin filaments and contraction ceases.