3.6.3 Skeletal Muscles are Stimulated to Contract by Nerves and act as Effectors

Question 1

What are the three types of muscle?

Answer 1

Cardiac: exclusively in the heart
Smooth: walls of blood vessels and the heart
Skeletal: responsible for conscious control

Question 2

What are myofibrils?

Answer 2

Groups of filaments which have been fused together to make up the larger muscle fibre
A bundle of muscle fibres share nuclei, a membrane called the sarcolemma and a cytoplasm called the sarcoplasm

Question 3

What is the sarcolemma membrane?

Answer 3

It encases a specialised cytoplasm called the sarcoplasm
On the inner surface there is sarcoplasmic reticulum
Inner foldings which form another membrane system called the T tubules, in contact with the sarcoplasmic reticulum

Question 4

What does the sarcoplasm contain?

Answer 4

Many mitochondria
Stores of glycogen
Myoglobin (store of oxygen with high association)
Numerous myofibrils
Numerous nuclei (for protein synthesis)

Question 5

What is the structure of muscle fibres?

Answer 5

Muscle fibres encase myofibrils which encase muscle filaments

Question 6

What is the simplified structure of muscle filaments?

Answer 6

See card

Question 7

What are the two types of muscle filaments?

Answer 7

Actin: thinner and consists of two strands twisted round one another
Myosin: thicker and consist of long rod shaped tails with bulbous heads that project to the side

Question 8

What are the two types of muscle fibres?

Answer 8

Slow - twitch

Fast - twitch

Question 9

What are the properties of slow - twitch fibres?

Answer 9

They contract slower
They provide less powerful contractions
Work over a longer period
Adapted for endurance work
Have a large store of myoglobin
Connected to a rich blood supply
Contain numerous mitochondria

Question 10

What are the properties of fast - twitch fibres?

Answer 10

They contract more rapidly
They produce powerful contractions for a short period
They are adapted for intense exercise
They are thicker and more numerous myosin filaments
Have a higher glycogen concentration
Have a higher enzyme concentration

Question 11

What is the process for the sliding filament theory?

Answer 11

1. The tropomyosin molecule prevents myosin heads from attaching to the binding site on the actin molecule
2. Calcium ions released from the ER cause the tropomyosin molecule to change shape and so pull away from the binding site on the actin molecule
3. The myosin head now attaches to the binding site on the actin filament
4. The head of myosin changes angle, moving the actin filament along as it does so. The ADP molecule is released
5. ATP molecules fix to the myosin head, causing it to detach from the actin filament
6. Hydrolysis of ATP to ADP by ATPase provides the energy for the myosin head to resume its normal position
7. Head of myosin reattaches to a binding site further along the actin filament and the cycle is repeated

Question 12

What is the role of actin in the sliding filament theory?

Answer 12

It is the thin filament
It is a part of the troponin tropomyosin complex
It provides binding sites for myosin heads during the power stroke

Question 13

How is calcium involved in contraction in the sliding filament mechanism?

Answer 13

The action potential travels throught the T tubules
The tubules are in contact with the ER of the muscle
The ER has actively transported calcium ions from the cytoplasm of the muscle leading to very low Ca 2+ concentration in cytoplasm
The action potential opens the calcium ion protein channels on the ER calcium ions diffuse into the muscle cytoplasm down a concentration gradient
The calcium ions cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away

Question 14

What are the different types of nervous system?

Answer 14

Sympathetic: stimulates effectors, speeds up activity
Parasympathetic: inhibits effectors, slows down activity

Question 15

Which part of the brain controls heart rate?

Answer 15

The medulla oblongata

Connected to the sinatorial node via the sympathetic and parasympathetic nervous system

Question 16

What is the process for the control of chemoreceptors?

Answer 16

When the blood has a higher concentration of CO2, it’s pH is lowered
Chemoreceptors in the wall of the arteries and aorta increase the frequency of nervous impulses to the centre in the medulla oblongata which increases the heart rate
This centre increases the frequency of impulses via the SA
This increases the rate of production of electrical waves by the SA
This increased the heart rate
The increased blood flow leads to more CO2 being removed from the lungs which decreases the concentration in the blood
The pH of the blood rises to normal and the chemoreceptors reduce the frequency of nerve impulses
This decreases the heart rate

Question 17

How do pressure receptor operate when the blood pressure is higher than normal?

Answer 17

Pressure receptors transmit more nervous impulses to the centre in the medulla oblongata which decreases heart rate
The centres sends impulses via the parasympathetic nervous system to the SA which leads to a decrease in heart rate

Question 18

How do pressure receptor operate when the blood pressure is lower than normal?

Answer 18

Pressure receptors transmit more nervous impulses to the centre in the medulla oblongata which increases heart rate
The centres sends impulses via the sympathetic nervous system to the SA which leads to an increase in heart rate