Chapter 15 The Nervous System and Muscles

Question 1

What is the difference between the nervous system and the hormonal system?

Answer 1

The nervous system has a rapid response but doesn’t last long and is restricted to one region of the body. Hormones can travel all over the body and the response can be quick or slow and long lasting.

Question 2

How is the resting potential across a nerve’s axon maintained?

Answer 2

The sodium-potassium pump actively transports 3 Na+ ions out of the axon and 2 K+ ions in. Furthermore, sodium gates are closed but potassium gates are more permeable allowing more K+ to then move out the axon.

Question 3

What happens in an action potential?

Answer 3

Stimuli causes sodium ion channels to open.

Some sodium ions diffuse in.

More sodium ions diffuse in.

This makes the inside membrane more positive +40mV. This is known as depolarisation.

Question 4

What happens in repolarisation?

Answer 4

Na+ channels close.

K+ channels open.

K+ move out of the axon causing the membrane to become negative again.

Too many K+ ions move out resulting in hyperpolarisation.

Question 5

What is the advantage of a myelinated neuron?

Answer 5

The impulses can travel faster because depolarisation does not need to happen across the whole membrane only at the Nodes of Ranvier. This is known as saltatory conduction.

Question 6

Apart from myelinated sheaths how else can the speed on transmission be increased?

Answer 6

Increasing the temperature.higher temperatures enhance ion mobility and the rates of the biochemical reactions underlying impulse propagation.

The greater the diameter of the axon, the more sodium ions can flow through, and the less resistance.

Question 7

What is the ‘all or nothing” principle?

Answer 7

Below the threshold value there is no action potential. Above the threshold value an action potential is generated and the strength of the action potential is always the same.

Question 8

What is the refractory period and why is it important?

Answer 8

Once an action potential is created there is a period afterwards where the inward movement of sodium ions is prevented. This ensures action potentials are unidirectional, discrete and of a limited number.

Question 9

What is the gap between neurons called?

Answer 9

Synaptic cleft.

Question 10

What is spatial summation?

Answer 10

Different presynaptic knobs all contribute neurotransmitters to one post-synaptic neuron.

Question 11

What is temporal summation?

Answer 11

Temporal summation is when a presynaptic neuron releases neurotransmitters many times in a short period.

Question 12

How do inhibitory synapses work?

Answer 12

Neurotransmitters from pre-synaptic neurons bind on chloride channels on a post-synaptic neuron opening them.

This causes Cl - to move in by facilitated diffusion.

This causes potassium channels to open and K+ ions to diffuse out of the post-synaptic neuron. This makes it more negative in the axon and means it needs more Na+ ions to generate an action potential.

Question 13

In cholinergic synapses what does the arrival of the action potential cause?

Answer 13

1. Action potential arrives. When an action potential reaches the synaptic knob, the membrane of the pre-synaptic neuron changes permeability by opening voltage-dependent calcium channels.
2. Calcium ions rush into the pre-synaptic neuron. Calcium ions diffuse into the synaptic knob down the concentration gradient.
3. The synaptic vesicles fuse with the pre-synaptic membrane. The excess of calcium ions causes the vesicles to move towards and then fuse with the membrane, releasing acetylcholine (the contents of the vesicles) into the synaptic cleft. This process is exocytosis.
4. Acetylcholine diffuses across the synaptic cleft. The neurotransmitters diffuse down the concentration gradient.
5. Acetylcholine binds with receptors in the post-synaptic membrane. These receptors change shape, opening a channel to allow sodium ions to diffuse in. As the receptors are only present on the post-synaptic membrane, the impulse cannot travel back, so it is unidirectional.
6. Sodium ions rush into the post-synaptic neuron. If enough sodium ions diffuse in, the action potential threshold is reached and this triggers depolarisation of the neuron.
7. An action potential in the post-synaptic neuron is generated

Question 14

In cholinergic synpases what effect does calcium ion influx have?

Answer 14

In the pre-synaptic neuron it causes vesicles to fuse with the membrane releasing acetylcholine into the synaptic cleft which then diffuses across the synaptic cleft.

Question 15

In cholinergic synapses what effect do acetylcholine molecules have?

Answer 15

They bind to receptors on sodium ion channels. Causing Na+ ions to diffuse in rapidly along a concentration gradient and generates an action potential.

Question 16

What is the function of acetylcholinesterase?

Answer 16

Break down acetylcholine into ethanoic acid and choline. This stops it from continuously generating action potentials.

Question 17

What ensures synapses are unidirectional?

Answer 17

Vesicles are only produced in pre-synaptic neurons and receptors are only found on post-synaptic neurons.

Question 18

How does the movement of bones take place?

Answer 18

Muscles act in antagonistic pairs against an incompressible skeleton.

Question 19

What are the three proteins that make up muscle fibre?

Answer 19

Actin, myosin and tropomyosin.

Question 20

Describe actin.

Answer 20

Actin is a thin globular protein consisting of two strands twisted around one another.

Question 21

Describe myosin.

Answer 21

Myosin is thicker than actin and made up of long rod shaped tails and bulbous heads that project to the side.

Question 22

What is the difference between a slow-twitch muscle fibre and a fast-twitch muscle fibre?

Answer 22

STMFs contract more slowly and are less powerful. They are built for endurance.

Question 23

Give 3 adaptations of slow twitch muscle fibres.

Answer 23

They have a large store of myoglobin.

They have a rich supply of blood vessels to deliver oxygen and glucose.

They contain lots of mitochondria to produce ATP.

Question 24

Give 4 adaptations of fast twitch muscle fibres.

Answer 24

They have a large store of glycogen.

Myosin filaments are thicker and more numerous than in STMS.

There is a high concentration of enzymes involved in anaerobic respiration.

They contain a store of phosphocreatine.

Question 25

What does phosphocreatine do?

Answer 25

Phosphocreatine rapidly generates ATP from ADP in anaerobic conditions.

Question 26

What is the purpose of tropomyosin?

Answer 26

Tropomyosin stops myosin heads binding to actin by covering the binding site until calcium ions are released from the ER causing the tropomyosin molecule to change shape.

Question 27

How does muscle contraction occur?

Answer 27

Question 28

What happens to the A band, I band, H zone and Z line in contraction?

Answer 28

A band - no change
I band - shortens in contraction
H zone shortens in contraction
Z lines move closer together

Question 29

What is the role of calcium ions and ATP in muscle contraction?

Answer 29

Ca+ ions cause tropomyosin to move and stimulate ATPase. ATP causes myosin heads to detach and return to normal position. ATP also actively transports Ca+ ions back to the sarcoplasmic reticulum when muscle is relaxed.