Neurones and muscles

Question 0

What is meant by the all or nothing nature of action potentials?

Answer 0

Once a threshold is reached an action potential will always fire no matter how big a stimulus is. A bigger stimulus won’t cause a bigger action potential it will just cause them to fire more frequently.

Question 1

What is a refractory period, why do they occur?

Answer 1

Period after a neurone has fired when another action potential can’t be generated.
It ensures that an action potential can only move in one direction from an active region to a resting region.
It produces discrete impulses meaning new action potentials can’t be generated directly after the first and ensures action potentials are separate.
It limits the number of action potentials – action potentials are separated from one another, therefore there is a limited amount that can pass along a neuron in a given time.

Question 2

What factors affect the speed of a nerve impulse?

Answer 2

1. The myelin sheath – Prevents action potential forming in myelinated areas of the axon. The action potential jumps from one node of Ranvier to another (salutatory conduction) – this increases the speed of the impulse as less action potentials need to occur
2. The greater the diameter of the axon the greater the speed of conductance – due to less leakage of ions from the axon
3. Temperature – Higher temperature, faster nerve impulse. Energy for active transport comes from respiration. Respiration like the sodium potassium pump is controlled by enzymes.

Question 3

What is spatial summation?

Answer 3

When different presynaptic neurons together release enough neurotransmitter to exceed the threshold value to form an action potential.

Question 4

What is temporal summation?

Answer 4

When one neuron releases neurotransmitters many times over a short period. Eventually the neurotransmitter will accumulate so as to overcome the threshold value of the postsynaptic membrane. Therefore generating a new action potential

Question 5

What happens when an action potential reaches the end of a neurone?

Answer 5

1. Causes neuro-t’s to be released into the synaptic cleft.
2. Neuro-t’s diffuse across the post synaptic membrane and bind to specific receptors.
3. When neuro-t’s bind to receptors they may trigger an action potential causing a muscle contraction or hormones to be secreted.

Question 6

What do calcium ions do in the presynaptic neurone?

Answer 6

1. An action potential stimulates voltage-gated ca ion channels in ps neurone to open.
2. Ca ions diffuse into synaptic knob. (Are pumped out again by active transport).
3. Influx of ca ions cause synaptic vesicles to fuse with presynaptic membrane.

Question 7

What does the neurotransmitter acetylcholine (ACh) do?

Answer 7

1. Is released from synaptic vesicles (exocytosis).
2. ACh diffuses across the synaptic cleft and binds to specific cholingeric receptors on the post synaptic membrane.
3. Causing sodium ion channels in the post synaptic neurone to open.

Question 8

What do sodium ions do in the post synaptic neurone?

Answer 8

1. Sodium ion channels open when neuro-t’s reach the post synaptic neurone.
2. In influx of na ions causes an action potential (if the threshold is reached).

Question 9

What charge is a neurone at rest?

Answer 9

The outside is positively charged compared to the inside so the membrane is polarised (there is a difference of voltage across it).

Question 10

What is the threshold for depolarising a neurone, what happens when it’s depolarised?

Answer 10

If a potential difference reaches the threshold (around -55mV) more sodium ion channels open, more na ions diffuse into the neurone.

Question 11

What is the threshold for repolarisation, what happens when a neurone is repolarised?

Answer 11

At a potential difference of around +30mV the na ion channels close and potassium ion channels open. The membrane is more permeable to potassium so K ions diffuse out of the neurone. This starts to get the neurone back to its resting potential.

Question 12

What are transverse tubules?

Answer 12

Bits of the sarcolemma that fold inwards and stick into the sarcoplasm. They help spread electrical impulses through the sarcoplasm so they reach all parts of the muscle fibre.

Question 13

What is the dark band, what is it made of?

Answer 13

Also called the A-band, contains thick myosin filaments and some overlapping thin actin filaments.

Question 14

What is the light band, what does it contain?

Answer 14

Also called the I-bands, contain thin actin filaments.

Question 15

What is the H zone, what does it contain?

Answer 15

Is around the M zone only contains myosin.

Question 16

What happens to the bands when muscles contract?

Answer 16

The H and I bands get shorter. The A bands stay the same.

Question 17

What are three ways energy is provided for muscle contraction?

Answer 17

Aerobic respiration, anaerobic respiration and the ATP-Phosphocreatin (PCr) system (where ATP is made by phosphorylating ADP and adding a phosphate group from PCr.

Question 18

Describe slow twitch muscles.

Answer 18

Red as they are rich in myoglobin. Energy released slowly through aerobic respiration, lots of blood vessels supply the muscles with oxygen. Good for endurance.

Question 19

Describe fast twitch muscles.

Answer 19

White in colour as they don’t have much myoglobin. Good for short bursts. Energy released quickly trough anaerobic respiration using glycogen.

Question 20

Describe the stages of muscle contraction.

Answer 20

-Acetylcholine fits into receptor sites -causes an increased permeability in the sarcoplasmic reticulum causing an influx of Ca ions -Ca ions bind with troponin on actin causing tropomyosin to move so the myosin head can bind to the binding site (as long as myosin head has been activated by breakdown of ATP) -ADP and Pi released so the myosin head returns to its relaxed position causing the thin filament to be pulled (power stroke) -ATP reattaches to the myosin head & actin myosin bond breaks. -breakdown of ATP to ADP and Pi allows myosin head to be activated & return to the cocked position.

Question 21

What is the advantage of nervous communication over hormonal communication?

Answer 21

Electrical impulses travel faster than hormones so nervous communication is faster.

Question 22

What is a generator potential?

Answer 22

The change in potential difference due to a stimulus.

Question 23

What is potential difference?

Answer 23

The difference in voltage across the membrane of a neurone.

Question 24

What kind of receptors are found at neuromuscular junctions?

Answer 24

Nicotinic cholingeric receptors

Question 25

What do tendons do?

Answer 25

Attach skeletal muscle to bone.

Question 26

Where are Ca ions stored in a muscle?

Answer 26

In the sarcoplasmic reticulum.

Question 27

What is a motor unit?

Answer 27

A motor neurone and all the muscle fibres attached to it.

Question 28

Synapses are unidirectional. Explain how acetylcholine contributes to a synapse being unidirectional.

Answer 28

1. Acetylcholine released from presynaptic side;
2. Diffusion from higher concentration/to lower concentration;
3. Receptors in postsynaptic (side) / binds on postsynaptic (side);

Question 29

Describe an excitatory synapse?

Answer 29

Has neuroreceptors that are Na ion channels. When they open, +ve ions diffuse in causing a depolarisation called an excitory postsynaptic potential (EPSP). This makes an action potential more likely.

Question 30

Describe an inhibitory synapse?

Answer 30

Have neuroreceptors that are Cl ion channels. When the channels open -ve ions diffuse in causing local hyperpolarisation called an inhibitory postsynaptic potential (IPSP). Making an action potential less likely.