Nerves

Question 1

Describe the structure of a neuron.

Answer 1

Neurons are specialised cells that allow communication between different parts of the body as nervous impulses.

Question 2

Compare the hormonal and nervous system (8 points).

Answer 2

1. Hormonal uses chemicals to communicate; nerves use impulses as communication.
2. Hormones are transported in the blood; impulses are transmitted by neurons.
3. Hormonal: transmission is relatively slow; transmission is very rpaid in the nervous system.
4. Hormones travel to all areas of the body but only target cells respond; nerve impulses travel to a specific part of the body.
5. Hormonal: response is wide-spread; reponse is localised in the nervous system.
6. Hormonal: response is slow; response is rapid in the nervous system.
7. Hormones are long-lasting; nerve impulses are short-lived.
8. Hormone effect may be permanant and irreversible; nerve impulse effects are usually temporary and reversible.

Question 3

Definition of excitable

Answer 3

Can detect and respond to stimuli.

Question 4

Definition of conductive.

Answer 4

Can transmit impulses or action potentials.

Question 5

What are the three types of neurons?

Answer 5

1. Sensory- impulse from receptor to central nervous system.
2. Intermediate- Impulse between neurons.
3. Motor- Impulse from central nervous system to effector.

Question 6

Describe the function of Schwann cells.

Answer 6

Surrounds the axon and forms the myelin sheath.

Question 7

Describe the myelin sheath.

Answer 7

Covers the axon and is rich in lipid, known as myelin.

Question 8

Definition of electrical impulses.

Answer 8

A brief change in the distribution of electrical charges across the axon membrane of neurons.

Question 9

Definition of resting potential.

Answer 9

The difference in electrical charge maintained across the membrane of an axon of a neuron hen not stimulated.

Question 10

Describe how to produce a resting potential (7 points).

Answer 10

1. Sodium-potassium pump actively pumps 3 Na+ ions out of the cell.
2. 2 K+ ions into the cell at the same time requires ATP.
3. Some K+ ions diffuse back out the neuron through open K+ channels. The membrane is more permeable to K+ / less permable to Na+ ions.
4. More positive ions are outside the cell than inside the cell.
5. This sets up an electrochemical gradient.
6. The potential difference outside the cell compared to inside the cell is -65mV.
7. The membrane is polarised.

Question 11

Definition of action potential.

Answer 11

A brief reversal of the resting potential across the axon membrane.

Question 12

Definition of threshold potential.

Answer 12

Minimum intensity that a stimulus must reach in order to trigger an action potential in a neuron.

Question 13

Describe the resting potential of a neuron briefly.

Answer 13

Polarised at -65mV.
Na+ / K+ ions are actively transported; 3 Na+ ions out, 2 K+ in the cell.
Few K+ channels are open, few K+ ions diffuse out the cell.
Na+ gated channels are closed.

Question 14

Describe deplarisation (7 points).

Answer 14

1. Stimulus causes some Na+ channels to open.
2. Some Na+ diffuse out the cell.
3. The membrane depolarises and the inside becomes less negative compared with the outside and reaches a threshold value of -50mV.
4. If threshold is reached, more Na+ channels open and Na+ flood into the cell.
5. The inside of the cell becomes positive compared with the outside of the cell.
6. The potential difference across the membrane reaches +40mV.
7. The Na+ channels close, the K+ channels open.

Question 15

Describe repolarisation (2 points).

Answer 15

1. The membrane become repolarised and positive inside compared with the outside of the cell.
2. More K+ channels open, causing more K+ ions to diffuse out the cell.

Question 16

Describe the refractory period (4 points).

Answer 16

1. The membrane is hyper-polarised.
2. The potential difference becomes more negative than -65mV due to the flood of K+ ions out the cell.
3. The K+ channels close and the Na+/K+ pumps to restore the resting potential of of -65mV.
4. This re-establishes the ion concentration across the membrane.

Question 17

Definition of the all or nothing response.

Answer 17

If threshold is reached, an action potential will fire with the same change in voltage, no matter how big the stimuli is.

Question 18

How does the brain interpret the size of the stimulus?

Answer 18

By having different neurons with different threshold potentials allows the brain to interpret the number and type of neurons that are transmitting an action potential to determine the size of the stimulus.

Question 19

Explain how the refractory period ensures the action potential only goes in one direction (3 points).

Answer 19

1. Action potential passes from active to resting region.
2. Action potential cannot be produced in refractory period.
3. Prevents action potential from spreading in both directions along the axon.

Question 20

Explain how the refractpry period produces discrete impulses (3 points).

Answer 20

1. New action potential cannot be formed immediately behind first action potential.
2. Allows action potential to be separated.
3. Helps show strength / frequency of stimulus.

Question 21

Explain how the refractory period limits the number of action potentials (3 points).

Answer 21

1. Action potentials are discrete.
2. Limits the strength of the stimulus that can be detected.
3. Body / brain not overloaded by stimulus.

Question 22

Describe non-myelinated transmission of an action potential (6 points).

Answer 22

1. During an action potential, gated Na+ channels are open.
2. There is a high concentration of Na+ ions in the depolarised region of the axon and low concentration of Na+ in the resting regions to either side.
3. This sets up a concentration gradient.
4. Na+ diffuses sideways from a high to a low concentration along the axon.
5. Diffusion of Na+ depolarises the next region causing Na+ channels to open (when it reaches -50mV threshold potential).
6. This causes the generation of a new action potential in this region. The action potential is therefore a travelling wave of depolarisation.

Question 23

Describe myelinated transmission of an action potential (6 points|).

Answer 23

1. Myelin electrically insulates axons which increases the speed of transmission of conduction.
2. Sodium ions and potassium ions cannot flow through the myelin sheath.
3. Ion exchange can only occur across the axon membrane at the Nodes of Rnvier.
4. Action potentials appear to jump from one Node to the next which is called saltatory conduction.
5. This can increase the speed of transmission by up to 50 times.
6. Local currents are elongated and Na+ diffuse along the inside of the neuron from one Node of Ranvier to the next.

Question 24

Describe and explain how temperature affects the speed at which action potentials can travel.

Answer 24

A lower temperature causes a slower rate of diffusion of ions, so a slower rate of conduction of ions. Less kinetic energy, so a slower response.

A higher tmeperature causes a faster rate of diffusion of ions, so a faster rate of conduction of ions. More kinetic energy, so a faster response.

Question 25

Describe and explain how diameter of the axon affects the speed at which action potentials can travel.

Answer 25

The smaller the diameter, the slower the the diffusion of ions. More resistance to the flow of ions in cytoplasm. Therefore, a slower response.

The larger the diameter, the quicker the diffusion of ions. Less resistance to flow of ions in cytoplasm, so quicker response.

Question 26

Describe and explain how the presence of a myelin sheath affects the speed at which action potentials can travel.

Answer 26

A myelinated neuron has a quicker diffusion of ions as there is a steeper gradient. Depolarisation only at Node of Rnvier. A quciker response.

A non-myelinated neuron has a slower diffusion of ions so a shallower concentration gradient. Depolarisation along length of membrane. A slower respsonse.

Question 27

Definition of a synapse.

Answer 27

The junction between 2 neurons. There is a very narrow gap of about 20nm between neurons called the synaptic cleft.

Question 28

Definition of a cholingeric synapse.

Answer 28

Synapses that use acetylcholine.

Question 29

Describe sequences of events at a synapse (10 points).

Answer 29

1. Action potential arrives at synaptic knob causing depolarisation of membrane.
2. Ca^2+ channels open and Ca^2+ ions enter the axon by facilitated diffusion.
3. Causes vesicles containing neurotransmitter to move and fuse.
4. With pre-synaptic membrane.
5. Neurotransmitter released into the synaptic cleft and diffuses across cleft.
6. Neurotransmitter binds with specific receptors on the post-synaptic membrane.
7. Na+ channels open and Na+ flood in.
8. Post-synaptic membrane is depolarised.
9. Acetylcholinesterase hydroylses acetylcholine into acetyl and choline.
10. Product diffuses back to pre-synaptic membrane to be re-synthesised.

Question 30

State and explain a unidirectional synapse.

Answer 30

State: Impulses are only transmitted in one direction.
Explain: Neurotransmitter is only produced in the pre-synaptic membrane in vesicles. Action potential causes movement of vesicles. Neurotransmitter binds to receptor only found on post-synaptic membrane.

Question 31

State and explain a temporal summation synapse.

Answer 31

State: Synaptic transmission of an impulse can use temporal summation.
Explain: Pre-synaptic neuron stimulated to release neurotransmitter. Low frequency of impulses doesn’t exceed threshold potential, so no action potential. High frequency of impulses does exceed threshold potential, so action potential is generated.

Question 32

State and explain a spatial summation synapse.

Answer 32

State: Synaptic transmission of an impulse can use spatial summation.
Explain: Many pre-sysnaptic neurons connected to one post-synaptic neuron. If both pre-synaptic neurons release neurotransmitters and threshold potential is reached, action potential is fired.

Question 33

State and explain an inhibitory nsynapse.

Answer 33

State: synapses can be inhibitory.
Explain: Less likely for an action potential to be fired in the post-synaptic neuron.

Question 34

Describe how neurons can be inhibitory (6 points).

Answer 34

1. Pre-synaptic neurons release neurotransmitter which binds to Cl- channels on post-synaptic membrane.
2. Neurotransmitter alos binds to K+ channels.
3. Cl- channels open, Cl- ions diffuse into the post-synpatic neuron into the cleft.
4. The movement of ions causes hyperpolarisation of post-synaptic membrane.
5. More Na+ channels must be opened to allow more Na+ to diffuse in.
6. To reach threshold.

Question 35

Explain how a drug that creates more action potentials effects synapses.

Answer 35

More action potentials are created.
Stimulates the release of more neurotransmitters or inhibits enzymes which break down the neurotransmitters. The body’s repsonse to impulse is enhanced.

Question 36

Explain how a drug that creates fewer action potentials effects synapses.

Answer 36

Fewer action potentials are created.
Some drugs inhibit the release of neurotransmitters / blocks receptors on the post-synaptic membrane. The body’s response to impulse is reduced.

Question 37

Describe endorphins and provide two examples of drugs that mimic enorphines’ effects.

Answer 37

Used to block pain pathways.
Morphine and codeine bind to specific receptors in the brain where endorphins normally work.