Unit 6.5

Question 1

1. Neurones are?
2. 3 basic components of nerve cells?
3. Myelin sheath function?
4. How do neurons generate and conduct electrical signals?
5. What does unequal distribution of ions on different sides of the membrane create?
6. Explain what resting potential is.

Answer 1

1. Specialised cells that function to transmit electrical impulses within the nervous system.
2. Dendrites - short branched fibres.
   Axon - elongated fibre that transmits electrical signals to terminal regions for communication with other neurons.
   Soma - a cell body containing the nucleus and organelles.
3. Improves conduction speed of electrical impulses along the axon but it requires additional space and energy.
4. By pumping positively charged ions (Na+ and K+) across their membrane.
5. Creates a charge difference called a membrane potential.
6. The difference in charge across the membrane when a neuron is not firing. In a typical resting potential, the inside of the neuron is more negative relative to the outside (-70mV).

Question 2

1. What type of process is maintaining resting potential?
2. How does the sodium-potassium pump exchange sodium and potassium ions?
3. What does this create and what is it?
4. What does the exchange of sodium and potassium ions require?
5. What are action potentials?
6. Name the 3 stages action potentials occur in.

Answer 2

1. An active process (ATP dependent)
2. It expels 3 Na+ ions for every 2 K+ ions admitted.
3. Am electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment.
4. It requires the hydrolysis of ATP.
5. Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing.
6. Depolarization, repolarization, refractory period.

Question 3

1. What is Depolarization?
2. What does depolarization refer to?
3. Explain the process of depolarization.
4. What is Repolarization?

Answer 3

1. When electrical impulses are transmitted across the cell, it is depolarized to become more positive inside the cell as there is an increase in sodium.
2. The sudden change in membrane potential, usually from a relatively negative to positive internal change.
3. Signal initiated at a dendrite - so sodium channels open within the membrane of the axon. As Na+ ions are more concentrated outside the neuron, the opening of sodium channels causes a passive INFLUX of SODIUM - this influx creates a more positive membrane potential (depolarization) and becomes 30mV.
4. This refers to the restoration of a membrane potential following depolarization (restoring a negative internal charge).

Question 4

1. Explain the process of Repolarization?
2. What is the Refractory Period?
3. Explain the process of the Refractory Period.
4. State the mV inside the neuron during repolarization.
5. State the mV inside the neuron during depolarization.

Answer 4

1. Following the sodium influx, potassium channels open within the membrane of the axon. Bc K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium (so decrease of potassium inside the cell). The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarization).
2. This refers to the period of time following a nerve impulse before the neuron is able to fire again.
3. In normal resting state, sodium ions are predominantly outside the neuron and potassium ions are mainly inside (resting potential). Following depolarisation (sodium influx) and repolarisation (potassium efflux), this iconic distribution is largely reversed. Before a neuron can fire again, the resting potential must be restored via the antiport of action of the sodium-potassium pump.
4. Inside -80mV
5. Inside -70mV

Question 5

1. Name of the ion channels that occupy the length of the axon.
2. How does depolarisation spread along the length of the axon?
3. What is the minimum stimulus? What is this known as?
4. What occurs if the threshold potential is not reached?
5. Equipment used t measure membrane potential across and neuronal membrane?
6. What is the purpose the the refractory period?

Answer 5

1. Voltage-gated
2. Spreads along the length of the axon as an unidirectional ‘wave’.
3. It is the level required to open voltage-gated ion channels. It is known as the threshold potential -55mV.
4. An action potential can’t be generated so the neuron will not fire.
5. Oscilloscopes - data is displayed as a graph, with time (milliseconds) on the X axis and membrane potential (in millivolts) on the Y axis.
6. Limits the amount of action potentials that a given nerve cell can produce per unit time.

Question 6

1. Non-myelinated axons have?
2. What do non-myelinated fibres not show?
3. What do non-myelinated nerve fibres not have?
4. Myelinated nerve fibres are what colour?
5. What is white matter composed of?
6. What is grey matter composed of?

Answer 6

1. Nerve fibres that are grey and don’t have a myelin sheath.
2. Nodes and internodes
3. Nodes of Ranvier
4. White
5. Regions of nervous system composed of myelinated axon tracts.
6. Neuronal cell bodies, dendrites, synapses and support cells (glial cells).

Question 7

1. Movement of Na and K at resting?
2. Minimum millivolts at threshold potential and what will not occur if this is not reached?
3. Millivolts at depolarization?
4. Millivolts at repolarization?
5. What is hyperpolarization?
6. When are electrical signals unable to be conducted?
7. How do neurons transmit information across a synapse?

Answer 7

1. 3 sodium go out and 2 potassium go out.
2. -50/55 millivolts - if this is not reached the cell will not reach action potential.
3. 30+
4. 30+
5. Voltage overshoots to -80, refractory period must occur, period of time required to return from -80 to -70 via sodium-potassium pumps.
6. When a semi-permeable membrane is absent.
7. By converting the electrical signal into a chemical signal.

Question 8

1. Explain the process of chemical transfer across synapses.

Answer 8

1. a) action potential reaches axon terminal, triggering opening of voltage-gated calcium channels. Calcium enters through facilitated diffusion, signalling vesicles to migrate to the pre-synaptic membrane.

b) Calcium ions diffuse into the cell to promote fusion of vesicles (containing neurotransmitters) in the cell membrane.

c) vesicles bind to pre-synamtic neuron.

d) Neurotransmitters release from axon terminal by exocytosis (atp) across synaptic cleft.

e) Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels.

f) The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal.

g) The neurotransmitters released into the synapse are either recycled (reuptake pumps) or degraded by enzymatic activity - to stop an increase of neurotransmitters in the synaptic cleft.

Question 9

1. What are neurotransmitters?
2. What are they released in response to?
3. How do neurotransmitters trigger a variety of responses?
4. What are these target cells?
5. What responses are created?

Answer 9

1. Chemical messengers released from neurons and function to transmit signals across the synaptic cleft.
2. To the depolarisation of the axon terminal of a presynaptic neuron.
3. Depending on the type of target cell they activate.
4. Neuron, glandular cell, muscle fibre.
5. Neuron - Stimulate or inhibit electrical signal

Glandular - Stimulate or inhibit secretion

Muscle fibre - Stimulate or inhibit muscular contraction/ relaxation.

Question 10

1. Example of a neurotransmitter used by both CNS and PNS?
2. It is commonly released at? and binds to what? for what purpose?
3. Why is it commonly released within the autonomic nervous system?
4. How is it created?
5. Where is it stored and how is it released?
6. How does it activate a postsynaptic cell?
7. Why must it be continually removed from the synapse? and how?

Answer 10

1. Acetylcholine
2. Released at neuromuscular junctions and binds on receptors on muscle fibres to trigger muscle contractions.
3. To promote parasympathetic responses (rest and digest).
4. Created in the axon terminal by combining choline with an acetyl group (derived from mitochondrial Acetyl CoA).
5. Stored in vesicles in axon terminal, released via exocytosis.
6. Binds to either two classes of specific receptor (nicotinic or muscarinic).
7. As overstimulation can cause fatal convulsions and paralysis (removed via recycling or enzyme breakdown).

Question 11

1. What enzyme breaks down Acetylcholine?
2. Where is this enzyme present?
3. What occurs to the liberated choline?
4. What type of pesticides irreversibly bind to nicotinic acetylcholine receptors?
5. What do they trigger?
6. What does low activation of acetylcholine receptors promote?
7. Why are neonicotinoids more toxic to insects rather than mammals?

Answer 11

1. Acetylcholinesterase
2. Either RELEASED into the synapse from the presynaptic neuron OR EMBEDDED on the membrane of the post-synaptic cell.
3. Returns to the presynaptic neuron where it’s coupled with another acetate to reform acetylcholine.
4. Neonicotinoid pesticides.
5. A sustained response causing overstimulation of target cells as these pesticides can’t be broken down.
6. Promotes nerve signalling
7. They have a different composition of acetylcholine receptors which bind to neonicotinoids more strongly making them an effective pesticide.

Question 12

1. Disadvantages of neonicotinoids?
2. How do excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation?
3. How do inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation?
4. What does the combined action of all neurotransmitters acting on a target neuron determine?
5. What causes the neuron to fire?
6. What stops the neuron from firing?
7. (retrieval question) what is the threshold potential (required to open voltage-gated channels)?

Answer 12

1. Linked to reduced bee populations and reduced bird populations.
2. By opening ligand-gated sodium or calcium channels.
3. By opening ligand-gated potassium or chlorine channels.
4. It determines whether a threshold potential is reached.
5. If there is more depolarisation than hyperpolarization and a threshold potential is reached.
6. If there is more hyperpolarisation than depolarisation and a threshold potential is not reached.
7. Approximately -55mV.