6.5 - Neurons and Synapses

Question 1

What is a neuron?

Answer 1

Cells that which transmit electrical impulses within the nervous system.

Question 2

Draw a neuron. Include dendrites, axons, cell body, nucleus and myelin sheath.

Answer 2

:D

Question 3

Which ions play a role in generating a resting membrane potential?

Answer 3

Sodium and potassium - BOTH POSITIVELY CHARGED.

Question 4

What is a membrane potential?

Answer 4

The unequal distribution of ions on different sides of the neuron membrane creates a membrane potential.

Question 5

What is a resting membrane potential?

Answer 5

The difference in charge across a neuron membrane when it is NOT firing an elecrical impulse. This is about -70 mV.

Question 6

Is the inside of a neuron more negative or positive compred to the outside?

Answer 6

Negative, about -70 mV.

Question 7

How is the resting potential maintained at -70 mV?

Answer 7

Using a membrane protein called the sodium-potassium pump!

• It expels 3 sodium ions and takes up 2 potassium ions
• This means that the inside of the neuron is slightly more NEGATIVE, since more positive ions are exiting the neuron

Question 8

Define an action potential.

Answer 8

The rapid changes in charge across the membrane that occur when a neuron is firing.

Question 9

What are the three main stages in an action potential?

Answer 9

Depolarization, Repolarization and a Refractory Period

Question 10

What happens during depolarisation, the first stage in an action potential?

Answer 10

Depolarisation is the sudden change in membrane potential – usually from a negative to positive charge

In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon.

Since Na+ ions are more concentrated outside of the neuron, the opening of SODIUM CHANNELS (NOT the sodium/potassium pump!!!!) causes a passive influx of sodium. NO ENERGY NEEDED.

The influx of sodium causes the membrane potential to become more positive.

Question 11

What happens during repolarisation, the second stage in an action potential?

Answer 11

Repolarisation is the restoration of a membrane potential following depolarisation, from positive back to negative.

Following an influx of sodium, potassium channels open within the membrane of the axon.

As K+ ions are more concentrated inside the neuron, opening potassium channels (NOT THE SODIUM POTASSIUM PUMP!) causes a passive efflux of potassium. Again NO ENERGY NEEDED.

The efflux of potassium causes the membrane potential to return to a more NEGATIVE internal charge, below the resting membrane potential of -70 mV.

Question 12

What happens during the refractory period, the third stage in an action potential?

Answer 12

This is the period of time following a nerve impulse before the neuron is able to fire again.

Before a neuron can fire again, the resting potential must be restored to -70 mV via the action of the sodium-potassium pump.

Question 13

What is a nerve impulse?

Answer 13

Nerve impulses are ACTION POTENTIALS that move along the length of an axon as a wave of depolarisation.

Depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon.

This causes depolarisation to spread along the length of the axon as a UNIDIRECTIONAL ‘wave’.

Question 14

What is the “all or nothing” principle in the propagation of a nerve impulse?

Answer 14

A minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels.

If the threshold potential is not reached, an action potential CANNOT BE GENERATED and hence the neuron will not fire!

Question 15

Draw a graph showing an action potential.

Answer 15

Perfect!

Question 16

What is myelination?

Answer 16

Axons may be covered by a fatty substance called myelin which functions as an INSULATING sheath layer.

The main purpose of the myelin sheath is to increase the speed of electrical transmissions.

Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation.

In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin heath called the nodes of Ranvier. THIS IS KNOWN AS SALTATORY CONDUCTION.

This results in an increase in the speed of electrical conduction by a factor of up to 100-fold

Question 17

What is saltatory conduction?

Answer 17

The propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction speed of action potentials.

Question 18

What is a synapse?

Answer 18

Synapses are the physical GAPS that separate neurons from other neuronal cells.

Question 19

What is a pre and post-synaptic neuron?

Answer 19

A presynaptic neuron is a neuron (nerve cell) that fires the neurotransmitter as a result of an action potential entering its axon terminal.

A postsynaptic neuron is a neuron that receives the neurotransmitter.

Question 20

What is a neurotransmitter?

Answer 20

A chemical which is released at the end of a nerve cell by the arrival of a nerve impulse. This substance can diffuse across the synapse and be received by the next neuron.

Question 21

How do neurotransmitters function?

Answer 21

1. When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels
2. Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitter) with the cell membrane
3. The neurotransmitters are released from the axon and cross the synaptic cleft
4. Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ion channels
5. The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal
6. The neurotransmitters released into the synapse are then degraded.

Question 22

What is an example of a neurotransmitter?

Answer 22

!! ACETYLCHOLINE !!

This neurotransmitter triggers MUSCLE CONTRACTION.

Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse.

Acetylcholine activates a post-synaptic cell by binding to receptors.

However…. acetylcholine must be continually REMOVED from the synapse, as overstimulation can lead to fatal convulsions and paralysis.

Acetylcholine is broken down into its two component parts by the synaptic enzyme acetylcholinesterase.

Acetylcholinesterase is released into the synapse from the presynaptic neuron and breaks down acetylcholine.

Question 23

A nerve impulse can only be generated if the _____________ potential is met.

Answer 23

Threshold

Question 24

Excitatory neurotransmitters (such as noradrenaline) cause ____________ . This leads to an action potential!

Answer 24

Depolarisation