Neurons

Question 1

What do neurons do?

Answer 1

Mediate internal communication by transmitting nerve impulses

Question 2

What are nerve impulses?

Answer 2

Action potentials that travel along the axons of neurons

Question 3

Draw the structure of neurons

Answer 3

See image in textbook

Question 4

What is the myelin deposited by?

Answer 4

Schwann cells which grow around the nerve fibre

Question 5

What are the gaps in the myelin sheath called?

Answer 5

Nodes of Ranvier

Question 6

What is the function of myelin?

Answer 6

It provides insulation and speeds up the transmission of the signal. In myelinated axons, the impulse jumps from one node of Ranvier to the next. This is called saltatory conduction and is much quicker than continuous transmission in unmyelinated nerve fibres

Question 7

How is transmission initiated?

Answer 7

When an input is received (neurotransmitter binding to receptor site)

Question 8

What is the resting potential?

Answer 8

The electrical potentials across the plasma membrane of a neuron when it is not conducting an impulse

Question 9

Describe resting potential

Answer 9

-The inside of the neuron has a negative charge compared to the outside
- A typical resting potential is -70mV
- The resting potential is generated by the pump Na+/ K+ ATPase by active transport
-The Na+/ K+ ATPase uses the energy from the hydrolysis of one ATP molecule to pump two potassium ions in the cell and three sodium ions
- Concentration gradients of Na+ and K+ ions

Question 10

Describe action potential

Answer 10

• The transmission is initiated by a stimulus
  -In response the membrane potential at a part of the axon rises from -70mV to -50mV. The potential of -50mV is called the threshold potential. An action potential is initiated only if the threshold potential is reached
• When the threshold potential is reached, depolarisation happens
• Depolarisation is the reversal of the electrical potential across the membrane- the inside becomes positive and the membrane potential rises to +40mV
• Depolarisation happens because voltage-gated sodium channels open and sodium ions diffuse into the neuron down the concentration gradient
• Repolarisation is the reversal of the electrical potential across the membrane
• Repolarisation happens rapidly after depolarisation. It is due to the closing of sodium channels and opening of potassium channels and the diffusion of K+ ions out of the axon down the concentration gradient.
• THere is a delay in the closing of potassium channels, which results in hyperpolarisation: the potential falls close to -80mV
• The resting potential is restored by the Na+/ K+ pump after some milliseconds. This period is called the refractory period. This ensures that impulses only travel in one direction

Question 11

Describe local currents

Answer 11

Propagation of the action potential happens because when one part of a neuron depolarises, it triggers depolarisation in the neighbouring part. The propagation happens due to diffusion of Na+ ions between the region with an action potential and adjacent region with a resting potential, in both sides of the membrane. The diffusion of Na+ ions is called local currents. Local currents reduce the concentration gradient in the part that has not yet depolarised, rising the membrane potential from -70mV to the -50mV threshold. This causes the sodium channels to open, propagating the impulse

Question 12

How can membrane potentials be measured?

Answer 12

Oscilliscope

Question 13

What are synapses?

Answer 13

Junctions between neurons and between neurons and receptor or effector cells

Question 14

Draw a synapse

Answer 14

Take a look it from the notes

Question 15

What are neurotransmitters?

Answer 15

Chemicals used to send signals across synapses.

Question 16

Describe the steps of a synaptic transmission

Answer 16

• A nerve impulse is propagated along the pre-synaptic neuron until is reaches its end
• Depolarisation of the pre-synaptic membrane causes calcium ions to diffuse into the neuron
• Influx of Ca2+ ions causes vesicles containing neurotransmitters to move to the pre-synaptic membrane and fuse with it
• The neurotransmitter is released by exocytosis into the synaptic cleft
• The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the post-synaptic membrane
• Binding of the neurotransmitter to the receptor causes Na+ channels to open
• Na+ diffuse into the post-synaptic neuron, causing the post-synaptic membrane to reach the threshold potential
• To prevent continuous transmission, the neurotransmitter is rapidly broken down by enzymes and removed from the synaptic cleft.

Question 17

What is acetylcholine?

Answer 17

Neurotransmitter used in many synapses including the neuromuscular junction

Question 18

What are cholinergic?

Answer 18

Synapses that use acetylcholine

Question 19

Where is acetylcholine is prodcued?

Answer 19

In pre-synaptic neurons by combing choline and an acetyl group

Question 20

Where is acetylcholine secreted from?

Answer 20

It is secreted in the synaptic cleft where it diffuses and binds to acetylcholine receptors in the post-synaptic cell

Question 21

Describe the enzyme acetylcholinesterase

Answer 21

Acetylcholine only remains bound for a short time because the enzyme acetylcholinesterase breaks it down into choline and acetate. The choline is reabsorbed by the pre-synaptic neuron where it is converted again into active acetylcholine.

Question 22

What are neonicotinoids?

Answer 22

Synthetic compounds that are used as insecticides

Question 23

Describe the action of neonicotinoids

Answer 23

They bind to acetylcholine receptors in cholinergic synapses of insects, but cannot be broken down by acetylcholinesterase. As a result, acetylcholine receptors are blocked, acetylcholine cannot bind, synaptic transmission is prevented and the insects paralyse and die.