Neurons and Synapses 6.5

Question 1

What do neurons do?

Answer 1

• Carry messages at high speed in the form of electrical impulses.
• Many are very elongated and carry impulses long distances in a very short time.

Question 2

What are the 2 main devisions of the nervous system?

Answer 2

• Peripheral nervous system (PNS) = peripheral nerves

- Central nervous system (CNS) = brain and spinal cord

Question 3

What is saltadory conduction?

Answer 3

• myelinated fibres have a myelin sheath eith small gaps called nodes of Ranvier
• this allows the nerve impulse to jump from node to node
• speeds up the transmission

Question 4

What is the structure of a motor neuron?

Answer 4

1. dendrites (short branched nerve fibres)
2. cell body
3. nucleus
4. axon
5. nodes of Ranvier (myelin sheath) (shwann cells)
6. exon terminals

Question 5

What is a synapse?

Answer 5

• a junction between two neurons or a junction between neruons and receptor or effector cells.
• the plasma membranes of the neurons are separated by a narrow fluid-filled gap called the synaptic cleft.
• messages passed across the synapse in the form of chemicals called neurotransmitters
• pre-synaptic——–>post-synaptic

Question 6

How does a synapse function?

Answer 6

1. Nerve impulse reaches the end of the pre-synaptic neuron.
2. Vesicles of neurotransmitters move to the membrane and release their contents.
3. Neurotransmitter diffuses across the synaptic cleft and binds to receptors.
4. Sodium ions enter the post-synaptic neuron and cause depolarization.
5. Nerve impulse setting off along the post-synaptic neuron.
6. Neurotransmitter is broken down in the cleft and reabsorbed into the vesicles.

Question 7

What is a cholinergic synapse?

Answer 7

• Synapses that use acetylcholine as their neurotransmitter.

Question 8

What are neonicotinoid pesticides and why are they dangerous?

Answer 8

• Neonicotinoids bind to acetylcholine receptors in the postsynaptic membranes of cholinergic synapses in insects.
• Cholinesterase does not break down the pesticides, they remain bound to the receptors preventing acetylcholine from binding.
• This blocks synaptic transmission, killing the insect.
• Honey bee are killed along with other insects that are the intended target of neonicotinoids.

Question 9

What are membrane potentials?

Answer 9

• Neurons have a difference in charge across their membranes due to the distribution of positively charged ions (Na+ & K+)
• Electrical signals are created by changing membrane polarity.
• Resting Potential (Polarity of a neuron when it is at rest) -70mV
• Action Potential (Polarity of a firing neuron) +30mV

Question 10

What is myelination?

Answer 10

• Nerve impulses are Action Potentials propagated via axons.
• Action Potentials are only propagated if a certain threshold potential is reached (~-55mV)
• The axon is covered by a myelin sheath.
• Enables saltatory conduction (speeds up transmission)
• The Action Potential jumps between gaps in the myelin sheath (nodes of Ranvier) for faster transmission.

Question 11

What is the Resting Potential?

Answer 11

• Maintained by a Na+/K+ pump

- It exchanges sodium ions (3 out) and potassium ions (2 in) so that the membrane potential becomes slightly negative.

Question 12

What is the Action Potential?

Answer 12

• Changes the resting membrane potential
• The opening of sodium channels causes a sodium influx
• This creates a positive membrane potential (depolarization)
• Opening potassium channels causes a potassium efflux
• This restores a negative membrane potential (repolarisation)

Question 13

How is a nerve impulse propagated along an axon?

Answer 13

1. A nerve impulse is an action potential that travels along the bacon of a neuron from one end to the other.
2. An action potential in one part of the axon triggers an action potential in the next part.
3. This is called propagation of the nerve impulse.
4. This is due to diffusion of sodium ions between a region with an action potential and the next region that is still at a resting potential.
5. The diffusion of spdoum ions along the axon, both inside and outside the membrane, is called local currents.
6. This changes the voltage across the membrane from -70mV to the threshold potential -50mV.
7. Causing an action potential because voltage-gated sodium channels open.

Question 14

How does a nerve message pass from one neuron to the next?

Answer 14

1. Nerve impulse reaches the end of the presynaptic neuron
2. Depolarization causes calcium channels in the membrane to open
3. Calcium diffuses into the presynaptic neuron
4. Vesicles containing the neurotransmitter move and fuse with the presynaptic membrane
5. The neurotransmitter is released into the synaptic cleft by exocytosis
6. The neurotransmitter diffuses across the synapse and attaches to receptors on the postsynaptic neuron
7. Receptors cause ion channels to open and sodium diffuses into the postsynaptic neuron
8. The postsynaptic neuron is depolarized which causes a new action potential
9. The neurotransmitter on the psotsynaptic membrane is broken down and reabsorbed into the presynaptic neuron