6.5 - Neurons And Synapses

Question 1

Resting potential (5)

Hint SOPIPOANA

Answer 1

• sodium ions are actively pumped OUT OF the neuron
• potassium ions are actively pumped INTO the neuron
• potassium ions diffuse out of the axoplasm through an open facilitated diffusion channel
• negative ions remain in the axoplasm
• this generates the resting potential

Question 2

Action potential (2)

Answer 2

• two parts: depolarisation and repolarisation

- nerve impulses are action potentials propagated (passed) along the axons of neurons

Question 3

Depolarisation (5)

Hint: STCOSIANP

Answer 3

• stimulus causes sodium-gated facilitated diffusion channel to open briefly
• this allows a few sodium ions to diffuse into the axoplasm
• the axoplasm starts to become less negative
• if insufficient sodium ions diffuse in, it fails to reach the threshold potential and it returns to the resting potential
• if it reaches the threshold potential, it continues and changes from a negative potential to a positive potential

Question 4

Repolarisation (4)

Answer 4

• potassium-gated facilitated diffusion channel opens briefly
• more potassium ions can now diffuse out of the axoplasm making the axoplasm less positive
• the sodium ions that diffuse in are now pumped out
• the axoplasm returns to the resting potential

Question 5

Propagation of action potentials (2)

Answer 5

• travel in one direction has there is a refractive period after depolarisation preventing backward movement
• it must end in the opposite end to which it was initiated

Question 6

Transmission of an impulse

Answer 6

• when depolarisation occurs at a node of Ranvier, local currents stimulate the next node to become depolarised
• this is done by causing the voltage-gated sodium channel to open
• thus the impulse jumps from node to node
• this is called saltatory conduction

Question 7

Local currents (5)

Answer 7

• once nerve depolarised at axon, sodium ions concentration is high
• so ions diffuse to the neighbouring part that is still polarised
• outside concentration is opposite so they diffuse from the polarised part back to the part just depolarised
• reduces sodium concentration in the part of the neurone not yet depolarised, rising potential from -70 mV to -50 mV
• this is known as threshold potential as Na+ channels open, causing depolarisation

Question 8

Threshold potentials

Answer 8

Sometimes not enough neurotransmitter is secreted for this to be reached in the post-synaptic knob and this can sometimes be because several neurons joining are required to secrete neurotransmitter

Question 9

Synaptic transmission (9)

Answer 9

1. Action potential reaches the pre-synaptic knob
2. Depolarisation causes voltage-gated calcium channels to open so Ca+ ions move in by diffusion
3. Vesicles combing neurotransmitter (acetylcholine) move to and fuse with the membrane
4. Exocytosis occurs and acetylcholine diffuses across the synaptic cleft
5. The acetylcholine binds to receptors on the post-synaptic cleft and Na+ channels open
6. Na+ ions diffuse in causing an action potential in the post-synaptic neuron
7. Acetylcholinesterase breaks down acetylcholine into acetyl and choline
8. Acetyl and choline are reabsorbed into the pre-synaptic knob
9. Acetylcholine is regenerated in the Krebs cycle in mitochondria

Question 10

Neonicotinoids features (4)

Answer 10

• chemically similar to nicotine
• very effective insecticides
• not highly toxic to humans and other mammals
• the consequence in insects is paralysis and death

Question 11

Neonicotinoids (3)

Answer 11

• bind to the acetylcholine receptor in cholinergic synapses in the central nervous system of insects
• acetylcholinesterase does not break down neonicotinoids so binding is irreversible
• receptors are blocked, so acetylcholine is unable to bind and synaptic transmission is prevented

Question 12

Myelinated nerves (4)

Answer 12

• a nerve with an extra coating of myelin
• deposited by Schwann cells, consisting of many layers of phospholipid bilayer
• gaps between the myelin deposited by adjacent Schwann cells, known as the node of Ranvier
• impulses jump from one node to another, a process called saltatory conduction which increases the rate of transfer by 100x

Question 13

Myelin sheath

Answer 13

Insulating covering that surrounds axon with multiple spiral layers of myelin, discontinuous at the nodes of Ranvier

Question 14

Node of Ranvier

Answer 14

A gap in the myelin sheath of a nerve, between adjacent Schwann cells

Question 15

Axoplasm

Answer 15

The cytoplasm of of nerve axon

Question 16

Depolarization

Answer 16

A change from negative to positive

Question 17

Repolarisation

Answer 17

A change from positive to negative

Question 18

Saltatory conduction

Answer 18

In myelinated nerve fibres the nerve impulse can jump from one node of Ranvier to the next

Question 19

Synapse

Answer 19

Gap between neurons

Question 20

Cholinergic

Answer 20

Relating to nerve cells in which acetylcholine acts as a neurotransmitter

Question 21

Acetylcholinesterase

Answer 21

Breaks down acetylcholine into acetyl and choline

Question 22

Sensory neurons

Answer 22

Sensory neurons carry electrical impulses from receptors to CNS

Question 22

Motor neurons

Answer 22

Motor neurons carry electrical impulses away from the CNS to effectors

Question 23

What are neurons covered by

Answer 23

Neurons are covered by special cells which act as insulation, called the myelin sheath

Question 24

What are the small gaps between the myelin sheath called

Answer 24

Nodes of Ranvier

Question 25

When does depolarization only occur

Answer 25

Depolarisation occurs only if this is reached

Question 26

What are nerve impulses?

Answer 26

Nerve impulses are action potentials propagated along the axons of neurons, they move along as the depolarisation of one area triggers depolarisation in the neighbouring area