6.5 Neurons and Synapses

Question 1

neurons

Answer 1

cells that carry rapid electrical impulses

Question 2

Dendrites

Answer 2

short, branched nerve fibres that receive impulses from other neurons

Question 3

Axons

Answer 3

elongated nerve fibres that transmit impulses throughout a neuron

Question 4

Resting Potential

Answer 4

A neuron is at rest when it is not transmitting a signal
There is still a membrane potential = potential or voltage difference across the membrane
Membrane potential is due to an imbalance of positive and negative charges across the membrane
Resting potential is maintained by active transport by the sodium-potassium (Na+/K+) pump (antiport)
3 Na+ ions pumped OUT
2 K+ ions pumped IN
This creates a concentration gradient for each ion

The membrane is ~50X more permeable to K+ ions than Na+ ions
K+ ions leak back across the membrane (OUT) faster than Na+ ions
Na+ gradient is steeper than the K+ gradient, which creates a charge imbalance
Additionally, negatively charged proteins in the neuron increases the charge imbalance in the axon
Resting membrane potential = -70 mV

Question 5

Action potential

Answer 5

Rapid change in membrane potential
Depolarization = reversal of charge from negative to positive
Repolarization = restoration of charge from positive to negative

Question 6

Depolarization

Answer 6

Some Na+ channels open
Na+ ions diffuse INTO neuron
Inside the cell becomes positive relative to the outside
If a sufficient change in membrane potential is achieved (threshold potential) all the voltage gated Na+ channels open.
Membrane potential = +30mV

Question 7

Repolarization

Answer 7

Na+ channels close
K+ channels open
K+ ions diffuse OUT OF neuron
Inside the cells becomes negative relative to the outside
Membrane potential = -70mV

Question 8

Refractory Period

Answer 8

Membrane potential (electrical gradient) is restored (-70mV) but ions are not in the correct location
Na+/K+ pump re-establishes the chemical gradient
This resetting ensures that impulses can only travel in one direction

Question 9

Threshold Potential

Answer 9

Nerve impulses follow an all-or-nothing principle
An action potential is only initiated if the threshold potential is reached → causes voltage gated Na+ channels to open
If threshold is reached → full depolarization

Question 10

Propagation of a nerve impulse in un-myelinated axons

Answer 10

Depolarization causes the first part of the neuron to have different Na+ concentrations than the neighbouring part
Causes Na+ ions to diffuse between these regions both inside and outside → local currents
Local currents reduce the concentration gradient in the part of the neurons that is not yet depolarized
Changes the membrane potential from -70mV to -50mV → threshold potential
Causes a wave of depolarization that propagates the action potential

Question 11

Propagation of a nerve impulse in myelinated axons

Answer 11

Myelin acts as an insulator
Myelinated axons only allow action potentials to occur at the unmyelinated nodes of Ranvier.
This forces the action potential to jump from node to node (saltatory conduction).
The “jump” along the axon is actually just the very rapid conduction inside the myelinated portion
Result = impulse travels much more quickly (up to 200 m/s) along myelinated axons compared to unmyelinated axons (2 m/s).
Reduces degradation of the impulse → allows the impulse to travel longer distances
Reduces energy expenditure over the axon as the quantity of Na+/K+ ions that need to be pumped to restore resting potential is less

Question 12

synapses

Answer 12

junctions between neurons and other neurons/effector cells

Electrical signals “jump” across a synapse as chemical signals (neurotransmitters)
Synapses span from axon terminals of the pre-synaptic neurons to dendrites of the post-synaptic neuron

Question 13

neurotransmitters

Answer 13

chemical signals

Question 14

Synaptic Transmission

Answer 14

Nerve impulse is propagated along the pre-synaptic neuron → reached the end of neuron
Depolarization causes Ca2+ ions to diffuse into the pre-synaptic neuron
Influx of Ca2+ ions causes vesicles containing neurotransmitters (NTs) to exit the pre-synaptic cell via exocytosis
NT diffuses across the synaptic cleft and binds to receptors on the post-synaptic neuron
Binding of the NT causes Na+ channels to open
Na+ ions diffuse into the post-synaptic neuron → neuron reaches threshold potential
Action potential is triggered in the post-synaptic neuron and is propagated along the neuron
NT is broken down and removed from the synaptic cleft

Question 15

Synaptic Transmission and Threshold Potential

Answer 15

Amount of NT secreted may not enough to cause the threshold potential to be reached in the post-synaptic neuron
Na+ ions pumped out of the post-synaptic neuron by the Na+/K+ pump
Post-synaptic neuron returns to resting potential
Typical post-synaptic neurons have synapses with many pre-synaptic neurons
May need many pre-synaptic neurons to release NTs at the same time to ensure threshold potential is reached

Question 16

Acetylcholine

Answer 16

neurotransmitter used in many synapses through the nervous system

Question 17

neonicotinoid pesticides

Answer 17

Neonicotinoids are synthetic compounds similar to nicotine. They
bind to the acetylcholine receptor in cholinergic synapses in the
central nervous system o insects. Acetylcholinesterase does not break down neonicotinoids, so the binding is irreversible. The receptors are blocked, so acetylcholine is unable to bind and
synaptic transmission is prevented. The consequence in insects is paralysis and death.

Question 18

advantages o neonicotinoids

Answer 18

not highly toxic to humans and other mammals. This is because
a much greater proportion o synapses in the central nervous
system are cholinergic in insects than in mammals and also because
neonicotinoids bind much less strongly to acetylcholine receptors in
mammals than insects.