6.5 Neurons and synapses

Question 1

What is a synapse?

Answer 1

A synapse is the junction between two neurons

Question 2

What are neurons?

Answer 2

Cells that carry rapid electrical impulses

Question 3

Explain how a nerve impulse passes along a non-myelinated neuron (8 marks)

Answer 3

Action potential activates voltage gated sodium channels; sodium ions rush in to axon
Potential increases; If it increases beyond threshold, more sodium channels open; Axon depolarises, stimulating adjacent sections; Potassium channels open, potassium ions rush out; Potential is reduced (repolarisation); Refractory period ensures one-way conduction of action potential; sodium-potassium pump returns axon section to resting potential.

Question 4

Explain principles of synaptic transmission (8 marks)

Answer 4

nerve impulse (AP) travels to end of presynaptic neuron
triggers influx of Calcium ions
causes synaptic vesicles to fuse with membrane
release neurotransmitter molecules into synaptic celft
neurotransmitter crosses/diffuses across channel
neurotransmitter binds to receptors on next / postsynaptic neuron
causes ion channels to open on post-synaptic neuron
Sodium ion diffuse into postsynaptic neuron
can inhibit/excite
by hyperpolarising/depolarizing
neurotransmitter degraded
Calcium ion pumped back into the synaptic cleft by active transport
acetylcholine / GABA / dopamine / serotonin / other examples of neutrotransmitter

Question 5

Outline the use of four methods of membrane transport in nerves and synapses (8 marks)

Answer 5

Active transport
- sodium-potassium pump resets resting potential in the axon following nerve impulse
- re-uptake of neurotransmitters to the pre-synaptic neuron following synaptic transmission
- removal of calcium ions from pre-synaptic neuron following synaptic transmission

Simple Diffusion
- diffusion of NT across synaptic celft
- diffusion of potassium ions out of axon in resting potential

Facilitated diffusion
- opening of voltage gated sodium and potassium ions channels in action potential
- opening of voltage gated calcium ion channels at pre-synaptic terminal
- sodium ions channels activated at post-synaptic terminal to propagate AP

Vesicle Transport
- influx of calcium activates vesicles of neurotransmitters
- exocytosis of NT from pre-synaptic neuron to synaptic cleft

Question 6

What are the three main basic components of neurons?

Answer 6

Dendrites – Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals
Axon – An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors
Soma – A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

Question 7

What is resting potential?

Answer 7

A resting potential is the difference in charge across the membrane when a neuron is not firing

Question 8

The maintenance of a resting potential is an ________ process

Answer 8

active

Question 9

Explain the process which maintains the resting potential

Answer 9

The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps

The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)
It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)
This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)
The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)