1.6 Neurones and Synapses

Labe the structure of a neurone

What state is a neurone in at rest?

Inside negatively charged WRT the outside - resting potential -70mV

Membrane is polarised bc of the difference in charge

Caused by excess of positively charged Na ions in fluid surrounding neurone

Difference in charge is maintained because at rest the membrane is impermeable to the flow of ions

What is an action potential & what happens?

A temporary reversal of the resting potential when the neurone is stimulated

Happens because some gated ion channels open, making the membrane more permeable to ions

Pos ions diffuse into the axon down the conc. gradient

• Difference in charge is reduced

• This change in voltage is the generator potential

Weak stimulus → few ions diffuse in & generator potential quickly declines

Strong stimulus → Ions diffuse in until voltage reaches -40mV

At the threshold value of -40mV permeability increases sharply (more channels open) so more ions diffuse in

A difference in charge of +40mV is created (pos inside WRT outside)

• This is the action potential

• Membrane is depolarised

Sudden influx of ions at 1 point on the axon membrane creates local circuits which increases permeability of the next bit of membrane to ions

• This is repeated along the neurone

• Wave of depolarisation

• Impulse travels down the axon

What is the all or nothing law?

An action potential will only occur if the initial stimulus exceeds a threshold value -40mV

A stronger stimulus will leader to a greater frequency of impulses, not a bigger action potential

What is the refractory period?

Hyperpolarisation prevents the neurone from being restimulated instantly

• Ions are diffused out of the membrane using ion pumps

• The quickly repolarises the membrane

• Membrane is now impermeable again

Pumps overcompensate until equilibrium is reached again

• Voltage will drop below -70mV for a short time

• This is hyperpolarisation

This comes with a period of inexcitability where inward movement of ions is prevented

Why is the refractory period important?

• Ensures the action potential is unidirectional which prevents action potential from spreading out in both directions

• Limits number of action potentials so there’s a maximum frequency achievable

• A new action potential can’t be immediately formed after the first one

What is a nerve impulse?

A wave of depolarisation spreading rapidly along a neurone

How is a nerve impulse propagated?

As each region of the membrane becomes depolarised it sets up a local circuit that triggers an action potential in the next part of the membrane

Positive ions from the depolarised zone pass along the inside of the membrane towards to polarised zone in front

On the outside of the membrane positive ions move back from the polarised zone to the depolarised zone

What are the factors affecting speed of propagation?

• Large nodes of Ranvier increases speed because the impulse jumps from node to node- Saltatory conduction

• Large diameter transmits impulses quicker because of a greater surface area for ion exchange & proportionally less leakage of ions

Label the structure of a synapse

Describe the process of synaptic transmission

• When an impulse arrives at the end of the presynaptic neurone, the membrane becomes permeable to calcium ions which diffuse into the bulb

• Influx of calcium ions causes vesicles to fuse with the presynaptic membrane

• Thus releasing the neurotransmitter molecules by exocytosis into the synaptic cleft

• These molecules diffuse across the synaptic cleft to the post synaptic membrane

• The molecules fit into receptor sites on the sodium ions channels

• This causes them to open, allowing ions to diffuse into the postsynaptic neurone

Altering the potential difference in the post

Magnitude of this change depends on amount of transmitter released

• Influx of ions begins to depolarise the membrane & generates an excitatory post synaptic potential (EPSP)

If threshold value is reached → an action potential is produced

• Acetylcholinesterase (on the post) hydrolyses the acetylcholine into acetyl and choline

• They diffuse back across the cleft & are taken up again by the pre

The breakdown prevents post neurone from continuously generating new action potentials

• Mitochondria in the synapse produces ATP to recombine the acetyl and choline. The it’s stored in the vesicles for future use

• The sodium ions channels on the post membrane have closed

Outline the functions of synapses

• Allow an impulse to be transmitted from one neurone to another

• Pass impulses unidirectionally

• Act as junctions- several neurones converge at one synapse & together generate an action potential whereas they individually wouldn’t

• Can be inhibitory

Outline the development of an IPSP

The neurotransmitter GABA is released from the pre membrane by exocytosis, diffuses across the cleft & fits into receptors on the post membrane → causing chloride channels in the post membrane to open → the chloride ions diffuse into the post neurone, causing the development of an inhibitory post synaptic potential. Inside of neurone becomes hyperpolarised, making it less likely for an action potential to be generated in the post neurone

Outline the stimulating effect of chemicals in synapses

Stimulate the nervous system by creating more action potentials in post synaptic neurones

The drug may:

• be a similar shape to a neurotransmitter

• stimulate the release of more neurotransmitter

• inhibit the enzyme breaking down the neurotransmitter

eg: nicotine stimulates release of acetylcholine so more action potentials occur

Outline the inhibitory effect of chemicals in synapses

Inhibit the nervous system by creating fewer action potentials in the post synaptic neurones

The drug may:

• Inhibit the release of a neurotransmitter

• Block receptors on the post membrane so the normal transmitter can’t fit

eg: opioids block the calcium ion channels in the pre neurone so less neurotransmitter is released → action potential less likely