Neurones and synaptic transmission

What is the resting potential of a neurone

-70mv

How is the resting potential maintained

Sodium potassium pump actively transports 3Na+ ions out of the axon and 2K+ into the axon

Creates an electrochemical gradient so K+ diffuses in and Na+ out

The membrane is more permeable to K+ ions because there are more K+ channels. so K+ ions diffuse out of the axon faster less permeable to Na+ so Na+ diffuses out slower

There are negatively charged proteins in the axon and a potential difference or -70 Mv

Define action potential

The momentary positive potential difference across the cell surface membrane of a neurone following its depolarisation

Describe how an axon becomes depolarised

Na+ enters the axon due to a stimulus

If enough Na+ ions enter the threshold is exceeded

This causes Na+ voltage gated channels to open

So Na+ ions move into the axon by facilitated diffusion down their electrochemical gradient until am action potential is reached

Fully depolarised 40

Describe what happens after action potential is reached until the axon returns to resting potential

Na+ voltage gated channels close and K+ voltage gated channels open

K+ move out of the axon by facilitate diffusion causing the axon to redepolarise

Too many k+ move out so axon becomes more negative than resting potential difference across-hyper polarisation K+ voltage gated channels close

The axon returns to resting potential

Describe how an action potential is generated and propagated in an unmyelinated axon

Na+ ions move into the axon by facilitated diffusion due to a stimulus /neurotransmitter

These ions repel positive ions already present in axon

There is diffusion of Na+ along the axon in both directions

Threshold reached and exceeded at adjacent region

VG Na+ channel open

More Na+ move in by FD

process repeats

Describe how an action potential is propagated in a myelinated axon

Na+ ions enter the axon at the first node of rangier causing the axon to become more positive inside

Influx of positive Na+ ions repels the positive ions already in the axon so they move toward the next node of ranvier

This causes threshold to be reached at the next node ,Na+ voltage gated channels open and Na+ moves by facilitated diffusion.

This process repeats down the axon

This is called salutatory conduction because the action potential jumps from one node of ranvier to the next

What is meant by the all or nothing principle

A threshold value must be reached im order for an action potential to occur . If depolarisation does not exceed -55 AP not produced

All action potentials same size/ peak at the same maximum voltage +40mv

How does the strength of a stimulus affect the frequency of action potentials

Stronger the stimulus the higher the frequency of the action potentials transmitted along a neurone

Above a certain strength of stimulus frequency of AP cannot get any higher

What is the refractory period

The period of time during which a new action potential cannot be generated

What factors contribute to the refractory period

The sodium ion voltage gated channels become inactive immediately after they were opened, so for a short period of time they cannot open again

The neuron is hyperpolarised so it is further away from threshold level required to generate an action potential

How does the refractory period ensure that impulses are kept discrete

It prevents a new action potential being generated immediately after the previous one because Na+ voltage gated channels will not open

-travel in one direction

How does the refractory period limit the frequency of impulse transmission

When +why does this matter

Prevents a new action potential being generated immediately after the previous one

This is relevant because if a stimulus strength increases the frequency of impulses increases. Because frequency of impulses cannot exceed a certain level it is not possible to differentiate between stimuli above a certain strength

Describe and explain how myelination and saltatory conduction affect the speed of conductance

Speed of conduction higher in myelinated neurones

Myelination provides electrical insulation

Ions only diffuse in and out at nodes of ranvier

So depolarisation can jump from one node of ranvier to the next by saltatory conduction

-in non myelinated depolarisation along the whole length of the axon

Describe and explain how axon diameter affects speed of conductance

The bigger the diameter the higher speed of conductance

This is because a bigger diameter means there is less resistance of movement of the action potential down axon

There also less leakage of potassium ions

Describe and explain how temperature affects the speed of conductance

The higher the temperature the higher speed of conductance

This is because the ions have more kinetic energy so the rate of difffusion of ions into the axon increases

Describe what happens at cholinergic synapses e

• action potential causes depolarisation of the pre synaptic membrane

• calcium ion voltage gated channels open so Ca2+ ions move in by facilitated diffusion

• Ca2+ cause synaptic vesicles to move towards the pre synaptic membrane and fuse with it

• Neurotransmitter acetylcholine is released by exocytosis

• Acetylcholine diffuses across the synaptic cleft

• Acetylcholine binds to the receptors on ligand gated Na+ channels on the post synaptic membrane

• Na+ channels open

• Na+ ions move into post synaptic neurone by facilitated diffusiom down their electrochemical gradient causing it to depolarise

• If threshold is exceeded voltage gated Na+ ion channels open so more Na+ ions move in by facilitated diffusiom and a new action potential is generated in the post synaptic neurone by

• Acetylcholinesterase hydrolyses acetylcholine to form ethanoic acid and choline

• These are reabsorbedd into the pre synaptic neurone by

• ATP is used to regenerate Acetylcholine using ethanoic acid and choline

What is meant by unidirectionality

Impulses only travel in one direction across synapses and along axons

Describe how a synapse ensures the unidirectionality of impulses

Only the pre synaptic neurone contains synaptic vesicles with neurotransmitter

Only post synaptic neurone has receptors which neurotransmitters can bind to

What is an excitatory synapse

When Na+ ions move into the post synaptic neurone so it becomes more positive inside making it more likey that an action potential is generated in post SN

What is an inhibitory synapse

When post synaptic neurone becomes more negative

*may bind to receptors because complementary on post sm prevent binding

Due to K+ ions moving out or Cl- ions moving in

More Na+ required in order for an action potential to be reached

Less likey that an action potential trial is generated im the post synaptic neurone

What is spatial summation

When there is more than one pre synaptic neurone synapsedto the same post synaptic neurone

-more than one neuron at same synapse

Describe how spatial summation makes it more likely for an action potential to occur in the post synaptic neurone

Neurotransmitter from both pre synaptic neurones bind to receptors on ligand gated channels on post synaptic neurone

So sufficient sodium ion ligand gated channels open

So sufficient sodium ions move into the post synaptic neurone by facilitated diffusion

So threshold is exceeded and an action potential is generated

What is temporal summation

When the pre synaptic neurone has a high frequency of action potentials

Describe how temporal summation makes it more likey for an action potential to occur in the post synaptic neurone

A large amount of neurotransmitter is released by the same pre synaptic neurone in a short space of time

So sufficient sodium ion ligand gated channels open

So sufficient sodium ions move into the post synaptic neurone by facilitated diffusion so threshold can be exceeded and action potential generated