How Nerves Work 3
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When is an action potential sent?
When the resting membrane potential of -70mV depolarises to threshold at around -55mV
What causes the depolarisation of the resting membrane potential?
External stimuli acting on specific sets of ion channels to create graded potential where the size of potential is related to the size of the stimulus
What is a synapse?
A junction/gap between 2 neurones where the action potential releases transmitter molecules that activate receptor on the second cell which open ion channels creating another graded potential
What happens at an NMJ?
Motor neurone depolarises the muscle to threshold by evoking a graded potential known as the endplate potential
What are examples of graded potential?
• Generator potentials at sensory receptors • Postsynaptic potentials at synapses • Endplate potentials at NMJ (skeletal muscle) • Pacemaker potential in pace maker tissues
What are properties of graded potentials?
• Electrotonic potentials • Decremental potentials • Non-propagated potentials • Local potentials
Why are graded potentials decremental and non-propagated?
As you further away from the site of stimulus, more current leaks out along the membrane and as a result the membrane potential gets smaller and smaller
Where can graded potential signal stimulus intensity?
In their amplitude
What is meant by graded potentials being graded?
A stronger stimulus would open more channels, creating a bigger current flow and therefore a bigger potential
Why can graded potentials be depolarising or hyperpolarising?
• Firing an AP depends on reaching a firing threshold. - Graded potentials can therefore excite or inhibit a cell
What do EPSPs or IPSPs depends on?
Which ion channels are open
Opening which ion channels would cause hyperpolarisation?
Potassium and chlorine
What would opening potassium and chlorine channels do?
• Causes hyperpolarisation • Takes them away from threshold • IPSP
What would opening Na and Cl channels do?
• Causes depolarisation • Takes then towards threshold • EPSP
What are ligand-gated ion channels?
Postsynaptic potentials are produced by a neurotransmitter opening or closing ion channels
What are voltage-gated ion channels?
Action potentials are produced by depolarisation of the membrane potential opening ion channels
Why are graded potentials important in synaptic integration?
Graded potentials can summate
What is temporal summation?
One stimulus that is stimulated again quickly and the second EPSP adds on.
What is spatial summation?
When 2 separate stimuli add together
What is integration
Looking at all of the inputs and deciding whether or not to send one of those long distance signals (AP)
What is synaptic integration?
The process of summing all those inputs in space and time to determine whether or not the initial segment reaches threshold
What do IPSPs tend to do?
Stop the cell reaching threshold