Action potentials and synapses

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synapses

• communication between the neuron

• the connection between the neuron

graded potentials

• signals travelling down the dendrite and cell body

action potential or nerve impulse

• signal that does down the axon, which leaves at the axon terminals and starts the whole process again

Synapse can be electrical or chemical

Electrical

• gap junctions

• faster

• closer together —> ion channels close together

• as mammals, not very common

Chemical

• slower

• lead to much more complex connection

• common

How a chemical synapse works out

1. Nerve impulse arrives at the axon terminal

   1. action potential has come down the neuron, and that will lead that neuron to synapse with the next neuron

   2. this is an increase elec current

2. voltage gated calcium channels open, and calcium floods the cell

   1. open ups when it is positively charged

3. Increase in calcium activates the synaptic vesicles (little bubble things)

   1. these contain chemicals called neurotransmitters

   2. activation of the vesicles leads to release of NT in the synaptic cleft

   3. the little things merge with the cell (process called exocytosis) wall int o the synaptic cleft

4. the neurotransmitter in the synaptic cleft is free to bind to receptors on the postsynaptic neuron (little green trees)

   1. specific neurotransmitters will bind to specific types of receptors

5. When the neurotransmitter binds to a receptor, in the example shown a ligand-gated ion channel opens.

   1. this allows the movement of ions from outside the cell to inside the cell

   2. this example allows sodium to enter the cell (this is positively charged) - this is called excitation, and more likely to fire

   3. other neurotransmitters e.g chloride channels which are negatively charged inhibits the cell, less likely to fire

6. This excitation of inhibition of a cell is known as a post synaptic potential (or graded potential)

Neurotransmitters are generally returned to the presynaptic cell (recycling!)

Forms of synaptic communication:

• we’ve talked about synapsing onto a dendrite

This is not the only type of communication that can happen!

a) axondendritic —> you synapse onto a dendrite (as the dendrite’s main purpose is to pick up information

b) axosomatic —> onto the soma

c) axoaxonic —> onto another axon

Graded potentials

• neurons get information from other cells (at synapses), changing the electrical charge of the cell membrane

• Decreases in voltage are inhibitory Post-Synaptic Potentials (IPSPs) of the cell membrane

• Increases in voltage are Excitatory Post-Synpatic Potentials (EPSPs)

  

• Graded potentials (EPSPs and IPSPs) can move across the cell membrane towards the axon hillock (vrooommmm)

• information is summed so IPSPs and EPSPs can cancel each other out

  • inhibatory and exitatory information, and no action potential will fire

Action potentials

• An action potential will only fire once the membrane potential threshold is reached at the axon hillock

• all that information from multiple inputs is being summed together

• It is a nerve impulse, a rapid electrical signal that travels down a single cell

• it is about the movement of ions

Membrane resting potential is ~-70mV

Membrane threshold is ~-55mV

It all starts with the cell membrane:

• the soma or cell body, the phosphholipid bilayer maintains the negative charge within the cell

• forms a phospholipid Bilayer

The cell membrane is impermeable to movement

• outside the cell there is a lot of sodium

• inside the cell there is a lot of potassium

• the cell membrane is negatively charged (-)

• resting potential is -70mV

Concerntration gradient

• cell membrane precents ions moving around

• permeable membrande will diffuse evenly

• Positive ions are drawn to neg charged spaces

Diffusion: based on the concerntration gradient

Elec Potential Diff: Whether it is POS or NEG

Important things to remmeber about ions

• the cell is resting potential is negatively charged (-70mV)

• the ions are prevented from moving around by the phospholid bilayer

• sodium wants to get into the cell (55mV)

• Potassium is pretty close to its equilibrium potential (-90mV), but wants to drive cell into more negative space

how the action potential looks

When you get a large enough post synaptic potential, and its HIT THE THRESHOLD for generating an action potential.

1. voltage gated sodium channels opening up

2. we know that sodium loves being in the cell, so sodium floods in

3. this leads to depolarization of the membrane which means the membrane gets more positively charged to around 40 mV

   1. this is below where sodium wants to be, this is known as OVERSHOOTING, region above 0.

   2. the reason it doesn hit 55mV where it wants to be is the channels only open very briefly, and also the Potassium is hating the change. It wants the cell to be negatively charged

4. Potassium pushes its way through the channels into the cell to make it negatively charged

   1. this is repolarisation, rapid reduction in voltage of the cell

   2. this is hyperpolarisation, or an UNDERSHOOT.

5. Refractory period when it levels out

   1. think le chatelier’s principle

Message over 1m can be sent in 7ms. the process explained above of the graded potentials happens down the axon tail, until it hits the next neuron.

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