NMS Lecture 5 - Synapses

Where Are neurotransmitters located in the neuron

Inside vesicles at the pre-synaptic terminal

Where are neurotransmitter receptors located?

On the post synaptic membrane

Two types of chemical synaptic transmission

Directly gated and indirectly gated, synaptic transmission

What is the most common type of chemical synaptic transmission?

Directly gated, chemical synaptic transmission

Two excitatory neurotransmitters

acetylcholine and glutamate

2 inhibitory neurotransmitters

GABA and glycine

What neurons are excitatory neurons

Afferent and efferent neurons

What type of neurons are inhibitory neurons?

Interneurons

Directly gated, chemical synaptic transmission

Pre-synaptic neurotransmitters are released, bind to post synaptic receptors, which directly opens up an ion channel

What type of ion is an excited ion channel?

Sodium - as this depolarizes the cell

What type of ion enters and inhibitory ion channel?

Chloride or potassium - as this hyper polarizes the cell

excitatory postsynaptic potential (EPSP)

a slight depolarization of a postsynaptic cell, bringing the membrane potential of that cell closer to the threshold for an action potential

inhibitory postsynaptic potential (IPSP)

a slight hyperpolarization of a postsynaptic cell, bringing the membrane potential of that cell further to the threshold for an action potential

Voltage of post synaptic potentials

is only a small voltage change not enough to reach the threshold

Effects of directly gated chemical, synaptic transmission

Effects are fast in onset and short lasting.

Indirectly gated, chemical synaptic transmission the receptor and effector are the same molecule. What does this mean?

The receptor is located on the ion channel neurotransmitter binding opens up the ion channel

Indirectly gated, chemical synaptic transmission

Neurotransmitter binds to receptor and activates secondary Messenger system in post synaptic cell. This messenger system is usually G proteins.

Steps of indirectly gated chemical synaptic transmission

1. Neurotransmitters binds to receptor converting GDP to GTP.
2. GTP binds to adenyl cyclase, converting AMP to cAMP
3. cAMP activates cAMP dependent protein kinase
4. Protein kinase phosphorylates a protein channel
5. Protein channel opens, allowing for depolarization or hyperpolarization.

Effects of indirectly gated chemical synaptic transmission

Slow onset and long lasting. More complex can lead to longer-term changes in a cell such as learning

Chemical synapses vs. electrical synapses

Electrical synopsis cannot change, Are inflexible, And are always excitatory

Chemical synopsis are flexible, more complex (directly and indirectly gated) And can generate inhibitory signals

What is plasticity?

The process of indirectly gated chemical synaptic transmission leading to long-term changes in neurons. This is how learning occurs.

Steps of synaptic transmission

1. Action potential in pre-synaptic neuron reaches the axon terminal.
2. Axon terminal depolarizes opening up voltage-gated calcium channels.
3. Calcium enters the pre-synaptic cell, causing vesicles containing neurotransmitters to fuse to the terminal of the pre-synaptic membrane
4. Now transmitter is released into the synaptic gap by exocytosis
5. Neurotransmitter diffuses across the synaptic cleft and binds to the post synaptic receptor channels
6. An EPSP or IPSP is generated in the post synaptic cell
7. Neurotransmitter is released from the receptor, and is degraded or recycled

Neurotransmitter recycling

Take it up by pre-synaptic cell to be recycled

Neurotransmitter degradation

Taken by glial cells to be degraded

Excitatory pre-synaptic neuron

Releases glutamate or acetylcholine to be taken up by post-synaptic cell. Binding of neurotransmitter opens up sodium channels, sodium rushes into post synaptic cell and create an EPSP.

Inhibitory pre-synaptic neuron

Releases gabba or glycine to be taken up by post synaptic cell. Binding of neurotransmitter opens up chloride channels. Chloride rushes into the cell and creates an IPSP

How strong are EPSPs/IPSPs?

They are sub threshold, only around 0.5mV

If EPSP/IPSP are so small, how do they generate an action potential?

Synaptic integration

Synaptic integration

Summation of post synaptic potentials. Many post synaptic potentials can occur on a neuron at once and they are additive.

EPSP/IPSP decay

Snick potential decay as they move away from the synaptic cleft.

Temporal summation

Post synaptic potentials from a single neuron arrive around the same time this is seen when the same neuron keeps refiring

Spatial summation

Post synaptic potentials from different pre-synaptic neurons arrive at the same time

What happens if an EPSP and an IPSP arrive at a neuron around the same time

They subtract from each other

Which part of the neuron calculates the total amount of excitation or inhibition

The axon hillock

If the integrated number of PSP's is high enough what happens

The cell is brought to threshold, sodium channels open, and an action potential occurs

If the integrated number of PSP's is not high enough, what happens

Cell does not reach threshold no action potential occurs

Which synopsis have more influence on the axon hillock

Synapses close to the axon hillock have more influence, as their PSP signal does not decay

We are at -70 mV, and need to reach -50mV to get an action potential. 25 EPSPs worth 1mV each reach the hillock, and 10 IPSPs worth 0.5 mV reach the hillock. What happens?

We have 25 mill volts of EPSP and we subtract five mill volts of IPSP. Our total PSP voltage is +20 mV. The cell reaches threshold and fires in action potential.

Why don't we fire an action potential with every stimulus?

Synaptic integration increases complexity of behaviour. The same stimulus can result in different responses based on the situation.

Comparison of amplitude of post, synaptic, potentials, and action potentials

Post synaptic potentials can be depolarizing or hyperpolarizing and are a small voltage. Action potentials are in all or none response based on a threshold.

Comparison of PSP versus action, potential duration

PSP's are between 10 ms to a couple seconds and action potentials are between 2 to 3 ms

Comparison of PSP and action potential location

PSP's are evoked on the dendrites of the post synaptic neuron. Action potentials are evoked at the axon hillock

Comparison of PSP versus action potential conduction

PSP's are passive travel, short distances and decay over distance Action potentials are active travel, long distances and are regenerated at every point/note of Ranvier

Comparison of PSP versus action potential function

PSP's function is too slightly change the polarization of the post synaptic cell inhibitory neurotransmitters, cause a hyper polarization and excitatory neurotransmitters cause a depolarization.

Action potentials are generated if threshold potential is reached travel down the neuron and initiate neurotransmitter release