Exam Notes
Electrical Synapse
- Direct transfer of ionic current.
- Direct connection between pre- and postsynaptic neurons.
- Ions (e.g., sodium) flow directly from presynaptic to postsynaptic neuron.
- Bidirectional: can go from pre- to post-synaptic or vice-versa.
- First identified by First and Potter.
- When current is injected into one neuron, the effect is almost instantaneous in the connected neuron.
- There is a very slight delay.
- Attenuation of the signal.
- Passive: no voltage-gated channels are involved.
- Same shape and features in the graph.
Experiment Setup (First and Potter)
- Two neurons are connected.
- Recording electrodes in both neurons.
- Inject current into one neuron and observe the membrane potential change in both.
- The change in membrane potential will have the same shape, marginal delay, and slight attenuation.
Ionic Current Properties:
- Attenuation occurs due to spatial resolution.
- In synchrony among cells connected via electrical synapses.
- Requires no activation energy.
Chemical Synapse
- Involves a synaptic cleft: a space between the presynaptic and postsynaptic neurons.
- Involves various types of channels:
- Leak channels: Always open with no control.
- Voltage-gated channels: Respond to changes in membrane potential (e.g., sodium, potassium).
- Ligand-gated channels (Metabotropic): Neurotransmitter binds to a receptor, causing a change inside the cell that alters ion permeability.
Metabotropic Receptors
- Two-step Process:
- Neurotransmitter (ligand) binds to the receptor.
- A signal is triggered inside the cell, which opens the channel.
- Ligand: A term derived from Latin, referring to something that binds.
Chemical Synapse Mechanism
Presynaptic Terminal:
- Contains voltage-gated calcium channels.
- Vesicles containing neurotransmitters are docked, waiting for a signal.
Action Potential Arrival:
- Action potential depolarizes the presynaptic terminal.
- Voltage-gated calcium channels open, allowing calcium to flow in due to its concentration gradient.
Neurotransmitter Release:
- Influx of calcium triggers the release of neurotransmitters into the synaptic cleft.
- The amount of neurotransmitter released must be above a certain threshold to affect the postsynaptic cell.
Postsynaptic Action:
- Neurotransmitter binds to receptors on the postsynaptic side. This is the action to reach over the electrical changes across to the other side.
Key Differences Between Electrical and Chemical Synapses
| Feature | Electrical Synapse | Chemical Synapse |
|---|---|---|
| Directionality | Bidirectional | Unidirectional |
| Speed | Almost instantaneous | Has a delay |
| Signal Transfer | Can transfer sub-threshold information | Requires reaching a threshold |
| Other functions | Synchronization, metabolic signaling | Facilitates large proteins such as hormones |
| Postsynaptic | Postsynaptic densities asymmetry | Can look at the function through the anatomy of the asymmetry |
Postsynaptic Density
- Thickness of the postsynaptic density indicates whether the synapse is excitatory or inhibitory.
- Asymmetry: A larger postsynaptic side indicates an excitatory synapse.
Neurotransmitter Synthesis and Release
- Transmitter Synthesis: Neurotransmitters (e.g., dopamine, acetylcholine, norepinephrine) are synthesized and stored in vesicles.
- Action Potential Arrival: Action potential invades the presynaptic terminal.
- Calcium Influx: Voltage-gated calcium channels open, allowing calcium to enter.
- Vesicle Fusion: Calcium influx triggers the docked vesicles to fuse with the presynaptic membrane and release the neurotransmitter.
Vesicle Membrane
- Vesicle membrane is the same as the cell membrane.