Neurophysiology Part 2 Flashcards
Synaptic Transmission: Part 1
Electrical synapse involved in synchronization of neurons
Synapses can be either electrical or chemical
Electrical synapses (or gap junctions) are always excitatory, and very fast.
Chemical synapses are excitatory or inhibitory
Synapse is used both as a noun and a verb!
The synapse is the junction where a signal is transmitted from one nerve cell to another
•Electrical synapses are characterised by very tiny synaptic gaps crossed by ion channels from the pre- and postsynaptic neurons
•Chemical synapses involve the release of neurotransmitter by the presynaptic cell
Synaptic Transmission: Part 2
1) An action potential travels the length of the axon and arrives at the terminal button
2) Calium (Ca+) enters the cell through voltage dependent Ca+ channels
3) Exocytosis: synaptic vesicles migrate towards the pre-synaptic membrane
4) Vesicles fuse with the pre-synaptic membrane & rupture
5) Neurotransmitters are released into the synaptic cleft
6) Transmitter diffuses across the synaptic cleft towards the postsynaptic membrane
7) Neurotransmitter binds with post-synaptic receptors
8) Post-synaptic membrane changes in permeability
9) Excess transmitter either re-uptaken or degraded
Structure of Synapses
–Axodendritic – axons on dendrites
–Axosomatic – axons on cell bodies
•Dendrodendritic – capable of transmission in either direction
•Axoaxonic – may be involved in presynaptic inhibition
Synthesis, Packaging, and Transport of Neurotransmitter Molecules
•Neurotransmitter molecules
–Small
•Synthesized in the terminal button and packaged in synaptic vesicles
–Large
•Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal
Release of Neurotransmitter (NT) Molecules
- Exocytosis – the process of NT release
- The arrival of an Action Potential at the terminal opens voltage-activated Ca2+ channels
- The entry of Ca2+ causes vesicles to fuse with the terminal membrane and release their contents
Activation of Receptors by NT Molecules
- Released neurotransmitter molecules produce signals in postsynaptic neurons by binding to receptors
- Receptors are specific for a given neurotransmitter (lock-and-key analogy)
- Ligand – a molecule that binds to another
- A neurotransmitter is a ligand of its receptor
Synaptic Receptors
• Postsynaptic receptors open the gates of neurotransmitter dependent ion channels
• Postsynaptic receptors come in two forms:
1) Simple receptors: transmitters bind to postsynaptic receptors which then open the ion channel gates
2) G protein receptors: transmitter binds to receptor which triggers a series of intracellular cellular processes
Secondary messengers then change the shape of proteins associated with ion channels
1) Ionotrophic Receptors
A direct opening channel, in which the
binding site and the physical channel
are integrated into the same structure
•NT binds and an associated ion channel opens or closes, causing a PSP
•If Na+ channels are opened, an EPSP occurs
•If K+ channels are opened, an IPSP occurs
2) Metabotrophic Receptors
The receptor is physically distinct from the channel: The G-protein can open the Ion channel by either binding with it or by activating a second messenger.
Metabotrpoic receptors: effects are slower, longer-lasting, more diffuse, and more varied
•(1) NT 1st messenger binds. (2) G protein subunit breaks away. (3) Ion channel opened/closed OR a 2nd messenger is synthesized. (3) 2nd messengers may have a wide variety of effects
Small-molecule neurotransmitters
Synthesized in the cytoplasm of the terminal button and packaged in synaptic vesicles
• Action potentials trigger the entry of Ca++ into the button, causing the vesicles to fuse with the presynaptic membrane, rupture, and empty their contents into the synaptic cleft
Large-molecule neurotransmitters
Consist of polypeptides and proteins. Often released into the extracellular fluid and have wide-spread effects
Amino acids
> . Glutamate (excitatory)
>. GABA (inhibitory)
Monamines
> . Dopamine (generally inhibitory)
. Serotonin (generally inhibitory)
•Effects tend to be diffuse
•Catecholamines – synthesized from tyrosine
–Norepinephrine
–Epinephrine
•Indolamines – synthesized from tryptophan
Acetylcholine
Found at neuromuscular junctions.
•Unconventional neurotransmitters – soluble gases and endacannabinoids
•Soluble gases – exist only briefly
–Nitric oxide and carbon monoxide
–Retrograde transmission – backwards communication
•Acetylcholine (Ach):
–Acetyl group + choline
–Cholinergic neurons widely distributed in the brain.
–Nicotinic (ionotropic) and muscarinic (metabotropic)
–Peripherally at neuromuscular junctions (Nicotinic - fast ionotropic)
Neuropeptides
•Large molecules •Example – endorphins –“Endogenous opioids” –Produce analgesia (pain suppression) –Receptors were identified before the natural ligand was
