Summary: Chemistry of Behaviour


Synaptic Transmission Requires Sequence of Events

  1. action potential arrives at presynaptic axon terminal
  2. voltage-gated calcium channels in membrane of axon terminal open, allowing calcium ions (Ca2+) to enter
  3. calcium causes synaptic physicals filled with neurotransmitter to fuse with presynaptic membrane and rupture, releasing transmitter molecules into synaptic cleft
  4. transmitter molecules bind to special receptor molecules in postsynaptic membrane leading directly/indirectly to opening of ion channels in postsynaptic membrane. Resulting flow of ions creates local EPSP or IPSP in postsynaptic neuron
  5. IPSPs and EPSPs in postsynaptic cell spread toward axon hillock (if sum of EPSPs and IPSPs depolarize axon hillock enough to reach threshold, action potential will arise
  6. Synaptic transmission stops, so message is brief and accurately reflects activity of presynaptic cell
  7. Synaptic transmitter may activate presynaptic receptors, resulting in decrease in transmitter release

-action potential reaches presynaptic terminal

-synaptic vesicles release neurotransmitters into synaptic cleft

Synaptic Cleft: space between presynaptic and postsynaptic cells at synapse

-influx of calcium ions into axon terminal through voltage-gated Ca2+ channels

Synaptic Delay: time needed for calcium to enter terminal, transmitter to diffuse across synaptic cleft, and transmitter molecules to interact with receptors before postsynaptic responds

-transmitter acetylcholine (ACh) is a ligand that fits into the ligand binding site in neurotransmitter receptor molecules in postsynaptic membrane

Ligand: molecule of correct shape fitting into receptor protein to activate or inhibit it (key in a lock)

-ACh acting as excitatory neurotransmitter at excitatory synapses = open channels for Na+ and K+

-ACh acting as inhibitory neurotransmitter at inhibitory synapses = open channels that allow Cl- ions to enter and hyperpolarize the membrane (make it more negative, less likely to create action potential)

-Curare and bungarotoxin are poisons that block ACh receptors

-curare: arrowhead poison extracted from plant

-bungarotoxin: found in venom of snake native to China and Southeast Asia

Agonist: substance that mimics or boosts actions of transmitter ex. nicotine and ACh

Antagonist: substance that blocks/reduces actions of transmitter ex. curare

Cholinergic: referring to cells that use ACh as synaptic transmitter

-nicotinic cholinergic receptor

-found at synapse on muscles and autonomic ganglia

-blockade causes paralysis

-resembles lopsided dumbbell with tube running down central axis

-when ACh is released into synaptic cleft, postsynaptic action is brief to maximize how much info is transmitted (faster = more info)

Two processes that stop transmitter effects

  1. Degradation: chemical breakdown of neurotransmitter into inactive metabolites

-ex. acetylcholinesterase (AChE) is the enzyme that inactivates Ach

  1. Reuptake: process where released synaptic transmitter molecules are absorbed back into axon terminal and reused by presynaptic neuron, stopping synaptic activity

-ex. norepinephrine, dopamine, serotonin

Transporters: located on presynaptic axon terminal, special receptors for transmitters that return transmitters to presynaptic neuron for reuse

Axo-Dendritic Synapse: axon to dendrite (presynaptic terminal to postsynaptic neuron)

Background pattern

Description automatically generatedAxo-Somatic Synapse: axon to cell body/soma

Axo-Axonic Synapses: presynaptic axon terminal to postsynaptic axon terminal

Dendro-Dendritic Synapses: presynaptic dendrite to postsynaptic neuron

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