Chapter 5 (MT2) Flashcards
How do Neurons Communicate and Adapt?
Basis statement of neuron communication
Communication between neurons occurs across the synapse and is largely chemical, spurred by electrical action potentials
Neurotransmitter
Chemical messengers released by a neuron onto a target to cause an excitatory or inhibitory effect
- many of these same chemicals circulate outside the bloodstream outside the CNS as (slower acting) hormones
Aside from action speed, how else do neurotransmitters and hormones differ?
Distance travelled before encountering their receptors
When was the synaptic structure first revealed?
1950’s using electrical microscopy
Upper part of the synapse
Axon terminal, or end foot
Lower part of the synapse
Receiving dendrite
Round granular substances in the terminal, containing neurotransmitter molecules
Synaptic vesicles
How are neurotransmitters transferred at the synapse?
Storage granules hold vesicles containing neurotransmitter that travel to presynaptic membrane
Neurotransmitter is expelled into synaptic cleft, and binds to receptor proteins on the postsynaptic membrane
Presynaptic membrane (axon terminal)
Where the action potential terminates to release the chemical message
Postsynaptic membrane (dendritic spine)
The receiving side of the chemical message, where EPSPs or IPSPs are generated
Synaptic cleft (space between dendritic spine and axon terminal)
Small gap where the chemical travels from presynaptic to postsynaptic membrane
Synaptic vesicle (presynaptic)
Small membrane-bound spheres that contain one or more neurotransmitters
Storage granule (presynaptic)
Membranous compartment that holds several vesicles containing neurotransmitter(s)
Postsynaptic receptor (postsynaptic)
Site to which a neurotransmitter
molecule binds
Anterograde synaptic transmission
The five-step process of transmitting information across a chemical synapse from the presynaptic side to the postsynaptic neuron
Five steps of neurotransmission
(1) The neurotransmitter is synthesized somewhere inside the neuron (neurotransmitter synthesis)
(2) It is packaged and stored within vesicles at the axon terminal (neurotransmitter packaging and storage)
(3) It is transported to the presynaptic membrane and released into the cleft in response to an action potential (neurotransmitter transportation and release into cleft)
(4) It binds to and activates receptors on the postsynaptic membrane (neurotransmitter binding and activating receptors)
(5) It is degraded or removed, so it will not continue to interact with a receptor and work indefinitely (neurotransmitter degradation)
What are the two ways neurotransmitters are derived?
Synthesized in axon terminal
- building blocks from food pumped into cell, protein molecules embedded in cell membrane
Synthesized in cell body
- according to DNA instructions
Transporters
Protein molecules that move substances across cell membranes
- responsible for packaging some neurotransmitter classes into vesicles
Peptide transmitters
Synthesized in cell body according to DNA instructions
Lipid transmitters
Cannot be packaged and stored in vesicles (which are composed of lipids) but are synthesized ‘on demand’
Gaseous transmitters
Generated within the cells by enzymes
Ion transmitters
Not synthesized at all
Where can neurotransmitters that are packaged into vesicles be found?
Three locations at axon terminal
- in granules
- attached to microfilaments
- attached to presynaptic membrane
Transmitter-activated receptors
Protein embedded in the membrane of a cell that has a binding site for a specific neurotransmitter
What happens to a neurotransmitter after diffusing to the postsynaptic side?
(1) May cause excitatory action (EPSP, depolarize the postsynaptic membrane)
(2) May cause inhibitory action (IPSP, hyperpolarize the postsynaptic membrane)
(3) Initiate other chemical reactions
How may neurotransmitters interact with receptors on the presynaptic membrane?
Through autoreceptors
- a self-receptor on the presynaptic membrane that responds to the transmitter that the neuron releases
Ionotropic receptors
Associated with a pore that can open to allow ions to pass through the membrane, rapidly changing membrane voltage in one of two possible ways (allow Na+ to enter and depolarize the postsynaptic membrane, or allow K+ to leave or Cl- to enter to hyperpolarize the postsynaptic membrane)
Metabotropic receptor
When bound by a transmitter, may initiate intracellular messenger systems; this, in turn, may open an ion channel, thus modulating either excitation or inhibition or influencing other functions of the receiving neuron
How is neurotransmitter inactivation accomplished?
Diffusion
Degradation
Reuptake
Astrocyte uptake
Diffusion (neurotransmitter inactivation)
Some of the neurotransmitter simply diffuses away from the synaptic cleft and is no longer available to bind to receptors
Degradation (neurotransmitter inactivation)
Enzymes in the synaptic cleft break down the neurotransmitter
Reuptake (neurotransmitter inactivation)
Transmitter is brought back into the presynaptic axon terminal; by-products of degradation by enzymes also may be taken back into the terminal to be used again
Astrocyte uptake (neurotransmitter inactivation)
Nearby astrocytes take up neurotransmitter; can also store transmitters for re-export to the axon terminal
Electric synapse (or gap junction)
Where two neurons’ intracellular fluids or cytoplasm can come into direct contact
Gap junction = Fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next
What are the two ways a neurotransmitter acting through its receptors has an immediate effect?
It influences transmembrane ion flow either to increase or to decrease the likelihood that the cell with which it comes in contact will produce an action potential. Thus, despite the wide variety of synapses, they all convey messages of only these two types, excitatory or inhibitory
Do neurotransmitters determine excitation or inhibition?
No - the ion channel associated with the receptor does
Excitatory synapse
Located on dendrites
Round vesicles
Dense material on membranes
Large active zone
Wide cleft
Inhibitory synapse
Located on cell body usually
Flat vesicles
Sparse material on membranes
Narrow cleft
Small active zone
Where may have chemical transmission originated?
In feeding behaviour of single-celled creatures
