synaptic transmission Flashcards
what happens when the action potential reaches the end of a neuron?
information is transferred from one neuron to another, at specialised points of contact called synapses.
synapse: define
the specialised junction where a neuron contacts and communicates with another neuron or cell.
2 types of synapses:
- electrical: current (ions) flows through gap junctions
2. chemical: neurotransmitters pass through synaptic cleft
which type of synaptic transmission is faster: electrical or chemical?
electrical.
what is the main function of electrical synapses? in comparison, what is the main function of chemical synapses?
electrical synapses mainly transmit information. chemical synapses mainly amplify information.
_____ synapses serve neuron to neuron, and glia to neuron transmission. ______ synapses can serve neuron to neuron, and neuron to cell transmission.
electrical; chemical.
what does it mean for gap junctions to be electrically coupled?
one channel on a cell opens, the corresponding channel on another cell opens with a brief delay.
features of electrical synapse
transfer is very fast, fail-safe, almost instantaneous
is electrical synapse common in the mammalian brain?
yes.
locations of electrical synapses and examples
often found where normal function requires the activity of neighboring neurons to be highly synchronised.
eg. CNS, heart
what are glial cells (glia)?
are non-neuronal cells in the CNS and the PNS that do not produce electrical impulses. They maintain homeostasis, form myelin, and provide support and protection for neurons.
synaptic integration
most neurons in the CNS make connections with many neurons. If several postsynaptic potentials (PSPs) occur simultaneously, the neuron integrates them and produce an output.
______ synapses form the main mechanism of synaptic transmission
chemical
synaptic cleft
the space between neurons at a nerve synapse, across which a nerve impulse is transmitted by a neurotransmitter.
The two cells are not connected across the synaptic cleft.
how wide is the synaptic cleft?
20-50nm.
what is in the synaptic cleft?
the synaptic cleft is filled with matrix of extracellular protein, which makes pre- and post-synaptic membrane adhere to each other.
what are synaptic vesicles?
membrane enclosed spheres, each about 50nm in diameter, which store various neurotransmitters.
what are active zones in the presynaptic axon?
where synaptic vesicles cluster
what is the zone called postsynaptic density (motor endplate for skeletal muscle)?
a zone in the postsynaptic cell where neurotransmitter receptors are present.
process of information transfer across chemical synapse
- action potential reaches axon terminal. cell membranes become polarised.
- voltage gated Ca2+ channels open.
- an influx of Ca2+ enters axon terminal at the active zone, down the concentration gradient.
- this triggers neurotransmitters to be released and diffuse into the cleft.
- neurotransmitter binds to postsynaptic receptors, causing different effects.
- neurotransmitters removed from synaptic cleft.
how does Ca2+ entry cause neurotransmitter release?
Ca2+ surge acts as an intracellular messenger, resulting in the fusion of synaptic vesicles with axon membrane. They empty their contents by exocytosis.
Ca2+ is then removed from the terminal. It is taken up by mitochondria, or is removed via active Ca2+ pump. This resets voltage for another depolarisation.
what is the effect of the binding of neurotransmitters?
the binding of neurotransmitters opens ion channels, resulting in graded potentials. The effect is transient and usually last a few milliseconds before the NT is removed.
How are NTs removed?
- reuptake (active): by astrocytes/glial cells, or the presynaptic terminal.
- degradation (active): broken down by enzymes associated with the presynaptic membrane. then they can be transported back to the synapse.
- diffusion away from synapse (passive).
synaptic delay: define
the time for NT release, diffusion across the cleft, and binding to receptors.
Synaptic delay is the rate-limiting step of (chemical) neural transmission.
neurotransmitter release is ___ dependent.
Ca2+
there are more than ___ substances that act as neurotransmitters
100
to reduce transmission time, NT is synthesised ______.
in advance.
how are peptide NT synthesised?
- a precursor peptide is synthesised in the rough ER.
- the precursor peptide is split in the Golgi apparatus to yield the active neurotransmitter.
- secretory vesicles containing the peptide bud off from the golgi apparatus
- the secretory granules are transported down the axon to the terminal where the peptide is stored.
how are amine and amino acid NT synthesised?
- enzyme converts precursor molecules into NT in the cytosol.
- transporter proteins load the NT into synaptic vesicles in the terminal, where they are stored.
what are glutamate and glycine, and where are they found?
they are amino acids, and are NT in the CNS.
they are readily available in the cells.
what is GABA and where are they found?
GABA is an amine that is an inhibitory NT in the CNS. it is made only by the neurons that secrete them.
what is excitatory postsynaptic potential (EPSP)?
EPSP is a graded potential that depolarises the neuron and brings its membrane potential closer to the threshold.
where and how does EPSP occur?
in excitatory chemical synapses.
caused by opening Na+ and K+ channels.
what does EPSP cause?
it causes a sodium influx and increases the likelihood of AP firing.
what is inhibitory postsynaptic potential (IPSP)?
IPSP is a graded potential that hyperpolarises the postsynaptic neuron and brings its membrane potential further from the threshold.
where and how does IPSP occur?
in inhibitory chemical synapses.
caused by opening of Cl- channels.
what does IPSP cause?
it lowers the likelihood of reaching the threshold and AP firing.
what are the 2 types of NT receptors?
- ionotropic: receptor itself is an ion channel
2. metabotropic: receptor and ion channel are separate molecules
how do metabotropic receptors affect ion channels?
- NT binds to metabotropic receptors
- intermediate molecules called G-proteins are activated
- a series of intracellular events are triggered
- ion channels open due to a range of second messenger chemicals
what is the function of metabotropic receptors?
they typically amplify the response of the initial NT.
metabotropic receptors lead to _____ responses.
slower
SSRIs (selective serotonin reuptake inhibitors) are a type of antidepressant. How does it work?
it prevents serotonin from being reuptaken back into the presynaptic neuron, thus increasing serotonin levels in the synaptic cleft.
examples of excitatory neurotransmitters
- ACh
- norepinephrine
- epinephrine
- dopamine
- glutamate
- serotonin
examples of inhibitory neurotransmitters
- GABA
- glycine
_______ is the most prevalent excitatory neurotransmitter in the brain.
glutamate
afferent neurons convey information _______ the CNS; efferent neurons convey information ________ the CNS.
interneurons ____________ within the CNS.
towards; away from; connect neurons
why is action potential unidirectional?
graded potential is caused by external stimulus, while action potential is caused by membrane depolarisation.
when an AP occurs, the absolute refractory period prevents the initiation of an AP in a region of membrane that has just produced an AP.
______ is the main inhibitory neurotransmitter in the adult vertebrate brain. ______ is the main inhibitory neurotransmitter in the spinal cord.
GABA; glycine
the direction of information flow for electrical synapse is ______, while for chemical synapse it is _______.
bidirectional; unidirectional.
