Neuroscience – Chapter 5 – synaptic transmission
Overview
Synapse is the contact point between 2 neurons.
Two classes of synapses
1. Electrical synapse; direct exchange of ions and small
molecules by passive flow between pre- and
postsynaptic cell.
Gap junctions between pre- and postsynaptic
membrane (membranes close together). They
contain connexon channel
2. Chemical synapse; indirect transmission of electrical
signals trough chemical signalling molecules
(neurotransmitters).
Synaptic cleft is between the pre- and postsynaptic membrane (membranes
not close together)
Synaptic vesicles in the presynaptic terminal, filled with neurotransmitter
Signalling transmission at electrical synapses
The connexon channels are composed of ion channel proteins
connexins (21 different types). A connexins has 4 transmembrane
domains. In total 6 connexins are needed.
Characteristics:
• Extremely fast (<0,1 ms delay)
• Bi-directional
• One-to-one transmission (pre-synaptic cell is active than the
post-synaptic cell is also active
• Exchange of small molecules
• Use: synchronisation of cells in a network fast response
Signalling transmission at chemical synapses
The process:
1. Neurotransmitter is synthesized and then stored in
vesicles
2. An action potential invades at the presynaptic terminal
3. Depolarization of presynaptic terminal causes opening
of Ca2+ channel
4. Influx of Ca2+
5. Ca2+ causes vesicle to fuse with presynaptic
membrane
6. Transmitter released into synaptic cleft via exocytosis
7. Transmitter binds to receptor molecule in postsynaptic membrane
8. Opening (or sometimes closing) of postsynaptic channels
9. Postsynaptic current causes excitatory or inhibitory postsynaptic potential that
changes the excitability of the postsynaptic cell
10. Removal of neurotransmitter by enzymatic degradation or by neurotransmitter
transporter to reuptake the neurotransmitter into nerve terminals or
surrounding glial cells
, Characteristics:
• Fast (few ms delay)
• Mostly one-directional
• Tuneable transmission (you can regulate the number of neurotransmitters
released and the sensitive of post-synaptic membrane (amplification,
suppression or alteration of signals = plasticity (kunnen veranderen!))
• Variety of neurotransmitters
• Use: regulated activity plasticity (learning,
memory)
Properties of neurotransmitters
There are more than 100 different neurotransmitters.
Classified into 2 categories:
1. Small-molecule neurotransmitters; mediate rapid
synaptic actions (especially low frequency stimulation
of Ca because not far from Ca channel and high
frequency stimulation)
Synthase of this occurs locally within presynaptic
terminals. The enzymes that synthesize these
neurotransmitters are present in cytoplasm of
presynaptic terminal.
They are packed in small clear-core vesicles (SVs)
2. Peptide neurotransmitters; mediate slower, ongoing neuronal functions (high
frequency stimulation of Ca because far from Ca channel)
Synthase of this occurs in the cell body of a neuron. The
peptide filled vesicles are transported along axon down to
synaptic terminal via axonal transport.
They are packed in large dense-core vesicles (DCVs)
Co-transmitters; neurons synthesize and release 2 or more different
neurotransmitters. These co-transmitters are not always released
simultaneously (tegelijk) but at different frequencies of synaptic
activity (see high/low frequency)
Quantal release of neurotransmitter
Quantal release: neurotransmitter released into synapse in packaged vesicles called
quanta. One quantum generates MEPP. The sum of many MEPPs is known as EPP
Motoric end plate (neuromuscular junction); specialised chemical synapses formed at
sites where the terminal branches of the axon of a motor neuron contact a target
muscle cell
End plate potential (EPP); are the voltages which cause depolarization of skeletal
muscle fibers caused by neurotransmitters binding to postsynaptic membrane in the
motoric end plate (neuromuscular junction)
Miniature end plate potentials (MEPPs); spontaneously small differences in the end
plate potential
Overview
Synapse is the contact point between 2 neurons.
Two classes of synapses
1. Electrical synapse; direct exchange of ions and small
molecules by passive flow between pre- and
postsynaptic cell.
Gap junctions between pre- and postsynaptic
membrane (membranes close together). They
contain connexon channel
2. Chemical synapse; indirect transmission of electrical
signals trough chemical signalling molecules
(neurotransmitters).
Synaptic cleft is between the pre- and postsynaptic membrane (membranes
not close together)
Synaptic vesicles in the presynaptic terminal, filled with neurotransmitter
Signalling transmission at electrical synapses
The connexon channels are composed of ion channel proteins
connexins (21 different types). A connexins has 4 transmembrane
domains. In total 6 connexins are needed.
Characteristics:
• Extremely fast (<0,1 ms delay)
• Bi-directional
• One-to-one transmission (pre-synaptic cell is active than the
post-synaptic cell is also active
• Exchange of small molecules
• Use: synchronisation of cells in a network fast response
Signalling transmission at chemical synapses
The process:
1. Neurotransmitter is synthesized and then stored in
vesicles
2. An action potential invades at the presynaptic terminal
3. Depolarization of presynaptic terminal causes opening
of Ca2+ channel
4. Influx of Ca2+
5. Ca2+ causes vesicle to fuse with presynaptic
membrane
6. Transmitter released into synaptic cleft via exocytosis
7. Transmitter binds to receptor molecule in postsynaptic membrane
8. Opening (or sometimes closing) of postsynaptic channels
9. Postsynaptic current causes excitatory or inhibitory postsynaptic potential that
changes the excitability of the postsynaptic cell
10. Removal of neurotransmitter by enzymatic degradation or by neurotransmitter
transporter to reuptake the neurotransmitter into nerve terminals or
surrounding glial cells
, Characteristics:
• Fast (few ms delay)
• Mostly one-directional
• Tuneable transmission (you can regulate the number of neurotransmitters
released and the sensitive of post-synaptic membrane (amplification,
suppression or alteration of signals = plasticity (kunnen veranderen!))
• Variety of neurotransmitters
• Use: regulated activity plasticity (learning,
memory)
Properties of neurotransmitters
There are more than 100 different neurotransmitters.
Classified into 2 categories:
1. Small-molecule neurotransmitters; mediate rapid
synaptic actions (especially low frequency stimulation
of Ca because not far from Ca channel and high
frequency stimulation)
Synthase of this occurs locally within presynaptic
terminals. The enzymes that synthesize these
neurotransmitters are present in cytoplasm of
presynaptic terminal.
They are packed in small clear-core vesicles (SVs)
2. Peptide neurotransmitters; mediate slower, ongoing neuronal functions (high
frequency stimulation of Ca because far from Ca channel)
Synthase of this occurs in the cell body of a neuron. The
peptide filled vesicles are transported along axon down to
synaptic terminal via axonal transport.
They are packed in large dense-core vesicles (DCVs)
Co-transmitters; neurons synthesize and release 2 or more different
neurotransmitters. These co-transmitters are not always released
simultaneously (tegelijk) but at different frequencies of synaptic
activity (see high/low frequency)
Quantal release of neurotransmitter
Quantal release: neurotransmitter released into synapse in packaged vesicles called
quanta. One quantum generates MEPP. The sum of many MEPPs is known as EPP
Motoric end plate (neuromuscular junction); specialised chemical synapses formed at
sites where the terminal branches of the axon of a motor neuron contact a target
muscle cell
End plate potential (EPP); are the voltages which cause depolarization of skeletal
muscle fibers caused by neurotransmitters binding to postsynaptic membrane in the
motoric end plate (neuromuscular junction)
Miniature end plate potentials (MEPPs); spontaneously small differences in the end
plate potential
