Physiology of synapses, Physiology of synapses, interneuronal connectionsinterneuronal connections
What is a synapse?What is a synapse?
A A synapsesynapse is the junction between 2 is the junction between 2 neurones.neurones.
A specialized junction that transfers A specialized junction that transfers
nerve impulse information between nerve impulse information between
neuronsneurons
A junction that mediates information transfer from one A junction that mediates information transfer from one neuron:neuron:– To another neuronTo another neuron
Called neuro-synapses or just synapseCalled neuro-synapses or just synapse
– To an effector cellTo an effector cell Neuromuscular synapse if muscle involvedNeuromuscular synapse if muscle involved Neuroglandular synapse if gland involveNeuroglandular synapse if gland involve
Presynaptic neuron – conducts impulses toward the Presynaptic neuron – conducts impulses toward the synapsesynapse
Postsynaptic neuron – transmits impulses away from Postsynaptic neuron – transmits impulses away from the synapsethe synapse
Two major types:Two major types:– Electrical synapsesElectrical synapses– Chemical synapsesChemical synapses
SynapsesSynapses
Anatomical Types of SynapsesAnatomical Types of Synapses
Axo-dendriticAxo-dendritic – synapses between the axon of – synapses between the axon of one neuron and the dendrite of anotherone neuron and the dendrite of another
Axo-somaticAxo-somatic – synapses between the axon of one – synapses between the axon of one neuron and the soma of anotherneuron and the soma of another
Other types of synapses include:Other types of synapses include:– Axo-axonicAxo-axonic (axon to axon) (axon to axon)– Dendro-dendritic (Dendro-dendritic (dendrite to dendrite)dendrite to dendrite)– Dendro-somaticDendro-somatic (dendrites to soma (dendrites to soma))
Functional classificationFunctional classificationor Types of comnicationor Types of comnication
A.Chemical synapseA.Chemical synapse Almost all synapses used for signal transmission Almost all synapses used for signal transmission
in the CNS of human being are chemical in the CNS of human being are chemical synapses.synapses.
First neuron secretes a chemical substance First neuron secretes a chemical substance called neurotransmitter at the synapse to act on called neurotransmitter at the synapse to act on receptor on the next neuron to excite it, inhibit receptor on the next neuron to excite it, inhibit or modify its sensitivity.or modify its sensitivity.
The chemical synapseThe chemical synapse is a specialized junction that is a specialized junction that transfers nerve impulse information from a presynaptic transfers nerve impulse information from a presynaptic membrane to a postsynaptic membrane using membrane to a postsynaptic membrane using neurotransmitters.neurotransmitters.
Axo-dendritic synapse Axo-somatic synanpse Axo-axonic synapseAxo-dendritic synapse Axo-somatic synanpse Axo-axonic synapse
The Chemical SynapseThe Chemical Synapse
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NeurotransmittersNeurotransmitters Properties of neurotransmitters:Properties of neurotransmitters:
1) synthesized in the presynaptic neuron1) synthesized in the presynaptic neuron
2) Localized to vesicles in the presynaptic neuron2) Localized to vesicles in the presynaptic neuron
3) Released from the presynaptic neuron under 3) Released from the presynaptic neuron under physiological conditionsphysiological conditions
4) Rabidly removed from the synaptic cleft by uptake or 4) Rabidly removed from the synaptic cleft by uptake or degradationdegradation
5) Presence of receptor on the post-synaptic neuron.5) Presence of receptor on the post-synaptic neuron.
6) Binding to the receptor elicits a biological response6) Binding to the receptor elicits a biological response
R.E.B, 4MedStudents.com, 2003
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Neurotransmitters found in the nervous system
EXCITATORY Acetylcholine Dopamine Histamine
Nonepinephrine Epinephrine Glutamate Serotonin
INHIBITORY GABA Glycine
TThe he SSynapseynapse
Structure of a synapseStructure of a synapse
Excitatory postsynaptic potentialExcitatory postsynaptic potential
Single stimuli applied to the sensory nerves Single stimuli applied to the sensory nerves in the experimental situation described in the experimental situation described above characteristically do not lead to the above characteristically do not lead to the formation of a propagated action potential formation of a propagated action potential in the postsynaptic neuron. Instead, the in the postsynaptic neuron. Instead, the stimulation produces either a transient, stimulation produces either a transient, partial depolarization or a transient partial depolarization or a transient hyperpolarization.hyperpolarization.
If positive ion gates open (which allow more If positive ion gates open (which allow more Na+ and Ca2+ to enter than K+ to exit), the Na+ and Ca2+ to enter than K+ to exit), the membrane becomes depolarized, which results in membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action the threshold potential is exceeded, an action potential is generated.potential is generated.
If K+ or chlorine ion (Cl−) gates open (allowing If K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane K+ to exit or Cl− to enter), the membrane becomes more depolarized (hyperpolarized), becomes more depolarized (hyperpolarized), which results in an inhibitory postsynaptic which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more potential (IPSP). As a result, it becomes more difficult to generate an action potential on this difficult to generate an action potential on this membrane.membrane.
Summary of Synaptic
Transmission
Chemical SynapseChemical SynapseEvents at a chemical synapse1. Arrival of action potential on
presynaptic neuron opens volage-gated Ca++ channels.
2. Ca++ influx into presynaptic term.3. Ca++ acts as intracellular messenger stimulating synaptic vesicles to fuse
with membrane and release NT via
exocytosis.4. Ca++ removed from synaptic knob by mitochondria or calcium-pumps.5. NT diffuses across synaptic cleft and binds to receptor on postsynaptic
membran6. Receptor changes shape of ion
channel opening it and changing membrane
potential7. NT is quickly destroyed by enzymes
or taken back up by astrocytes or
presynaptic membrane.Note: For each nerve impulse reaching
the presynaptic terminal, about 300 vesicles are emptied into the cleft. Each vesicle contains about 3000 molecules.
NT affects the postsynaptic membrane potentialNT affects the postsynaptic membrane potential Effect depends on:Effect depends on:
– The amount of neurotransmitter releasedThe amount of neurotransmitter released
– The amount of time the neurotransmitter is The amount of time the neurotransmitter is bound to receptorsbound to receptors
The two types of postsynaptic potentials are: The two types of postsynaptic potentials are:
– EPSP – excitatory postsynaptic potentials EPSP – excitatory postsynaptic potentials
– IPSP – inhibitory postsynaptic potentialsIPSP – inhibitory postsynaptic potentials
Postsynaptic PotentialsPostsynaptic Potentials
Neurotransmitter binding to a receptor at Neurotransmitter binding to a receptor at inhibitory synapses: inhibitory synapses: – Causes the membrane to become more permeable to Causes the membrane to become more permeable to
potassium and chloride ions potassium and chloride ions – Leaves the charge on the inner surface more negative Leaves the charge on the inner surface more negative
(flow of K+ out of the cytosol makes the interior more (flow of K+ out of the cytosol makes the interior more negative relative to the exterior of the membranenegative relative to the exterior of the membrane
– Reduces the postsynaptic neuron’s ability to produce Reduces the postsynaptic neuron’s ability to produce an action potentialan action potential
Inhibitory SynapsesInhibitory Synapses
Electrical SynapsesElectrical Synapses Pre- and postsynaptic Pre- and postsynaptic
neurons joined by neurons joined by gap gap junctionsjunctions – allow local current to flow allow local current to flow
between adjacent cells. between adjacent cells. ConnexonsConnexons: protein tubes in : protein tubes in cell membrane.cell membrane.
Rare in CNS or PNSRare in CNS or PNS Found in cardiac muscle and Found in cardiac muscle and
many types of smooth many types of smooth muscle. Action potential of muscle. Action potential of one cell causes action one cell causes action potential in next cell, almost potential in next cell, almost as if the tissue were one cell.as if the tissue were one cell.
Important where contractile Important where contractile activity among a group of activity among a group of cells important.cells important.
The SynapseThe junction between two neurons is termed a synapse(synapsis = point of contact)
The narrow gap between the two neurons at the synapse is the synaptic cleft; the cleft is filled with extracellular fluid and spans an area of approximately 20 nm
A neuron that conducts impulses toward a synapse is called a pre-synaptic neuron
A neuron that conducts impulses away from a synapse is called a post-synaptic neuron
= synapse
The Synapse
synapticknob
axon terminal
mitochondrion
pre-synapticmembrane
synapticcleft
post-synaptic membrane (with receptors for neurotransmitter)
A single neuron may have many thousands of synaptic junctions on its dendrites and cell body
synaptic vesicle(contains neurotransmitter)
An action potential travels downthe axon of the neuron to the synaptic knob and depolarises
the pre-synaptic membrane
Events at the synapse
post-synapticmembrane calcium ions
in theextra-cellular
fluid
Voltage-gated calcium ionchannels open in the
pre-synaptic membrane
Calcium ions diffuseinto the synaptic knob
An action potential travels downthe axon of the neuron to the synaptic knob and depolarises
the pre-synaptic membrane
post-synapticmembrane
Voltage-gated calciumchannels open in the
pre-synaptic membrane
Calcium ions diffuseinto the synaptic knob
Events at the synapse
The uptake of calcium ions triggers the fusion of
the synaptic vesicles with the pre-synaptic membrane
Neurotransmitter is releasedinto the synaptic cleft
by EXOCYTOSIS
post-synapticmembrane
Voltage-gated calciumchannels open in the
pre-synaptic membrane
Calcium ions diffuseinto the synaptic knob
The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane
Events at the synapse
Neurotransmitter diffuses across the cleft
and binds to specific protein receptors embedded in the
post-synaptic membrane
receptors in the post-synaptic
membrane
Depolarisation of the post-synaptic membrane
post-synapticmembrane
Events at the synapse Neurotransmitter diffuses across the cleft and binds to specific
protein receptors embedded in the
post-synaptic membrane
Binding of neurotransmitter opens Na+ gates in the membrane and there is an influx of Na+ into the
post-synaptic neuron
sodium ions
An excitatory post-synaptic potential (EPSP) buildsup across the membrane
and if this reachesthreshold, an action
potential is triggered inthe post-synaptic neuron
Depolarisation of the post-synaptic membrane
post-synapticmembrane
Events at the synapse The neurotransmitter, acetylcholine,is hydrolysed by the enzyme acetylcholinesterase, which islocated at the surface of the
post-synaptic membraneFollowing activation of thepost-synaptic membrane,
neurotransmitter is removed from the synaptic cleft to enablefurther stimulation to occur
sodium ions
The neurotransmitter, noradrenaline, is actively
transported back intothe axon terminals
Unidirectionality
post-synaptic membrane (with receptors for neurotransmitter)
Unidirectionality describes the one-way transmission of nerve impulses between neurons
Neurotransmitter is stored and released only on the pre-synaptic
side of the synaptic cleft
This arrangement allows for the transmission of
impulses between neurons in one direction only
synaptic vesicle(contains neurotransmitter)
Receptors for neurotransmitter are only located on the
post-synaptic membrane
Inhibition occurs at synapses where transmitter
release results in the hyperpolarisation of the
post-synaptic membrane
During hyperpolarisation, the post-synaptic membrane
potential becomes more negative than its resting potential and
results from either the efflux of positive charge or the influx
of negative charge
The nature of the neurotransmitter determines
the response of thepost-synaptic membrane
OcclusionOcclusion On account of divergence one neuron may pass excitive On account of divergence one neuron may pass excitive
signals on the other neurons. Another neuron may excite signals on the other neurons. Another neuron may excite several neurons. But if from both neurons which is several neurons. But if from both neurons which is divergented excitement will be simultaneously the total divergented excitement will be simultaneously the total quantity of excited neurons will be decrease.quantity of excited neurons will be decrease.
Opposite inhibitionOpposite inhibition
Lateral inhibitionLateral inhibition If in a neurons' chain, which secure opposite inhibition If in a neurons' chain, which secure opposite inhibition
collaterals of axons of inhibition neurons form synaptic collaterals of axons of inhibition neurons form synaptic connection with neighboring excitive cells in these cells connection with neighboring excitive cells in these cells develop lateral inhibition develop lateral inhibition
Spatial summation
The adding together of EPSPs The adding together of EPSPs
generated simultaneously at generated simultaneously at
many different synapses on a many different synapses on a
dendrite.dendrite.
Two or more presynaptic inputs Two or more presynaptic inputs
are active at the same timeare active at the same time A space (spatial) dependent A space (spatial) dependent
process.process. Occurs in a Convergent SynapseOccurs in a Convergent Synapse
Temporal summationTemporal summation
The adding together of EPSPs The adding together of EPSPs
generated at the same synapse if generated at the same synapse if
they occur in rapid succession, they occur in rapid succession,
within 1-15 msec of one another.within 1-15 msec of one another.
The same presynaptic fiber fires The same presynaptic fiber fires
AP in quick successionAP in quick succession
A Time (Temporal) dependent A Time (Temporal) dependent
processprocess
Occurs in a Divergent SynapseOccurs in a Divergent Synapse