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[SYNAPSE]-
• synapse is the junction between two neurons.
• it is only a physiological continuity between two Nerve cells.
CLASSIFICATION OF SYNAPSE
Synapse is classified by two methods-
1. Anatomical classification
2. Functional classification
ANATOMICAL CLASSIFICATION
• generally synapse is formed by axon of one neuron ending on the cell body, dendrite or axon of the next neuron.
> On the basis of ending of axon, synapse is classified into three types-
1. Axo-axonic synapse
2. Axo-dendritic synapse
3. Axo-somatic synapse
1. Axo-axonic synapse
Axon of one neuron terminates on axon of another neuron.
2. Axo-dendritic synapse
Axon of one neuron terminates on dendrite of another neuron.
3. Axo-somatic synapse
Axon of one neuron Ends on cell body (soma) of another neuron.
FUNCTIONAL CLASSIFICATION
•it is based on mode of impulse transmission.
2 types
1. Electrical synapse
2. Chemical synapse
(Generally synapse word refers to chemical synapse)
1. Electrical synapse
• in this synapse between presynaptic and postsynaptic neuron physiological continuity is provided by gap junction between two neuron.
• in this synapse the action potential reaching the terminal portion of presynaptic neuron directly enters into postsynaptic neuron because there is direct exchange of Ions between the two neurons through the gap Junction.
✓important feature of electrical synapse-
•Syneptic delay is very less because of direct flow of current.
• impulses are also transmitted in same direction through the electrical synapse.
Electrical synapse present in-
- cardiac muscle fibers
- smooth muscle fibers of intestine
- epithelial cells of lens in eye
2. Chemical synapse
• it is a Junction between a nerve fiber and a muscle fiber or between to nerve fibers.
• signals are transmitted by the release of chemical transmitter.
• in this synapse there is no continuity between two neurons because of the presence of a space between two neuron that is called synaptic cleft.
• action potential reaching the presynaptic terminal and release the neurotransmitter from the vesicles.
• neurotransmitters reaches to postsynaptic neuron through synaptic cleft and causes the production of potential change.
FUNCTIONAL ANATOMY OF CHEMICAL SYNAPSE-
> Pre-syneptic neuron
Neuron from which axon arises.
> Post-synaptic neuron
Neuron on which the axon ends.
> Pre-syneptic axon terminals
Before forming the synapse the axonaxo presynaptic neuron divides into many small branches these branches is called presynaptic axon terminals.
AXON TERMINALS-
2 types
1. Terminal knob
2. Terminal coil or free endings
1. Terminal knob
It is perform excitatory function of the synapse.
2. Terminal coils ( free endings)
• this terminal do not have knob.
• this knob is coiled and wavy ( wave like) and have free endings.
PRE-SYNAPTIC MEMBRANE
pre- Syneptic axon terminal has a intact membrane that is called presynaptic membrane.
> Axon terminal have these str.-
a. Mitochondria - help in synthesis of neurotransmitter substances.
b. Syneptic vesicles- store neurotransmitter substances.
POST-SYNAPTIC MEMBRANE
it is membrane of post-synaptic neuron.
• it contains receptor protein.
SYNAPTIC CLEFT-
Space between pre-syneptic membrane and post-synaptic membrane.
• basal lamina of Syneptic cleft contains cholinesterase enzyme which destroys acetylcholine (Ach).
FUNCTIONS OF SYNAPSE-
• transmission of impulses.
( Action potential transfer from one neuron to another)
Types of SYNAPSE-
On the basis of function
2 types
1. Excitatory synapse-
Transmits the Impulses.
(Excitatory function)
2. Inhibitory synapses-
Inhibits the transmission of impulses.
(Inhibitory function)
*EXCITATORY FUNCTION-
[EPSP]=excitatory postsynaptic potential
• it is non-propagated electrical potential, resulting from local change in ionic conductance.
• it develops during the process of synaptic transmission.
a. opening of ca+ channel
when the action potential reaches to the presynaptic axon terminal the calcium channels at presynaptic membrane are opened.
b. enterance of ca+ ions
the calcium Ion enters into axon terminal from ECF ( extra cellular fluid).
c. release of neurotransmitter
By exocytosis calcium Ion release the neurotransmitter from vesicles.
d. Transmission of neurotransmitter
excitatory neurotransmitter passes through pre-syneptic membrane and synthetic left and reaches to postsynaptic membrane.
e. Binding of neurotransmitters
Neurotransmitter binds with receptor protein that are present in post synaptic membrane and form neurotransmitter-receptor complex.
f. Production of EPSP
Neurotransmitter receptor Complex causes production of non propagated EPSP.
{ Acetylcholine is a common excitatory neurotransmitter)
MECHANISM OF DEVELOPMENT OF EPSP-
1. neurotransmitter receptor Complex causes opening of ligand gated sodium channels.
2. Sodium ions enters into cell body of postsynaptic neuron from ECF.
3. Development of mild depolarization because sodium ion is positively charged and resting membrane potential (RMP) inside the cell body is altered.
This type of mild depolarization is called EPSP.
Properties of EPSP-
° it is a graded potential.
° it is non propagated.
° it does not obey all-or-none law.
° it is similar to receptor potential and end plate potential.
Significance of EPSP-
° EPSP causes Development of action potential.
° it is not transmitted into axon of postsynaptic neuron.
° when EPSP is strong it opens the Na+ channels in initial part of axon.
° action potential develops in initial part of axon ( depolarization) due to enterance of Na+ ions.
° action potential transfer to other part of axon.
*INHIBITORY FUNCTION
[IPSP]= inhibitory postsynaptic potential
Inhibition of synaptic transmission is classified into five types-
1. Postsynaptic or direct inhibition
2. Presynaptic or indirect inhibition
1. POSTSYNAPTIC OR DIRECT INHIBITION
• it occurs due to release of inhibitory neurotransmitter from presynaptic terminal.
✓ inhibitory neurotransmitters are
° GABA ( gamma amino butyric acid )
° dopamine
° glycine
Development of IPSP-
Action of GABA
a° it develops during postsynaptic inhibition.
b° it is electrical potential in the form of hyperpolarization.
c° inhibitory neurotransmitter binds with receptor on postsynaptic membrane.
d° transmitter receptor Complex opens the K+ channel. ( Not the Na+ channel)
e° potassium ion Move to ECF from cell body of postsynaptic neuron.
f. Chloride channel also open and chloride ions (Cl-) move inside the cell body of postsynaptic neuron from ECF.
g° exit of potassium ion and influx of chloride ions cours more negativity inside that leads to hyperpolarization.
h° hyperpolarized state of synapse inhibit synaptic transmission.
2. Pre-syneptic or indirect inhibition
• it occurs if release of excitatory neurotransmitter fails from presynaptic axon terminal.(Syneptic knob)
• synapse is the junction between two neurons.
• it is only a physiological continuity between two Nerve cells.
CLASSIFICATION OF SYNAPSE
Synapse is classified by two methods-
1. Anatomical classification
2. Functional classification
ANATOMICAL CLASSIFICATION
• generally synapse is formed by axon of one neuron ending on the cell body, dendrite or axon of the next neuron.
> On the basis of ending of axon, synapse is classified into three types-
1. Axo-axonic synapse
2. Axo-dendritic synapse
3. Axo-somatic synapse
1. Axo-axonic synapse
Axon of one neuron terminates on axon of another neuron.
2. Axo-dendritic synapse
Axon of one neuron terminates on dendrite of another neuron.
3. Axo-somatic synapse
Axon of one neuron Ends on cell body (soma) of another neuron.
•it is based on mode of impulse transmission.
2 types
1. Electrical synapse
2. Chemical synapse
(Generally synapse word refers to chemical synapse)
• in this synapse between presynaptic and postsynaptic neuron physiological continuity is provided by gap junction between two neuron.
• in this synapse the action potential reaching the terminal portion of presynaptic neuron directly enters into postsynaptic neuron because there is direct exchange of Ions between the two neurons through the gap Junction.
✓important feature of electrical synapse-
•Syneptic delay is very less because of direct flow of current.
• impulses are also transmitted in same direction through the electrical synapse.
Electrical synapse present in-
- cardiac muscle fibers
- smooth muscle fibers of intestine
- epithelial cells of lens in eye
2. Chemical synapse
• it is a Junction between a nerve fiber and a muscle fiber or between to nerve fibers.
• signals are transmitted by the release of chemical transmitter.
• in this synapse there is no continuity between two neurons because of the presence of a space between two neuron that is called synaptic cleft.
• action potential reaching the presynaptic terminal and release the neurotransmitter from the vesicles.
• neurotransmitters reaches to postsynaptic neuron through synaptic cleft and causes the production of potential change.
> Pre-syneptic neuron
Neuron from which axon arises.
> Post-synaptic neuron
Neuron on which the axon ends.
> Pre-syneptic axon terminals
Before forming the synapse the axonaxo presynaptic neuron divides into many small branches these branches is called presynaptic axon terminals.
AXON TERMINALS-
2 types
1. Terminal knob
2. Terminal coil or free endings
1. Terminal knob
It is perform excitatory function of the synapse.
2. Terminal coils ( free endings)
• this terminal do not have knob.
• this knob is coiled and wavy ( wave like) and have free endings.
PRE-SYNAPTIC MEMBRANE
pre- Syneptic axon terminal has a intact membrane that is called presynaptic membrane.
> Axon terminal have these str.-
a. Mitochondria - help in synthesis of neurotransmitter substances.
b. Syneptic vesicles- store neurotransmitter substances.
POST-SYNAPTIC MEMBRANE
it is membrane of post-synaptic neuron.
• it contains receptor protein.
SYNAPTIC CLEFT-
Space between pre-syneptic membrane and post-synaptic membrane.
• basal lamina of Syneptic cleft contains cholinesterase enzyme which destroys acetylcholine (Ach).
FUNCTIONS OF SYNAPSE-
• transmission of impulses.
( Action potential transfer from one neuron to another)
Types of SYNAPSE-
On the basis of function
2 types
1. Excitatory synapse-
Transmits the Impulses.
(Excitatory function)
2. Inhibitory synapses-
Inhibits the transmission of impulses.
(Inhibitory function)
*EXCITATORY FUNCTION-
[EPSP]=excitatory postsynaptic potential
• it is non-propagated electrical potential, resulting from local change in ionic conductance.
• it develops during the process of synaptic transmission.
when the action potential reaches to the presynaptic axon terminal the calcium channels at presynaptic membrane are opened.
b. enterance of ca+ ions
the calcium Ion enters into axon terminal from ECF ( extra cellular fluid).
c. release of neurotransmitter
By exocytosis calcium Ion release the neurotransmitter from vesicles.
d. Transmission of neurotransmitter
excitatory neurotransmitter passes through pre-syneptic membrane and synthetic left and reaches to postsynaptic membrane.
e. Binding of neurotransmitters
Neurotransmitter binds with receptor protein that are present in post synaptic membrane and form neurotransmitter-receptor complex.
f. Production of EPSP
Neurotransmitter receptor Complex causes production of non propagated EPSP.
{ Acetylcholine is a common excitatory neurotransmitter)
MECHANISM OF DEVELOPMENT OF EPSP-
1. neurotransmitter receptor Complex causes opening of ligand gated sodium channels.
2. Sodium ions enters into cell body of postsynaptic neuron from ECF.
3. Development of mild depolarization because sodium ion is positively charged and resting membrane potential (RMP) inside the cell body is altered.
This type of mild depolarization is called EPSP.
Properties of EPSP-
° it is a graded potential.
° it is non propagated.
° it does not obey all-or-none law.
° it is similar to receptor potential and end plate potential.
Significance of EPSP-
° EPSP causes Development of action potential.
° it is not transmitted into axon of postsynaptic neuron.
° when EPSP is strong it opens the Na+ channels in initial part of axon.
° action potential develops in initial part of axon ( depolarization) due to enterance of Na+ ions.
° action potential transfer to other part of axon.
*INHIBITORY FUNCTION
[IPSP]= inhibitory postsynaptic potential
Inhibition of synaptic transmission is classified into five types-
1. Postsynaptic or direct inhibition
2. Presynaptic or indirect inhibition
1. POSTSYNAPTIC OR DIRECT INHIBITION
• it occurs due to release of inhibitory neurotransmitter from presynaptic terminal.
✓ inhibitory neurotransmitters are
° GABA ( gamma amino butyric acid )
° dopamine
° glycine
Development of IPSP-
Action of GABA
a° it develops during postsynaptic inhibition.
b° it is electrical potential in the form of hyperpolarization.
c° inhibitory neurotransmitter binds with receptor on postsynaptic membrane.
d° transmitter receptor Complex opens the K+ channel. ( Not the Na+ channel)
e° potassium ion Move to ECF from cell body of postsynaptic neuron.
f. Chloride channel also open and chloride ions (Cl-) move inside the cell body of postsynaptic neuron from ECF.
g° exit of potassium ion and influx of chloride ions cours more negativity inside that leads to hyperpolarization.
h° hyperpolarized state of synapse inhibit synaptic transmission.
• it occurs if release of excitatory neurotransmitter fails from presynaptic axon terminal.(Syneptic knob)
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