Synapse

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1. Definition

• The junction between two neurons is called a synapse.
• It is a specialized junction where transmission of information takes place between a nerve fibre and another nerve, muscle or gland cell.
• It is not the anatomical continuation. But, it is only a physiological continuity between two nerve cells.

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2. Structure

The synapse consists of:

1. A presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles.

2. A postsynaptic ending that contains receptor sites for neurotransmitters.

3. A synaptic cleft or space between the presynaptic and postsynaptic endings. It is about 20nm wide.

• Function
• The main function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another.
• They allow integration, e.g. an impulse travelling down a neuron may reach a synapse which has several post synaptic neurons, all going to different locations. The impulse can thus be dispersed. This can also work in reverse, where several impulses can converge at a synapse

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Types

Synapses are usually classified as follows.

1. According to the part of neuron involved.

• Axo-dendritic- axon with dendrite.
• Axo- somatic- axon with cell body (soma)
• Axo- axonic- axon with axon.
• Dendro- dendritic- dendrite with dendrite.

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2. According to the nature of transmission

Chemical synapse – through neurotransmitter

• In a chemical synapse, electrical activity in the presynaptic neuron is converted into the release of a chemical called a neurotransmitter that binds to receptors located in the plasma membrane of the postsynaptic cell.

Electrical synapse – through gap junctions.

• In these synapses the membranes of the two cells actually touch, and they share proteins. This allows the action potential to pass directly from one membrane to the next. They are very fast, but are quite rare, found only in the heart and the eye.
• Conjoint synapse – partly electrical and partly chemical

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3. According to the number of neuron involved

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• One neuron ends on another (one to one)
• Multiple neurons ending on a single neuron (many to one)
• One neuron ends on multiple neurons (one to many)

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• Convergence refers to the phenomenon of termination of signals from many sources (i.e. many pre-synaptic neurons on a single post-synaptic neuron).
• Divergence refers to one pre-synaptic neuron terminating on many post-synaptic neurons. (i.e. single impulse is converted into a number of impulses going to a number of post-synaptic neurons.)

• Chemical synapse:
• Majority of synapses are of this type. Here synaptic transmission needs chemical mediators.
• The neuron from which the information passes through the synapse is called presynaptic neuron and the neuron which receives the information is called post synaptic neuron.
• The part of presynaptic neuron forming the synapse is called presynaptic membrane and that of the post synaptic neuron is called the post synaptic membrane.

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• The synapse is thus formed by the presynaptic membrane, postsynaptic membrane and the synaptic cleft in between them.
• The synaptic cleft is the gap in between these two membranes and is about 20 -50 nm wide.
• The presynaptic membrane is usually the part of an axon terminal, also called synaptic knob. Which contains neurotransmitter.

• It shows thickening called active zone which contain voltage gated Ca²⁺channels along with other proteins related to transmitter release.
• The postsynaptic membrane may be a part of dendritic spine, part of a cell body (soma) or part of an axon of neuron ( and in some cases muscle and gland cells), which contain receptors for the neurotransmitter. Sometimes it shows thickening called post synaptic density which contains receptors, binding proteins etc.

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• Neurotransmitter: is the chemical substance which is used to transfer of information through the synapse.
• This neurotransmitter amplifies the effect of the AP coming to the synapse.

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• Only due to this amplification the AP in the presynaptic neuron can stimulate the post synaptic neuron. Otherwise the AP, through usual way of conduction (I. e by local current) would not be able to depolarize the postsynaptic membrane after passing through the synaptic cleft.

• The arrangement in the synapse is such that the neurotransmitter can act on an wide area of the post synaptic membrane and a large number of receptors (ion channels) are activated.
• The protein neurexins in the presynaptic membrane help a lot to maintain the normal activities of a synapse.

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Synaptic transmission:

• It is the process by which the information passes through release of neurotransmitter.
• When an AP comes along the axon of presynaptic neuron it increases Ca²⁺ entry from the ECF into the synaptic knob through voltage gated Ca²⁺ channels.
• Increased Ca²⁺ in the synaptic knob leads to exocytosis of the neurotransmitter stored in the vesicles in the synaptic knob.

• The proteins called neurexin influence the synaptic activity and also help to hold the synapses together.
• The released transmitter now crosses the synaptic cleft by diffusion and binds to large number of its receptors on the postsynaptic membrane.
• These receptors are ligand gated ion channels. After the binding, ion channels in the receptors open up and movement of ions occurs.

• Depending upon the (Cation or Anion) and direction of their movement, the membrane potential MP of postsynaptic membrane changes either towards depolarization or hyperpolarization.
• This change of MP, also called synaptic potential creates the signal in post synaptic neuron.

• AP in the presynaptic ending entry of Ca²⁺in the synaptic ending release of neurotransmitter binding with receptors on the postsynaptic membrane opening of the ion channels movement of ions through the postsynaptic membrane change of MP in the postsynaptic membrane (synaptic potential)

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Synaptic potential may be of two types

• Excitatory postsynaptic potential (EPSP)
• Inhibitory postsynaptic potential (IPSP)

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• Synaptic potential is of longer duration than an action potential and if it is excitatory, can cause repeated firing of the initial segment of the postsynaptic neuron.

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• Electrical synapse
• This type is formed through gap junction, through which electrical activities of one neuron can pass to the other directly.
• This is found between some neuron in the lateral vestibular nucleus, neocortex and cerebellum etc. in chemical synapse the transmission is usually unidirectional.
• An electrical synapse may be bidirectional and transmission is fast.

• Slow electrical events are also easily transmitted by them.
• As stated above due to magnification of signal, the chemical synapses are superior to the electrical synapses in the nervous system.
• One to one synapse is the neuromuscular junction, many to one is the usual type found in CNS and one to many is not frequent .
• One to one synapse is typical of parasympathetic and one to many types are found in sympathetic system.

Properties of synapse

• The synapses or rather the synaptic transmission shows the following properties.

1. Law of forward conduction:

• Through a synapse impulse can travel in one direction only i.e., from pre to postsynaptic neuron(exception electrical synapse). This is called law of forward conduction.

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2. Synaptic delay

• Transmission in the synapse suffer a delay.
• This synaptic delay is the time required for the impulse to cross the synapse.
• It is about 0.5 ms. This time is required for Ca²⁺entry in the presynaptic knob, release of neurotransmitter and for its action on the postsynaptic membrane.
• The knowledge about synaptic delay helps to find out the number of synapses present in neural pathway.

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3. Law of divergence and convergence

• Through the synapses, information from one neuron can pass to many neurons and from many neurons information can pass to a single neuron. These are respectively called divergence and convergence.

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4. Excitatory postsynaptic potential(EPSP):

• If the potential change in the postsynaptic membrane due to synaptic transmission is towards depolarization then it is called EPSP, e.g. initially the RMP was -70 mV, after transmission it becomes say, -60mV, so the EPSP = 10 mV. EPSP of optimum magnitude leads to excitation (AP formation) of postsynaptic neuron.
• EPSPs are caused by transmitters like acetylcholine, nor adrenalin etc.
• Ionic basis: the neurotransmitter causing EPSP increases permeability of the postsynaptic membrane to various cations through its receptor. For example, acetylcholine receptors allow Na+ entry and K+ exit.

5. Inhibitory postsynaptic potential (IPSP)

• Due to synaptic transmission, if the potential of postsynaptic membrane is carried towards hyperpolarization, then it is called IPSP.
• This is because the hyperpolarization leads to inhibition of the postsynaptic neuron.
• Suppose the RMP was -70 mV, after transmission it becomes say -80 mV, so IPSP = 10 mV.

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Ionic basis Here the neurotransmitter usually causes opening of Cl^- channels and entry of Cl^- leads to hyperpolarization of the postsynaptic membrane, this carries the MP away from the firing potential and thus causes inhibition. An increase of K⁺efflux, a decreased N⁺ or Ca²⁺influx will lead to IPSPs

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