Synapse

Synapse is the functional junction between two neurons.By the formation of synapse impulse is transmitted from one part of the body to the distant area through the series of neurons.

Types of synapses

There are mainly two types of synapse they are the chemical synapse and the electrical synapse.

Chemical synapse

Almost all synapse used in the central nervous system in man belongs to the chemical synapse.It is usually axodendritic in direction.In this axon carry impulse away from the cell body and Dendron carry impulse towards the cell body and thus the axon act as the pre synaptic nerve while the dendron act as the post synaptic nerve.The  direction of transmission in chemical synapse is usually unidirectional that is axodendritic  because only the axon terminal or the pre synaptic terminal is capable of producing  the neuro transmitters.

Structural components of synapse

It is mainly composed of presynaptic nerve terminal and the postsynaptic nerve terminal

 A.Presynaptic nerve terminal

The pre-synaptic terminal terminates as oval or round knobs and are called as synaptic knobs or terminal knobs or boutons or end-feet.It is separated from the post synaptic terminal by the synaptic cleft having width of 200-300 angstroms.The pre synaptic terminal also contain  the synaptic vesicles  and the mitochondria.The synaptic vesicles conain the neuro transmitters which may be either fascilitatory or inhibitory in nature.The important neuro transmitters used in synapse includes the Acetylcholine,Dopamine,Sertonin,Glycine,Gamma Amino Butyric Acid (GABA),Nor-epinephrine etc..The mitochondria provide energy to synthesize new neuro- transmitters.

Mechanism of release of  neuro transmitters at the pre synaptic terminal

When an action potential reach the pre synaptic terminal it results in inflow of calcium ions from the synaptic cleft into the pre synaptic terminal.The calcium ions  thus entered  will combine with the protein molecule(release sites) along the inner surface of the pre synaptic membrane. This in turn results in the binding of the synaptic vesicles with  the pre synaptic membrane and they fuse with it.  Finally the content of the synaptic vesicles will be released into the synaptic cleft through exocytosis.

B.Post synaptic nerve terminal

The post synaptic nerve terminal contain the receptor proteins for the neuro transmitters released.The receptor protein has two important components  and are the binding component and the ionophore component.The binding component helps to bind the transmitter released and the ionophore component either will have an ion channels or a second messenger activator.

Ion channels: The ion channels in the post synaptic nerve are of two types and are the cation channels and the anion channels.The cation channels helps in conduction of the sodim ions while the anion channels conduct the calcium ions.The cation channels which conduct the sodium ions are lined with negative charges .These charges attract the positively charged sodium ions  and repels the negatively charged chloride ions and other anions and there by prevent their passage.

When the diameter of the anion channel increased the anions such as chloride ions pass into  and through the channels.The flow of cations through this channel is blocked mainly because of their large size.

The opening of sodium channels excites the post synaptic membrane.The transmitter substance that open the sodium channel is called as an excitatory transmitter and those transmitter which open the chloride channel is called the inhibitory transmitter.

When a transmitter substance activates an ion channel,the channel usually opens with a fraction of a millisecond  and when the transmitter substance is no longer present the channel closes rapidly.There fore the opening and the closing of ion channels provide a means for the rapid activation or rapid inhibition of the post synaptic neuron.

Second messenger system in post synaptic neuron

The ion channels are not sufficient to cause prolonged post synaptic neuronal changes because this channel close within millisecond in the absence of transmitter substances so in many instance the prolonged neuronal action is achieved by activating a second messenger chemical system inside the post synaptic nerve.

There are several types of second messenger systems and one of the most prevailing type is the G-proteins.The important changes that a second messenger can exert inside the post synaptic nerve includes the opening of the specific ion channels in the post synaptic nerve membrane,activation of cAMP or cGMP, activation of intra cellular enzymes and the activation of gene transcription.

Removal of neurotransmitter at the synapse

After a neurotransmitter is released at the  nerve endings, it is either destroyed or removed in order to prevent their continued action and this is established either by diffusion of the transmitter from the synaptic cleft in to the surrounding fluids or by the enzymatic destruction of the transmitter(Eg.destruction of acetylcholine by the enzyme acetylcholine esterase) or by the active transport of transmitters back into the pre synaptic terminal itself (Transmitter re-uptake)

Excitatory and inhibitory receptors in the postsynaptic membrane.

Some post synaptic receptors ,when activated  cause excitation of the post synaptic nerve and other cause inhibition.

Excitation

1.Opening of sodium channels allowed inflow of  large number of positive electrical charges to the interior of the post synaptic nerve cell.This raises the membrane potential in the positive direction  up toward the threshold level for excitation.This is the most widely used mean of causing excitation.

2.Depressed conduction through chloride or potassium channelor both.This decreases the diffusion of positively charged ions to the outside.In either instance the effect is to make the internal membrane potential more positive,which is excitatory.

3.Various changes in the  internal metabolism of the cell to excite cell activity or in some instances increase in the number of excitatory membrane receptors or decrease in the number of inhibitory membrane recptors.

INHIBITION

1.Openig of chloride ion channels through the receptor molecule.This allows rapid diffusion of negatively charged chloride ions from outside the post synaptic neuron to the inside,thereby carrying negative charges inward and increasing the negativity inside,which is inhibitory.

2.Increase in the conductance of potassium ions through the receptor.This allows positive potassium ions to diffuse to the exterior,which is also inhibitory.

3.Activation of receptor enzymes that inhibit cellular metabolic functions or that increase the number of inhibitory synaptic receptors or decrease the number of excitatory receptors.

Synaptic delay

Transmission of impulse along a synapse is slower than that of its conduction along a neuron. This is because of the time needed for the release of a neurotransmitter, its diffusion through the synaptic cleft and its action on the post synaptic membrane.The difference in  the rate is called the synaptic delay. It amounts to about half a millisecond at body temperature (37⁰C)

Synaptic fatigue

Repeated stimulation of the pre synaptic knob may deplete the neurotransmitter and this may fails to stimulate the post synaptic membrane. This condition of synapse is called  synaptic  fatigue. It lasts for several seconds during which the neurotransmitter is re synthesized. Synaptic fatigue is the only fatigue which affects the nervous tissue. Conduction of the nerve impulse along the neurons is not subject to fatigue.