Nerve impulse conduction at synapse

 

Synapse is junction between two Cell in which one cell is neuron and another cell may be nerve, muscle or gland.

In Cardiac muscle, Junction between muscle and muscle is synapse.The neuron Sending the signal is called presynaptic neuron while neuron receiving the message is Called post synaptic neuron.

synapse may be;

• Axodendritic: From axon to dendrite.

• Axosomatic: From axon to Cell body.

• Axoaxonic: From axon to axon.

The pre-synaptic cell is generally a neuron while postsynaptic  cell may be neuron, muscle or gland.

The synapses are essential for homeostasis because they allow information to be filtered and integrated. During learning, the structure and function of a synapse Changes new synapses are formed and previous synapses are destroyed.

synapse are of two types;  Chemical synapse and electrical synapse.

Chemical Synapse: Though the Plasma membrane of presynaptic and post synaptic membrane are close to each other , they do not touch each other. They are separated by synaptic cleft filled with interstitial fluid.

When Nerve impulse reaches presynaptic membrane , it releases neurotransmitter which diffuses through fluid in synaptic cleft and binds to receptors in post-synaptic  membrane. The Postsynaptic neuron receives chemical signal and produces Post synaptic potential.

The Chemical synapse relay signals slowly than electrical synapse due to synaptic delay of about 0.5 ms.

Mechanism of impulse Conduction:

when a nerve impulse arrives at pre-synaptic end , the voltage geted calcium Channels open and there is inflow of Calcium ions. An increase in Calcium concentration inside presynaptic neuron Serves as Signal that triggers exocytosis of synaptic vesicles and neurotransmitter within vesicles are released into the synaptic cleft. The neurotransmitter diffuse across the Synaptic cleft and bind to neurotransmitter receptor  in post synaptic membrane. The binding of neurotransmitter on their receptors causes ligand gated ion  Channels to open on Post synaptic membrane and allows Particular ions to flow across the membrane. Due to this flow of ions , voltage across the Post synaptic membrane Changes and this Change may be depolarisation or hyperpolarisation. If the depolarisation in postsynaptic membrane reaches thresold potential, it triggers an action potential in postsynaptic neuron.

At most Chemical synapse ,only one way information transfer Occurs from Presynaptic neuron to postsynaptic neuron. It is from axon to dendron not from dendron to axon because dendrons cannot secrete neurotransmitter and axon has no Chemoreceptors.

The most common neurotransmitter is Acetyl choline (Ach). To avoid Continued Stimulation of next neuron by neurotransmitter ,it is inactivated. The ACh is inactivated by Acetylcholinesterase(AChE) present in Synaptic cleft. This enzyme hydrolyses Acetyl choline into acetic acid and Choline. They diffuse into presynaptic membrane and enter inside mitochondria where they combine with each other in presence of enzyme Choline Acetyl transferase to form Acetylcholine Which is stored in synaptic vesicles.

Neurotransmitters: Are Chemical substances which Carry  impulse across synapse. The neurotransmitter may be small molecule neurotransmitters and Neuropeptides.

The small molecule neurotransmitters are Acetylcholine, Aminoacids, biogenic amines, nitric oxides etc.

Neuropeptides Consists of 3-40 aminoacids linked by peptide bonds. Brain neurons have plasma membrane receptors for opiate drugs such as morphine and heroin. The neuropeptide , enkephalins ( First identified neuropeptide) bind to such receptors which has analgesic effect 200 times stronger than that of morphine. The opoid peptides also include endorphins and dynosphins. These are natural Painkillers.

The neurotransmitters are also classified as Excitatory neurotransmitter and Inhibitory neurotransmitters.

Those neurotransmitters which open ligand gated Channels of positive ions (Cations) and Cause depolarisation in Postsynaptic neuron and Cause impulse transmission are called excitatory neurotransmitters. Eg. Glutamate, Acetyl choline, nor-adrenaline etc.

Glutamate ( Glutamic acid) is primary excitatory neurotransmitter in CNS.

Those neurotransmitters which open ligand gated Channels for negative ions ( anions) like CI- and Cause hyperpolarisation in postsynaptic membrane and inhibits impulse transmission are Called inhibitory neurotransmitters. eg: GABA ( Gamma Amino butyric Acid ),Dopamine , glycine, serotonine etc.

GABA is primary inhibitory neurotransmitter in CNS.

☆ Deficiency of Acetylcholine Causes Alzimers.

☆ Deficiency of Dopamine Causes Parkinson's disease.

☆ Nitric oxide plays Crucial role in  vasodilation by stimulating the production of Cyclic GMP. NO is natural vasodilator. viagra(sildenafil citrate) Which is used for the treatment of erectile dysfunction inhibits break down of cyclic  GMP, allowing nitric oxide's vasodilation for longer time. it decreases blood pressure.

 

Electrical synapse: In electrical synapse the presynaptic and postsynaptic membrane are very close to each other and remains Connected by gap junctions .Each gap junction Contains hundreds of tubular structures Called connexons which act like tunnel to Connect the cytosol of two Cells directly. The gap junctions are found in visceral Smooth muscles, Cardiac muscles, developing embryo, CNS etc.

It's main advantage is faster Communication and synchronization (Coordinate the activity of group of neurons or muscle cells).