Synapse is a junction between two neurons or a neuron and a target or effector cell such as a muscle cell. It permits the transmission of electrical or chemical signals.
The synapse is formed between presynaptic and postsynaptic neurons. It is known as the neuromuscular junction between a neuron and muscle.
The conduction of nerve impulses from an axon terminal of a neuron to the dendrites of the next neuron occurs through a synapse. It can be electrical or chemical.
Context
1. Types of Synapses
2. Transmission along chemical synapses
3. Property of chemical synapses
There are two types pf synapses:-
1. Types of synapses
(i) Chemical synapses
(ii) Electrical synapses
Chemical Synapse
- Chemical synapses are more common. The transmission of nerve impulses through chemical synapses is mediated by neurotransmitters.
- There is a fluid-filled space between the two neurons called the synaptic cleft. The nerve impulse cannot jump from one neuron to another.
- Axon terminals have a knob-like structure, which contain synaptic vesicles.
- When the action potential reaches the terminals, the synaptic vesicles from the terminal of the presynaptic neuron release neurotransmitters at the synaptic cleft.
- Neurotransmitters bind to the receptors present in the postsynaptic membrane.
- This results in the opening of voltage-gated channels and the flow of ions. This causes a change in the polarisation of the postsynaptic membrane and the electric signal is conducted across the synapse.
- Neurotransmitters can be inhibitory or excitatory. One neurotransmitter can initiate different responses in different cells.
- If there is a net flow of positively charged ions inside the cell, then the neurotransmitter is excitatory and it results in generating the action potential. This is called EPSP or excitatory postsynaptic potential.
- When the membrane potential becomes more negative, the membrane becomes hyperpolarized and the action of neurotransmitter is inhibitory. They generate IPSP or inhibitory postsynaptic potential.
- Once the neurotransmitters get attached to the receptor they are either acted on by enzymes or taken back and recycled to terminate the signal after it is conducted forward.
Electrical Synapse
- Electrical synapses are faster than chemical synapses.
- The presynaptic and postsynaptic neurons are in close proximity and they form gap junctions. There is a physical connection between pre and postsynaptic neurons at the gap junction by protein channels.
- These gap junctions allow the direct flow of ions and the transmission of an electric signal across the electrical synapse is similar to the conduction of impulse in an axon.
- Electrical synapses are not as flexible as chemical synapses as they cannot turn the excitatory signal to the inhibitory signal.
- It is found in some lower invertebrates. In humans, it is found between glial cells
2. Transmission along chemical Synapses
(i) Wave of depolarisation reaches presynaptic membrane.
(ii) Calcium ion from exreacellulaqr bfluid diffuses into axon terminal.
(iii) Calcium stimulates the fusion of synaptic vesicles with pre synaptic membrane and neurotransmitter are released in synaptic cleft.
(iv) Neurotransmitters bi8nds with specific recertors on post synaptic cleft.
(v) Binding makes sodium ion channels open.
(vi) New action Potential is generated.
Neurotransmitters
The Chemicals which are only present in chemical synapsis ghelps in transmission of impulses. It are vof two types -
(A) Excitatory
Acetylcholine, Epinephrine, Norepinephrine, Glutamate
(B) Inhibitory
Dopamine, Seotonine, Glycine, GABA.
Properties of Synapse:
1. One-way conduction (unidirectional conduction):
In a chemical synapse, since neurotransmitter is present only in the presynaptic region, impulse gets conducted from pre- to postsynaptic region only and not vice versa.2. Synaptic delay is for the neurotransmitter to:
a. Get released from synaptic vesicles when action potential has reached the presynaptic region.b. Pass-through synaptic cleft.
c. Act on a postsynaptic region to bring about the production of action potential in the postsynaptic region.
For all the above events to be brought about, sometimes is required. This is known as synaptic delay, which is normally about 0.5 msec at every synapse.
3. Fatigability:
When synapses are continuously stimulated, after some time, due to exhaustion of neurotransmitters at presynaptic terminals, impulses fail to get conducted. This results in fatigue occurring at the level of the synapse. Fatigue is a temporary phenomenon. If some rest is given to neurons, resting facilitates the resynthesis of neurotransmitters for further conduction of impulse across the synapse.4. Convergence and divergence:
Impulses from one presynaptic nerve fibre may end on the postsynaptic region of a large number of neurons and this is called divergence. When nerve fibres of different presynaptic neurons end on a common postsynaptic neuron, this is known as convergence. In CNS, on average, about 10000 synapses are found on any one neuron.5. Summation:
When a stimulus of subthreshold strength is applied, there will not be the development of action potential in the postsynaptic region. But if many subthreshold stimuli are applied at presynaptic region, effects of these stimuli can get added up and lead to action potential development in the postsynaptic region. This is known as summation.There are two types of summation namely spatial and temporal. In temporal summation, presynaptic neuron stimulated will be the same, but many stimuli are applied in rapid succession (timing of stimuli will be different, but a place of stimulation will be the same).
In spatial summation, presynaptic neurons stimulated will be different but stimuli will be applied simultaneously (time of stimulation shall be same, but places of stimulation will be different). This is possible because of the property of convergence.
