Synapse is a term that was first coined by Charles S. Sherrington in the year 1897 and derived from the Greek word “Synapsis” which means to conjugate or clasp. The communication between the neurons is through synapses only that helps in the transmission of a nerve signal from one to the next cell. A scientist named Sanford Palay observed the ultrastructure of neural tissue to prove the subsistence of synapse. Synapses function as a key junction in the nervous system, without which a signal cannot reach the brain directly.
A nerve impulse can directly pass to the next cell via electrical synapses, during the time of fast defence responses. Generally in chemical synapses, a nerve impulse must excite the release of neurotransmitters so that it can carry the signal further via binding with specific cell receptors.
Definition of Synapse
Synapses can define as a junction that helps in the conjugation and coordination of signal transmission activity between the two adjoining neurons. It also refers as “Neuronal junction” as it forms a neuronal network to coordinate the tasks performed by the central nervous system and peripheral effector cells. Synapses comprise of the following elements like:
Presynaptic terminal: It contains the synaptic vesicles encapsulated around the neurotransmitter substance.
Synaptic cleft: It also refers as “Synaptic gap” that is 20 nm wide and separates the two adjacent neurons.
Postsynaptic terminal: It contains receptor sites for the binding of a neurotransmitter, which can either inhibit or promote the passage of nerve signal from one cell to the next.
Classification of Synapses
Based on the attachment of neurons, the synapses can classify into the following types:
In this type, a synaptic junction is in between the axon endings of one neuron with the dendritic spines of another neuron.
Here synapses occur between the axon terminal of one nerve fibre with the soma or cell body of an adjacent neuron.
In this kind, synapses occur in between axon’s terminal ending of neighbouring neurons.
Based on neurotransmitter and neuroreceptor in a neuron, synapse can classify into the following types:
Excitatory Ion Channel Synapses
It contains neuroreceptor in the form of sodium ion channels that influx the electropositive sodium ions into the cytosol. The movement of sodium ions will depolarize the membrane potential inside the cytosol and generates the action potential respective to the stimulus. For the conduction of an action potential, a stimulus should reach the maximum threshold.
When an action potential reaches the presynaptic terminal, it will excite the synaptic vesicles having neurotransmitters like acetylcholine, glutamate or aspartate to fuse with the plasma membrane. The fusion results in the diffusion of neurotransmitters that stimulate the adjacent neuron.
Inhibitory Ion Channel Synapses
It contains the neuroreceptor in the form of chloride channels that influx the chloride ions into the cell cytoplasm. The movement of chloride ions will bring out hyperpolarization of the membrane potential and slow down the conduction of an action potential. In this synapse, the diffusion of neurotransmitter from presynaptic neuron inhibits the postsynaptic neuron. It includes glycine or GABA as inhibitory neurotransmitters.
These are having neuroreceptors in the form of membrane-bound enzymes, instead of ion channels. When the membrane-bound enzymes get activated by the neurotransmitters, they release the chemical messenger that performs an essential role in cell metabolism and cell response. It mediates the long-lasting responses (learning and memory) and comprises neurotransmitters like acetylcholine, epinephrine, dopamine etc.
The transmission of a nerve signal between the adjoining neurons is not a simple process. The nerve impulse must excite the synaptic vesicles to integrate it with the axon’s membrane. The dissolution of synaptic vesicles ensures the propagation of neurotransmitter molecules through exocytosis.
The chemical messengers or neurotransmitters can further carry the nerve signal from the presynaptic terminal beyond the synaptic gap. After its release, it binds with the specific cell receptors of the postsynaptic neuron or the target cell. The action of neurotransmitter can be either inhibitory or excitatory, i.e. it may either excite or inhibit the neuron they bind with.
Here, we can take a reference of neurotransmitter as key and cell receptors as a lock. Therefore, neurotransmitter functions as a key that can open and close the cell receptors or lock. The specific binding of neurotransmitter with the cell receptor will initiate the further movement of the nerve signal.
Types of Synapse
Synapses define as the functional links between the neural network, which can be either electrical or chemical.
It produces nerve impulse which can freely travel between the adjacent cells through gap junctions without any carrier molecules. Electrical synapses facilitate faster conduction of the nerve signal. It does not require neurotransmitters, and it can conduct the transmission of information bidirectionally.
In this, the ions move vigorously through the tiny apertures between gap junctions. The movement of ions occurs in a synchronized manner, without collision of ions. The gap between electrical synapses is approximately 3.5 nm. Electrical signals are generally excitatory. Destruction in signal strength occurs when it travels between the neighbouring neurons. To overcome the loss, it requires big presynaptic neuron to excite much smaller postsynaptic neuron.
In this type, the transmission occurs via synaptic knob containing synaptic vesicles. The vesicles store a chemical messenger (neurotransmitter) which gets fuse and releases the neurotransmitter outside the neuron. The nerve impulse along with the carrier molecule or neurotransmitter is carried across the membrane via the voltage-gated calcium channels.
An opening of voltage-gated calcium channels allows rapid influx of calcium ions whose concentration in the presynaptic neuron increases, resulting in a fusion of the presynaptic vesicle with the plasma membrane by releasing chemical messenger out of the neuron. Chemical synapses can further classify into the excitatory and inhibitory synapse based on its effect on nerve signal.
Excitatory Chemical Synapses
It promotes the propagation or conduction of an action potential. The binding of the neurotransmitter to the excitatory synapse leads into an opening of non-voltage gated channels that allow an influx of sodium or sometimes both sodium and potassium ions into the plasma membrane. The opening of the channel facilitates depolarization of the presynaptic plasma membrane that generates an action potential.
Inhibitory Chemical Synapses
It inhibits the propagation or conduction of an action potential. Binding of neurotransmitter with the inhibitory synapse results into an opening of potassium and chloride channels. Then it leads to the hyperpolarization of the postsynaptic membrane that ceases the further movement of an action potential.