The primary function of the nervous system is the transmission of signals and communication. This is achieved through transmission through synapses. A neuron is the structural and functional unit of the nervous system.
Structure of neuron
A neuron has three parts- a cell body, dendrites, and an axon ending at an axon terminal.
Dendrites are the numerous short extensions from the cell body. They receive the incoming signals and transmit them to the cell body.
Axon is the long tubular process that transmits output signals. The axon ends at synaptic knobs. The axon may be unmyelinated (no sheath) of myelinated. The myelin sheath provides electrical insulation and formed by Schwann cells. The naked space between two successive sheaths on the neuron is called the node of Ranvier.
Synapses are the junctions between two or more neurons where impulses pass from one neuron to others. Neuronal messages are conveyed to the appropriate structures in the CNS. Command messages from the CNS are transmitted through the synapses to the peripheral organs.
Each neuron forms about 2,000 synapses. There are about 1011 neurons in the CNS. The synapses are of different types and can be classified on the following bases.
Parts of neurons involved in the synapse
- Axodendritic synapse- The axon of the presynaptic neuron connects to the dendrite of the postsynaptic neuron. This is the most common synapse in the CNS.
- Axosomatic synapse- The axon of the presynaptic neuron synapses with the cell body(soma) of the postsynaptic neuron. This type of synapse is also common.
- Axoaxonic synapse- The axon of presynaptic neuron synapses with the axon of the postsynaptic neuron.
- Dendrodendritic synapse- The dendrite of presynaptic neuron connect to the dendrite of the postsynaptic neuron.
According to the nature of transmission
- Chemical synapse- through the lysosomes
- Electrical synapse -through gap junctions
- Conjoint synapse -partly electrical partly chemical
According to the number of neurons involved
- One to one- one neuron to another neuron
- Many to one- multiple neurons to a single neuron
- One to many- one neuron to multiple neurons
Functional anatomy of a synapse
A presynaptic neuron is a neuron from which information passes through the synapse. The neuron which receives the information is the postsynaptic neuron. The presynaptic membrane is formed by the part of the presynaptic axon terminal forming the synapse and that of the postsynaptic neuron is called the postsynaptic membrane. The space between the presynaptic and postsynaptic membrane is called the synaptic cleft. These three structures together form the synapse.
A neurotransmitter is a chemical substance that is used to transfer information through the synapse amplify the effect of the action potential arriving at the synapse this amplification causes the stimulation of the postsynaptic neuron when the action potential at the presynaptic passes through that cleft. Some examples of neurotransmitters are acetylcholine, catecholamines, Gamma-aminobutyric acid, glutamate, glycine, serotonin, histamine, substance P etc.
Presynaptic Axon Terminal
The terminal of presynaptic neurons usually ends in a small bulbous enlargement called the terminal button or synaptic notch. This notch contains numerous synaptic vesicles and dense tufts.
- Synaptic vesicles -they contain neurotransmitters like acetylcholine, gamma-aminobutyric acid, glycine, catecholamines, neuropeptides etc. They are of three types of small vesicles with clear code, small vesicles with dense code and large vesicles with a dense core.
- Dense tufts- these are filamentous projections present close to the vesicles. They help in exocytosis of the vesicles. They are present mainly in the active zone.
- Active zone- a modified region of the presynaptic membrane which contains many proteins and calcium channels. Vesicles containing neurotransmitters release their contents mainly through the active zone.
The synaptic knob contains many mitochondria that provide energy for the different processes that occur in this region. This energy is also used to maintain the resting membrane potential so that the membrane is available for the next potential change.
This gap is about 20 to 50 nanometre wide and is filled with extracellular fluid. The neurotransmitter molecules released from presynaptic terminal diffuse across the cleft to reach the postsynaptic receptors.
The area of the postsynaptic membrane modified for synaptic transmission is called the postsynaptic density. This area has a cluster of receptors for neurotransmitters embedded within the membrane.
Steps of synaptic transmission
Synaptic transmission is a process through which information from a presynaptic neuron passes on to the postsynaptic neuron to the synapse. The mechanism of synapse transmission can be divided into the presynaptic and postsynaptic mechanisms.
Vesicles containing neurotransmitter molecules are concentrated at the active zone of the presynaptic axon terminal. These vesicles attach with the membrane(with the help of synaptobrevin and syntaxin protein) and become ready to discharge their content in response to a stimulus.
Action potential arrives at presynaptic axon terminal and depolarizers the presynaptic membrane. The depolarization causes the opening of voltage-gated calcium channels so that calcium can enter the axon terminal through the active zone. The increase in calcium ion concentration in the presynaptic terminals causes the discharge of the neurotransmitters from the vesicles
Within the synaptic cleft, the neurotransmitter diffuses across and goes to the postsynaptic membrane.
The neurotransmitter binds with the receptors in the postsynaptic membrane, and conformational change occurs in the receptor. This either opens an ion channel or triggers a cascade reaction that generates a secondary messenger, which can generate a change in ionic permeability of the cell.
Thus the ion channels in the postsynaptic membrane open up, and movement of ions occurs. Depending on the ions and direction of the movement, there is a change in the membrane potential of the postsynaptic membrane. It may either be depolarized or hyper-polarized. This change is called synaptic potential which creates a signal and the action potential travels through the axon and process is repeated.