Last week we studied the concept of action potential. It is responsible for transmitting information between the sensory neurons and the central nervous system. This week we shed some light on synapses and how they transfer information between two neurons.
Synapse is the site of transmission of electric nerve impulses (or simply information) between two neurons. The synapse consists of
1. a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles
2. a postsynaptic ending that contains receptor sites for neurotransmitters
3. a synaptic cleft or space between the presynaptic and postsynaptic endings.
Types of Synapses
Synapses connect two neurons together and carry the information from the presynaptic neuron to the postsynaptic neuron. There are electrical synapses which are located at gap junctions. The membranes of two neurons have only about 3 nm between them, which allows ions to pass directly from one cell to another through protein channels. During an action potential, ions move to the next cell causing it to generate an action potential. Electrical synapses are bidirectional and electrical transmission moves very fast.
Most of the synapses in the brain are chemical. In a chemical synapse a synaptic cleft separates the presynaptic and postsynaptic membrane. The presynaptic neuron has vesicles that contain parts of neurotransmitters. Neurotransmitters are the chemicals that carry the messages to the postsynaptic neuron. They attach to receptors which respond to only a specific neurotransmitter.There are different types of chemical synapses; central nervous system (CNS) and neuromuscular junction synapses. The CNS synapses can be axodendritic, axosomatic, axoaxonic, axospinous or dendrodendritic. CNS synapses can also be classified as asymmetrical or Gray’s type I synapses. The differences of the chemical synapses are described in Table 1.
Types of chemical synapses and their descriptions:
- Axodendritic: The postsynaptic membrane is on a dendrite
- Axosomatic: The postsynaptic membrane is on the cell body
- Axoaxonic: The postsynaptic membrane is on another axon
- Axospinous: A presynaptic axon contacts a postsynaptic dendritic spine
- Dendrodendritic: Dendrites form synapses with one another
- Asymmetrical or Gray’s type I: The membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side
- Symmetrical or Gray’s type II: The membrane differentiations are of similar thickness
Types of neurotransmitters and their synthesis
Neurotransmitters can be categorized based on their chemical composition into amino acids, amines and peptides. All major neurotransmitters categorized in this manner can be seen from figure 2.
The synthesis of peptides happens on a different site in the cell compared to amines and amino acids. The precursor peptides are made in the rough endoplasmic reticulum and the finalization of the synthesis happens in the Golgi apparatus from where the synthesized neurotransmitters are loaded into vesicles and transported through an axon into a storage site. For amines and amino acids, the synthesis happens in the cytosol of the axon terminal and this procedure is done by certain enzymes. After the synthesis, the transporter proteins load amines and amino acids into vesicles and transport them to the storage site of the neurotransmitters.In general, the synapses in the central nervous system are transmitted by gamma-aminobutyric acid, glycine or glutamate, whereas acetylcholine is responsible for mediating the neuromuscular junctions. There is fast and slow synaptic transmission from which the fast is usually mediated by amine or amino acid neurotransmitters interacting with the transmitter-gated ion channels, which are presented in the chapter below.
Neurotransmitter receptors and effectors
There are three types of neurotransmitter receptors: autoreceptors, G-protein-coupled receptors and transmitter-gated ion channels. G-protein-coupled receptors act via second messengers, molecules which participate in control and activation of certain functions i.e. regulation of ion channel activity and cellular metabolism. Transmitter-gated ion channels open or close in response to the binding of a neurotransmitter. In other words, the protein structure becomes more permeable to certain kind of ions, such as calcium.
Effectors or effector proteins are related to the G-protein-coupled receptors and they can be G-protein induced ion channels in the membrane or enzymes which form second messengers. Autoreceptors are G-protein-coupled receptors which are found in the membrane of the presynaptic axon terminal. They participate in the regulation of second messenger synthesis and typically in the inhibition of neurotransmitter release.
The release of neurotransmitters
The release of neurotransmitters is initiated as the action potential arrives at the axon and the depolarisation activates the voltage-gated ion channels in the membrane. These channels have a structure like previously mentioned sodium channels, except instead of sodium, they transport calcium ions. Calcium concentration is low inside the cytoplasm of the axon terminal, so the ions will flow freely to the cytoplasm if the channel is open. The increased calcium ion concentration (number 2 in figure 3) in the cytoplasm of the axon terminal acts as a signal, which causes the neurotransmitters to be released into the cytoplasm (number 3 in figure 3). The vesicles release the transmitters into the synaptic cleft by exocytosis, during which the vesicles fuse momentarily with the plasma membrane.Learning about the nervous system has brought up many surprising ideas including the following ones. The synapse is able to transform an electrical message to a chemical one, a concept that our group found very interesting. Synapses transmit information faster than we expected because the synaptic cleft is very small to diffuse over. Finally, nerve cells are capable of initiating action potentials and combining them to create complex movements.
1. Creative Biomart. Neurotransmitter G Protein-Coupled Receptors. Available at: https://www.creativebiomart.net/researcharea-neurotransmitter-g-protein-coupled-receptors_2219.htm
2. Bear, Connors, Paradiso. 2015. Neuroscience: Exploring the Brain.