Types of Neurotransmitters
Neurotransmitters are described as molecules that communicate information between neurons and target cells and chemical synapses. They are organized into groups categorized by amino acids, peptides, and monoamines. Amino acids consist of amino groups and carboxylic acid groups. Peptides consist of chains of amino acid, and much larger molecules compared to other groups. Monoamines are biogenic amines with organic molecules connected to aromatic groups, which are then connected by 2 hydroxyl groups. The three most common amino acids consist of the glutamine, GABA, and glycine neurotransmitter. Glutamine is the most common excitatory neurotransmitter to cause depolarization in target cells. GABA is a inhibitory neurotransmitter found in the brain, while Glycine is an inhibitory neurotransmitter found in the spinal cord. The common peptides include: serotonin, histamine, dopamine, epinephrine, and norepinephrine. They function mostly in the brain, and are the sources for most drugs. Opioids are the most common form of endorphins, which help for pain perception. Acetylcholine commonly works with the CNS and PNS, and counts for contraction and movement.
Types of Neurotransmitter Receptors
The synapse is the one which is excitatory or inhibitory. The receptors on the target membrane depends on whether or not the response is excitatory or inhibitory. There are two types of neurotransmitter receptors: ionotropic and metabotropic. Ionotropic is described as a ligand gated ion channel. The lugand, or the neurotransmitter, binds to the receptors. It allows certains ions to pass when it binds. When the ligand leaves, the channel is closed. The target cell becomes excited when sodium/calcium ions pass in. The target cell is inhibitory when chloride and potassium pass in. Metabotropic works differently. When neurotransmitters bond to these receptors, “second messengers” are activated. These can change the behavior of ion channels, change activity of protein inside the neuron, or change gene expression. The effects in metabotropic neurotransmitters are slower, but more widespread and amplified because of the second messengers.
Synapse Structure
The synapse is the location at which neurons contact and communicate with the target cell. From the greek word meaning “to be clasped together”, the synapse is usually located at the end of the axon terminal connecting to the target cell. Overall, synapses typically cover the whole neuron. There are two types of synapses, chemical and electrical. Chemical synapse has a gap for the release of chemicals at the synaptic cross. Electrical synapse is physically connected to the target cell with gap junctions to let inside of the neuron communicate with the target cell. On a microscopic level, there are three parts: presynaptic membrane, the synaptic cleft, and postsynaptic membrane. At the presynaptic membrane, are synaptic vesicles which hold neurotransmitters. These transmitters cross the cleft to bind to receptors on the postsynaptic membrane.
Neuronal Synapses (Chemical)
A trigger on the neurological, microscopic identification, means that a certain channel is opened. A certain amount of sodium or potassium decides the triggers, boosts on potentials, etc. In terms of chemical synapses, however, calcium is a big player in this game. At the presynaptic membrane. Action potentials trigger voltage potentials, which in turn trigger certain channels to open. These are the calcium channels. SImilar to the sodium-potassium pump, there is a calcium ion pump. It takes 1 ATP, for calcium ions to bond, and to be released into the membrane. At the cusp of the membrane, are vesicles which hold neurotransmitters. In addition, there are SNARE proteins docking the vesicles to the outer membrane. The calcium ions bond to the SNARE proteins, pulling them in closer to the outer membrane of the presynaptic, and then pulling it apart. In a process known as exocytosis, neurotransmitters get dumped into the synaptic cleft. The neurotransmitters then bond to proteins on the postsynaptic membrane, which then triggers certain channels to open up, leaving the neurotransmitters to flow in. Potassium inhibits the entering, sodium allows it.
Saltatory Conduction
Neurons transmitting signals is a complex biological process. Once a stimulus occurs, sodium positive ions flood into the soma from the dendrites. In addition, other stimuluses may occur, which leads to a summation at the axon hillock. Once the threshold millivolt is reached, the axon fires down the electrical impulse. Since the current is traveling electrically, there are certain factors that are required. These include good conductivity for low resistance, and a good insulator for high resistance. This is required so that the current would be concentrated and would account for minimal energy loss. Hence, the axon is surrounded by an insulator, the myelin sheath, made up of schwann cells. However, the signal is traveling via an electrotonic potential, and from previous information, we know that it disappears as you go further along. Hence, nodes of ranvier exist between myelin sheaths to give the impulse a boost. This process altogether is known as saltatory conduction, coming from the latin word “saltare” meaning to jump or to hop.
Credit: Khan Academy Nervous System Intro. All entries are written by Jadon-Sean Sobejana
