Process of Neurotransmission
Living organisms derive their emotional responses, thoughts, feelings, sensations, motor responses, memory and learning, and functions and dysfunctions of the brain from the communication between nerve cells. Nerve cells, known as neurons, are highly differentiated and specialized for their role. They need to maintain communication for an organism to maintain their senses. However, neurons do not communicate through physical contact with each other. Instead, electrical impulses sent between the synapses between them. Chemical messengers (neurotransmitters) are essential for this transfer of impulses, a process known as neurotransmission. The process of neurotransmission understood through the concept of the synapse, the chemical events at a synapse, types of neurotransmitters, and the activating receptors of the postsynaptic cell.
The synapse is the point of communication between two nerve cells. Charles S. Sherrington’s reflective observations concluded that there are synapses and many of their properties. Since reflex arc transmission is slower than transmission through a corresponding axon length, Sherrington concluded that a process at the synapse delays transmission. EPSP and IPSP are graded responses. Postsynaptic depolarizations are referred to as excitatory postsynaptic potentials (EPSP) because they increase the likelihood of triggering neurons and stimulates the brain. Postsynaptic hyperpolarization is referred to as inhibitory postsynaptic potentials (IPSP) because they reduce the risk of a neuron firing and calms the brain. EPSP happens when the gate opens to permit sodium to go into the neuron’s membrane. When the gate opens to let the potassium out or allows the chloride in, an IPSP has occurred. The balance between EPSPs and IPSPs increases or decreases at the neurons’ frequency of action potentials, thence EPSPs on a neuron compete with the IPSPs. Graded potentials like EPSPs allow positively charges ions while IPSPs allow negatively charged ions to summate their effects. Temporal summation is the summation of graded potential from stimuli at a separate time. Spatial summation is the summation of potentials from distinct locations. Neurotransmitters exert ionotropic effect to bind the receptor, opening the gate for a specific ion such as sodium to pass through the membrane. The ionotropic effects are fast and shorts (Lisman, Raghavachari, & Tsien, 2007). Neurotransmitter exerts metabotropic effects to activate a second messenger in the postsynaptic cell, causing slower but more lasting changes (Greengard, 2001). Most synapses work by transferring a neurotransmitter from the presynaptic cell to the postsynaptic cell. Otto Loewi demonstrated this point by electrically stimulating the heart of a frog and then transferring fluids from the stimulated heart to another frog’s heart. Loewi discovered that this transfer of these fluids would modify the reactivity of the heart muscle.
Understanding chemical events at a synapse are essential to understanding the nervous system. Each year, researchers discover more details about synapses, their structure, and the link between these structures and their functions. These are the most important events: the neuron synthesizes chemicals that act as neurotransmitters. It synthesizes the smallest neurotransmitters at axon terminals and neuropeptides in the cell body (soma). The action potential crosses the axon. At the presynaptic terminal, an action potential allows calcium to enter the cell. Calcium releases neurotransmitters from the terminals to the synaptic cleft, the space between the presynaptic and postsynaptic neurons. The released molecules diffuse through the cleft, bind to the receptors on the dendrites of another neuron, and modify the postsynaptic neuron activity. Neurotransmitter molecules disconnect from their receptors. Neurotransmitter molecules maybe return to the presynaptic neuron for recovery or release. Most postsynaptic cells send reverse messages to control the subsequent delivery of presynaptic cell neurotransmitters.
Hundreds of chemicals are suspected to be neurotransmitters (Borodinsky et al., 2004). These are the main categories,
Amino acids- that contain an amino group (NH2) include Glutamine, GABA, Aspartate, maybe other Modified Amino Acid, which is Acetylcholine.
Monoamines- Chemicals formed by a change in certain amino acids such as indoleamines: Serotonin, catecholamines: dopamine.
Acetylcholine (a one-member “family”)- A chemical like amino acid, except that it includes an N(CH3)3 group instead of an (NH2).
Neuropeptides- Chains of amino acids-endorphins, substance P, neuropeptide Y and other.
Purines- A category of chemicals including adenosine and its derivatives are ATP, adenosine, maybe others.
Gases- Nitric oxide and possibly others — No (Nitric Oxide), Nitric oxide (NO) is the oddest transmitter released by some small local neurons. Its function relates to blood flow to the brain area when it becomes actively high, blood flow to that area increases (Dawson, Gonzalez-Zulueta, Kusel, & Dawson, 1998).
There are two-way drugs effects on synaptic transmissions. Agonists facilitate it, example is a Parkinson’s – patients have too little dopamine; they take a chemical called L-dopa, which can also bind to dopamine receptors. Antagonists inhibit it; an example is antipsychotic medications that block the dopamine actions at the postsynaptic membrane. Various drugs, including LSD, nicotine, and opiates, exert their behavioral effects by binding to receptors on the postsynaptic neuron (O’Rourke, Weiler, Micheva, & Smith, 2012). After a neurotransmitter activated the receptor, numerous transmission molecules re-enter the presynaptic cell through transporter molecules in the membrane. The process refers to reuptake, which allows the presynaptic cell to recycle its neurotransmitter. Stimulants and many antidepressants inhibit this process (Beuming et al., 2008; Schmitt & Reith, 2010; Zhao et al., 2010). Postsynaptic neurons send chemicals to receptors to inhibit the subsequent release of neurotransmitters.
Conclusion
The process of neurotransmission involves the synapse, chemical events at a synapse, types of transmitters, and the activating receptors of the postsynaptic cells. Communication between neurons occurs at the synapse. When stimulation reaches the synapse, it builds a fleet graded potential in the postsynaptic cell. Two summations are form temporal and spatial summations. Neurons synthesize neurotransmitters, the smallest at the axon, and the neuropeptides in the cell body. The resulting action potential at the presynaptic terminal permits calcium release of neurotransmitters that diffuse through the cleft and bind to their receptors. There are different types of neurotransmitters, whose chemical compositions determine how the receptors are activated. Variations in chemical compositions relate to the numerous messages that need transmitting from one neuron to another.

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