In addition to carrying afferent signals, dendrites on each neuron may also be involved in protein synthesis and independent signaling functions with other neurons. Axons, on the other hand, terminate in an axon terminal where neurotransmitters, neurohormones, and neuromodulators are released in a process that converts electrical energy to chemical signals at the neuromuscular junction or synapses (Mochida, 2022). At a synapse, a neuron terminates in close proximity to another neuron, and at neuromuscular junctions, neurons terminate directly on muscle cells. Axonal transport (neuronal signaling) is aided by proteins such as kinesin and dynein. Potential change via ion movement through voltage-gated channels also plays key role in signal transmission in the neurons. Potassium, sodium, and chloride ions play principal roles in controlling the membrane potentials of the neuron. The different types of neuronal transmissions controlled by graded potentials allow the brain to undertake different physiological functions and regulations in the body. Neurons communicate by two basic mechanisms: Fast point- to-point information transfer and slow signaling. Point-to- Synaptic Communication A nerve impulse traveling along the axonal membrane of the presynaptic neuron cannot directly cross or jump the synaptic cleft. Neurotransmitters at the synaptic junction make action along the presynaptic axonal membrane happen. These transmitting molecules are stored in the synaptic vesicles and manufactured by the presynaptic neurons. Synaptic vesicles make direct contact with a part of the presynaptic plasma membrane in a region called the active zone. At this zone, vesicles fuses, and exocytosis of neurotransmitters from the presynaptic axon into the receptors of the postsynaptic neuron occurs. Although the process occurs quickly, the control mechanisms involved are biologically complex (Montchenegro- Venegas et al., 2022). The docking and fusion of the vesicles at the presynaptic membrane are regulated by a soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complex (Chen et al., 2021). SNARE is aided by many other presynaptic proteins that regulate the formation of the complex and the control of calcium-dependent neurotransmitter release. At the postsynaptic neuron, the process of signal transmission is slightly different. Neurotransmitters released from the
point transfers are mediated by synaptic transfer, and the slower signaling method is mediated by a range of chemical messengers, including neuropeptides, endocannabinoids, and monoamines. At the synapse, presynaptic axons house synaptic vesicles containing neurotransmitters. These neurotransmitters are deposited on postsynaptic terminals (usually dendrites) containing neurotransmitter receptors. Both terminals are separated by a gap of about 20–25 nm in a region referred to as the synaptic cleft (Caire et al., 2022). A variety of cell adhesion molecules hold the presynaptic and the postsynaptic membranes together to ensure structural integrity is maintained during neuronal transmission. Early research in behavioral physiology implicated alterations in the gene coding for these cell adhesion molecules in the pathology of many cognitive impairments. The literature has confirmed the important function of neuronal communication and signaling in mental health. Psychopharmacological research has since focused on the most important of these adhesion molecules, including N-cadherin, neuroligin, and neurexin (Liu et al., 2022). presynaptic vesicles act on the receptors of the postsynaptic neuron. Depending on the type and function of the postsynaptic neuron, the actions mediated by a neurotransmitter may be different. In ligand-gated postsynaptic neurons (ionotropic receptors), the binding of glutamate, a neurotransmitter, to the amino-3-hydroxy-5-methyl-4-isoazolepropionate (AMPA)-type and N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors mediates excitatory neuronal transmission (Zhu et al., 2022). However, GABA binding to these receptors triggers an influx of negatively charged chloride ions that mediated inhibitory synaptic transmission. Structurally, a region of interlinked proteins called the postsynaptic density (PSD) is located just after the postsynaptic membrane. This region houses a concentrated assembly of adhesion molecules, receptors, and associated signaling proteins. Primarily, the PSD functions as a postsynaptic organizing structure, maintaining the normal positioning of receptors, adhesion molecules, and gated channels in the postsynaptic membrane as neuronal transmission takes place. Alterations in the composition and functioning of PSD have been implicated in the early stages of many psycho- behavioral disorders (Feng et al., 2022).
NEUROTRANSMITTERS IN BEHAVIORAL SCIENCE
Neurotransmitters are chemical messengers released from presynaptic neuron vesicles to receptors on the postsynaptic neuron. Primarily, they regulate neuronal function by mediating adequate receptor–ligand interactions. Molecules in neurotransmitters convert, amplify, and transmit neuronal signals in the different complex processes responsible for the regulation of behavior and cognition in humans. Based on recent estimates, more than 200 neurotransmitters have been identified and characterized in humans. Although there are many chemical messengers in the human body direct associated with behavior and cognition, only a handful fit the biological roles played by neurotransmitters. In neuroscience, a chemical messenger is classified as a neurotransmitter only after fulfilling a few requirements, including: • It must be produced and released by a neuron and stocked at the presynaptic terminal of the same neuron. • It must induce a specific behavior in the postsynaptic neuron. • Its exogenous administration must produce the same effect. • Its induced action on the postsynaptic cell can be stopped by a specific mechanism.
Research investigating the biological implication of these neurotransmitters has revealed how subtle changes in the quality and quantity of these molecules can directly influence alterations in behavior. These changes may result from genetic mutations, toxicity, or congenital malformations of the brain circuitry. Other useful conclusions drawn from these studies include that not all neurotransmitters are uniform, and their abundance depends primarily on the regional differences in the brain. Leveraging this research information, recent efforts in psychopharmacology have been focused on developing clinical intervention models and the formulation of new therapeutics. Among the neurotransmitters, dopamine, serotonin, noradrenaline, and acetylcholine (ACh) seem significant for the mechanism regulating psycho-behavioral events in humans. However, in many instances, these roles are reliant on inputs from several other proteins and peptides, including neurotropic factors, growth factors, and endogenous chemical compounds. With the major neurotransmitters, these proteins and peptides selectively control the maintenance of proper brain functioning, including the feedback mechanism in the synthesis of principal neurotransmitters.
CLASSIFYING NEUROTRANSMITTERS
Different neurotransmitters are produced in distinct parts of the brain for different physiological functionalities. In behavioral science, neurotransmitters are broadly classified as canonical or noncanonical. The small molecules widely accepted as
neurotransmitters are canonical, and neuroactive compounds that have been recently classified as neurotransmitters under controversial conditions are noncanonical.
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