10. Neurotransmitters

Introduction

• Neurotransmitters: Definition and Mechanisms
• Definition:
• Neurotransmitters are chemical messengers that neurons use to communicate with each other. They are released from the presynaptic neuron (the sending neuron) into the synaptic cleft (the small space between neurons) and bind to specific receptors on the postsynaptic neuron (the receiving neuron). typically small molecules, similar in size to a single molecule such as an amino acid or a monoamine. This small size allows them to diffuse quickly across the synaptic cleft to reach their target receptors.
• Release Mechanism:
• Synthesis: Most neurotransmitters are synthesized in the presynaptic nerve terminals from various precursors. The synthesis pathways can vary significantly depending on the type of neurotransmitter.
• Storage and Release: Once synthesized, neurotransmitters are stored in vesicles within the presynaptic neuron. Upon receiving an electrical signal, these neurotransmitters are released into the synaptic cleft through a process called exocytosis, where vesicles fuse with the presynaptic membrane to release their contents.
• Types of Neurotransmitters:
• Classic Neurotransmitters: These include substances like acetylcholine, dopamine, serotonin, and glutamate. They are generally synthesized in the nerve terminal and are quick in their action.
• Classification of Neurotransmitters
• Neurotransmitters are crucial chemical messengers in the nervous system, grouped based on their structure and function into four main categories:
• 1. Choline Esters:
• Acetylcholine (ACh): This is the primary neurotransmitter used at neuromuscular junctions, crucial for muscle activation.
• Interaction with Toxins: Curare competes with ACh for nicotinic receptors on the motor end plate of muscles, reducing the efficacy of excitatory postsynaptic potentials (EPSP), thus weakening muscle contractions.
• 2. Biogenic Amines:
• Tyrosine Derivatives: Includes Dopamine, Norepinephrine, and Epinephrine, all synthesized from the amino acid Tyrosine. Dopamine can be converted into Norepinephrine, which in turn can be methylated to produce Epinephrine.
• Other Amines:
• Histamine: Derived from Histidine, involved in immune responses and regulation of physiological function in the gut.
• Serotonin (5-HT): Synthesized from Tryptophan, plays roles in mood, appetite, and sleep regulation.
• 3. Amino Acids:
• Glutamate: The primary excitatory neurotransmitter in the CNS, facilitating cognitive functions such as learning and memory. It interacts with three ionotropic receptors and one metabotropic receptor.
• Gamma-Aminobutyric Acid (GABA): An inhibitory neurotransmitter that helps regulate neuronal excitability throughout the nervous system. It is synthesized from Glutamic Acid. Notably, a deficiency in GABA is linked to the uncontrolled movements seen in Huntington’s disease.
• Glycine: Another inhibitory neurotransmitter in the CNS, it increases chloride ion influx in the postsynaptic neuron, leading to hyperpolarization and inhibition.