Neurotransmitters: Types, Structures, and Functions Explained

Action potentials propagate along neurons through sequential depolarization and repolarization of adjacent membrane segments. The axon contains high densities of voltage-gated sodium channels (350-500 per segment) that enable this propagation. The axon hillock serves as the trigger zone where action potentials are initiated. The mechanism involves: depolarization reaching threshold, opening voltage-gated sodium channels and causing rapid sodium influx, sodium channels closing while potassium channels open, potassium exiting to repolarize the membrane, and potassium channels closing slowly causing brief hyperpolarization before returning to resting potential.

Action potentials are rapid, all-or-nothing electrical signals that propagate along neurons. They begin with depolarization when voltage-gated sodium channels open, allowing Na+ influx and membrane potential to become positive. Repolarization follows as potassium channels open, allowing K+ efflux and restoring negative charge. Hyperpolarization briefly makes the membrane more negative than resting potential. The sodium-potassium pump then restores original ion distribution. This sequence enables rapid, long-distance signal transmission.

Action potentials propagate along neurons through a process where the depolarization at one segment of the membrane causes adjacent voltage-gated sodium channels to open. The entry of sodium ions makes the adjacent membrane region more positive, which then triggers sodium channels in that region to open. This creates a wave of depolarization that travels along the neuron. The propagation is bidirectional because the depolarization affects adjacent regions on both sides of the stimulus point. This propagation mechanism allows electrical signals to travel long distances along neurons without losing strength.

Action potential represents electrical changes in cells during nerve stimulation. The resting membrane potential is -70 millivolts (outside positive, inside negative). When stimulated, sodium channels open, allowing positive sodium ions to enter the cell, causing depolarization to +35 millivolts. The spike lasts about 2 milliseconds with a 105-millivolt change. The threshold potential at -55 millivolts triggers rapid sodium channel opening. Sodium channels have activation gates (outside) and inactivation gates (inside). Potassium channels open gradually during depolarization, allowing potassium to exit and begin repolarization. Some potassium channels remain open, causing hyperpolarization to -72 millivolts. Action potentials propagate between adjacent neurons through the local circuit mechanism, where a resting cell depolarizes a polarized cell, creating a chain reaction. This propagation is unidirectional because cells that have just fired cannot respond immediately. Myelinated neurons conduct action potentials much faster through saltatory conduction, where the myelin sheath acts as an electrical insulator and action potentials only occur at the nodes of Ranvier, allowing the signal to 'jump' between nodes.

Synaptic vesicles containing neurotransmitters move toward the presynaptic membrane and fuse with it. This fusion releases the neurotransmitter contents into the synaptic cleft. The neurotransmitters then diffuse across the synaptic cleft to reach the postsynaptic membrane.

Once vesicles reach the active zone, they fuse with the cell membrane and release their chemical messages into the synapse. This process represents the fundamental mechanism of cell-to-cell communication in the nervous system, allowing neurons to transmit signals to one another.

Vesicle fusion requires coordinated protein interactions: syntaxin on the presynaptic membrane recognizes synaptobrevin on vesicles, bringing them together. Synaptotagmin initiates fusion, while synaptophysin and phycophilin form the fusion pore. During pore formation, neurotransmitters exit vesicles into the synaptic cleft. This SNARE-mediated fusion mechanism ensures rapid and efficient neurotransmitter release, completing the synaptic transmission cycle.

Chemical synaptic transmission involves a coordinated sequence of events: (1) Nerve impulses depolarize the presynaptic axon terminal, opening voltage-gated calcium channels; (2) Calcium influx triggers vesicle fusion via SNARE proteins and synaptotagmin; (3) Neurotransmitters are released into the synaptic cleft; (4) Postsynaptic receptors bind transmitters, generating excitatory or inhibitory responses; (5) Transmitters are rapidly cleared via enzymatic breakdown (acetylcholinesterase) or reuptake by transporters; (6) Vesicle membranes are recycled through endocytosis for reuse. This ensures brief, accurate signal transmission between neurons and muscle cells.

When the synaptic vesicle fuses with the cell membrane, it empties its contents (neurotransmitters) into the synaptic cleft, which is the small gap between neurons where neurotransmitters diffuse to communicate with adjacent neurons.

Amino acids are organic compounds containing both amino (NH2) and carboxyl (COOH) groups, making them multifunctional. They are classified by amino group position: alpha (NH2 on C-alpha), beta (NH2 on C-beta), and gamma (NH2 on C-gamma). Most natural amino acids are alpha amino acids, with 20 standard amino acids forming proteins. Essential amino acids cannot be synthesized by humans and must be obtained from diet, while non-essential amino acids can be synthesized. The five basic amino acids are glycine, alanine, valine, glutamic acid, and lysine. Amino acids exhibit amphoteric behavior, forming zwitterions with both positive and negative charges, making them highly water-soluble with high melting points. They react with acids and bases, undergo esterification, and can polymerize through condensation reactions. Peptides are formed by linking alpha amino acids through peptide bonds (CONH), classified by amino acid count: dipeptide (2), tripeptide (3), tetrapeptide (4), pentapeptide (5), hexapeptide (6), and polypeptide (many). Each peptide has an N-terminus (free NH2) and C-terminus (free COOH), and sequence matters. Peptides undergo hydrolysis through enzymatic, acid, or base methods. The Biuret test detects peptides with two or more peptide bonds. Determining amino acid structures involves reaction stoichiometry, percentage composition, and mass spectrometry. Electrophoretic behavior depends on pH: neutral (no migration), acidic (migrates to negative electrode), basic (migrates to positive electrode). Melting points follow: amino acids > carboxylic acids > alcohols > esters > ketones > hydrocarbons.

This section introduces the foundational concepts of amino acids and peptides. It explains the distinction between peptides (fewer than 50 amino acids) and proteins (more than 50 amino acids), including specific classifications like dipeptides, tripeptides, and tetrapeptides. The general structure of amino acids is presented, consisting of an amino group, carboxyl group, hydrogen atom, and R group (side chain) attached to a central alpha carbon. The section also covers L-amino acids, which are the stereoisomers that constitute proteins in the human body, bacterial cell walls, and antibiotics, distinguished by the amino group's position to the left of the alpha carbon.

Amino acids are organic compounds containing both a carboxyl group (acidic) and an amino group (basic) attached to a central alpha carbon, with approximately 20 natural amino acids existing in nature; they can form zwitterions at physiological pH (around 7.3) where both groups ionize, and they react through condensation reactions to form peptide bonds, creating dipeptides, tripeptides, and polypeptides (proteins with over 80 amino acids), which serve structural functions (like collagen and keratin) or catalytic functions as enzymes that accelerate reactions by lowering activation energy with substrate specificity.

Peptides are compounds containing 2-50 alpha amino acid residues linked by peptide bonds (-NH-CO-). Classification: dipeptides (2-3 residues), tripeptides (3-4 residues), polypeptides (>10 residues). Proteins are high-molecular-weight polypeptides (10,000-1,000,000+ amu). Simple proteins contain only amino acids (albumin, silk fibroin); complex proteins contain additional components (nucleoproteins, lipoproteins). Key chemical properties: hydrolysis (complete to amino acids, partial to smaller peptides) and biuret test (purple color with Cu(OH)2 in alkaline medium).

Proteins are complex structures formed by amino acids linked through peptide bonds. Amino acids serve as the fundamental building blocks of proteins, similar to how bricks build a structure. The simplest amino acid is glycine, containing a carboxylic acid group, an amino group, a hydrogen atom, and a carbon atom. Amino acids contain both acidic and basic functional groups, and when both are present in the same molecule, they can form a zwitterion (dipolar ion) with both positive and negative charges. Alpha amino acids, the standard form in biological systems, have both functional groups attached to the same alpha carbon, which is bonded to four different groups including a variable side chain (R group).