Synapses and neurotransmitters Knowledge-Based Study

Study notes

1. The Synapse and Its Components

   A synapse is a specialized junction through which a neuron communicates with another cell, such as another neuron, a muscle cell, or a glandular cell. While some electrical synapses connect cells directly via gap junctions, the majority of communication in the human nervous system occurs across chemical synapses.

   A chemical synapse consists of three main structural components:

   • Presynaptic terminal (axon terminal): The sending end of the upstream neuron, which contains mitochondria and membrane-bound synaptic vesicles packed with chemical messengers called neurotransmitters.
   • Synaptic cleft: A narrow extracellular fluid-filled gap, typically 20 to 40 nanometers wide, separating the sending and receiving membranes.
   • Postsynaptic membrane: The plasma membrane of the target cell (dendrite, soma, or effector cell), which contains specific protein receptors that recognize and bind the released neurotransmitters.

   Step-by-Step Transmission Across the Synapse

   Signal transmission across a chemical synapse converts an electrical signal into a chemical signal and then back into an electrical response through a sequence of coordinated events:

   1. Arrival of the Action Potential: An action potential travels down the axon and depolarizes the presynaptic axon terminal.
   2. Calcium Influx: Depolarization triggers the opening of voltage-gated Ca2+ channels in the presynaptic membrane, allowing calcium ions to rush down their concentration gradient into the terminal.
   3. Vesicle Exocytosis: The elevated intracellular Ca2+ concentration causes synaptic vesicles to dock with and fuse to the presynaptic membrane, releasing neurotransmitter molecules into the synaptic cleft via exocytosis.
   4. Diffusion and Receptor Binding: Neurotransmitter molecules diffuse across the synaptic cleft and bind reversibly to specific receptor proteins on the postsynaptic membrane.
   5. Ion Channel Response: Receptor activation opens or closes ligand-gated ion channels, altering the membrane potential of the postsynaptic cell.

   Postsynaptic Responses and Neurotransmitter Clearance

   The binding of neurotransmitters produces graded potentials in the postsynaptic membrane, which can be excitatory or inhibitory:

   • Excitatory Postsynaptic Potential (EPSP): Binding opens channels that permit the influx of positive ions (such as Na+), depolarizing the postsynaptic membrane and bringing it closer to the threshold for firing an action potential.
   • Inhibitory Postsynaptic Potential (IPSP): Binding opens channels that allow either K+ efflux or Cl− influx, hyperpolarizing the membrane and moving it further away from the firing threshold.

   For synaptic signaling to remain precise and capable of receiving new signals, neurotransmitters must be rapidly cleared from the cleft by one of three mechanisms:

   1. Enzymatic degradation: Specific enzymes in the cleft break down the transmitter (for example, acetylcholinesterase cleaves acetylcholine into acetate and choline).
   2. Reuptake: Transport proteins in the presynaptic membrane or surrounding glial cells actively pump the neurotransmitter back inside for recycling or degradation.
   3. Diffusion: Neurotransmitter molecules diffuse away from the synaptic cleft into surrounding extracellular fluid.

   Key Neurotransmitters and Their Physiological Roles

   Neurotransmitters are categorized based on their chemical structure and primary physiological actions:

   • Acetylcholine (ACh): Functions at neuromuscular junctions to stimulate skeletal muscle contraction and plays central roles in memory and attention within the central nervous system.
   • Glutamate: The primary excitatory neurotransmitter in the brain, essential for learning, long-term potentiation, and synaptic plasticity.
   • GABA (Gamma-Aminobutyric Acid): The primary inhibitory neurotransmitter in the brain; by hyperpolarizing postsynaptic neurons, it prevents overexcitation and regulates anxiety and motor control.
   • Dopamine: A monoamine involved in reward pathways, motivation, mood, and the regulation of voluntary movement.
   • Serotonin (5-HT): A monoamine regulating mood, sleep, appetite, and emotional states.
   • Norepinephrine: Acts as both a neurotransmitter and a hormone, mediating arousal, alertness, and the fight-or-flight response.