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The two types of synapses formed are excitatory (depolarizing) and inhibitory (hyperpolarizing) synapses.
Excitatory and inhibitory signals from each synaptic bouton are integrated, or algebraically summed, at the dendritic and somatic levels.
An action potential is generated when the spatial and temporal algebraic sum of excitatory and inhibitory potentials reaches the activation threshold at the axon hillock.
Calcium ions enter the presynaptic terminal through voltage-gated Ca²⁺ channels, triggering the fusion of synaptic vesicles with the membrane.
The sites of vesicle anchoring are highly organized structures known as active zones, located near voltage-gated Ca²⁺ channels.
Synaptotagmin is a membrane protein that acts as a calcium sensor, binding Ca²⁺ and facilitating the interaction with the SNARE complex to initiate vesicle fusion.
In unmyelinated axons, the action potential propagates along the entire axonal membrane at high speeds, ranging from 5 to 20 m/s, without changing its shape or amplitude.
Myelin, formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system, provides excellent electrical insulation, enhancing the speed of action potential propagation.
Nodes of Ranvier are gaps in the myelin sheath, approximately every 1–1.5 mm, where the axonal membrane is exposed and contains a high density of Na⁺ channels, allowing for action potential generation.
Saltatory conduction is the mechanism by which action potentials are regenerated at each node of Ranvier in myelinated axons, allowing for rapid propagation speeds of 20 to 80 m/s.
Chemical synapses involve the release of neurotransmitters across a synaptic cleft, while electrical synapses allow direct ion and small molecule diffusion between adjacent cells, enabling bidirectional transmission.
The synaptic delay in chemical synapses ranges from 0.3 to 1.5 ms, which is longer than the approximately 0.1 ms delay in electrical synapses.
Neurotransmitters are released from the presynaptic terminal and bind to specific receptors on the postsynaptic membrane, transmitting chemical signals and influencing neuronal excitability.
The nicotinic acetylcholine receptor is an ionotropic receptor composed of five protein subunits (2α, β, γ, δ) that opens in response to acetylcholine binding, allowing Na⁺ and Ca²⁺ influx.
GABA (gamma-aminobutyric acid) binds to GABA_A receptors, which are ion channels permeable to Cl⁻. This leads to hyperpolarization of the postsynaptic membrane, inhibiting neuronal activity.
Synaptic integration is crucial for all brain functions, including cognitive, sensory, and motor processes, as it determines the overall excitability of neurons based on the balance of excitatory and inhibitory inputs.
Synaptic pores are large enough to allow the passage of relatively large molecules, such as ATP and second messengers, facilitating metabolic coordination and intracellular signaling among groups of neurons.
Microfilaments guide the movement of synaptic vesicles containing neurotransmitters toward the synaptic membrane, facilitating their release during synaptic transmission.
Neurotransmitters include acetylcholine, catecholamines, amino acids and their derivatives, as well as neuropeptides, each playing distinct roles in synaptic transmission.
Neurotransmitters can act directly on ionotropic receptors, causing immediate changes in ion flow, or indirectly on metabotropic receptors, leading to longer-lasting effects through intracellular signaling pathways.