Neurophys L3: SYNAPTIC TRANSMISSION AND AP

Neurons and Synapses

• Neurons are the core components of the brain, spinal cord, and nerves.
• Dendrites integrate incoming information and determine whether action potentials will be produced by the neuron.
• Dendritic branching patterns can change and may increase or decrease in response to environmental stimuli.

Axons

• Axons can be only a few micrometers or over 10 meters long, depending on the species and location.
• Axons contain the majority of the cell cytoplasm and several organelles, including neurofibrils, neurotubules, small vesicles, lysosomes, mitochondria, and enzymes.
• Axonal proteins are synthesized in the soma and transported to the axon through anterograde transport.

Synaptic Transmission

• Chemical synapses are the most common form of synapses, usually involving unidirectional transmission.
• The presynaptic neuron secretes a neurotransmitter that acts on receptor proteins in the postsynaptic neuron.
• Neurotransmitters can be either excitatory or inhibitory.

Neurotransmitters

• There are two main types of neurotransmitters: conventional and unconventional.
• Conventional neurotransmitters share basic features, are stored in vesicles, and released in response to action potentials.
• Examples of conventional neurotransmitters include:
  • Small molecule neurotransmitters (e.g., amino acids, amines, purines)
  • Large molecule neurotransmitters (e.g., endorphins, substance P, neuropeptide Y)
• Unconventional neurotransmitters do not follow traditional rules and can carry signals backward or act directly on molecules inside the cell.
• Examples of unconventional neurotransmitters include:
  • Endocannabinoids (e.g., anandamide, 2-arachinonyl-glycerol)
  • Gasotransmitters (e.g., nitric oxide, carbon monoxide, hydrogen sulfide)

Receptors

• Ionotropic receptors directly gate ion channels, while metabotropic receptors act through second messenger systems.
• Examples of ionotropic receptors include nicotinic acetylcholine receptors.
• Examples of metabotropic receptors include G-protein-coupled receptors.

Electrical Synapses

• Electrical synapses involve direct connection of the cytoplasm of adjacent cells through gap junctions.
• They allow for free movement of ions between cells and are bidirectional.
• Electrical synapses can coexist and interact with chemical synapses, promoting synchronous firing of interconnected neurons.

Graded Potentials

• Graded potentials are brief local changes in postsynaptic membrane potentials generated in response to neurotransmitters.
• Their amplitude is directly proportional to the intensity of the stimulus applied at synaptic sites.
• Graded potentials can be either excitatory or inhibitory.

Integration of Graded Potentials

• The axon hillock integrates graded potentials by algebraic processing, adding or subtracting potential changes.
• The sum of all graded potentials must stay under the threshold for an action potential to occur.
• There are two modes of summation: spatial and temporal summation.

Action Potentials

• Action potentials are generated at the trigger zone and propagate without losing strength due to the "domino effect".
• The speed of conduction depends on axon diameter and the degree of myelinization.
• In myelinated fibers, action potentials occur only at the Nodes of Ranvier, "jumping" from node to node.

Saltatory Conduction

• Saltatory conduction is the process of action potentials "jumping" from node to node in myelinated fibers.
• The impulse is transmitted from node to node, and the action potential is regenerated at each node.