Neuron Structure and Electrochemical Gradients

Neuron Structure and Function

• Neurons are specialized cells responsible for communication in the body.
• The cell body acts as the control center, processing signals and maintaining cell health.
• Major components of the cell body include:
  • Nucleus: Contains DNA.
  • Cytoplasm: Houses organelles like mitochondria and ribosomes.
  • Dendrites: Receive signals from other neurons.
  • Axon: Transmits electrical impulses away from the cell body.

Intracellular and Extracellular Fluids

• Intracellular Fluid (ICF) major ions: Potassium (K+), Phosphate (HPO4^2-), Proteins, Sodium (Na+).
• Extracellular Fluid (ECF) major ions: Sodium (Na+), Chloride (Cl-), Calcium (Ca2+), Potassium (K+).

Electrochemical Gradients

• Chemical Gradient: Concentration differences of ions drive movement from high to low concentration.
• Electrical Gradient: Charge differences across the membrane attract ions to oppositely charged areas.

Action Potentials

• Action potentials are rapid, transient changes in membrane potential.
• Threshold for firing an action potential is around -55 mV.
• Depolarization: Membrane potential becomes less negative as Na+ ions enter.
• Repolarization: K+ ions exit to restore negative resting potential (-70 mV).
• Hyperpolarization: Membrane potential becomes more negative than resting potential.
• The all-or-nothing principle indicates that action potentials either occur fully or not at all.

Components of Axons

• Axon Hillock: Initiates action potentials, cone-shaped region.
• Myelin Sheath: Insulates the axon, increasing signal transmission speed.
• Nodes of Ranvier: Gaps in myelin sheath where action potentials jump.
• Axon Terminals: Release neurotransmitters to communicate with other cells.

Synaptic Transmission

• Synapse: Junction between two cells (e.g., neuron and muscle) for communication.
• Types of Synapses include:
  • Electrical Synapses: Fast communication via gap junctions.
  • Chemical Synapses: Slower, using neurotransmitters to relay signals.

Neurotransmitter Systems

• Neurotransmitters play a crucial role in communication between neurons.
• Two main types:
  • Excitatory Neurons: Promote action potentials (e.g., Glutamate).
  • Inhibitory Neurons: Decrease likelihood of action potentials (e.g., GABA).

Excitatory and Inhibitory Mechanisms

• Excitatory Mechanism:
  • Glutamate binds to AMPA and NMDA receptors, facilitating Na+ and Ca2+ influx leading to depolarization.
• Inhibitory Mechanism:
  • GABA binds to GABA_A receptors, allowing Cl⁻ influx and generating rapid inhibitory signals.

Neurotransmitter Release and Recycling

• Neurotransmitter release is triggered by action potentials at synapses.
• Neurotransmitter recycling: Breakdown and reuptake of neurotransmitters for reuse.

Studying Synaptic Function

• Techniques for studying neurotransmitters and receptors:
  • Immunocytochemistry: Visualizes specific proteins in cells.
  • In Situ Hybridization: Detects mRNA related to neurotransmitter synthesis.
  • Optogenetics: Activates specific synapses with light.
  • Microiontophoresis: Tests neurotransmitter effects on postsynaptic responses.

Neurotransmitter Types

• Glutamate: Main excitatory neurotransmitter in the CNS.
• GABA: Main inhibitory neurotransmitter in the brain.
• Endocannabinoids: Retrograde signaling molecules for modulating synaptic transmission.
• Gaseous Neurotransmitters: Nitric Oxide (NO), a retrograde messenger regulating blood flow.

Synaptic Plasticity

• The process by which frequent use of synapses increases their strength, essential for memory and learning.