Neurons: Electrical & Chemical Signals

• Individual neural cells integrate sensory information for reflexes.
• Reflex pathways involve signals to the brain, but also a simultaneous and direct route.

Squid Axon Research

• Giant axons from squids could be extracted and maintained in a saline solution that mimicked bodily fluids.
• Hodgkin and Huxley received a Nobel Prize for their work describing the neuron's electrical activity.

Neuronal Communication Methods

• Electrical messages occur within a neuron via action potentials.
• Chemical messages occur between neurons via neurotransmitters.

Potentials Defined

• Membrane potential arises from the balance of ions across the cell membrane, enabling the cell to function.
• Resting potential refers to when the neuron is at rest.
• Action potential (AP) refers to when the neuron is actively communicating.

Ions Defined

• An ion represents an electrically charged molecule.
  • Anions are negatively charged
    • Examples include organic anions (A-) and chloride (Cl-).
  • Cations are positively charged
    • Examples include sodium (Na+), potassium (K+), and calcium (Ca+).

Membrane Structure

• A membrane is a thin, porous outer layer of a neuron formed by a lipid bilayer.
• It separates intracellular fluid from extracellular fluid.
• It separates ions, which diffuse during action potential.

Neuronal Membrane Properties

• It is semi-permeable, allowing certain chemicals to pass through while blocking others.
• It contains protein molecules, including transporters and receptors.
• It maintains electrical polarization, which refers to an unequal balance of ions on both sides and creates a membrane potential.

Membrane Potential in Detail

• It results from the unequal distribution of ions across the membrane.
  • The inside of the cell has a negative charge of -70mV at rest.
• It is a store of potential energy.
  • Potential refers to the ability to use stored power.

Maintaining Resting Potential Details

• The sodium-potassium pump constantly exchanges ions across the membrane, moving 2 K+ ions in for every 3 Na+ ions out.
  • This pump keeps the cell inactive at rest.
• Concentration gradient and diffusion play a role.
• Efflux refers to movement out of the cell.
• Influx refers to movement into the cell.

How a Neuron Fires

• An action potential occurs when the inside of the cell becomes depolarized until it reaches the threshold of excitation.
• Above the threshold the cell always fires, according to the all-or-none law.

Depolarization Defined

• During this process, Na+ channels open when the threshold of excitation is reached.
• Depolarization is an all-or-none process.
• The charge inside the cell becomes less negative.

Refractory Periods Explained

• Absolute refractory period refers to when the cell is unable to fire.
  • Na+ channels are unable to open and must reset.
• Relative refractory period refers to when it is more difficult for the cell to fire.
  • Open K+ channels require a stronger stimulus for an AP.

Repolarization Described

• The inside of the cell becomes positively charged (+40mV).
• Na+ channels become refractory.
• Potassium channels open, and K+ leaves the cell.

Hyperpolarization Defined

• K+ channels close slowly after the resting potential is reached.
• The potential overshoots, causing the inside of the cell to become more negative.
  • It goes lower than -70mV.
• The charge inside the cell becomes more negative.

Reestablishing Resting Potential

• Resting potential occurs through diffusion and the Na+/K+ pump.
• This process employs an active transport mechanism using ATP.

Ligand Gated Channels

• Sodium ligand gated ion channels are crucial for EPSPs (Excitatory postsynaptic potential)
• Potassium: Opening results in efflux of K+, causing an IPSP (Inhibitory postsynaptic potential)
• Chloride: Opening depends on the membrane potential; often neutralizes EPSPs.

Synaptic Event Brevity

• Occurs via enzymatic degradation and reuptake.

Checks and Balances in Neural Communication

• Autoreceptors, located on the presynaptic cell, regulate the synthesis and release of NTs (Neurotransmitters)
  • These are metabotropic receptors.
• Axoaxonic synapses
  • Alters NT release.
    • Presynaptic inhibition decreases NT release from the postsynaptic terminal.
    • Presynaptic facilitation increases NT release from the postsynaptic terminal.