Neuroscience: Ion Movement and Signaling

Ion Movement Across Cell Membranes

• Factors influencing ion movement:
  • Chemical concentration gradient
  • Electrical gradient
  • Cell membrane structure

Neuronal Signaling

• Input signal: Dendrites
• Trigger signal: Axon hillock/cell body
• Conducting signal: Axon
• Output signal: Terminal button

Channels, Gates, and Pumps

• Channels: Pores in the membrane, selective for specific molecules. Vary greatly in size and structure.
• Gates: Proteins changing shape to regulate molecular entry/exit.
• Pumps: Actively transport molecules across the membrane, often against gradients.

Ion Concentrations and the Resting Membrane Potential (RMP)

• Proteins: Higher concentration inside the cell; too large to quickly cross the membrane.
• Chloride: Higher concentration outside the cell; contributes to RMP.
• Sodium: Higher concentration outside the cell; limited ability to enter the cell.
• Potassium: Higher concentration inside the cell; leak channels allow ions to flow out down the concentration gradient.
• RMP: Result of ions not reaching equilibrium due to concentration and electrical gradients. Maintained by the Na+/K+ pump.

Graded vs. Action Potentials

• Graded potential: EPSPs and IPSPs; not all-or-none; passive spread.
• Action potential: All-or-none; reaches threshold; active propagation.

Synaptic Summation

• Temporal summation: Rapid-fire impulses from one neuron.
• Spatial summation: Simultaneous impulses from multiple neurons.

Excitatory and Inhibitory Postsynaptic Potentials (EPSPs and IPSPs)

• EPSPs: Depolarize the cell.
• IPSPs: Hyperpolarize the cell.

Action Potential Frequency and Stimulus Intensity

• Action potential frequency encodes stimulus strength.

Action Potential Propagation

• Voltage-gated Na+ channels: Two gates (one opens, one closes).
• Voltage-gated K+ channels: One gate, slower opening/closing, allows prolonged K+ outflow.
• Axon diameter affects AP speed. Larger diameter = faster.

Refractory Periods

• Absolute refractory period: Impossible to generate another AP.
• Relative refractory period: Hyperpolarization; a larger stimulus needed for another AP.

Synaptic Transmission

• Postsynaptic effect depends on the receptor.
• Neurotransmitter removal: Glial cell degradation, diffusion, reuptake, enzymatic deactivation.

Neural Function and Pathway

• Function of a region is determined by its inputs and outputs.
• Retrograde NT: e.g., anandamide; not released from vesicles, but crosses the membrane.

Neurotransmitters and Pathways

• Tyrosine -> L-dopa -> Dopamine -> Norepinephrine -> Epinephrine.
• Serotonin: Derived from tryptophan; affects mood and aggression.
• Histamine: Derived from histidine; arousal, waking, allergic reactions.
• Cholinergic: Involved in waking, attention, and memory; dysfunction linked to Alzheimer's.
• Seratonergic: Wakefulness, learning; linked to disorders like depression/schizophrenia, OCD.
• Nigrostriatal pathway: Voluntary movement (substantia nigra to forebrain); damage = Parkinson's.
• Mesolimbic pathway: Reward behavior and addiction (ventral tegmentum to forebrain).
• Excess dopamine: Schizophrenia.
• Insufficient dopamine: Parkinson's.
• Noradrenergic: Emotion; imbalance linked to depression/mania.

Glial Cells

• Glial cells support neurons.

  • Astrocytes: Blood-brain barrier, blood flow regulation, neuron guidance.
  • Oligodendrocytes (CNS): Myelin production, one segment for multiple axons.
  • Schwann cells (PNS): Myelin production, one axon, tube formation for nerve regeneration.
  • Microglia: Phagocytosis; facilitate synaptic connections.

• Excessive glial cells: Possible tumors, neurodegenerative diseases.

• Neural threesome: Neurons, astrocytes, oligodendrocytes working together.