Nervous System Physiology and Glial Cells

Action Potential Phases

  • Resting State

    • Only Na and K channels are open, allowing ions to pass through continuously.

    • Current is negative as 3 Na ions exit the neuron while 2 K ions enter.

    • Both Na and K voltage-gated channels are closed.

  • Depolarization Phase

    • Triggered when a neuron is stimulated, causing the membrane potential to reach the threshold due to the accumulation of graded potentials.

    • Na voltage-gated channels open, resulting in Na ions rushing into the cell.

    • Influx of Na ions occurs due to the concentration gradient, making the membrane potential more positive and it overshoots.

  • Repolarization Phase

    • Na voltage-gated channels close after depolarization completes.

    • K voltage-gated channels open, allowing K ions to exit the cell.

    • This efflux of K ions causes the membrane potential to become more negative.

  • Hyperpolarization Phase

    • K voltage-gated channels close slowly, allowing extra K ions to escape.

    • This results in an undershoot where the membrane potential dips below the resting potential.

    • Both voltage-gated Na and K channels close to reset the state of the neuron.

Glial Cells

Glial Cells of the Peripheral Nervous System (PNS)

  • Satellite Cells

    • Located in Dorsal root ganglia (sensory neuron cell bodies) and Autonomic ganglia (sympathetic and parasympathetic).

    • Form a protective and continuous layer around neuronal cell bodies.

    • Function similarly to astrocytes in the central nervous system (CNS).

  • Schwann Cells

    • Long, flat cells that form the myelin sheath around a segment of an axon.

    • Myelination increases the velocity of action potential conduction.

    • Allows for smaller diameter nerves due to the insulating properties of myelin.

Glial Cells of the Central Nervous System (CNS)

  • Oligodendrocytes

    • Form the myelin sheath in the CNS by wrapping extensions around axons.

    • Provide electrical insulation to improve signal transmission.

  • Astrocytes

    • Most abundant glial cell type with many subtypes.

    • Play a critical role in forming the blood-brain barrier (BBB).

    • Regulate which molecules can enter the CNS, shielding it from potential toxic substances in the blood.

  • Microglia

    • Resident immune cells within the CNS.

    • Function as phagocytic immune cells to protect the brain from infections.

  • Ependymal Cells

    • Line the cavities of the brain and spinal cord.

    • Some ependymal cells are responsible for the secretion and circulation of cerebrospinal fluid (CSF).