Neuroglial Cells

Overview of Neuroglia

Neuroglial cells, also known as glial cells, are essential support cells in the nervous system. They originate from the same type of stem cell as neurons and play critical roles in both the central and peripheral nervous systems.

• Definition: Neuroglia are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons.

• Location: Found in both the CNS (Central Nervous System) and PNS (Peripheral Nervous System).

• Main Types in CNS:

  • Ependymal cells

  • Astrocytes

  • Microglial cells

  • Oligodendrocytes

• Main Types in PNS:

  • Schwann cells

  • Satellite cells

Neuroglia in the CNS

Ependymal Cells

Ependymal cells are specialized glial cells lining the fluid-filled passageways in the brain and spinal cord.

• Shape: Cuboidal or columnar.

• Function:

  • Produce and circulate cerebrospinal fluid (CSF).

  • Types:

    • CSF-producing ependymal cells – secrete CSF.

    • Ependymocytes – have motile cilia to circulate CSF.

    • Tanyocytes – contain microvilli, transport substances between CSF and brain.

• Location: Line ventricles of the brain and central canal of the spinal cord.

Microglia

Microglia are small, mobile glial cells in the CNS that act as the main immune defense.

• Origin: Related to white blood cells (WBCs).

• Function:

  • Proliferate during inflammation (disease/injury).

  • Remove cellular debris, waste, and pathogens by phagocytosis.

Astrocytes

Astrocytes are star-shaped glial cells found abundantly in the CNS, especially between neurons and blood vessels.

• Functions:

  • Maintain the blood-brain barrier.

  • Provide structural support.

  • Aid metabolism of substances (e.g., glucose).

  • Regulate ion and nutrient content of interstitial fluid.

  • Absorb and recycle neurotransmitters.

  • Form scar tissue after CNS injury.

Oligodendrocytes

Oligodendrocytes are glial cells in the CNS responsible for stabilizing axons and producing myelin.

• Function:

  • Produce myelin – a membranous wrapping that insulates axons and speeds up signal transmission.

  • Form Nodes of Ranvier – gaps in myelin sheath that facilitate rapid conduction.

  • Contribute to white matter (myelinated axons) vs. grey matter (unmyelinated axons and cell bodies).

  • Not all axons are myelinated.

Neuroglia in the PNS

Schwann Cells

Schwann cells are the principal glial cells of the PNS, responsible for myelinating axons.

• Function:

  • Wrap axons in multiple thin layers of plasma membrane.

  • Insulate and speed up signal transmission.

  • Support axon regeneration after injury (Wallerian degeneration).

Satellite Cells

Satellite cells surround neuron cell bodies in PNS ganglia and regulate the environment around neurons.

• Function:

  • Provide nutritional support.

  • Regulate ion concentrations.

• Ganglion: A group of neuron cell bodies in the PNS.

Neuron Injury and Regeneration

Neurons have limited ability to regenerate, especially in the CNS.

• Cell body injury: Usually kills the neuron; mature neurons do not divide.

• Axon injury in PNS: May be repaired via Wallerian degeneration (slow process, ~4 mm/day).

• Regeneration in CNS: Unlikely to occur.

Membrane Potential

Overview

Membrane potential refers to the electrical potential difference across a cell membrane, crucial for neuron and muscle cell function.

• Selective permeability: Cell membranes allow only certain ions to pass, creating a charge difference.

• Excitable cells: Neurons and muscle cells can rapidly change their internal electrical environment in response to stimuli.

Resting Potential

The resting potential is the membrane potential of an undisturbed cell.

• Typical value in neurons: About -70 mV.

• All neural activities begin with a change in the resting potential.

Graded Potential

Graded potentials are temporary, localized changes in membrane potential caused by stimuli.

• Proportionality: Change is directly proportional to stimulus intensity.

• Excitatory stimulus: Opens chemically-gated Na+ channels, allowing Na+ influx and making the cell less negative.

• Threshold: If strong enough, graded potential can bring the cell to threshold (about -55 mV).

• Effect decreases with distance from stimulus.

Gated Channels

Types of Gated Channels

Gated channels are membrane proteins that open or close in response to specific stimuli, altering membrane permeability.

• Chemically-gated (ligand-gated) channels: Open when binding specific chemicals (e.g., acetylcholine at neuromuscular junctions). Most common on dendrites and cell bodies.

• Voltage-gated channels: Open or close in response to changes in membrane potential. Common in axons; sodium channels have activation and inactivation gates.

• Mechanically-gated channels: Open in response to physical distortion of the membrane. Important in sensory receptors (touch, pressure, vibration; e.g., hair cells in the inner ear).

Distribution of Gated Channels

• Chemically-gated channels: Dendrites and cell body.

• Voltage-gated sodium and potassium channels: Along the axon.

• Voltage-gated calcium channels: At synaptic terminals.

• Most gated channels are closed at resting potential.

Summary Table: Neuroglial Cell Types and Functions

Key Equations

Membrane Potential Equation

The Nernst equation calculates the equilibrium potential for a particular ion:

Where: = equilibrium potential for the ion = universal gas constant = temperature (Kelvin) = charge of the ion = Faraday's constant , = ion concentrations outside and inside the cell

Threshold Potential

Threshold for action potential initiation in neurons:

Additional info:

• Neuroglia are essential for nervous system health and function, not just passive support.

• Wallerian degeneration is a process unique to the PNS, allowing some axon regeneration.

• Membrane potentials are foundational for understanding nerve impulses and synaptic transmission.