Neuroglia and Neuronal Support Cells

Microglia

Microglia are specialized immune cells that reside in the central nervous system (CNS). They play a crucial role in protecting and preserving neuronal cells from pathogens and facilitating recovery from metabolic insults.

• Function: Act as the primary immune defense in the CNS, removing debris and dead neurons through phagocytosis.

• Activation: Microglia are activated by signals from damaged or apoptotic neurons. Once activated, they can change their morphology and function.

• Threshold Response: Microglia remain in a resting state until a threshold of activating signals is reached. Prolonged activation can lead to altered properties and may contribute to neurodegenerative diseases.

• Clinical Relevance: Dysfunctional microglia are implicated in conditions such as Alzheimer's disease, amyotrophic lateral sclerosis (ALS), and neuropathic pain.

• Example: In response to neuronal injury, microglia migrate to the site, engulf debris, and release factors that modulate inflammation and repair.

Ependymal Cells

Ependymal cells line the fluid-filled cavities (ventricles) in the brain and the central canal of the spinal cord. They are essential for the production, circulation, and regulation of cerebrospinal fluid (CSF).

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

• Functions:

  • Circulate CSF, which cushions the brain and spinal cord.

  • Provide a barrier between the CSF and nervous tissue.

  • Participate in chemical signaling and waste clearance.

• Structure: Ependymal cells are ciliated, aiding in the movement of CSF.

• Additional info: Ependymal cells may also serve as a source of neural stem cells in certain regions.

Peripheral Neuron Repair

Mechanisms of Repair

Peripheral neurons have a limited ability to repair after injury, largely due to the supportive role of glial cells such as Schwann cells.

• Glial Response: After injury, glial cells can seal off damaged tissue and form a supportive environment for regeneration.

• Schwann Cells: In the peripheral nervous system (PNS), Schwann cells can create a tube or pathway to guide the regrowth of regenerating axons.

• Growth Cone: The tip of a regenerating axon forms a growth cone, which is guided by chemical and physical cues to re-establish synaptic connections.

• Limitations: Natural repair is less likely in the CNS due to inhibitory factors and lack of supportive glial response.

• Example: After a peripheral nerve injury, Schwann cells proliferate and align to form a regeneration tube, facilitating axonal regrowth.

Electrical Signals in Neurons

Membrane Potential and Ion Distribution

Neurons and muscle cells are excitable due to their ability to propagate electrical signals in response to stimuli. The membrane potential is influenced by the distribution of ions and membrane permeability.

• Key Factors:

  • Uneven distribution of ions (concentration gradients) across the cell membrane.

  • Selective membrane permeability to different ions.

• Nernst Equation: Calculates the equilibrium potential for a single ion:

• Resting Membrane Potential: In most neurons, the resting membrane potential is approximately -70 mV, mainly due to potassium (K+) permeability.

• Ion Movements: Small changes in ion concentration or permeability can significantly alter the membrane potential.

• Example: To change the membrane potential by 100 mV, only a tiny fraction of total K+ ions need to move across the membrane.

Goldman-Hodgkin-Katz (GHK) Equation

The GHK equation predicts the membrane potential resulting from the combined effect of all permeant ions, considering both their concentration gradients and membrane permeabilities.

• Equation:

• Variables:

  • = permeability of the membrane to ion x

  • and = concentration of ion x inside and outside the cell

• Difference from Nernst Equation: GHK considers multiple ions and their relative permeabilities, while Nernst is for a single ion.

Ion Channels and Gated Channels

Ion channels are proteins that allow specific ions to cross the cell membrane, altering membrane potential and generating electrical signals.

• Types of Ion Channels:

  • Mechanically Gated Channels: Open in response to physical deformation (e.g., pressure, stretch).

  • Chemically Gated Channels: Open in response to binding of neurotransmitters or other signaling molecules.

  • Voltage-Gated Channels: Open or close in response to changes in membrane potential.

  • Monovalent Cation Channels: Allow passage of Na+ and K+.

• Channel Properties: Channels vary in their selectivity, gating mechanisms, and kinetics. Some channels inactivate spontaneously or in response to specific signals.

• Channelopathies: Diseases caused by dysfunctional ion channels, such as cystic fibrosis, congenital insensitivity to pain, and certain muscle disorders.

Current Flow and Ohm's Law

The movement of ions through channels generates an electrical current, which follows Ohm's law.

• Ohm's Law:

• Variables:

  • = current (amperes)

  • = voltage (potential difference, volts)

  • = resistance (ohms)

• Sources of Resistance:

  • Membrane resistance (phospholipid bilayer)

  • Internal (cytoplasmic) resistance

• Significance: Resistance determines how far current can travel within a neuron before dissipating.

Types of Electrical Signals in Neurons

Neurons generate two main types of electrical signals: graded potentials and action potentials.

• Graded Potentials:

  • Variable strength, travel short distances, and decrease in strength as they spread.

  • Can be depolarizing (make the inside less negative) or hyperpolarizing (make the inside more negative).

  • If large enough, can trigger an action potential.

• Action Potentials:

  • Brief, large depolarizations that travel long distances without losing strength.

  • Essential for rapid signaling in the nervous system.

• Example: Sensory neurons generate graded potentials in response to stimuli, which, if sufficient, initiate action potentials that travel to the CNS.

Summary Table: Types of Glial Cells

Additional info: Table includes glial cell types not explicitly listed in the original notes for completeness.