Nerve cells, electrochemistry and biosignalling

Excitatory and inhibitory nerve fibers

• Excitatory inputs drive the membrane potential closer to threshold, promoting depolarization.
• Inhibitory inputs shift the membrane potential more negative, hindering depolarization.

Multiple subthreshold excitatory inputs can summate, reaching threshold jointly, while co-occurring inhibitory signals can offset excitatory impulses. Repetitive firing at a high frequency can also combine with residual depolarization from prior action potentials, incrementally boosting the membrane potential to threshold

Glial cells, neuroglia

Glial cells—also known as neuroglia—are the non-neuronal cells that play a variety of crucial support roles in both the central and peripheral nervous systems. Although they were once thought to be merely the “glue” holding neurons together (an idea that gave rise to the term “neuroglia”), modern research shows that they are dynamic and indispensable players in maintaining overall brain health and function.

Key glial types and their functions

Astrocytes: Star-shaped and abundant, they balance ions, maintain the blood–brain barrier, support neurons, modulate synapses, and aid in repair.
Oligodendrocytes & Schwann Cells: In the CNS, oligodendrocytes myelinate axons for fast impulse transmission; Schwann cells perform this role in the PNS.
Microglia: Acting as the nervous system’s immune cells, they patrol for pathogens and damage, coordinating inflammatory responses.
Ependymal Cells: These line the brain’s ventricles and spinal cord canal, producing and regulating cerebrospinal fluid to cushion and clear waste.

Glial cells are not only the scaffold for the nervous system—they also:

• Maintain homeostasis: By regulating the chemical composition around neurons, glia help sustain the delicate environment required for proper neuronal signaling.
• Modulate synaptic function: Astrocytes, in particular, can influence how neurons communicate by controlling neurotransmitter levels at synapses.
• Aid in repair and regeneration: After injury, certain glial cells help clear debris and create conditions that promote neuronal recovery, although this response can sometimes also contribute to scarring or other complications.
• Contribute to neurodevelopment: During brain development, glial cells guide the formation of neural circuits and support the migration of neurons to their proper locations.

Electrochemistry

Concentration cells and electron flow

• In a concentration cell, the difference in ion concentration drives the redox reaction. Although the electrodes are identical (meaning their standard potentials cancel out), the side with the higher ion concentration has a greater chemical potential.
  Electrons move from the electrode in the less concentrated solution (lower chemical potential) to the electrode in the more concentrated solution (higher chemical potential). This electron flow helps the system work toward equilibrium by equalizing the concentration difference between the two half-cells.

The Nernst equation

• The Nernst equation relates the cell’s electrode potential to the ion concentrations and temperature:

$E$= $E^{\circ }$ - $\frac{RT}{nF}$ ln Q

Where:

• $E$ is the electrode potential,
• $E^{\circ }$ is the standard electrode potential,
• $R$ is the universal gas constant,
• $T$ is the absolute temperature in Kelvin,
• $n$ is the number of electrons transferred,
• $F$ is Faraday’s constant, and
• $Q$ is the reaction quotient representing the ratio of ion concentrations.

This simplified form shows that even a small difference in ion concentration (Q) can generate a measurable voltage, as the potential varies logarithmically with the concentration ratio.

Biosignalling (BC)

Biosignalling involves the processes that allow cells to detect, process, and respond to external signals. Biosignalling relies on several specialized proteins embedded in the cell membrane:

Gated ion channels:
These channels control the flow of ions into and out of cells, thereby influencing electrical activity and cellular responses. They come in different types:

• Voltage-gated channels: Open or close in response to changes in the electrical potential across the cell membrane. This mechanism is essential in neurons and muscle cells, where rapid changes in voltage trigger the transmission of signals.
• Ligand-gated channels: Respond to the binding of specific molecules (ligands) such as neurotransmitters. When a ligand attaches to the channel, it induces a conformational change that opens the channel, allowing ions to pass through and alter the cell’s electrical state.

Receptor enzymes:

• These receptors combine signal detection with enzymatic activity. When a ligand binds to a receptor enzyme, it activates an intrinsic catalytic function that can modify other proteins (often through phosphorylation). This action triggers a cascade of intracellular signals, enabling the cell to mount a coordinated response to the initial stimulus.

G Protein-Coupled Receptors (GPCRs):

• Representing one of the largest families of cell-surface receptors, GPCRs mediate a wide variety of physiological processes. Upon ligand binding, a GPCR undergoes a structural change that activates an associated G protein. The activated G protein then interacts with other intracellular effectors, setting off a chain of events that can alter cellular metabolism, gene expression, or other functions.