Impairments in Protein Synthesis and Fluid-Electrolyte Balance
Normal neuronal function depends on continuous protein synthesis. Deficits in amino acids, decreased enzyme activity, or destruction of the endoplasmic reticulum (Nissl substance) lead to irreversible damage. Similar disruptions occur in glial cells, such as astrocytes, which are responsible for pinocytosis.
Functional and structural changes often begin with plasma membrane ion channel dysfunction, driving a pathological cycle:
- Neurons rapidly lose potassium ions (K⁺), which accumulate in the extracellular space.
- Sodium ions (Na⁺) are retained inside the cell.
- Water follows sodium into the cell along the osmotic gradient.
- Hyperosmolarity, hyperhydration, and severe swelling develop, inevitably leading to neuronal destruction.
At the whole-organism level, neurological dysfunctions may present as systemic failures such as hyporexia (decreased appetite), hyposalivation, and gastric hyposecretion.
Role of Calcium and the Glutamate Cascade
A crucial irreversible stage of neuronal alteration is the accumulation of excess Ca²⁺ ions in the cytoplasm. This is driven by neurotransmitter imbalance, primarily excess glutamate, which is massively released during ischemia.
Excess glutamate causes sustained overactivation of NMDA receptors. This opens membrane channels, causing a massive influx of calcium. Persistent plasma membrane depolarization ensues, preventing the cell from functioning. Intracellular calcium activates destructive enzymes:
- Protein kinase C — disrupts protein phosphorylation processes.
- Phospholipase — degrades membrane phospholipids, further releasing calcium from intracellular stores.
- Endonuclease — begins destroying nucleic acids.
Rapid free radical reactions complete the destruction process.
Neuronal Membrane Damage
Alteration of membrane structures is recognized as the universal mechanism of neural impairment. Membranes are damaged by several factors:
- Generation of reactive oxygen species (ROS) and enhancement of lipid peroxidation (LPO).
- Attack by activated endogenous phospholipases.
- Mechanical overstretching due to hyperhydration and cell swelling.
- Incorporation of amphiphilic compounds into the lipid bilayer.
Fatal Consequences of Membrane Damage: Normal cellular activity is severely disrupted. Synaptic transmission and the synthesis of neurotransmitters (peptides, amino acids, catecholamines) fail, and axonal transport halts. The neuron's specialized function—electrogenesis—ceases. Ultimately, the cell dies.
Mechanisms and Pathways of Neural Cell Death
The pathway by which a neuron dies depends on multiple factors: the intensity and duration of the pathological stimulus, the cell's phylogenetic characteristics, the selectivity of the damage, and the integrity of the blood-brain barrier (BBB).
Two fundamentally different pathways of cell death are distinguished:
- Necrosis (Destruction). Occurs during severe, massive injury and is typically accompanied by a breach of the BBB. Classic examples include ischemic or hemorrhagic strokes, where acute hypoxia and intracellular acidosis cause rapid destruction of brain tissue.
- Genetically Controlled Death (Apoptosis and Necroptosis). Most commonly seen in neurodegenerative disorders. This pathway characterizes Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), and various forms of dementia.