"Nerve" - what is it, definition of the term
In rats and mice, a neural bundle comprises grouped axons encased in connective tissue that conduct electrochemical impulses between the central nervous system and peripheral organs, thereby mediating sensory input and motor output.
Detailed information
The peripheral neural bundle in rodents consists of myelinated and unmyelinated fibers that transmit sensory and motor signals between the central nervous system and distal tissues. In both laboratory rats and mice, the largest such structure in the forelimb is the brachial plexus, while the hindlimb is served by the sciatic bundle, which contains thousands of axons of varying diameters.
Microscopic examination reveals a layered organization: an outer epineurium provides mechanical protection; the perineurium encloses fascicles; the endoneurium surrounds individual axons. Myelination is mediated by Schwann cells, which express proteins such as myelin basic protein and peripheral myelin protein 22. Unmyelinated fibers are ensheathed by non‑myelinating Schwann cells that support metabolic exchange.
Physiological properties differ between the two species. Rats exhibit larger axonal diameters and higher conduction velocities, typically 55–70 m s⁻¹ in the sciatic bundle, compared with 35–50 m s⁻¹ in mice. These differences affect experimental outcomes in electrophysiological recordings and nerve injury models.
Common experimental manipulations include:
- Transection: complete severing to study regeneration; regeneration rates average 1–2 mm day⁻¹ in rats, slightly slower in mice.
- Crush injury: calibrated force applied for a defined period, preserving connective sheaths while disrupting axons.
- Chemically induced demyelination: administration of cuprizone or lysophosphatidylcholine to assess remyelination dynamics.
Molecular analyses show conserved expression of neurotrophic factors (NGF, BDNF) across both species, but quantitative PCR indicates higher baseline NGF mRNA in rat dorsal root ganglia. Genetic tools available in mice, such as Cre‑loxP lines targeting Schwann cell genes, enable cell‑type‑specific manipulation not readily achievable in rats.
In summary, the neural conduit in rodents presents a well‑characterized anatomy, distinct electrophysiological parameters between species, and a versatile platform for injury and regeneration studies. Understanding these nuances is essential for translating findings from rodent models to broader neurobiological contexts.