Biology Reference
In-Depth Information
consequence of peripheral nerve injury, cell bodies in dorsal root ganglia
(DRGs) and anterior horns of the spinal cord undergo adaptive changes that
involve a chromatolytic reaction associated with a shift in protein synthesis
from a “signaling mode” to a “growing mode” and protein synthesis
switches from neurotransmitter-related substances to those required for axo-
nal reconstruction. Moreover, the peripheral and central nervous systems
(CNSs) are functionally integrated and a peripheral nerve lesion always
results in long-lasting central modifications and reorganization (
Kaas,
1991; Kaas & Collins, 2003; Wall, Xu, & Wang, 2002
). The mechanisms
of plasticity and reorganization of brain circuits that occur after nerve injury
are complex; they may result in beneficial adaptive functional changes or
contrarily cause maladaptive changes, such as pain, dysesthesia, hyperre-
flexia, and dystonia (
Lundborg, 2000, 2003
).
2. CHANGES AT THE NERVE LEVEL
In 1943, Sir Herbert Seddon introduced a classification of three dis-
crete types of nerve injury: neurapraxia, axonotmesis, and neurotmesis
(
Seddon, 1943
):
i.
Neurapraxia is a mild injury characterized by local myelin damage.
Axon continuity is preserved, and the nerve does not undergo
Wallerian degeneration. It may result from exposure to a wide range
of conditions such as heat, cold, irradiation, or electrical injuries, but
is most commonly due to mechanical stress, such as concussion, com-
pression, or traction injuries. Recovery may occur within hours, days,
weeks, or up to a few months.
Axonotmesis involves additional damage to peripheral axons, but con-
nective tissue structures remain intact. The interruption of axons is
often the result of nerve pinching, crushing, or prolonged pressure.
Wallerian degeneration occurs, but subsequent axonal regrowth may
proceed along the intact endoneurial tubes. Recovery depends upon
the degree of internal disorganization in the nerve as well as the distance
to the end organ.
iii.
Neurotmesis is the most severe injury, equivalent to physiologic dis-
ruption of the entire nerve. Functional recovery does not easily occur
because of the extent of endoneurial tube disruption. Nonetheless, suc-
cessful regeneration might result with surgical intervention.
In 1951, Sunderland expanded Seddon's classification to five degrees of
peripheral nerve injury instead of three (
Sunderland, 1951
). He divided
ii.
