V. LESIONS IN PERIPHERAL NERVES
A. Transection
- 1. Wallerian degeneration occurs in the distal part of
the axon following transection, crush or local loss of blood supply. In
peripheral nerves, 1 day following injury, myelin retraction is observed
at nodes of Ranvier. At 2 days, fatty droplets are present in the myelin;
the axon is pale, varicose, granular and fragmented; Schwann cells are undergoing
proliferation and enlargement and there is phagocytosis of myelin debris.
By 25 days most of the debris has been removed, and the size and number
of Schwann cells begin to decrease. In central nerve tracts, by contrast,
including even those central portions of primary sensory neurons, the time
course of Wallerian degeneration is very slow and prolonged.
- 2. Chromatolysis describes the retrograde changes seen
in the nerve cell body following trauma to its axon. Within 48 hours changes
are evident in the ventral horn motor cells (and to a lesser extent in the
dorsal root ganglion cells), consisting of swelling of the cell, decreased
staining of the Nissl substance, margination of Nissl substance peripherally,
and nuclear swelling and eccentric migration. Maximal changes are evident
at 1-2 weeks, followed by recovery during the next 3 months.
- 3. Axon regeneration starts as early as 1 day after injury.
Large amounts of protein are synthesized and flow through the axon to the
site of injury. Complete regeneration cannot occur but up to 80% of fibers
may be restored in 6 months - 2 years. Axons persist only if innervation
is achieved.
B. Compression
- Compression of the peripheral nerve is more common than acute traumatic
transection. It is of a more chronic nature and is a common cause of disability,
especially of the upper limb. An example is the compressed median nerve
in the carpal tunnel.