Degeneration and regeneration of nerves and muscles after trauma Flashcards
What is neruopraxia
Temporary short lived paralysis and loss of function
Recovery is rapid and complete
Pressure on the nerve is the most common cause
e.g. pins and needles
What are Axonotmesis
Nerve lesions in which the axon is served but the surrounding connective tissues are not (endoneurium etc remains in tact)
Wallerain degeneration occurs on distal axonal segment
functional recovery more rapid and more complete
crushing injuries most common cause
What is neruotmesis?
Complete sectioning of the nerve and all its connective tissues
Permanent damage to the nerve never fully recover
What events occur immediately after nerve trauma?
impulse transmission immediately stops
cut ends of axon begin to ooze axoplasm but are quickly sealed
What happens to the distal segment of damaged axons?
Non viable therefore dies
Microglia and macrophages attracted to segment begin to hydrolyses and break down and phagocytose the distal segment axon this is called Wallerian Degeneration
Within a week segment completely degnenerated
endoneurium and Schwann cells remain
Schwann cells being to divide rapidly within the endoneurium
What happens to the proximal segment and the cell body?
Undergo Retrograde degeneration
cell body starts to undergo chromatolysis Nissl substance (RER) disappears nucleus is displaced to the periphery and cellular oedema occurs
Synaptic stripping may also occur
Proximal segment of axon is degenerated up to first Node of Ranvier
What determines whether a cell body survives or dies?
The proximity of the lesion to the cell body
The more distal the greater the chance of the cell body surviving
The more proximal ie close the more likely the cell body is to die and be degenerated itself
If any damage occurs to the cell body at all the whole neurone dies
What does regeneration depend on?
Neuronotropic factors (produced by Schwann cells)
Growth associated proteins
Endonurial sheath and Schwann cell cords to guide sprouts
Inhibitory factors in the epinerurium
What happens in the regeneration of the cell body?
Increased protein and RNA synthesis to prepare for axonal repair
nucleus returns to the centre of the cell and the Nissl substance reappears
cell swelling (oedema decreases)
What is the process of regeneration of axons (in axonotemesis)
sprouts emerge from proximal stump in response to neruronotropic growth factors
sprouts are guided by parallel cords of Schwann cells as well as the endonurial sheath (collectively a band fibre) towards there target effector
Inhibitory factors in the perineurium prevent the sprouts leaving the nerve
This often leaves to reinnervation of the effector (muscle)
over time Schwann cells beign to remylelinate the axon
The growth of sprouts is slow 1.5 mm a day so can take months to heal
What happens to regenerating axons following neruotemisis?
poor prognosis
because no connective tissue network is there to guide the sprouts they often become tangled forming a nuroma
nerves often don’t reinnervate there correct effectors
return to normality is highly unlikley
What about brain and SC injuries
No Schwann cells or connective tissue therefore any damage severe and usually permeant
What happens to muscles supplies by nerves immediately after they are severed?
immediately paralysed
muscle starts to fasciculate uncontrolled contractions due to release of ACh from damaged neurone
can last for weeks more ACh receptors are formed
What are the longer term effects of nerve damage on the muscles?
fasciculation’s will eventually subside
muscles that are denervated begin to deteriorate and die
Bulk of muscle lost due to denervation atrophy
use bulk due to lack of use disuse atrophy
muscle eventually dies and is replaced by connective tissue
How can muscle damage kill neurones and people?
Crushing injuries can result in muscle death and Traumatic Rhabdomyolysis
Death of muscle damage and death of neurones
Toxic metabolites produced can lead to death
