Degeneration and regeneration of peripheral nerves Flashcards
What are descending pathways?
The brain communicates with effectors through relay chains of neurones.
What are ascending pathways?
The brain communicates with tissues for sensory experiences via ascending pathways.
What does the nervous system express its actions through?
Skeletal muscles - somatic nervous system.
Smooth muscles - autonomic nervous system.
Glands - autonomic.
Sensory brain - via afferents that originate in the periphery.
What are the causes of degeneration of nerves?
Genetics
Natural processes of aging
Acquired through injury.
What is the boundary of the peripheral nervous system?
The boundary between CNS and PNS is defined by Pia Mater of the Meninges.
CNS do not regenerate following injury.
Peripheral nerves have capacity to regenerate and re-innervate depending on the location of injury.
What is dysfunction?
If a neurone or its immediate environment are disturbed, the function of the neurone might be disrupted.
The extent of dysfunction if dependent on nature of insult.
What are the structural features of the nerve?
Axon
Myelin Sheath
Endoneurium - ensheaths an axon
Perineurium - ensheaths a nerve fasicle (multiple axons).
Epineurium - superficial - ensheaths entire nerve.
What is Seddon’s classification of nerve injury?
Neuropraxia - confined to epineurium
Axonotomesis - partially transects nerve
Neurotmesis - completely transects nerve
What is neuropraxis?
A neurone temporarily loses its ability to function.
Usually at the level of the myelin sheath.
Restoration of function would be complete upon recovery.
e.g. temporary compression of fibre leading to blood supply loss.
What is axonotomesis?
Usually a result of severe crush injury.
Axons and myelin sheath are damaged.
Endoneurium, perineurium and epineurium are intact.
Restoration of function is expected.
What happens in axonotomesis?
If endoneurium, perineurium and epineurium are intact, new nerve sprouts would use guidance cues from tubes of the ensheating membranes that form.
This re-supplies the target effector to restore normal function.
What is neurotmesis?
The entire nerve fibre is completed transected.
Axon and connective tissue are damaged.
No recovery of function.
How is a nerve divided after injury?
Divided into proximal segment and distal segment of axon.
What is the proximal segment of the axon?
This is attached to the cell body, it survives the injury.
It is likely to continue to receive support - nutrients etc, from the cell body.
What is the distal segment of the axon?
Cut off from cell body.
Loses potential for repairs and nutritional support.
Becomes vulnerable to phagocytosis by glia.
How does the environment of injured neurones change?
Astrocyes create an environment in which neurones thrive.
Microglia are immune cells which are activated and digest the distal segment of the axon.
The axon leaks intracellular fluid containing nutrients into the extracellular environment.
What happens to the distal segment minutes after the injury?
The neurone immediately stops conducting action potentials beyond the axon.
The two ends of the axon are exposed and start leaking intracellular fluid - losing electrolytes and neurotransmitters - toxic.
The cut ends pull apart, sealing themselves and swelling.
What happens to the distal segment hours after injury?
The synaptic terminal degenerates - accumulation of neurofilaments and vesicles.
Astroglia surround the terminal - cause the terminals to be pulled away from postsynaptic cell.
Microglia begin digesting the distal segment.
What happens days after the injury to the distal segment?
Wallerian Degeneration - digested by phagocytes - microglia.
What happens to the proximal segment after nerve injury?
Chromatolysis: The cell body produces lots of proteins for repairing the cell.
The volume of the cell body increases and becomes bloated with new products.
The nucleus is displaced to the side of the body.
The nerve seals the wounded stump to form a neuroma.
What happens to the neuroma of the proximal segment?
Regenerating axons form many sprouts, some of which find Schwann cell tubes.
What does innervation mean?
The normal state of a nerve supply to a muscle or other type.
e.g. alpha motor neurones innervate skeletal muscle.
What does re-innervation mean?
Re-growth of the nerve to re-supply the muscle, after denervation occurs.
However, sometimes the nerves re-innervate the wrong muscle, leading to unusual symptoms.
What happens in the acute phase of a denervated muscle?
The muscle is immediately paralysed.
The muscle becomes areflexic (no tendon reflexes) and atonic.
The muscle starts to fasciculate (twitch).
If the muscle is not reinnervated, the fasciculations will subside.
