Injured nervous system Flashcards
Why can the peripheral nervous system (PNS) regenerate after injury/degeneration but the central nervous system (CNS) not?
The peripheral nervous system (PNS) can regenerate after injury due to factors such as supportive Schwann cells, a lack of strong inhibitory factors, and a more permissive environment. In contrast, the central nervous system (CNS) exhibits limited regeneration because of inhibitory factors like myelin-associated inhibitors, scar formation by reactive astrocytes, and the intrinsic lower growth potential of CNS neurons. Additionally, the differential response of support cells (Schwann cells vs. oligodendrocytes) and the proximity of target cells contribute to the contrasting regenerative abilities between the PNS and CNS. Ongoing research aims to understand these differences and develop strategies to enhance CNS regeneration.
Can we prevent degeneration and enhance regeneration?
Promoting nerve regeneration and preventing degeneration involves strategies such as neuroprotective measures (antioxidants, anti-inflammatory agents), stem cell therapy, nerve growth factors, rehabilitation, electrical stimulation, genetic approaches, drug development, and lifestyle modifications. Research in these areas aims to enhance neural function and treat neurodegenerative conditions.
Different glial cells of the CNS and PNS (histology)
Glial cells (glia, neuroglia) support neurons with structure and nutrients, maintain their environment, protection and also participate in neurotransmission/synaptic activity (tripartite synapse).
astrocytes= a type of glial cells
Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, they provide structural support, contribute to blood-brain-barrier, electrical
active (tripartite synapse; gap junctions). Radial glia serve as scaffolds for developing neurons of the cortex as they migrate to their end destinations. Microglia (specific microphages) scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions
the neurons.
Astrocytes:
Function: Provide structural support, regulate nutrient and ion concentrations, and participate in the blood-brain barrier. They are also involved in synaptic regulation and repair processes.
Microglia:
Function: Act as the immune cells of the central nervous system, defending against pathogens and removing dead or damaged cells through phagocytosis. They play a role in inflammation and the response to injury.
Oligodendrocytes (in the central nervous system) / Schwann
Cells (in the peripheral nervous system):
Function: Produce myelin, a fatty substance that wraps around axons to insulate and facilitate faster nerve impulse conduction. Oligodendrocytes myelinate multiple axons, while Schwann cells myelinate a single axon.
Ependymal Cells:
Function: Line the ventricles of the brain and the central canal of the spinal cord, contributing to the production and circulation of cerebrospinal fluid. They also provide a barrier between cerebrospinal fluid and nervous tissue.
Schwann cells, which form the myelin sheath, have
phagocytic activity and promote regrowth of axons.
Satellite cells, smalls cells which provide nutrients and
structural support to neurons in ganglia.
Enteric glial cells in digestive system ganglia support
homeostasis
Nerve damage within PNS and CNS may be generated by
different cause
Neurodegenerative diseases
(Alzheimer disease AD,
Parkinson disease PD- dopaminergic neurons neurons (helps with memory, sexual activity)
Amyotrophic lateral sclerosis ALS)
Stroke
Nutrition Neuropathies
(Diabetic/
vitamin B12 liver shortage)
Viral infection
(HIV/CMV-neuropathy,
Post Herpetic Neuralgia,
COVID-19)
Multiple Sclerosis MS
Autoimmune disease EAE
Trauma
(physical injury caused by an
external force:
cut or crushed
Regeneration of the PNS
What are some nerve guidance cues
- Self-repair of damaged peripheral nerves is good in the skin and if larger nerves are only mild/small parts affected.
- Surgical repair of damaged peripheral
nerves is good
Functional outcome is good, if small gap between nerve ends, help is needed with larger gap (>7cm)
- Nerve guidance cues
- Hydrogels
- Fibrin glue
- re-anastomosis = reuniting (as by surgery or healing)
- autologous nerve graft repair = part of other nerve of
recipient - non-autologous nerve graft repair = non-nerve part of
recipient - allograft = tissue not from recipient
Regeneration of the CNS – spinal cord injury (SCI)
no cure
* 800 people have a SCI each year in the UK
* SCI primarily affects young adults approximately 80% are males
* There is an increasing incidence in the elderly in whom rehabilitation can be more difficult
* 40,000 people live with paralysis in the UK
* Cost to the nation estimated at £1 billion per annum
Spinal cord injury:
A lesion to the spinal cord has debilitating effects, it
disrupts sensory and motor signals passing between brain and body. This results in essentially no feeling or control of function below the injury site.
Other Medical Issues
Bladder/bowel function management
Decubitus/wound infections
Assuring respiration
Treatment for spinal cord injury
Spinal stabilization
High dose steroid therapy
Until recently, treatment for spinal cord
injury has had only 2 main aims:
- Prevent further damage at the time of injury using high dose steroids
- Rehabilitation of remaining function
Wallerian degeneration (WD)
degeneration= the progressive loss of structure and/or function of neurons that culminates in cellular atrophy and death
Wallerian degeneration is an important concept that is useful in mapping the anatomic components of peripheral nerves and spinal cord segments, in recognizing peripheral or central nervous system disorders microscopically, and in understanding and predicting reinnervation by peripheral nerves.
- The active degeneration process of the axon distal to the injury site (cut or crushed) taking place in the CNS and PNS, first description by Augustus Waller in 1858 “The Case of the
Disappearing Axon”. - Neurodegeneration is a ‘Wallerian-like degeneration’ process a highly stereotyped dying back or retrograde degeneration process.
- the distal section of the axon tends to remain
electrically excitable for some time (phase I) - WD usually begins within 24–36 hours of a
lesion (lag phase II) - Anterograde degeneration of distal axon
followed by myelin sheet degradation and
macrophage infiltration (fragmentation phase III) - Regeneration phase IV only for PNS not CN
PNS – Peripheral Nerve Injury (PNI) Wallerian degeneration
In most cases, the lag phase is then followed by a rapid fragmentation phase, in which the axon breaks into many individual pieces, which are then phagocytosed by glia and immune cells. WD likely entails a cell autonomous chain of events that occur within the distal axon itself and, hence, can be considered as a “self-destruction” pathway, akin to apoptosis.
However, WD appears to involve a molecular pathway that is quite distinct from apoptosis. Entry of extracellular calcium and release of intracellular calcium stores, with consequent activation of calcium-dependent proteases
schwann cells can dedifferentiate back so they lose there myelination job.
What are the steps of wallarien degeneration
(A) intact myelinating Schwann cells enwrap an intact axon and fibroblasts are scattered between nerve fibres
(B-E) injured PNS nerve that undergoes normal Wallerian degeneration:
(B) Traumatic injury produces immediate tissue damage at the lesion site a gap may be formed between the proximal and distal nerve stumps, and Galectin-3/MAC-2+ macrophages accumulate at the lesion site within 24 hours after the injury.
(C) Destruction of distal axons is detected during normal Wallerian degeneration 36 hours after the injury.
(D) Recruitment of Galectin-3/MAC-2+ macrophages, myelin disintegration, and Galectin-3/MAC-2 expression by
dedifferentiating Schwann cells begin 48 to 72 hours after injury during normal Wallerian degeneration.
(E) Galectin-3/MAC-2+ macrophages and Schwann cells scavenge degenerated myelin during normal Wallerian degeneration, and Schwann cells proliferate further and form Büngner bands (basement membrane remains
after axons deg. & helps guiding new axons;
German doctor Otto von Büngner around 1900
Degeneration and regeneration in the PNS
In PNS
Permissive lesion environment:
- Schwann cells de-differentiate and produce neurotrophic
factors and secrete extracellular matrix (ECM) molecules for
axonal growth (integrins, interleukin-1, PDGF etc), form
skeleton of parallel-arranged microscopic tubes as basement membrane = basal lamina (Büngner bands), axons follow them = remyelination
- Scavenger cells = macrophages phagocytose peripheral
nerve debris.
- Fast removal of nerve debris
Significant intrinsic neuronal regenerative response:
- Neuronal axon tip forms growth cone, up regulation of
GAP43, CAP23 & transcription factors (c-jun, c-fos, ATF-3)
by injured neuron, regeneration-associated genes (RAGs),
Degeneration and regeneration in the CNS
Non-permissive/distructive lesion environment:
- Oligodendrocytes do not rejuvenate or regenerate, they do not secrete growth factors, inhibit axonal growth with endogenous myelin inhibitory molecules (nogo, MAG, OMgp), disordered tubes, no basement membrane to follow for axons
- Astrocytes secrete growth inhibitors including proteoglycans and form dense scar
- Slow removal of nerve debris
No significant intrinsic neuronal regenerative response:
- Neuronal axon tip forms retraction bulb disorganized
microtubules, which is associated with the accumulation of
mitochondria and anterograde transported vesicles.
Pseudounipolar dorsal root ganglia (DRG) neuron proximal
and distal axon regeneration ≈ CNS and PNS regeneration
PNS axons vs. CNS axons. (A) Proximal/central and distal/peripheral axon of pseudounipolar DRG axon.
If the peripheral branch is injured, it forms growth cone at its tip and regenerates. If the central branch is injured
in SCI, it forms dystrophic endball and cannot regenerate. Reactive astrocytes at the site of SCI produce many
factors including proteoglycans, which make a gradient and may induce dystrophic endball formation. PG,
proteoglycan; HSPG, heparan sulfate proteoglycan. (B) A comparison between CNS and PNS injuries.
Proximal/central dorsal root ganglia (DRG) axons as model
for CNS regeneration
The central collaterals of myelinated DRG cells form the ascending dorsal column projection system.
A myelinated tract, responsible for fine, discriminative, touch. It may be discretely lesioned and regenerating axons may be labelled from the periphery with a tracer.
Proximal/central dorsal root ganglia (DRG) axons as model
for CNS regeneration
Differences in regenerative ability between the central and peripheral DRG neuron axons. (A) The peripheral DRG neuron
axon (orange) is capable of growing long distances beyond the lesion site (green), possibly reinnervating its targets. (B) After injury to the dorsal column tract (left), central DRG neuron axons (orange) are not capable of regenerating beyond the lesion site.
In contrast, if the dorsal root is injured (right) the central axons can slowly grow beyond the lesion site (green) but they are not capable
of regenerating across the dorsal root entry zone. (C) Schematic drawing of the conditioning lesion effect. If an injury to
peripheral DRG neuron axons is performed some days before the central lesion (conditioning lesion), the regeneration ability of the
central axons increases (green axon): dorsal column tract fibers are able to regenerate beyond the lesion site and dorsal root axons
grow faster (injury dorsal root later, Richardson and Verge, 1987; spinal cord later, Neumann and Woolf, 1999; central DRG earlier, Ylera et al., 2009).
