no edit summary
Line 20: Line 20:  
==[[General CNS Responses to Injury]]==
 
==[[General CNS Responses to Injury]]==
   −
===Ischaemic Damage===
  −
  −
* The CNS is particularly sensitive to ischaemia, because it has few energy reserves.
  −
* The CNS is protected by its bony covering.
  −
** Despite offering protection, the covering also makes the CNS vulnerable to certain types of damage, for example:
  −
*** Damage due to fractures and dislocation.
  −
*** Damage due to raised intracranial pressure.
  −
**** Raised intracranial stimulates a compensatory increase in blood flow, further raising intracranial pressure. This stimulates a further increase in blood flow, and the cycle continues until intracranial pressure is so high that blood flow is impeded.
  −
***** The result of this is '''ischaemia'''.
  −
* Survival of any cell is dependent on having sufficient energy.
  −
** Ischaemia causes cell death by impeding energy supply to cells.
  −
*** Cells directly affected by ischamia die rapidly.
  −
**** For example, those suffering a failure of pefusion due to an infarct.
  −
*** Neurons surrounding this area of complete and rapid cell death exist under sub-optimal conditions and die over a more prolonged period.
  −
**** This area of gradual death is known as the '''lesion penumbra'''.
  −
**** There are several mechanisms implicated in cell death in the penumbra:
  −
****# Increase in intracellular calcium
  −
****# Failure to control free radicals
  −
****# Generation of nitrogen species (e.g NO and ONOO) are the main damaging events.
  −
  −
===Oedema===
  −
  −
* There are three types of cerebral oedema:
  −
*# '''Vasogenic oedema'''
  −
*#* Vasogenic oedema follows vascular injury.
  −
*#* Oedema fluid gathers outside of the cell.
  −
*#* This is the most common variation of cerebral oedema.
  −
*# '''Cytotoxic oedema'''
  −
*#* Cytotoxic oedema is due to an energy deficit.
  −
*#** The neuron can’t pump out sodium and water leading to swelling within the cell.
  −
*# '''Interstitial oedema'''
  −
*#* Associated with hydrocephalus.
  −
*#* This type of cerebral oedema is of lesser importance.
  −
* One serious consequence of oedema is that the increase in size leads to the brain trying to escape the skull.
  −
** This causes herniation of the brain tissue.
  −
** The most common site of herniation is at the foramen magnum.
  −
*** The medulla is compressed at the site of the respiratory centres, leading to death.
  −
  −
===Demyelination===
  −
  −
* Demyelination is the loss of initially normal myelin from the axon.
  −
* Demyelination may be primary or secondary.
  −
  −
====Primary Demyelination====
  −
  −
* Normally formed myelin is selectively destroyed; however, the axon remains intact.
  −
* Causes of primary demyelination:
  −
** Toxins, such as hexachlorophene or triethyl tin.
  −
** Oedema
  −
** Immune-mediated demyelination
  −
** Infectious diseases, for example canine distemper or caprine arthritis/encephalitis.
  −
  −
====Secondary Demyelination====
  −
  −
* Myelin is lost following damage to the axon.
  −
** I.e. in [[CNS Response to Injury - Pathology#Wallerian Degeneration|wallerian degeneration]]
  −
  −
===Vascular Diseases===
  −
  −
* Vascular diseases can lead to complete or partial blockage of blood flow which leads to ischaemia.
  −
** Consequences of ischaemia depend on:
  −
**# Duration and degree of ischaemia
  −
**# Size and type of vessel involved
  −
**# Susceptibility of the tissue to hypoxia
  −
* Potential outcomes of vascular blockage include:
  −
** Infarct, and
  −
** Necrosis of tissue following obstruction of its blood supply.
  −
* Causes include:
  −
** Thrombosis
  −
*** Uncommon in animals but may be seen with DIC or sepsis.
  −
** Embolism. e.g.
  −
*** Bone marrow emboli following trauma or fractures in dogs
  −
*** Fibrocartilaginous embolic myelopathy
  −
** Vasculitis, e.g.
  −
*** Hog cholera (pestivirus)
  −
*** Malignant catarrhal fever (herpesvirus)
  −
*** Oedema disease (angiopathy caused by E.coli toxin)
  −
  −
===Malacia===
  −
  −
* Malacia may be used:
  −
** As a gross term, meaning "softening"
  −
** As a microscopic term, meaning "necrosis"
  −
* Malacia occurs in:
  −
** Infarcted tissue
  −
** Vascular injury, for example vasculitis.
  −
** Reduced blood flow or hypoxia, e.g.
  −
*** Carbon monoxide poisoning, which alters hemoglobin function
  −
*** Cyanide poisoning, which inhibits tissue respiration
  −
  −
  −
[[Category:CNS Response to Injury]]
       
Author, Donkey, Bureaucrats, Administrators
53,803

edits