Peripheral Nerve Damage Flashcards
Causes of peripheral nerve damage
- Infections such as leprosy and HIV-AIDS
- Nutritional deficiencies such as vitamin B12
- Nerve injury sure to trauma
- side effects of drugs such as some anticancer drug
- disruption is blood supply to the nerve occurs with vasculitis, arteriosclerosis
- Systemic diseases such as diabetes
Chronaxie
Duration that a stimulus twice the strength of rheobase needs to be applied to elicit a response
Denervation hypersensitivity
Also known as supersensitivity.
It is a heighten response of the target organ /tissue following Denervation due to an upregulation of receptors on the postsynaptic membrane
Rheobase
It’s the minimum strength of stimulus that can elicit a biological response in the strength - duration curve
Utilization time
Duration for which a stimulus of rheobase strength needs to be applied in order to elicit a biological response
Types of nerve injuries
- Neuropraxia
- Axonotmesis
- Neurotmesis
Neuropraxia
- Least serve form of nerve injury
- Nerve is compressed or blood supply to the nerve is disrupted
- Recovery rapid with a few hours
- Serve forms need several months
Axonotmesis
- More serve form
- Axon is disrupted, myelin sheath is intact
- Wallerian degeneration takes place in Distal segment
Neurotmesis
- Most serve form
- Axon, and surrounding encapsulating connective tissue is affected
- Last chances of recovery
Changes that occur in Wallerian degeneration
Is the Distal segment
Distal segment :
- Formation of ovoids by schwann cells which fragment the myelin sheath and scavenge myelin debri
- change in schwann cell properties from Myelinating to demyelinating
- Infiltration of leukocytes into the Distal seg.
Changes that occur in Wallerian degeneration
Is the proximate segment
- The first node of ranvier is degenerated
- Chromatolysis: breakup of the RER and shift of the nucleus from the center of the cell body. Changes in RER alter rRna synthesis, gene expression that supports neuronal recovery.
What mediates Wallerian degeneration
Calcium influx and activation of axonal proteases
Effect of nerve degeneration
- Reduced /absence of neurotransmitters
2. Upregulation (increased number or affinity) of receptors on the postsynaptic membrane
What brings about Denervation hypersensitivity
Nerve regeneration or Exogenous administration of neurotransmitter agonist
How are motor nerves tested
Assessed clinically by evaluating the muscle strength of the muscles they supply
How are sensory neurons tested
Assessed by testing for sensation in the area of the specific nerve supply
strength and duration curve
Relies on the varying strength and duration of a stimulus applied to a nerve and evaluating the biological response
How is the strength and duration curve formed
An electrode is placed over the point where a motor nerve is entering a muscle this point is called the motor point. And the strength and duration of the stimulus can be regulated until biological response is achieved
Parameters of a strength duration curve
- Rheobase
- Chronaxie
- Utilization time
Damaged nerve SD curve
The Utilization time and Chronaxie are prolonged and rheobase is increase. Showing a right shift away from the normal curve
Nerve regeneration is characterized by
- Development and prolongation of a growth cone from the proximal nerve stump
- Extension along the surfaces schwann cell column in the Distal segment
Difference between nerve regeneration in PNS and CNS
Nerve regeneration occurs in PNS but not in CNS
Differences in nerve degeneration and regeneration of neurons in PNS and CNS
The rate of removal of myelin debris, cytokins release, regeration association gene expression are relatively faster in PNS than in the CNS. Also there is a response from nonnueronal cells, in PNS schwann cells dedifferenciate into a “growth supporting” mode but CNS oligodentrocytes show no change
