Spinal Nerves & Their Distribution Flashcards
What are some reasons for needing to access the vertebral canal?
\+ Sample CSF for blood or organisms \+ Measure CSF pressure \+ Injection of anaesthetics (subarachnoid and epidural) \+ Injection of antibiotics \+ Injection of chemotherapeutic agents
Why is it safest to access the vertebral canal in the lumbar region?
+ The spinal cord ends at L1/L2
+ Subarachnoid space extends to S2
+ Cauda equina unlikely to be damaged
When is one instance when a spinal tap should not be performed?
When patient has increased intracranial pressure
What are features of the lumbar cistern?
+ Area of subarachnoid space
+ Surrounds cauda equina
+ Extends from disc between L1 and L2 to the lower border of S2
Where is the optimum area for needle for lumbar puncture to pass through?
Between spinous processes of L3 and L4 or L4 and L5
What layers must the needle for a lumbar puncture pass through?
- Skin
- SC fascia and fat
- Supraspinous ligament
- Interspinous ligament
- Ligamentum flavum
- Epidural space and extradural fat
- Dura mater
- Arachnoid mater
What are somites?
+ Paired divisions of paraxial mesoderm
+ Give rise to musculoskeletal elements: somites will develop into individual muscles
+ Nerves always follow the muscle developing from the adjacent somite
+ Muscles will receive motor nerved from the adjacent segment of the spinal cord
What are myotomes?
The part of a somite that develops into the muscles:
E.g deltoid develops from C5 and C6 somite and is supplied by nerves arising from spinal segments C5 and C6.
Other muscles that bring about abduction will also be supplied by nerves arising from spinal segments C5 and C6 = MYOTOME
Where are motor neurons found during embryonic development?
Cell bodies lie anteriorly within the neural tube
Somatic motor efferents carry impulses that stimulate muscle contraction
Where are sensory neurons found during embryonic development?
Cell bodies lie in the dorsal root ganglia which develop from neural crest cells
Somatic sensory afferents carry sensation of temperature, pain, touch, proprioception
Give an example of dermatomes?
Skin that develops from ectoderm adjacent to a specific somite is innervated by sensory neurones arising from the adjacent specific spinal level
The lateral wall of each somite in a vertebrate embryo, giving rise to the connective tissue of the skin e.g:
Skin on the lateral side of the forearm develops adjacent to the C6 somite
The skin is therefore supplied by nerves that develop from spinal segment C6 = DERMATOME
What are the features/functions of dermatomes and myotomes?
+ Areas supplied by single spinal nerves/segments
+ Proves sensory and motor supply of an adjacent muscle mass (myotome) and the cutaneous supply of an area of skin (dermatome)
Dermatomes overlap - what does this mean in regards to damage to a single spinal nerve?
It would rarely result in the loss of sensation over the area illustrated by a dermatome map
What causes shingles?
Varicella zoster virus
After an infection with the virus (chicken pox) the virus lies dormant in a DRG. The virus can be reactivated in later life causing shingles.
The rash develops in the skin supplied by the spinal nerve affected i.e within a dermatome
Each segment of the spinal cord gives rise to a pair of spinal nerves. What do they consist of?
- Axons of motor neurons that innervate a group of muscles arising from the same somite (they share a function) (myotome)
- Axons of sensory neurons that innervate a defined area of skin (dermatome)
- Axons of sympathetic neurons to structures in the body wall that control body temperature (blood vessels, sweat glands etc.)
