S2: the lumbar spine Flashcards
Describe the embryonic development of dermatomes and myotomes
By day 30, each embryo has approximately 34-35 pairs of somites. As soon as these somites form, they begin to differentiate. Hence, during the fourth week after fertilisation, each somite differentiates into the sclerotome, which is ventral and gives rise the vertebrae and ribs, and the dermatomyotome which is dorsal and forms the dermis (via the dermatomes) and the muscle tissue (via the myotomes)
Dermatomyotomes develop in association with a specific neural level of the spinal and take their nerve supply with them from the neural tube as a spinal (segmental) nerve
The skin and muscle derived from a single dermatomyotome therefore have a common spinal nerve supply
Define dermatome
A dermatome is an area of skin that is supplied by a single spinal nerve.
What is a spinal nerve?
A spinal nerve is a mixed nerve that carries motor, sensory and autonomic signals between the body and the spinal cord
Dorsal roots contain afferent/sensory nerve fibers only
Ventral roots contain efferent/motor and autonomic nerve fibers only
Exist briefly as they pass through the intervertebral foramen (marks the division between the CNS + PNS
Describe the structure of the spinal cord
The spinal cord commences at the inferior margin of the medulla oblongata, at the base of the brain stem, and ends at the conus medullaris at L2. Due to differential growth between the spinal cord and the vertebral column, the spinal cord is shorter than the vertebral canal.
Below the conus medullaris is the cauda equina
Describe how many cervical vertebrae and cervical spinal roots there are
There are eight pairs of cervical spinal roots and only seven cervical vertebrae. Therefore, the cervical spinal roots emerge above the corresponding vertebral body with the exception of the 8th cervical spinal root, C8, which emerges between the C7 and T1 vertebrae
What are the rami?
After emerging through the intervertebral foramen, each spinal nerve divides into rami. The posterior (or dorsal) ramus supplies the deep muscles and skin of the dorsal trunk. In humans, the dorsal rami are very small. The anterior (or ventral) ramus supplies the muscles and skin of the upper and lower limbs and also the lateral and ventral trunk (much larger than posterior ramus)
What does the dorsal ramus divide into? What does this supply?
The posterior (dorsal) rami divide again into medial and lateral branches that supply the skin of the back in a ‘tidy’ segmental manner. Each dorsal ramus supplies a narrow strip of muscle and skin in line with the intervertebral foramen
What does the ventral ramus distrubute to?
The distribution remains segmental for the skin and muscles of the trunk, but the anterior rami of C5-T1 and L1 – S5 enter the limb buds instead and ultimately supply the muscles and skin of the upper and lower limbs respectively
The anterior rami that supply the limbs immediately enter complex networks of nerves called the brachial plexus (C5-T1) and the lumbosacral plexus (L1-S5).
Note: the dermatomes for C4 and T2 are adjacent to each other on the anterior trunk
Define myotome
Group of muscles supplied by a single spinal nerve (or spinal nerve root)
What does the functional overlap between dermatomes mean? What is the axial line?
There is functional overlap between adjacent dermatomes. Hence, if a single spinal nerve is damaged, the area of anaesthesia (loss of sensation) will likely be smaller than you might imagine from the dermatome map. However, this functional overlap does not extend across an axial line.
Axial line = junction of two dematomes supplied from discontinuous spinal levels
What are the pre-axial and post-axial borders? What are they marked by in the upper limb and lower limb?
Boundaries of dorsal and ventral compartments
Marked by superficial veins
Upper limb: cephalic vein (pre-axial) + basilic vein (post-axial)
Lower limb: long saphenous vein (pre-axial) + short saphenous vein (post-axial)
What is the relationship between spinal nerves and peripheral nerves? What is a plexus?
Peripheral nerves often contain neurones from several spinal nerves
Fibres from one spinal nerve can enter multiple peripheral nerves
The plexuses are complex networks of nerves that divide and merge with other bundles of nerve fibres multiple times
Fibres from mutliple spinal nerves often combine to form a peripheral nerve
What are peripheral nerve territories?
These are the areas of skin supplied by the peripheral nerves
Often overlap sections of multiple dermatomes (consistent with their spinal nerve content)
Note: these are NOT dermatomes
What is a motor unit?
A motor unit is a motor neuron and the skeletal muscle fibres it innervates
1 spinal nerve contains the neurones of many motor units
1 spinal nerve supplies one myotome
Myotome actions in the upper limb
C5: shoulder abduction + external rotation
C6: elbow flexion, wrist extension + supination
C7: elbow extension, wrist flexion + pronation
C8: finger flexion + finger extension
T1: finger abduction + adduction
