Spinal Nerves, Dermatomes and Myotomes Flashcards
What does gray matter contain ?
Contains neuronal cell bodies within the horns.
Receives sensory information from the periphery and sends motor stimuli to the periphery.
What does white matter contain ?
Myelinated and un-myelinated nerve fibres.
Conduction of information inside the nervous system.
Describe the horns of the gray matter
Dorsal (posterior) horns : inter-neurons, sensory neurons
Lateral horns : visceral (autonomic) motor neurons
Ventral (anterior) horns : somatic motor neurons
Where are lateral horns found ?
Lateral horns are only found in thoracic and upper lumbar regions.
Function of visceral motor neurons
Innervate smooth muscles and glands
What divides the spinal cord ?
Posterior median sulcus - found at dorsal horn side
Anterior median fissure - found at ventral horn side
How many spinal nerves are there ?
31
8 Cervical
12 Thoracic
5 Lumbar
5 Sacral
1 Coccygeal
Where do the spinal nerves emerge from ?
The C1 nerve passes between the occipital bone and the atlas.
The C2-L5 spinal nerves emerge through their own intervertebral foramen.
S1-S4 from anterior and posterior sacral foramina.
Describe the ventral root
Contains :
Anterior root
Ventral root
Motor root
Describe the dorsal root
Contains :
Posterior root
Dorsal root
Sensory root
What does the ventral horn contain ?
Somatic neuron
(lower motor) in ventral horn
What does the lateral horn contain ?
Visceral motor (preganglionic sympathetic) neuron
in lateral horn
Found between T1 and L2
What does the dorsal root ganglion contain ?
Contains 1st order sensory pseudo-unipolar neuron
Difference between root and rami
Roots are pure
Rami are mixed
What are rootlets ?
Rootlets contain axons of one type of neuron (either motor or sensory, irrespective of somatic or visceral)
Describe formation of spinal nerves
Rootlets
Roots (dorsal and ventral - posterior and anterior)
Spinal nerve
Dorsal ramus and Ventral ramus
Describe the dorsal ramus
Posterior branch
Dorsal ramus
Motor and Sensory neurons
What do the dorsal rami of the thoracic spinal nerves carry ?
Motor fibres to erector spinae (back muscle)
Sensory fibres from the skin of the back and from vertebral joints.
Sympathetic neurons to blood vessels and sweat glands of the skin of the back.
What do the ventral rami of the thoracic spinal nerves carry ?
Motor fibres to intercostal muscles
Sensory fibres from the thoracic skin and parietal pleura and vertebral joints.
Sympathetics fibers to blood vessels and sweat glands of the skin.
T1 also contributes to a nerve plexus. (brachial)
What is the function of the motor fibres of the dorsal ramus ?
Innervate the deep muscles of the back
What is the function of the sensory fibres of the dorsal ramus ?
Innervate the skin of the back
Describe the ventral ramus
Anterior branch
Ventral ramus
Motor and Sensory neurons
Ventral ramus vs Dorsal Ramus
They both carry motor and sensory information.
Ventral ramus is thicker.
What are nerve plexuses ?
Ventral rami of spinal nerves may join with others to form nerve plexuses.
What do adjacent rami communicate to form ?
Large plexuses e.g. :
- Cervical plexus : C1-4
- Brachial plexus : C5-T1
- Lumbar plexus : L1-4
- Sacral plexus : L4-S4
What is different about the nerves of the thoracic region ?
The nerves of the thoracic region remain distinct from each other.
They are continuous with the intercostal nerves.
What do the nerves of the plexuses and intercostal nerves provide ?
Sensory innervation to the :
- upper and lower limbs
- ventral abdominal wall
- thoracic wall
What is reflex ?
Reflex is a rapid, involuntary motor response to a stimulus.
What is a reflex arc ?
Receptor
Sensory neuron
Integration centre
Motor neuron
Effector organ
Function of the spinal reflex
Somatic spinal reflexes provide information on integrity of the reflex pathway and degree of excitability of the spinal cord.
Describe the development of somites
Around day 20 of gastrulation, in the occipital region, paraxial mesoderm becomes segmented and organises into somites.
What do somites differentiate into ?
(new somites appear in a cranio-caudal sequence)
Sclerotome
Myotome
Dermatome
Describe the formation of sclerotome
Cells in the ventral and medial walls of the somite shift their position to surround the neural tube and notochord.
They form sclerotome which forms the vertebrae and ribs.
What does sclerotome form ?
Vertebrae and ribs
Each vertebral body is derived from 2 adjacent somites.
Describe the formation of dermatome (dermatomyotome)
Cells at the dorsomedial and ventrolateral edges of the upper region of the somite, form precursors and muscle cells, whereas cells from these 2 groups form the dermatome.
What is dermatome ?
Dermatome is an area of skin innervated by a single spinal segment.
What does the dermatome give rise to ?
The dermis.
What does the dorsal root ganglia contain ?
The cell bodies of somatic sensory neurons are located in the dorsal root ganglia, which develop from neural crest.
What is a dermatomyotome ?
Myotome + Dermatome
What does a dermatomyotome give rise to ?
Dermatomyotome of each somite gives rise to skeletal muscle and dermis of skin.
What does dermatome contain ?
Axons of sensory neurons that innervate a defined area of skin.
What does myotome contain ?
Axons of motor neurons that innervate a group of muscles arising form the same somite.
i.e. they share the same function
What is myotome ?
Dorsomedial muscle cells
A group of muscles innervated by the same spinal nerve.
What do dorsomedial muscle cells give rise to ?
Intrinsic back muscles
What do ventrolateral muscle cells give rise to ?
All other trunk and limb muscles.
What does myotome give rise to ?
Skeletal muscle.
The cell bodies of motor neurons that will supply this myotome are located anteriorly in the neural tube.
Axons of motor neurons (to become spinal nerves later) from the spinal cord segment adjacent to their corresponding somite follow the dermatomyotome.
Myotome examination
Most muscles of the body are innervated by more than one spinal cord level.
Selected joint movements are used to test myotomes.
What causes shingles ?
Varicella / Herpes Zoster virus
After initial infection (chickenpox) the virus lies dormant in a dorsal root ganglion.
The virus can be reactivated in later life causing shingles.
C4
Tip of shoulder
C6
Thumb
C7
Middle finger
C8
Little finger
T10
Umbilicus
L5
Big toe
Why do you need to perform a lumbar puncture ?
Sample CSF for blood or organisms
Measure the pressure of CSF
Injection of anaesthetics
Injection of medications
Why is it safest to carry out lumbar puncture from the lumbar cistern ?
Spinal cord ends at L1/2
Subarachnoid space (dural sac) extends to S2
Cauda equina is unlikely to be damaged.
What is the supracristal plane ?
A plane (line) between the superior most points of iliac crests
Crossing L4 spinous process and L4/5 intervertebral disc.
Clinical Relevance of Lumbar puncture
Contraindicated (not used) in patients with raised intracranial pressure. (as may cause harm)
Can lead to coning (herniation of the brain)
Layers of the back
Skin
Superficial fascia
Supraspinous ligament
Interspinous ligament
Ligamentum flavus
Extradural space
Dura mater
Arachnoid mater
Subarachnoid space
Pia mater
Paresthesia
Altered sensation (pins and needles)
What is the function of the L5 root ?
One of the roots that provides motor innervation to the anterolateral muscles of the calf.
Altered motor function and tendon reflexes.
Where are sympathetic neurons found ?
T1-L2 segments, although the axes of these neurons are found in all spinal nerves.
