18-11-21 - Spinal Nerves and their Distribution, Dermatome and Myotomes Flashcards
Learning outcomes
- Review lumbar puncture including its optimal anatomical location
- Define dermatome and myotome
- Label a diagram of a cross section of the spinal cord
- Label a diagram of a cross section of the distribution of a thoracic spinal nerve
- List the branches of a thoracic spinal nerve
- Describe the embryological origin of dermatomes and myotomes
What are 5 reasons we may have to access the vertebral canal?
What are 3 reasons why access for a lumbar puncture is the safest in the lumbar region?
What is a contraindication in medicine?
What is an example of a contraindication of a lumbar puncture?
• Reasons to access the vertebral canal:
1) Sample CSF for blood or organisms
2) Measure the pressure of CSF
3) Injection of anaesthetics (subarachnoid and epidural)
4) Injection of antibiotics
5) Injection of chemotherapeutic agents
• Access for lumbar puncture is the safest in the lumbar region because:
1) The spinal cord ends at L1/L2, so is unlikely to get damaged
2) The subarachnoid space that contains CSF extends to S2
3) The cauda equina is unlikely to be damaged, as the nerves move out of the way
• A contraindication in medicine is a condition that serves as a reason not to carry out a certain medical treatment due to harm it may cause to the patient
• Raised intracranial pressure can serve as a contraindication for a lumbar puncture
• If the subarachnoid space is accessed further down the spinal cord, this can cause coning (brain herniation - swelling)
What is the lumbar cistern?
Where does it extend from?
What are the 3 steps of a lumbar puncture?
• The lumbar cistern is an area of subarachnoid space that surrounds the cauda equina
• The lumbar cistern extends from the IV disc between L1 and L2 to the lower borders of S2
• Steps of a lumbar puncture:
1) Line drawn between the most superior points of the iliac crests. This forms the supracristal line which crosses the L4 spinous process
2) The patient lies in the foetal position (on their side, with back and hips flexed) to increase the space bewteen spinous processes
3) The needle is then inserted between the spinous processes of L3 and L4 or L4 and L5
What are the 9 layers that need to be penetrated for a lumbar puncture?
What layer would we stop at for an injection for epidural anaesthesia?
1) Skin
2) Superficial fascia
3) Supraspinous ligament
4) Interspinous ligament
5) Ligamentum flavum
6) Extradural (epidural) space – stop here for injection of epidural anaesthesia
7) Dura mater
8) Arachnoid mater
9) Subarachnoid space
Describe the steps involved in the formation of the neural tube.
What are cells associated with the neural tube?
What are these cells responsible for?
What forms next to the neural tube?
1) There is a notochord that is formed during gastrulation
2) The notochord induces changes in the overlying ectoderm, which thickens to form the neural plate
3) The ectoderm then begins to fold and deepen, until it closes off to form the neural tube
4) The ectoderm then seals itself off to become a complete layer again
5) There is now a neural tube deep to the ectoderm and posterior to the notochord
- There are separate cells associated at the tip of the folding tube called neural crest cells, which form the neural crest above the neural tube
- These cells are responsible for forming many different structures, such as dorsal root ganglion, sympathetic ganglion
- Next to the neural tube (red structures in diagram B), parts of the mesoderm aggregate and organise to form into a segmental structure called a somite
What are somites?
How are they arranged?
How do they develop?
What are the 3 different parts of the somite?
What are dermatomyotomes?
What do dermatomyotomes give rise to?
What do sclerotomes give rise to?
How do somatic nerves arise?
- Somites are paired aggregations of paraxial mesoderm (situated alongside an axis)
- Somites are arranged sequentially along each side of the neural tube
- Somites develop cranially to caudally
- The dermatome is divided into the sclerotome, the dermatome, and the myotome
- Dermatomyotome is the term for myotomes and dermatomes
- Dermatomyotomes give rise to the musculoskeletal elements of the limbs and body walls e.g dermis of the skin (peripheral areas)
- Sclerotomes give rise to the axial skeleton (central areas)
- Somatic nerves arise segmentally in association with somites
Where are cell bodies of motor efferents found?
Where do they grow into?
What are they destined to do?
What are the 2 muscle divisions?
What do they form?
- Cell bodies of motor efferents are found in the neural tube
- The motor efferents grow out of the neural tube into a somite, so a spinal segment is associated with a somite
- These motor efferents are destined to go on and carry impulses from the CNS to muscles
- There is a division into epaxial and hypaxial muscles
- Epaxial muscles go on to form the intrinsic back muscle of dorsum of the embryo
- The division leading to the epaxial muscles will form the dorsal ramus
- The hypaxial back muscles will form the lateral and anterior body walls and the limbs
- The vision leading to the hypaxial muscles will form the ventral ramus
What gives rise to individual skeletal muscles?
How are these muscles formed?
How are they innervated?
What does this mean for the type of movement done by these muscles?
What is a myotome?
- The dermatomyotome of each somite gives rise to individual skeletal muscle
- Myotomes from dermatomyotomes migrate out into the periphery and give rise to various skeletal muscles (e.g deltoid muscle)
- As they do this, they drag with it the nerve supply (e.g C5 and C6 spinal segments for the deltoid)
- This means skeletal muscles receives somatic motor fibres from the spinal cord segment adjacent to their corresponding somite
- This means muscles like the deltoid which also bring about abduction will also be supplied by spinal nerves from the C5 and C6 spinal nerve segments
- A myotome is a group of muscles innervated by the same spinal nerve
What are certain actions of limbs associated with?
Why is this?
What is an example of this?
How does this allow us to localise spinal segment defects?
What spinal nerve segments are associated with which segments?
- Certain actions of the limb are associated with certain spinal cord levels
- This is because the nerves that supply the muscles that perform this action have come from common spinal segments
- The action of abduction in the glenohumeral joint is associated with spinal level C5 as all the muscles providing this action come from the C5 somite
- This allows us to see if certain actions are present, absent, or weakened in patients, allowing us to localise a deficit to a particular spinal level
What are the cell bodies of somatic sensory neurons found?
What do they develop from?
Where do the dermatomes migrate?
What are the 2 places these axons can go?
What are the roles they are responsible for?
- Cell bodies of somatic sensory neurons develop in and lie in the dorsal root ganglia
- These cell bodies develop from the neural crest
- The dermatome part of the dermatomyotome migrates
- Axons will grow out into the CNS and the periphery
- The part that that goes out into the periphery will be responsible for sensation e.g temperature, pain, touch, proprioception
- The part that goes into the CNS will relay the signal from the periphery into the CNS
What is the difference between the place where motor and sensory cell bodies develop?
- Motor cell bodies develop in the CNS
- Sensory cell bodies develop in the neural crest cells
- This is why sensory cell bodies develop in a ganglion and not a nucleus – as they are outside of the CNS
What does the dermatomyotome of each somite give rise to?
What does this send back?
What is an example of this?
What is a dermatome?
- The dermatomyotome of each somite gives ride to the dermis of the skin
- The dermis sends somatic sensory fibres to the spinal cord segments adjacent to its corresponding somite
- Example
- The dermis on the lateral side of the dorearm and the thumb develops from the C6 somite
- The skin is therefore supplied by spinal nerves that develop from the spinal segment of C6
- A dermatome is an area of skin innervated by a single spinal segment
What is a myotome?
What is a dermatome?
- A myotome is a group of muscles innervated by the same spinal nerve
- A dermatome is an area of skin innervated by a single spinal segment
What is shingles caused by?
What occurs after initial infection?
What can occur later in life?
- Shingles is caused by varicella zoster virus (VZV)
- After initial infection (chickenpox) the virus lies dormant in a dorsal root ganglion
- The virus can then be reactivated later in life, which causes shingles
- The rash develops in the skin area supplies by the spinal nerve affected
What is the reason for spinal nerves emerging above their associated vertebrae through the intervertebral foramen at the top of the spine, then below their associated vertebrae as we move down the vertebrae?
Why does this also contribute to having more cervical spinal nerves than vertebrae?
- This is due to the wav the nerves come through the somites
- The nerves grow physically through somites in order to get to the dermatomyotome part of the somites
- In doing this, it splits the sclerotome part of the somite, so that the vertebral bodies (axial skeleton) are actually formed from 2 parts of adjacent somites
- The spinal nerves follow the pattern of the somites e.g 8 cervical spinal nerves due to their being 8 cervical somites
- Because the somites are split by the nerve coming through them, you end up with 7 cervical vertebrae and the part of the occipital bone, which is formed by an occipital somite
Describe the cross section of the spinal nerve
What are dorsal rami of thoracic spinal nerves?
Where do they carry:
• Motor neurons
• Sensory neurons
• Sympathetic neurons
- Dorsal rami of thoracic spinal nerves are mixed nerves that will carry:
- Motor neurons to erector spinal muscles (intrinsic back muscles – epaxial muscles)
- Sensory neurons from the skin of the back and from vertebral joints
- Sympathetic neurons to blood vessels and sweat glands of the skin
What are dorsal rami of thoracic spinal nerves?
Where do they carry:
• Motor neurons
• Sensory neurons
• Sympathetic neurons
What is a plexus?
Where will they form?
What branches do ventral rami have?
Why is this?
- Ventral rami of thoracic spinal nerves are mixed and will carry:
- Motor neurons to intercostal muscles (supplying lateral and anterior body wall and limbs – hypaxial muscles)
- Sensory neurons from thoracic skin, pleura and vertebral joints
- Sympatric neurons to blood vessels and sweat glands of the skin
- A plexus is an interweaving network of nerves that come in as spinal nerve roots, do some mixing, and change their names
- Ventral rami will form plexuses , which will give rise to peripheral nerves consisting of fibres from multiple spinal nerves
- Ventral rami have a lateral cutaneous and anterior branch
- This is to supply the lateral aspect of the anterior wall
- The anterior branch also has a lateral branch and a medial branch to supply under the body wall
What is a reflex?
Describe the 5 stages of the reflex arc.
Describe the 4 steps of the reflex that occurs when hitting the patellar tendon with a tendon hammer.
What is this process for?
• A reflex is a rapid involuntary motor response to a stimulus • Stages of the reflex arc: 1) Receptor 2) Sensory neuron 3) Integration centre 4) Motor neuron 5) Effector
1) Hitting the patellar tendon with a hammer causes a stretch receptor to see the muscles have been stretched
2) The stretch receptor sends an action impulse back via an affecer (sensory) neuron, which heads into the spinal cord, but doesn’t ascend to the brain and get processes
3) This impulse then passes through the spinal cord and synapses onto a motor neuron, as the body has evolved to have a subconscious response to these stretches
4) An efferent branch is sent back to the muscle, which will then contract,
• This process is mainly for maintaining posture and balance
Where do reflexes occur?
What are 2 examples of reflexes we use to test certain spinal cord segments?
What can the reflex be?
What is an example of a reflex involving an interneuron?
How does this compare with simple stretch reflexes?
- Reflexes occur at a particular spinal level
- We use the knee to test L3
- We use the ankle to test the sacral spinal segment
- The reflex can be absent, present or exaggerated
- Pain reflexes involve interneurons e.g touching a hot pan
- Simple stretch reflexes are monosynaptic
Describe the diagram of the spinal nerve – motor neurons (efferent)
Describe the diagram of the spinal nerve – sensory neurons (afferent)
Where are sympathetic cell bodies found?
Where do they travel?
Describe the spinal nerve – sympathetic neurons diagram
• Cell bodies of sympathetic neurons are only in the lateral horns of the T1-L2 region, but travel in all spinal nerves
