15-11-21 Spinal Cord and Meninges Flashcards
Learning outcomes
- List the boundaries of the vertebral canal
- Define the extent of the spinal cord within the vertebral canal
- Explain the development of the cauda equina
- Describe the arrangement of the meninges
- Define the extent of the subarachnoid space
- Describe the blood supply and venous drainage of the spinal cord
- List the structures through which the needle must pass when performing a lumbar puncture
How is the vertebral canal formed?
What sits in the vertebral canal?
Where do spinal nerves come out of?
- The vertebral canal is formed from the stacking of vertebral foramen of each vertebra
- The spinal cord sits in the vertebral canal
- Spinal nerves come out of the intervertebral foramen
What forms the anterior and posterior boundary of the vertebral canal?
- The anterior boundary of the vertebral canal is formed by the posterior longitudinal ligament
- The posterior boundary of the vertebral canal is formed by ligamentum flavum, which are thick segmental ligament that runs between the lamina of adjacent vertebrae
Where does the spinal cord begin?
Where does it end?
What is the inferior end of the spinal cord?
Where are the 2 enlargements of the spinal cord?
Why are there enlargements?
- The spinal cord begins at the foramen magnum of the skull as a continuation of the medulla oblongata (lowest part of the brainstem)
- The spinal cord ends at the L1/L2 vertebrae in adults
- The inferior end of the spinal cord is the conus medullaris, which narrows as the filum terminale
- Enlargements of the spinal cord:
1. Cervical enlargement - Between C4-T1 (brachial plexus)
- For the innervation of upper limbs
- Lumbosacral enlargement
- Between T11-S1 (lumbosacral plexus)
- For the innervation of lower limbs
- There is much more fine motor control in the upper and lower limbs than the trunk, so more neurons are required, hence the enlargements
How does the length of the spinal cord and vertebral canal compare in the embryo at 8 weeks?
Where does the spinal cord end in neonates (new-borns)?
Where does the spinal cord end in adults?
What causes this?
- In the embryo at 8 weeks, the spinal cord and vertebral canal are the same length
- In neonates, the spinal cord ends at the L3 vertebrae
- In adults, the spinal cord ends at the disc between L1/L2 – it exists in the upper 2/3rds of the vertebral column
- This is because the vertebral column grows faster than the spinal cord
What are the 5 main external features of the spinal cord?
- Cervical and lumbosacral enlargements - areas involved in innervation of the limbs
- Conus medullaris - End of spinal cord
- Posterior median sulcus – location of posterior spinal arteries
- Anterior median fissure – location anterior spinal artery
What are the 3 main parts of Gray matter in the spinal cord?
What do they each contain?
What is at the centre of the grey matter?
What is white mater of the spinal cord?
What does it contain?
What are the 3 parts of white mater in the spinal cord?
What do they each contain?
- Gray matter in the spinal cord:
1. Dorsal/posterior horns - Where sensory neurons come in and where we find interneurons (neurons between sensory and moto neurons)
- Sensory neurons may come in and synapse or come in and pass through depending on the type of neuron
- Lateral horns
- Associated with the sympathetic nervous system (part of visceral/autonomic nervous system)
- Where we find pre-ganglionic sympathetic motor neurons before they head out
- Ventral/anterior horns
- Where we find cell bodies of somatic motor neurons
- Central to the gray matter is the central canal, which contains CSF, and is continuous with the ventricular system of the brain (communicating cavities within the brain)
- White mater are columns containing a number of ascending and descending tracts
- All tracts are paired, and most dessucate (cross to form an X)
- Grey mater of the spinal cord:
- Dorsal (posterior) columns
* Associated with sensory ascending tracts - Lateral columns
* Mix of ascending and descending tracts - Ventral (anterior) columns
* Mix of ascending and descending tracts
How many pairs of spinal nerves do we have?
Where do they exit from?
What does each nerve relate to?
How many vertebrae and spinal nerves do we have at each segment?
What causes a mismatch in vertebrae and spinal nerves?
How does this differ from the top of the spinal cord compared to lower in the spinal cord?
What does this lead to the formation of?
- We have 31 pairs of spinal nerves
- They emerge from the intervertebral foramen, where the intervertebral disc forms the anterior boundary
- Each nerve relates to a spinal segment
- Because of the way nerves and the bones around them develop, we get a mismatch between the vertebrae and the spinal nerves
- e.g C1 spinal nerve comes out above C1 and C8 spinal nerves come out below C7
- At the top of the spinal cord, spinal nerves come out at relatively the same level as the associate spinal segment e.g C1 spinal nerve around the same level as C1 vertebrae
- As we move down the spinal cord, the mismatch becomes greater
- A lumbar segment of the spinal cord could be associated with T10 vertebrae, but may descend in the vertebral canal and come out at the L2 vertebra
- The leads to the formation of the caudal equina in the bottom 3rd of the vertebral canal (horses tail), which contains roots of spinal nerves associated with the L2 of the spinal cord and below
What are roots formed?
What are the 2 different kinds of roots?
How are spinal nerves formed?
What do spinal nerves divide in to?
- Rootlets converge to form roots
- There are 2 kinds of roots:
- Ventral – motor (efferent)
- Doral – sensory (afferent)
- Ventral and dorsal roots converge to become a spinal nerve, which is mixed motor and sensory
- Spinal nerves divide to form ventral and dorsal rami, which are mixed motor and sensory nerve fibres
What are the 5 steps of spinal nerve reflex?
- Sensory (afferent) information comes from the periphery, which would come through to the dorsal root ganglion
- After the dorsal root ganglion, the fibres form the dorsal root, which enters the dorsal horn of the spinal cord (this information ascends the spinal cord to the brain)
- After being processed by the brain, the motor (efferent) fibres descend the spinal cord and will exit from the ventral horn, which forms the ventral (motor) root to the periphery
- If the output is somatic (conscious) it will go straight to the periphery
- If the output is visceral (sympathetic nervous system – green), the pre-ganglionic fibres will exit the lateral horns and synapse in the sympathetic ganglion with post-synaptic fibres before entering the periphery (may be an effector)
- Ganglion – collection of cell bodies out with the CNS
- Sympathetic ganglion right next to spinal cord
- Parasympathetic ganglion right next to target structure
Where does the spinal cord end?
What is this rea known as?
What structure is beyond this point?
What is the purpose of this structure?
What must lumbar nerves do in the vertebral canal?
What does the lower third of the vertebral canal contain?
What does this form?
- The spinal cord ends at around the L1/L2 vertebrae
- The end of the spinal cord tapers to form a cone known as the conus medullaris
- Beyond the conus medullaris, there is the filum terminale, which is an extension of pia mater that extends from the terminal end of the spinal cord to the coccyx
- The filum terminale helps to anchor the spinal cord in place
- Lumbar nerves must travel down the vertebral canal to their own intervertebral foramen to exit the canal
- The lower third of the vertebral canal contains the roots of the spinal nerves of L2 and lower, as well as the filum terminale
- These roots form the cauda equina (horses tail)
Where do lower spinal segments of the spinal cord lie in relation to their vertebra?
- Lower spinal segments of the spinal cord lie adjacent to the vertebrae above their own level
- E.g L1 spinal segment lies adjacent to T11
- E.g S1 spinal segment lies adjacent to T12
What are meninges?
What is their function?
What are the 3 layers from outermost to innermost?
Where are all 3 of these layers contained?
How are they separated from the bone?
Where is CSF contained?
- Meninges are three membranes which surround, protect, and suspend the brain, spinal cord, and spinal nerves
- Outer to innermost meninges:
- Dura mater
- Arachnoid Mater
- Pia mater – indistinguishable from surface of spinal cord
- All 3 of these layers are contained within the vertebral canal
- They are separated from the vone by extradural/epidural space, which is mostly filled with fat
- CSF is contained in the subarachnoid space
What is the Dura mater?
How does it differ from spinal cord and skull?
Where is it attached?
What is the dura mater called in the vertebral canal?
What does it do at the S2 vertebrae?
How is dura associated with spinal nerves?
- The dura mater is the outermost, thick protective layer
- In the skull, it consists of 2 layers: the outer periosteal layer and inner meningeal layer
- It is attached the inner surface of the cranium
- As it descends through the vertebral canal is it called the Dural sac and is not attached to bone
- At the S2 vertebra, the dural sac narrows extensively where it fuses with the filum terminale
- The Dura covers roots of spinal nerves for a short distance as they pass through the intervertebral foramen by fusing with the epineurium and connective tissues on the outside of nerves
What is the arachnoid mater?
Where does it lie in relation to the dura/dural sac?
Why is this?
How does the arachnoid mater connect with the pia mater?
What is the subarachnoid space continuous with?
Where does it end?
What does it contain?
What does it enclose?
- The arachnoid mater is a delicate avascular surface
- It lies deep to the dura (lining the Dural sac) with no space between the layers
- The CSF in the subarachnoid space pushes the arachnoid mater against the dura, leaving no space (but still a potential space)
- The arachnoid mater connects to pia mater via arachnoid trabeculae, which are mini bridging fibres, with the subarachnoid space in between the layers
- The subarachnoid space is continuous with the space around the brain
- It ends at the lower border of the 2 vertebrae, like the dura mater
- The subarachnoid space contains CSF and blood vessels supported by web-like structures
- The subarachnoid space encloses the caudal equina as the lumbar cistern, which is a slightly wider space containing a lot of CSF
What is the Pia mater?
Where is the pia mater found?
What does the pie mater form at the inferior end of the spinal cord?
What is the function of this structure?
How does the pia mater connect to the arachnoid mater?
Where does this structure lie?
- The pia mater is the inner most layer, and is vascular
- The pia mater is found closely adhered to the surface of the brain and spinal cord, and also covers the surface of spinal roots
- Beyond the conus medullaris, there is the filum terminale, which is an extension of pia mater that extends from the terminal end of the spinal cord to the coccyx
- The filum terminale helps to anchor the spinal cord in place
- In the vertebral canal, triangular extensions of pia called denticulate ligaments are laterally attached to the arachnoid mater that lines the Dural sac, which suspends the spinal cord
- Denticulate ligaments lie between dorsal and ventral rootlets
What does the extradural/epidural space contain?
What is its structure like?
Where does the extradural space run from?
Where is it located in relation to the periosteum of vertebrae?
What does it contain?
- The extradural/epidural space contains all of the meninges and the spinal cord
- The extra dural space is a fatty matrix
- The extradural space runs from the magnum foramen to the sacral hiatus on the sacrum (around S2)
- The extradural space is internal to the periosteum of the vertebrae
- The epidural space contains the internal vertebral plexus
What are the 2 directions of blood supply to the spinal cord?
What are they branches of?
Where are they found?
- Directions of blood supply to the spinal cord:
1. 3 longitudinal arteries - Derived form branches of the subclavian artery
- Lie on the surface of the spinal cord running all the way down the length of the spinal cord
- Numerous segmental arteries
- Derived from branches of the aorta e.g intercostal arteries in the thoracic region
- They enter the vertebral canale via the intervertebral foramen
- They come in at various vertebral levels, and are needed to reinforce longitudinal supply
- Examples are branches of vertebral arteries in the neck, and branches of the aorta known as lumbar arteries in the lumbar region
What are the 3 arteries in the longitudinal supply?
Where do they arise from?
How are they formed from vertebral arteries?
What volume of the spinal cord is supplied by each type of arteries?
- In the longitudinal supply there are:
1. 2 posterior spinal arteries - Arise from vertebral and posterior cerebellar arteries
2. 1 Anterior spinal artery - Arises from vertebral arteries
- Vertebral arteries ascend in the neck via the transverse foramen of cervical vertebrae and branch after the foramen magnum
- About 2/3rds of the spinal cord is supplied by the anterior spinal artery, making it more important in terms of blood supply
What is the role of segmental arteries?
What are the different branches of segmental supply?
Where do these arteries enter?
What are 3 different examples of segmental arteries?
Where do they arise from?
What is the artery of Adamkiewics?
What is it also known as?
Where does it usually enter?
What does it act as?
What can damage to this artery cause?
Why are we segmental creatures?
- The role of segmental arteries is to reinforce the blood supply of the longitudinal supply
- There may be segmental or radicular branches in the segmental supply
- These arteries enter through the intervertebral foramen
- Examples of segmental arteries:
- Cervical arteries – arise from vertebral arteries
- Thoracic arteries – arise from intercostal arteries
- Lumbar arteries – arise from lumbar arteries
- The artery of Adamkiewics (aka the great segmental medullary artery) is a very big and important segmental supply
- This artery usually enters via the intervertebral foramen on the left of the T10 vertebrae
- The artery of Adamkiewics acts as a big reinforcer of longitudinal supply
- Damage to this artery can cause ischaemia in the lower end of the spinal cord
- We are segmental creatures because of embryology
What is the role of segmental spinal arteries?
What could these arteries be?
How are radicular arteries formed?
What are segmental medullary arteries?
- Segmental arteries come through the intervertebral foramen to accompany a spinal nerve
- These arteries could be a branch of intercostal, vertebral, or lumbar arteries
- Radicular arteries are formed when segmental spinal arteries branch and run along side the roots of spinal nerves
- Segmental medullary arteries are found when a radicular artery continues long enough to the point where it anastomoses with the anterior spinal artery
Where are longitudinal venous channels found?
How does blood flow in these venous channels?
How are they matched with arteries?
Where is the internal vertebral plexus found?
Where is the external vertebral venous plexus found?
What does it drain?
Where does it connect?
What is a problem associated with venous connections in the vertebrae/spinal cord?
- Longitudinal venous channels are found on the surface of the cord
- There are no valves in these longitudinal venous channels, so blood flows slowly in either direction
- Longitudinal venous channels are not matched to any arteries, and aren’t very consistent (form a variable plexus)
- But as we go laterally, there is some matching to arteries, such as the anterior and posterior segmental medullary veins
- The internal vertebral plexus is found in the extradural fat of the epidural space
- The external vertebral venous plexus is found surrounding vertebrae
- It drains the medullary cavity of the vertebral bodies
- The medullary cavity is the central cavity of bone shafts where barrow is stored, hence it is also known as the marrow cavity
- Th external vertebral venous plexus connects to segmental veins e.g lumbar or azygos veins
- Venous connections allow the metastatic spread of tumours
- Examples are breast and prostate cancer, which can spread to vertebral bodies as bony metastases due to these venous connections
What is lumbar puncture a method for?
Where is lumbar puncture performed?
Why is it performed here?
What are all the layers we need to get through to sample CSF?
What layer would we stop at to register an epidural anaesthetic?
- Lumbar puncture is a method for taking CSF or administering epidural anaesthetic
- Lumbar puncture is performed between L3/L4 or L4/L5
- This is because the spinal cord has ended by this point
- When putting a needle through here, the nerve rootlets in the caudal equina are more likely to move out the way, rather than hitting solid spinal cord
- We would stop at the extradural space, aspirate to ensure we aren’t in a vessel, and register an epidural anaesthetic
Label the structures of this diagram
