Spinal Cord and Nerves Flashcards
Subdivisions of the nervous system
What is gray matter? What is it called in the CNS/PNS?
A collection of nerve cell bodies.
CNS: Cortex and nucleus
PNS: Ganglion
What is white matter? What is it called in the CNS/PNS?
Axons with glial sheath (myelin).
CNS: pathway e.g. tract/fascicle
PNS: nerve
How many spinal nerves are there?
31 pairs:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
Where do the spinal nerves emerge from the spinal cord?
Intervertebral formaina (space between each vertebrae)
how many spinal vertebrae are there?
- 7 cervical
- 12 thoracic
- 5 lumbar
- Sacrum (5 fused)
- Coccyx (3-4 fused)
- Where does the spinal cord extend from/to?
- How does it terminate?
- What is it continuous with superiorly?
- Extends from: foramen magnum
- Continuous with medulla oblongata superiorly
- Extends to: vertebral levels L1-L2
- Terminates as conus medullaris
What is the conus medullaris?
The tapered, lower end of the spinal cord. It occurs near lumbar vertebral levels 1 (L1) and 2 (L2), occasionally lower.
Cross section of spinal cord
- Spinous process posteriorly
- Vertebral body anteriorly
Dorsal view of spinal cord. What is cauda equina?
The long collection of nerves that leave the spinal cord at the lower end of the spinal cord (i.e. below the conus medullaris)
View of termination of spinal cord as the conus medullaris as well as cauda equina
What is the filum terminale?
- A delicate strand of fibrous tissue proceeding downward from the conus medullaris to insert into the coccyx
- Provides an inferior attachment for spinal cord –> secures it into the coccyx to prevent excessive movement
Describe the meningeal arrangement surrounding the spinal cord
- Dura mater –> only one layer: the meningeal layer
- Dural sac:
- Membranous sheath of dura mater that surrounds the spinal cord and caudal equina
- Contains CSF
- Extends lower than termination of spinal cord –> encompasses cauda equina nerves
- Under dura mater is the arachnoid layer
- Under the arachnoid layer is the subarachnoid space, filled with CSF
What are the cauda equina nerves suspended in?
CSF
What 2 enlargements are found along the spinal cord?
- Cervical enlargement
- Lumbar enlargement
What are the regions of the spinal cord?
Based on spinal nerves: each spinal nerve segment gives rise to 1 pair of segmental spinal nerves.
- 8 Cervical segments (C1-C8)
- 12 Thoracic segments (T1-T12)
- 5 Lumbar segments (L1-L5)
- 5 Sacral segments (S1-S5)
- 1 Coccygeal segment (Co1)
- Conus medullaris
- Cauda equina
- Filum terminale
Are the pairs of spinal nerves part of the CNS or PNS?
PNS
Where does the dural sac extend to?
S1/S2
What is the lumbar cistern? What does it contain?
- Refers to the subarachnoid space in the lower lumbar spinal canal.
- The cistern is an enlargement of the subarachnoid space in the dural sac, distal to the conus medullaris.
- It contains CSF and the nerve roots of the cauda equina.
Why is the cervical spinal region thicker (cervical enlargement)?
- Cervical region is where you find all the motor cell bodies (gray matter) that innervate the upper limbs
- Including collections of nerves that form the brachial plexus (C1-C8)
What is the lumbosacral enlargement?
- A widened area of the spinal cord that gives attachment to the nerves which supply the lower limbs
- Lumbosacral plexus (L1-S4)
- It commences about the level of T11 and ends at L2
What roots does the cauda equina contain?
Dorsal and ventral roots of lower lumbar, sacral and coccygeal spinal nerves
Why is there 7 cervical vertebrae (C1-C7) but 8 cervical nerves (C1-C8)?
C1–C7 emerge above their corresponding vertebrae, while C8 emerges below the C7 vertebra (and just above T1 vertebrae)
how do the cervical spinal nerves emerge in relation to their corresponding cervical vertebrae?
C1–C7 emerge above their corresponding vertebrae, while C8 emerges below the C7 vertebra (and above T1)
how do the thoracic spinal nerves emerge in relation to their corresponding thoracic vertebrae?
Emerge below their corresponding vertebrae
View of cervical and thoracic spinal nerves leaving in relation to their corresponding vertebrae
Meninges of spinal cord view.
- is the subdural space real or potential?
- is the subarachnoid space real or potential?
- subdural: potential
- subarachnoid: real
What are denticulate ligaments?
- Lateral extensions of pia mater that secure the spinal cord each side
- Found interspersed between spinal nerve roots
- Pass from the pia mater, through the arachnoid mater and into the dura mater
- Function: prevent excessive movement
As the spinal nerve roots leave the spinal cord, what do they take with them?
A layer of meninges
As the spinal nerve roots leave the spinal cord, they take with them a layer of meninges. What do these go on to form?
The epineurium: a protective outer sheath of connective tissue for each spinal nerve.
How does the dura mater in the cranium differ from that of the spinal cord?
- Cranium: dura mater has 2 layers (meningeal and periosteal)
- Space between these 2 layers is potential
- Spinal cord: dura mater has 1 layer (meningeal only)
- Space between vertebrae and dura mater is real –> filled with fat
What is the epidural space (between vertebrae and meningeal dura mater) filled with?
Fat
Why does the spinal cord end before the vertebral column ends? Describe changes from foetus, to birth, to adult
- Differential growth of vertebral column:
- During development, the dural sac grows along the vertebrae but not the spinal cord
- Vertebral column grows faster (so spinal cord ascends)
In foetus (2-3 months):
- Spinal cord extends all the way along length of vertebrae
- S1 spinal nerve root exits near level or origin
- Spinal cord and dura terminate near end of vertebral canal
At birth:
- Vertebral column grows faster compared to spinal cord
- Spinal cord regresses and ascends upwards
- Filum terminale remains attached to coccyx
- Spinal cord (conus medullaris) ends around L3 vertebral level
- S1 spinal nerve root now originates near lumbar region but still extends down to exit below S1
- Spinal cord regresses and ascends upwards
Adult:
- Vertebral column keeps growing faster than spinal cord (cord keeps ascending)
- Spinal cord now terminates between L1 and L2
- S1 now originates around L1
- Lumbar and sacral nerves extend down as the cauda equina to reach their corresponding vertebral level exits
- Lumbar cistern: filled with CSF
- Filum terminale continues to attach to coccyx and secure spinal cord
Describe the changes in disparity between the vertebral level of the emergence of spinal roots from the spinal cord and their level of exit via intervertebral foramina
Spinal nerve roots show disparity between their segmental attachment to the cord and the corresponding vertebral levels:
- Cervical spinal nerves emerge and then exit close to cervical segment
- BUT as you continue down the spinal cord, spinal nerves start to emerge and then descend before exiting the spinal cord below their corresponding vertebrae
- This is due to differential growth
View of cauda equina
Travel through the lumbar cistern in order to reach their corresponding vertebrae
What is a lumbar puncture? What is the function? Where must the needle be inserted?
- The CSF within the lumbar cistern is the target of a lumbar puncture
- Cistern is an enlargement of the subarachnoid space in the dural sac, distal to the conus medullaris.
- It contains cerebrospinal fluid and the nerve roots of the cauda equina
- Cistern is an enlargement of the subarachnoid space in the dural sac, distal to the conus medullaris.
- Purpose:
- Take a sample of CSF from your spinal cord
- Measure the pressure of the CSF – to help diagnose a condition
- Inject medications – such as painkillers, antibiotics or chemotherapy.
- Needle must be inserted below L3 (to be safe, as conus medullaris may extend further in anatomical variations)
- Spinal cord normally ends between L1-L2
During a lumbar puncture, the needle is inserted into the lumbar cistern which contains the cauda equina. Why is there a low risk of inserting a needle into the spine at the level of the cauda equina?
these loose nerve fibres (cauda equina) gently move out of the way of the needle without being damaged e.g. like inserting needle into spaghetti
When is a lumbar puncture not indicated? Why?
In cases of increased intracranial pressure –> as soon as you insert the needle, the CSF will escape (as a jet) due to pressure differential, brain can also herniate (tonsilar herniation through foramen magnum)
What is the typical position of the patient for a lumbar puncture? Why?
- Lying on side with knees to chest (most common)
- Sitting down and arching over
In these positions, the patient bends their back and stretches the ligaments between the vertebrae (specifically the ligamentum flavum) so their is more room to insert the needle
What is the ligamentum flavum?
The ligamenta flava are a series of ligaments that connect the ventral parts of the laminae of adjacent vertebrae. Each ligamentum flavum connects two adjacent vertebrae.
What layers does a lumbar puncture go through?
- Skin
- Subcutaneous tissue
- Supraspinous ligament
- Interspinous ligament
- Ligamentum flavum
- Epidural space
- Dura
- Arachnoid
- Finally into the subarachnoid space
When doing a lumbar puncture, how can you tell you have passed the ligamentum flavum?
Can hear a ‘pop’
What are some abnormalities of the CSF that may be found during a lumbar puncture?
- Blood cells in CSF
- May indicate subarachnoid haemorrhage
- Cell composition of CSF may indicate presence of infection (e.g. meningitis)
- N.B. this space may also be punctured to inject drugs
- What is an epidural anaesthesia?
- Which part of your body is anaesthetised?
- How does it differ from a lumbar puncture?
- What is found in the epidural space of the spinal cord?
- Needle is inserted here to inject anaesthetic
- Anything below is anaethetised (e.g. motor and sensation from lower limb)
- Ligamentum flavum is still punctured but you don’t go as deep as a lumbar puncture
- Fat is found here
What is the location of the epidural given during childbirth? Why?
Given at the sacral hiatus (usually S5) –> only anaethetises nerves around perineum
What is the anterior median fissure? What is found posteriorly?
- Anterior median fissure is a groove located anteriorly and centrally in the spinal cord in which the anterior spinal artery sits
- Posterior median sulcus is found posteriorly and centrally
What sulcus is present either side of the anterior median fissure? What emerges here?
- Anterolateral sulcus
- Anterior nerve roots exit here
What sulcus is present either side of the posterior median sulcus? What emerges here?
Posterolateral sulcus - posterior nerve roots emerge here
What artery is found in the anterior median fissure? What is this a branch of? What does this supply?
- Anterior spinal artery (unpaired) - branch of the 2 vertebral arteries
- Descends all the way down to supply anterior aspect of spinal cord
Is the anterior spinal artery paired or unpaired?
Unpaired
Is the posterior spinal artery paired or unpaired?
Paired
Where does the posterior spinal artery (dorsal spinal arteries) arise from?
- Vertebral artery (25% of humans) or PICA (75% of humans)
Describe the vascular supply of the spinal cord
The spinal cord receives vascular supply through three major arteries. The single anterior spinal artery and two paired posterior spinal arteries
Where do the posterior spinal arteries descend?
Each vessel descends on the posterior surface of the spinal cord along the posterolateral sulcus.
What are segmental spinal arteries?
These segmental spinal/medullary branches reinforce the blood supply to the spinal cord
What are the segmental spinal arteries a branch of?
Depends on where you are in the spinal cord. From superior to inferior, they can be a branch of:
- Vertebral artery (close to neck)
- Deep cervical artery
- Posterior intercostal artery (thorax region)
- Lumbar arteries
- Lateral sacral artery
The segmental spinal arteries approach the spinal cord laterally. As it does, what branches does it give? What do these branches supply?
- The medullary artery branches
- Reinforces anterior spinal artery (anterolateral aspect of cord)
- Anterior and posterior radicular arteries
- Supply spinal nerve rootlets
Coronal section through brain vs through spinal cord: gray and white matter differences
Brain:
- Gray matter (nerve cell bodies) generally confined to outside with some deep collections of gray matter
- White matter (nerve axons) tends to be underlying gray matter (inside)
Spinal cord:
- Gray matter confined to inside of cord
- White matter confined to outside of cord
Spinal cord: transverse section.
Description and explanation of gray matter
White matter outside and gray matter inside
-
Dorsal gray horn: dorsal-most aspect of gray matter
- Many synapses occur here –> mainly sensory neurones
-
Lateral gray horn: only exists between T1-L2 and S2-S4 levels of cord
- Forms origin of preganglionic visceral motor neurones of the symp and paraymp nervous systems
-
Ventral gray horn: ventral-most aspect of gray matter
- Contains somatic motor neurones that innervate muscles
Spinal cord: transverse section.
Description and explanation of white matter
- Dorsal white columns: on either side of midline
- Lateral white column
- Ventral white column
- Ventral white commissure: allows communication between right and left side of cord (crossing fibres)
What is found in the dorsal gray horn of the spinal cord?
Many synapses occur here –> mainly sensory neurones
What is found in the lateral gray horn of the spinal cord? Between which levels of the cord does this exist?
only exists between T1-L2 and S2-S4 levels of cord
Forms origin of preganglionic visceral motor neurones of the symp and paraymp nervous systems
What does the ventral gray horn of the spinal cord contain?
Contains somatic motor neurones that innervate muscles
Another view of transverse section through spinal cord
How does each spinal nerve begin?
- As a collection of ventral spinal rootlets (leaving anterolateral sulci) and dorsal spinal rootlets (leaving posterolateral sulci)
- These then merge to form the ventral and dorsal roots
- These dorsal and ventral roots then combine to form a segmental spinal nerve (mixed)
The dorsal root is associated with a dorsal root ganglion. What is this? Is there a ventral root ganglion?
- Dorsal root ganglion: Cluster of cell bodies for these sensory neurons
- These cell bodies belong to 1ary sensory neurons
- No ventral root ganglion
Transverse section of spinal cord and spinal nerve: explained
- Sensory (afferent) fibres (can be somatic or visceral) travel in spinal nerve and then segregate to dorsal root to enter dorsal grey horn of spinal cord
- These fibres have their cell bodies in the dorsal root ganglion
- N.B. these are travelling TO the CNS
- From the ventral grey horn, cell bodies of somatic motor neurones leave the spinal cord (efferent fibres) via the ventral root and then join the spinal nerve
- Travelling FROM the CNS
- From the lateral grey horn, cell bodies of preganglionic visceral motor neurones (from autonomic nervous system) send their axons out of spinal cord via the ventral root
What type of neurones arise from the lateral horn of the spinal cord?
autonomic neurons innervating visceral and pelvic organs
What type of neurones arise from the ventral horn of the spinal cord?
motor neurons that innervate skeletal muscle
What is a reflex arc?
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain.
Describe the reflex arc
In a reflex (e.g. touching a hot object), the information does not have time to travel all the way to the brain
- Person touches hot object
- Sensory (afferent) neurons travel towards CNS via mixed spinal nerve, then dorsal root (with cell bodies in dorsal ganglion) to enter dorsal horn of spinal cord
- In dorsal horn of spinal cord, these sensory neurons connect/synapse with an interneuron (interneurons connect motor and sensory neurons)
- Interneuron then relays information to the motor (efferent) cell body
- Motor fibres then leave spinal cord via the ventral horn of spinal cord to enter the ventral root and then join the mixed spinal nerve
- Travels towards the muscle it innervates which then contracts to withdraw body part from painful stimuli
How does white matter change throughout the spinal cord? Why is this?
- White matter increases as you ascend the spinal cord (most white matter in cervical area)
- Due to increasing fibres from sensation joining
How does grey matter change throughout the spinal cord?
- Ventral grey horn is larger in the cervical and lumboscaral regions
- Due to more cell bodies of motor neurones leaving from these areas
- Ventral grey horn of thoracic region is quite small
- Lateral grey horns are only found between:
- T1-L2
- S2-S4
What is found anywhere in the spinal cord above segment T6?
There are 2 dorsal columns on each side of the midline:
- Gracile fascicle medially
- Cuneate fascicle laterally
Below T6:
- Only the gracile fascicle is present
Where is the cuneate/gracile fascicle located?
- Cuneate fascicle:
- Located laterally and dorsally in the upper thoracic and cervical (C1 to T6) spinal cord segments, contains afferents from the upper trunk and extremities.
- Lies between the gracile fasciculus and the posterior column
- Gracile fascicle:
- Located dorsally throughout the whole length of the spinal cord
What do the cuneate and gracile fasciculi expand superiorly as?
The gracile and cuneate tubercles
What is the function of the gracile and cuneate fasciculus?
- Gracile fasciculus: carries sensory information from the lower half of the body entering the spinal cord at the lumbar level
- Cuneate fasciculus: carries sensory information from the upper half of the body (upper limbs, trunk, and neck) entering the spinal cord at the cervical level.
How is the grey matter of the spinal cord grouped?
- Rexed’s laminae based on the cellular composition of each of these levels (which correlates with fuction)
- Sites of sensory or motor nuclei
- Extend varying length of cord
E.g. Lamina IX: motor neurons (ventral horn)
What is Lissauer’s tract?
Region that sensory fibres use to enter the spinal cord (ie. sensory neurones that enter here travel briefly in this tract before synapsing in their corresponding lamina of grey matter)
Outside of the grey matter of the spinal cord is the white matter (columns). Within these columns are specific tracts.
What tracts are found at the level of the dorsal columns? What are these responsible for?
- The gracile fascicle (whole length of spinal cord) and cuneate fascicle (above T6 only)
- Responsible for carrying ascending sensory information (e.g. fine touch, vibration, pressure, joint position sense of right) from the body
- Proprioception: refers to the body’s ability to perceive its own position in space (joint position)
- Responsible for carrying ascending sensory information (e.g. fine touch, vibration, pressure, joint position sense of right) from the body
I.e. dorsal columns is responsible for this
What tracts are found at the lateral white matter? What are these responsible for?
- Lateral cortiospinal tract
- Carries axons of descending motor neurons for voluntary control of muscles
What is the ventral corticospinal tract responsible for?
- Smaller tract in anterior white matter of cord
- Carries descending motor fibres for voluntary movement
What is the spinothalamic tract responsible for?
- Extend from ventral/lateral region of cord
- Carry ascending sensory information from crude touch, pain and temperature
Look at which side of the body corresponds to the different tracts
Left spinothalamic tract –> Crude touch, pain and temperature of right
What is the anterior (ventral) white commissure?
a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray commissure
How is the grey matter of the spinal cord divided up?
Into 3 horns:
- Ventral (anterior)
- Dorsal (posterior)
- Lateral grey horn only present:
- T1-L2 (sympathetic outflow)
- S2-S4 (parasympathetic outflow)
How is the white matter of the spinal cord divided up?
3 columns:
- Ventral
- Lateral
- Dorsal
Function of dorsal grey horn of spinal cord?
The dorsal horn contains neurons that receive somatosensory information from the body, which is then transmitted via the ascending pathways, to the brain.
Sensory (afferent) fibres (can be somatic or visceral) travel in spinal nerve and then segregate to dorsal root to enter dorsal grey horn of spinal cord.
Function of lateral grey horn?
- Contains neurons that innervate visceral and pelvic organs.
- Mainly contains the neuronal cell bodies of the sympathetic nervous system (T1-S2) to innervate viscera
