Spinal Cord Function and Dysfunction

Question 1

How many spinal segments are there?

Answer 1

31

Question 2

How many spinal nerves are there? State how many in each vertebra region.

Answer 2

31 (paired nerves)

• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• 1 Coccygeal

Question 3

Where do spinal nerves leave the vertebral column?

Answer 3

Through the intervertebral foramina

Question 4

What happens to the distance between the spinal nerve and its corresponding vertebral column the further you go down?

Answer 4

The distance between them increases - especially when you get to the lumbar and spinal nerves, they have to travel downwards before they get to their corresponding vertebra and exit through the vertebral column

Question 5

What are the two enlargements in the spinal cord and what is their significance?

Answer 5

Cervical enlargement (C4-T1) – increased number of motor nerve fibres here which innervate the muscles of the upper limbs

Lumbosacral enlargement (L2-S3) – increased number of motor nerve fibres here which innervate the muscles of the lower limbs

So these spinal segments are thicker due to the increased number of nerve fibres

Question 6

Describe the meningeal covering of the spinal cord.

Answer 6

• THREE layers of meninges
• Dura mater (only meningeal layer), arachnoid mater and pia mater
• CSF circulates in the subarachnoid space

Question 7

What are denticulate ligaments?

Answer 7

• Bilateral triangular extensions of the pia mater (adherent to spinal cord)
• These extensions are fibrous - made up of collagen fibres
• Attaches to dura mater (i.e. anchors spinal cord to dura, stabilising it within verterbra foramen/canal)

Question 8

Which space is present in the spinal meninges but not in the cranial meninges?

Answer 8

• Epidural space
• Full of venous plexuses and fatty tissue
• This space is clinically useful because you can inject anaesthetic by giving an epidural

Question 9

Explain grey and white matter in the spinal cord.

Answer 9

Grey matter (unmyelinated) = cell bodies (and nerve endings)

White matter (myelinated = tracts/axons

Question 10

What is a dermatome?

Answer 10

Area of skin innervated by a single sensory spinal nerve root

NOTE: spinal nerve is mixed - has sensory (dorsal) and motor (ventral) roots which together make up the spinal nerve root

Question 11

What is a myotome?

Answer 11

Muscles innervated by a single motor spinal nerve root

NOTE: spinal nerve is mixed - has sensory (dorsal) and motor (ventral) roots which together make up the spinal nerve root

Question 12

What are the anterior and posterior rami?

Answer 12

Mixed spinal nerves split into anterior and posterior rami - both have mixed motor and sensory fibres

• Posterior ramus (thinner) - innervates deep/intrinsic back muscles and skin of the back
• Anterior ramus (thicker) - everything else

Question 13

Describe the typical structures on the cross section of the spinal cord.

Answer 13

Grey matter:

• Dorsal (posterior) horn - contains sensory nerve fibre endings and cell bodies of interneurones
  
  • sensory info received from body via somatic & visceral efferents via dorsal root
  • sensory neurones synapse with interneurons here to send info to brain or for reflex arc
• Ventral (anterior) horn - contains motor nerve fibre cell bodies
  
  • motor info sent to body via somatic efferents via ventral root

White matter:

• Posterior column
• Lateral column
• Anterior column
• These contain different nerve tracts (carrying info to and from the brain)

Other key structures - labelled on diagram

Question 14

What is the lateral horn and on which spinal cord segments is it present?

Answer 14

Contains the cell bodies of the preganglionic autonomic efferent nerve fibres

• T1 - L2: sympathetic
• S2 - S4: parasympathetic

NOTES:

• Both autonomic and somatic efferent nerve fibres exit the spinal cord through the ventral root and form the mixed spinal nerve
• There is also parasympathetic outflow of the brainstem cranial nerves (III, VII, IX, X)

Question 15

What are the major tracts of the spinal cord?

Answer 15

Tracts can be ascending or descending

• Ascending = nerve fibres sending info to the brain (sensory - afferent)
• Descending = nerve fibres bringing info from the brain (motor - efferent)

NOTE: tract/fascicle = bundle of nerve fibres

Question 16

What are the two major pathways in somatic sensory perception?

Answer 16

• Dorsal columns-medial lemniscal (DC-ML) pathway
• Spinothalamic pathway

NOTE: both have 1° (i.e. 1st order neurones), 2° and 3° neurones

Question 17

What sensory info does the DC-ML pathway carry?

Answer 17

• Discriminative touch
• Vibration
• Proprioception (being able to sense the position and movement of your body)

Question 18

Explain the 1° neurones in the DC-ML pathway.

Answer 18

• Cell bodies of 1° neurones are in the dorsal root ganglia
• 1° neurones transmit information from the sensory receptors, ascend ipsilaterally (on the same side) up the spinal cord to the medulla
• So these neurones do NOT terminate in the dorsal horn of the spinal cord
• These primary neurones are ascending up the posterior/dorsal column
• Within the dorsal column there are two fasciculi
  
  • Fasciculus cuneatus - ascending bundle of nerve fibres from ipsilateral the upper limbs
  • Fasciculus gracilis - ascending bundle of nerve fibres from ipsilateral the lower limbs
• There are two nuclei within the medulla where the 1° neurones synapse with the 2° neurones
  
  • Fasciculus cuneatus → Nucleus cuneatus
  • Fasciculus gracilis → Nucleus gracilis

Question 19

Explain the 2° and 3° neurones in the DC-ML pathway.

Answer 19

• 2° neurones cross over to the other side at the medulla before ascending
  
  • Crossing over is also known as decussation
• The 2° neurones ascend (known as the medial lemniscus) and synapse with 3° neurones in the contralateral thalamus
• 3° neurones will ascend to the primary somatosensory cortex (where info will be processed)

Question 20

What sensory info does the spinothalamic pathway carry?

Answer 20

• Pain
• Temperature

Question 21

Explain the 1° neurones in the spinothalamic pathway.

Answer 21

• Cell bodies of 1° neurones are in the dorsal root ganglia
• 1° neurones enter the dorsal horn via the dorsal root and immediately synapse with 2° neurones (i.e. synpase in the dorsal horn of the spinal cord)

Question 22

Explain the 2° and 3° neurones in the spinothalamic pathway.

Answer 22

• 2° neurones from the dorsal horn will immediately decussate (cross over to the other side) across the spinal cord
• The 2° neurones will then ascend contralaterally
  
  • These 2° nerve fibres make up the spinothalamic tract
  • The spinothalamic tract is in the ventral/anterior column of the spinal cord
• The 2° neurones then synapse with 3° neurones in the contralateral thalamus
• 3° neurones will ascend (straight upwards) to the primary somatosensory cortex (where info will be processed)

Question 23

What is a major descending pathway?

Answer 23

Corticospinal pathway - main voluntary movement pathway

Question 24

How many orders of neurones are there in the corticospinal pathway?

Answer 24

There are two (so only 1° and 2°)

Upper motor neurones (UMNs):

• Cell bodies are the betz cells (large pyramidal neurones) in the layer 5 of the primary motor cortex
  
  • NOTE: cerebral cortex has 6 layers
• Axons can either travel in the lateral OR anterior/ventral corticospinal tracts to synapse with LMN

Lower motor neurones (LMNs):

• Cell bodies in the ventral horn of the spinal cord
• Axons travel out of the ventral root to effector (contralateral to primary motor cortex)

Question 25

What are the two types of corticospinal pathways?

Answer 25

• Lateral corticospinal pathway
  
  • 90-95% of UMNs
  • Innervate skeletal muscles of the limbs (appendicular muscles)
• Anterior corticospinal pathway
  
  • 5-10% of UMNs
  • Innervate skeletal muscles of the head, neck and trunk (axial muscles)

Question 26

Describe the lateral corticospinal pathway.

Answer 26

• UMNs starts off in primary motor area of cerebral cortex
• Goes through the midbrain and pons
• Some of the UMNs cross over to the other side a the medulla oblongata (i.e. pyramidal decussation)
  
  • ​At this point these fibres form the lateral corticospinal tract (in the lateral column of the spinal cord)
• The fibres as part of the lateral corticospinal tract descends down contralaterally until they reaches their relevant segment of the spinal cord
• At the spinal segment, they synapses with LMNs in the ventral horn

Question 27

Describe the anterior corticospinal pathway.

Answer 27

• UMNs starts off in primary motor area of cerebral cortex
• They travel downwards and go through midbrain, pons and medulla WITHOUT crossing over in the medulla
  
  • At this point, the fibres which have not undergone pyramidal decussation form the anterior corticospinal tract (in the anterior column of the spinal cord)
• The fibres of anterior corticospinal tract descends down ipsilaterally the spinal cord to their relevant spinal segment
• Once the fibres reaches the spinal segment, they cross over to the spinal cord and and synpases with LMNs in the ventral horn

Question 28

Describe the muscle stretch reflex pathway and explain why it would be used.

Answer 28

NOTE:

• 1a - type of sensory neurone
• α-motor neurone = LMN (of corticospinal pathways innervating skeletal muscle)

Explanation:

• This reflex is essentially a muscle contraction in response to involuntary stretching within that muscle
• For example, if you are standing upright and begin to lean to one side, the postural muscles that are closely connected to the vertebral column on the opposite side will stretch
• Therfore, the stretched muscles will contract to correct posture

Question 29

Describe the reflex pathway in response to a noxious stimulus.

Answer 29

Noxious stimulus = a stimulus which is actually or potentially damaging to body tissue

Question 30

Which factors affect the severity of a spinal lesion (tissue damage)?

Answer 30

• Loss of neural tissue
• Vertical level
• Transverse plane

Question 31

Explain how loss of neural tissue affects severity.

Answer 31

The more neural tissue which is lost, the greater the severity

• If due to trauma, usually small
  
  • The most common causes of spinal cord injury is a broken neck or back (broken cervical or lumbar vertebrae affect those spinal cord segments)
• Metastasis or degenerative diseases can do lots of damage
  
  • Metastasis - cancerous tumours can spread and compress large portions of the spinal cord, leading to damage
  • Degenerative diseases - causes degeneration of neurones within the spinal cord

Question 32

Explain how vertical level affects severity.

Answer 32

The higher the lesion, the greater the disability

• This is because there are ascending and descending tracts connecting spinal segments to the brain
• If there is lesion high up in the spinal cord, this affects the the tracts which are connected to all the vertebrae below it, so would have a more damaging effect

Question 33

Explain how transverse plane affects severity.

Answer 33

Depends on whether the lesion was:

• Complete = affecting whole spinal segment
• Partial = affecting part of the spinal segment

NOTE:

• Most lesions are not complete
• Damage to white matter is usually more important than damage to grey matter because of which ascending and/or descending tracts are affected (would affect the tracts connecting other segments too)

Question 34

Which tracts are the main ones involved in spinal injury?

Answer 34

• Lateral corticospinal
• Dorsal columns (i.e. the fasciculi)
• Spinothalamic

Question 35

What is the significance of the vertical level of the injury in terms of symptoms?

Answer 35

Because depending on where the lesion took place (i.e. whether or not crossing over has occured yet), it would effect either the contralateral OR ipsilateral side of the body (to the lesion)

REMEMBER points of decussation (crossing over):

• DC-ML pathway decussates in the upper medulla
• Spinothalamic tract decussates in the spine
• Lateral corticospinal tract decussates in the medulla

Question 36

What does damage to the DC-ML pathway result in?

Answer 36

Loss of discriminative touch and conscious proprioception:

• Ipsilateral - becuase the 1° neurones (the ones in the dorsal column as part of the tracts) ascend upwards ipsilaterally and cross over at the medulla
• Below the level of the lesion - because the fibres in the tracts from all the spinal segment below would have to pass through the affected segment, so would be damaged in the process

Question 37

What does damage to the spinothalamic tract result in?

Answer 37

Loss of temperature and pain sensation

• Contralaterally - because the 2° neurones cross over at the spinal cord segment as ascend upwards as part of the spinothalamic tract
• Below the level of the lesion - because the fibres in the spinothalamic tract from all the spinal segment below would have to pass through the affected segment, so would be damaged in the process

Question 38

What does damage to the lateral corticolspinal tract result in?

Answer 38

Results in ipsilateral symptoms - because the UMNs have crossed over in the medulla and are descending down the spinal cord in the tract which is on the same side of the body it is going to innervate

Damage results in paralyis (due to disruption of motor function)

Paralysis occurs in 2 stages:

• Stage 1
  
  • Spinal shock: loss of reflex activity below the lesion lasting for days or weeks = flaccid paralysis (limp, reduced muscle tone)
• Stage 2
  
  • Return of reflexes: hyperreflexia and/or spasticity = rigid paralysis (excessive stretch reflexes lead to increased muscle tone/stiffness and involuntary contractions which cause jerking movements)

Question 39

Explain what Brown-Séquard syndrome is.

Answer 39

• This is when you have hemisection of the spine - i.e damage to one half of the spinal cord
• Because the DC-ML pathway dessucates in the medulla the sensory deficit will be ipsilateral
  
  • Dorsal column tracts are ipsilateral
  • Left hemisection → left sensory deficit
• Because the spinothalamic pathway dessucates in the spinal cord (segment), the sensory deficit will be contralateral
  
  • ​Spinothalamic tract is contralateral
  • Left hemisection → right sensory deficit