Spinal cord

Question 1

Spinal cord gross anatomy: define the terms nerve, nerve root and ramus; explain the relationship between spinal and vertebral levels and the clinical significance; define the cervical and lumbar enlargements and recall the spinal segments involved

Answer 1

Spinal cord segments

31 spinal segments - 31 pairs of spinal nerves

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

Nerves leave the vertebral column through intervertebral foramina

Enlargements for innervation of the limbs:

1. Cervical (C3-T1)
2. Lumbar (L1-S3)

Discrepancy between spinal levels and vertebral levels:

• Difference of number of nerves and vertebrae
• C1-c7 emerge above the bone that corresponds to them / C8 nerve comes down below the corresponding vertebra
• Vertebral column and spinal cord are not the same length – difference in nomenclature between vertebrae and spinae
• At the top – the nerves do emerge close to the corresponding vertebrae
• The nerves become more angled the more you go down

Dorsal/Ventral

1. Dorsal Rootlets are axons of Sensory Neurons (Input/Afferent) which combine to form the dorsal route
2. Ventral Rootlets are axons of Motor Neurons (Output/Efferent) which combine to form the ventral route

The rootlets extend out of the spinal cord medially and combine to form roots. The roots combine to form the spinal nerve and then they split apart again and are now called rami (ramus for singular) (photo) –> rami are mixed (both afferent and efferent)

Autonomic Ganglia

1. Cranial - parasynthetic ganglia:

• On the head
• 4 ganglia (ciliary, submandibular, otic, pterygopalatine)

2. Paravertebral - sympathetic ganglia:
3. Collateral - sympathetic ganglia:
   - In contrast to dorsal root ganglia, these have synapses

Nerve plexuses

• Networks of successive ventral rami that exchange fibers
• Mainly innervate the limbs

Dermatomes (Skin is a continuous organ)

• Innovated by dorsal root
• Areas of skin supplied by a single sensory spinal nerve root. Skin is a continuous organ
• Stacked along the thorax and abdomen; longitudinally along the limbs
• Clinical significance: can determine the site of spinal damage by simple pin prick exam

Myotomes (muscles are discrete organs)

• Muscles that are supplied by a single motor spinal nerve root
• Clinical significance: Testing for root level muscle weakness

There are TWO enlargements of the spinal cord

Cervical Enlargement

• Has extra motor neurones that go to the muscles of the upper limb

Lumbosacral Enlargement

• Has extra motor neurones that go to the muscles of the lower limb

The spinal cord stops growing early whereas the vertebral column keeps on growing into adulthood

The lumber cistern can be used to sample CSF

Question 2

Spinal cord horns: define the function of the cells in the dorsal, ventral and intermediate horns of the spinal cord

Answer 2

• The spinal cord consists of a core of grey matter surrounded by white matter
• Grey matter consists of cell bodies of interneurons and motor neurons. It also consists of neuroglia cells and unmyelinated axons.
• If a spinal injury damages the grey matter, motor neurons may be lost but the effects won’t be disastrous.
• The grey matter is also arranged into dorsal and ventral “horns”
• The dorsal horns receive sensory information from the body via spinal nerves and dorsal roots.
• This information is used in spinal reflexes or projected to the brain for further processing
• The ventral horns contain motor neurones whose axons control the muscles of the body via the ventral roots and spinal nerves.
• In the thoracic and upper lumbar region, the intermediate horns contain the sympathetic preganglionic motor neurones whose axons control visceral functions via the ventral roots and spinal nerves

Question 3

Explain the structure of the spinal cord

Answer 3

Spinal cord:

• Column of nervous tissue that is protected and enclosed within the vertebral column
• Grey matter: cell body of the neurons surrounded by white matter: myelinated axons
• Grey matter (butterfly-shaped): each side has a dorsal horn where the sensory neurons make their connections and a ventral horn which consists of the cell bodies of the motor neurons connected to muscles etc.
• White matter: some bundles will carry information upwards towards the central nervous system (ascending tracts) and bundles carrying information to the periphery (descending tracts)
• Vertebral column: stack of vertebrae that allows us movement and has an opening in the middle where spinal cord sits
• Intervertebral foramina: little holes on the sides of the vertebral column where the spinal nerves come out from –> 2 roots: one dorsal and one ventral that join up to form the nerve (also connected to the sympathetic ganglia)
• Spinal cord consists of 31 nerves which come out just below the corresponding vertebrae (cervical area C1-C7 nerves come out above and that’s why we have one less vertebra = 30)
• Nerves: 8 cervical, 12 thoracics, 5 lumbar, 5 sacral, 1 coccygeal
• The spinal cord is shorter than the vertebral column (in the lumbar area there is no nervous tissue but only fibers called cauda equina) –> lumbar puncture to collect fluid happens in that region as it is safer
• Each nerve segment innervates a particular segment of the body (dermatome) and a set of muscles (myotome)

Spinal cord function :

1. Connects the PNS and ANS to the brain
2. Carries sensory signals to the brain
3. Carries motor signals to the muscles
4. Coordinates reflexes (eg. the patellar reflex)

Question 4

Meninges: recall the layers of the meninges and explain their function

Answer 4

• Meninges: another protective layer
• 3 layers
• First: Dura mater
  
  • an outer, hard and fibrous layer which consists of two layers, one connected to the bone (periosteal) and another connected with the lower meninges (meningeal)
  • In between periosteal and meningeal layers, there is the superior sagittal sinus (where venous blood collects
  • There is a little extradural/epidural space between the outside of the dura and the bone - you don’t find this in the cranial meninges
  • This epidural space is full of venous plexuses and fatty tissue
  • This space is clinically useful because you can inject anaesthetic by giving an epidural
• Arachnoid mater:
  
  • connective tissue that looks like a spider web that holds blood vessels in place
  • space between arachnoid and pia mater is called subarachnoid space is where the vessels run (when they break –> subarachnoid haemorrhage)
  • subarachnoid space is filled with cerebrospinal fluid
• Pia mater:
  
  • thinner layer, that is not superficial and follows all the gyrus and sulci
  • flexible
  • There are little protrusions of pia mater called denticulate ligaments that tether the spinal cord and hold it in the middle of the subarachnoid space

Question 5

Spinal cord cross-sectional anatomy: demonstrate on a diagram the main areas of grey matter, and the main ascending and descending tracts

Answer 5

• The white matter of the spinal cord contains short pathways which interconnect adjacent segments of the spinal cord via ascending + descending tracts, and longer tracts which convey information to and from the brain
• It consists almost totally of myelinated motor and sensory axons
• Damage to the white matter is likely to have severe consequences, as a lesion may damage the ascending/descending tracts, therefore every part of the body supplied by nerves below the lesion will be damaged.

Question 6

Motor sensory Pathways

Answer 6

You need to know where the various tracts are within the white matter in order to determine deficits caused by spinal injury.

With regards to spinal cord injury, 3 tracts are important…

1. Dorsal columns pathway -

SENSORY, Touch, vibration, pressure

2. Spinothalamic pathway -

SENSORY, Pain, temperature

3. Lateral Corticospinal pathway MOTOR

Important for fine movements

Somatosensory organisation

In the dorsal columns pathway

• if the primary axon enters below spinal level T6 it will travel in the fasciculus gracilis (gracial pathway = medial part of the column)
• if the primary axon enters above T6, then it travels in the fasciculus cuneatus (cuneate pathway = lateral part of the column)
• As the sensory neurones enter the spinal cord they travel up the same side of the spinal cord as the part of the body innervated
• In the medulla, the primary axons synapse with a secondary neuron (internal arcuate fibres), and these fibres cross over at the decussation of the lemniscus, and continue ascending on the contralateral side.
• Secondary axons terminate in the ventral posterolateral nucleus (VPL) of the thalamus, where they synapse with tertiary neurons.
• These tertiary neurones then ascend via the posterior limb of the internal capsule and end in the primary sensory cortex.

With regards to spinal lesions; decussation (crossing over) is in the brainstem, so if a lesion only affects part of the spinal cord in the mid-thoracic region, sensory loss will only be below the level of the lesion and ipsilateral (on the same side)

In the spinothalamic pathway (aka the anterolateral system)…

• Pain nerves cross over IMMEDIATELY as they enter the spinal cord
• When the neurone gets into the dorsal horn it synapses with a second cell and

then that cell sends its axon immediately across the midline

• Left-sided unilateral lesion at midthoracic level will not interrupt the spinothalamic tract coming from the left leg but it will interrupt the spinothalamic tract that’s going up the left side and coming from the right leg
• In this case, a lesion on the left will cause loss of pain sensation in the right leg

With regards to spinal lesions; decussation occurs at the midline almost immediately in the cord, therefore a mid-thoracic lesion will result in a contralateral loss of sensory information.

Motor organisation is in the form of the corticospinal tract..

• Cortical upper motor neurons descend in the posterior limb of the internal capsule through the crus cerebri, down through the pons, and to the medullary pyramids, where 95% of the axons cross to the contralateral side at the pyramidal decussation.
• The lateral corticospinal tract neurones decussate at the level of the medulla
• If you have a unilateral lesion in the mid-thoracic region then the deficit will be on the same side as the lesion

• They then descend as the lateral corticospinal tract.

With regards to spinal lesions, because crossing over takes place in the brainstem, a partial lesion in the mid-thoracic region will result in ipsilateral deficits.

Question 7

Spinal lesions: define the sensory and motor deficits resulting from spinal cord lesions

Answer 7

Spinal lesions

1. Sacral damage = loss of bladder + bowel function, spina bifida, neural tube defect
2. Thoracic damage = loss of lower limb function + incontinence (paraplegia)
3. Cervical damage = loss of lower limb + upper limb function, incontinence (quadriplegia)

• High spinal lesion, e.g. C1/2 = cannot breathe unassisted, as the phrenic nucleus controls the diaphragm which is innervated by C3,4+5
• Most spinal injuries aren’t complete sections of the spinal cord - the extent of the damage depends on which part of the spinal cord (particularly which part of the white matter) has been severed

Ascending - Sensory

Descending - Motor

A severing of the spinal cord just below the lumbosacral enlargement would affect the roles of the spinal nerves below the lesion

This lesion would stop the pathway carrying the motor command from the brain to the bladder and bowel so this person would be incontinent

Lesion in the mid-thoracic region will result in the loss of voluntary control of the lower limbs

Paraplegia - loss of voluntary control of lower limbs

Lesion above C3-C5 will result in not being able to breathe

SPINAL DYSFUNCTION
The degree of the deficit following a spinal cord lesion depends mainly on three factors.

1. loss of neural tissue - usually small if due to trauma but could be more extensive e.g. metastases, degenerative disease.
2. the vertical level of the lesion - generally the higher the lesion the more severe the disability.
3. the transverse plane - which and how many tracts are affected. Damage to the ascending and descending spinal tracts may produce motor and/or sensory loss (e.g. paralysis, anaesthesia)…

Effect of injury to the lateral corticospinal tract

Injury to this area results in two stages of a response

STAGE 1 - Spinal Shock

• You lose all reflex activity below the level of the lesion - there is no direct damage to them, they just stop working
• This leads to flaccid paralysis - the limbs become floppy and there is very little muscle tone
• Eventually this changes and the reflexes come back with a vengeance

STAGE 2 - Return of Reflexes

• Hyperreflexia
• This means that if you test the knee-jerk reflex the leg will go mad You also get spasticity where the patient experiences spontaneous muscle contraction
• There is very high muscle tone - RIGID paralysis
• This is probably because the lower motor neurones below the level of the lesion have been cut off from their normal activating pathway so they have lowered their threshold and become more sensitive
• Spinal gliomas may also cause problems in the same way as other lesions.
• There are some diseases that result in quite dispersed damage to the spinal cord 


With these cases where a lot of tissue is affected, it is very difficult to predict the pattern of the deficit 


This MRI shows secondary lymphomas 


The top of the spinal cord looks ok but it is 
very disturbed lower down 


It is caused by lymphoma that has spread to 
the vertebral bodies and this has had an effect on the spinal cord behind it 


The important thing isn’t the amount of grey 
matter that has been lost but in fact the WHITE MATTER that has been cut through 


In the white matter surrounding the brain you 
get the ascending and descending pathways that are connecting all the levels 
of the spinal cord with the brain 


The effect on the WHITE MATTER is the most important in spinal injury 


Brown-Séquard syndrome

• With unilateral lesions the relationship of the deficit to the lesion depends on where the tract decussates
• Why sensations are lost differentially from the side of injury

Question 8

Syringomyelia

Answer 8

Syringomyelia

Syringomyelia refers to a disorder in which a cyst or cavity forms within the spinal cord. This is usually seen in the cervical region, so upper limbs are affected.

1. Loss of temperature sensation in the arms but not the legs is caused by the formation of a space in the spinal cord called a syrinx
2. There is a large space in the middle that is selectively damaging the spinothalamic axons that are crossing over at the level of the lesion but it does not affect the fibres that have already crossed over and are travelling up in the spinothalamic tract
3. The fibres from the legs would already have crossed over and would be travelling up the spinothalamic tract
4. Syringomyelia is caused by an enlargement of the central canal