2. Spinal cord function and dysfunction

Question 1

What is a spinal cord segment?

Answer 1

a portion of the spinal cord that gives rise to a pair of spinal nerves (31 pairs altogether that emerge from the vertebral canal).

Question 2

How does the alignment of nerves change as you go down the spinal cord? Why does this happen?

Answer 2

At the top the vertebrae and nerves line up very well but as you go down then the place where the nerves emerge from the vertebral column are further away from the point at which the same nerves emerged from the spinal cord

There are 7 cervical vertebrae but there are 8 cervical nerves because all the cervical nerves emerge above the vertebra. So the C2 nerve is between the c1 and c2 vertebra. However, the C8 nerve runs below the 7th cervical vertebra making there one more cervical nerve than there are cervical vertebra
After that all the nerves come out from below their corresponding vertebra

Question 3

How many layers of meninges does the spinal cord have and what are they?

Answer 3

Pia mater - adheres to the spinal cord
Arachnoid mater - below this is the subarachnoid space where the CSF is found
Dura mater- outermost layer

Question 4

What is special about the pia mater.

Answer 4

It adheres to the spinal cord but 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

What do the meninges form in the spinal cord that is not present in the brain? Why is this not present in the brain?

Answer 5

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.

Filling of a potential epidural space in the brain would be pathological such as a haemorrhage. This occurs because there are two layers of dura mater in the brain but only one layer in the spinal cord. The second layer is lost as the medulla emerges from the foramen magnum

Question 6

What is the epidural space filled with?

Answer 6

This epidural space is full of venous plexuses and fatty tissue

Question 7

Why is the epidural space useful in the lumbar spine? What happens if you penetrate this layer and enter the one below?

Answer 7

This space in the lumbar spine is clinically useful because you can inject anaesthetic by giving an epidural. If you go deeper into this layer then you’ll be in the sub arachnoid space which is used to take CSF out or put anaesthetics for a spinal block.

Question 8

How many spinal nerves are there? How do they leave the vertebral column?

Answer 8

31 pairs - 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal

They leave the vertebral column through the intervertebral foremen

Question 9

Why are there enlargements in some places of the spinal cord? Where do these occur?

Answer 9

There are enlargements for innervation of the upper and lower limbs. These occur as there are a large quantity of nerves entering/leaving at a particular point in the spinal cord

Cervical (C3-T1)
Lumbar (L1-S3)

Question 10

Why is the cervical spinal cord larger in size than the thoracic spinal cord?

Answer 10

As there is an increased amount of nerves that supply the upper and lower limbs, the cervical part of the spinal cord is larger in area than the thoracic part and there is also a lumbar enlargement.

Question 11

What are dermatomes and myotomes? How are they determined and why are they clinically important?

Answer 11

• Dermatomes – relating to the skin
  
  • Myotomes – relating to muscle
  • A pair of spinal nerves goes to a set of muscles or an area of skin
  • Allows us to know how severe and widespread the damage to a nerve can be
  • The dermatomes are tested for the last level where they have normal function e.g. if there is an injury to the spinal cord
  • They relate to the spinal segments not the vertebral level

Question 12

What is the difference between roots, horns, rami and spinal nerves?

Answer 12

• Anterior horn – motor information out
  
  • Dorsal horn – sensory information in
  • Rootlets join to make roots
  • It is when sensory and motor roots join that a mixed spinal nerve is created. When the nerve splits again it is called a ramus. The larger of the two branches is the anterior ramus
  • Anterior ramus – both sensory and motor fibers as it is a branch of the mixed spinal nerve. Anterior root – only sensory info

Question 13

How is the spinal cord oraganised? What is the difference between the anterior and posterior divisions of the spinal cord?

Answer 13

• Divided into different regions – these are the white matter tracts outside of the horns containing cell bodies
  
  • The anterior side the division between the left and right side of the cord is slightly larger as there is a fissure but the posterior side has a division that is not too deep so it is called a sulcus

Question 14

Outline the different tracts of the spinal cord

Answer 14

• Corticospinal tract (lateral and anterior) – major voluntary movement pathway. Anterior innervates the more proximal muscles like the trunk. Lateral is mainly the limbs
  
  • Spinothalamic – takes pain and temperature back to the brain
  • Dorsal columns – ascending in the posterior aspect of the spinal cord carry fine touch and discriminative ability

Question 15

What determines the crossing over of sensory nerves?

Answer 15

• Sensory nerves come in through the back. Then they either cross over then come up or they go up then cross over. This is usually determined by whether or not there is a synapse. Generally the nerves cross over if a synapse has occurred.
  
  • If a nerve comes into the spinal cord and doesn’t synapse it is likely to go up on the same side. This usually synapses further up in the medulla. After the synapse the secondary neurone will cross over to the other side. If it comes into the spinal cord and synapses then it is likely to cross over to the other side of the spinal cord

Question 16

Outline the pathway of the corticospinal tract

Answer 16

Upper motor neurone in the brain crosses over in the medulla at the pyramids. So a motor neurone in the left motor cortex crossed over at the medulla and travels down the spinal cord in the lateral corticospinal tract, synapses on a lower motor neurone. This emerges from the spinal cord and becomes part of another peripheral nerve.

Question 17

Outline the main sensory pathway

Answer 17

• 3 neurones: skin, CNS and one going up to the brain
  
  • Neurone has cell body in the dorsal root ganglion. Another neurone either in spinal cord or ganglia (depending on the pathway), and a third neurone from the thalamus to the cortex
  • Second order neurones are the ones that cross over from one side of the body to another; for one tract this happens in the spinal cord and for the other it happens in the medulla
  • It then synapses in the thalamus
  • The third order neurone is the one that projects up from the thalamus to the sensory cortex
  • Pain and temp pathway (lateral spinothalamic pathway) – crosses in the spinal cord. Touch/dorsal column pathway – crosses in the medulla

Question 18

Outline the dorsal column pathway and what type of information they transmit

Answer 18

• Brings info about fine touch, discriminative ability and proprioception (position in space)
  
  • First neurone has its cell body in the dorsal root ganglion, ascends in the dorsal column, synapses in the medulla. Second neurone then crosses to the other side, goes up to the thalamus where it synapses and the third neurone then goes to the sensory cortex

Question 19

Outline the general structure of the spinothalamic tract

Answer 19

First order neurone in the skin, cell body in the dorsal root ganglion but synapses to the second order neurone in the spinal cord and so crosses over there

Question 20

What are the factors that affect the severity of spinal lesions?

Answer 20

Loss of neural tissue (how much tissue is damaged), vertical level (level of cord that is damaged) and transverse plane (where it is damaged

Question 21

What is the difference between flaccid and rigid paralysis?

Answer 21

• Flaccid paralysis – being floppy so can’t move

* Rigid paralysis – being stiff and having hypertonic muscles

Question 22

What happens if there is injury to the lateral corticospinal tract?

Answer 22

There will be motor impairment of the limbs on the same side of the body

Question 23

What is brown sequard syndrome?

Answer 23

A rare neurological condition where there is a lesion in the spinal cord which results in weakness or paralysis on one side of the body and a loss of sensation on the other