Anatomy of the Spinal Cord

Question 1

What is the nervous system and how is it divided?

Answer 1

The nervous system is a complex collection of nerves and specialised cells that receives and transmits information to and from various parts of the body.

The nervous system is structurally divided into main two systems: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

The CNS is composed of the brain and spinal cord. The PNS is composed of all the nerves that branch off from the CNS such as the cranial nerves and the spinal nerves.

Question 2

What are the components of the CNS?

Answer 2

Spinal cord

Brain

Question 3

What are the components of the PNS?

Answer 3

Spinal nerves

Cranial nerve

Question 4

What is the function of neurons?

Answer 4

The basic structural and functional unit of the nervous system is the neuron. A neuron is a specialised cell that transmits nerve impulses.

Question 5

What is a dendrite?

Answer 5

Dendrites receive signals from other neurons. At the end of these projections are the synapses, where information is transferred.

Question 6

What is the neuron cell body?

Answer 6

The cell body contains the cell nucleus and connects the dendrites (which receive information from other neurons) and the axon (which sends information to other neurons).

Question 7

What is the axon terminal?

Answer 7

The axon terminal consists of long, slender nerve fibres that conduct electrical signals to a nerve synapse.

Question 8

What is a Schwann cell?

Answer 8

Schwann cells are of glial cells that keep peripheral nerve fibres (both myelinated and unmyelinated) alive. In myelinated axons, Schwann cells form the myelin sheath.

Question 9

What is the myelin sheath?

Answer 9

Myelin is an insulating layer, or sheath that forms around nerves. It is made up of protein and fatty substances. This myelin sheath allows electrical impulses to transmit quickly and efficiently along the neurons.

Question 10

What is the brain composed of and what does it do? What separates the regions of the brain?

Answer 10

The brain is made up of billions of neurons and is a central component of the nervous system, a network that transmits signals to and from different parts of the body, and regulates involuntary and voluntary actions.

Folding of the cerebral cortex creates sulci (singular: sulcus) and gyri (singular:gyrus) which separate brain regions.

Question 11

What is the brainstem, what are its three parts and what emerges from it?

Answer 11

The brainstem is the most distal part of the brain and is continuous with the spinal cord. It is composed of three components: the midbrain (1); the pons (2) and the medulla oblongata (3). Each of these components has its own unique structure and function. Cranial nerves III to XII emerge from the brainstem.

Question 12

Where is the spinal cord and what surrounds it?

Answer 12

The spinal cord extends from the medulla oblongata to the lumbar regions. It passes through the bony vertebral canal and like the brain it is covered by the meninges and surrounding cerebrospinal fluid.

Question 13

What are the the meninges and what are its three layers?

Answer 13

Surrounding the central nervous system are 3 protective layers. From the outer most layer to the inner most layer these include the dura mater, arachnoid mater and pia mater.
The three meningeal layers are continuous with the spinal cord.

Question 14

Where is the needle inserted during an epidural? What about a lumbar puncture?

Answer 14

An epidural is administered into the epidural space (between the vertebrae and dura mater).

A lumbar puncture is performed in the subarachnoid space (between the arachnoid mater and the pia mater)

Question 15

What is grey matter composed of?

Answer 15

Mainly neuronal cell bodies and unmyelinated axons

Question 16

What is white matter composed of?

Answer 16

Mainly myelinated axons

Question 17

How are white and grey matter arranged in the spinal cord?

Answer 17

White matter surrounds the centrally distributed grey matter

Question 18

How are white and grey matter arranged in the brain?

Answer 18

White matter is found in deeper areas with surrounding grey matter
Other grey matter structures, such as the basal ganglia, lie deeper with the grey matter

Question 19

What are afferent and efferent nerve fibres? How do they enter and exit the spinal cord, and what structures do they form?

Answer 19

Information has to either exit the spinal cord to send signals to peripheral tissues or information from peripheral tissues needs to be carried into the spinal cord. This is done via efferent (green) and afferent (red) fibres.

Sensory (efferent) and motor (afferent) fibres enter and exit the cord via rootlets that arise from both the posterior and anterior aspects of the spinal cord. Ventral (anterior) rootlets carry motor information out of the spinal cord (i.e. they contain efferent fibres) while the dorsal (posterior) rootlets carry sensory information into the spinal cord (i.e. they contain afferent fibres). Several dorsal rootlets merge together to form the dorsal root, while several ventral rootlets similarly converge to form the ventral root.

The dorsal root ganglion is a collection of cell bodies found in the dorsal root. The ventral root does not contain a ganglion. Ganglia are only found where neuron cell bodies are outside the CNS.

Distal to the dorsal root ganglion, the fibres of the ventral and dorsal root merge together and pass through the dura to become the spinal nerve. Spinal nerves contain both sensory (efferent) and motor (afferent) fibres and are therefore a mixed nerve.

Question 20

What are ascending tracts and what do they do?

Answer 20

The ascending tracts transport sensory information from the body to the brain. There are three orders of neurons that carry this information to the brain.

The first order neurons conducts impulses from peripheral receptors to the spinal cord, where is will synapse with second order neurons in the spinal cord or brainstem. The second order neurons will carry impulses up the spinal cord to the brain, usually the thalamus. The third order neurons carry information from the thalamus to the primary sensory cortex.

Question 21

What are the descending tracts and what do they do?

Answer 21

The ascending tracts transport sensory information from the body to the brain. There are three orders of neurons that carry this information to the brain.

The first order neurons conducts impulses from peripheral receptors to the spinal cord, where is will synapse with second order neurons in the spinal cord or brainstem. The second order neurons will carry impulses up the spinal cord to the brain, usually the thalamus. The third order neurons carry information from the thalamus to the primary sensory cortex.

Question 22

What structure transmits information from the spinal cord to the extremities of the body?

Answer 22

Ventral roots

Question 23

Which functional division of the nervous system would be responsible for the physiological changes seen during exercise (e.g., increased heart rate and sweating)?

Answer 23

Autonomic

Question 24

What does the peripheral nervous system contain?

Answer 24

The peripheral nervous system includes all neural structures outside of the brain and spinal cord.

Question 25

What is a spinal nerve?

Answer 25

A mixed nerve containing sensory and motor fibres

Question 26

How many pairs of spinal nerves are there?

Answer 26

31

Question 27

What type of information is transmitted through the dorsal roots?

Answer 27

Sensory information

Question 28

What type of information is transmitted through the ventral roots?

Answer 28

Motor information

Question 29

What is the dorsal root ganglion?

Answer 29

It contains cell bodies of sensory neurons. These are cell bodies of first order neurons.

Question 30

What are spinal nerves and where are they located? What different types are there?

Answer 30

Spinal nerves are nerves originating from the spinal cord that contain sensory and motor neurons. Each spinal nerve travels to a specific region / segment of the body. There are 31 pairs of spinal nerves and are named according to their origin:
8 pairs cervical (C1-C8)
12 pairs thoracic (T1-T12)
5 pairs lumbar (L1-L5)
5 pairs sacral (S1-S5)
1 pair coccygeal (C0)

Question 31

Where does the spinal cord extend from/to? Where does it terminate? Where are its enlargements located?

Answer 31

The spinal cord extends from the foramen magnum at the base of the skull to the L1/L2 vertebrae where it terminates as the conus medullaris (medullary cone). A thin thread called filum terminale extends from the tip of the conus medullaris all the way to the 1st coccygeal vertebra (Co1) and anchors the spinal cord in place. Below L1/L2 the spinal nerves are bunched together as the cauda equina. Along its length, the spinal cord has two defined enlargements; a cervical enlargement and a lumbosacral enlargement.

Question 32

Why are there two well-defined enlargements along the length of the spinal cord?

Answer 32

They provide innervation to the limbs.

Question 33

What is a dermatome and how can it vary between people?

Answer 33

A dermatome is an area of skin that is supplied by a single spinal nerve and these nerves transmit sensations, such as pain, from the skin to the CNS. Dermatomes have a segmented distribution throughout the body. The exact dermatome pattern can vary from person to person and some overlap between neighbouring dermatomes may also occur.

Question 34

A patient is experiencing numbness across the lateral aspect of their shoulder, what spinal level do you think the injury is at?

Answer 34

C5

Question 35

Where on the hand would you test dermatome C8?

Answer 35

Little finger

Question 36

What is a plexus and what do they do?

Answer 36

Spinal nerves contribute to the formation of plexi (singular: plexus). A plexus is a network of nerves that intermingle and become redistributed so that the peripheral nerve leaving the plexus contains fibres from multiple spinal nerve roots. Each limb muscle receives motor innervation from multiple spinal nerve roots.

Question 37

How many plexus groups branch off the spinal cord?

Answer 37

4 main plexus groups, found in the cervical, brachial, lumbar and sacral regions of the spinal cord

Question 38

What is the arrangement of nerves in the thoracic region?

Answer 38

Spinal nerves in the thoracic region are arranged segmentally and generally do not contribute to the plexi

Question 39

What plexus supplies the upper limbs?

Answer 39

Brachial plexus

Question 40

What plexus supplies the lower limbs?

Answer 40

Lumbosacral plexus

Question 41

What is the function of the somatic nervous system?

Answer 41

Sensory impulses from the skin are relayed to the CNS
Acts mainly on skeletal muscle
Responsible for the voluntary control of body movements

Question 42

What is the function of the visceral nervous system?

Answer 42

Acts on glands
Responsible for involuntary control of bodily functions
Autonomic nervous system is the motor part of this system
Acts on cardiac and smooth muscle
Sensory information comes from the internal organs

Question 43

What is the autonomic nervous system and what does it do?

Answer 43

The autonomic nervous system (ANS) regulates body homeostasis. It in involuntary control of body processes such as breathing, heart beat and digestive processes. It can be divided into the sympathetic nervous system and the parasympathetic nervous system. Generally, the sympathetic nervous system prepares the body for stressful or emergency situations i.e. the fight or flight response. Whereas the parasympathetic nervous system controls body processes during ordinary situations i.e. the rest and digest response.

Question 44

Where do the sympathetic and parasympathetic nervous systems arise from?

Answer 44

The sympathetic nervous system emerges from thoracolumbar regions and parasympathetic emerges from from craniosacral regions.

Question 45

What is the function of the parasympathetic nervous system?

Answer 45

Stimulates urination
Stimulates saliva production
Stimulates stomach, pancreas and intestines
Constricts pupils
Slows heart rate
Stimulates erection (males)

Question 46

What is the function of the sympathetic nervous system?

Answer 46

Stimulates ejaculation (males)
Inhibits stomach, pancreas and intestines
Increases heart rate
Inhibits urination
Dilates pupils
Increases blood flow to skeletal muscles

Question 47

What is the vertebral column and what does it consist of? What are the characteristics of the vertebrae?

Answer 47

The vertebral column is part of the axial skeleton. It is made up of 33 vertebrae stacked on top of each other, and has many functions, including transmission of the spinal cord, weight bearing, and serving as the attachment points for various muscles and ligaments. The vertebrae can be separated into 5 regions: there are 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 3-5 coccygeal vertebrae. These will be explored in more details in the labelled tabs.

Despite each level having characteristic features, the vertebrae are composed of the same basic elements (see picture). Anteriorly is the vertebral body, which is the weight-bearing part. The transverse processes are connected to the body via the pedicle. Inferior to the pedicle is the intervertebral notch, which is where spinal nerves exit. The transverse processes are connected to the spinous process posteriorly via the lamina. Note the space centrally, the vertebral foramen, which transmits the spinal cord.

Question 48

What is the vertebral canal?

Answer 48

The vertebral canal is defined by the vertebral foramina in sequence. We cannot see any large bony impingements on the space, but there are degenerative changes at multiple levels which are encroaching slightly

Question 49

What is the vertebra prominens?

Answer 49

This is the spinous process of C7, which is longer than its neighbours and hence is more easily palpable. For this reason it is known as the vertebra prominens

Question 50

Which structure is at risk of compression by a very posteriorly displaced dens?

Answer 50

Spinal cord
As the dens moves posteriorly, it reduces the space in the vertebral canal and risks pinching off the spinal cord at this level. Although this is too caudal to compress the brainstem directly, upper cervical spine fractures can cause angulation which may damage the brainstem as it exits the foramen magnum.

Question 51

What may be a feature of a high cervical spine fracture?

Answer 51

Due to the level, it certainly may cause paralysis of both upper (A) and lower (B) limbs, but may also involve the diaphragm if above C5, causing breathing difficulty (C). Dysregulation of heart rate (D) may result from denervation of sympathetic neurons, and severe damage to the brainstem or spinal cord above C3 can be fatal (E).

Question 52

Which structure, running in the transverse foramen, may also be affected by cervical vertebral fractures?

Answer 52

Vertebral arteries
The vertebral arteries run in the transverse foramina for C6-C1, and may therefore be affected by fracture or displacement of the cervical vertebrae.

Question 53

Damage to the vertebral arteries is most likely to affect which part of the brain?

Answer 53

Brainstem
Occipital lobe
Parietal lobe
The vertebral arteries unite to form the basilar artery, which supplies the posterior circulation to the brain, giving off branches to supply the brainstem, cerebellum and occipital lobe. The other regions listed are supplied by the anterior circulation, via branches of the anterior and middle cerebral arteries.

Question 54

Where does the spinal cord begin? Where does it travel to and where/how does it terminate? What enlargements does it have an why?

Answer 54

The spinal cord begins at the foramen magnum, where it is continuous with the brainstem rostrally. It travels through the vertebral canal and ends at L2. This most caudal part of the cord is known as the conus medullaris. Below L2, the vertebral canal contains instead the cauda equina, which is a bundle of spinal nerves travelling inferiorly to exit below their vertebral level.

There are two enlargements of the spinal cord: one in the cervical region, and one in the lumbar region. This is to meet the demand for more comprehensive motor and sensory innervation to the upper and lower limbs.

Question 55

What is the spinal cord covered with? How many layers are there and what are they called? What does each of these layers do?

Answer 55

Just like the brain, the spinal cord is covered by three layers of meninges: the dura most superficially, the arachnoid beneath this, and the pia deepest.

The dura covers the whole cord, and also invests little sleeves to the exiting nerve roots. It is separated from the periosteum of the vertebral canal by a ‘potential space’, the epidural space. The arachnoid abuts the dura, separated only by some lubricating fluid. In the subarachnoid space, there is CSF. The pia is intimately related to spinal cord. It continues at the cona medullaris as the filum terminale, which attaches to the coccyx to anchor the cord. Similarly, there are denticulate ligaments along the length of the cord, which begin in the pia and attach to the dural to stabilise the cord.

The dura ends at S3 by blending with the filum terminale.

Question 56

At what spinal level should you insert a needle to obtain a sample of CSF and why?

Answer 56

This is approximately the L4/L5 space, which is ideal for lumbar puncture. The key here is that it is below L2, which is where the spinal cord ends. The cauda equina is much less at risk as the nerves will move out the way as the needle is advanced.

Additionally, the CSF space is comparatively larger here in the lumbar cistern.

The L3/L4 and L5/S1 spaces are suitable alternatives if the L4/L5 is unsuccessful.

Question 57

Why should a lumbar puncture not be given at the C5/6 level?

Answer 57

This is approximately at the level of the C5/C6 space. The spinal cord is large here and the subarachnoid space is small. Obtaining CSF here will be very difficult and the procedure could lead to catastrophic complications, including tetraplegia.

Question 58

Why should a lumbar puncture not be given at the T4/5 level?

Answer 58

This is approimately the T4/T5 level. It is dangerous to pass a needle blindly at any point along the spinal cord due to risk of injury.

Bear in mind also that we are approaching the spinal cord from the posterior, so in the thoracic region, it will be technically difficult to pass the overlapping spinous processes with the needle.

Question 59

Why should a lumbar puncture not be given at the L1/2 level?

Answer 59

This is approximately the L1/L2 space. This is just a little too high for a lumbar puncture, as at this level, of course, we still find spinal cord.

Question 60

Why should a lumbar puncture not be given at the S2/3 level?

Answer 60

This is approximately S2/S3 level. As the sacral vertebrae are fused here, it is not possible to access the vertebral canal from this approach.

Question 61

Why might lumbar puncture be more challenging in a young child?

Answer 61

The spinal cord ends lower down the vertebral column
In young children, the spinal cord extends caudally to the level of L3. It grows very little past infancy, while the vertebral column grows considerably, and therefore, in adults the spinal cord can be found ending at L2 level. The fact that the spinal cord is relatively longer in children makes it more difficult to perform lumbar puncture, as the number of safe vertebral levels are reduced.

Of course, there are added challenges such as trying to keep a child still and trying not to scare them too much, which must also be considered.

Question 62

Why might lumbar puncture be more challenging in an elderly person?

Answer 62

As all of us age, we accumulate wear and tear strains on our bones and joints. In the lumbar vertebrae, this may be especially evident as it bears a lot of weight. There will be loss of space between vertebrae as the intervertebral disc compresses and dehydrates. There may also be osteoarthritis affecting the facet joints, and osteophytes occupying the space between adjacent laminae. This makes it very difficult to pas the needle.

Additionally, elderly people are more likely to have other medical problems, and may be on medications such as blood thinners, which will increase the risk of the procedure.

Question 63

Which meningeal space is targeted in lumbar puncture?

Answer 63

In lumbar puncture, the tip of the needle should be in the subarachnoid space. This is where the CSF circulates, and is continuous with the cranial subarachnoid space.

Question 64

What are the risks associated with lumbar puncture?

Answer 64

Lower limb paralysis
- Although the risk of damaging the cauda equina is lower than the spinal cord, it is not zero. One or more lumbosacral nerve roots may be damaged, especially if there is a large bleed.
Bowel and bladder dysfunction
- The bowel and bladder are innervated by sacral nerves
Sexual dysfunction
- The external genitalia receive parasympathetic innervation via the sacral nerves
Meningitis
- Introducing a foreign object into the body always carries the risk of infection. In this case, the infection may be introduced to the meninges or the CSF. To mitigate this risk, lumbar puncture is a strictly aseptic procedure

Question 65

Louise’s friend, Rodrigo, is a third year medical student. He is worried about Louise having the lumbar puncture as he has learnt that the pressure change can start to pull the brain out of the skull base. What is Rodrigo referring to here?

Answer 65

Rodrigo is worried about cerebellar tonsillar herniation, or ‘coning’. This is a risk of lumbar puncture in the context of obstructive hydrocephalus.

Normally, CSF will circulate freely around both brain and spinal cord as it is produced and reabsorbed.

Where there is obstructive hydrocephalus, however, there is a blockage in the cerebral ventricular system, effectively creating two closed CSF compartments. In this situation, when CSF is drawn off during lumbar puncture, a pressure differential is created, with spinal pressure much lower than cranial. This pulls the cerebellar tonsils out through the foramen magnum, compressing the brainstem - invariably an immediately fatal event.

If obstructive hydrocephalus is a possibility, imaging should always be undertaken to rule it out prior to lumbar puncture.

Question 66

What are the white matter tracts? What types are there?

Answer 66

The white matter tracts of the spinal cord are bundles of myelinated axons which transmit information from brain to periphery (descending) and from periphery to brain (ascending).

Question 67

What is the most important descending tract?

Answer 67

The most important descending tract is the corticospinal tract, which transmits motor information from the primary motor cortex to the muscles peripherally. The upper motor neurons decussate in the medullary pyramids.

Question 68

What are some examples of ascending (sensory) tracts?

Answer 68

The ascending (sensory) tracts include the dorsal column medial lemniscus (DCML), the spinothalamic and spinocerebellar tracts. DCML ascends ipsilaterally and decussates in the medulla. Spinothalamic ascends 1-2 spinal levels ispilaterally, then decussates and ascends contralaterally. Spinocerebellar either ascends entirely ipsilaterally without decussating, or decussates at the point of entry, ascends to the cerebellum and then decussates again.

Question 69

What type of signal is the corticospinal tract associated with?

Answer 69

Voluntary movement

Question 70

What type of signal is the DCML tract associated with?

Answer 70

Discriminatory touch
Conscious proprioception
Vibtration

Question 71

What type of signal is the spinothalamic tract associated with?

Answer 71

Pain
Temperature
Crude touch

Question 72

What type of signal is the spinocerebellar tract associated with?

Answer 72

Unconscious proprioception

Question 73

What position in the spinal cord best represents the corticospinal tract?

Answer 73

This is the lateral corticospinal tract, in which travel the upper motor neurons on their way to synapse with the lower motor neurons in the ventral horn at the relevant spinal level

Question 74

You discover that Nathan has complete ‘flaccid’ paralysis and loss of reflexes in the entire left lower limb. The right lower limb has full muscle power throughout. What is the pathophysiology underlying this (temporary) flaccid paralysis and arreflexia immediately following spinal cord injury?

Answer 74

Spinal shock
Spinal shock is a pathophysiological state seen immediately after a spinal cord injury. Sudden loss of descending information from the brain to lower motor neurons and reflex interneurons is thought to cause hyperpolarisation of those neurons, desensitising them to stimuli, causing flaccid paralysis and arreflexia.

Over days to weeks, reflexes are restored as synapses are strengthened by recruiting more receptors. Over weeks to months, hyperreflexia and spasticity develop as denervated axons try to form new synapses.

This is not core knowledge but is an interesting clinical phenomenon which ties in a lot of anatomy and physiology!

Question 75

Now you test Nathan’s sensation, starting with the left lower limb. The first thing you notice is he has complete loss of light touch, vibration and proprioception up to the level below: (whole left leg from hip to foot)
The upper margin of this distribution represents which dermatome?

Answer 75

T10

The upper margin is at the level of the umbilicus, which is within the T10

Question 76

Which tract carries information on light touch, proprioception and vibration sense?

Answer 76

DCML
The dorsal columns, comprising the gracile and cuneate fasciculi, carry these sensory modalities. Specifically, below the level of T6 as in this case, the gracile fasciculus is responsible.

Question 77

Which tract carries pain and temperature sensation?

Answer 77

Spinothalamic

Question 78

Why might the right lower limb be affected and not the left?

Answer 78

The primary afferent activated by a thermal or noxious stimulus enters the spinal cord, ascends 1-2 spinal levels and synapses in the dorsal horn. The secondary afferent then decussates and ascends in the contralateral spinothalamic tract. In Nathan, pain and temperature information from the left leg is able to cross over and bypass the injury, while similar information from the right crosses over and ascent is blocked by the injury.
This type of injury might be caused by a complete hemisection of the left side of the spinal cord, commonly known as Brown-Sequard syndrome and classically results from penetrating trauma to the cord, like stabbing injuries

Question 79

What provides most of the arterial supply to the spinal cord? What supplies the posterior third? What are these arteries supported by?

Answer 79

The spinal cord requires a rich blood supply to support its highly metabolically active neurons. Most of the arterial supply is via the anterior spinal artery (ASA), which arises from the vertebral arteries cranially and descends to supply the anterior two thirds of the cord.

The posterior third of the cord is supplied by the two posterior spinal arteries (PSA), which are may be distal branches of the vertebral arteries, or may originate from the posterior inferior cerebellar arteries.

Regularly along its length, the spinal cord is additionally supported by radicular arteries and segmental medullary arteries.

Question 80

If a patient has full upper limb function, but has complete paralysis and is insensate to pain and temperature on the lower limb, what damage has likely been done?

Answer 80

A spinal cord lesion in the thoracolumbar region could cause bilateral lower limb signs but spare the upper limbs

Question 81

Which structures are supplied by the anterior spinal artery?

Answer 81

Corticospinal
Spinothalamic
Spinocerebellar

Question 82

Which structures are supplied by the posterior spinal artery?

Answer 82

DCML

Question 83

If Bernard has full upper limb function, but has complete paralysis and is insensate to pain and temperature on the lower limb, which spinal tracts are involved?

Which spinal artery is therefore most likely to be involved?

Answer 83

Spinothalamic and corticospinal
Bernard has bilateral loss of voluntary movement (corticospinal tract) and pain and temperature sensation (spinothalamic tract) in the lower limbs. There is preservation of light touch and proprioception (DCML) throughout.

Anterior spinal artery
This clinical picture is known as anterior cord syndrome. It is most commonly due to disruption of the blood supply from the ASA. The tracts involved are those in the ventral portion of the spinal cord, namely the spinothalamic, corticospinal and spinocerebellar tracts. The DCML pathway is spared as it travels dorsally and hence is primarily supplied by the PSAs.

Question 84

Imaging demonstrates infarction of the ventral spinal cord betwen T10 and L2 due a clot in a large, left sided segmental medullary artery at T10. What is the name of this artery?

Answer 84

This artery is commonly known as the artery of Adamkiewicz. It is the largest and most important segmental medullary artery and is responsible for most of the arterial supply to the lower thoracic and lumbar spine. Damage to this artery therefore can cause catastrophic neurological sequelae.