Spinal cord and periphery

Question 1

On the spinal cord, where is the grey and white matter in relation to each other?

Answer 1

Central grey matter and peripheral white matter

Question 2

Where do axons of sensory fibres enter the spinal cord?

Answer 2

Through the dorsal root into the dorsal horn of grey matter

Question 3

Where are the cell bodies of motor neurons in the spinal cord?

Answer 3

Ventral horn

Question 4

label the cross sectional anatomical image of the spinal cord

Answer 4

Question 5

What are the 3 columns of white matter in the spinal cord?

Answer 5

Posterior

Lateral

Anterior

Question 6

What is found in each column of the spinal cord?

Answer 6

Various bundles (tracts) of axons

Question 7

What kind of tracts are ascending pathways?

Answer 7

Sensory

Question 8

What kind of tracts are descending pathways?

Answer 8

Motor

Question 9

What are 3 important tracts of the spinal cord?

Answer 9

Corticospinal/pyramidal tract

Posterior/dorsal column

Lateral spinothalmic tract

Question 10

What does the corticospinal/pyramidal tract carry?

Answer 10

Motor impulses from motor cortex to skeletal muscles

Question 11

What does the posterior/dorsal column carry?

Answer 11

Touch, tactile localisation, vibration sense, proprioception

Question 12

What does the lateral spinothalmic tract carry?

Answer 12

Pain and temperature

Question 13

Where does the corticopsinal/pyramidal tract begin?

Answer 13

Motor cortex (area 4)

Question 14

Do corticobulbar fibres go to the ipsilateral or contralateral nuclei?

Answer 14

Contralateral

Question 15

Where do the corticospinal fibres decussate?

Answer 15

Decussation of pyramids

Question 16

What are the different corticospinal tracts?

Answer 16

Lateral and anterior corticospinal tract

Question 17

Do the anterior corticospinal tracts decussate?

Answer 17

No, only the lateral

Question 18

What does the lateral and anterior corticospinal tracts innervate?

Answer 18

Lateral - limb muscles

Anterior - axial muscles

Question 19

What are the fibres before anterior/lateral corticospinal fibres called?

Answer 19

Corticobulbar fibres

Question 20

what is the internal capsule supplied by?

Answer 20

branch of middle cerebral artery

Question 21

what is the middle cerebral artery often occluded by?

Answer 21

middle cerebral artery is a direct continuation of the ICA,
quite frequently this is implicated in occlusions, which can lead to strokes

an occlusion of this branch here that supplies the internal capsule will lead to
a dense stroke, because all these fibres are coming together in this region

Question 22

describe the movement of axons from the left precentral gyrus to the medulla?

Answer 22

from the left precentral gyrus
you got the axons that travel down the corona radiata, down the internal capsule and enters into your brain stem. In the brain stem
they pass anteriorly, in the midbrain, in the pons to reach the medulla.

It has to cross to the opposite side because, remember, your left precentral gyrus is taking care of the right half of your body.

Question 23

describe the corticobulbar tract?

Answer 23

So we’re talking about fibres here that are coming from the precentral gyrus, but then are leaving this tract to go to cranial nerve nuclei.

Now, why are they doing that? You know that the head and neck region is going to be innervated by cranial nerves.
And you know that all cranial nerves from 3 up until 12 arise from the brain stem.

So motor fibres that are destined to the head and neck region do not continue down into the spinal cord.
Instead, they leave this tract at the region where those cranial nerve nuclei are.

For instance, you know that your facial nerve that innervates the muscles of facial expression arises from just below the pons.
So there is a nucleus for the facial nerve within the pons and the fibres
destined for that are going to leave this tract here go to the opposite side
to synapse at those neurons

Question 24

what happens to the fibres which dont leave via the corticobulbular tract?

Answer 24

But the rest of the fibres from the corticospinal tract, they go down in the medulla and in the medulla
they’re forming this projection, which you find,
which you can see and which is called as the pyramids.

At the lower part of the pyramids,
these fibres are going to cross over and
that is called as the decussation of the pyramids.

And that takes this tract to lie in the lateral column of the spinal cord and therefore forming the lateral corticospinal tract.

Now, you will again find that some fibres don’t confine to this normal pattern.
They do not cross until much later and they form something called as the anterior corticospinal tract.

Let’s just talk about this main bundle, which has come down from the precentral gyrus through the internal capsule,
through the front of the midbrain, the pons, to the pyramids of the medulla,
cross over to the opposite side, form the lateral corticospinal tract.
This is the very same axon of the neurone that began in the grey matter here, and that’s travelling all the way down to your spinal cord.

Now, what happens to it? You know that it has to go to supply the skeletal muscles.
And in order to get to the skeletal muscles, it needs to go through one of those 33 pairs of spinal nerves.
In order to do that, this axon is going to enter into the grey matter where it’s going to synapse. Another neurone,
the neuronal cell body of which is present in the ventral horn of that grey matter,
is going to pick up that impulse and then pass out through the ventral root, through the spinal nerve and to the skeletal muscles.

Question 25

describe the movement of neurons if you want to move your, let’s say your thumb muscles
or your hand muscles?

Answer 25

You know that the nerves that reach the hand have to pass through the brachial plexus and the brachial plexus comes out from C5 to T1.
The root value of the brachial plexus, remember, is the ventral rami of C5 to T1, and therefore,
in order to get to the hand, these specific neurones are going to fire, passes through the corona radiata, internal capsule,
front of the midbrain, front of the pons to the pyramids of the medulla, crosses over to the opposite side,
forms the lateral corticospinal tract. But when it comes to the cervical segments of the spinal cord,
those fibres that are destined to the hand are going to leave the corticospinal tract, synapse in the grey matter in the regions of C5 to T1.
The neurones from C5 to T1 are going to pick it up, go through the ventral
rami, ventral root, spinal nerve, ventral ramus, brachial plexus and then to the upper limb.

Question 26

What about the neurones that are going to supply the leg, for instance?

Answer 26

That’s coming from the sciatic nerve. That’s coming through the sacral plexus. So these neurones are firing.
Axons carry those impulses through the corona radiata, through the internal capsule, through the midbrain, through the pons,
through the pyramids of the medulla, crosses over at the lower part of the pyramids, forms the lateral corticospinal tract, goes down into the spinal cord,
does not leave it at the cervical segments though, still travels down the spinal cord until you reach the sacral segments,
because the sacral plexus is coming from L4,5 to S2,3 and then goes out there in the sacral segments.
That’s where it is going to synapse.

That’s where this lower motor neurone will go up ventral root now, spinal nerve, ventral ramus, sacral plexus, sciatic nerve
and then to the lower limb. So you can think of this lateral cortico-spinal tract very much like a bus
carrying information down the spinal cord. At various points or various bus stops,
you’ve got some nerves leaving it to go and synapse and go out.
But the tract itself carries on becoming smaller and smaller until it reaches the very
lowermost parts where the sacral and the coccygeal spinal nerves then emit.

Question 27

So as it comes down into the brain stem, in the midbrain, these motor fibres are going to sit in the very front of the midbrain.

This bit here is called as the crus cerebri.
It sits just in front of this substantia nigra.

We know that the substantia nigra is functionally part of the basal ganglia and your motor fibres are sitting just in front of that.
When you look at this gross specimen of the brain stem and look at it anteriorly, this
V-shaped tract of fibres that you see in front is nothing but the crus cerebri.
When it comes into the pons, once again, it is going to remain anterior, is depicted here by these brown little bundles,
but they are going to be separated because in this region of the pons,

Answer 27

Question 28

What is the site of the lower motor neurons?

Answer 28

Ventral horn

Question 29

describe the path of first order neurons?

Answer 29

First order neurones start off from the peripheral receptor, come through the spinal nerve, the dorsal root, with the dorsal root ganglia
having the neuronal cell body of that spinal nerve, acts on, climbs up forming the dorsal column pathway, reaches the gracile
and cuneate nuclei of the medulla, where it synapses.

Question 30

describe the path of second order neurons?

Answer 30

Second order neurone is a multipolar neurone whose
cell body is in the gracile and cuneate nuclei, crosses over to the opposite side to form the medial
lemniscus, travels up the brain stem and up to the thalamus where it synapses in the ventral posterolateral nucleus.
23:23
The ventral posterolateral nucleus of the thalamus has neuronal cell bodies,

Question 31

describe the path of third order neurons?

Answer 31

third order neurone now, which radiate through the internal capsule, through the corona radiata to the left postcentral gyrus.
Once again, looking at where things lie. Once it reaches the medulla, in the lower part of the medulla,
you’re going to have the gracile and cuneate nuclei, not shown here.
But once it crosses over and forms the medial lemniscus, that’s where the medial lemniscus is going to lie in the medulla.
In the pons. And in the brainstem.

Question 32

what is the function of posterior/dorsal column?

Answer 32

carries sensation of fine touch tactile, localisation, vibration, sense, proprioception

Question 33

how does the 1st order neuron end?

Answer 33

synapsing with the cell body of the second neuron

Question 34

where does the 1st order neuron synapse with the 2nd order neuron?

Answer 34

lower part of the medulla

Question 35

where does the 2nd order decessates?

Answer 35

medulla

Question 36

what is the tract called followiing dessucation?

Answer 36

medial lemniscus and passes through the medulla, pons, and midbrain to reach the thalamus

Question 37

where does the third order neuron pass and end?

Answer 37

starts from the hypothalamus and axons pass through the internal carotid and radiate to post central gyrus

Question 38

What are the common characteristics of pathways reaching conscious level (ascending spinal tracts)?

Answer 38

There are 3 neurons between peripheral receptor and cortex

Receptor -> first order neuron -> second order neuron -> third order neuron -> contralateral parietal cortex

Question 39

In ascending spinal tracts, where is the first order neuron?

Answer 39

Dorsal root ganglion