Download Anatomy Lecture 3 Descending Motor Tracts In the last lecture the

Anatomy Lecture 3
Descending Motor Tracts
In the last lecture the doctor started talking about the descending motor tracts
and here I’m going to continue what he started.
We have two major motor tracts:
1- Pyramidal (aka direct or oligosynaptic): passes through the pyramid
(part of medulla oblongata; thus the name)
2- Extra-pyramidal (aka indirect or polysynaptic)
Both of these tracts start in the brain (cerebral cortex) and end in the Lower
motor neurons; in any motor tract the beginning is in the upper motor neurons
and the end is in the lower motor neurons.
The Pyramidal is divided into two tracts:
1- Corticospinal
2- Corticobulbar
Corticospinal Tract:
- Begins at the cerebral cortex (areas 4 mainly, 6, (3,1,2) and (5,7))
- The cell bodies of this tract are present mainly here and these neurons
send their axons downwards. At first the axons are spread forming what
is known as corona radiate, then these axons come together in a narrow
are called the internal capsule (specifically its posterior limb)
- Then the fibers pass through the internal capsule to the brainstem and
its parts (midbrain, pons and medulla oblongata).
- After the end of the pyramid of the medulla gradual decussation of the
fibers of the corticospinal tract occurs by which:
1- 90% of the fibers go backwards and laterally forming the
lateral corticospinal tract.
2- 10% continue without crossing forming the ventral
(anterior) corticospinal tract.
NOTE:
- The internal capsule is the most common site of a stroke (CVA).
- Porta Cerebri: is another name for the internal capsule. Any nerve fiber
that descends from the cortex (motor) or ascends towards it (sensory)
MUST pass through the internal capsule; thus the name.
- In area 4 the body is presented upside down; face below and lower limb
above and they’re distributed precisely but disproportionately.
Lateral Corticospinal Tract:
- Passes through the lateral column of the white matter of the spinal cord.
- Originates in the medulla oblongata below the pyramid.
- It descends down through all the segments of the spinal cord (cervical,
thoracic, sacral, lumbar)
- Its fibers synapse gradually with the lower motor neurons (alpha and
gamma) mostly through interneurons.
- Although they mostly synapse through interneurons, some fibers (very
minimal in number) can synapse directly with alpha and gamma motor
neurons.
- Has a facilitatory (excitatory) effect mainly on the alpha and gamma
motor neurons of the lateral group (as we know, the lateral group is
responsible for the movement of the distal muscles; flexors of the hand,
that are important in fine skilled movements).
Decussation:
- The fibers that originate from the left side of the cortex, after
decussation, will become in control of the muscles on the right side of
the body (contra-lateral) and the opposite is true (for the right side).
- If we damage the corticospinal tract
1- Above the decussation: the effect would be on the other side of
the medulla (contra-lateral)
2- Below the decussation: the effect would be on the same side of
the body (ipsi-lateral).
3- At the beginning of the left cervical region of the spinal cord: the
effect would be on the left side of the body affecting both the
upper and the lower limb.
4- At the level of the umbilicus (T10): the effect would be on the
same side of the damage but affecting only the lower limb.
NOTE:
- Remember that: the upper limb is supplied by the brachial plexus
(C5T1) and the lower limb is supplied by the lumbosacral plexus.
- Damage means preventing the effect of the corticospinal tract on the
alpha and gamma motor neurons below the level of damage (or cut).
- Although the lateral corticospinal tract mainly affects the distal muscles,
it also could affect the proximal one but less commonly.
Suppose we have 1000 fibers in the corticospinal tract, then:
- 55% will synapse in the cervical region: the brachial plexus originates from
this region, this high percentage implies that the corticospinal tract
although affects the proximal muscles mainly affects the distal muscles
used for skilled movement like tying your shoes (the flexors of the hand are
supplied by branches from the brachial plexus (C8 and T1)).
- 20% will synapse in the abdomen and thorax: the small percentage is due
to the fact that there is no fine skill in moving the intercostal muscles.
- 25% will synapse in the lumbosacral region
If we take cross sections in the cervical, thoracic, sacral, lumbar spinal segments,
we will notice that the number of the lateral corticospinal tract fibers will
decrease gradually due to their synapsing in the gray matter.
SLIDE 12B:
- The lateral corticospinal tract may synapse on interneurons of the dorsal
horn where it will modulate sensory transmission. The doctor explained
this by saying the following:
The fibers of the corticospinal tract while going down may pass by
laminae 4,5,6,7,8 and may synapse with the interneurons there. If you
“in3’azait be dabboos” you will move your hand away rapidly, why do you
do that?
This movement is to “fool the brain”. Moving the hand rapidly will
cause impulses to flow down from the cortex (area 3,1,2) through the
corticospinal tract to move the hand and on the way, some of these
impulses will pass through the fibers synapsing in the dorsal horn (sensory);
thus inhibiting the pain sensation.
Why do the fibers of the corticospinal tract synapse in interneurons?
Interneurons are divided into excitatory and inhibitory neurons; its (the
division’s) importance is explained in the following example: if a person wants to
flex his fingers, there must be an excitatory effect on the flexors and inhibitory
effect on the extensors. So the fibers that will synapse on the excitatory
interneurons will affect the flexors and those that will synapse on the inhibitory
interneurons will affect the extensors. As we said before, this tract is mainly
excitatory for the agonist but it also causes inhibition for the antagonist.
Figure 12D:
We notice that the arrows are thick at first then gradually decrease in
thickness; this suggests that the lateral corticospinal tract mainly affects the
lateral group (distal muscles), keeping in mind that it also affect the proximal
group but to a lesser extent.
NOTE:
- In our body, muscles always have a tone, even during sleep, so an
excitatory tract will increase the tone of contraction. If there is damage in
that tract, the tone will decrease causing hypotonia.
Ventral corticospinal tract:
- It passes in the anterior column in the white matter of the spinal cord
segment.
- It affects the medial group (axial and proximal muscles).
- It supplies the same area and the opposite one as well; bilateral (if this tract
is at the right side it will affect the alpha and gamma motor neuron that
supply the right and left axial muscles).
NOTE:
- A person developed a stroke at the internal capsule, the 1st day the patient
is paralyzed, then after a couple of days the patient started to walking and
moving his arms but with NO fine movement in the hand. EXPLANATION:
Since the stroke occurred at the internal capsule on a certain side,
say left, it means that the ventral and lateral tracts are both damaged
leading to right side paralysis. Then after a couple of days, the proximal
muscles (the hips and shoulders) will start moving due to the effect of the
other (right) ventral corticospinal tract. Keep in mind that the fine skilled
movement is lost completely.
- If the lateral corticospinal tract was damaged then we will have hemiplegia
(palaysis of the arms and legs) or hemiparesis (weakness of the arms and
legs) depending on the number of fibers affected; the more fibers affected
by the stroke, themore it shifts towards paralysis.
- We said before that the corticospinal tract mainly synapses with alpha and
gamma through interneurons, however some of the fibers will synapse with
alpha and gamma directly, this helps in moving only 1 or 2 of the distal
flexor muscles of the hand (lumbricles, etc..) not a whole group of muscles.
This occurs because sometimes fine movement needs only 2 muscles to be
achieved.
Alpha and gamma motor neurons:
These motor neurons have a spontaneous activity; action potential keeps
on discharging thus if they are not controlled there will be continuous
contractions causing convulsions. Since this over-activity is dangerous, these
neurons must be limited.
Renshaw cells:
- Present in laminae number 7
- If an alpha neuron is hyperactive, discharging occurs. Impulses will pass
through collateral branches exiting from this neuron and entering into the
renshaw cells; activating these cells. In return, renshaw cells will inhibit the
alpha neurons, this is called recurrent inhibition.
NOTE:
- Strychnine is a drug that inhibits renshaw cells thus an overdose causes
convulsions.
Corticobulbar tract:
- Starts at the cortex and ends in the nuclei of certain cranial nerves (5, 7, 9,
10, 11, 12) in the brain stem.
- Supplies the same and other sides of the body (contra- and ipsi-lateral). If
damage occurs to one of the corticobulbar tract, the cells in the cranial
nuclei don’t get damaged, they only weaken (the occurrence of paralysis or
paresis is very minimal).
NOTE:
- All these cranial nuclei have motor fibers (5:mandibular, 7:facial, the rest:
pharynx, larynx and tongue).
- Alpha and gamma motor neurons are replaced with the nuclei of the cranial
nerve.
NOTE:
- An examination of a stroke patient neurologically revealed that this patient
has left hemiplegia or hemiparisis, the corticobulbar tract must be
examined by testing the cranial nerves.
What happens is that the medical student knows that it is rare for a
person to get cranial nerve paralysis because the nuclei are supplied by
both corticobulbar tracts, so the student doesn’t truly examine the cranial
nerves.
The examiner doctor asks the patient to smile revealing that the mouth
shifts towards the right. Explanation of the student’s mistake:
The stroke occurred in the right internal capsule, paralysis occurred on
the left side and the lower part of the facial nucleus on the left side is
affected leading to weakening of the lower part of the face; mouth shifts to
the right. Why is facial nerve different???
All the cranial nerves’ nuclei are supplied by both right and left corticobulbar tract
except for part of the motor nucleus of the facial nerve (and maybe hypoglossal
nerve)
The facial nerve nucleus has 2 parts:
1- Upper part which activate the upper facial muscles
2- Lower part which activates the lower facial muscles (those around the
mouth).
The upper part receives contra- and ipsi-lateral corticobulbar innervations;
however the lower part only receives from the contra-lateral corticobulbar tract.
NOTE:
- If 5, 7, 9, 10, 11, 12 nuclei were damaged on both sided, the patient won’t
be able to move his face, talk, swallow, etc…
Slide 14:
- If we damaged the left cranial nerve itself after it exits the skull (at the
parotid for example), the upper and lower left half of the face will be
paralyzed. If we damaged the internal capsule (supranuclear damage) only
the lower muscles of the face will be weakened (the upper will become
weak but to a much lesser extent).
NOTE:
- If there is a lesion in the midbrain affecting the corticospinal tract and the
occulomotor nerve, there will be contralateral hemiparesis (or paralysis)
and ipsilateral eye paralysis this is called cross hemiplasia.
Regards,
Jumana Abbadi 