NB7-4 - Corticospinal and Corticobulbar Fibers and DLAs

Question 1

Answer 1

A

Question 2

What is the anterior paracentral lobule? What is it responsible for? What is its blood supply?

Answer 2

The anterior paracentral lobule is the medial portion of the precentral gyrus that is responsible for the motor activity of the lower limbs. Its blood supply comes from the anterior cerebral artery.

Question 3

From where within the primary motor cortex do the upper motor neurons (UMNs) receive their information?

Answer 3

From the pyramidal cells directly or indirectly (via interneurons) in layer 5 (internal pyramidal layer) or the primary motor cortex.

Question 4

From what other regions of the brain will the primary motor cortex receive input? Describe the significance of this input.

Answer 4

1. The limbic association cortex is what causes movements based upon emotion. It will first input to the prefrontal cortex which will allow/disallow an action based upon the emotion. If allowed, the prefrontal cortex will signal the premotor cortex, which will find the instrutions for how to perform the action and then signal the primary motor cortex.
2. The visual cortex will communicate with the primary motor cortex to coordinate movements based upon what is seen. It will first input to the posterior parietal cortex which will integrate the sensory information for motor planning in concert with the premotor cortex.
3. Primary somatosensory cortex allows for actions to be taken based upon somatosensory input
4. Basal ganglia and Cerebellum via the thalamus to initiate and modulate movement

Question 5

Answer 5

Question 6

List the Brodman areas we need to know and what they do.

Answer 6

• 1,2, & 3 - Somatosensory cortex
• 4 - Primary motor cortex
• 5 & 7 - Posterior parietal cortex (integrates sensory information for motor planning in concert with the premotor cortex)
• 17 - primary visual cortex
• 22 - Wernicke’s area (language comprehension)
• 44 & 45 - Broca’s area (pars opercularis and pars triangularis respectively) for language production

Question 7

List the different sections of the premotor cortex, any other names they may have, and their general functions.

Answer 7

• The medial premotor area, or supplementary area, is involved in planned actions and mental rehearsal. For example, if you just decided, unprompted, to clap and then started clapping.
• The lateral premotor area is involved with sensory guided movements (all types of sensation). For example, if someone told you to clap and then you clapped

Question 8

Draw out a horizontal section of the caudate nucleus, globus pallidus, thalamus and internal capsule. Indicate the general layout of fibers within the internal capsule.

Answer 8

Question 9

Which fibers cross at the caudal end of the medulla? What tract is formed after the decussation of these fibers?

Answer 9

Most (~90%) of the corticospinal fibers decussate to form the lateral corticospinal tract

Question 10

What do the corticospinal fibers that don’t decussate become?

Answer 10

The anterior corticospinal tract

Question 11

What are the origins of the two predominant fiber types in the lateral corticospinal tract? Where do these fibers synapse? Which neurons do they synapse upon? What is the point of this?

Answer 11

The lateral corticospinal tract fibers originating from the primary motor cortex will synapse on LMNs in the ventral horn to recruit muscle activity.

The lateral corticospinal tract fibers originating from the primary somatosensory cortex will synapse on sensory afferents in the dorsal horn to inform the brain about whether an action is being carried out correctly

Question 12

Answer 12

A

Question 13

What are the general clinical features of an UMN syndrome?

Answer 13

• Hyperreflexia
• Babinski Sign
• Clonus (repeated muscular spasms)
• If there is bilateral damage, none of the above symptoms will be seen. Instead, the patient will have spinal shock which is a loss of sensation and motor paralysis. Over time, some sensation and motor ability will return and the previous symptoms will become evident.

For the unilateral symptoms, they will be contralateral if the lesion occurs rostral to the decussation and ipsilateral if the lesion occurs caudal to the decussation.

Question 14

Answer 14

F

Question 15

What are the general clinical features of a LMN syndrome?

Answer 15

• Hyporeflexia
• Hypotonia
• Fasciculations (visible muscle twitches)
• Fibrillations (invisible muscle twitches)
• Profound atrophy/muscle wasting

Signs are always ipsilateral to the lesion

Question 16

Answer 16

C

Lesions in the right crus cerebri would cause symptoms in more than just the lower limbs

Question 17

What is Brown Sequard syndrome? What major clinically significant tracts does the syndrome affect?

Answer 17

Brown Sequard syndrome results from a lesion to the right or left half of the spinal cord. It most notably affects the corticospinal, spinothalamic, and dorsal column-medial lemniscus (DCML) pathways.

Question 18

List the motor symptoms seen with Brown Sequard Syndrome.

Answer 18

UMN Syndrome symptoms are seen ipsilateral and below the level of the lesion because just the UMNs are affected for those spinal levels.

LMN Syndrome symptoms will be seen ipsilaterally at the levels of the lesion because the ventral horns are affected at those levels.

Question 19

What is and what causes paraplegia? What are its clinical features?

Answer 19

Paralysis of the legs and lower body typically caused by a bilateral spinal cord injury. Its clinical features are:

• Spinal shock for the first 4 weeks
• Loss of all somatosensory perception below the lesion which will also lead to urine retention and leakage
• When motor capability begins to return, UMN syndrome symptoms will present

Question 20

Describe the types of symptoms seen with an internal capsule infarction. Be sure to include the exremely evident symptoms.

Answer 20

Contralateral UMN syndrome symptoms will be seen. Most notable symptoms are:

• Contralateral hemiplegia (paralysis of one side of the body)
• Contralateral deviation of tongue (so away from lesion but towards hemiplegia)
• Ipsilateral Gaze Palsy
• Contralateral paralysis of lower facial muscles

Question 21

What are the rules for facial paralysis as it pertains to UMN and LMN lesions? Why?

Answer 21

The facial nucleus has a section that innervates the upper face and a section that innervates the lower face. The upper face sections supply neurons to both sides of the upper face while the lower sections only supply their ipsilateral sides. Because of this, upper spares upper. UMN syndromes will only affect the motor to the lower half of the face while lesions of the facial nerve (LMN) will affect the entire face

Question 22

Describe the differences in central and peripheral paralysis of the genioglossus muscle.

Answer 22

A lesion of this muscle’s UMNs causes paralysis of the contralateral muscle which causes a contralateral (away from lesion) deviation of the tongue.

A lesion of this muscle’s LMNs causes paralysis of the ipsilateral muscle which causes a ipsilateral deviation of the tongue.

Question 23

Which cranial nerves decussate?

Answer 23

CN2 - Optic

CN4 - Trochlear

CN7 - Facial

CN12 - Hypoglossal

Question 24

Describe what gaze palsy is and what causes?

Answer 24

A gaze palsy is a horizontal deviation of the eyes caused by some kind of central lesion (eg - to frontal eye field or internal capsule). The gaze is always towards the side of the lesion.

Question 25

Answer 25

C

Question 26

List the extrapyramidal pathways.

Answer 26

Rubrospinal

Lateral Reticulospinal

Medial Reticulospinal

Lateral Vestibulospinal

Medial Vestibulospinal

Question 27

What are the flexor and extensor biased pathways?

Answer 27

The ruburospinal and lateral reticulospinal pathways are flexor biased.

The medial reticulospinal, lateral vestibulospinal, and medial vestibulospinal pathways are extensor biased

Question 28

Which of the extramedullary tracts receive cortical input?

Answer 28

• Rubrospinal
• Lateral Reticulospinal
• Medial Reticulospinal

Question 29

Describe the route of the rubrospinal tract, including any input it receives. Describe this tracts location and where it synapses in the spinal cord. What is this tract’s purpose and which neurons does it synapse on?

Answer 29

The rubrospinal tract originates in the red nucleus of the midbrain where it also receives ipsilateral cortical input. It immediately decussates before descending on the contralateral side of the spinal cord. This tract only spans the cervical levels of the spinal cord. This tract is found lateral to the lateral horn and will synapse on alpha and gamma motor neurons in the dorsal aspect of the ventral horn. This tract serves to facilitate flexion in the upper limbs.

Question 30

Describe the route of the lateral reticulospinal tract, including any input it receives. Describe this tracts location and where it synapses in the spinal cord. What is this tract’s purpose and which neurons does it synapse on?

Answer 30

The lateral reticulospinal tract (LRST) originates at the medullary reticular nucleus where it also receives bilateral cortical input. Some of these fibers will immediately decussate while others will not before they bilaterally descend down the spinal cord. This tract is located just ventral to the ventral horn and synapses on interneurons in the dorsal aspect of the ventral horn which then synapse on alpha and gamma motor neurons. The major function of this pathway is to powerfully suppress extensor spinal reflex activity and facilitate contraction of flexors.

Question 31

What is the LRST aka and why?

Answer 31

The Medullary Reticulospinal Tract because it originates in the medullary portion of the reticular formation unlike the medial reticulospinal tract (MRST or pontine reticulospinal tract) which originates in the pontine portion

Question 32

Describe the route of the medial reticulospinal tract, including any input it receives. Describe this tracts location and where it synapses in the spinal cord. What is this tract’s purpose and which neurons does it synapse on?

Answer 32

The medial reticulospinal tract (MRST) originates at the pontine reticular nucleus where it also receives bilateral cortical input and input from the spinoreticular tract. These fibers will not decussate and will descend down the ipsilateral spinal cord. This tract is located just medial to the ventral aspect of the ventral horn and synapses on gamma motor neurons in the medial ventral horn to activate extensor muscles.

Question 33

Describe the route of the spinoreticular tract. Describe this tracts location and where it synapses. What is this tract’s purpose?

Answer 33

These fibers are collateral axons from the neurons of the anterolateral system (making this a sensory tract). These fibers will synapse in the reticular nuclei of the pons and medulla. The major function of this tract is to relay sensory information to reticular nuclei about pain, temperature, and crude touch in the truck and limbs on the contralateral side. It exerts an excitatory infulence predominantly on the pontine reticular nuclei to facilitate contraction of the extensor muscles of the limb and trunk.

Question 34

Describe the route of the lateral vestibulospinal tract, including any input it receives. Describe this tracts location and where it synapses in the spinal cord. What is this tract’s purpose and which neurons does it synapse on?

Answer 34

The lateral vestibulospinal tract arises from the lateral vestibular nuclei in the pons and medulla where they’ll receive additional input from the inner ear and cerebellum. This tract does not decussate and descends on the ipsilateral side of the spinal cord. It is the most medial and ventral tract in the spinal cord. It synapses on alpha motor neurons for extensor muscles that regulate balance.

Question 35

Describe the route of the medial vestibulospinal tract, including any input it receives. Describe this tracts location and where it synapses in the spinal cord. What is this tract’s purpose and which neurons does it synapse on?

Answer 35

The medial vestibulospinal tract arises from the medial vestibular nuclei in the pons and medulla where they’ll receive additional input from the inner ear and cerebellum. This tract does not decussate and descends on the ipsilateral side of the spinal cord. This tract is only seen in the cervical and upper thoracic sections of the spinal cord. It is located very medially and just ventral to the anterior white commisure. It synapses on alpha motor neurons responsible for some neck and eye movements to coordinate the movement of the head and eyes (important for balance). This tract is extensor biased.

Question 36

Why is alpha-gamma co-activation important?

Answer 36

Without simultaneous activation, the muscle spindle will either become slack (no gamma activation) which will make it unable to detect muscle length changes, or it will become too taught (no alpha activation) which will send incorrect information about muscle length changes to the CNS.

Question 37

Which spinal tracts are responsible for alpha-gamma coactivation and which tracts only activate either alpha or gamma?

Answer 37

Co-activation of the alpha & gamma neurons is mostly done by the corticospinal tract (also lateral reticulospinal and rubrospinal)

Individual activation of the alpha motor neurons is done by the lateral vestibulospinal (also medial vestibulospinal in head and neck)

Individaul activation of the gamma motor neurons is done by the medial reticulospinal tract.

Question 38

What are decorticate and decerebrate posturing?

Answer 38

Decorticate Posturing - upper limbs flexed at elbow and wrist; lower limbs extended and internally rotated

Decerebrate Posturing - upper limbs and lower limbs are extended

Refer to image

Question 39

What information can be immeditaely garnered from observing that a patient is in a decorticate or decerebrate posture? Why do we know this?

Answer 39

If in a decorticate posture, a patient likely has a brainstem lesion above the red nucleus. This posture is caused by a lesion to the corticospinal and corticobulbar tracts only. This lesion can only occur above the red nucleus.

If in a decerebrate posture, a patient likely has a brainstem lesion below the red nucleus. This posture is caused by a lesion to the corticospinal, corticobulbar, and rubrospinal tracts only. This lesion can only occur below the red nucleus

Question 40

Why does a lesion to the corticospinal and corticobulbar tracts cause a decorticate posture?

Answer 40

When these tracts are lesioned, only the LVST, MRST, LRST, and rubrospinal tracts remain.

The rubrospinal tract causes strong flexion of the upper limbs as does the LRST (flexion of all extremeties). The extensor activation of the the LVST and MRST cannot overcome this, hence upper extremity flexion.

The rubrospinal tract does not extend into the lower limbs so here the LVST and MRST can overcome the flexion activation of the LRST, hence lower extremity extension.

Question 41

Why does a lesion to the corticospinal, corticobulbar, and rubrospinal tracts cause a decerebrate posture?

Answer 41

When these tracts are lesioned, only the LVST, MRST, LRST remain.

The flexor activity of the LRST cannot overpower the extensor activity of the MRST and LVST, hence extension of the upper and lower limbs.