Module 2 - corticospinal tracts

Question 1

Where is Rexed’s laminae IX, and what is its function?

Answer 1

It makes up the majority of the anterior horn of gray matter and gives rise to motor neurons. The medial aspect of the anterior horn contains axons that innervate proximal muscles, while the lateral aspect contains axons that innervate distal muscles.

Question 2

How many spinal segments are there, and how many vertebrae are in each region? Where does the SC end?

Answer 2

31 spinal segments.

7 cervical (8 cervical nerves), 12 thoracic, 5 lumbar, 5 sacral, 1 coccyx.

The SC ends at L1-L2 vertebrae.

Question 3

Define the conus medullaris, cauda equina, and filum terminale.

Answer 3

Conus medullaris: the tip of the spinal cord.

Cauda equina: from L2 and below, a bundle of nerves like a ‘horse’s tail’

Filum terminale: a thin continuation of pia mater connecting to the base of the coccyx that holds the SC in place.

Question 4

Where do cervical nerve roots exit?

Answer 4

Above the associated vertebrae. Since there are 7 cervical vertebrae and 8 cervical nerves, C8 exits below C7 and above T1.

Question 5

Where do thoracic and lumbar nerve roots exit?

Answer 5

Below the associated vertebrae.

Question 6

Where is the cervical enlargement?

Answer 6

From C3-T1 segments. It has a larger proportion of white matter, as there is more communication with the brain higher up in the spinal cord.

Question 7

Where is the lumbosacral enlargement?

Answer 7

From L1-S3 segments. It has a larger proportion of gray matter, as there is more communication with the periphery.

Question 8

What is a myotome?

Answer 8

All the muscles innervated by a single spinal cord segment.

Question 9

What are the clinically relevant nerve roots for the arm?

Answer 9

C5: innervates the shoulder muscles.
C5 + C6: innervates elbow flexors.
C6: innervates wrist extensors.
C7: innervates elbow extensors.

Question 10

What are the clinically relevant nerve roots for the leg?

Answer 10

L4: innervates knee extensors.
L5: innervates ankle dorsiflexors.
S1: innervates ankle plantar flexors.

Question 11

What are lower motor neurons (LMNs)?

Answer 11

Neurons that project from the spinal cord to the periphery. They are the final common pathway from the CNS to the muscle.

Question 12

Where are LMNs located?

Answer 12

In the anterior horn of the SC, also in cervical and thoracic regions.

Question 13

What are the symptoms of LMN lesions?

Answer 13

1. Weakness
2. Fasciculations
3. Atrophy
4. Hyporeflexia
5. Hypotonia (AKA flaccid paralysis)

Question 14

What is seen in high steppage gait?

Answer 14

Happens when L5 neurons are damaged (dorsiflexors).

1. Excessive knee flexion ipsilateral to the injury to avoid dragging of the foot
2. lack of dorsiflexion during the swing phase, so the toe hangs down
3. Foot lands toe first and sets down carefully.

Question 15

What are the causes of LMN lesions?

Answer 15

Injury or disease that affects motor neurons in the anterior horn, ventral nerve roots, or spinal nerves.

Question 16

What is polio?

Answer 16

A virus that attacks cell bodies in the anterior horn (LMN symptoms). Primarily affects muscles in the lumbar region.

Question 17

How can spinal cord damage cause LMN damage?

Answer 17

If disc herniations impinge on ventral nerve roots.

If a tumor grows into the anterior horn.

**There will only be LMN symptoms AT the level of the injury.

Question 18

How do ventral roots and spinal nerves get injured?

Answer 18

Through compression, traction, laceration, or entrapment.

Question 19

Where are upper motor neurons (UMN) located, and what do they do?

Answer 19

Cell bodies are in the motor regions of the cortex. They synapse onto LMNs.

Question 20

Where is the primary motor cortex located (PMC)? What is the somatotopic organization?

Answer 20

The precentral gyrus.

Leg areas are most medial, face areas are most lateral. Think of a person lying on their back with their arms out.

Question 21

What are corticospinal tracts?

Answer 21

Bundles of UMN axons from the motor cortex travelling to the SC.

Question 22

Recite the path of the corticospinal/corticobulbar tracts from the cortex to the spinal cord.

Answer 22

Corona radiata -> posterior limb of the internal capsule -> basis pedunculi in the midbrain -> medullary pyramids -> cross the midline at the pyramidal decussation -> SC

Question 23

What is the function of the lateral corticospinal tract, and which side(s) of the body does it control?

Answer 23

Controlling the periphery, primarily fine motor control of distal muscles.

The neurons control contralateral sides of the body.

Question 24

Where does the lateral corticospinal tract enter, and where does it terminate?

Answer 24

It enters into the lateral column of the SC. It terminates at the spinal enlargements (cervical and lumbosacral).

Question 25

What are the components of the internal capsule?

Answer 25

Anterior limb, genu, and the posterior limb.

Question 26

What is the function of the anterior corticospinal tract, and which side(s) of the body does it control?

Answer 26

It primarily controls axial muscles (neck and trunk). It controls bilaterally, as the axon bifurcates at the spinal level it wants to innervate. Not all axons bifurcate, but most do.

Question 27

How do the anterior corticospinal tract axons cross the midline of the SC?

Answer 27

Via the anterior commissure.

Question 28

What are corticobulbar tracts?

Answer 28

Tracts that originate in the cortex and go to the brainstem.

Question 29

How many cranial nuclei are there? What do they give rise to?

Answer 29

12. They give rise to the 12 cranial nerves.

Question 30

Which cranial nerve is considered the 'facial nerve'?

Answer 30

Cranial nerve VII.

Question 31

Where do facial nuclei originate?

Answer 31

In the pons.

Question 32

Which side(s) of the body does the facial nerve control? How does this change between the upper and lower face?

Answer 32

The facial nerve controls ipsilateral muscles. For muscles above the eyes, the UMN input is bilateral to the facial nerve. For muscles below the eyes, UMN input is contralateral to the facial nerve.

Question 33

Where is the injury located if there is weakness on the entire side of the face?

Answer 33

There is damage to the facial nuclei on the side ipsilateral to the weakness. There will be LMN symptoms.

Question 34

Where is the injury located if there is weakness just on the lower side of the face?

Answer 34

There is an injury to the UMNs contralateral to the weakness. There is some preserved function in the upper face, as there is bilateral input to the upper face, and the other UMNs are still intact.

Question 35

What is Bell's Palsy?

Answer 35

A condition with an unknown cause that attacks the facial nerve. Damage is to the entire ipsilateral side of the face. Symptoms subside after 2-3 wks, but it takes 3-6 months to fully recover.

Question 36

How does the somatotopic organization of the CS and CB tracts change as they descend to the SC?

Answer 36

1. In the cortex, the head areas are most lateral, and the legs are most medial. 2. When the tracts reach the internal capsule, they rotate so the face becomes most anterior, and the legs become most posterior. 3. At the midbrain, they have fully rotated so the face is most medial and the legs are most lateral. 4. The tracts get off in order. Face at the pons, arms at the cervical region, trunk at the thoracic region, and legs at the lumbar region.

Question 37

What is the function of the rubrospinal tract, and where does it travel?

Answer 37

It travels from the red nucleus in the midbrain to the lateral column of the spinal cord, and it terminates at the cervical level. It has contralateral arm control, facilitates flexor muscle tone of the arms, and inhibits extensor tone of the arms.

Question 38

What is the function of the vestibulospinal tract, and where does it travel?

Answer 38

It travels from the vestibular nuclei in the pons and medulla and down through the anterior column of the SC. It is important for arm and leg tone, and neck + balance reflexes.

Question 39

What is the function of the lateral vestibulospinal tract, and where does it travel?

Answer 39

It travels from the lateral vestibular nucleus in the pons, and goes ipsilaterally down to all levels of the SC. It facilitates extensor muscles tone of arms and legs, and inhibition of flexor muscle tone of arms and legs.

Question 40

What is the function of the medial vestibulospinal tract, and where does it travel?

Answer 40

It splits immediately from the medial vestibular nuclei, and runs bilaterally down to the cervical and thoracic levels of the SC. It controls the tone of neck and axial muscles.

Question 41

What is the function of the reticulospinal tract, and where does it travel?

Answer 41

It travels from the reticular formation (central brainstem) and ipsilaterally down through the anterior column of the SC to all levels. It modulates general muscle tone and reflexes (i.e. the startle reflex).

Question 42

What are UMN lesions?

Answer 42

Injury or disease affecting upper motor nuclei in the motor cortex, or axons descending through the brain, brainstem, or SC.

Question 43

Depending on where the injury is, where will you see symptoms?

Answer 43

Lateral CST lesions: symptoms in distal muscles. Cortical lesions: symptoms will be seen contralateral to the injury. Lesions past the cervicomedullary junction: symptoms will be ipsilateral to the injury.

Question 44

What are the symptoms of UMN lesions?

Answer 44

1. Weakness 2. Hyperreflexia 3. Hypertonia 4. Positive Babinski's sign

Question 45

What is decorticate posture?

Answer 45

It happens when the lesion is above the midbrain. It involves the rubrospinal tract, so there a net flexion effect. You will see upper body flexion and lower body extension. If the damage is on one side, the symptoms will be contralateral to the lesion.

Question 46

What is decerebrate posture?

Answer 46

It happens when the lesion is below the midbrain. It involves the vestibulospinal tract, so there a net extension effect. You will see upper body extension and lower body extension. If the damage is on one side, the symptoms will be contralateral to the lesion.

Question 47

What is hemiplegic gait?

Answer 47

When someone is walking with decorticate posture. One arm is in flexion and there is circumduction of the extended leg.

Question 48

What are the causes of UMN lesions?

Answer 48

1. Injury to the motor cortex where the cell bodies are 2. Damage to the internal capsule or cerebral peduncles 3. SC lesion or compression 4. Diseases attacking UMNs

Question 49

What is PLS?

Answer 49

A disease with an unknown origin that attacks UMNs specifically. It primarily affects cells in the motor cortex. It is progressive, but non-fatal, and there is not cure.

Question 50

What is ALS?

Answer 50

AKA Lou Gehrig's disease. It involves the degeneration of UMNs and LMNs. The origin is potentially genetic, but otherwise unknown. It results in problems with breathing, swallowing, and speaking. It is fatal in 2-4 years. LMN symptoms: 1. Atrophy 2. Fasciculations UMN symptoms: 1. Hypertonia 2. Hyperreflexia

Question 51

What is the deep tendon reflex scale?

Answer 51

0 – absent reflex 1 – decreased but present 2 – normal 3 - Brisk but excessive 4 – Non-sustained Clonus 5 – Sustained Clonus

Question 52

What is the muscle strength scale?

Answer 52

0 – no movement 1 – Slight contraction, no movement 2 – movement possible, without gravity 3 - movement possible, with gravity 4 – movement possible, some resistance 5 – movement possible, full resistance