Anatomy Of Pyrimidal System

Question 1

What are upper motor neurons?

Answer 1

whose cell bodies lie in the higher motor centers in the brain and brain stem, and their axons constitute the descending motor pathways.

Question 2

What are Lowe motor neurons?

Answer 2

• whose cell bodies lie in the spinal ventral horns or the corresponding cranial motor nuclei , and include both a-and y-MNs
• Axons of the lower motor neurons proceed through the peripheral somatic nerves to innervate skeletal muscles.

Question 3

Upper motor neurons control?

Answer 3

Are the descending supraspinal pathways that influence the activity of the LMN

1-voluntary motor activity
2-maintenance of posture & equilibrium
3- muscle tone and
4- reflex activity
• generally exert their effect on groups of muscles ( not on one specific muscle )
reciprocally on agonist and antagonist muscle group

Question 4

Functions of lower motor neurons?

Answer 4

Are motor neurons that innervate the voluntary muscles located in
1-anterior grey column of the spinal cord, and
2- motor nuclei of brainstem form the final common pathway

Question 5

Pyramidal tracts?

Answer 5

Originates from the cerebral cortex and descends to the spinal cord (the corticospinal tract) passes through the pyramids of the medulla

Question 6

Extrapyramidal tracts?

Answer 6

The rest of the descending motor pathways do not travel through the medullary pyramids

Question 7

Name descending motor tracts?

Question 8

Corticobulbar tract origin?

Answer 8

• The corticobulbar tracts arise from the lateral aspect of the primary motor cortex
• They receive the same inputs as the corticospinal tracts
• The fibers converge and pass through the internal capsule to the brainstem.

Question 9

The corticobulbar tract terminate in the motor nuclei of?

Answer 9

” The neurons terminate on the motor nuclei of the crani nerves. Here, they synapse with lower motor neurons, whic carry the motor signals to the muscles of the face and neck.

1. 3rd and 4th cranial nerves in the midbrain
2. 5th, 6th, 7th and 8th cranial nerves in the pons
3. 9th, 10th, 11th, and 12th cranial nerves in the medulla

Question 10

Course of corticobulbar tract?

Answer 10

Corticobulbar tract from one side of the brain terminates mostly in the cranial motor nuclei of both sides of the brain stem (Many of these fibers innervate the motor neurons bilaterally)
- Except, the lower part of the facial nerve nucleus, and the hypoglossal nerve nucleus receive only contralateral innervation from the cerebral

Question 11

Function of corticobulbar tract?

Answer 11

Involved in control of facial and jaw musculature, swallowing and tongue movements
control vol. movement of muscles of larynx, pharynx, palate, face, jaw and eyes.

Question 12

Function of corticospinal tract?

Answer 12

The corticospinal tracts are particularly concerned with the control of voluntary, discrete, skilled movements, especially those of the distal parts of the limbs.

Question 13

Origin of corticopinaltract?

Answer 13

Corticospinal tract neurons arise from cell bodies in the cerebral cortex
• It originate from
1. PYRAMIDAL CELLS in layer V of the cerebral cotex.
2. PRIMARY MOTOR CORTEX
3. SUPPLEMENTARY MOTOR AREA
4. PREMOTOR CORTEX
5. SOMATOSENSORY CORTEX
6. PARIETAL LOBE
7. CINGULATE GYRUS
- The cells of origin are widely distributed in the motor and sensory cortices, including the precentral gyrus or primary motor cortex.

Question 14

Course of corticospinal tract?

Answer 14

• The nerve axons travel from the cortex through the posterior limb of internal capsule (located between the thalamus and the basal ganglia)
• The neurons pass through the crus cerebri of the midbrain, the pons and into the medulla.
- The axons form two prominences called the pyramids of the medulla.

Question 15

What is the internal capsule?

Answer 15

Bundle of projection fibers, passes through the interval between the
thalamus and the basal ganglia

Question 16

Parts of the internal capsule?

Answer 16

• Has 5 parts:
1. Anterior limb: Thalamocortical &
Frontopontine fibers
2. Genu: corticobulbar fibers
3. Posterior limb: Corticospinal, Corticobulbar & Thalamocortical fibers
4. Retrolenticular part:
Geniculocalcarine fibers
5. Sublenticular part (not shown):
geniculo-temporal fibers

Question 17

Parts of the corticospinal tract?

Answer 17

In the most inferior part of the medulla, the tract divides into two:

1. lateral corticospinal tract
   - fibers decussate (cross over to the other side), resulting in muscles being controlled by the opposite side of the brain
   - They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels).
   Note: About 75-90% of fibers decussate and enter the contralateral lateral corticospinal tract, which is located in the lateral part of the spinal white matter
2. The anterior corticospinal tract
   - fibers remain ipsilateral, descending into the spinal cord
   - They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels
   Note: 10-25% of pyramidal fibres remain ipsilateral and enter the ventral corticospinal tract located lateral to the ventral median fissure.

Question 18

Describe the lateral corticospinal tract?

Answer 18

• 80-90% of fibers
• Descends contralaterally in the lateral column of the opposite side)
Q Along the whole length of the spinal cord.
O Terminate gradually on the laterally situated neurons on the ventral horn directly or more common
indirectly through
interneurons.

Question 19

Describe ventral corticospinal tract?

Answer 19

• Remaining 10% of fibers
• Descends ipsilaterally in the ventral column on the same side.
• Till the mid-thorathic region only
• Terminate gradually by crossing at various levels of the spinal cord to terminate on the medially-situated neurons directly or more common
indirectly through interneurons
• control axial muscles in both sides

Question 20

Termination of corticospinal tract?

Answer 20

Termination :-
Cervical region - 55%
Thoracic region - 20%
Lumbosacral region - 25%

Question 21

Significance of corticospinal tracts?

Answer 21

Significance -
• Lateral & Anterior Corticospinal Tracts indicates that certain muscles in the body must have bilateral control from both cerebral hemisphere e.g -
Respiratory muscles.

Question 22

Role of corticobulbar system in initiation of voluntary movements?

Answer 22

This is particularly mediated by the CBS tract fibers that join the lateral motor system. These fibers include:
1) The corticobulbar tract fibers that end in: (i) The lower part of the facial nerve motor nucleus which supplies the muscles of the lower part of the face, and (il) The hypoglossal nucleus that innervate the tongue muscles. These facial and tongue muscles are concerned with highly skilled movements such as those involved in the act of speech.

Question 23

Time of corticospinal tract on initiation of voluntary movements?

Answer 23

The lateral corticospinal tract fibers that descend in the spinal cord for control of muscles of the distal parts of the limbs, especially the hand and digits muscles, which subserve fine skilled movements used in manipulation by hand and fingers, and other accurate motor actions done by the limbs.

Question 24

Role of corticobulbar system in automatic and postural movements?

Question 25

Role of corticobulbar system in facilitations of the muscle tone?