Lecture 5 - Overview of Motor Pathways

Question 1

how many fibres cross at the pyramidal decussation before entering the spinal cord?

Answer 1

about 85%

Question 2

what is the pyramidal decussation?

Answer 2

the dividing line between medulla and spinal cord

Question 3

where do the fibres go below the decussation?

Answer 3

they descend to the appropriate level of the spinal cord, enter gray matter and synapse onto dendrites of LOWER MOTOR NEURONS in the anterior horn.

Question 4

what do the axons of LMNs form

Answer 4

anterior roots (GSE) and join spinal nerves to innervate skeletal muscles

Question 5

what is the corticospinal pathway

Answer 5

the pathway providing voluntary motor function.

in the pre central gyrus (location of motor cortex)

axons extend from UMNs to into the posterior limb of the internal capsule

and enter the pyramidal decussation where 85% cross,

then they trace within the white matter of spinal cord forming the lateral cotrticospinal tract

that innverates the LMNs to innervate skeletal muscle

Question 6

what are the 3 anatomical divisions of the internal capsule

Answer 6

the anterior limb

the genu (“knee”)

and the posterior limb contains the descending axons of UMNs.

Question 7

why is it important that so many axons of the UMNs are packed into such a small space in the posterior limb of the internal capsule?

Answer 7

Even a small lesion could damage many axons.

Question 8

where do the axons go once they leave the internal capsule?

Answer 8

the midbrain via the crus cerebri on the superficial anterior surface

then they dive deep in the pons in the basis pontis

then into madulla on the anterior surface forming the medullary pyramids

Question 9

where do the 85% of the fibres that cross over go

Answer 9

the lateral corticospinal tract in the lateral funiculus of spinal cord white matter

Question 10

where do the 15% of fibres that do not cross at the pyramidal decussation go

Answer 10

they descend in the anterior/medial corticospinal tract in the anterior funiculus and then travel to the target level of spinal cord then innervate LMNs

Question 11

what are Lateral motor nuclei

Answer 11

in the anterior horn of grey matter in spinal cord

receive ipsilateral innervation from fibres in the lateral corticospinal tract) innervate limb muscles

Question 12

what are medial motor nuclei

Answer 12

in the anterior horn of grey matter in spinal cord

receive mostly contralateral innervation from fibres in the anterior corticospinal tract) innervate trunk muscles.

Question 13

do all of the fibres in the anterior corticospinal tract not cross over?

Answer 13

90% of them end of crossing over so only a small percent actually stays on the ipsilateral side

Question 14

where do 90% of the anterior cortiospinal fibres cross?

Answer 14

the anterior white commissure

same spot as pain and temperature crossing

Question 15

How do UMNs reach the LMNs that innervate the muscles of the head and neck?

Answer 15

the corticobulbar tract.

they travel together with the other UMN, but through the genu instead of the posterior limb

Question 16

where do the UMNs of that innervate the head and neck meet their LMN parters?

Answer 16

instead of going into the decussation and into the spinal cord, they find their LMN partners in certain cranial nerve nuclei

Question 17

where are muscles in the body derived by

Answer 17

Most skeletal muscle of the body (including muscles that move the eye and tongue) are derived from myotomes (GSE)

Some skeletal muscles in the head & neck are derived from branchial/pharyngeal (gill) arches. (SVE)

Question 18

what is the Trigeminal motor nucleus

Answer 18

in pons

CN V, muscles of mastication, SVE

Question 19

what is the Facial motor nucleus

Answer 19

pons

CN VII, muscles of facial expression, SVE

Question 20

what is the Nucleus ambiguus

Answer 20

in medulla

Glossopharyngeal (CN IX) & Vagus (CN X) – muscles of the pharynx and larynx (swallowing, speech), SVE

Question 21

what is the Hypoglossal nucleus

Answer 21

in medulla

CN XII, muscles of the tongue, GSE

Question 22

what is the Spinal accessory nucleus

Answer 22

cervical spinal cord, anterior horn
CN XI, trapezius & sternocleidomastoid, SVE

Question 23

is the innervation of these cranial nerve motor nuclei (LMNs) by corticobulbar fibres typically unilateral or bilateral

Answer 23

bilateral. every nucleus receives input from both sides of the brain

Question 24

what is the exception to the bilateral aspects of the nucleus

Answer 24

neurons in the CN VII (facial) motor nuclei responsible for innervation of the lower part of the face receive is only unilateral from contralateral (crossed) fibres.

Question 25

what is the difference between pyramidal systems and extrapyramidal systems

Answer 25

pyramidal system subserves volitional movements, whereas the extrapyramidal systems are automatic

UMNs are not in the cortex but in brainstem!

Question 26

what are the 3 cranial nerves (and corresponding nuclei) involved with eye movements (all GSE):

Answer 26

CN III oculomotor nerve (upper midbrain)

CN IV trochlear nerve (lower midbrain)

CN VI abducens nerve (lower pons)

Question 27

are the neurons of the oculomotor, trochlear and abducens nuclei innervated by neurons in the primary motor cortex.

Answer 27

no

Other cortical areas like the frontal lobe and the parieto-occipito- temporal area are cortical regions associated with volitional eye movements.

Question 28

is the primary motor cortex is the actual start of volitional motor control.

Answer 28

no

“Motor association” areas lie just anterior to the primary motor cortex. which are involved in higher motor planning.

-supplementary motor area
-premotor cortex