motor 1: pyramidal system - 2

Question 1

Premotor cortex (PMC)
- anatomy

Answer 1

prefrontal & dorsal parietal sensory cortices > PMC > Primary motor cortex (M1)

• is located anterior to precentral sulcus, but only on LATERAL SURFACE of the hemispheres - is located inferior to supplementary motor cortex in middle & inferior frontal gyri
• is a middle-order association motor cortex (similar to supplementary motor cortex)
• contributes to pyramidal pathway

Question 2

Premotor cortex (PMC)
functional

Answer 2

• integrates sensory information into motor plans (sensorimotor transformation)
• Dorsal parietal (sensory) cortex is important in transformation of somatosensory & visual information into relevant motor commands
• anticipates voluntary movement, hence coordinates on-going movements
• reacts more to externally delivered cues and works more closely with the cerebellum

Question 3

Apraxia

Answer 3

Lesions of premotor & supplementary motor cortices DOES NOT lead to paresis, instead to aprexia
- (= inability to execute a voluntary motor movement despite being able to demonstrate normal muscle function; it also includes inability to imitate a movement)

Question 4

The primary motor system cortex (M1)

location

Answer 4

in the precentral gyrus, which is most caudal part of the frontal lobe & rostral to central sulcus
PFC > SMC/PMC>M1

Question 5

The primary motor system cortex (M1)

functional

Answer 5

• execution of movements/motor output (contributes to pyramidal pathway)
• contains the giant cells of Betz in layer V of the cortex (major source of pyramidal pathways) - is somatotopically organized as the motor homunculus (“little man”; see figure above)
• Motor homunculus: the size of cortical surface responsible for a part of the body is proportional to the degree of motor control exercised in that part

Question 6

Primary motor cortex and long descending motor pathways

Answer 6

§Pyramidal tract = corticonuclear + corticospinal tracts
§Corticonuclear(bulbar) tract modulates motoneurons of cranial nerves nuclei §Corticospinal tract modulates motoneurons in ventral horn of the spinal cord

Question 7

Tracing the pyramidal tract within the brain

Answer 7

§ The pyramidal pathway is part of the projection fibres
§ The projection fibers consist of efferent (exiting) and afferent (arriving) fibers connecting the cerebral cortex with the cerebral nuclei of the brain, brainstem, and the spinal cord

Question 8

Location of motor tracts:

Answer 8

• corticobulbar in posterior limb of IC

- corticospinal in posterior limb of IC

Question 9

The cortical origin of the pyramidal pathway

Answer 9

the pyramidal tract originates from the following different cortical areas:

• 40% from the primary motor cortex
• 40% from the supplementary and premotor cortices
• 20% from the primary sensory cortex (postcentral gyrus) of the parietal lobe
• In humans, the corticospinal tract contains about 1 million axons (fibres)

Question 10

Damage of the pyramidal tract

Answer 10

As already mentioned the pyramidal tract is composed of:
§40% from the primary motor cortex: lesion leads to paresis (muscle weakness)
§40% from the premotor cortices (SMC & PMC): lesion does NOT lead to paresis, instead to lack of skilled movements (apraxia)
§ 20% from the primary sensory cortex (postcentral gyrus) of the parietal lobe:
- synapses with interneurons and neurones of posterior horn of the spinal cord to
modulate incoming sensory information that could potentially affect the ongoing action of the pyramidal tract
- lesion does NOT lead to paresis, instead to degeneration (disturbance) of motor actions

Question 11

Pathway of the lateral corticospinal tract

Answer 11

§Most fibers originate from the motor cortex and terminate in the ventral horn of the spinal cord (a-motoneurons)
§Constitute a significant part of the corticospinal tract (~ 90%)
§Primarily concerned with precise movements involving distal parts of the limbs (mostly hand)
§terminate at the cervical & lumbosacral levels to Innervate upper & lower limbs

Question 12

Pathway of the anterior corticospinal tract

Answer 12

§Constitutes only 10% of the corticospinal tract
§Do not decussate at the pyramidal decussation
§decussates at different spinal cord levels §innervates mainly axial (neck & trunk) muscles

Question 13

THE LATERAL CORTICOSPINAL TRACT

Answer 13

(90%):

• innervates the limbs (distal more than proximal)
• therefore, it targets α-motoneurons that feed intothe brachial & lumbosacral plexuses

Question 14

THE ANTERIOR CORTICOSPINAL TRACT

Answer 14

(10%)
- innervates axial muscles (neck, thoracic & abdominal)
- therefore, it synapses with α-motoneurons that are
located at spinal levels C1-C4 and T2-T12

Question 15

Diseases affecting the motor system: Poliomyelitis

Answer 15

1. Poliomyelitis (infantile paralysis) : motoneurones of the anterior horn are
   specifically affected by polio virus

Question 16

Diseases affecting the motor system: Amyotrophic Lateral Sclerosis (ALS

Answer 16

2. Amyotrophic Lateral Sclerosis (ALS), motor neuron disease, Lou Gehrig’s disease: - fatal, progressive neurodegenerative disease
   - degeneration of cortical and spinal motoneurons
   - genetic and sporadic forms
   - leads to muscle weakness and atrophy

Question 17

Diseases affecting the motor system: Stroke (ischemic & Hemorrhagic) - UL

Answer 17

§Upper limbs:
- adducted posture at the shoulder
- a flexed posture at the elbow and wrist - flexed fingers into the palm
(hence hand muscles more affected = loss of skilled movement)

Question 18

Diseases affecting the motor system: Stroke (ischemic & Hemorrhagic) - LL

Answer 18

§Lower limbs:

• hip and knee extension
• slight circumduction at hip joint - plantar flexion at the ankle

Question 19

Babinski Sign (reflex)

Answer 19

dorsal extension of the big toe and fanning of little toes after stimulating the lateral surface of the sole.