Motor Systems III

How can UMNs influence movements?

By affecting the activity of local circuit neurons and LMN

Where are the motor control centers in the brain stem and what do they control?

Vestibular nuclei - body position

Reticular formation - body position, posture, tone

Red nucleus - arm movements

Superior colliculi - head and eye movements

Where are the motor control centers in the cortex and what do they control?

Motor area - precise control of complex movements

Premotor & supplementary motor area - motor planning

What is fractionation?

The ability to activate individual muscles independently of other muscles

Why is fractionation important?

It is essential for normal movement of the hands

What are some examples of fractionation?

Buttoning, knitting, find dexterity, holding a pen; any fine/skilled motor control movement

What is the lateral corticospinal tract essential for?

Skilled precise movement, independent fractionated movements, movements involving distal joints, visually-guided movements

What is the primary descending pathway for voluntary control of fractionated movements?

Lateral corticospinal tract

What is the path of the lateral corticospinal tract (name all structures)?

1. PMC

2. Posterior limb of internal capsule

3. Cerebral peduncle

4. Anterior pons

5. Medullary pyramids

6. Medullary decussation

7. Lateral white column of SC

8. Synapse w/ LMN in ventral gray horn of SC

Where do UMNs primarily reside in the frontal lobe?

The PMC

What are the UMNs in the PMC responsible for?

Precise control of contralateral limb movements and have a low threshold for eliciting movements in response to electrical stimulation

What are the UMNs in the premotor & supplementary motor areas responsible for?

Movement initiation, bimanual movements, sequent movements; have extensive reciprocal connections with PMC & project axons to brain stem and SC

Where is the origin of UMNs?

Pyramidal cells of layer V of the cerebral cortex

Which limb of the internal capsule does the corticospinal tract travel through?

Posterior

How do the fibers of the corticospinal tract decussate?

About 88% percent cross the midline and enter the lateral columns of the SC (lateral) while the rest continue without crossing (ventral)

Which portion of the CST is responsible for controlling axial and trunk muscles?

Ventral CST

What are the potential termination sites of the UMNs?

The ventral horn of the SC (lateral portions), on interneurons, or directly on AMNs

What is the significance of the different sizes on the motor homunculus?

The size of the representation is proportional to the complexity or precision of the movements performed by the body part

What is shown in the motor homunculus?

Organized movements; not individual muscle stimulation

How do motor areas communicate with each other?

They are interconnected via commissural and association fibers; they also receive info from somatosensory cortex

What is the importance of the rubrospinal tract?

Controls posture and muscle tone and is involved in the control of UE muscles

Which descending paths control posture and gross movements?

Reticulospinal, medial & lateral vestibulospinal, medial & lateral CST

What is the function of the reticular formation?

Cardiovascular & respiratory control of eye movements, regulation or sleep and wakefulness, temporal and spatial coordination of movements

Where do UMNS originating in reticular formation terminate?

Medial parts of spinal gray matter for the control of axial and proximal muscles

What do the UMNs of the RcST do?

Make adjustments to posture and equilibrium, receive control form cortical and brain stem motor centers, initiate adjustments to stabilize posture during ongoing movements

What is the function of the RcST?

Facilitate bilateral LMNs innervating postural and gross limb movement muscles, coordinate activity of trunk and proximal muscles, involved in anticipatory postural control, involved in synergistic movements and neck/startle reflex

What is anticipatory postural adjustments (APA)?

A feedforward system where simple movements are accompanied by activation of postural muscles that precede the intended activity in order to ensure stability

What is an example of APA?

Activation of the calf muscles before the biceps during curls; the forward movement of the body’s COM results in gastroc activation to pull the body backwards as a counteraction

What is the significance of feedforward mechanisms?

To predict the resulting disturbances in body stability and generate a stabilizing response

What is the difference between feedforward and feedback responses?

Feedback responses are initiated by sensory inputs that detect postural instability while feedforward responses are preprogrammed and precede the onset of limb movement

Do feedforward and feedback mechanisms happen at the same time?

Yes, they work together for postural control

How is the RCST affected by a stroke?

Studies show RSCT hyperexcitability in post-stroke spacticity