motor control and UMNs

what is voluntary movement

1. specific behavioral tasks

2. purposeful

3. same stimulus can elicit different response

4. improves with experience and learning

5. unlike reflexes, can be elicited without external stimuli (internally generated)

what we are learning:

how the primary motor cortex drives the brainstem & spinal cord

what tracts are there from the primary motor cortex to the LMNs

1. lateral corticospinal

2. medial corticospinal

3. rubrospinal

4. reticulospinal

5. vestibulospinal

what does it mean that there is parallel and hierarchical control of the primary motor cortex to the LMNs

1. descending pathways modulate activity of LMNs

2. these inputs are termed UMNs

3. can either:

   1. directly activate LMNs from motor cortex

   2. indirectly activate LMNs from interneurons, brainstem centers, or modulation of reflex activity

what is the parallel control from the motor cortex to the LMNs

many pathways the cortex uses to control LMN

what is the hierarchical control from the motor cortex to the LMNs

just that… there is a top down approach with the cortex as the controller telling the LMN what to do

-hierarchical control within the motor cortex too

the movement plan is influenced by inputs from:

1. basal ganglia: movement modulation, initiation

2. cerebellum: error correction, motor learning

3. association cortex: posterior parietal cortex, information about internal & external environment

in the hierarchies of control, what areas are in charge of movement goal and strategy

1. motor association areas

2. basal ganglia

for movement goal and strategy, the basal ganglia and motor association areas help with:

1. intended goal of movement

2. environment in which movement will occur

3. locate movement target

4. choose best strategy

5. make postural adjustment prior to movement

6. start movement

7. use feedback to improve future movement

in the hierarchies of control, what areas are in charge of tactics and timing of movement

1. primary motor cortex

2. cerebellum

for movement tactics and timing, the primary motor cortex and cerebellum areas help with:

1. which muscle groups to use

2. in what order should they contract

3. how much force is needed

4. current excitability of the LMNs

5. duration - when they should turn off/how long to contract

6. amount of braking needed at end of movement

7. using feedback to modify on-going movements

in the hierarchies of control, what areas are in charge of execution of movement

1. spinal cord

2. brainstem

for execution of movement, the spinal cord and brainstem help with:

1. directly activating muscles needed for task

2. recruiting specific motor units

3. control balance between agonist and antagonist

4. provide some input for balance control

5. control specific motor tasks such as gait

what are upper motor neurons (UMNs)

neurons that innervate the LMNs AND originate in the brainstem or cerebral cortex

is innervation direct or indirect with UMNs

can be either direct (from cortex to LMN) or indirect (via interneuron in the SC or brainstem)

where is a major source of UMNs

primary motor cortex

what Brodmanns area is the pre-central gyrus (part of primary motor cortex)

area 4

how is the primary motor cortex arranges

Somatotopically!

-LE medial

-UE lateral

-face lateral inferior

Not as distinctive as somatosensory cortex

maps of MOVEMENTS not muscles

what was added to the homunculus of the primary motor cortex in 2022

additional complexity - regions that integrate joint movement

what layers of the cortex receive inputs from cortical areas and brainstem

Layers I, IV, V

what layer of the cortex projects to contralateral cortex

layer III

what layer of the motor cortex gets input from the Thalamus

layer IV

what layer of the motor cortex is very developed, has output neurons to thalamus/SC, and where the corticospinal tract starts

layer V

what layer has output neurons to Thalamus

layer VI

what are Betz cells and what layer of the motor cortex are they located in

large, pyramidal in structure (pyramidal tract)

layer V

what areas of the brain are inputs to the primary motor cortex

1. per-motor cortex

2. supplementary cortex

3. basal ganglia via thalamus

4. cerebellum via thalamus

5. primary motor cortex of opposite hemisphere

6. sensory cortex

7. hippocampus

where are the motor cortex outputs to sub-cortical motor areas (everything below the cortex)

1. spinal cord lower motor neurons via corticospinal tract

2. spinal cord interneurons via corticospinal tract

3. cranial nerve motor neuron- corticobulbar tract

4. brainstem motor centers

   1. red nucleus

   2. reticular formation

   3. inferior olivary nucleus

where are the motor cortex outputs to sub-cortical sensory areas

1. thalamus (talks to motor cortex)

2. dorsal column nuclei in medulla (anticipate sensory info during motor)

what are the motor cortex outputs of the UMN output

activates multiple muscles for a given movement

what is the corticospinal tract

UMNs that connect to LMNs and interneurons in the spinal cord

where do the collaterals of the corticospinal tract synapse at

1. red nucleus in midbrain

2. reticular formation - especially in medulla

3. gives cortical fiber “access” to other brainstem motor areas

4. part of the corticobulbar tracts (cortex→brainstem)

what layer does the primary motor cortex originate and what % of corticospinal fibers is there

primarily layer V

30% fibers

what % of corticospinal fibers is at the Pre-Motor and Supplementary cortex

30%

what % of corticospinal fibers is at Primary Somatosensory & Cingulate Gyrus

40% fibers

what is the pathway of the corticospinal tract

1. cell bodies in cortex

2. axons enter into cortical radiations

3. then into the internal capsule

   1. somatotopically arranged

   2. IC contains multiple axon types

   3. CS fibers in the posterior limb and genu

4. continues to descend through brainstem

5. in the medulla (pyramids)

6. synapse with LMNs and interneurons in SC

capsular strokes are common due to _________

hypertension

what percent of axons of the corticospinal tract do/don’t cross over in the medulla

1. 90% of axons cross over

   1. lateral corticospinal tract

2. 10% of axons don’t cross

   1. 8% cross over in SC = anterior corticospinal tract

   2. 2% remain uncrossed = medial corticospinal tract

where do most corticospinal tract axons synapse

in cervical and lumbar cord, but all cord levels are innervated

fibers from Primary motor, pre-motor, and supplementary motor cortex synapse in _____ _____ on MNs and interneurons

ventral horn

the lateral corticospinal tract is responsible for

fractionated movement of digits and limbs

the anterior and medial corticospinal tract is responsible for

trunk and proximal muscle control

fibers from the sensory cortex synapse in _____ ______

dorsal horn

-modulation of sensory information

where are other motor tracts

1. cortex also innervates other brain stem areas that then act as motor pathways = corticobrainstem tracts

2. reticular formation = reticulospinal tract

3. vestibular nuclei = vestibulospinal tract

4. midbrain superior and inferior colliculi = tectospinal tract

5. midbrain red nucleus = rubrospinal tract

begin in brainstem and facilitate trunk control and postural responses

what happens if there is a loss of corticospinal tracts

-monkeys with only brainstem motor tracts

1. use axial and proximal muscles

2. able to walk, run, stand, climb

3. unable to use distal limbs

4. able to have gross movement of arms and develop mass grasp

these may be implications for CVA

where does the corticobulbar tract arise from

lateral aspect of motor cortex

1. also inputs from cingulate gyrus

2. emotional vs. voluntary movements

what is the corticobulbar tract

UMN to cranial nerve nuclei

1. bilateral innervation of upper face (above eyebrows)

2. lower face innervated from opposite side

the primary motor cortex controls:

movements not single muscles

1. speed of movement

2. force of muscle contraction

3. direction of movement

what is directional tuning

neurons have a “preferred” direction in which they are tuned to be most active in

-does not mean that anytime this movement is needed it will fire this cell

-parallel pathways (multiple ways - other neurons have diff preferred movement)

what happens when there is stimulation of primary motor cortex neurons

1. controls muscles on opposite side of the body

2. complex motor maps

3. groups of muscles are activated (ex: elbow flexors), while others are inhibited (ex: elbow extensors)

   1. simple movements, oriented to movements in personal space

4. any given UMN increases activity in a number of muscles

   1. each UMN has a “muscle field”

   2. 2-3 muscles which it activates

what Brodmann’s area is the pre-motor and supplementary motor cortex together

area 6

what are the inputs of the pre-motor and supplementary motor cortex

1. basal ganglia via thalamus → supplementary motor cortex

2. cerebellum via thalamus → pre motor cortex

3. posterior parietal cortex → area 6

   1. integration of somatosensory and visual information

what are the outputs of the pre motor and supplementary motor cortex

1. primary motor cortex

2. spinal cord

what do the pre-motor cortex and supplementary motor cortex do

programming for muscle sequence

mental rehearsal

what is the function of the supplementary motor cotex

1. coordinate self-initiated movements

2. governs postural adjustments - even preparatory

3. complex motor patterns

4. unilateral and bilateral influence

5. both axial and distal muscles

what can happen with a lesion of the supplementary motor cortex

1. ideomotor apraxia

2. ideational apraxia

what is ideomotor apraxia

inability to execute movement on command

what is ideational apraxia

inability to conceptualize movement, cannot identify sequence of movements needed

what is the function of the pre-motor cortex

1. complex movements using multiple joints

   1. oriented to extra-personal space (more than arms length)

2. direct connections on LMNs of hand and proximal limb muscles

3. control movements that are visually guided with input from posterior parietal cortex

   1. movements generated by external stimuli

what apraxia can lesions to the pre-motor cortex cause

1. unable to coordinate bilateral movements

2. loss of strength in proximal muscles

what is the function of the pre-frontal cortex

1. important for working memory

2. stored info on object location in space long enough to guide movement

3. motor planning functions- strategic planning

4. important in motor learning as well

what movements result from the medial corticospinal tract

contraction of neck, shoulder and trunk muscles