BIO448 - Upper motor neurons

local circuit neurons

lower motor neuron integration

lateral

cortex motor neurons decussate immediately and go down the ___ tract

they have control over distal muscles for fine movement

anterior-medial

brainstem motor neurons go down the ___ tract and have control over axial/postural muscles because they are close to medial motor pools.

both

information from brainstem motor neurons synapse on long-distance circuit interneurons and is spread to ___ sides

upper cortical motor neurons

decussate in the brainstem

go down lateral tract

synapse on interneurons

interneurons connect to lower motor neurons in lateral ventral horn of spinal cord

movement of distal muscles for skilled/fine movement

upper brainstem motor neurons

go down on same side anterior-medial tract

synapse on interneurons spreading info to both sides

interneurons synapse on lower motor neurons in medial ventral horn

axial and proximal limb muscles for posture and balance

primary motor cortex

first gyrus after central sulcus

premotor cortex

region in front of primary motor cortex

controls the motor neurons in the brainstem

some axons go into primary motor cortex as well as

Betz cells

largest neurons

5% of upper motor neurons

influence distal muscles, especially hands and digits

pyramidal neurons

remaining 95% of upper motor neurons

control over muscles other than hands and digits, mostly for postural control

layer 5

OUTPUT layer of the cortex

corticobulbar tract

• carry axons from cortex to brainstem to control head and neck muscles

• no clear decussation, all nerves decussate as needed based on the face region they control (all decussating neurons look like a fan kind of)

• info goes through cranial nerves

• synapse on cranial motor neurons that control muscles of the head and neck

• synapse on neurons of reticular formation

• synapse on neurons in red nucleus

corticospinal tract

• decussate at the lower/caudal medulla level

• go through lateral white matter and synapse in spinal cord

• synapse on local circuit interneurons or directly on lower motor neurons

• the 10% which do NOT decussate go to medial white matter of spinal cord

90

___% of upper cortical neurons decussate/cross over in caudal/lower medulla and travel in lateral white matter of spinal cord to control distal muscles

10

___% of upper cortical motor neurons do NOT decussate/cross over and travel in the anterior white matter of spinal cord to control axial/postural muscles and some proximal

interneurons

Most upper motor neurons synapse on local ___ firs with the exception of direct contact with motor neurons innervating hands and digits

alpha

direct contact from upper to lower ___ motor neurons is restricted to muscles of hands and digits

aka only BETZ CELLS synapse directly on neurons

overrepresented

musculature used in tasks requiring fine motor control are ___ in the motor cortex

rougher

somatotopic map of the motor cortex is ____ than the somatic sensory cortex

muscle fields

# of muscles affected by the discharge of a given upper motor neurons

one upper motor neuron affects 2 or more muscles

movements

• purposeful ___ are mapped in motor cortex

• somatotopic organization is best understood in terms of behavioral ___

Examples: hand to mouth, defensive, reach, climbing, etc.

upper motor neurons encode ___ rather than individual muscles

preferred direction

Each neuron has highest activity for movement in a specific direction

However, each neuron is tuned too broadly

neuronal population vector

movement encoded by the simultaneous discharge of a large population of upper motor neurons

averaging the preference of many neurons gives an overall vector for movement

brace

because you have to ___ yourself before a dynamic movement, postural muscles contract before the distal muscles, usually subconsciously

feedforward

aka anticipatory

controlled by upper motor neurons in premotor cortex

provides info to spinal cord that maintain posture in response to environmental disturbance of body position

initiate ___ adjustment that stabilizes posture during on-going movement

feedforward pathway

1. premotor cortex

2. brainstem aka reticular formation

3. both sides of spinal cord

4. synapse on interneurons in both sides of spinal cords

5. control medial/axial/postural muscles

reticular formation

heterogenous cluster of neurons

a set of interconnected nuclei in the brainstem that spans from the lower end of the medulla oblongata to the upper end of the midbrain

premotor coordination of lower somatic and visceral motor neurons (caudal pontine and medullary)

modulate forebrain activity (mesencephalic and rostral pontine)

feedback pathway of postural control

1. postural instability detected by hair cells in semicircular canals

2. info goes to vestibular nuclei

3. info sent to lower motor pools that control proximal/postural muscles via LONG-DISTANCE CIRCUITS interneurons

medial local circuit

all responses/pathways ends by synapsing on ____ neurons

babinski sign

reflects incomplete motor neuron control of local motoneuron activity

Example: extensor plantar response in babies