Upper motor neurons

Upper motor neurons

Motor cortex

Brainstem

Medial white matter spinal cord

Lateral white matter spinal cord

Proximal muscles

Distal muscles

Posture

Balance

Visual orientation

Skilled movements

Bilateral projection

Projects to one side of body

Pathways

Lateral & medial vestibulospinal tracts/ vestibular nuclei

Recticulospinal tract/ reticular formation

Colliculospinal tract/ superior colliculus

Lateral vestibular nuclei

Medial vestibular nuclei

Terminates bilaterally in MWM of SC

Terminates in medial lower neuronal pools that govern proximal limbs

Balance & posture in response to environment

Feedforward

Rapid feedback

Contains direct projects to spinal cord

Indirect projects from premotor cortex to spinal cord

Pathways

Primary motor cortex

Brodmann's area 4

Layer 5

Betz cells

Non betz cells

Pyramidal

Plays a role in activation of lower motor neurons in distal extremeties

Corticospinal & corticobulbar tracts

Premotor cortex

Corticobulbar tracts terminate at different areas of the brainstem, including the reticular formation

90% of axons in corticospinal tract cross the midline

Ends up in lateral cortico spinal tract and terminates ipsilaterally

10% of axons in corticospinal tract do not cross midline

Ends up in ventral corticospinal tract and terminates bilaterally

It terminates onto lower motor neurons, bypassing interneurons. This can only be seen in certain primates and could account for why humans are so dexterous with their hands.

Cortical mapping

Ethologically meaningful movements (pattern)

Mirror neurons

Fires AP during observation and performing the motor movement

Encodes the actions of others --> learning, goal directed

Neurons that detect movements also carry out those movements

Damage to descending motor pathways

Upper motor neuron syndrome

Manifests itself as overactivity of muscles such as

- Spasticity

- Deep reflexes (tendon tap where knee has a great reaction)

- Babinski's sign

-Increased tone

- Loss of voluntary fine movement

- Weakness (due to loss of voluntary movement)

The brain has an overall inhibitory effect on motor movement. Loss of that inhibition would result in activity.

Lower motor neuron syndrome

Inactivity of muscles

- Weakness or paralysis

- Decreased tone/ flaccid (resting tone is lost)

- Muscle atrophy

Note: Pathways for upper motor neurons

Motor cortex

1. Direct to lower motor neurons

2. Indirect to brainstems

Brainstem

1. Reticular formation - Feedforward

2. Vestibular nuclei - Feedback

3. Superior colliculus

Direct

Indirect

At the caudal medulla

Pyramidal decussation