Spinal Motor Organization & Brainstem Supraspinal Paths

Focus on the organization of spinal motor neurons and supraspinal pathways that influence motor control.

Classification:
- Corticospinal Tract (CST): Functions as the "elevator system" for motor commands.
- Cell Body Organization:
  - Medial Cell Bodies: Control proximal muscles (close to the center of the body).
  - Lateral Cell Bodies: Control distal muscles (further from the center, e.g., limbs).

Overview: Propriospinal neurons are critical for coordinating multiple levels of lower motor neurons (LMNs) for gross movements.
Examples of Movements:
- Gross Movements:
- Example: Reaching overhead that requires trunk stabilization and control of shoulder, elbow, wrist, and hand muscles.
- Fine Motor Movements:
- Requires few levels of neuron control; primarily involves C8 and T1 segments for finger motion.
Structure:
- Links intersegmental neurons across different spinal cord levels to perform motion.
- Divided into three levels:
1. Long propriospinal neurons.
2. Intermediate propriospinal neurons.
3. Short propriospinal neurons.

1. Long Propriospinal Neurons:
- Ascend or descend in the anterior fasciculus proprius.
- Access all spinal cord levels.
- Provide bilateral influence on medial motor neurons controlling axial muscles.
2. Intermediate Propriospinal Neurons:
- Located in the ventral portion of the lateral fasciculus proprius.
- Innervate motor neurons of proximal limb muscles.
3. Short Propriospinal Neurons:
- Found only in cervical and lumbosacral enlargements.
- Innervate motor neurons of distal limb muscles.

Vestibular Nuclear Complex:
- Principal influencer of spinal motor activity in the brainstem.
- Comprises four nuclei: medial, lateral, superior, and inferior, located in the rostral medulla/caudal pons.
- Functions to maintain balance and equilibrium.
- Projection: Influences spinal motor nuclei via lateral and medial vestibulospinal tracts.
- Lateral Vestibulospinal Tract: Strongly influences extensor muscles of ipsilateral limbs.
- Medial Vestibulospinal Tract: Influences bilateral control of head, neck, trunk, and proximal limb muscles.

Pontine Reticular Formation:
- Responsible for facilitating extensor movements while inhibiting flexor movements.
- Discussed in relation to Decerebrate Posturing in comatose patients.
- Under inhibitory control from higher centers.
Medullary Reticular Formation:
- Comprises lateral reticulospinal fibers.
- Facilitates flexor movements while inhibiting extensors, facilitated by higher centers.
- Influenced by corticoreticular fibers, cerebellum, vestibular nuclei, and ascending pain fibers.

Anatomical Location: Found in the tegmentum of the midbrain, at the superior colliculus level.
Inputs:
- Cerebral cortex through corticorubral tracts.
- Cerebellum via cerebellorubral tracts.
Output:
- Rubiospinal tract which facilitates flexor movements of contralateral (CL) upper limbs both directly and through the medullary reticular formation.

Pathway Division: The descending pathways from higher centers are categorized into three major groups:
1. Ventromedial
2. Lateral
3. Cortical

Location: Anterior funiculus of the spinal cord.
Components:
- Includes medial vestibulospinal fibers.
- Includes medial reticulospinal fibers.
Synapses: Form in the medial aspect of the anterior horn.
Influences:
- Affects long propriospinal and LMNs in the medial anterior horn.
- Strongly involved in controlling axial muscle movements.

Location: Lateral funiculus of the spinal cord.
Components:
- Contains the rubrospinal tract, lateral reticulospinal fibers, and lateral vestibulospinal fibers.
Input: Strong input from the red nucleus.
Synapses: Occur in the lateral aspect of the anterior horn.
Influences:
- Primarily affects proximal and distal limb muscles.

Location: Lateral corticospinal tract.
Components:
- Comprises corticospinal tract fibers.
Synapses: Form in the dorsolateral aspect of the anterior horn.
Influences:
- Major influence on all lower motor neuron (LMN) innervation, particularly affecting distal muscles.
- Most distal muscles, for instance, intrinsic hand muscles, are directly influenced by the corticospinal tract.

Ventromedial Group:
- Responsible for bilateral axial movements, strongly influenced by medial vestibulospinal and reticulospinal tracts.
Lateral Group:
- Controls unilateral proximal limb muscles, influenced by lateral reticulospinal and vestibulospinal tracts, as well as some corticospinal tract influence.
Cortical Group:
- Controls distal muscles, especially flexors of upper extremities, heavily influenced by corticospinal and rubral tracts.
- The control of distal muscles (like fingers) comes directly from the cerebral cortex.

Patient Recovery Dynamics:
- Rapid recovery observed in neck and trunk movements as these are not reliant on the pyramidal tract for innervation.
- This supraspinal control stems from the ventromedial group.
- Recovery of proximal and distal movements occurs gradually due to involvement of the corticospinal tract (CST).
- Recovery of more complex distal muscles (e.g., independent finger motion) may not occur as these are strictly controlled by the CST, and damage here can inhibit function restoration.

Decerebrate Posturing:
- Occurs with brainstem damage affecting motor nuclei, specifically if damage is between the red nucleus and vestibular nuclei.
- Signs/Symptoms (S/S):
- Extensor posturing of both upper and lower limbs.
- Mechanism: Impairment in extensor inhibition usually exerted on the reticular formation leads to overactivity in extensor facilitation.
- Lateral vestibular nuclei play a significant role in this response.
Decorticate Posturing:
- Appears if damage occurs rostral to the red nucleus.
- Signs/Symptoms (S/S):
- Upper limbs flexed, lower limbs extended.
- Mechanism: Red nucleus exerting facilitation on flexor movements predominantly affecting the upper limbs.

Relation of posturing to damage locations can determine prognosis:
- From Decerebrate to Decorticate: Better prognosis, indicates recovery of upper limb control.
- From Decorticate to Decerebrate: Worse prognosis, indicating further caudal progression of damage. - If damage reaches the respiratory and cardiovascular centers in the medulla, it poses severe life-threatening risks.