VMM 812 Spinal Cord Lesion Localization and Motor Neuron Physiology

The motor system is categorized into two primary divisions: Upper Motor Neurons and Lower Motor Neurons.
Upper Motor Neurons (UMN):
- These neurons are entirely contained within the Central Nervous System (CNS).
- The cell bodies are located in the motor cortex of the forebrain or in motor nuclei within the brainstem.
- Axons from UMNs project to and control the activity of LMNs.
- They typically belong to the corticospinal, corticomedullary, or descending brainstem motor pathways.
- UMNs function by initiating and terminating LMN activity and providing inhibitory modulation to spinal reflex activity.
- Loss of UMN commands results in paresis or paralysis, while the loss of inhibitory modulation leads to increased muscle tone and exaggerated spinal reflexes.
Lower Motor Neurons (LMN):
- LMN cell bodies are located in the CNS, specifically in the ventral horn of the spinal cord or in cranial nerve nuclei.
- Their axons project out of the CNS into the Peripheral Nervous System (PNS) via spinal or cranial nerves.
- These neurons connect directly with skeletal muscles at the neuromuscular junction.
- LMNs are responsible for inducing muscle contraction when stimulated.
- Muscle tone, muscle bulk, and the execution of reflexes are entirely dependent on functional LMNs.
- Loss of LMNs result in paresis or paralysis, but unlike UMN lesions, these are characterized by decreased to absent muscle tone and reflexes.

Paresis: Defined as weakness or a reduction in voluntary movement.
- Clinical signs include trouble standing, dragging limbs while walking, or a short stride (hypometria).
- Ambulatory Paresis: The patient can stand and walk at least $10$ steps without external support.
- Non-ambulatory Paresis: The patient can move their legs but lacks the strength to support their own weight or walk.
Paralysis or Plegia: The total loss of voluntary movement; the animal cannot move the affected limb(s) and cannot stand or walk if multiple limbs are involved.
Proprioceptive Ataxia: Uncoordinated movement characterized by truncal sway (wobbliness), unsteady gait, and abnormal limb stances.
- Specific abnormalities include circumduction, abduction, adduction, crossing over of limbs, and a delay in the start of the swing phase of the gait.

Motor Function: Both UMN and LMN lesions result in paresis or paralysis.
Reflexes:
- UMN Lesion: Normal to increased (hyperreflexia).
- LMN Lesion: Decreased to absent (hyporeflexia to areflexia).
Extensor Muscle Tone:
- UMN Lesion: Normal to increased (hypertonicity/rigidity).
- LMN Lesion: Decreased to absent (hypotonicity/flaccidity).
Muscle Atrophy:
- UMN Lesion: Mild and chronic (disuse atrophy).
- LMN Lesion: Severe and fast (neurogenic atrophy).

An intact reflex (whether normal or exaggerated) indicates that the lesion is not located within that specific reflex arc.
The Reflex Arc Components: Receiver $\rightarrow$ Sensory Neuron $\rightarrow$ Spinal Cord (Intumescence) $\rightarrow$ Motor Neuron $\rightarrow$ Muscle.
Reflexes will persist even if the neuroaxis cranial or caudal to the reflex circuit is severely damaged.
Exaggerated reflexes or increased extensor tone occur because the UMNs cranial to the reflex arc normally provide an overall inhibitory influence; when this is lost, the LMN becomes overactive.
Diminished or absent reflexes occur when the lesion directly affects the LMNs at the specific spinal cord segments involved in the reflex (the intumescences).

Four Limbs Affected (Tetraparesis/Tetraplegia):
- If all four limbs show UMN signs (increased tone/reflexes): Lesion is in the $C1-C5$ region.
- If thoracic limbs show LMN signs (decreased tone/reflexes) and pelvic limbs show UMN signs: Lesion is in the $C6-T2$ region.
Pelvic Limbs Affected (Paraparesis/Paraplegia):
- If pelvic limbs show UMN signs (normal/increased tone and reflexes): Lesion is in the $T3-L3$ region.
- If pelvic limbs show LMN signs (decreased/absent tone and reflexes, such as an absent patellar reflex or decreased withdrawal): Lesion is in the $L4-S3$ region.
Diffuse LMN Signs (Flaccid Paralysis in all four limbs):
- This indicates a diffuse problem in the Peripheral Nervous System (nerves or neuromuscular junction) rather than a focal spinal cord lesion.
- Common causes include: Polyradiculoneuritis (Coonhound paralysis), tick paralysis, botulism, and fulminant myasthenia gravis.

Schiff-Sherrington is an exception to standard localization rules occurring with severe lesions usually in the $T3-L3$ region (though it can also occur with $L4-S3$ lesions).
Clinical Presentation: Extensor rigidity (increased tone) in the thoracic limbs accompanied by paralysis in the pelvic limbs.
Key Diagnostic Feature: The thoracic limbs have normal function; they are not weak, paralyzed, ataxic, and they do not have proprioceptive deficits. The rigidity is purely postural.
Pathophysiology: It is caused by the interruption of inhibitory neuron input (border cells) originating in the lumbar spinal cord that normally travels cranially to inhibit the extensors of the thoracic limbs.

Question: What is the neuroanatomic cause of extensor rigidity in the forelimbs of a patient with Schiff-Sherrington posture?
Answer: Interruption of inhibitory neuron input from the lumbar spinal cord.
Neuro Trivia: How do you know the exact moment a gecko has brain death?
Hint 1: There is an actual neurophysiological explanation, not a joke.
Hint 2: It occurs a little bit earlier than the point where it would be obvious to a casual observer.