UMN vs. LMN Lesions Part 2

The differentiation between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) lesions is achieved through a comprehensive neurological examination.
Understanding these distinctions is essential for localized deficit diagnosis and improving clinical outcomes in canine and feline neurology.
General Rule of Thumb:
- Upper Motor Neuron (UMN): Characterized by a "loss of inhibition." These units reside within the Central Nervous System (CNS), specifically the brain and spinal cord.
- Lower Motor Neuron (LMN): Characterized by a "loss of function." These units constitute the Peripheral Nervous System (PNS), including peripheral nerves, the neuromuscular junction, and the muscles themselves.

Upper Motor Neuron (UMN) Components: Includes the brain and the spinal cord.
Lower Motor Neuron (LMN) Components: Includes peripheral nerves, the neuromuscular junction (NMJ), and the muscle.
Paresis: General term for partial loss of voluntary movement or weakness.
- UMN Paresis: Manifests as abnormal gait generation.
- LMN Paresis: Manifests as an inability to bear weight.
Paralysis: The complete loss of voluntary movement.
Ataxia: Characterized by disordered movement and the loss of proprioception or postural reactions.
Reflex: An immediate motor response to a specific sensory input that does not require higher-level integration from the brain. An example is the Patellar Reflex.
Reaction: A response that requires either conscious or unconscious cerebral integration. An example is the Menace Response.
Nociception: Also known as "deep pain," this refers to a conscious reaction to noxious stimuli. It is distinct from a simple withdrawal reflex.

Gait:
- UMN: Characterized by delayed generation and a long-strided gait.
- LMN: Characterized by weakness, a short-strided gait, and an inability to support weight.
Muscle Tone:
- UMN: Results in Hypertonia (increased tone) due to the loss of inhibitory signals.
- LMN: Results in Hypotonia or Atonia (decreased or absent tone).
Muscle Mass:
- UMN: Typically appears normal.
- LMN: Results in Neurogenic Atrophy, which occurs rapidly if the lesion persists for more than $7\,d$ .
Spinal Reflexes:
- UMN: Normal to Hyperreflexia (exaggerated reflexes).
- LMN: Hyporeflexia or Areflexia (decreased or absent reflexes).
Postural Reactions/Proprioception:
- UMN: Abnormal or absent.
- LMN: Usually normal, though may appear abnormal if the lesion is exceptionally severe.

Two primary junctions facilitate the innervation of the nerves to the limbs:
Brachial Plexus:
- Responsible for the movement (LMN) and sensation of the thoracic limbs.
- Comprised of the ventral branches of spinal segments $C_6-T_{1(2)}$ .
Lumbosacral Plexus:
- Responsible for the movement (LMN) and sensation of the pelvic limbs.
- Provides perineal and visceral innervation.
- Comprised of the ventral branches of spinal segments $L_4-S_3$ .

UMN Lesions:
- The loss of inhibition leads to increased muscle tone and hyperreflexia.
- Clinical signs include Spastic Paresis or Spastic Paralysis.
LMN Lesions:
- The loss of direct innervation results in decreased muscle tone and diminished or absent reflexes.
- Clinical signs include Flaccid Paresis or Flaccid Paralysis.
- LMN lesions are usually motor-related; sensory perception often remains normal.
- These lesions may present as either focal (localized) or generalized.
Diagnostic Tools: A reflex hammer is a standard instrument used to test the presence and strength of the patellar reflex to aid in localization.

Clinical images and schematic representations of neurological dysfunction are credited to:
- Suzy Gray, Dip ACVIM: Provided imagery of a hopping dog performing postural reaction tests.
- Catherin Reiss, DVM: Provided imagery of the patellar reflex assessment.
- Rcchang16: Provided schematic representations comparing the CNS vs. PNS and UMN vs. LMN units.