Peripheral and Lower Motor Neuron Lesions

• The motor system is the part of the central nervous system that is involved with movement.
• It consists of:
• Pyramidal and
• Extrapyramidal system tracts from UMN & LMN
• A motor neuron is a nerve cell (neuron) whose cell body is located in the spinal cord and whose fiber (axon) projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.
• Motor neurons' axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects.

LOWER MOTOR NEURON

• Are neurons whose cells bodies are located in either the ventral (anterior) horn of the spinal cord gray matter or in the motor nuclei of the cranial nerves in the brainstem.

1. Upper Motor Neuron
2. Lower Motor Neuron

• All voluntary movement depends upon excitation of lower motor neuron by upper motor neuron.
• These are the only neurons that innervate the skeletal muscle fibers; they function as the final common pathway, the final link between the CNS and skeletal muscles.

Lower Motor Neurons Classification

• Lower motor neurons are classified based on the type of muscle fiber they innervate:
• Alpha motor neurons (α-MNs)
• Gamma motor neurons (γ-MNs)

Alpha Motor Neurons (α-MNs)

Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most numerous type of muscle fiber and the one involved in muscle contraction.

Gamma Motor Neurons (γ-MNs)

Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which together with sensory afferents compose muscle spindles. These are part of the system for sensing body position (proprioception).

Lesion

• A lesion is any abnormal damage or change in the tissue of an organism, usually caused by disease or trauma.
• Lesion is derived from the Latin word laesio meaning injury.

Destruction of Motor Neurons

Destruction of motors which supply the muscles.

It starts from anterior horn cell ends at the muscles.

• Above Anterior Horn Cell – UMN
• Below – LMN (Final Common Pathway)

	UMN	LMN
M mass	Slightly ↓ dt disuse	Severely ↓ ↓
M tone	Spastic	Flaccid
M power	↓	↓ ↓
Distribution	Individual Mm never affected	Individual Mm maybe affected
DTRs	↑	Lost
Babinski	↑	Absent
Superficial reflexes	Lost (maybe regained later)	Lost

Spinal Cord Lesion:

• LMN signs at the level of lesion + UMN signs below
• Acute lesions lead to spinal shock followed by recovery in a few weeks
• Bladder & bowel involvement

Anterior Horn Cell/Ventral Root/Plexus Lesions:

• Weakness in specific myotomes
• Slow degeneration of anterior horn cells leads to fasciculations

Peripheral Nerves:

• Single nerve lesion leads to mononeuritis resulting in weakness in distribution
• Polyneuritis:
• Distal weakness
• Early loss of reflexes – may not correlate with the degree of weakness

Neuromuscular Junction:

• Predilection for ocular/pharyngeal or proximal muscles
• Reflexes lost late in affected muscles

Muscle:

• Proximal weakness
• Deep reflexes maybe decreased but elicitable
• Myotonia

1. Destruction in the anterior horn cell: Poliomyelitis, motor neuron diseases
2. Motor nerve (peripheral) is affected: Traumatic stress, peripheral neuropathy
3. Abnormal stimulation at Neuromuscular junction: Myasthenia gravis
4. Myopathy muscles: Neoplasm, Trauma, muscular dystrophies

Motor Affection:

1. Flaccid Paralysis: Defection in the muscle. Loss of ability to a body part.
2. Muscle Wasting: Atrophy in the muscle due to losing of muscle function (Muscle cannot contract voluntarily but reflex).
3. Atonia: Complete loss of muscle tone as the nerve fibers are affected.

• NCS/EMG: Nerve Conduction Studies and Electromyography
• Evoked Potentials
• MRI: Magnetic Resonance Imaging
• Lumbar Puncture
• Nerve Biopsy
• Muscle and Skin Biopsy
• Systemic Tests: Fasting Blood Sugar (FBS), HIV test, serology, etc.

• Mainly by treating underlying conditions.