Difference Between Upper and Lower Motor Neuron

The main difference between upper and lower motor neuron is that upper motor neuron is the motor component of the central nervous system that transmits impulses from the brain to the synapses of the lower motor neurons whereas lower motor neuron is the motor component that connects with the muscles. 

Upper and lower motor neuron form the motor part of the somatic nervous system. They are responsible for the movement of muscles that are voluntary. Voluntary muscular movements are initiated and coordinated by the motor cortex, a posterior part of the frontal lobe of the brain. 

Key Areas Covered 

1. What is Upper Motor Neuron
     – Definition, Structure, Transmission
2. What is Lower Motor Neuron
     – Definition, Structure, Transmission
3. What are the Similarities Between Upper and Lower Motor Neuron
     – Outline of Common Features
4. What is the Difference Between Upper and Lower Motor Neuron
     – Comparison of Key Differences

Key Terms: Lower Motor Neuron, Somatic Nervous System (SNS), Upper Motor Neuron, Voluntary Muscular Movements

What is Upper Motor Neuron 

The upper motor neuron is a motor neuron that originates from the motor region of the cerebral cortex or from the brainstem. It transmits nerve impulses from the brain to the lower motor neurons. Thereby, it is not involved in the transmission of nerve impulses to the muscles. The transmission of the nerve impulses from upper motor neurons to the lower motor neurons occurs by a neurotransmitter called glutamate via glutamatergic receptors. 

Figure 1: Upper Motor Tract

The six pathways of the upper motor tract are the corticospinal tract, corticobulbar tract, colliculospinal tract, rubrospinal tract, vestibulospinal tract, and the reticulospinal tract. 

What is Lower Motor Neuron 

The lower motor neuron is a motor neuron that transmits nerve impulses from the upper motor neurons to the effector muscles. It may originate from anterior gray column, anterior nerve roots or cranial nerve nuclei of the cranial nerves or the brainstem. The main function of lower motor neurons is to connect spinal cord or the brainstem to the muscles. Therefore, the lower motor neurons are the cranial and the spinal nerves. The formation of spinal nerves at the spinal cord is shown in figure 2.

Figure 2: Formation of Spinal Nerves at the Spinal Cord

Similarities Between Upper and Lower Motor Neuron 

• Upper and lower motor neuron are parts of the somatic nervous system. 
• They transmit nerve impulses from the brain to the muscles. 
• Both are responsible for voluntary muscular movements. 

Difference Between Upper and Lower Motor Neuron 

Definition 

Upper Motor Neuron: A motor neuron that originates from the motor region of the cerebral cortex or from the brainstem

Lower Motor Neuron: A motor neuron that transmits nerve impulses from the upper motor neurons to the effector muscles. 

Significance 

Upper Motor Neuron: Located near the central nervous system  

Lower Motor Neuron: Located near muscles 

Transmission 

Upper Motor Neuron: Transmits nerve impulses from the brain to the synapses of lower motor neurons 

Lower Motor Neuron: Transmits nerve impulses from upper motor neurons to the muscles 

Form Synapses with 

Upper Motor Neuron: Lower motor neurons 

Lower Motor Neuron: Muscles 

Found in 

Upper Motor Neuron: Cerebral cortex or in the brainstem 

Lower Motor Neuron: brainstem and in the spinal cord 

Cell Bodies 

Upper Motor Neuron: Bigger; located in the brain cortex 

Lower Motor Neuron: Small; located in the gray matter of the spinal cord and the brainstem 

Classified Based on the  

Upper Motor Neuron: Pathway they travel in 

Lower Motor Neuron: Type of muscle fiber they innervate  

Classification 

Upper Motor Neuron: Has six pathways 

Lower Motor Neuron: Cranial and spinal nerves 

Symptoms of Damage 

Upper Motor Neuron: Increased muscle tone and hyperactive deep reflexes  

Lower Motor Neuron: Decreased muscle tone, hyperactive deep reflexes, and muscle atrophy  

Conclusion 

Upper motor neuron originates from the central nervous system and transmits nerve impulses to the lower motor neurons that transmit the nerve impulses to the muscles. Both upper and lower motor neurons make up the somatic nervous system that controls the voluntary muscular movements. The main difference between upper and lower motor neuron is the origin and the function. 

Reference:

1. McCaffrey, Patrick. “ Chapter 10. Upper Motor Neuronal Tracts.” Upper Motor Neuronal Tracts – CSU, Chico, Available Here
2. “Lower Motor Neuron:” Lower Motor Neuron – Definition — Neuroscientifically Challenged, Neuroscientifically Challenged, Available Here

Image Courtesy:

1. “Gray764” By Henry Gray (1918) Anatomy of the Human Body (See “Book” section below)Bartleby.com: Gray’s Anatomy, Plate 764 (Public Domain) via Commons Wikimedia
2. “Spinal nerve” By Mysid (original by Tristanb) – Vectorized in CorelDraw by Mysid on an existing image at en-wiki by Tristanb. (CC BY-SA 3.0) via Commons Wikimedia