Difference between Upper and Lower Motor Neurons

Although the term “motor neuron” brings up visions of a single type of movement-controlling neuron, this is not the reality. Both upper and lower motor neurons are categorized as motor neurons, despite the fact that their origins, synapse locations, pathways, neurotransmitters, and lesion aspects are completely different. Motor neurons (or motoneurons) are a complex set of neurons that innervate effector muscles and glands, allowing for both voluntary and involuntary actions. The top and lower motor neurons constitute a two-neuron circuit. Lower motor neurons begin in the spinal cord and travel throughout the body to innervate muscles and glands, whereas upper motor neurons begin in the cerebral cortex and go down to the brainstem or spinal cord. Understanding the differences between upper and lower motor neurons, as well as the pathways they travel, is critical for not just diagnosing but also efficiently localizing these neuronal damage.

Difference between upper and lower motor neurons

Upper motor neuron: The upper motor neuron is a type of motor neuron that originates in the cerebral cortex’s motor region or in the brainstem. It is responsible for transmitting nerve signals from the brain to lower motor neurons. As a result, it isn’t engaged in nerve impulse transmission to the muscles. Through glutamatergic receptors, a neurotransmitter called glutamate sends nerve impulses from top motor neurons to lower motor neurons.

A motor neuron that arises in the cerebral cortex’s motor area or in the brainstem.

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

Lower motor neuron: The lower motor neuron transfers nerve impulses from the upper motor neurons to the effector muscles. It could come from the brainstem, the anterior gray column, anterior nerve roots, or cranial nerve nuclei of the cranial nerves. Lower motor neurons’ primary role is to connect the spinal cord or brainstem to the muscles. As a result, the cranial and spinal nerves are the lowest motor neurons.

A motor neuron that connects the higher motor neurons to the effector muscles and delivers nerve impulses.

Neuroanatomy

The brain is the command center of the body. Every idea, every emotion, and the great majority of our behaviors are controlled by this unique organ. Its one-of-a-kind (and complicated) three-dimensional architecture is crucial in deciding on and executing those crucial directives. Scientists have discovered that the brain has designated regions responsible for various functions such as understanding and producing speech or processing visual and spatial information during the last several centuries. The intricate design of each portion of the brain helps us make sense of the world around us by governing sensation and perception, information processing, and the initiation of a wide range of activities. While a comprehensive study of neuroanatomy would require a dense textbook with several pictures, here are some of the fundamentals.

Motor neuron

Upper and lower motor neurons are segregated and form a variety of carefully controlled, intricate circuits throughout the body. The innervation of effector muscles and glands regulates both deliberate and involuntary actions. A two-neuron circuit is formed by the upper and bottom motor neurons.

• The cerebral cortex gives rise to higher motor neurons, which migrate down to the brainstem or spinal cord.

• Lower motor neurons originate in the spinal cord and travel throughout the body, innervating muscles and glands.

Structure and function

A two-neuron circuit is responsible for movement, and it is made up of upper and lower motor neurons. Different neurotransmitters are used by upper and lower motor neurons to transfer their signals. Lower motor neurons use acetylcholine, while upper motor neurons use glutamate.

                            Fig: upper motor tract

A signal must originate in the brain’s primary motor cortex, which is located in the precentral gyrus, in order to make a movement. Betz cells are the cell bodies of higher motor neurons found in the primary motor cortex.  These cells are found in the motor cortex’s layer 5 and feature lengthy apical dendrites that branch up into layer 1.  The upper motor neuron is in charge of integrating all excitatory and inhibitory impulses from the cortex into a signal that will either initiate or prevent voluntary movement. The axons of the upper motor neuron go along the internal capsule’s posterior limb. They then go through the cerebral peduncles of the midbrain, longitudinal pontine fibers, and the medullary pyramids. The bulk (about 90%) of the fibers decussate at this point and continue down the lateral corticospinal tract on the opposite side of the body. The lateral corticospinal tract, which is located in the lateral funiculus, is the greatest descending pathway. The lower motor neuron in the anterior horn of the spinal cord will synapse directly onto this tract. The anterior corticospinal tract is made up of pyramidal tract fibers that did not decussate at the medulla and is much smaller than the lateral corticospinal tract. This tract, which lies near the anterior median fissure, is in charge of axial and proximal limb movement and control, which aids posture. This tract decussates at the spinal level being innervated, even if it does not decussate in the medulla.

                  Fig: Formation of spinal nerves at the spinal cord

The signal from the upper motor neuron is transmitted to the effector muscle to perform a movement by the lower motor neuron. Lower motor neurons include somatic motor neurons, specialized visceral efferent (branchial) motor neurons, and general visceral motor neurons.

Conclusion

Despite the fact that both upper and motor neuron lesions cause muscle weakness, they are clinically distinct due to a variety of additional symptoms. Muscle atrophy, fasciculations (muscle twitching), diminished reflexes, decreased tone, negative Babinsky sign, and flaccid paralysis are all symptoms of LMN lesions, unlike UMNs. These features are critical in determining the difference between UMN and LMN lesions, and they must be separated from UMN characteristics in order to create a good differential diagnosis. Poliomyelitis and spinal muscular atrophy are two famous examples of isolated LMN disease, despite the fact that lower motor neurons are involved in a variety of disorders.