Upper vs Lower Motor Neuron
The conduction of motor and sensory nerve impulses to and from the brain is basically carried out by sensory (ascending) and motor (descending) tracts, and pathways in the spinal cord. The names of the pathways are given according to their position in the white matter, and their beginning and ending places. In the human nervous system, there are two types of spinal cord tracts; (1) somatic sensory pathways and (2) somatic motor pathways. The somatic sensory pathways carry sensory impulses from somatic sensory receptors to the cerebral cortex, and the somatic motor pathways carry motor impulses from the cerebral cortex to skeletal muscles. There are two basic sets of neurons present in these somatic motor pathways, namely; upper motor neuron and lower motor neuron. These two types of neurons mostly synapse with association neurons. However, there are very few pathways, which directly synapse both upper and lower motor neurons.
Upper Motor Neuron
Upper motor neurons are the motor fibers present within the central nervous system (CNS). These motor neuron fibers make synaptic connections with the motor neurons in the peripheral nervous system (PNS). Upper motor neurons include two pathways of the pyramidal system (the corticospinal and corticobulbar tracts) and the four pathways of the extrapyramidal system (the ruborspinal, reticulospinal, vestibulospinal, and tectospinal tracts).
Lower Motor Neuron
Lower motor neurons are the motor neurons located in both the CNS and PNS. These neurons receive both excitatory and inhibitory signals from many presynaptic neurons, thus called the ‘final common pathway’. Most of the nerve impulses are conveyed to lower motor neurons through the association neurons. Very few impulses are received directly from upper motor neurons. Hence, the total signal input of lower motor neurons is determined by the sum of the input from upper motor neurons and association neurons.
What is the difference between Upper and Lower Motor Neuron?
• Upper motor neurons modulate the activity of lower motor neurons.
• Upper motor neurons located entirely within the CNS, whereas lower motor neurons located either in the spinal cord gray matter or within the cranial nerve nuclei in the brain stem.
• Lower motor neuron lesions produce flaccid muscle weakness, muscular atrophy, fasciculation, and hyporeflexia, whereas upper motor neuron lesions result spastic muscle weakness, and hyperreflexia.
• Upper motor neurons pass nerve impulse to either association neurons or directly to lower motor neurons. In contrast, lower motor neurons convey the nerve impulses to receptors in skeletal muscles.