Motor neurons are specializednerve cellsthat transmit signals from thecentral nervous system(brain and spinal cord)to muscles and glands throughout the body.
These cells belong to a larger category calledefferent neurons, which carry information away from the central nervous system to target tissues.
Motor neurons can be classified into two main types:upper motor neurons, which connect the brain to the spinal cord, andlower motor neurons, which extend from the spinal cord to the body’s muscles.
Motor neurons serve as essential communication bridges between the nervous system and the body’s muscles and glands. Their primary functions include:
Involuntary Response Management
These neurons also control automatic responses, including:
Autonomic Functions
Through connections with smooth muscles and glands, motor neurons help regulate:
Motor neurons achieve these functions throughtwo key mechanisms: direct muscle activation via lower motor neurons, and coordination of complex movements through the interaction between upper and lower motor neurons.
Motor neurons communicate with muscles through chemical messengers calledneurotransmitters,primarily acetylcholineat the neuromuscular junction.
Each function relies on precise timing and careful regulation of neurotransmitter release at specialized junctions between the neuron and its target tissue.
Structure
Motor neurons are known asmultipolar neuronsin terms of their structure. This means that they have a single axon and multiple dendrites. Motor neurons are the most common structure for neurons.
The structure of a motor neuron can be categorized into three components:the soma, the axon, and the dendrites.
Motor neurons are located in thecentral nervous system(CNS), specifically in the motor cortex, brainstem, and spinal cord.
While the cell bodies (soma) of motor neurons remain within the CNS, their axons, calledefferent fibers, project outward to reach muscles and other peripheral systems such as organs and glands.
These neurons are notable for havingsome of the longest axons in the body. In fact, the efferent fibers that extend from the base of the spinal cord to the toes represent one of the longest axon pathways in the human body.
With approximately500,000 motor neuronsin total, these cells form an extensive network carrying information from the CNS to peripheral organs, muscles, and glands.
Motor neurons are divided into two main categories:upper motor neurons and lower motor neurons.These neurons form distinct pathways through different regions of the nervous system.
Upper Motor Neurons
Upper motor neurons originate in either the brain’s motor cortex or the brainstem and travel through several distinct tracts:
Lower Motor Neurons
Lower motor neurons are categorized into three main subtypes based on their characteristics and connections:
Located in the brainstem, somatic motor neurons include three distinct types:
Also called branchial motor neurons, these are found in the brain stem and form parts of severalcranial nerves:
These neurons are components of both thesympatheticandparasympatheticdivisions of theautonomic nervous system. They connect to all peripheral structures except skeletal muscles.
Damage to upper and lower motor neurons produces distinct patterns of symptoms and can result from various conditions.
Upper Motor Neuron Damage
Upper motor neuron damage, also called pyramidal insufficiency, can result from conditions such as multiple sclerosis (MS), stroke, cerebral palsy, and spinal or brain injury.
When electrical signals cannot reach the muscles, prolonged inactivity leads to:
Lower Motor Neuron Damage
Primary symptoms include muscle paralysis and weakness.
Motor Neuron Disease
ALS is a neurodegenerative disease with symptoms typically worsening over time. While there is no cure for motor neuron diseases, various treatments can help manage symptoms, including:
References
Liden, D. (n.d.).What is a Motor Neuron?(with pictures). Info Bloom. Retrieved June 16, 2021, from https://www.infobloom.com/what-is-a-motor-neuron.htm
McLaughlin, K. (2020, May 16).Motor Neuron. Biology Dictionary. https://biologydictionary.net/motor-neuron/
Moini, J. & Piran, P. (2020)Histophysiology. Functional and Clinical Neuroanatomy. Academic Press. https://doi.org/10.1016/B978-0-12-817424-1.00001-X
Physiopedia. (n.d.).Motor Neurone. Retrieved June 16, 2021, from https://www.physio-pedia.com/Motor_Neurone
Slater, C. R. (2009). Neuromuscular junction (NMJ): mammalian development.DEVELOPMENTAL NEUROBIOLOGY, 529.
Zayia, L. C., & Tadi, P. (2020). Neuroanatomy, Motor Neuron.StatPearls[Internet].
Further Reading
Saul McLeod, PhD
BSc (Hons) Psychology, MRes, PhD, University of Manchester
Saul McLeod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.
Olivia Guy-Evans, MSc
BSc (Hons) Psychology, MSc Psychology of Education
Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.
