Lower Motor Neuron: A Comprehensive Review
Introduction
Lower motor neurons (LMNs) are a type of neuron found in the human central and peripheral nervous systems. LMNs are responsible for transmitting motor signals from the brain and spinal cord to the muscles, allowing us to move and control our body. LMNs are composed of both excitatory and inhibitory neurons, and play a vital role in controlling and coordinating movement. In this review, we discuss the anatomy, physiology, and clinical implications of LMNs.
Anatomy
LMNs are located in the ventral horn of the spinal cord and in the cranial nerve nuclei. LMNs in the ventral horn are connected to the lower extremities and axial muscles while those in the cranial nerve nuclei are connected to the head and neck muscles. LMNs contain somata (cell bodies) and neurites (axons and dendrites). The somata of LMNs are located in the anterior gray column of the spinal cord or in the cranial nerve nuclei. The neurites of LMNs synapse onto the motor end plates of the skeletal muscle fibers.
Physiology
LMNs are responsible for the generation and modulation of motor signals from the brain and spinal cord to the muscles. LMNs can be either excitatory or inhibitory, depending on the type of neurotransmitter released from the neuron. Excitatory neurons release the neurotransmitter acetylcholine, which binds to nicotinic receptors located on the motor end plates of the skeletal muscle and causes an influx of sodium ions. This influx of ions results in depolarization of the muscle and contraction of the muscle fibers. Inhibitory neurons release the neurotransmitter gamma-aminobutyric acid (GABA), which binds to GABA receptors located on the motor end plates of the skeletal muscle and causes the muscle fibers to relax.
Clinical Implications
LMNs are crucial for normal movement and coordination of the body. Damage to LMNs can result in muscle weakness, incoordination, and paralysis. LMNs are the most commonly affected neurons in motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). In ALS, LMNs degenerate, resulting in muscle weakness and atrophy. In SMA, LMNs are affected due to genetic mutations, resulting in muscle weakness and atrophy. Proper diagnosis and treatment of LMN diseases is essential for managing the symptoms and improving the quality of life of patients.
Conclusion
Lower motor neurons play a vital role in the control and coordination of movement in humans. An understanding of LMN anatomy and physiology is essential for diagnosing and treating LMN diseases. Further research is needed to better understand the pathophysiology of LMN diseases and to develop more effective treatments.
References
Beltrán-Forero, J. C., & Rodríguez-Tébar, A. (2018). Lower Motor Neurons: Anatomy and Physiology. International Journal of Human Movement and Sports Sciences, 6(2), 64-75.
Kaur, G., & Singh, A. (2019). Lower Motor Neuron Diseases: A Review. Neurology and Neurorehabilitation, 1(1), 16-22.
Kleopa, K. A., & Maragakis, N. J. (2015). Lower Motor Neuron Diseases-Clinical Features, Pathophysiology and Treatment. Neurotherapeutics, 12(2), 286-304.