Lower motor neuron

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Overview

Lower motor neurons (LMNs) are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles.

Classification

Lower motor neurons are classified based on the type of muscle fiber they innervate:

• Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most numerous type of muscle fiber and the one most involved in muscle contraction.
• Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which are involved with muscle spindles and the sense of body position.

Physiology

Glutamate released from the upper motor neurons triggers depolarization in the lower motor neurons in the anterior horn which in turn causes an action potential to propagate the length of the axon to the neuromuscular junction where acetylcholine is released to carry the signal across the synaptic cleft to the postsynaptic receptors of the muscle cell membrane, signaling the muscle to contract.

Consequences of lesions

Damage to lower motor neurons (lower motor neurone lesions) is indicated by abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and neurogenic atrophy of skeletal muscle.

v • d • e Histology: nervous tissue
Neurons (gray matter)	soma - axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament) dendrite (Nissl body, dendritic spine, apical dendrite, basal dendrite) types: bipolar - pseudounipolar - multipolar - pyramidal - Purkinje - granule
Afferent nerve/Sensory nerve/Sensory neuron	GSA - GVA - SSA - SVA - fibers (Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain)
Efferent nerve/Motor nerve/Motor neuron	GSE - GVE - SVE - Upper motor neuron - Lower motor neuron (α motorneuron, γ motorneuron)
Synapses	neuropil - synaptic vesicle - neuromuscular junction - electrical synapse - Interneuron (Renshaw)
Sensory receptors	Meissner's corpuscle - Merkel nerve ending - Pacinian corpuscle - Ruffini ending - Muscle spindle Free nerve ending Olfactory receptor neuron - Photoreceptor cell - Hair cell - Taste bud
Glial cells	Astrocyte (Radial glia) - Oligodendrocyte - Ependymal cells (Tanycytes) - Microglia
Myelination (white matter)	CNS: Oligodendrocyte PNS: Schwann cell - Neurolemma - Nodes of Ranvier/Internode - Schmidt-Lanterman incisures
Related connective tissues	epineurium - perineurium - endoneurium - nerve fascicle - meninges

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Acknowledgement and Attribution Regarding Sources of Content

Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .