Slide 1

Concept of upper and lower motor neurons

NEUROPHYSIOLOGY

UPPER MOTOR NEURONUpper motor neuronsaremotor neuronsthat originate either in themotor regionof thecerebral cortexor in thebrain stemand carry motor information down to thelower motor neurons The main effector or motor neurons (efferent neurons) for voluntary movement lie within layer of theprimary motor cortexand are a type of giantpyramidal cellcalledBetz cellsThe cellbodies of theseneuronsare the largest in the brain, approaching nearly 0.1mm indiameter.

Betz cellsBetz cells(also known aspyramidal cells of Betz) are giantpyramidal cells(neurons) located within the fifth layer of thegrey matterin theprimary motor cortexThese neurons are the largest in thecentral nervous system, sometimes reaching 100min diameterBetz cells areupper motor neuronsthat send theiraxonsdown to thespinal cordvia thecorticospinaltract, where inhumanstheysynapsedirectly withanterior horncells, which in turn synapse directly with their targetmuscles

UPPER MOTOR NEURONThe upper motor neuron descends in the spinal cord to the level of the appropriate spinal nerve rootAt this point, the upper motor neuron synapses with the lower motor neuron, each of whose axons innervate a fiber of skeletal muscle. These neurons connect thebrainto the appropriate level in the spinal cord, from which pointnerve signalscontinue to the muscles by means of thelower motor neurons.

UPPER MOTOR NEURONTheneurotransmitterglutamatetransmits the nerve impulses from upper to lower motor neurons, where it is detected byglutamatergicreceptors.

LOWER MOTOR NEURONLower motor neurons(LMNs) aremotor neuronslocated in either theanterior grey column,anterior nerve roots(spinal lower motor neurons) or thecranial nervenuclei of the brainstemand cranial nerves with motor function (cranial nerve lower motor neurons)

All voluntary movement relies on spinal lower motor neurons, which innervate skeletal muscle fibersand act as a link betweenupper motor neuronsand muscles

Cranial nerve lower motor neurons control movements of the eyes and tongue, and contribute to chewing, swallowing and vocalization Damage to the lower motor neuron can lead toflaccid paralysis.

CLASSIFICATIONClassification Lower motor neurons are classified based on the type of muscle fiber they innervate:Alpha motor neurons(-MNs) innervateextrafusal muscle fibers, the most numerous type of muscle fiber and the one involved inmuscle contraction.Beta motor neurons(-MNs) innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers (type of slow twitch fibers).Gamma motor neurons(-MNs) innervateintrafusal muscle fibers, which together with sensory afferents composemuscle spindles. These are part of the system for sensing body position(proprioception)

LOWER MOTOR NEURON

Glutamatereleased from the upper motor neurons triggersdepolarizationin the lower motor neurons in the anterior grey column, which in turn causes an action potential to propagate the length of theaxonto theneuromuscular junctionwhereacetylcholineis released to carry the signal across thesynaptic cleftto the postsynaptic receptors of themuscle cellmembrane, signaling the muscle to contract.

CONSEQUENCE OF LESIONDamage to lower motor neurons,lower motor neuron lesions(LMNL) causemuscle atrophy, decreased strength and decreasedreflexesin affected areas. These findings are in contrast to findings inupper motor neuron lesions. LMNL is indicated by abnormalEMGpotentials,fasciculations,paralysis,weakening of muscles, and neurogenicatrophyof skeletal muscle.) are all pathologies associated with lower motor neuron dysfunction.