Upper Motor Neuron (UMN) lesions are lesions occurring anywhere in the CNS from the brain up to the spinal cord before the alpha motor neurons arise from the spinal cord. The lesion could arise from the cerebral cortex, internal capsule, midbrain, pons, medulla, and the cortico-spinal tract in the spinal cord. Lower motor neuron (LMN) lesions may arise from disease processes affecting the anterior horn cell or the motor axon and/or its surrounding myelin. Neuromuscular junction pathology and muscle disorders may mimic an LMN disorder and form part of the differential diagnosis. In an LMN lesion, the muscle becomes hypersensitive to neurotransmitter as it is denervated. Similarly, the damaged lower motor erratically discharges the neurotransmitter stored within itself as the neuron degrades. So, both increased hypersensitivity and erratic release of neurotransmitter cause fasciculations. However, in UMN lesions, there is regular firing to prevent the atrophy of muscles. LMN syndromes are clinically characterized by muscle atrophy, weakness, and hyporeflexia without sensory involvement. Neuromuscular Junctions, the junction between a motor neuron and muscle fiber is a specialized synapse. The motor neuron releases a flood of acetylcholine (Ach) neurotransmitters upon stimulation from the axon terminals from synaptic vesicles that bind with the post-synaptic receptors at the plasma membrane. This response is contractile causing muscle contraction and inhibition does not require a neurotransmitter release. UMC & LMN lesions cause very different clinical findings. UMN lesions are lesions anywhere from the cortex to the descending tracts. This lesion causes hyperreflexia, spasticity, and a positive Babinski reflex, presenting as an upward response of the big toe when the plantar surface of the foot is stroked, with other toes fanning out. LMN lesions are lesions anywhere from the anterior horn of the spinal cord, peripheral nerve, neuromuscular junction, or muscle. This type of lesion causes hyporeflexia, flaccid paralysis, and atrophy. Satyendra Dhar MD, @DharSaty #UMN #LMN #Muscleweakness #musclearc #flaccid #atrophy #peripheralnerve #NeuromuscularJunctions #NMJ
