UMN send fibers to the LMN, and that exert direct or indirect supranuclear control over the LMN.
Lower motor neurons (LMN) directly innervate the skeletal muscle
Motor cortex: the UMNs are located in the primary motor cortex, Brodmann’s area 4, and the premotor areas, Brodmann’s area 6 (secondary motor complex and premotor complex).
Betz’s giant pyramidal neurons are the distinct group of neurons in layer 5 and other smaller neurons initiate the contraction of the small groups of the skeletal muscles and control the force and contraction.
Signs and Symptoms of UMN Disorders
Loss of dexterity, voluntary skillful movements. It may be expressed as stiffness, slowness, and clumsiness, in particular, rapid repetitive motions.
weakness is mild. UMN: UEx extensors, and LEx flexors.
Spasticity is hallmark of the UMN disease. Spasticity is a state of sustained increase in muscle tension in response to muscle lengthening, in particular, with passive movements.
Pseudobulbar palsy is hallmark of the UMN disorder, which is characterized by sudden unmotivated crying or laughing.
The LMNs are located in the brainstem and spinal cord
The spinal LMNs are also known as anterior horn cell. The neurons are clustered in nuclei, forming longitudinal columns.
Dorsal anterior horn cells innervate distal muscles, ventral located cells- proximal muscles, medially located neurons- truncal and axial muscles. Markedly enlarged lateral parts of the cervical and lumbar (lower thoracic) anterior horns innervate arm, hand, and leg muscles.
Large spinal cord LMNs are called alpha neurons.
Signs and Symptoms of Lower Motor Neuron Dysfunction
Weakness: denervation as well as decreased number of functional LMN units reduces overall muscles strength.
Muscle atrophy and Hyporeflexia
Muscle hypotonicity and flaccidity
Primary Lateral sclerosis: a diagnosis of exclusion
Hereditary spastic paraplegia: AD disorder
HTLV-1 associated myelopathy: X-linked recessive inheritance, increased serum of very-long-chain fatty acids
Lathyrism: history of consumption of chickpeas
Disorders of LMNs
Multifocal Motor Neuropathy
Benign Focal Amyotrophy
Hopkins’ syndrome: Acute post-asthmatic amyotrophy
Bulbo-Spinal Muscular Atrophy (BSMA; Kennedy's Syndrome; X-linked)
Primary Muscular Atrophy (PMA)..