ORGANUM
Weakness is a decreased ability to voluntarily and actively move muscles. This may be generalised or localised to one aspect of the body. Hypotonia is a state of low muscles resistance to movement. Hypotonia can be associated with weakness, but in some cases, is present with normal motor strength.
Weakness and hypotonia may be due to disorders of upper motor or lower motor neurons. UMN originate in the cerebral motor cortex; their axons form the corticospinal tract ending in the spinal cord and control voluntary motor activity. The anterior horn cells, their motor roots, peripheral motor nerves, neuromuscular junctions, and muscles represent the lower motor neurons and muscle units. Maintenance of normal strength, tone and coordination requires integrated communication throughout this complex system, including the cerebral cortex, cerebellum, brainstem, thalamus, basal ganglia and spinal cord.
Weakness caused by upper motor neuron disease differs from weakness produced by lower motor units. Disease of the upper motor neuron causes loss of control, but not total loss of movement as motor nuclei of the basal ganglia, thalamus, and brainstem have tracts that produce simple or complex stereotyped patterns of movement. The corticospinal tract permits fine motor activity and is best tested by rapid alternating movements of the distal extremities. Mild dysfunction produces slowed, stiff motions. More severe dysfunction produces stiff, abnormal involuntary postures (spasticity) consisting of forearm flexion at the elbow and wrist and adduction close to the chest, with leg extension and adduction.
Damage to the spinal cord leaves residual simple, stereotyped relfex movements corrdinated by local spinal reflexes below the. level of the lesion. Destruction of the lower motor neuron, the final common pathway producing muscle activity, leads to total absence of movement with hypotonia. Function is best tested by measuring the strength of individual muscle groups, or in young child, observing the ability to perform tasks requiring particular muscle groups (walk up or down stairs, arise from the ground, walk on toes or heels, raise the hands above the head, squeeze a ball).