Why Are Upper Motor Neuron Signs Difficult to Elicit in ALS?
Why Are Upper Motor Neuron Signs Difficult to Elicit in ALS?
In ALS, in addition to loss of functional α motor neurons that innervate striated muscle fibres, there are other abnormalities in the cord. In the anterior horns of the spinal segments, motor neurons are arranged such that neurons innervating axial muscles are located in medially placed longitudinal nuclei, and neurons innervating distal muscles, such as small hand muscles, are situated laterally. Each spinal nucleus innervating a muscle contains interneuronal connexions from related flexor and extensor muscles, including internuncial connexions from the opposite side of the cord, presynaptic Renshaw inhibition, descending propriospinal interneuronal connexions, and input from muscle spindle and tendon afferents (figure 1). In humans, the interneurons associated with the segmental α motor neurons are located in close proximity. Small γ motor neurons innervating muscle spindles are located in relation to their functional associations with the spinal motor nuclei representing the innervation of specific muscles. Histological studies of the cord in ALS have revealed that interneurons degenerate alongside loss of α motor neurons and that the γ and β innervation to muscle spindles is also lost in the disease, although the primary and secondary sensory innervation of muscle spindles is intact. The α–γ coactivation during voluntary movement therefore is no longer possible. Exactly what effect this might have on dynamic stretch reflexes during voluntary movement, and the generation of spasticity, is uncertain, but de-efferented spindles have been shown to continue to function normally to muscle stretch, tendon vibration and tendon percussion. The response to a dynamic tendon stretch is principally dependent on the level of segmental excitation of motoneurons innervating the tested muscle.
Loss of Spinal Interneurons and Gamma Motor Neurons in ALS
In ALS, in addition to loss of functional α motor neurons that innervate striated muscle fibres, there are other abnormalities in the cord. In the anterior horns of the spinal segments, motor neurons are arranged such that neurons innervating axial muscles are located in medially placed longitudinal nuclei, and neurons innervating distal muscles, such as small hand muscles, are situated laterally. Each spinal nucleus innervating a muscle contains interneuronal connexions from related flexor and extensor muscles, including internuncial connexions from the opposite side of the cord, presynaptic Renshaw inhibition, descending propriospinal interneuronal connexions, and input from muscle spindle and tendon afferents (figure 1). In humans, the interneurons associated with the segmental α motor neurons are located in close proximity. Small γ motor neurons innervating muscle spindles are located in relation to their functional associations with the spinal motor nuclei representing the innervation of specific muscles. Histological studies of the cord in ALS have revealed that interneurons degenerate alongside loss of α motor neurons and that the γ and β innervation to muscle spindles is also lost in the disease, although the primary and secondary sensory innervation of muscle spindles is intact. The α–γ coactivation during voluntary movement therefore is no longer possible. Exactly what effect this might have on dynamic stretch reflexes during voluntary movement, and the generation of spasticity, is uncertain, but de-efferented spindles have been shown to continue to function normally to muscle stretch, tendon vibration and tendon percussion. The response to a dynamic tendon stretch is principally dependent on the level of segmental excitation of motoneurons innervating the tested muscle.