Why is there hypotonia in UMN lesions?

Damage to Descending Motor Pathways: The Upper Motor Neuron Syndrome. This initial period of “hypotonia” after upper motor neuron injury is called spinal shock, and reflects the decreased activity of spinal circuits suddenly deprived of input from the motor cortex and brainstem.

Why does upper motor neuron lesion causes Hypertonia?

Through mechanisms that are not well understood, the loss of descending inputs tends to result in an increased firing rate of alpha and/or gamma motor neurons. The higher firing rate causes an increase in the resting level of muscle activity, resulting in hypertonia.

Why is there hypotonia in cerebellar lesions?

It is typically seen with acute hemispheric lesions and is often accompanied by hyporeflexia. Hypotonia likely results from decreased fusimotor activity resulting in decreased muscle spindle afferent response. It is usually a transient phenomenon after an acute lesion but can be seen in chronic lesions as well.

Is hypotonia upper or lower motor neuron?

Lower Motor Neuron Lesions Flaccidity, or hypotonia, is a typical feature of lower motor neuron damage, and in the extreme form of total flaccid paralysis, all resistance to passive muscle stretch is lost and the limbs become flail-like.

Why does UMN cause Hyperreflexia?

Hyperreflexia and hypertonia are the classic upper motor neuron (UMN) signs thought to occur from the loss of corticospinal motor tract suppression of the spinal reflex arc.

How does UMN lesion cause Hyperreflexia?

Hyperreflexia indicates an upper motor neuron lesion, and reflects a loss of inhibitory modulation of the motor pathways. It is often associated with increased muscle tone (spasticity).

What is UMN lesion?

UMN lesions are designated as any damage to the motor neurons that reside above nuclei of cranial nerves or the anterior horn cells of the spinal cord. Damage to UMN’s leads to a characteristic set of clinical symptoms known as the upper motor neuron syndrome.