Increased tone from disease of the corticospinal system is usually called spasticity. Rigidity usually implies extrapyramidal disease.
With a unilateral corticospinal system lesion above the brain stem, there are no changes in tone evident in the head and neck.
In the upper limb the increased tone is in the flexors of the fingers, wrist, and elbow. It is greatest in the early part of stretching. It can be abolished temporarily by repeated stretches and will return after the muscle has been put at rest. The posture of the upper limb is the consequence of flexor muscle spasticity and extensor muscle weakness.
In the lower limb, spasticity is first and greatest in the quadriceps, gastroc-nemius, and soleus. The footdrop posture is also a combined manifestation, the foot being dropped down by dorsiflexor weakness and pulled down by plantar flexor spasticity.
A hemiparesis resulting from a cortical lesion (a very rare occurrence) will be flaccid and will remain this way. Many acute intracranial and all acute spinal, corticospinal tract lesions present initially with flaccid weakness. In the latter case this is called spinal shock. After days or weeks the limbs become spastic. An acute lesion in one cerebellar hemisphere will present with a flaccid, ipsilateral weakness, ataxia, and often absent reflexes.
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