Upper motor neuron lesion is characterized by: March 2013
**Core Concept**
Upper motor neuron (UMN) lesions result from damage to the motor pathways that originate in the cerebral cortex and descend to the spinal cord or brainstem. This type of lesion affects the corticospinal tract, which is responsible for voluntary motor control.
**Why the Correct Answer is Right**
A UMN lesion leads to a characteristic pattern of weakness and spasticity. The affected limb is typically characterized by **increased muscle tone**, **hyperreflexia**, and a **positive Babinski sign**. This is due to the loss of inhibition from the cerebral cortex on the lower motor neurons, resulting in an unopposed effect of excitatory neurotransmitters such as glutamate.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because lower motor neuron lesions typically result in flaccid paralysis, decreased muscle tone, and hyporeflexia.
**Option B:** This option is incorrect because UMN lesions do not typically cause muscle atrophy, which is more characteristic of lower motor neuron lesions.
**Option C:** This option is incorrect because UMN lesions do not typically cause loss of proprioception, which is more characteristic of peripheral neuropathy.
**Clinical Pearl / High-Yield Fact**
A key feature of UMN lesions is the presence of **spasticity**, which can be relieved by administering drugs such as baclofen or diazepam.
**Correct Answer: C. Loss of proprioception.**