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ultrasonic Sounds with frequencies above 20,000 Hz. ultrasonic aspirator An apparatus commonly used in human neurosurgery to remove tissue; the tip ultrasonically vibrates at about 25KHz to break up tissue which is removed by an attached piece that provides irrigation and suction. Related Words ultrasonic upper motor neuron A neuron that is part of the motor nerve pathway that orignates in the brain and terminates at a peripheral motor neuron; damage to this pathway releases the peripheral nerve from central control, often causing exaggerated responses.    Related Words brain  ;  central nervous system (CNS)  ;  motor nerve  ;  motor neuron  ;  neuron  ;  peripheral nervous system (PNS) urine specific gravity (USG) Measures the concentrating ability of the kidney tubules and hydration status of the patient;  in general, the greater the USG, the more concentrated the urine. ►click here to learn more-link provided by: Cornell University, College of Veterinary Medicine Related Words kidney