Upper motor neuron
A neuron that starts in the motor cortex of the brain and terminates within the medulla (another part of the brain) or within the spinal cord. Damage to upper motor neurons can result in spasticity and exaggerated reflexes. There is no standard laboratory test for upper motor neuron disease.
- Upper respiratory infection
An infection of the upper part of the respiratory system which is above the lungs. An upper respiratory infection can be due to any number of viral or bacterial infections. These infections may affect the throat (pharyngitis), nasopharynx (nasopharyngitis), sinuses (sinusitis), larynx (laryngitis), trachea (tracheitis) or bronchi (bronchitis). Upper respiratory infection is commonly abbreviated URI.
An increase in the number of receptors on the surface of target cells, making the cells more sensitive to a hormone or another agent. For example, there is an increase in uterine oxytocin receptors in the third trimester of pregnancy, promoting the contraction of the smooth muscle of the uterus.
A canal that connects the urinary bladder to the umbilicus (bellybutton) during fetal development. The urachus is normally obliterated, so it is usually a solid cord. Failure for the urachus to fill in leaves it open. The telltale sign of an open urachus is leakage of urine through the umbilicus. An open urachus is a […]
- Uracil (U)
A nucleotide base and one member of the A-U (adenine-uracil) base pair in RNA. The other base pair in RNA is G-C (guanine-cytosine). Uracil takes the place in RNA that thymine (T) occupies in DNA.
A metallic element that is used as nuclear fuel and is highly toxic and radioactive. Exposure to radiation from uranium can occur in various ways. The breakdown of uranium products creates radon daughters. These can attach to dust particles and, if workers inhale the dust, the particles lodge in their lungs, where they release high […]