1 Extrapyramidal tracts â These tracts originate in the brain stem, carrying motor fibres to the spinal cord. 2  They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion.his article is about the descending tracts of the central nervous system. The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurones then directly innervate muscles to produce movement.
The word oxygen comes from Greek words: oxus (acid) and gennan (generate). Oxygen comes to earth from all parts of the universe in meteorites. It is also found in minerals and plants that grow on the earth and other planets.
By Rachelle Oblack. Definition: Oxygen in the atmosphere is the second most abundant gas in the atmosphere behind nitrogen. Oxygen makes up 29.047% of the atmosphere by volume in dry air. Earthâs atmosphere did not always have oxygen.The addition of oxygen occurred slowly from about 3.5 billion years ago to about 2.5 billion years ago.y Rachelle Oblack. Definition: Oxygen in the atmosphere is the second most abundant gas in the atmosphere behind nitrogen. Oxygen makes up 29.047% of the atmosphere by volume in dry air. Earthâs atmosphere did not always have oxygen.