The cerebral peduncles are two cylinders composed entirely of nerves, with one on each side of the brain. These bundles of nerves carry information between the higher brain (the cortex) and the lower part of the brain (the brain stem). The information that the cerebral peduncles carry is information about body movement. Damage to the cerebral peduncles usually shows up in some other part of the body.
Functions of the Cerebral Peduncle
The cerebral (meaning having to do with the brain) peduncles (meaning a stem like connection between organs) are two tracts of neural fibers that are usually hidden by the temporal lobes of the brain. The cerebral peduncles run from the base of the cortex to the pons (bridge) and carry information about movement. The pons is a round swelling on the front of the brain stem that serves as a connection between the lower brain (the brain stem) and the higher brain (the cortex). The cerebral peduncles are only two of the bundles of nerve fibers that connect to the pons. The cerebral peduncles refine movements. If the simple commands to move that come from the cortex were actually sent to the body, the movements would be jerky and uncoordinated. With the help of the midbrain, especially the cerebellum, the cerebral peduncles refine these commands by adjusting them to account for the present positions of the body parts. The refined commands are then sent to the body along with information about how to accelerate movement and how to decelerate movement depending on how the movement is going. The cerebral peduncles are composed of two very different tracts: the crus cerebri and the tegmentum.
The Crus Cerebri
The crus (leg shaped) cerbri (of the brain) is the front portion of a cerebral peduncle. This tract of nerves carries the actual commands to the body parts to control movement. This information is the result of the interaction between the conscious decision to move that originated in the cortex and the modifications and refinements made by the brain stem based on the present state and position of the body. For example, when you decide to reach behind you without turning around, the cortex makes the decision to move, but the brain stem adds on the part about rotating your arm so that the palm turns up, because that makes the reach easier. The brain stem also monitors the movement to make sure that it is smooth. The crus cerebri then carries this "refined" information to the body. Damage to the crus cerebri usually paralyzes some part of the body.
The tegmentum (covering) is the back part of a cerebral peduncle. It appears very early in embryonic development. It sends information back and forth between the cortex and brain stem during the "consulting process" that leads to the development of the refined information that will be put onto the crus cerebri. When the gegmentum is damaged, the body produces jerky, robot-like movements.