The Golgi tendon organ is an organ that can sense changes in muscle tension and send information to the brain about them. It can be found where the skeletal muscle fibers join the tendons. It's made up of nerves and strands of collagen, an elastic protein.
One end of the collagen strands is connected to muscle fibers and the other end merges into the tendon. The tendon organ itself is enclosed in a capsule that's pierced by nerve fibers.
The Golgi tendon organ was discovered by Camillo Golgi, an Italian doctor who studied the nervous system around the turn of the last century.