Skeletal muscle contraction is ultimately initiated by activity in the nervous system. Muscle receives both sensory and motor nerve fibres. The sensory nerves convey information about the state of the muscle to the nervous system. This includes information about muscle length detected by the muscle spindles and tension detected by the Golgi tendon organs. There are also a variety of free nerve endings in the muscle tissue, some of which convey sensations of pain. Of the motor fibres in mammals, the γ-motoneurons provide a separate motor nerve supply for the muscle fibres of the muscle spindles. However, the bulk of the muscle fibres are supplied by the α-motoneurons. Each α-motoneuron innervates a number of muscle fibres, from less than ten in the extraocular muscles, which move the eyeball in its socket, to over a thousand in a large limb muscle. The complex of one motoneuron plus the muscle fibres which it innervates is called a motor unit. Since they are all activated by the same nerve cell, all the muscle fibres in a single motor unit contract at the same time. However, muscle fibres belonging to different motor units may well contract at different times. Thus, most mammalian muscle fibres are contacted by a single nerve terminal, although sometimes there may be two terminals originating from the same nerve axon. Muscle fibres of this type are known as twitch fibres, since they respond to nervous stimulation with a rapid twitch.