Signalling properties of muscle spindles and tendon organs

Some important issues for muscle receptors remain unresolved. For muscle spindles it is uncertain how responses to combined static and dynamic fusimotor stimulation may summate. Such summation may occur during certain phases of locomotion. Two mechanisms considered here include electrotonic spread of generator current between sources of impulse activity and mechanical interactions between contracting intrafusal fibres. For tendon organs, it remains uncertain what aspects of muscle tension they signal. Here they were tested for their ability to respond to rises in whole-muscle passive tension after eccentric contractions. It was found that only when motor units were contracted which had a direct action on a tendon organ did it signal the rise in tension. The finding raises questions about the role of tendon organs as monitors of muscle tension.