Lecture #10: Golgi Tendon Organs PDF

University of British Columbia Lecture #10: Golgi Tendon Organs The source of “the code”  University of British Columbia Goals for Today – Where we have been, where we are going…. – GTO morphology – GTO physiology: encoding properties feedbac...

University of British Columbia Lecture #10: Golgi Tendon Organs The source of “the code”  University of British Columbia Goals for Today – Where we have been, where we are going…. – GTO morphology – GTO physiology: encoding properties feedback pathways  University of British Columbia Golgi Tendon Organs (GTO) GTO’s are located at the tendon junction. Spindles are spread throughout the muscle belly. Spindles lie in parallel to muscle fibres, GTO’s in Series.  University of British Columbia morphology Structure  University of British Columbia Golgi Tendon Organ Capsule (1 mm by 0.1mm) 10-20 muscle fibers Not all muscle fibers in a muscle pass through a GTO. Therefore some are in series and some are in parallel. The afferent fiber that innervates the golgi tendon organ is the 1B afferent.  University of British Columbia each of these muscle fibers - usually from different motor units  University of British Columbia Sensory Innervation single Ib afferent (2% of GTOs have 2 Ib afferents) nerve endings then interdigitate among collagen fibers  University of British Columbia So, what do GTO’s Code for? ………………muscle length? …………………….muscle velocity? …………………………….muscle what? muscle receptors that provide feedback to the CNS about muscle force or Tension  University of British Columbia Sensitivity and Unloading Once thought that GTO’s designed to signal dangerously high force levels. However, Level of force necessary to excite a GTO depends on mode of activation. Passive stretch requires ~2 Newtons, whereas activation of “a” single muscle fiber (30-90 millinewtons), can activate GTO. Therefore, GTO’s much more sensitive to actively generated forces than passive (external?) ones.  University of British Columbia GTOs are sensitive to muscle contraction In series versus in Parallel response to muscle shortening and increased tension during a twitch.  University of British Columbia passive The GTO is designed to monitor the force produced by the GTO firing rate muscle actively, rather than force generated passively. active In trace A, the GTO is insensitive to passive GTO firing rate muscle stretch through joint angle change. With active contraction (see EMG) the GTO reflects better the level of muscle activity.  University of British Columbia GTO’s discharge rate is approximately linear for force. Each GTO’s discharge rate will scale linearly With increasing force in Muscle…. Figure 35-6B Kandel  University of British Columbia Population code of GTO’s Instantaneous frequencies of a population of GTO’s (6). MU A activates all 6 GTO’s, MU B activates 4 GTO’s, BUT when MU A + B, get all 6 GTO’s again activated, and a very, very good representation of muscle force.  University of British Columbia GTOs provide motor feedback feedback to spinal cord via Ib afferent disynaptic connection to motoneuron Ib inhibitory interneuron Perhaps an inappropriate name as this interneuron gets input from cutaneous, joint, and many descending sources. autogenic inhibition – inhibits agonist muscle.  University of British Columbia summary 1. GTOs monitor muscle force/tension 2. active contraction > passive stretch 3. GTO population code (ensemble code) 4. provide feedback to the CNS via negative feedback pathway  University of British Columbia morphology Goals for Today – Where we have been, where we are going….. – GTO morphology – GTO physiology: encoding properties feedback pathways