Golgi Tendon Organ

• • golgi tendon organ is a much simpler sensory receptor than the muscle spindle
• Golgi Tendon Organ structure pt1:

  • the golgi tendon organ is found in the muscular tendinous junction - transition period between the muscle belly and the tendon
  • the golgi tendon organ is a specialist adapted axonal ending within the muscular tendinous junction
  • the muscular tendinous junction is formed by connective tissue, which is bundles of collagen

  
• Golgi Tendon Organ structure pt2:

  • golgi tendon organ's axon is entwined among the collagen which allows it to be exposed to the mechanical perturbation that arises when the tendon is under tension - the collagen fibres would be aligned and squashed and place pressure (mechanical perturbation) upon all these tendon that make up the golgi tendon organ
  • so the location and they way theyre entwined amongst the connective tissue enables the axons to respond to tension within this area

  
• Golgi Tendon Organ structure pt3:

  • the axons of the golgi tendon organ are also encapsulated to keep them away from the ionic concentrations (just like the muscle spindles)

  
• Golgi Tendon Organ function pt1:

  • the golgi tendon organ is a mechanoreceptor (just like the muscle spindle)
  • muscle spindles responds to the length of a muscle
  • golgi tendon organ responds to the amount of tension going through a muscle, specifically the musculotendinous junction

  
• Golgi Tendon Organ function pt2:

  • the axons of the golgi tendon organ are located in the connective tissue and pick up tension of the collagen getting squashed together, squeezing the axons and increasing the permeability of their membranes, permitting the influx of sodium, causing depolarisation, causing voltage gated channels to open, further influx of sodium and then action potentials to be generated

  
• Golgi Tendon Organ function pt2:

  • the golgi tendon organs will produce a frequency of action potentials that is directly proportionate to the amount of tension going through the musculotendinous junction
  • if theres a lot of tension, they'll get squashed a lot, so theres a big change in permeability, so theres a big influx of sodium, lots of depolarisation, lots of action potentials
  • if theres not much tension, they wont get squashed a lot, therefore there wont be much depolarisation
  • so its a simple linear relationship between tension and frequency of action potentials

  
• Golgi Tendon Organ:

  • the golgi tendon organ is innervated by 1B afferents - not as big as 1A, so golgi has a smaller axon than muscle spindle's axon, and action potentials will travel a bit slower
  • action potentials travel up through the dorsal root, past the dorsal root ganglion, where the soma is, into the spinal cord where the axon synapses with interneurons
  • the golgi also sends branches up the ascending afferent pathways to the CNS to help with motor control regarding tension within a muscle

  
• • muscle spindle provides information about length of a muscle and the speed its lengthening at and the golgi provides information about the amount of tension going through the muscle
  • the combination of those 3 things is all the information needed to know how the body is moving - such as its position, rate of change of movement and the actual tension going through the muscles
• Golgi Tendon Organ afferents pt1:

  • diagram shows the presence of an interneurons in a reflex circuit
  • golgi tendon organ's afferent 1B comes in the spinal cord and synapses with an interneuron, forming an excitatory synapse, causing depolarisation of the interneuron which generates action potentials
  • these travel down their axon and go to an alpha motor neurone, which together form an inhibitory synapse, making it less likely for it to generate action potentials
  • this alpha motor neurone goes out through the ventral root to the same muscle the golgi innervates

  
• Golgi Tendon Organ afferents pt2:

  • tension of the muscle causes inhibition of the alpha motor neurone, which causes further tension in the muscle
  • but this little loop causes tension to stop - there's multiple theories forming from this

  
• Golgi Tendon Organ afferents hypothesis:

  • the alpha motor neurone causing inhibition to stop tension in muscle is thought to have been a protective mechanism
  • when theres a large force going through the muscle that could be damaging, this mechanism will switch the muscle off, reducing the force, minimising the risk of damage
  • under a lot of question as the strength of this loop in terms of its ability to switch of a muscle is not that great and the timing doesnt fit

  
• Golgi Tendon Organ & descending tracts pt1:

  • the interneuron that the 1B afferent synapses to has further inputs, not just one to the alpha motor neurone
  • some of these inputs come down the descending tracts, altering the level of activity and ability of that interneuron to effect the alpha motor neurone and therefore the level of force in a muscle

  
• Golgi Tendon Organ & descending tracts pt2:

  • if the descending input into the interneuron were excitatory, the excitability of the interneuron would increase, depolarising it, making it generate a higher frequency of action potentials in response to anything coming from the golgi, and would have to effect of enhancing the golgi's inhibition on the alpha motor neurone
  • if the descending input into the interneuron were inhibitory, hyperpolarising it, it would make it less likely that this golgi loop would be able to inhibit the alpha motor neurone

  
• Golgi Tendon Organ & descending tracts pt3:

  • so this is a mechanism where the ability of the golgi tendon to inhibit the alpha motor neurone is under descending control and it fluctuates (hypothesis) depending on the task and behaviour we're doing and its another spinal mechanism where we control output from a muscle

  
• Golgi Tendon Organ & descending tracts pt4:

  • so if we're in a situation where the task or behaviour at hand requires maximal force output, the descending inputs will adjust the interneuron accordingly to inhibit the interneuron, so that we could not get inhibition of the alpha motor neuron
  • whereas if we're doing a fine dextrous task that doesnt want excessive force output, the descending inputs will adjust the interneuron accordingly, depolarising them, making it more likely that the golgi feedback loop could reduce force output

  
• Golgi Tendon Organ & descending tracts pt5:

  • so this is a hypothesis mechanism where force production from a muscle can be adjusted through spinal interneurons = descending inputs modulate the interneuron, adjusting our force output according to the task we're undertaking
  • this mechanism is dependent on descending tracts
  • so pts with damage to descending pathways can have impairments to this mechanism

  
• Golgi and Spindle Provide:

  • Proprioception / Kinasthesia:
  • The position of our body and limbs
  • The speed and amplitude of movement
  • The level of force being applied
  • Afferent Drive of spinal reflexes