Sensory Receptors

• Muscle Spindle - Sensory Receptors:

  • sensory receptors lay upon the intrafusal fibres

  
• Muscle Spindle - Sensory Receptors:

  • afferent (sensory) receptors can be categorised in terms of their size
  • Type 1A = have the largest myelinated axon and the fastest conduction speed - the sensory information generated in this receptor is communicated rapidly up to the spinal cord and to the CNS at about 150 metres per second
  • Type 2 = have a smaller axon and are slower when conducting

  A) type 1A

  B) type 2

  
• Muscle Spindle - Sensory Receptors - Type 1A:

  • afferent (sensory) receptors are also different in terms of which parts of the intrafusal fibres they attach to
  • Type 1A:
  • the axon comes into the spindle
  • the axons very specialised and wraps around the central portion of the all the intrafusal fibres via annulospiral endings
  • they wrap around tightly and attach to the membrane of the intrafusal fibres - so if the membrane of the intrafusal fibre is pulled/tensioned, the membrane of the axon is pulled/tensioned

  A) type 1A

  
• Muscle Spindle - Sensory Receptors - Type 2:

  • afferent (sensory) receptors are also different in terms of which parts of the intrafusal fibres they attach to
  • Type 2:
  • the axons comes in more distal from the central portion
  • attach to the membrane of the intrafusal fibres, but do not wrap around
  • instead they branch out in way called flower-spray endings

  A) type 2

  
• Muscle Spindle - Sensory Receptors - Action Potential Generation pt1:

  • key point is that the axons are attaches to the membrane of the intrafusal fibres
  • these sensory endings are called mechanoreceptors - respond to mechanical perturbation
  • mechanical deformation occurs, causes tension
  • this puts the intrafusal fibres membrane under tension
  • and so the axons membrane come under tension
  • this increases the axons membranes permeability allowing sodium to follow its electrochemical gradient in the membrane - causes depolarisation of the axons membrane

  
• Muscle Spindle - Sensory Receptors - Action Potential Generation pt2:

  • if depolarisation of the axons membrane reaches -55, all the voltage gated channels on the axon to open - this causes a further influx of sodium, generating an action potential
  • this is called sensory transduction - where a force is transduced into creating an action potential
  • in simple terms, stretch upon a spindle produces action potentials

  
• Muscle Spindle - Sensory Receptors - Action Potential Generation pt3:

  • the more stretch (mechanical deformation) = the more frequency of action potentials - a lot of stretch on a spindle will increase amount of tension on membrane of spindles intrafusal fibres and that will increase the tension on the axon membrane which increases the permeability even more, allowing even more sodium to flow in, causing more depolarisation and the more depolarisation there is means the higher frequency of action potentials that are generated

  
• Muscle Spindle - Sensory Receptors - Action Potential Generation pt4:

  • so these sensory receptors generate action potentials at a frequency thats directly proportionate to the degree of stretch occurring
  • and that frequency of action potentials will travel up the axon (e.g. at 150 metres per second - type 1a) to the CNS to provide with information about the length of a muscle and the speed its lengthening at - relates to both the annulospiral and flower-spray endings as theyre both mechanoreceptors