reflexes

• Reflexes

  Involuntary coordinated patterns of muscle contraction and relaxation that involves an afferent signal into the spinal cord and an efferent signal out to the muscle

  
• Rhythmic movements


  Repetitive rhythmic motor patterns like chewing, swallowing, scratching, and alternative activation of flexors and extensors during locomotion
• Voluntary contractions


  Actions generated by the cerebral cortex in response to a perceived need, which improve with practice as a result of feedback and feed-forward mechanisms
• Reflex arc

  • Sensory receptor: site of stimulus
  • Sensory neuron:transmit afferent impulses to CNS
  • Integration centre in the CNS
  • Motor neuron: conducts efferent impulses from integration centre
  • Effector (muscle fibre or gland cell): produces responses (muscle contraction / gland secretion)

  
• Muscle receptors

  • Muscle spindle
  • Tendon organ

  
• Muscle spindle

  • Detects changes in skeletal muscle length and rate of change in length
  • consist of 2 sensory afferent fibres (1a, II) : motor efferent fibre + intrafusal muscle fibre
  • provide regulatory function for movement and maintenance of posture

  
• Tendon organ

  Detects changes in muscle force

  
• Stretch reflex pathwaybrain send signal down to spinal chord

  1. Muscle spindle activation
  2. Ia afferent signal to spinal cord
  3. Alpha motor neuron activation
  4. Muscle contraction: sensed by muscle spindle afferent
  5. 1A afferent send signal to spinal chord
  6. generate reflex response: resist strength in muscle / increase contractile activity of skeletal muscle

  
• Alpha-gamma coactivation

  
  

  • Voluntary contractions involve concurrent activation of alpha and gamma motor neurons by the cerebral cortex
  • alpha motor neurons generate contractile force in muscle
  • signal to gamma motor neurons at the same time which sensitize muscle spindles
  • change in muscle length sensed by 1a afferent motor neuron
  • muscle generates stretch response

  
• Tendon tap reflex

  • an example of stretch reflex
  • Tapping the patellar tendon stretches the muscle spindles in the quadriceps femoris muscle
  • stretch reflex in contraction of extensor muslce , resulting in a knee jerk response
  • at as preliminary assessment of nervous system function
• Golgi tendon organs


  • Connect with up to 25 extrafusal fibers near tendon's junction to muscle
  • Detect tension generated by active muscle to protect the muscle and surrounding connective tissue from sudden or excessive load
  • when stimualted by excessive tension, golgi receptors transmit signals to spinal cord via 1b afferent to elicit reflex inhibition of muscles they supply
  • protect muscle from injury due to sudden + excessive load

  
• Flexor withdrawal reflex


  1. Painful stimulus

  2. Coordinated excitation of flexors and inhibition of extensors in the stimulated limb, removing the body part from the stimulus

  3. variable depending on which afferents are activated

  4. slower than stretch reflex
• Crossed extensor reflex


  1. Ipsilateral flexor withdrawal reflex

  2. Contralateral extensor reflex

  3. Excitation of extensors and inhibition of flexors in the opposite limb

  4. important in maintaining balance when removing foot from a painful stimulus while standing
• Masticatory reflexes


  • Stretch reflexes from muscle spindles
  • Golgi tendon organ reflexes
  • Periodontal reflexes
  • Jaw opening reflexes

  
• Jaw jerk reflex


  Monosynaptic reflex excitation of jaw-closer muscles in response to stretch or increased resistance

  
• Unloading reflex


  Reflex decrease in jaw-closer muscle activity in response to decreased resistance, e.g. biting through a nut
• Periodontal reflexes


  Inhibition or excitation of jaw-closer muscles in response to tapping on a tooth or rapid increase in load force

  
• Jaw opening reflexes


  Inhibition of jaw-closer muscles in response to painful oral and perioral stimuli
• The stretch reflex involves a monosynaptic connection and resists muscle lengthening

  
• Golgi tendon organs produce reflex inhibition of the activated muscle

  
• Withdrawal and crossed extensor reflexes are triggered by a variety of sensory receptors
• Periodontal reflexes can produce either excitation or inhibition, depending on the type of stimulus
• Muscle receptors

  • largest reflex system in body
  • 2 sensory receptors within muscle : muscle spindle and tendon organ
  • provide rapid feedback signals to nervous system
• Muscle spindle sensitivity 


  • weight applied to muscle
  • muscle is stretched
  • sensed by muscle spindle
  • 1a afferent send action potential to spinal cord
  • resist stretch by generation of stretch reflex pathway
  • steady tension elicits steady firing of 1a afferent sensory fibre
• muscle spindle sensitivity 2

  • alpha motor neuron alone is stimulated: 1a afferent stops firing because spindle is unloaded by contraction
  • if gamma motor neuron is also stimulated: spindle is not unlaoded during contraction and 1a afferent activity is maintained
  • efferent innervation maintains spindle activity during muscle shortening
• • Golgi tendon organ reflexes
  • force sensors at origins and insertions of jaw muscles
  • normal role is inhibition of parent muslce
  • Periodontal ligament receptors at articulation of teeth
  • Jaw opening reflexes
• periodontal reflexes 


  • tapping on tooth / rapid increase in load force
  • evokes reflex inhibitions of jaw closer muscles
  • di-synaptic inhibition of jaw clopser muscles
  • weal pressure on a tooth evokes reflex excitation of jaw-closer muscles
  • presumably plays a role in helping jaw muscles to keep food between teeth
  • periodontal reflexes important in guiding teeth into occlusion + coordinating chewing to allow crushing of food
• jaw opening reflexes

  • painful oral and perioral stimuli inhibit jaw closing muscle
  • high-threshold mechanoreceptors and nociceptors in periodontal ligament, gingiva, oral mucosa, tongue, lips, facial skin, nociceptors in tooth pulp
  • di-synaptic excitation of jaw-openers in humans (only animals)
  • plays a protective role (e.g. fishbone in gingiva)