Lecture 17- Basic reflexes and the maintenance of posture Flashcards
What does the reflex arc consist of?
Sense organ
Afferent neuron > dorsal horn of spinal cord (control centre): synapses with effector neuron/ interneurons > ventral root/ motor cranial nerve
What are the components of the stretch reflex?
receptor – muscle spindle afferent fibre – muscle spindle afferent integration centre – lamina IX of spinal cord efferent fibre – α-motoneurones effector – muscle
Process of the stretch reflex e.g knee jerk reflex
starts when muscle spindle stretched (caused by tap stimulus in knee jerk reflex). muscle spindles are responsible for detecting the length of the muscles fibres.
when stretch is detected- action potentials to be fired by Ia afferent fibres.
Ia fibres synapse within spinal cord with α-motoneurones (innervate extrafusal fibres.)
antagonistic muscle inhibited and the agonist muscle contracts i.e. in the knee jerk reflex the quadriceps contract and the hamstrings relax.
What control the sensitivity of the stretch reflex?
The sensitivity of the reflex is regulated by gamma motoneurones – these lead to tightening or relaxing of muscle fibres within the muscle spindle.
What is the golgi tendon organ
Monitors force of the muscle- consists of knobbly nerve endings embedded within the fascicles of a tendon
Endings supplied by Ib afferent fibres
Lie in series with skeletal muscle fibres, discharge during passive stretching/ muscle contraction due to tendon stretch
How do each influence motor control
Muscle spindle afferents transduce (ie convert the message into another form) muscle length, whereas GTO afferents transduce muscle force.
What may be seen with increased Y discharge to muscle?
Tremor- due to sensitised muscle spindles
Clonus- exaggerated tremor with very little stimuli
UMN lesions when descending inhibitory input to Y fibres absent, amytrophic lateral sclerosis, MS, spinal cord legions and hepatic encephalopathy
What may be seen in hypertonic (overly excitable stretch reflexes)?
Clasp-knife effect- arm pushed back and is then pulled into elbow rapidly and uncontrollably
UMN lesion and concomitant spastic paralysis- passive flexion resisted by triceps stretch reflex but replaced by inverse stretch response when tendon stretched
What occurs in the flexor reflex?
Stimulation of flexor muscle inhbits antagonistic enxtensor muscle of same limb- in opposing limb the muscles reflexively act the opposite way (crossed extensor reflex)
What occurs in the flexor reflex?
Stimulation of flexor muscle inhbits antagonistic enxtensor muscle of same limb- in opposing limb the muscles reflexively act the opposite way (crossed extensor reflex)
What occurs in the positive supporting reflex?
Leg extends to push down on a finger touching the sole- foot can follow the finger (even without supraspinal influence)
Usually overridden with supraspinal relfexes in conscious humans, but not in UMN lesions
