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The reflex arc is composed of 5 parts:

sensory receptors, afferent neuron(s), synapses within the central integrating station, efferent neuron(s) and the effector (includes transmission across the neuro-muscular junction and contractile mechanism - Assuming this is a myotatic reflex).

Which part takes the longest time & why?

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I am providing this answer considering myotatic reflex arc (as you have specified in the question).

There are so many factors on which it depends...I have listed as many as I could - Here the sensory receptor is intrafusal muscle fibres. Afferent neurons are type Ia or type II. Central processing may involve mono- or poly- synaptic junction(s). Efferent neuron is alpha or gamma motoneurons. Effector is intra- or extra- fusal muscle spindle.

Considering the human or animal to be of normal health and condition (absence of hypoxia, pyrexia, high or low pressure and anesthesia) sensory receptor takes the shortest time in reflex arc. Central synaptic junction may be mono- or poly- synaptic. Poly-synaptic junctions take longer time, but still it is shorter than neuro-muscular transmission.

So the competition now is between neurons and neuro-muscular junction. In case of neurons it mainly depends on the length, diameter and myelination (provided the above mentioned conditions about the sample is "normal"). Neuro-muscular transmission speed depends on the length of synaptic cleft, body calcium and chloride level, type of muscle fiber innervated and the type of reflex arc (alpha-gamma co-activation, reciprocal innervation etc) to name a few. So, it can be said that longest time would depend on the reflex arc you are considering, as this will decide the speed of transmission through the afferent and efferent neurons and neuro-muscular junction.

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The answer to this question depends which reflex arc was activated, without knowing the exact reflex arc one should assume the answer as multifactorial and relate to this (or similar) table:

enter image description here

(Source: Fundamental Neuroscience for Basic and Clinical Applications, by Duane E. Haines)

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  • $\begingroup$ This provides only information on the conduction velocities. Thing is that peripheral sensory transduction, synaptic delays in the reflex arc and in the neuromuscular junction may add considerable amounts of time too, and not to forget the time it takes for the muscle to contract. Conduction velocities are just part of the equation. $\endgroup$ – AliceD Dec 15 '15 at 12:07
  • $\begingroup$ @Christiaan In other words, this question could not be answered. $\endgroup$ – Ilan Dec 15 '15 at 17:23
  • $\begingroup$ Then why did you O.o ? $\endgroup$ – AliceD Dec 15 '15 at 19:48
  • $\begingroup$ @Christiaan I gave a direction... Sometimes it is better than exact answer. $\endgroup$ – Ilan Dec 15 '15 at 19:53
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    $\begingroup$ The nerves in the gray matter would expect to conduct slower. Also synaptic transmission is generally slower than saltatory conduction. $\endgroup$ – rhill45 Dec 17 '15 at 1:48

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