The question you're asking here seems more of a question concerning neural circuitry, or as mentioned above, how certain neural routes are dictated by genes. By this, I basically mean, as an example, how transduced electromagnetic radiation makes it to the visual cortex: light stimulates the neural tunic (the retina) of the eye, an AP propagates via the optic nerve, crosses at the optic chiasm, synapses with the lateral geniculate nucleus of the thalamus, and eventually processes up the visual cortical hierarchy in the occipital lobe. This, right here, is and abridged example of a type of neural
circuit you're referring to.
The patellar reflex is precisely the same thing. Addressing your question, afferent impulses from the 'tap' travel along axons bundled within the femoral nerve. Eventually, that nerve diverts into its sensory and motor components in the spinal cord, termed the ventral root and dorsal root ganglion region. There, sensory neurons will directly synapse with motor neurons where the impulse will regroup down the efferent pathway of the femoral nerve, stimulating a sudden jerk. This is a prime example of a monosynaptic reflex arc. There are several other reflexes that involve more synaptic activity and are therefore termed polysynaptic reflexes. Also, one last thing which I did not add because it was already mentioned, the jerk is caused my stimulation/relaxation of the quadriceps and hamstring.
Hopefully this helps and sorry if this thread was dead, I'm new to the form!
The sensory neuron directly synapses on the motor neuron. It is -so to say- hard-wired. The only thing the sensory neuron "decides" is whether the sensory input is strong enough to elicit an action potential, but not to which motor neuron it has to send the impulse. Same thing with the interneuron, which leads to a relaxation of the hamstring muscle: direct synapses "don't leave the sensory neuron a choice".
Exactly, nice post.
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