In we humans, (leaving aside the knee jerk and other such jerks), most responses involve a longer CIRCUIT with numerous SYNAPSES and NEURONS arranged in series and in parallel.

Our big brains help accommodate this expanded synaptic bridge that links SENSATION to ACTION.  Thus, bigger and more evolved the brain, more the number of neurons and connections and hence more the repertoire of available responses. 

 Although the total number of NEURONS is, more or less, fixed at our birth, we can always form new and numerous CONNECTIONS/SYNAPSES to improve our brain’s processing capacity. In other words, we can upgrade our hardware (brain connections/synapses) for newer software (behavioural responses)