By , 4 months ago

In a general case, the pathway of a nerve impulse may be summarized as in the following diagram:

Stimuli --->Sensory Receptors ---> Sensory neuron ---> C.N.S.--->Relay Neuron --------> Brain ---> Motor Neuron ---> Effector (Muscle/gland)



This indicates that the first event in this sequence is some "Stimulus". ("Stimuli" is the plural form, referring to more than one stimulus.)
      In this context, a stimulus is something that human sensory receptors are able to detect.
      Examples may include: Sounds; Physical contact; Tastes; Visual stimuli, etc..
    * The next stage in the pathway is the "Sensory Receptors" sensing the stimulus.
      These receptors are located all over the body but some types of receptors are in specific areas of the body (e.g. taste receptors in the       mouth).
    * Sensory neuron(s) then transmit information from the sensory receptor(s) to the Central Nervous System (i.e. the brain and spinal cord.). This is happens because peripheral nerves all connect to the spinal cord via the network of nerves within the nervous system.
    * The information so received by the C.N.S. is further transmitted by relay neurone(s) with the C.N.S.

So from the above explanation it is clear that the first receptor of the reflex arc is Sensory Receptor.