This indicates that the first event in this sequence is a '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, e.g. sounds, physical contact, tastes, visual sensation, 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 (CNS), i.e. the brain and spinal cord. This is happens because peripheral nerves connect to the spinal cord via the network of nerves within the nervous system.Information so received by the CNS is further transmitted by relay neurone(s) within the CNS.
This is shown in more detail in Figure (2), below.The information may or may not be processed in the brain.Some stimuli lead to 'reflex responses' that can be described in terms of the 'Simple Reflex Arc', whereas other stimuli (such as all visual stimuli) always involve processing by the brain. More information about the simple reflex arc is shown in Figure (2), below.Following either simple reflex arc response, or processing by the brain, neural 'instructions' may be sent via a motor neurone to an effector (usually a muscle or gland). In this way, the effector is instructed to take action. The action may be physical movement of a muscle - hence perhaps also a body part such as a limb, or e.g. a chemical action by a gland. Whatever the consequent action, this has occurred due to the function of the nervous system, in response to the initial stimulus or stimuli.A 'Simple Reflex Arc'The following diagram shows the sequence of events described above in more detail, specifically for the case of the 'simple reflex arc'. i'm unsure if this helps at all or is even truley fully related as im not the best with this kind of thing but when i was doing studies about the human bodies i noted that.