The simplest level of integration in the nervous system is the reflex. Reflexes are specific, automatic responses to their specific adequate stimuli (eliciting or triggering stimuli). They adapt the body to the effect of the adequate stimulus. The body as a general rule tries to counteract anything done to it.

For example, a tap to the tendon under the knee cap elicits (triggers) the "knee jerk" reflex. The diagram below shows how this reflex works.

The tendon below the knee cap attaches the muscle that straightens the knee to the lower leg bone. The tap to the tendon stretches that muscle, which stimulates receptors that detect muscle stretch. The stimulation triggers neural signals that go into the spinal cord, where they connect to (among other places) neurons that control the muscle that was stretched. These neurons send signals to that muscle, which contracts to kick the knee out. This reflex counteracts the stimulus that triggered it.

All reflexes have 5 parts:

1. The sensory receptor, which is tuned to the reflex's adequate (specific triggering) stimulus (salivary reflex is triggered by taste in mouth; eye blink reflex is triggered by air puff to eye, etc.).
2. the sensory (or afferen) axon, which carries the neural signal from the receptor into the spinal cord over a dorsal root.
3. one or usually more synapses (connections to other nerve cells) in the grey matter of the cord.
4. motor (or efferent) axons that carry signals from the cord over a ventral root to the effector.
5. the effector, the muscle or gland that actually produces the reflex response you can observe.

The body contains many reflexes besides the stretch reflex. You probably recognize these:

• Withdrawal reflex: Touching something painfully hot elicits (triggers) automatic jerk of the hand away from the source of burning heat.
• Sneeze: A tickle in the nose elicits (triggers) a complex pattern of muscle contractions in throat, chest, and abdomen, which together produce a sneeze to get rid of the tickle.
• Salivary reflex: A sour lemon elicits salivation to dilute the acid.
• Pupillary reflex: Light in the eye makes the pupil (dark disk in middle of colored iris) get smaller. You can check this in the bathroom mirror. Cover one eye; the pupils of both eyes get bigger, which you can see in the uncovered eye. Uncover the eye, and both pupils get smaller.
• Blink reflex: A puff of air or other irritation around the eye triggers blinking.

Other reflexes adjust blood pressure, heart rate, and other vital functions in response in increased or decreased demand for oxygenated blood, make eyes jump left or riight as you turn your head, increase or decrease actions of gut depending on food content, etc.