In the 1800s, scientists knew a little about the nervous system, although a lot less than they know today. They knew the body had nerves. They knew the nerves were involved in reflexes that were input/output circuits built into the nervous system. In a reflex, some stimulus (an input to the nervous system) produces a response (an output from the nervous system). Here is a classic diagram of a spinal motor reflex from the 1920s. It shows the essential elements.
In a reflex, there is a sensory component (stimulus) and a motor component (response).
What components are found in all reflexes?
All reflexes have a stimulus component and a response component, corresponding to sensory and motor neural circuits, respectively. The simplest examples are spinal motor reflexes such as the knee jerk reflex (patellar reflex) or the finger withdrawal reflex. In the knee jerk reflex, a sensory neuron sends signals to the spine when the knee is tapped with a rubber hammer. In the spine, the sensory neuron synapses on the dendrites of a motor neuron, which sends nerve impulses back out to the muscle fibers, making the leg jerk. In the finger withdrawal reflex, pain such as the heat from a candle causes the finger to be withdrawn. The following figure, from the frontispiece of William James's Psychology: The Briefer Course (1892), shows the neural circuitry underlying the finger withdrawal reflex.
The finger-withdrawal reflex as depicted by William James in 1892. Inputs go through the sensory nerves (1 and S) and responses come out the motor nerves (M and 2).
The following table lists some reflexes in humans. In each case a stimulus activates sensory neurons, which trigger a motor response. The response is always biologically significant. It always helps to preserve the individual or the species.
Reflexes are stereotyped in form. They can be recognized in all normal members of a species. They are built in—they do not need to be learned. Snakes, for example, are born with all the reflexes they need. They emerge from an egg and slither away to hunt without any learning.
What did the word "reflex" mean to Pavlov and other Russian scientists?
The word reflex had a broader meaning in the late 1800s when Pavlov used the term. In those days the word reflex was used to describe almost everything we would today call biologically based behavior. Indeed, Russian psychologists called themselves reflexologists from the 1880s through about the 1920s. They believed reflexes were the basic building blocks of all behavior.
How did Pavlov's discovery seem to provide a missing link?
In the 1890s, an emphasis on reflexes seemed to make psychology more scientific, in the 1800s, by anchoring it in biology. However, this also presented a problem. Human beings were obviously influenced to a very great extent by learning. How could reflexes—patterns that seemed to be fixed into place by biology—be the building blocks of human behavior? The missing link seemed to be provided by Pavlov when he showed there was a way to modify reflexes...to bring them under the control of learning.