Re: When fishing, can the worm feel pain when stuck with the hook?

Dear Liz,

You've presented an interesting question. It is interesting because you are attempting to understand the nervous system of a worm in the context of a human nervous system. It is one thing to describe the neural pathways for a worm, but it sounds like you want to understand those pathways as they compare to a human's neural pathways. First I will describe for you the basics of how nerves work. Then I'll describe how the nervous systems of worms are similar to those of humans, and why I feel that the best answer to your question is that, yes, worms do feel pain.

Ultimately what we're comparing are reflex arcs. These arcs are combinations of sensory and motor neurons connected by interneurons. Sensory neurons carry messages from sensory organs, such as your skin or eyes. Motor neurons carry signals to muscles, instructing them how and when to move. Interneurons gather the signals from one or more sensory nerves and then decide if a movement should take place. If they decide a movement should take place, the interneurons cause the motor neurons to fire.

In humans, different kinds of sensation come from different groupings of sensory nerves. Your senses of touch, pain, and temperature all travel in separate nerve groups up to your central nervous system. Your central nervous system has many, many interneurons analyzing the signals of the sensory neurons, before deciding whether or not to move muscles by triggering motor neurons.

Like humans, worms do have a central nervous system with many interneurons. But are the number of interneurons linked with ability to perceive pain? I will give an example of human reflex arcs that suggests to me that the answer is yes.

If you put your hand on a hot stove, your hand will jerk back from the stove reflexively. A few instants later, you will begin to feel the pain of the burn. There are two reflex arcs involved: one that caused the movement (but caused no pain) and one that caused pain (after movement had already taken place). The reflex arc that caused the movement of your hand away from the stove has only one interneuron involved. It's fast and effective, requiring no other neurons to decide to move. The feeling of pain, which comes after the hand has moved away from the stove, involves a separate, slower neurual pathway. The painful pathway uses interneurons all through the brain to understand what's causing the injury. It is the involvement of many interneurons that allows for memory to occur. Memory is important so you keep your hand away from hot stoves in the future. Pain and memory are powerfully interrelated.

We do know that earthworms are capable of some memory (avoidance of repeated electrical stimuli) and that they have distinct central and peripheral nervous systems. They react quite differently to injurious stimuli and touch stimuli. So it seems that they do have the ability to feel pain. Certainly they don't possess memory to the degree that a human does. Nevertheless, pain comparable to human pain is likely a part of worm behavior.

Fantastic pictures of glowing earthworm nerve distibutions for the central and peripheral nervous systems can be found through links at the University of South Hampton's Wormland web page.

General information about worms, including how their nervous system works, can be found at Nick Musurca's Earthworm Web Page. In particular, there is a great illustration which shows the location of the worm's brain and major nerves.

I hope this information is helpful to you. Worms are very interesting creatures. If you'd like to know more about them, don't hesitate to ask us!


Your MAD Invertebrate Scientist,

John