We can use this demonstration to learn how reflexes work. Many important motor reflexes are processed by the spinal cord and occur without any input from our brain. Motor reflexes are involuntary, which we will see here. Make someone move without their control! (again)
Reflexes require no thought. They are automatic, fast, and of huge importance to a human's ability to successfully respond to their environment. Despite the magnificent information-processing power of the billions of neurons in our brain, we need a lot of stuff to be done automatically. Without reflexes, our brains would be overloaded with worrying about constantly updating the position of our unstable bodies to keep us upright. Without reflexes, our ability to engage in complex thought (black holes, neuroscience, what to do this weekend, how do I make an instrumented reflex hammer?) would be limited. Without reflexes, your reactions to painful stimuli would require thought, and. don't take it personally.. but you think very slowly. Don't feel bad, all humans are slow thinkers, and we need more speed to respond to dangerous painful stimuli. So we let our spinal cord do that fast work for us.
One example of a reflex is the patellar stretch reflex. Our spinal cord partners with sensors in our muscles, called muscle spindles, to keep track of where our bodies are in space and how stretched or contracted our muscles are. The way that these sensors interact with our spinal cord is through a reflex pathway. Stretching the muscle activates the muscle spindle at the end of the sensory neuron (embedded in your muscle) and starts the reflex. The reflex is to prevent overstretching of the muscle and compensates with a contraction.
As you can see, there is only one connection (a synapse) needed for the information from the sensory neuron to get to the motor neuron and cause a muscle contraction. Because of this single synapse, this can happen very fast. In a young, healthy person, it takes 15-30 milliseconds for the stretch stimulus to produce a muscle contraction, by comparison, it takes 5-10 times that long to blink your eye in response to a stimulus, or 150-300 milliseconds. This is super useful for correcting your muscle length in response to rapid changes such as a slip or trip. These situations require very fast corrections to prevent falling and injury. If you had to consciously flex your leg in response to the leg stretch (a reaction) it would be much slower than the 15-30 seconds of reflex.
Now let's try to measure this reflex! And perhaps let's measure a reaction too!
Before you begin, make sure you have the Backyard Brains Spike Recorder installed on your computer/smartphone/tablet. The Backyard Brains Spike Recorder program allows you to visualize and save the data on your computer when doing experiments. We have also built a simple lab handout to help you tabulate your data.
If you purchased your Muscle SpikerBox Pro before Feb 2019, you will need to follow the Firmware Update Instructions
