What is a reflex?
Quick answer: a quick movement that occurs in response to something before you even realize you are moving.
Alternative answer: a local motor response to a local sensation.
When the doctor taps on your knee, you respond by kicking your leg. This occurs quickly, before you even realize that you did it. Another reflex is pulling your hand away from a painful or hot or freezing item quickly, before realizing you did it. Another is blinking when something touches your eye. And gagging when you something is placed down your throat (like the doctor telling you to say "ah" while he/she shoves a popsicle stick into the back of your mouth). All of these reflexes are motor responses to sensory information... where the movement is of the same body region that sensed the stimulus.
You will see that these reflexes occur because the sensory neurons that bring the stimulus information into the spinal cord synapse directly or nearly directly onto motor neurons that lie in that area of the spinal cord that the sensory neurons entered. You will see on the spinal nerves page that each region of the body has sensory neurons for it and motor neurons that control it within the same region of the spinal cord.
A reflex may be monosynaptic, disynaptic, or polysynaptic
If the sensory neuron comes in and directly synapses on the motor neuron, that is called a monosynaptic reflex. Monosynaptic means "one synapse." So, in the monosynaptic reflex, there is only one synapse that occurs in the spinal cord: SN -----< MN (the line with the "<" represents an axon with axon terminals). This reflex is quick: it only takes as long to respond as it takes for the sensory neuron to carry the information to the spinal cord, cross the synapse to the motor neuron, run back out the motor neuron axon to the muscle, and cause contraction. Not even one second is necessary.
A disynaptic reflex requires that one interneuron be interposed between the sensory neuron and motor neuron (SN -----< IN -----< MN). A polysynaptic reflex has more than two synapses in the spinal cord (SN -----< IN -----< IN -----< MN). The disynaptic and polysynaptic reflexes are the most common. One example of a disynaptic reflex is shown in your book, and I have put the figure here, too. But in order to explain the details of reflexes to you, I am using my own drawings below.
The knee-jerk reflex is an example of a monosynaptic reflex
The knee-jerk reflex is called that because it is a reflex most commonly tested by tapping on the patellar tendon and seeing if the leg kicks out, or jerks, in response. In actuality, this particular test is a hard one to start off explaining, so I'm going to start with a different example of the same type of reflex.
In order to understand this reflex at all, I have to introduce you to proprioception. I've mentioned this sense in class before. Proprioception is the awareness of your body's position in space. Even with your eyes closed, you know where your hands and arms and legs are. Are your hands on your keyboard or on your lap? Are your legs crossed or uncrossed?
How do we sense this? One way we do it is by using sensory neurons that have their dendrites embedded within special muscle fibers within your skeletal muscles, called muscle spindles. When your muscles change length, that excites the dendrites of these proprioceptive sensory neurons within the muscle spindles. Consider your biceps brachii muscle for a moment. As your elbow extends, this muscle stretches. And as your elbow flexes, biceps brachii shortens. Using stick figures (since we saw how useful they were in lab this week), I drew a scenario that involves muscle spindles embedded within the biceps brachii muscle.
As this stick figure holds a cup steadily, the biceps brachii doesn't change length. But, no one can hold a cup perfectly still... with time, the arm may extend a teeny bit at the elbow. This is unintentional. But, if it were to go uncorrected, the stick figure might spill the liquid from the cup.
Luckily, as the arm extends, even just a teeny bit, the biceps brachii stretches a teeny bit. This stretch activates the muscle spindle sensory neurons, and they send this sensory information into the spinal cord to cause a correcting movement-- a small contraction of the biceps brachii. This tiny contraction is all that is needed to restore arm position.
Therefore, you can think of the knee-jerk reflex as a correctional system. Any deviations from planned movements (in the figure above, the planned movement was to remain still) will be corrected for.
Note that this type of movement should be a reflex, since we do not need to involve our brains in correcting every deviation from planned movements. It is handy that the spinal cord can take care of this for us. Could you imagine if you had to think about every movement all the time-- we would never have room for other, higher thoughts to occur at the same time as we moved around! Yikes.
What would this reflex look like in terms of cellular components within our nervous systems? Here's my drawing of what it would look like. First just look at the connection from the sensory neuron to the purple motor neuron.
The sensory neuron brings the information from the muscle spindle into the spinal cord. The SN axon synapses directly on motor neurons that innervate the muscle containing the muscle spindle. So, if the SN brought in information from spindles within the biceps brachii muscle, it would synapse directly on biceps brachii MNs. This monosynaptic connection is the knee-jerk reflex. The result would be contraction of the biceps brachii muscle.
Of course, if you try to contract your biceps brachii at the same time that your triceps brachii is active, you won't get any movement. Right? When antagonists are active together, that stabilizes position, making it more rigid. What we want to do is to correct the position... that's why there's a bit more to this reflex than just the SN and the purple MN.
Now look at the other part of this diagram. The SN sends its information to an IN within the spinal cord as well. These INs synapse onto MNs that innervate antagonists to the muscle containing the muscle spindle. And these synapses are inhibitory. Let me explain by using our example again... if the SN brings information from the biceps brachii muscle spindle into the spinal cord, it synapses on an interneuron which inhibits the triceps brachii muscle. If we have to flex at the elbow to correct our movement error, we need to turn off the triceps brachii muscle so that it doesn't oppose the elbow flexion. This second half of the knee-jerk reflex is disynaptic.
How come when the doctor taps your patellar tendon you kick in response? The patellar tendon is a tendon of the quadriceps muscle group. The quadriceps muscles all work to extend the knee. The tap on the tendon is an artificially-induced stretching of the quadriceps muscles. The quadriceps muscle spindles then sense that the muscles have been stretched (which would normally mean that the knee has flexed). The SNs from the spindles of the quadriceps muscles bring this information back to the spinal cord to cause the quadriceps muscles to contract (and to relax the hamstring muscles). The result: a kick.
The crossed-extensor reflex is another important reflex
This is the reflex that will make more sense to you as far as its function is concerned, but is a little trickier for how the cells contribute to it...
If a person touches something hot, or feels something sharp, that person tends to move away from the noxious stimulus quickly. If your hand touches a hot object, you remove your hand so fast that it is safe before your brain even registers that you moved. That's a reflex.
Here's how it works overall. If you step on a tack, you do that as you extend your leg to step. In response to that pain, you have to pick up your leg, or flex it. However, if all you do is flex your leg-- without extending the other leg-- you'll probably fall flat on your face!
It is because of this need to extend the contralateral leg (in order to stay upright) that this reflex is called the crossed- extensor reflex. And, like with all reflexes, it occurs locally in the spinal cord... your brain doesn't register what's going on until later. Only then can you say "ow!" or whatever else you might say after stepping on a tack.
Consider what has to happen, physically, one more time:
Ready for the cellular version within the spinal cord? I think you are. The SN in this figure is one that detects pain. It enters the spinal cord and has to stop ipsilateral extension as well as cause ipsilateral flexion. Can you follow the diagram and see that occurring? (the red interneuron is the one that inhibits cells it synapses on, while the black interneuron excites postsynaptic cells).
The information from this pain SN has to cross the spinal cord locally to get to the other side. It crosses via interneurons that send their axons across the midline. On the contralateral side, the pain information has to stop flexion and cause extension. Can you see that happening, too?
Note that in my drawing, all the connections appear to be disynaptic. I did that to simplify the drawing. Many of these connections require another IN... so they are polysynaptic. I just didn't draw them as polysynaptic connections because it looks so confusing.
Your book refers to a "withdrawal reflex." That is the left half of the drawing above of the crossed-extensor reflex. I don't bother with it, because it is rare that we only involve one half of our body in a reflex movement to get away from painful stimuli. If you touch something hot or painful with your finger, as you jerk your hand away quickly, your other hand moves, too, in order to keep your balance. In fact, if you are standing, your legs get involved, too, so that you don't fall over. The "withdrawal reflex" doesn't really happen in isolation. But it is one of those things that every textbook includes.
Why don't we realize that we felt something painful until after the reflex movement is over?
In order to realize that we feel something painful (or to realize that we have moved), information has to go to our brains and synapse quite a few times there. If you step on a tack, the SN sends information to all the local INs in the diagram above, as well as to other INs that take this information on up to the brain (in the spinal tracts of the white matter).
It takes very little time for the information to pass through a couple of synapses locally and then head back to the muscles and cause contraction; the reflex occurs quickly. It takes more time for the information to make it all the way up your spinal cord to your brain and through the many synapses it has to make there in order for you to come to the realization that you felt something painful or that you moved because of it. Since sending the information to the brain takes longer than sending the information through the reflex, we can't possibly realize that we felt something painful or moved because of it until after the reflex has occurred.
© 2011 STCC Foundation Press