One Monday we covered a very famous stretch reflex, the myotatic reflex.
Today I want to take you to another level of complexity with the flexor withdrawal/crossed extensor thrust reflex (that’s a mouthful!).
The flexor withdrawal/crossed extensor thrust reflex is another spinal reflex, but instead of involving only two neurons and one muscle, it involves 5 neurons, a bunch of interneurons and synapses and 4 muscles on each side of the body.
I like to talk about this reflex because it is simple enough to be explained in a straight forward manner, but still complex enough to show people that unconscious reflexes can involve quite complex behaviors.
The Flexor Withdrawal/Crossed Extensor Thrust Reflex
Let’s start with the stimulus. While the stimulus for the myotatic was a short and light stretch of the tendon, this one requires a much stronger stimulus. Think about pain.
- Lets say something hurts your leg strongly, you want to remove that leg from harms way (by flexing it away) and transfer your weight on the other leg so as not to loose balance.
So, as soon as the stimulus hits, the afferent neuron sends that information to the spinal cord much as in the myotatic reflex.
The difference starts here. Instead of synapsing on one motor neuron in the anterior horn of the spinal cord, it synapses mostly in the inter zone of the spinal cord (grey central laminae). There, it will synapse on a bunch on interneuron. For our purpose, let’s say it synapses on 2 interneurons: one on the ispsilateral side (same side as stimulus) and one on the contralateral side.
- To make things easier I will first talk about what happens on the ipsilateral side, then on what happens on the contralateral side.
On The Ipsilateral Side
On the ipsilateral side, the interneuron that just received the input from the afferent neuron will, itself synapse on 2 different neurons.
A. Inhibitory Interneuron
This first interneuron will synapse on an inhibitory interneuron, and that inhibitory interneuron will synapse on the motor neuron controlling the muscle that just got hurt.
In this case, when the inhibitory interneuron gets an impulse it effectively prevents the following subsequent interneuron from firing an action potential and contracting the muscle.
If you remember our overview of the knee jerk reflex, you remember that in that previous example, it’s the muscle that got tapped that ends up contracting (The quadriceps femurs muscle). Here it’s the muscle that got hurt that ends up NOT contracting.
B. Motor Neuron
The first ipsilateral interneuron will also synapse (and activate) the motor neuron of the opposite (antagonist) muscle on the same leg (the hamstring muscle).
Once this motor neuron fires an action potential, it will contract the hamstring, thus moving the leg that was hurt away from harm. (The other muscle is being inhibited so as to make sure that the leg flexes as fast as possible, without any resistance).
On The Contralateral Side
Here we have a similar but opposite situation as in the ispilateral side: the hamstring gets relaxed and the quadriceps get contracted to make sure that the other leg stays extended so we can put our weight on it while the other leg is bent.
The first interneuron on the contralateral side synapses on 2 neurons: one inhibitory interneuron, and one motor neuron.
A. Inhibitory Interneuron
The Inhibitory interneuron synapses on the motor neuron for the hamstring muscle thus preventing it from contracting.
B. Motor Neuron
The motor neuron connects to the quadriceps femurs muscle and the impulse ends up contracting it.
Minimum To Remember
The end goal of this reflex is to bend the hurt leg away from harm, and switch the weight on the other leg to support our weight.
- To bend the first leg we need to flex the hamstring and prevent the contraction of the quadricep (so we need an inhibitory interneuron in between)
- The other leg needs to stay straight, and so we need to contract our quadricep and prevent the contraction of the hamstring (and so we need an inhibitory interneuron in between)
Note: There are really a great number of interneurons involved in this reflex. The description above is a simplified version of what happens in reality. The most important point to understand is that there are inhibitory interneurons involved on both sides for this reflex.
If you can understand why we need inhibitory interneurons where they are, you understand the main point of this example.
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