#reflex arc

Receptor -- receptor end of a dendrite or a specialized receptor cell in a sensory organ -- senses specific type of internal or external change

Sensory Neuron -- dendrite, cell body, and axon of a sensory neuron -- carries information from receptor to brain or spinal cord

Interneuron -- dendrite, cell body, and axon of a neuron in the brain or spinal cord -- carries information from sensory neuron to motor neuron

Motor Neuron -- dendrite, cell body, and axon of a motor neuron -- carries instructions from brain or spinal cord out to effector

Effector -- muscle or gland -- responds to stimulation or inhibition by motor neuron -- produces reflex or action

Another Counterfactual Central Nervous System

In A Counterfactual on Central Nervous System Development I speculated on the possibility of an evolutionary trajectory in which central nervous system (CNS) clusters developed supervening upon sensory organs, so that the organisms of such a counterfactual biosphere had “smart” sensory organs—smart eyes, for example—and perhaps the entirety of a sophisticated brain evolved supervening directly upon sensory organs. Human eyes move in the sockets, so the analogy doesn’t work well in this case, but our ears and our noses don’t move (other than external cartilage), so that we could imagine a cluster of nerves supervening upon our inner ear neural mechanisms that eventually grow until they constitute the largest neural center of the organism in question.

The above scenario came to me by way of the concrete example of complex eyes. Another counterfactual CNS occurred to me as a result of thinking about reflexes that involve the CNS but do not bring the brain into the reaction. Reflexes have played a significant role in evolution; if we had to think about some dangers, there would be many times that we would not respond fast enough to survive a sudden danger. Reflexes allow organisms to respond almost instantly, and so have a high survival value; natural selection strongly favors reflexes that save the life of the selected organism.

Reflexes are possible due to reflex arcs, which are neural pathways that channel a nerve stimulation and its response through the spinal column before the nerve stimulation reaches the brain. When you experience a reflex action, your body has responded before the experience has even registered in your brain, and well before you can experience a conscious thought about the experience, because the reflex arc is significantly shorter than any neural pathway that would involve the brain in the circuit.  

There is a passage from Darwin that I have often thought about in which he discusses his attempt to suppress a reflex action:

“Another familiar instance of a reflex action is the involuntary closing of the eyelids when the surface of the eye is touched. A similar winking movement is caused when a blow is directed towards the face; but this is an habitual and not a strictly reflex action, as the stimulus is conveyed through the mind and not by the excitement of a peripheral nerve. The whole body and head are generally at the same time drawn suddenly backwards. These latter movements, however, can be prevented, if the danger does not appear to the imagination imminent; but our reason telling us that there is no danger does not suffice. I may mention a trifling fact, illustrating this point, and which at the time amused me. I put my face close to the thick glass-plate in front of a puff-adder in the Zoological Gardens, with the firm determination of not starting back if the snake struck at me; but, as soon as the blow was struck, my resolution went for nothing, and I jumped a yard or two backwards with astonishing rapidity. My will and reason were powerless against the imagination of a danger which had never been experienced.” (Charles Darwin, The Expression of Emotions in Man and Animals, chapter 1)

This was my point of origin for another thought experiment in a counterfactual CNS. Suppose that a form of life with a nascent CNS converges upon distributing much more of its behavior to reflexes than has been the case with terrestrial life. Suppose that the nerve fibers between the limbs and the CNS proliferate and grow more complex, so that a range of reflexes are controlled from a bundle of nerves like a spinal cord and allow for relatively sophisticated responses without a brain to anchor the CNS. Such structures up and down something like a spinal cord—perhaps growing from the integrating center of a reflex arc—could eventually evolve into a series of neuronal processing centers, no one of which was an equal to the human brain, or even to an average mammal brain, but which strung together along a spinal cord equivalent might be a powerful substitute for a brain.

The vertebral column protects the delicate spinal cord even as it allows for movement, and there is a selective value for these functions (flexibility and spinal cord protection), just as there has been a selective value for the brain to be protected by the skull in endoskeletal organisms. If a spinal cord-like structure evolved into a primary processing center of a CNS, there would be a similar selection pressure to protect this organ within bone or cartilage or chitin or some similarly robust organic structure that could evolve within the body of a biological being. We could imagine a much larger vertebral column, with a much larger spinal canal, though such an arrangement would probably limit motion while never fully allowing for the development of the nervous tissues it contained.

This might work in an elongated body, like a snake, that could string together a large number of integrating centers. In a strongly segmented body, like those of insects, the physiological structure of the kind of alternative CNS that I have described would need to be quite different—like the insect compound eye is different from the lens eyes of mammals, birds, and cephalopods. The point here is that CNS architecture seems to be a result of deep homology, as we see the structure of a spinal cord or a notocord with a brain at one end is found throughout the animal kingdom on Earth. In another biosphere, if the kind of CNS architecture I have described were to become the model of all life, genetically transmitted to all subsequent organisms, we would find it conserved by deep homology and variously expressed across a range of organisms.    

With more distributed functions, evolving from the elaboration of reflex mechanisms, and multiple centers for cognition evolving from the distributed nerve clusters supervening on integrating centers of the reflex arc, and many more reflex actions functions than those we possess, the experience such beings would have of their world would be different, their response to their world would be somewhat different than ours is to our world, and, if their CNS grew to the complexity of making conscious thought possible, it would be likely that this consciousness would have a different structure than the consciousness of terrestrial beings. Interesting possibilities would follow from these differences.    

One might ask what the value is of this kind of speculation. The more that I consider the problem, the more I have come to think that the only way for our anthropocentric minds to grasp radically different forms of life, consciousness, and thought is by starting with familiar forms of life, consciousness, and thought, working out possible variations on these themes of terrestrial emergent complexity, and then building on these thought experiments with variations on the theme of our previous variations. In this way, we can slowly build toward radically different forms of emergent complexity, first only quantitatively different from what we experience, and later, with many iterations of this process, arriving at emergent complexities qualitatively different from any we know.

I think of these kind of thought experiments as a form of conceptual preparation for what Carl Sagan called “deprovincialization,” as when in Cosmos he wrote, “The study of a single instance of extraterrestrial life, no matter how humble, will deprovincialize biology.” We are not yet in a position to study extraterrestrial life, consciousness, or thought, and there are endless debates on the degree of incommensurability of alien emergent complexities, but the actual work of trying to formulate truly alien conceptions of life, consciousness, and thought is mostly yet to be done. This present thought experiment is one of my ongoing attempts to break through the familiar, and, following the methodology I have described, it can be a stepping stone to further such efforts that move beyond what I have suggested here. 

Struggling to remember the order of the reflex arc? Well here’s a simple mnemonic to help you remember it:

Syndicate      (stimulus)

Reacted        (receptor cells)

Saltily            (sensory neurone)

Realising      (relay neurone)

Mianite         (motor neurone)

Expected      (effector)

Revenge      (response)

Making your stretching more effective. 

While I was making linguine and clam sauce for my family, one of my favorite foods that I haven’t had in quite some time( and listening to Dream Theater of course) I was thinking about this post.  Then I remembered about voice recognition on my iMac.  Talk about multitasking!

What do you agree that stretching is good or not, you or your client still may decide to do so possibly because of the “feel good” component. Make sure to see this post here on “feel good”  part from a few weeks ago. 

If you do decide to stretch, make sure you take advantage of you or your clients neurology.  There are many ways to do this. One way we will discuss today is taking advantage of what we call myotatic reflex.

The myotatic reflex is a simple reflex arc. The reflex begins at the receptor in the muscle (blue neuron above) : the muscle spindles (nuclear bag or nuclear chain fibers). This sensory (afferent) information then travels up the peripheral nerve to the dorsal horn of the spinal cord where it enters and synapses in the ventral horn on an alpha motor neuron.  The motor neuron (efferent) leaves the ventral horn and travels back down the peripheral nerve to the contractile portion of the myfibrils (muscle fiber) from which the the sensory (afferent) signal came (red neuron above).  This causes the muscle to contract. Think of a simple reflex when somebody taps a reflex hammer on your tendon. This causes the muscle to contract and your limb moves.

Nuclear bag and nuclear chain fibers detect length or stretch in a the muscle whereas Golgi Tendon organs tension. We have discussed this in other posts here.   With this in mind, slow stretch of a muscle causes it to contract more, through the muscle spindle mechanism.

Another reflex that we should be familiar with is called reciprocal inhibition. It states simply that when one muscle (the agonist) contracts it’s antagonist is inhibited (green neuron above).  You can find more on reciprocal inhibition here.

Take advantage of both of these reflexes?   Try this:

do a calf stretch like this: put your foot in dorsiflexion, foot resting on the side of the doorframe.

Keep your leg straight.

Grab the the door frame with your arms and slowly draw your stomach toward the door frame. 

Feel the stretch in your calf; this is a slow stretch. Can you feel the increased tension in your calf? You could fatigue this reflex if you stretched long enough. If you did, then the muscle would be difficult to activate. This is one of the reasons stretching seems to inhibit performance. 

Now for an added stretch, dorsiflex your toes and try to bring your foot upward.  Did you notice how you can get more stretch your calf and increased length? This is reciprocal inhibition at work!

There you have it, one neurological tool of many to give you increased length.The next time you are statically stretching, take  advantage of these reflexes to make it more effective.

 The Gait Guys. Teaching you more  about anatomy, physiology, and neurology with each and every post. 

The Reflex Arc