[00:00:04] Speaker A: Id the Future, a podcast about evolution and intelligent design.
[00:00:12] Speaker B: Welcome to id the future. I'm your host, Andrew McDermott. Today I'm welcoming back Dr. Jonathan McClatchy to discuss the intelligent design of muscles. Dr. McClatchy is a fellow and resident biologist at the Discovery Institute's center for Science and Culture. He was previously an assistant professor at Sadler College in Boston, where he lectured biology for four years. McClatchy holds a bachelor's degree in forensic biology, a master's degree in evolutionary biology, a second master's in medical and molecular bioscience, and a PhD in evolutionary biology. His research interests include the scientific evidence of design and nature, arguments for the existence of God, and New Testament scholarship. Jonathan is also founder and director of Talkaboutdoubts.com. Jonathan, it's great to have you with me again.
[00:01:02] Speaker A: Great to be back. Thanks very much for having me.
[00:01:04] Speaker B: You're welcome. Well, as we learn more about biological systems, we're also learning that perhaps the best way to study these systems is through an engineering lens. The bodies of animals and humans contain a vast network of integrated systems, all working together to sustain life. It's helpful to remember that the laws of nature by themselves don't tend toward life. They actually tend toward degradation and death. Without the ability to innovate and circumvent these natural inclinations, we'd all be toast. Jonathan, you and I have discussed a few key systems of the human body already on the podcast, including our hearing and the systems involved in sexual reproduction. Today I'd like to talk with you about the intelligent design of muscles you've written
[email protected] on this topic. So let's discuss some of your insights. In his book Darwin's Black Box, biochemist Michael Behe writes that in order to understand the barriers to evolution, we have to bite the bullet of complexity. He also says to appreciate complexity, you have to experience it. Now, your articles on muscles help us experience, if you will, the complexity of these systems. And although this process can get a bit technical, it's a necessary part of appreciating the design of the systems. So first, how many muscles are found in the human body, and what is their primary job?
[00:02:28] Speaker A: Well, Andrew, we actually have more than 600 muscles in our bodies. Most of them are attached by tendons to the skeleton, and their principal purpose is to move the skeleton. And when skeletal muscles contract, they shorten and pull the bone. And the contraction of muscles also creates heat, which contributes to the maintenance of our core body temperature.
[00:02:53] Speaker B: Okay, that's a lot of muscles. Now, in your article, you linked to a short animation that illuminates the anatomy of skeletal muscles. We'll include a link to that in the episode's show notes. But for now, can you give us a very brief overview of the key parts of muscles?
[00:03:08] Speaker A: Absolutely. So, each skeletal muscle possesses many thousands of cells. These are known as muscle fibers or otherwise called myocytes. The number of muscle fibers that contract depend on what task you're actually performing. So, for example, if you were to pick up a book, that would require the contraction of more finger flexing muscle fibers than if you were to pick up a pencil, for example, which requires the contraction of relatively few fibers. So the muscle fibers are organized into bundles that are known as fascicles. And these fascicles are given structural support by a connective tissue that surrounds them. It's called the paramesium. And the paramesium protects and distributes the blood vessels and nerves that supply the muscle, in addition to facilitating the transmission of forces generated by muscle contractions. And together with the endomesium, which is connective tissue that surrounds individual muscle fibers, and the epimesium, which is connective tissue that surrounds the entire muscle, the perimesium helps to maintain the overall integrity and indeed, the organization of skeletal muscle.
[00:04:15] Speaker B: Okay, now, there are two main types of muscles in our bodies. They're called opposing antagonists and cooperative synergists. What's the difference between those two? And can you give us a few examples?
[00:04:27] Speaker A: Sure. So, antagonistic muscles are pairs of muscles that have opposite actions at a joint. One muscle in the pair is responsible for producing a specific movement, while the other has an opposing action. So, for example, the biceps and the triceps in the upper arm are a classic example of antagonistic muscles. And when you flex your elbow, the biceps, which are located on the front of the upper arm, contract to bend the arm. While the triceps relax and lengthen, muscles are unable to push. They exert no force when they relax. And so the elbow can be flexed by the biceps but cannot be extended. And thus we need another muscle known as the triceps, which is located on the back of the upper arm. And when you extend your elbow, the triceps contract and the biceps relax. But synergistic muscles, by contrast, work together to produce the same movement as a joint. So they assist what's called the prime mover in performing a specific action. And that helps to stabilize the joint and provides additional force or control to the movement. So, for instance, when you flex your elbow, the biceps are the prime mover. But other muscles act as synergists to assist the biceps in generating the movement. And these synergistic muscles provide support and help to fine tune the motion. Now, you might wonder why we need three muscles to carry out the same task. And when your hand is positioned palm up, the prime mover, which does most of the work, is of flexing, is the biceps. And when your hand is positioned palmed down, the prime mover is the brachialis. And when your hand is positioned thumbed up, the prime mover is the brachial radialis. So thus, depending on the position of their forearm, different muscles can be more or less effective at generating force. Synergists can also serve to steady or stabilize a joint, which makes it possible to make more precise movements. So, for example, if you're drinking a glass of water, the prime mover for flexing the arm is the biceps, and to assist in getting the water into your mouth, not spilling down your chin or over your shoulder, that joint is stabilized by the shoulder muscles.
[00:06:34] Speaker B: Okay, so we have lots of muscles with different potentials, depending on the action we're doing and how heavy something is and even the angle of it. But it all works together depending on those factors. Well, we think of muscle contraction as this simple tightening and loosening of muscles to allow us to do things, but really it's much more involved than that. As I read your brief summary of the process, I was really struck by how many systems are involved to make contraction happen. Tell us how multiple systems work together to make that muscle contraction possible.
[00:07:09] Speaker A: Sure. So the muscular system is unable to perform its job of animating our skeleton without assistance from the nervous system, the respiratory system and the circulatory system. The electrochemical impulses that drive muscle contraction are transmitted by motor neurons, which is the nervous system. And muscle cells contain many mitochondria which perform cellular respiration. It creates the atp that's needed for contraction. And this depends critically on the exchange of the gases, oxygen and carbon dioxide between the blood and air by means of the respiratory system. So the oxygen is brought to the muscles and carbon dioxide is removed by means of the circulatory system. And so a tremendous amount of functional integration is required for muscle contraction to actually work.
[00:07:54] Speaker B: Fascinating. The nervous system, the respiratory. You do pronounce it a little differently. We're both from Scotland, but of course, we have different pronunciations of things and the circulatory and of course, everything to move the skeletal system. So it's just amazing how all these systems are working together as one sort of unit. Well, muscle contraction actually starts in the brain. Doesn't it explain how our brain contributes to the process of muscle contraction?
[00:08:22] Speaker A: Absolutely. So the region of the brain responsible for generating the nerve impulses for movement is the frontal lobes of the cerebrum. So the muscle fibers contract when they receive electrochemical impulses generated from the motor erons of the frontal lobes that travel along motor neurons, which are grouped into nerves. So a single neuron can innervate anywhere between a few to hundreds of muscle fibers, as its axon can branch extensively, and as this is known as a motor unit. Now, in muscles that carry out small and precise movements, movements of the fingers or the eyes, for example, muscles typically have small motor units, two to 100 fibers per neuron, whereas, in contrast, muscles that have to carry out powerful rather than precise movements, such as the large muscles of the hips and legs, for instance, have hundreds of muscle fibers per neuron. The cerebellum is the part of the brain that is responsible for regulating the coordination and motor control operating largely below the level of consciousness. So this means that many of its functions occur without our conscious awareness. So the cerebellum receives input from multiple sensory systems, including the proprioceptive system, which confers information about the body's position and movements. The vestibular system, which corresponds to balance the spatial orientation as well as the visual and auditory system. So these inputs help the cerebellum to establish a sense of where the body is in space and how it's moving. The cerebellum also receives information from the motor cortex, which provides efference copies of the motor command sent to muscles. And these efference copies are essentially predictions of the intended motor output and are used to compare with the actual sensory feedback. And this comparison helps the cerebellum to detect any discrepancies between the intended and actual movements. The cerebellum acts as an integration center for sensory and motor information. And it's constantly comparing the efference copies with the incoming sensory feedback, such as the proprioceptive signals from stretch receptors and Golgi tendon organs, as well as visual and vestibular input. And this comparison occurs at the subconscious level, and it allows the cerebellum to detect errors in movement even before we are consciously aware of them.
[00:10:35] Speaker B: I find that fascinating. Yeah. So, in the brain, the cerebellum is at work here with muscular function, but just the fact that it can predict and make a mental model of where you're going to be at a certain time and how that muscle is going to get used. It's really interesting. So you're saying that our brain can really predict our movements. Is that what's going on?
[00:11:00] Speaker A: Yeah, exactly. So when the cerebralum detects errors in movement or discrepancies between the intended and actual outcomes, it generates corrective signals. And these signals are sent to the motor cortex and other motor control centers in the brain. And the cerebellum adjusts the ongoing motor commands to correct the errors and improve the precision and accuracy of movements. The cerebellum is also involved in motor learning and adaptation. Through repetitive practice and learning, the cerebellum stores information about various motor tasks and their associated sensory feedback. And this allows it to refine movements over time, even without our conscious awareness. So, for example, when you are learning to ride a bike when you're a child, the cerebellum helps you automatically adjust your balance and coordination without needing to consciously think about it. The cerebellum also plays a role in feed forward control, where it predicts the sensory consequences of planned movements. So it can make anticipatory adjustments to movements based on the expected sensory feedback. So, for example, when you reach for an object, the cerebellum can adjust the motor commands to account for the expected weight and resistance of the object, allowing for smoother and more precise movements. The cerebellum also receives information from inner ear receptors for equilibrium and uses it to balance the contractions of antagonistic muscles, such that the contractions of one set don't cause the body to fall over.
[00:12:26] Speaker B: Okay. And what you just described is muscle sense, they call it. Right?
[00:12:30] Speaker A: Right.
[00:12:31] Speaker B: Okay. And then there's muscle memory. Give us another example of how muscle memory is used in everyday life. You talked about learning to ride a bike, right. Over time. So is it basically anything that gets practiced over and over that you do with your muscles and with your body? Is that getting stored then in the cerebellum?
[00:12:51] Speaker A: Yeah. So muscle sense, which is also known as proprioception, is the body's ability to sense and perceive the position, movement, tension of one's muscles and joints. So it plays an important role in maintaining our balance and coordinating our movements. It operates in the background of our conscious awareness, so we're able to perform tasks without having to think about it, constantly think about the position of our limbs or the force that's needed to carry out a particular task. Activities that involve fine motor skills, such as typing on a keyboard or playing on a musical instrument, depend heavily on muscle sense for control and accuracy, and these improve with repetition and practice due to what's known as muscle memory. So as neural pathways are controlled, the necessary movements become strengthened. The experienced pianist or typist doesn't need to constantly think about every movement I touch type, and I don't need to consciously think about where I'm putting my keys on the keyboard. And likewise for someone that plays the piano like yourself, Andrew. So muscles contain receptors known as stretch receptors, otherwise known as muscle spindles or proprioceptors, which detect changes in muscle length when it's stretched. So the brain interprets these sensory impulses to generate a mental image of where the muscle is in space. And the impulses responsible for muscle sense are received and processed by the cerebellum for unconscious muscle sense, and in the perietal lobes of the cerebrum for conscious muscle sense.
[00:14:18] Speaker B: Yeah, you mentioned the piano, wonderful instrument. I've loved it all my life. But obviously I've got a lot less time to create new neural pathways regarding piano pieces. And so I rely on just the same repertoire that I learned many years ago. Those were burned into my brain, and I'll be able to play those, but not too much new stuff, unfortunately. Yeah, it's just fascinating how the brain works to make all this happen. Now, most of our muscles exist in a state of slight contraction. What is that called, and how does it help us?
[00:14:52] Speaker A: Yeah, so this is known as muscle tone. So with the exception of certain stages of sleep, most of our muscles actually exist in a state of slight contraction. And this enables us to keep an upright posture.
Only a few muscle fibers in the muscle have to contract in order for the muscle to be in a slightly contracted state. And this allows us to maintain our upright posture, allows us to stand up and to sit up rather than constantly be on the floor. And so to prevent the muscle from becoming fatigued, alternative fibers actually take turns at contracting. And this is consciously regulated by the cerebellum. And the heat that's produced by muscle fibers during cellular respiration, which is necessary for production of atp, accounts for approximately 25% of the total body heat at rest. And so when you have a newborn baby, the baby has very little muscle tone, and so they're not even able to balance their head on their own, they're not able to sit up. And then over time, the baby will develop more and more muscle tones, that they're able to eventually lift their head by themselves. They're able to sit up and so forth.
[00:16:00] Speaker B: Okay. Yeah. So that sort of comes with age and with practice using these muscles. Interesting. Well, in a second article, you go deeper to explain the biochemistry involved in muscle contraction, nerve impulses from the brain activate a complicated set of biochemical reactions for the contraction and the relaxation of muscles to happen. Can you tell us a little bit about the structure of muscle fibers first?
[00:16:25] Speaker A: Yeah. So, muscles contain thousands of cylindrical cells called muscle fibers, or myocytes. The motor neuron terminates at the muscle fibers near your muscular junction. And the tip of the motor neuron is known as the axon terminal, and it contains sacs of acetylcholine, which is a neurotransmitter that plays a crucial role in muscle contraction. So the muscle fiber also has a membrane known as a sarcolema that contains acetylcholine receptor sites, in addition to an inactivator known as colinesterase. The small space between the sarcolema and the axon terminal is known as the synapse, or synaptic cleft. So the muscle fiber contains thousands of individual contracting units that are known as sarcomeres, and these are organized end to end in cylinders known as myofibrils. In the center of the sarcomere are thick filaments comprised predominantly of the protein myosin, which is a motor protein. And thin filaments containing actin can be found at the ends, attached to the end boundaries of the sarcomere, known as the z discs by the protein titan. So that's a little bit of detail on the structure of muscle cells.
[00:17:40] Speaker B: Yeah. Okay. Now, as simply as possible, and stay with us, listeners walk us through these reactions that power the muscle contraction at the biochemical level.
[00:17:51] Speaker A: Yeah. So muscle contraction is basically driven by two contractile proteins known as actin and myosin and myosin. Each myosin molecule consists of a long tail and a globular head. Myosin heads have atpas activity, which allows them to hydrolyze atp, and that generates energy for muscle contraction. Myosin heads also have binding sites for actin and atp. Actin has binding sites for myosin heads, but these binding sites are typically covered by two inhibitory proteins known as tropomyosin and tropinin when the muscle is relaxed. And these inhibitory proteins prevent the sliding of myosin and actin during relaxation of the muscle fiber. So the sarcomeres are surrounded by the sarcoplasmic reticulum, which is the muscular equivalent of the endoplasmic reticulum that we find in most of our cells, which serves as a reservoir of calcium ions. And calcium ions are actually very, very important, as we'll see, for muscle contraction to take place. So when a muscle fiber is in a state of relaxation, the sarcolemma has a resting potential, or it's said to be polarized. And this refers to the difference in electrical charges between the inside and the outside. And when the sarcolema is polarized, there is a positive charge outside relative to the negatively charged inside. And there is a greater concentration of sodium ions outside the cell and a greater concentration of potassium ions and negative ions inside the cell. And as a result of the concentration gradient, the sodium ions have a tendency to diffuse into the cell, whereas the potassium ions have a tendency to diffuse outside of the cell. And these are actively transported back out and in, respectively, by the sodium and potassium pumps, which are driven by, powered by ATP to maintain polarization and muscle relaxation until a change is stimulated by a nerve impulse. So what's the first step in muscle contraction? Well, the first step is the arrival of a nerve impulse at the axon terminal, stimulating the release of the neurotransmitter acetylcholine, that I referred to earlier. So, acetylcholine diffuses across the synapse and binds to acetylcholine receptors on the sarcolema. And that makes the sarcolema very permeable to sodium ions, which rapidly enter the cell. And this reverses the charges such that there's now a positive charge on the inside of the sarcolemma relative to the outside. And this charge reversal is known as depolarization.
Inward folds on the sarcolemma, which are known as transverse tubules, or ttubules, carry this electrical impulse, which is known as an action potential, to the interior of the muscle cell. And depolarization triggers the release of calcium ions from the sarcoplasmic reticulum. And these bind to the tropinin tropamycin complex, and that moves it away from the actin filaments.
Now the binding sites on actin are available, and so actin can be bound by the myosin heads, where it was inhibited from doing so previously. And so this now forms cross bridges. And once the cross bridges are formed, the myosin heads pivot, pulling the thin filaments towards the center of the sarcomere. And this action is known as the power stroke, and is powered by the energy that's released from atp hydrolysis. And following the power stroke, the myosin heads require atp to detach from actin. So ATP is hydrolyzed into adp, an inorganic phosphate which energizes the myosin head for the next cycle. And the cycle of crossbridge formation, power stroke, atp hydrolysis and detachment repeats as long as there are calcium ions present and there is an availability of atp. And this results in the shortening of the sarcomere and collectively, the entire muscle fiber. And that leads to muscle contraction. And the force that's generated by many muscle fibers contracting as a unit together allows for movement of the body.
[00:21:54] Speaker B: Okay, well, so that's the biochemistry in a few words. So what is a human doing to power this process? Or, as you say, is it all working at the level of unconscious activity?
[00:22:09] Speaker A: Well, this is when we're actually contracting voluntarily. Contracting our muscles voluntarily. So if I move my arm to pick up a pencil or a glass or something, then I'm engaging in muscle contraction.
[00:22:21] Speaker B: Yeah, but thankfully, we don't have to think about every part of the process. Our body takes care of it. Well, now, to the crux of the issue. You write that to contend that the phenomenon of muscle contraction arose through a blind and undirected process. One tiny darwinian step after the other seems to me to strain credulity. So why would a darwinian process like natural selection and random mutation struggle to produce muscle contraction? Can you summarize that for us?
[00:22:51] Speaker A: Sure. So there are multiple interdependent systems that are needed for the muscular system to work, right? So you need circulatory respiratory nervous systems to work together as well as you need, of course, the tendons that actually anchor the muscles to the bones. Not to mention, of course, the incredible structure and arrangement of the muscles themselves. So they possess many mitochondria that produce atp, bicellular respiration, and that is required to meet the energy demand. Muscles, of course, are also comprised of thousands of muscle fibers. The muscles also will have to be arranged antagonistically and synergistically for coordinated action, et cetera. The origin of the skeletal muscles thus depends on many codependent changes in order to come about. And, of course, the process of muscle contraction and relaxation requires the coordinated action of actin myosin, troponin, tropamycin, the acetylcholine, the iron channels, and much more. So you need a lot of different systems to come together to work in unison to bring about the contraction and relaxation of muscles.
[00:24:00] Speaker B: And obviously, that's hard for a darwinian process that moves in a stepwise fashion to do, right? I mean, either it's all there, ready and available to use, or it's coming about so slowly that an organism can't move until the respiratory system is in place, or the circulatory system has been perfected. And so you're in this catch 22 with a darwinian paradigm where you just cannot explain all of it at once through a stepwise process. Which leads me to my next question. Why is it not a surprise to look at muscle contraction from a design perspective? What would you say to that?
[00:24:38] Speaker A: Sure. So the sorts of processes that we observe that are responsible for muscle contraction exhibit foresight, coordination, and planning. It has multiple layers of interdependency and design. And so any process that is going to be capable of producing this sort of system is going to be ultimately indistinguishable from intelligence, because only intelligent agents are able to visualize a complex end goal and to bring everything together needed to realize that complex end goal, whereas unguided stochastic processes typically are not able to do that.
[00:25:15] Speaker B: Yeah. Well, where can listeners learn more about the incredible design of muscles and other systems in the human body?
[00:25:22] Speaker A: Absolutely. So you can go to evolutionnews.org, where, of course, there's plenty of articles, including my articles pertaining to this subject, as well as many other wonders of anatomy and physiology and cellular biology. You can also pick up just about any anatomy and physiology textbook and get all of this information there and much more. And of course, there's the book your design body, by Harold Glixman and Steve Loughman. So I highly recommend picking up a copy of that as well.
[00:25:47] Speaker B: Okay. Well, Jonathan, thank you for taking the time to unpack this complexity and this design of muscles, this amazing system. You're really helping us to get into these systems more and understand them and appreciate the design inherent in them.
[00:26:03] Speaker A: Thanks for having me on.
[00:26:04] Speaker B: Well, to find more of Jonathan's work, you can visit his website, jonathanmcclachey.com. And if you enjoy what you hear on the podcast, consider leaving us a written review at Apple Podcasts so new listeners can find a trustworthy source of news and information about evolution and intelligent design.
For now, I am Andrew McDermott for idthuture. Thanks so much for listening.
[00:26:28] Speaker A: Visit
[email protected] and intelligentdesign.org. This program is copyright Discovery Institute and recorded by its center for Science and Culture.
