Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
The brain is a very remarkable and versatile organ. It makes us conscious of our surroundings, controls our breathing and cardiovascular system, lets us swallow, controls our movements, and allows us to manipulate things. It also provides homeostasis of the body’s internal environment by controlling such things as appetite, thirst, fluid balance, and core temperature.
The brain has several interconnected regions for emotions like fear, anger, love, and pleasure. It also has regions for higher functions such as thought, memory, language, calculation, learning, reasoning, problem solving, and judgment. Scientists know that these functions are accomplished by billions of nerve cells interacting with each other through chemical means. But knowing that, for example, the visual cortex receives millions of impulses from light entering the eyes, which are split up, overlapping, turned around, and upside down, does not explain how we can see. The same applies for everything else we experience. What within us takes these nerve impulses and actually does the seeing, the hearing, the feeling, the moving, and the thinking?
Using our brain to try to figure out how our brain works is a mystery because, as Marcel Gabriel said, “a mystery is a problem that encroaches upon itself because the questioner becomes the object of the question.” Neuroscience tells us that when the mind decides it wants to do something, it sets off a chain of brain-controlled neural events. Clinical experience teaches that for our earliest ancestors to have survived within the laws of nature, they would have had to have been able to perform well-coordinated movements to achieve these goal-directed activities. How does the brain do it?
To appreciate how your muscles work, now would be a good time to test them. Slowly move your eyes and eyelids, mouth and jaw, then your neck, all the joints of your upper and lower extremities, and then your back, in every possible direction. Then go back and try it again, except this time go as fast as you can and see which parts of the body you can move the fastest and with the most precision and control.
You probably noticed that your eyes and eyelids, mouth and jaw moved very quickly with precision and control. Your fingers moved much faster, and with more precision and control, than your toes, your wrists more than your ankles, your elbows more than your knees, your shoulders more than your hips, and your neck more than your upper and lower back.
Each skeletal muscle consists of numerous muscle fibers. When stimulated by a motor neuron, they contract and the bones to which they are attached move toward each other. The muscle fibers controlled by a given motor neuron is called a motor unit. The motor units of different muscles contain different numbers of muscle fibers in relation to their function. For the coarse strong movements of the back, the legs and the arms, there are usually hundreds to several thousand muscle fibers per motor unit. In contrast, for the fine and precise movements of the eyes and the fingers, there are as few as five to ten muscle fibers per motor unit. In addition, compared to the muscles of the back, the legs, and the arms, the muscles of the fingers and the eyes usually have many more muscle spindles to provide the central nervous system with more information on muscle length and the rate of change. This helps them perform intricate movements.
Since muscles only work by contraction, a given muscle can only move the eyeball or the bones of a joint in one direction. To move them back requires a complementary muscle which must also stay relaxed to allow the given muscle to do its job in the first place, and vice versa. For example, when the lateral rectus of the right eye contracts, the eye looks to the right. But to move it back to the left requires it’s complementary muscle, the medial rectus, to contract. But the medial rectus must have stayed relaxed to allow the lateral rectus to have done its job in the first place, and vice versa. Similarly, the biceps contracts to flex the elbow, but the only way to straighten the elbow out again is for the triceps to contract. And the tricep must have remained totally relaxed for the biceps to do its job in the first place, and vice versa.
The main lesson to learn here is that to perform a well coordinated action, it is not only important for a given muscle to contract, but also that its counterpart relax. The muscle spindles within both muscles monitor the changing of each muscle’s length and the joint angle. In this way, they notify the brain of what is happening and verify that the correct actions are taking place. Without nervous control of these complementary muscles, there would be a continuous tug of war that would make maintaining the body’s position and performing well-coordinated, goal-directed actions impossible.
The regions of the brain responsible for the initiation and refinement of purposeful movement are primarily the motor areas of the cerebral cortex, the basal ganglia, and the cerebellum.
The motor cortex on one side of the brain controls the movement of the opposite side of the body. Messages from the motor cortex travel down the spinal cord to the motor neurons, telling them what to do. The motor cortex also sends signals to the basal ganglia and the cerebellum to inform them of what is happening. The motor cortex does not make decisions within a vacuum. It analyzes sensory input sent to it from other areas of the brain, which tell it about things like vision, touch, vibration, pressure, temperature, pain, balance, and the position and movements of the limbs. It also receives information from the basal ganglia and the cerebellum. It is this ongoing feedback from throughout the nervous system that allows the motor cortex to make adjustments in the force needed to achieve certain voluntary actions. Clinical experience shows that injuries and malfunction of the motor cortex on one side of the brain results in weakness and clumsiness of the muscles on the opposite side of the body.
The basal ganglia consist of several nerve-connecting centers that lie deep within the brain, just below the cerebral hemispheres. Due to their location, the basal ganglia have not been easily accessible to investigation, so our understanding of their function is somewhat limited. However, clinical experience from disturbances associated with injury to the basal ganglia indicates that they play a vital role in the body’s goal-directed activities. The basal ganglia are connected with neural circuits that involve both sensory and motor impulses that give feedback to both the sensory and motor regions of the cerebral cortex. The messages from the basal ganglia can either turn on (excite) or turn off (inhibit) the neurons they contact. It is thought that the basal ganglia are responsible for the processing and integration of sensory data that is used to regulate motor function. It appears that the basal ganglia are involved in some basic movement programs that are initiated by the cerebral cortex and acted upon by other higher centers as goal-directed activities take place. Clinical experience shows that diseases of the basal ganglia usually cause a constellation of symptoms and signs known as movement disorders, of which Parkinson’s Disease is the most common. This condition causes muscle rigidity, slow movements, and often a pill-rolling tremor at rest. All of these progress to weakness and marked debility over time.
The cerebellum (little brain), which lies under the occipital lobes and behind the brainstem, receives sensory information from the muscle spindles, Golgi tendon organs, and the receptors of the skin and joints. This means the cerebellum is aware of the status of the muscles and joints as they perform activities. The cerebellum also receives sensory data from the vestibular regions of the brainstem, so it’s involved in balance as well. The cerebral cortex informs the cerebellum of what actions are being planned, which allows it to have a moment-to-moment knowledge of all of the activity within the neuromuscular system. The cerebellum analyzes and integrates all of this sensory data so that it can support and modify the messages being sent to the muscles by the motor cortex. The cerebellum is therefore able to make moment-to-moment adjustments to allow coordinated voluntary actions and maintain balance, posture, and position. Injuries or degeneration of the cerebellum can result in dizziness, imbalance, and loss of muscle control, causing clumsiness, an intention tremor, and slurred speech.
Clearly, for our earliest ancestors to have lived long enough to reproduce required them to not only have this irreducibly complex neuromuscular system, but also have a natural survival capacity to react fast enough and know what to do to survive. Evolutionary biologists believe that somehow or other chance and the laws of nature alone brought about this incredible masterpiece of precision that allows us to “live and move and have our being.” All human experience says otherwise.
Image credit: Edgar Degas, via Wikimedia Commons.