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 nerves and muscles of our body allow us not only to breathe, but also to move around and manipulate things. But how do they do it? In my last I article showed that nerve cells (neurons) and muscle cells (myocytes) are excitable. This means that when adequately stimulated, they depolarize. By letting large amounts of Na+ ions enter, they cause the plasma membrane to go from being negatively charged to positively charged. Depolarization triggers large amounts of Ca++ ions to enter the cell, causing the neuron to release its neurotransmitter and the myocyte to contract.
The nervous system is set up like a military operation. Reconnaissance from sensory neurons within the peripheral nerves is sent to and organized by the spinal cord and transmitted to the brain. The brain, acting as headquarters, analyzes the sensory information, makes decisions, and sends out orders. The orders travel to and are organized by the spinal cord and sent through the motor neurons within the peripheral nerves. The messages tell the muscles to contract, which results in controlled and purposeful movement. The success of a military operation is dependent on having good information about the enemy and one’s own troops. Let’s look at some of the ways the body acquires the information it needs about its external and internal environment so it can know what to do and act accordingly.
Everyone knows that an odometer measures distance and a speedometer measures velocity. Each device is essentially a sensory transducer with a mechanism in place that enables it to sense a physical phenomenon and convert it into useful information. We have seen that the body has a whole host of sensory transducers, which provide the information necessary to maintain its internal environment. They are divided into three different categories: chemoreceptors, which respond to chemicals, like oxygen, glucose and calcium; mechanoreceptors, which respond to motion and stretch, like the baroreceptors in the walls of the arteries leading to the brain that monitor blood pressure; and physical sensors, which respond to natural phenomena, like the thermoreceptors in the hypothalamus that detect core temperature.
The skin not only protects the body from infection and injury. It also provides sensory information about its immediate surroundings. The skin has different sensory receptors that can detect light touch, pressure, motion, vibration, and temperature. It has pain receptors that react to chemicals related to nearby cell injury. They also react to too much pressure or motion and extreme temperatures. Adequate stimulation of any one of these sensory receptors causes the associated neuron to depolarize and release its neurotransmitter. The neurotransmitter then depolarizes a nearby connecting neuron in a cascade that transmits the message all the way to the brain, usually in the sensory region of the cerebral cortex.
In addition to the skin, which provides sensory information about the body’s external environment, there are pain and mechanoreceptors in and around the joints, ligaments, and deep soft tissues. These, in addition to chemoreceptors within the major organs, provide sensory information about the body’s internal environment. Although we are aware of many of the sensations caused by our external and internal environment, one set, the proprioceptors, acts unconsciously and without them life would be impossible.
Proprioception involves joint position awareness and kinesthesia, or the awareness of joint and limb movement through muscular effort. The proprioceptors tell the central nervous system about muscle length, joint position, and limb movement. If the body had no way of knowing what its muscles and joints were doing and where its bones were located in space, then how could it control its position and movements? In addition to the mechanoreceptors in and around the joints, the two main proprioceptors are the muscle spindles and the Golgi tendon organs, which provide sensory information on joint position and muscle movement.
The Golgi tendon organ joins the muscle to the tendon as it is attached on one end to the muscle fibers and on the other end to the tendon. Because it is directly connected, in series, to the muscle and the tendon, the Golgi tendon organ is sensitive to the degree of tension applied by muscle contraction.
An increase in tension causes the Golgi tendon organs to increase their impulses to the spinal cord, causing an immediate reflex inhibition of muscle contraction and a reduction in tension. This explains one of the important functions of the Golgi tendon organs: preventing muscle injury and tendon rupture by causing automatic muscle relaxation in the presence of dangerously high levels of tension.
Conversely, a decrease in tension causes the Golgi tendon organs to decrease their impulses to the spinal cord, which results in an immediate reflex reduction in inhibition causing an increase in muscle contraction. This explains another important function of the Golgi tendon organs, which is to help the body maintain its posture while performing goal directed activities.
Muscle fatigue, with its resulting diminished contraction, can lead to inadvertent falling or unexpected changes in position. But the information sent by the Golgi tendon organs to the spinal cord reflexively makes the flagging muscles contract more to allow the body to maintain its posture. Clearly, without the Golgi tendon organs to tell the central nervous system what’s going on at the level of where the muscle attaches to the tendon, our ability to move around would be impossible.
Muscle spindles are sensory organs consisting of modified muscle fibers positioned in between and parallel to the skeletal muscle fibers, allowing them to compare their respective lengths. Since each skeletal muscle is usually attached to two different bones across a joint, its length also determines the angle of the joint and its position.
For example, when the elbow is fully extended to zero degrees, the biceps are at their greatest length and the triceps are at their shortest. In contrast, when the elbow is fully flexed at about 160 degrees, the biceps are at their shortest length and the triceps are at their greatest. When the angle of the elbow is in between, at 80 degrees, so too are the lengths of the biceps and the triceps. This demonstrates one of the functions of the muscle spindles, which is to provide the central nervous system with information about the length of each skeletal muscle and the angle and position of its associated joint.
If the skeletal muscle fibers are longer than the muscle spindles (an indication of being stretched) the muscle spindles react by sending more impulses to the spinal cord, reflexively causing muscle contraction. Conversely, if the skeletal muscle fibers are shorter than the muscle spindles (an indication of muscle contraction) the muscle spindles react by sending less impulses to the spinal cord which reflexively results in muscle relaxation. This demonstrates another important function of the muscle spindles. Like the Golgi tendon organs, they help the body maintain its posture and position in space while performing purposeful movements.
In a static situation, such as carrying a load in front of the body, it is easy to understand how the muscle spindles maintain position through changes in muscle function. In this setting, the elbows must be maintained at about ninety degrees by the combined actions of the biceps and triceps. During this activity, stretching (lengthening) of the biceps, due to muscle fatigue, will at the same time cause a shortening (contraction) of the triceps. If this is not corrected quickly the angle of the elbow will decrease and allow gravity to take more effect which may cause the load to be dropped.
To prevent this from happening, the muscle spindles in the biceps detect the lengthening and send more messages to the spinal cord, reflexively causing an increase in the contraction of the biceps. This maintains the angle of the elbow at ninety degrees so as to not drop the load. At the same time, the muscle spindles in the triceps detect the shortening of its muscle fibers and send out fewer messages to the spinal cord, reflexively resulting in relaxation of the triceps so that the ninety-degree elbow angle is maintained.
Clinical experience with diabetics and others who suffer from sensory nerve malfunction indicates that without any one of the above-mentioned sensory devices, it would have been impossible for our earliest ancestors to survive. Evolutionary biologists must explain how each of them came about and their presence in the precise locations where they are needed in addition to how intermediate life forms could have survived throughout this developmental process.
Next time we will look at the special sense of vision and see what it requires.
Photo credit: © bst2012 — stock.adobe.com.