It is our muscles, controlled by our nerves, that allow us to breathe, swallow, move around, and handle things. The peripheral nerves send sensory information about what is going on outside and inside the body to the spinal cord and the brain and from them send back instructions to the muscles to tell them what to do. In my last few articles, I described the sensory receptors, which act as transducers, converting phenomena into information the body can use to survive.
The vestibular apparatus, which detects body motion, and the cochlea, which detects sound, are housed within the inner ear. Light is detected by the retina at the back of the eye. Pressure, light touch, motion, vibration, hot and cold, and pain are detected by sensory receptors in the skin that tell the body what’s going on around it. Other receptors keep tabs on what’s going on within many of the body’s organs.
To control the musculoskeletal system, so it can do what it needs to do, the body uses the proprioceptors located in the joints, tendons, and muscles. In general, purposeful movements are done voluntarily. But to maintain its position or avoid serious injury, sometimes the body must act quickly so that these reflexive movements are done unconsciously. Let’s look at how some of these automatic reflexes work to prevent organ damage and help maintain the body’s position so that goal directed activities can be done. Keep in mind that when evolutionary biologists tell us about how life arose, they only deal with how it looks and not how it must actually work within the laws of nature to survive. Ask yourself which is a more plausible explanation for how life arose: chance and the laws of nature alone or intelligent design?
A reflex is an involuntary, pre-programmed, automatic motor response to a stimulus that comes about without conscious direction from the brain. It works through the spinal cord and brainstem and serves to protect the body from injury and to maintain its position while the mind is focused elsewhere. For example, the slightest touch of the cornea in the eye triggers blinking immediately. Shining a bright light into the eyes instantaneously makes the pupils close. Without these automatic and quick reflexes, our eyes would be at high risk for injury because the time it would take for our brain to assess the situation and implement a plan to avoid injury would not be fast enough to prevent serious damage.
The simplest reflex, involving just one sensory nerve sending a message to just one motor nerve, is the stretch reflex. The patellar reflex, where tapping on the tendon of a bent knee suddenly straightens it out is the classic example. Tapping the patellar tendon stretches the muscle spindles in the quadriceps muscle, sending nerve impulses through the sensory nerve to the spinal cord. If the impulses are strong enough to stimulate the motor nerve, it instructs the quadriceps to contract, causing the knee to go into extension.
The stretch reflexes are important for maintaining posture and position. Standing with the knees extended for some time eventually results in fatigue of the quadriceps and puts the body at risk of falling. The resulting stretch of the muscle spindles activates the patellar reflex and stimulates the quadriceps to contract more, which stabilizes the knee and prevents a fall. This recovery of posture function takes place throughout the body because the stretch reflex operates in all of the muscles, whether in the head and neck, the arms and legs, or the spinal column.
However the ability for the stretch reflex to maintain the body’s position wouldn’t be possible without reflex inhibition. The movement of a bone across a joint in any direction usually depends on two complementary muscles working in opposite directions. The quadriceps extends the knee and the biceps femoris flexes it. The patellar reflex not only causes the quadriceps to contract but simultaneously sends nerve messages to relax the biceps femoris. Without reflex inhibition the complementary muscles would constantly be fighting a tug of war with each other and coordinated muscle function would be impossible.
Another important but more complex pattern used by the body to protect itself from injury while maintaining its position is the withdrawal (flexor) and crossed extensor reflexes. If you step on something hot or sharp, the pain messages quickly go to the spinal cord where it stimulates a series of flexor muscles to contract so that you immediately withdraw your foot. But when you lift your leg off the ground, you are at risk of falling. So a split second later, the crossed extensor reflex straightens out your other leg and your body weight shifts over it to maintain your balance. You barely think about any of this — your body knows to do it naturally.
When it comes to life, real numbers have real consequences. When you stand up your buttocks is about one meter off the ground. If your quadriceps weaken or you suddenly lift your leg off the ground to avoid pain, gravity immediately kicks in. Since gravity makes all things accelerate to the ground at 10 m/sec2, it is possible to calculate how long it would take for you to hit it from one meter up (0.45 sec). This means that your nervous system would have to be able to react fast enough to prevent a fall — less than half a second.
The impulse velocity of a nerve is faster if it is larger in diameter and insulated with a fatty substance called myelin, rather than smaller and unmyelinated. Normally, the sensory and motor nerves involved in the abovementioned reflexes are large and myelinated and have an impulse velocity of about 100 m/sec (> 200 mph). Since it is about one meter from the foreleg to the lower spinal cord, it only takes about 0.02 sec (0.01 + 0.01) for the nerve impulse to travel along the sensory nerve to the spinal cord and back along the motor nerve for these reflexes to work in time to keep you balanced and on your feet. This would give the neuromuscular system plenty of time to make changes to prevent a fall (0.45 sec).
In contrast, the impulse velocity of the smaller and unmyelinated nerves that inform the body about pain is only about one meter/sec. This means that if the sensory and motor nerves involved in these reflexes were like the pain nerve fibers, it would take at least two seconds for the impulses to go from the leg to the spinal cord and back again. Clearly, this would not be fast enough to prevent you from falling (0.45 sec). This also explains why, when you are injured you can react very quickly to avoid further tissue damage but you don’t experience the severe pain until a few seconds later.
The system of reflexes the body uses to protect itself from injury and maintain its position is irreducibly complex because of the different nerve and muscle cells it needs to work. But it also demonstrates natural survival capacity in that the impulse velocity of the sensory and motor nerves involved in these reflexes is sufficient to contend with the force of gravity. Without this, no matter how sophisticated their reflexes, it would have been impossible for our earliest ancestors to maintain their position and survive.
Evolutionary biologists imagine how these systems came into being by how they look, but they rarely seem to think that it’s also important to explain how they happen to work within the laws of nature. Next time we’ll look at what it takes for the body to keep its balance.