Bovine biology series
Part - 33 Nervous system (4/4)
The nervous system
In this final lesson about the nervous system, we explore the world of sleep. I begin it this way....
When I first became aware of animals at a very early age, animals seemed to need sleep just like me. The cats in the calf barn always seemed to sleep, curled up in a little ball of fluff. Dogs would sleep in the daytime too, as if a nap during the daylight hours let them stay awake at night.
I rarely ever found a cow asleep. That is, as I walked through the free stall barn at 3:00 am getting the first string ready for milking, the cows seemed all awake ready to go. I supposed two things: either they were accustomed to getting up at this hour of the day, or they did not sleep very hard and upon hearing my footsteps, and awakened easily.
Well, sleep is associated with the overall nervous system, so let's learn about it together, and maybe we can really figure out if cows need any.
Sleep appears to be required to rest the mind instead of resting the body. That is, we can rest the body just by lying down and relaxing. The mind, however, is not resting. It is going full speed, thinking, processing, dreaming, planning, and doing the other functions that come automatically, such as regulating breathing, body temperature, releasing hormones for body functions, and even making secretions such as milk or enzymes that digest of food.
Sleep is required for good health, for animals purposely deprived of it will get sick, fight, and especially their immune systems will be impaired. This must relate to stress, then, for stress can be manifested in an animal by certain physiological changes, the accumulation of them slowly wearing the body down until disease, weight loss, or productivity decreases become obvious.
Sleep may be divided into two components, deactivated and activated sleep.
Deactivated sleep is an inactive sleep, so to speak, in which the animal is largely immobile; the spinal reflexes and muscle tone is depressed. The visceral functions of an animal are slowed because of the dominance of the parasympathetic nervous system that regulates the visceral organs: the heart rate slows and the blood pressure is lower. Breathing is slowed, cellular output of carbon dioxide is reduced, the blood becomes slightly more acidic, the metabolic rate of the body slows and the body temperature drops. The blood flow to the brain during deactivated sleep is usually increased, thus indicating some brain activity that can occur because the rest of the body is really slowed down.
Scientists have not yet fully figured out the entire sleep mechanism, that is, what chemical or electrical pathways are required so that we may induce deactivated sleep. We know that being tired certainly helps us sleep, as well as being awake for a long period of hours.
But without a doubt, the brain has a pivotal role in putting animals to sleep. One brain hormone, seratonin, is found in very high amounts during sleep and low amounts otherwise, suggesting its role in turning off certain neurons in the brain, thus inducing sleep. Another hormone of the nervous system, acetylcholine, exhibits a similar effect upon the brain, inducing sleep. Since the parasympathetic nervous system is largely in control of the body during deactivated sleep, it may play a role in getting the other nerve systems to shut down for sleep periods.
The other form of sleep is the activated sleep. This is associated with the dreamtime of animals, when the brain is more active, but the body is not expressive behaviorally, rather during activated sleep the motor activity of the body is less inhibited. However, a portion of activated sleep can be a deep sleep.
Interestingly enough is this fact: the higher the animals are in terms of brain development, the higher the proportion of activated sleep time. That is, activated sleep time does not occur in fish or amphibians, is rarely expressed in reptiles and birds.
And since this is dreamtime sleep, when an animal is in the activated sleep mode, there is some twitching of muscles and body movement suggesting that some response to the dream is taking place.
The brain area that seems to be most concerned with producing activated sleep is the pons, that part of the brain stem next to the cerebellum.
Sleep scientists do not really know why this dreamtime of activated sleep is necessary, rather than having all of it be deactivated sleep. Some have suggested that activated sleep may promote maturity in higher animals, or it may serve as an internal reward so the brain can dream - play, or still another reason may be to process and store pertinent memories gained during the wake time. Still others suggest that it promotes emotional stability, and that activated sleep may be preparing the animal for awakening, because the majority of activated sleep occurs just before sleep is ended.
Cows do sleep mostly at night, we are told. In fact, as a percentage of the 24 hour day, wakefulness is 52 percent, 44 percent of the time these bovines are in deactivated sleep, while only 4 percent of the time they get to dream in activated sleep. Yes, you are reading this right, for cows can exhibit deactivated sleep standing up. But usually cows will be lying down in the position of sternal recumbency (lying slightly uphill so the rumen contents do not spill out.
Cattle are easily awakened, so as I walked through the free stall barn, I did not need to prod them awake.
Another interesting point is the relationship between predators and preyed upon species.
Cats are considered to be predators, so they may sleep more hours of activated sleep because they are relatively safe from danger. They have not too many dangers in their life, so they are safe to fall into a deep sleep of activated slumber.
Prey species, such as cows and rabbits sleep very lightly, because Nature has built into their physiology an awareness that they may have to react to danger at any moment so they may escape. Thus if a cow were in deep sleep, dreaming away, they may not escape the fleeting actions of a predator!
|