Bovine biology series
Part - 32 Nervous system (3/4)
The nervous system
The central nervous system, composed of the brain and spinal cord, largely serve the role of interpreting messages and then responding to them through the peripheral nervous system.
In this way the entire nervous system can be divided into parts; each require the other but each have different functions. Last month we examined the CNS. Let us learn about the peripheral nervous now.
May I introduce this lesson with a personal experience? During one fine summer day, in the milking parlor came a newly freshened cow with a right-front teat almost completely severed. There was blood everywhere. And I had not seen one if these before.
So Don came into the barn and cut the bottom the teat off. Well, I was astonished at how much blood there was, and milk all over, and goodness; I wondered why she - the cow - was jumping all over! But he had me hoist her tail straight over her back, and the surgery was over in a second or two.
Now on subsequent milkings, I watched this teat gradually heal. But what I remember most about this cow is when she would come in; I would very carefully reach up and gently milk the teat out. For during the previous hours, the teat end had closed with a clot of some sort, and in order to get the milk out, I would have to force the clot away so the teat would drain. She was, after all, a fresh cow and milking well.
Over the course of several weeks she gradually dried up and we milked her as a three-quartered cow. Too, I noticed that the teat end was not as delicate, for the tissue healed, and therefore her interpretation of pain did as well.
What lesson can we learn from this boyhood experience in a milking parlor?
Well, we will never know how a cow almost severs a teat, but that it happens is no cause for surprise. We do know that in all but the most unusual circumstances, such a teat is better removed than sewn back on, for the onset of mastitis because the teat end defensive barrier is gone.
We know too, that this is an example of how well the body is enervated, so to speak, in that when a part of the body is damaged, traumatized, or severed, it is the peripheral nervous system that signals, via electrical charges, to the location on the CNS found in the spinal cord, and this in turn is communicated to the brain and in her mind, the cow experiences pain. Thus she is under stress. And in a wonderful manner, a whole cascade of events are underway in order that she will not go into shock, or she will not stop breathing, or she will not have her immune impaired. [read: Homeostasis!]
While her pain tolerance is not easily quantified, we do know that various biochemical processes within her command are immediately given their task so that this animal will survive.
And I learned a lesson that as the body heals from a particular traumatic event like a severed teat end, the pain is lessened, and eventually the nerve endings are less reactive to touch.
Well, so much for my teat end story.
The peripheral system is actually made up of two different types of neurons (this is explained previously in part 30, March, 1996), the sensory or afferent neurons and the motor or efferent neurons.
The sensory neurons, by definition, bring messages from the body to the spinal cord. Thus, they sense the environment of the animal. When the cow cut her teat, and we removed it entirely, you can be sure that the multitude of sensory afferent neurons interpreted this message, for indeed, some of them were severed as well! The message traveled via the nerve cell body into a specific spot on the spinal cord.
The sensory afferent neurons belong to two subgroups. The first is the ones found in the body periphery, like the limbs and skin and head area, as well as the teat end. The other one is made up of visceral sensory neurons, which are found deeply within the body. They interpret the environment of the internal organs, the heart, lungs, liver and stomach, for instance, and thus function to communicate with the brain about the regulatory parts of the body.
The motor efferent neurons can also be divided into two subgroups. In both, however, their primary function is to receive a message, via interpretation and decision making in the brain, and through the spinal cord the CNS is thus linked to send signals out through the motor efferent neurons.
The first division is the autonomic or involuntary nervous system. These neurons largely regulate the smooth muscles, the cardiac - heart muscle, and the organs of the digestive, and gastrointestinal system, and some endocrine glands. They are involuntary, for the body is self-regulated in a manner that the mind cannot be thoughtfully made to change these regulatory functions. For instance, you cannot think nor will the heart to beat faster, or the digestive system to move digesta through the GI tract faster.
To complicate matters, the autonomic system is divided into the sympathetic and parasympathetic divisions. The sympathetic system largely speeds the body up, increases basal metabolic rate and heart rate in preparation for action, while the parasympathetic system slows the body down for conservation and efficiency purposes.
There is another kind of motor efferent neurons, the somatic or voluntary system. These nerve cells carry signals from the CNS to the skeletal muscles mainly in response to external stimuli as interpreted by the sensory afferent neurons incoming to the CNS. In fact the way we sit in a chair, or walk or run, are largely controlled by enervation of the musculature as directed by the somatic nervous system.
So, when I hoisted the cows tail thirty years ago in a milking parlor, I was short-circuiting the signal of incoming messages from the sensory efferent neurons in the teat end. If I had not, the CNS, via the tail head that was not pinched, would have signaled the leg to kick hard, as directed by the motor efferent side of the peripheral system, and through the somatic voluntary neurons, we might have been kicked.
We will conclude this exciting and stimulating section of the nerves next month.
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