Bovine biology series
Part - 30 Nervous system (1/4)
The nervous system
I am recalling one of the first times I learned to milk cows. I had been doing some outside work when I was told to go in the parlor and help prep cows for milking. There, I learned that if you touch a cow without her sensing you are there, she will be startled, may kick, may defecate, and most certainly will be uncomfortable.
I sensed that large animals had a keen sense of touch, at least in their hindquarters and in their udders and teats. I felt that as long as they knew we were there, they could be touched, their teats cleaned, and thus the machine could fill with milk.
Little did I realize that the relationship of touch and interpretation was largely under the control of the nervous system, and that the nerve cells in the body reached into almost every body part. Little did I know that these nerve cells traveled through the spinal column and ended in the brain region where synapses would be interpreted, all of this comprising the central nervous system.
And little did I know that pain was a way for the body to signal the brain that something was wrong. Thus pain could be our friend, for without it, we could not stay out of harms way.
With you, I will spend several months studying this topic of nerves, and the nervous system.
I begin with something very close to home. My sister, Madeline, suffers from Multiple Sclerosis. The disease is manifested in the nervous system, in that the junctures of nerve cells in the brain with interpretory brain cells found in the cerebellum, have failed.
Life in our family has not been fair. For at a time when my sister, at the wonderful age of 41, deals with a progressively degenerative disease for which there are no cures, just adjunct therapy, I ponder my athletic ability to deal with slowing down a few seconds per mile over the course of a Marathon. At times like this, we are humbled. And we attempt to place in perspective the blessings of our body in staying healthy, as the effects of age and time march on...for each of us. I will refer to my sister from time and time when appropriate.
We begin studying the nervous system with its basic functioning unit the cell. Another name for a nerve cell is neuron, Greek word that means "nerve."
A typical neuron consists of a cell body, containing the cell nucleus and the surrounding cytoplasm, several short radiating processes called dendrites, and one long process called the axon. This part of the nerve cell, the axon, terminates at the end point in branchlike twigs called telodendrons, and may have branches called collaterals projecting from its branch. The axon together with its sheath forms the nerve fiber.
Neurons are very specialized cells for transmitting signals from one location of the body to another. Although there are several types of neurons, they all share this similar function: communicating over distance certain messages and signals.
Neurons with mainly dendrites as processes convey signals from a body cell toward the neuron body. Dendrites are derived from the Greek word dendron, meaning tree. Aptly named, the neurons with many dendrites spread their branch-like processes over the surface of many cells. These cells can communicate with the nerve cell via the dendrites. How does this happen?
Well, let us say the nerve cell is found in the leg muscle. The dendrites spread out over the nearby muscle cells, and when they are hard at work, the muscle cell is exchanging oxygen into and carbon dioxide out of the muscle cell. Some water is generated, as well as other by-products, such as lactic acid and the net loss of sodium and potassium out of the cell as glucose is oxidized for fuel.
The dendrites interpret this change, one in which the muscle is working and no longer in a resting state.
The receptive site at the very end of the dendrite is called a synapse. Here, an electrical signal is made by the flow of ions across the plasma membrane of the cell. We will study this electrical potential in a further lesson.
But for now, remember that at the very end of the dendrite is the site of a synapse, and the electrical message generated here is carried into the dendrite at the receptor site, carried into the nerve cell body.
From this point, the signal leaves the nerve cell body via the axon process. The axon is usually a very long process, and in many cases, may run from the various parts of the peripheral body, a leg or arm, all the way to the brain via the spinal cord.
The sciatic nerve in our leg has axons that extend a distance of three feet or more!
The axon is surrounded by a protective layer of cells called the myelin sheath. Since these nerve cells are very important for communicating with the brain, they are protected with these myelin sheath cells. The end point of the axon is called the synaptic terminal. Many of these are found in the spinal cord, where the messages of nerve cells are sent via the synaptic terminals of neurons, all of them linked with the brain stem so that the signals can be interpreted.
There are three major classes of neurons. Sensory neurons communicate information about the external and internal environments from sensory receptors to the nervous systems, such as those in muscle tissue.
Motor neurons convey impulses from the central nervous system found in the brain to the effector cells. That is, messages that originate in the brain are sent back to the effector cells, such as muscle cells. Messages, like move faster, slow down, or stop moving. Thus, we have two classes of neurons that do opposite types of work....incoming as sensory neurons and outgoing as motor control neurons. The final class is the interneuron, serving the role of bridging the synaptic connections of other neurons, so those nerve cells can carry the message onward by linking many nerve cells together via interneurons along the neural pathway.
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