2005-02-18 / Columnists

Drawing On Science


by Stephen Yaeger

Everyone has had the experience of someone yelling, “Hey, catch!” When you turn around an object is on its way toward you. Without thinking you throw up your hands in front of you and, at the same time, blink. What you have experienced is called a reflex or involuntary action. Sneezing, coughing, breathing, and your heartbeat are other examples of involuntary actions. Reflexes are some ways in which our bodies protect themselves. These involuntary actions are controlled by the regulatory or nervous system. Our regulatory system is divided into two parts: the central nervous system composed of the brain and spinal cord. This system controls most of the activities of the body. The second part is the peripheral nervous system. This part consists of many nerves carrying messages between the central nervous system and the muscles and sensory organs of the body.

The brain is one of the most active organs of the body. It is here that we actually interpret what we see, hear, taste, smell, and touch. Because of its importance about 20% of the blood that leaves the heart is pumped to the brain. About every three weeks the brain replaces most of its protein and its energy comes almost exclusively from glucose (sugar). The brain has three major parts. The cerebrum or forebrain is the largest part of the brain. It makes up about two-thirds of the brain’s mass. It has a groove down the middle separating it into the right and left cerebral hemispheres. The outermost layer of the cerebrum is the cerebral cortex or gray matter, which has many folds or convolutions. These convolutions increase the surface area of the cerebrum. It is here that we sense things through our eyes, ears, nose, etc. and it is the region where we think, learn, and hold memories. The cerebrum also controls our movements such as walking, and picking things up. The layer below the gray matter is white matter, which is composed of nerve fibers.

Below and in back of the cerebrum is the cerebellum or mid brain. Its outer layer is gray matter and the inner layer is white matter. This part of our brain works together with the cerebrum to control voluntary movements as well as some involuntary movements. It also works with other parts of the body to control involuntary movements. It can work with the inner ear to maintain balance or with the muscles to maintain muscle fitness. Beneath the cerebellum is the medulla oblongata or hindbrain also composed of gray and white matter, but here the gray matter is the inner layer while the white matter is the upper layer. The medulla oblongata is connected to the spinal cord and is composed mainly of nerve fibers. These nerve fibers connect the spinal cord to other parts of the brain. Involuntary movements such as breathing, heartbeat, coughing, and blood flow are controlled by the medulla oblongata.

The spinal cord is found in the spinal column and extends to its base. It has an H-shaped inner region of gray matter surrounded by an outer layer of white matter. The white matter contains nerve fibers that extend to all parts of the body. The spinal canal runs through the middle of the spinal cord and is filled with cerebrospinal fluid. The spinal cord connects the nerves of the brain with the nerves of the peripheral nervous system. It also controls certain reflexes.

The peripheral nervous system is composed of all the neurons (=nerve cell) and nerve fibers outside of the brain and spinal cord. Bundles of neurons make up nerves. Spinal nerves are connected to the spinal cord and cranial nerves are connected to the brain. There are 31 pairs of spinal nerves and 12 pairs of cranial nerves. Each of these nerves or combination of nerves sends and receives impulses from different parts of the body. The path over which these impulses travel is called a reflex arc.

Questions/comments?

E-mail Steve: Drawingonscience @aol. com.

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