The Nervous System
The nervous system is the body’s communication system. It consists of two parts. The central nervous system includes the brain and spinal cord. The peripheral nervous system includes all other nerves.
Nearly all messages of the nervous system involve the brain. Messages may begin with one of the major sense organs. Nerve cells in the sense organs react to changes in the environment. Information is converted to electrical and chemical signals that travel in sensory nerves. These nerves lead to the spinal cord and brain or to the brain directly. Messages carried on motor nerves act to move distant body parts, such as the arms and legs.
The shape of the nerve cell helps it function. Dendrites spread out from the cell body. The axon provides a quick path to the next nerve cell. The synapse is a small gap between cells where the electrical message changes into a chemical message.
Nerve cells can react to changes in the environment quickly. This reaction is called a reflex. Suppose you touch a flame with your hand. Nerve endings in your skin pick up the “hot” signal and immediately send it to the spinal cord. The spinal cord sends a message to your brain and hand. You jerk your hand away before you know you did it!
The brain is the most complex part of the nervous system. It contains 90 percent of the body’s nerve cells. All actions, except for reflex actions, are controlled by the brain. The brain is protected by the skull.
The upper portion of the brain is called the cerebrum. It receives, stores, and recalls all the information picked up through the senses. It is also the processing center for memory, decision-making, thinking, speech, smell, taste, touch, vision, and hearing.
The cerebrum is divided into two halves. The right cerebrum controls the left side of the body while the left cerebrum controls the right side.
Below the cerebrum is the cerebellum, which controls muscle coordination and balance. The medulla controls involuntary actions, such as the heartbeat, breathing, and digestion.
The Endocrine System
The endocrine system is a group of glands that release chemical messengers into the blood. The messengers are called hormones.
Hormones control the body’s growth, its use of energy, and its ability to reproduce. Adrenaline is a hormone released into the body in times of danger. Some hormones control the levels of sugar or calcium in the blood. Others control the sexual maturation process of men and women.
The Reproductive System
The male reproductive system is controlled by hormones. The testes produce sperm, the male sex cells. Sperm are stored in the scrotum. The sperm leave the body through the penis.
The female reproductive system is also controlled by hormones. The ovaries produce female sex cells, or eggs. There an egg matures and is released into the uterus. If fertilized, it attaches to the wall of the uterus. If not, it passes out of the body through the vagina.
The growth and release of a mature egg is called the menstrual cycle. The cycle, roughly 28 days in length, starts when an egg begins to mature. The egg is released into the uterus. If the egg is not fertilized, uterine lining breaks down and is shed through the vagina as menstrual blood. The cycle begins again. If a mature egg and sperm unite and attach to the uterus, a pregnancy results.
Growth of the Fetus
As soon as an egg is fertilized, a sequence of changes begins-series of events occur, one following the other. The egg connects itself to the uterus through a membrane called the placenta. There the egg begins to divide and grow. The rapidly developing organism is called an embryo. During the first three months of pregnancy, the head and brain grow quickly, but the body growth is slow. The heart forms and begins to beat. Although all body systems are present, most of them cannot function yet. During this period, the embryo is very sensitive. A mother that uses nicotine, alcohol, or drugs can seriously damage it.
At the end of two months, the embryo begins to look somewhat human. At that stage it is called a fetus. During the next three months, the body begins to catch up with the head. Skin develops, and the mother may feel the fetus kick. During the last three months, the fetus gains a great deal of weight. Soon it can no longer move freely in the uterus. Bones, blood, and nerves develop rapidly. At this time, good nutrition-especially the addition of calcium, iron, and protein-is essential to the mother’s diet.
At the end of about nine months, the mother’s body produces hormones that control the baby’s birth. The smooth muscles of the uterus begin to contract and relax. This movement, called labor, pushes the baby down the birth canal and out of the mother’s body. The umbilical cord that joined the baby to the mother is cut. After a few more contractions, the placenta is forced out of the mother’s body.