Your nerves perform many vital functions, from controlling your muscles to helping you sense your body's movements and allowing you to hear and taste. The nervous system begins to develop early in pregnancy, eventually forming nerves that link all the parts of your body to the brain and spinal cord. Although this complex network continues to develop throughout childhood and adolescence, the essential framework is well formed and ready to function at birth.
During the third week of pregnancy -- when the fetus is less than 1 inch in length -- an indentation called the neural groove forms along the middle of the back. The edges of this groove eventually join to form a structure called the longitudinal neural tube, which develops into the brain and the spinal cord. During the next few weeks, the brain enlarges and folds on itself, until it resembles a fully formed brain. The spinal cord continues growing during this period. Nerve cells in both areas send out extensions that connect with other nerve cells, forming microscopic structures called synapses that communicate messages from one nerve to another. Parts of the brain and spinal cord also organize into different areas during this period. These areas are called gray and white matter. The gray matter contains mostly central parts of nerve cells, or cell bodies. The white matter contains primarily nerve cell extensions, or processes. Nerve processes continue to grow and eventually develop into the long, thin structures found outside the brain and spinal cord. These structures are generally called nerves.
Nerves outside of the brain and spinal cord are called peripheral nerves. Nerves containing nerve cell processes that control muscular movement are called motor fibers. Those that carry the sensations of touch, pain and temperature back to the brain and spinal cord are called sensory fibers. By the 16th week of pregnancy, peripheral nerves have grown and organized their activity to the extent that a baby can actively move. By week 25 of pregnancy, fetal nerves are able to manage fine movements, such as those needed for control of the facial muscles. This was shown in a study published in March 2013 in the "International Journal of Gynecology," in which 24 normal fetuses were studied with four-dimensional ultrasound examinations. The fetuses were found to produce common facial expressions, including smiling, sucking, blinking and yawning. These movements involve not only control of muscle activity, but also relaying of complex information back to the fetal brain about the position of the muscles and the parts of the face they control.
Fetuses also develop another highly specialized network called the autonomic nervous system. After birth, it controls organ and tissue functions that occur without conscious thought, including beating of the heart and breathing. The autonomic nerves begin to develop during approximately the fifth week of pregnancy. They originate from special nervous tissue called the neural crest, which begins to form outside the nervous system and moves into a position near the spinal cord or in the walls of certain organs. These nerves form complex connections with each other, the spinal cord and the brain. Autonomic nerves are also important for special functions, such as balance and hearing. By week 32 of pregnancy, nerves in the ears are functional, and a baby begins responding to sounds. This was shown in a study published in 2011 in the journal "Developmental Science," in which researchers found that a fetus's heart rate changes in response to the sound of her mother's voice.
In addition to their connections to the brain and spinal cord, nerves form complex structures called plexuses. These are sites where nerves connect with each other. From a plexus, interconnected fibers form nerves that travel to their destination, where they interact with muscles and other structures. Nerve plexuses begin to form as early as 14 weeks of pregnancy, as shown in a 2012 paper published in "Advances in Clinical and Experimental Medicine." Researchers studied the fetal brachial plexus, which contains nerves that control muscles in the arms and hands. They found that most growth occurs between weeks 14 and 18, and continues later in fetal development at a slower rate. In addition to the development of nerve cells, specialized cells called Schwann cells develop from neural crest tissue during fetal life. Schwann cells produce myelin, a fatty substance that surrounds many nerves. Myelin insulates nerves, helping them transmit messages smoothly and rapidly. Essential for normal muscle function, myelin begins appearing on peripheral nerves by about 20 weeks of pregnancy and continues to accumulate rapidly throughout the first year of a baby's life.
Joanne Marie began writing professionally in 1981. Her work has appeared in health, medical and scientific publications such as Endocrinology and Journal of Cell Biology. She has also published in hobbyist offerings such as The Hobstarand The Bagpiper. Marie is a certified master gardener and has a Ph.D. in anatomy from Temple University School of Medicine. Houston Chronicle