The Development & Defects Of The Fetal Nervous System
Throughout pregnancy the nervous system is continuously growing and developing more than any other system in the body. The nervous system can be divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all of the nerves outside the CNS. The PNS is made up of the cranial nerves and spinal nerves and the nerves to your upper and lower extremities (Cohen & Hull, 2015).
The general functions of the nervous system involve receiving information from the environment, known as sensory, and generating a response from that information, known as a motor response. The sensory neurons of the PNS gather or carry impulses to the spinal cord and brain, and the CNS interprets them. Motor neurons carry impulses from the CNS to muscles and glands so that when you touch something hot you can withdraw your hand (Cohen & Hull, 2015).
The cranial nerves are responsible for four types of signals: special sensory impulses, general sensory impulses, somatic motor impulses and visceral motor impulses. The special sensory impulses involve smell, hearing, taste, and vision. General sensory impulses involve those for temperature, touch, pain, pressure, vibrations and deep muscle sense. Somatic motor impulses control skeletal muscles and visceral motor impulses control smooth muscle, glands and cardiac muscle (Cohen & Hull, 2015).
The PNS is also further divided into two subdivisions the autonomic nervous system (ANS) and the somatic nervous system, soma meaning cell body. The ANS is involuntary and mainly helps to maintain homeostasis; its effectors are smooth muscle, cardiac muscle and glands. Whereas, the somatic nervous system is voluntary or conscious and is responsible for motor responses involving contractions of skeletal muscle (Cohen & Hull, 2015).
The fetal nervous system is the very first system to develop. One of the most important stages in embryonic development takes place in the third week. In the third week, which is known as the start of gastrulation, the primitive streak and the formation of the embryonic mesoderm and endoderm begin. We also see the emergence of the notochord and prechordal mesoderm; this promotes “the overlying ectoderm to thicken causing the neural plate to form” (Sadler, p71, 2015). These cells in the plate constitute as the neuroectoderm and their induction will begin neurulation.
Neurulation is when the neural plate forms the neural tube. The lateral edges of the neural plate elevate to form folds (Sadler, p92, 2015). These folds then continue elevate and morph into a groove that encloses and fuses to form the neural tube (Sadler, p92, 2015). After the development of the neural tube and neurulation is complete the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord (Sadler, p92, 2015).
The cranial neuropore closes on the 25th day after conception, if it fails to close this develops into a myelomeningocele. The embryonic period that is from the third to eighth week of development is the period where the germ layers develop and gives rise to tissues and organ systems (McLone & Bowman, 2018). More specifically the “ectodermal germ layer gives rise to the central and peripheral nervous systems, sensory epithelium of the ears, nose and eye, skin including hair and nails and mammary, pituitary, and seat glands as well as the enamel on the teeth” (Sadler, p92, 2015). With all of these amazing processes happening in the miracle of life, there are so many ways in which something can go wrong in any of the stages of development. One of the problems that can occur in this very early stage of development is neural tube defects.
Neural tube defects are birth defects that involve the central nervous system. One of the most common neural tube defects is spina bifida. Spina bifida occurs when the neural tube does not close or will not stay completely closed. This typically occurs during the embryonic stage of development, specifically around days 21 to 28 (Foster, 2017). There are three types of spina bifida: spina bifida occulta, meningocele and spina bifida cystica (myelomeningocele) (Foster, 2017). In spina bifida occulta or hidden, the defect can’t be seen and rarely has any complications, most of the time it is discovered by accident when the person has an x-ray or MRI (Foster, 2017).
Meningocele is when the membrane around the spinal cord enlarges, creating a lump, that is not seen through the skin and often times does not cause problems (Foster, 2017). In some instances the spinal canal is cleft or bifid, which means the cyst has expanded to the surface. This type of cyst can be surgically removed, leaving no permanent disability (Foster, 2017). Spina Bifida cystica is the most complex and severe form causing neurological problems. In this type a part of the spinal cord and nerves are exposed and visible outside of the body. If this is the case the infant will have surgery within the first 48 hours of life and be given antibiotics to prevent infection. (Foster, 2017).
With modern day treatment, most children are able to live functional lives with some degree of independence. Maternal-fetal medicine specialists are now performing in utero spina bifida surgery. Some of the qualifications needed to be considered for this surgery includes the “myelomeningocele must fall between the T1-S1 vertebra and there must be a Chiari II malformation along with no other known genetic defects or physical abnormalities. Also, the mother must not have a high-risk pregnancy and must have a body mass index of less than 35 kg/m2 before pregnancy” (Brownlee, 2017). Research from the past decade has shown better outcomes for babies with spina bifida if surgically repaired before birth (Brownlee, 2017). Some have a degree of leg paralysis and sometimes difficulty with bowel and bladder incontinence (Foster, 2017).
Spina Bifida can be caused by chromosomal disorders or environmental factors such as harmful exposure, medications or nutrition (Foster, 2017). It is extremely important for a woman who is trying to conceive or has conceived to take folic acid supplements. It is recommended that if a woman is trying to get pregnant she take a folic acid supplement even before conceiving. The “need for folic acid increases during rapid growth and the most crucial time is the first 4 weeks of pregnancy” (Foster, p11, 2017). Some ways a woman can get folic acid in her diet in through many breakfast cereals and grain products, dark green leafy vegetables such as broccoli, spinach, dark green lettuce, egg yolks, beans, whole grains, orange juice and citrus fruits (Foster, 2017).
Some other diseases associated with nutrition include Scurvy which is associated with lack of vitamin C, Rickets, which results from vitamin D deficiency, type 2 diabetes results from obesity and cardiovascular diseases results from a diet high in fat. Symptoms associated with spina bifida include meningocele or myelomeningocele at birth; spinal cord deformity and almost always the patients are asymptomatic (Foster, 2017). Even after surgical intervention the patient will have some disability to varying degrees such as spine curvature, leg paralysis, hip, foot or leg deformity and problems with bowel and bladder control (Foster, 2017).
Some other symptoms include imbalances in muscle strength and function, upper spine abnormalities such as Arnold-Chiari malformation, which causes coordination problems and hydrocephalus (Foster, 2017). Hydrocephalus is an accumulation of fluid in the brain, which leads to neurological problems or mental retardation. Finally, some secondary effects of spina bifida are obesity, due to inactivity, pathologic bone fractures, and growth hormone deficiency, resulting in short stature, latex allergy and psychological, social and sexual problems (Foster, 2017).
Detection of spina bifida is usually performed during a prenatal test where the level of maternal serum alpha fetal protein is measured, in a mother carrying a fetus with spina bifida it will be unusually high (Foster, 2017). Other tests used as well include ultrasound and amniotic fluid testing. Children with spina bifida occulta require no treatment, children with meningocele usually undergo surgical removal with little to no disability, but patients with myelomeningocele require extensive lifelong treatment and assistance (Foster, 2017).
This typically includes surgery followed by antibiotics and specialty care such as physical therapy, physical education, and assistance with bowel and bladder control and prevention of obesity by maintaining a healthy weight, making good dietary choice and staying physically active. There are support groups and counseling available for children and their families to help with the challenges spina bifida can bring such as stress, anger, guilt and anxiety and hopelessness. The outlook for a patient with spina bifida is dependent on the severity and complications, but with support and a multidisciplinary team to help, they are capable of living productive and happy lives (Foster, 2017).
References:
- Brownlee, Christine. (2017). Minimally Invasive Spina Bifida Surgery has Fewer Maternal Risks. Retrieved from https://www.hopkinsmedicine.org/news/articles/minimally-invasive-spina-bifida-surgery-has-fewer-maternal-risks
- Cohen, B.J. & Hull, K.L. (2015). Memmler’s The Human Body in Health and Disease (13th ed). Philadelphia, PA: Wolters Kluwer Health
- Foster,M.R.(2017). SpinaBifida. Retrieved from http://www.emedicinehealth.com/spina_bifida/article_em.htm
- McLone, D.G., & Bowman, R, M. (2018). Pathophysiology and clinical manifestations of myelomeningocele (spina bifida). Retrieved from [online periodical] http://www.uptodate.com/contents/pathophysiology-and-clinical-manifestations-of-myelomeningocele
- Sadler, T.W. (2015). Langman’s Medical Embryology (13thed.). Philadelphia, PA: Wolters Kluwer Health.
