Your spinal cord is a bundle of nerves that runs down the middle of your back. It carries signals back and forth between your body and your brain. It is protected by your vertebrae, which are the bone disks that make up your spine. If you have an accident that damages the vertebrae or other parts of the spine, this can also injure the spinal cord. Other spinal cord problems include

• Tumors
• Infections such as meningitis and polio
• Inflammatory diseases
• Autoimmune diseases
• Degenerative diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy

Symptoms vary but might include pain, numbness, loss of sensation and muscle weakness. These symptoms can occur around the spinal cord, and also in other areas such as your arms and legs. Treatments often include medicines and surgery.

Spinal cord infarction is a stroke within the spinal cord or the arteries that supply it. It is caused by arteriosclerosis or a thickening or closing of the major arteries to the spinal cord. Frequently spinal cord infarction is caused by a specific form of arteriosclerosis called atheromatosis, in which a deposit or accumulation of lipid-containing matter forms within the arteries.

Symptoms, which generally appear within minutes or a few hours of the infarction, may include:

• Intermittent sharp or burning back pain
• Aching pain down through the legs
• Weakness in the legs

• Paralysis, loss of deep tendon reflexes
• Loss of pain and temperature sensation
• Incontinence

Treatment is symptomatic. Physical and occupational therapy may help individuals recover from weakness or paralysis. A catheter may be necessary for individuals with urinary incontinence.

Recovery depends upon how quickly treatment is received and how severely the body is compromised. Paralysis may persist for many weeks or be permanent. Most individuals have a good chance of recovery.

Syringomyelia is a rare disorder that causes a cyst to form in your spinal cord. This cyst, called a syrinx, gets bigger and longer over time, destroying part of the spinal cord. Damage to the spinal cord from the syrinx can cause symptoms such as

• Pain and weakness in the back, shoulders, arms or legs
• Headaches
• Inability to feel hot or cold

Symptoms vary according to the size and location of the syrinx. They often begin in early adulthood.

Syringomyelia usually results from a skull abnormality called a Chiari I malformation. A tumor, meningitis or physical trauma can also cause it. Surgery is the main treatment. Some people also need to have the syrinx drained. Medicines can help ease pain. In some cases, there are no symptoms, so you may not need treatment.

Your spine, or backbone, protects your spinal cord and allows you to stand and bend. Spinal stenosis causes narrowing in your spine. The narrowing puts pressure on your nerves and spinal cord and can cause pain.

Spinal stenosis occurs mostly in people older than 50. Younger people with a spine injury or a narrow spinal canal are also at risk. Diseases such as arthritis and scoliosis can cause spinal stenosis, too. Symptoms might appear gradually or not at all. They include

• Pain in your neck or back
• Numbness, weakness, cramping, or pain in your arms or legs
• Pain going down the leg
• Foot problems

Doctors diagnose spinal stenosis with a physical exam and imaging tests. Treatments include medications, physical therapy, braces, and surgery.

Spinal stenosis happens when the spaces in the spine narrow and create pressure on the spinal cord and nerve roots. The spinal cord is a bundle of nerves that comes out of the base of the brain and runs down the center of the spine. The nerve roots branch out from the cord. The narrowing usually occurs over time and involves one or more areas of the spine:

• The spinal canal, the hollow space in the center of each vertebrae (bones in the spine that protect the spinal cord); the spinal cord and nerve roots run through the spinal canal.
• The canals at the base or roots of nerves branching out from the spinal cord.
• The openings between vertebrae, through which nerves leave the spine and go to other parts of the body.

There are many different structures in the anatomy of the back that work together to support your body. There are four regions of the spine:

• Cervical spine.
• Thoracic spine.
• Lumbar spine.
• Sacrum and coccyx.

Spinal stenosis most commonly develops in the lumbar spine and cervical spine.

Who Gets Spinal Stenosis?

Anyone can get spinal stenosis; however, the chances of developing the disorder increase with age. Spinal stenosis also can be present in younger people who are born with a narrow spinal canal or who have an injury to the spine.

Symptoms of Spinal Stenosis

Symptoms of spinal stenosis may develop when the spaces within the spine narrow, most often in the lower back and neck. The narrowing creates pressure on the spine and related structures, causing symptoms. For most people, symptoms develop and progress slowly over a period of time, and some people may not have any symptoms.

The symptoms you experience depend on the location of the narrowing in your spine. Symptoms of spinal stenosis in the lower back can include:

• Pain in the lower back.
• Burning pain or ache that radiates down the buttocks and into the legs, that typically worsens with standing or walking and gets better with leaning forward (flexion).

• Numbness, tingling, or cramping in the legs and feet. These may become more pronounced during standing or walking.
• Weakness in the legs and feet.

Symptoms of spinal stenosis in the neck may include:

• Neck pain.
• Numbness or tingling that radiates down the arms into the hands.
• Weakness in a hand, arm, or fingers.

Walking, standing, or extending the lumbar area of the spine can cause symptoms to worsen. Sitting or flexing the lower back or neck may relieve symptoms. The flexed position “opens up” the spinal column, enlarging the spaces between vertebrae at the back of the spine.

People with more severe stenosis may have problems with:

• Bowel function.
• Bladder function.
• Sexual function.

Causes of Spinal Stenosis

Several factors can contribute to the narrowing of the spinal canal, leading to spinal stenosis. Normally, the vertebral canal provides enough room for the spinal cord and cauda equina. However, aging and age-related changes in the spine, injury, other diseases, or inherited conditions can cause narrowing of the spaces.

Aging and Age-Related Changes in the Spine

Aging and age-related changes in the spine happen over a period of time and slowly cause loss of the normal structure of the spine. They are the most common causes of spinal stenosis. As people age, the ligaments that keep the vertebrae of the spine in place may thicken and calcify (harden from deposits of calcium salts). Bones and joints may also enlarge. When surfaces of the bone begin to project out from the body, these projections are called osteophytes (bone spurs).

For example:

• A herniated (bulging) disk may place pressure on the spinal cord or nerve root. Disks are cushion-like pads that lie between the vertebrae and act like shock absorbers and spacers for the spine. As the disk ages, it can dry out and crack, causing the disk to bulge.
• When a segment of the spine becomes too mobile, the capsules (membranes) of the facet joints at the back of the vertebrae thicken. This effort to try to stabilize the segment can cause bone spurs, which decrease the space around the nerve roots leaving the spinal cord.

• Spondylolisthesis happens when one vertebra slips forward on another vertebra. This creates poor alignment of the spinal column and can place pressure on the spinal cord or nerve roots.

Arthritis

Arthritis is also a common cause of spinal stenosis. Two forms of arthritis that may affect the spine are osteoarthritis and rheumatoid arthritis.

• Osteoarthritis is a degenerative joint disease in which the tissues in the joint break down over time. It is the most common type of arthritis and is more common in older people. Osteoarthritis can lead to disk degeneration and an enlargement or overgrowth of bone that narrows the central and nerve root canals, causing spinal stenosis.
• Rheumatoid Arthritis (RA) is a chronic (long-lasting) inflammatory disease that mostly affects joints. RA causes pain, swelling, stiffness, and loss of function in joints. It is an autoimmune disorder because the immune system attacks the healthy joint tissues. Although not a common cause of spinal stenosis, damage from RA can cause significant problems with joints in the spine, leading to spinal stenosis.

Other Conditions

The following conditions may cause spinal stenosis:

• Tumors of the spine are abnormal growths of soft tissue that may affect the spinal canal directly by causing inflammation or growth of tissue into the spinal canal. This can narrow the space and cause bone changes, leading to spinal stenosis. Some people develop a rare disorder called epidural lipomatosis, which happens when fat builds up on or around the lining of the spine.
• Fractures due to trauma (injury) or other medical conditions may either dislocate the spine and the spinal canal or cause fractures that produce fragments of bone that penetrate the canal.
• Paget's disease of bone is a chronic (long-lasting) disorder that causes bones to grow larger and become weaker than normal. As Paget’s disease progresses, new bone forms at a faster rate than the rate at which old bone is removed. However, the new bone does not form correctly, leading to larger bones that are misshapen, weaker, and softer than normal bone. This can cause problems with blood supply and bone structure, which changes the spaces in the spinal canal, leading to spinal stenosis.
• Ossification of the posterior longitudinal ligament happens when calcium deposits form on the ligament that runs up and down behind the spine and inside the spinal canal. These deposits turn the fibrous tissue of the ligament into bone and may press on the nerves in the spinal canal.

Inherited Conditions

Some people are born with a condition that can cause spinal stenosis. These conditions cause the spinal canal to narrow, leading to spinal stenosis.

For example:

• Congenital stenosis happens when you are born with a small and narrow spinal canal.
• Scoliosis is a curvature of the spine.
• Achondroplasia is an inherited condition that causes problems with bone formation in the spine and other bones in the body.

What is spinal muscular atrophy (SMA)?

Spinal muscular atrophy (SMA) is a group of genetic diseases that damages and kills motor neurons. Motor neurons are a type of nerve cell in the spinal cord and lower part of the brain. They control movement in your arms, legs, face, chest, throat, and tongue.

As the motor neurons die off, your muscles start to weaken and atrophy (waste away). The muscle damage gets worse over time and can affect speaking, walking, swallowing, and breathing.

What are the types of spinal muscular atrophy (SMA) and what are their symptoms?

There are different types of SMA. They are based on how serious the disease is and when the symptoms start:

• Type l is also called Werdnig-Hoffman disease or infantile-onset SMA. It is the most severe type. It is also the most common. Babies with this type usually show signs of the disease before 6 months of age. In more severe cases, the signs show up even before or just after birth (Types 0 or 1A). The babies may have trouble swallowing and breathing and may not move around a lot. They have chronic shortening of muscles or tendons (called contractures). They usually cannot sit up without help. Without treatment, many children with this type will die before 2 years of age.
• Type ll is a moderate to severe type of SMA. It usually first noticed between 6 and 18 months of age. Most children with this type can sit without support but cannot stand or walk without help. They may also have trouble breathing. They can usually live into adolescence or young adulthood.
• Type lll is also called Kugelberg-Welander disease. It is the mildest type that affects children. The signs of the disease usually show up after age 18 months. Children with this type can walk by themselves but may have trouble running, getting up from a chair, or climbing stairs. They may also have scoliosis (curvature of the spine), contractures, and respiratory infections. With treatment, most children with this type will have a normal lifespan.
• Type IV is rare and often mild. It usually causes symptoms after 21 years of age. The symptoms include mild to moderate leg muscle weakness, tremors, and mild breathing problems. The symptoms slowly get worse over time. People with this type of SMA have a normal lifespan.

What causes spinal muscular atrophy (SMA)?

Most types of SMA are caused by a change in the SMN1 gene. This gene is responsible for making a protein that the motor neurons need to be healthy and to function. But when part of the SMN1 gene is missing or abnormal, there isn't enough protein for the motor neurons. This causes the motor neurons to die off.

Most people have two copies of the SM1 gene - one from each parent. SMA normally only happens when both copies have the gene change. If only one copy has the change, there usually aren't any symptoms. But that gene could be passed down from parent to child.

Some of the less common types of SMA may be caused by changes in other genes.

How is spinal muscular atrophy (SMA) diagnosed?

Your health care provider may use many tools to diagnose SMA:

• A physical exam
• A medical history, including asking about family history
• Genetic testing to check for the gene changes that cause SMA
• Electromyography and nerve conduction studies and a muscle biopsy may be done, especially if no gene changes were found

Parents who have a family history of SMA may want to do a prenatal test to check to see whether their baby has an SMN1 gene change. An amniocentesis or in some cases a chorionic villi sampling (CVS) is used to get the sample for testing.

In some states, genetic testing for SMA is part of newborn screening tests.

What are the treatments for spinal muscular atrophy (SMA)?

There is no cure for SMA. Treatments can help manage symptoms and prevent complications. They may include:

• Medicines to help the body make more of the proteins that the motor neurons need
• Gene therapy for children under 2 years of age
• Physical, occupational, and rehabilitation therapy to help to improve posture and the mobility of the joints. These therapies may also improve blood flow and slow muscle weakness and atrophy. Some people may also need therapy for trouble speaking, chewing, and swallowing.
• Assistive devices such as supports or braces, orthotics, speech synthesizers, and wheelchairs to help people stay more independent
• Good nutrition and a balanced diet to help maintain weight and strength. Some people might need a feeding tube in order to get the nutrition they need.
• Breathing support for people who have muscle weakness in the neck, throat, and chest. The support may include devices to help with breathing during the day and to prevent sleep apnea at night. Some people might need to be on a ventilator.

Your spinal cord is a bundle of nerves that runs down the middle of your back. It carries signals back and forth between your body and your brain. A spinal cord injury disrupts the signals. Spinal cord injuries usually begin with a blow that fractures or dislocates your vertebrae, the bone disks that make up your spine. Most injuries don't cut through your spinal cord. Instead, they cause damage when pieces of vertebrae tear into cord tissue or press down on the nerve parts that carry signals.

Spinal cord injuries can be complete or incomplete. With a complete spinal cord injury, the cord can't send signals below the level of the injury. As a result, you are paralyzed below the injury. With an incomplete injury, you have some movement and sensation below the injury.

A spinal cord injury is a medical emergency. Immediate treatment can reduce long-term effects. Treatments may include medicines, braces or traction to stabilize the spine, and surgery. Later treatment usually includes medicines and rehabilitation therapy. Mobility aids and assistive devices may help you to get around and do some daily tasks.

A spinal cord injury (SCI) is damage to the tight bundle of cells and nerves that sends and receives signals from the brain to and from the rest of the body. The spinal cord extends from the lower part of the brain down through the lower back.

SCI can be caused by direct injury to the spinal cord itself or from damage to the tissue and bones (vertebrae) that surround the spinal cord. This damage can result in temporary or permanent changes in sensation, movement, strength, and body functions below the site of injury.

Injury and severity

The extent of disability depends on where along the spinal cord the injury occurs and the severity of the injury.

Loss of nerve function occurs below the level of injury. An injury higher on the spinal cord can cause paralysis in most of the body and affect all limbs (called tetraplegia or quadriplegia). A lower injury to the spinal cord may cause paralysis affecting the legs and lower body (called paraplegia).

A spinal cord injury can damage a few, many, or almost all of the nerve fibers that cross the site of injury. A variety of cells located in and around the injury site may also die. Some injuries having little or no nerve cell death may allow an almost complete recovery.

Type of injury

A spinal cord injury can be classified as complete or incomplete.

• An incomplete injury means the spinal cord is still able to transmit some messages to or from the brain. People with incomplete injuries retain some sensory function and may have some control of muscle activity below the injury site.
• A complete injurymeans there is no nerve communications and sensory and motor function below the injury site are lost.

Primary damage is immediate and is caused directly by the injury. Secondary damage results from inflammation and swelling that can press on the spinal cord and vertebrae, as well as from changes in the activity of cells and cell death.

Common causes

Motor vehicle accidents and catastrophic falls are the most common causes of SCI in the United States. The rest are due to acts of violence (primarily gunshot wounds and assaults), sports injuries, medical or surgical injury, industrial accidents, diseases and conditions that can damage the spinal cord, and other less common causes.

The spinal cord is a soft, cylindrical column of tightly bundled nerve cells (neurons and glia), nerve fibers that transmit nerve signals (called axons), and blood vessels. It sends and receives information between the brain and the rest of the body. Millions of nerve cells situated in the spinal cord itself also coordinate complex patterns of movements such as rhythmic breathing and walking.

The spinal cord extends from the brain to the lower back through a canal in the center of the bones of the spine. Like the brain, the spinal cord is protected by three layers of tissue and is surrounded by the cerebrospinal fluid that acts as a cushion against shock or injury.

Inside the spinal cord is:

• A collection of neurons and their projections that connect with other nerve cells to transmit and receive information, called gray matter.
• Bundles of axons, some of which are coated with a whitish mixture of proteins and fat-like substances, called white matter.

Other types of nerve cells sit just outside the spinal cord and relay information to the brain.

31 pairs of nerves, each of which contains thousands of axons, are divided into 4 regions having individual segments and link the spinal cord to muscles and other parts of the body:

• Cervical spinal nerves (C1 to C8) emerge from the spinal cord in the neck and control signals to the back of the head, the neck and shoulders, the arms and hands, and the diaphragm.
• Thoracic spinal nerves (T1 to T12) emerge from the spinal cord in the upper mid-back and control signals to the chest muscles, some muscles of the back, and many organ systems, including parts of the abdomen.
• Lumbar spinal nerves (L1 to L5) emerge from the spinal cord in the low back and control signals to the lower parts of the abdomen and the back, the buttocks, some parts of the external genital organs, and parts of the leg.
• Sacral spinal nerves (S1 to S5) emerge from the spinal cord in the low back and control signals to the thighs and lower parts of the legs, the feet, most of the external genital organs, and the area around the anus.

The spinal column, which surrounds and protects the spinal cord, is made up of 33 rings of bone (called vertebrae), pads of semi-rigid cartilage (called discs), and narrow spaces called foramen that act as passages for spinal nerves to travel to and from the rest of the body. These are places where the spinal cord is particularly vulnerable to direct injury.

Spina bifida is a neural tube defect - a type of birth defect of the brain, spine, or spinal cord. It happens if the spinal column of the fetus doesn't close completely during the first month of pregnancy. This can damage the nerves and spinal cord. Screening tests during pregnancy can check for spina bifida. Sometimes it is discovered only after the baby is born.

The symptoms of spina bifida vary from person to person. Most people with spina bifida are of normal intelligence. Some people need assistive devices such as braces, crutches, or wheelchairs. They may have learning difficulties, urinary and bowel problems, or hydrocephalus, a buildup of fluid in the brain.

The exact cause of spina bifida is unknown. It seems to run in families. Taking folic acid can reduce the risk of having a baby with spina bifida. It's in most multivitamins. Women who could become pregnant should take it daily.

Definition

Spina bifida (SB) is a neural tube defect (a disorder involving incomplete development of the brain, spinal cord, and/or their protective coverings) caused by the failure of the fetus's spine to close properly during the first month of pregnancy. Infants born with SB sometimes have an open lesion on their spine where significant damage to the nerves and spinal cord has occurred. Although the spinal opening can be surgically repaired shortly after birth, the nerve damage is permanent, resulting in varying degrees of paralysis of the lower limbs. Even when there is no lesion present there may be improperly formed or missing vertebrae and accompanying nerve damage. In addition to physical and mobility difficulties, most individuals have some form of learning disability. The types of SB are: myelomeningocele, the severest form, in which the spinal cord and its protective covering (the meninges) protrude from an opening in the spine; meningocele in which the spinal cord develops normally but the meninges and spinal fluid) protrude from a spinal opening; closed neural tube defects, which consist of a group of defects in which development of the spinal cord is affected by malformations of the fat, bone, or meninges; and and occulta, the mildest form, in which one or more vertebrae are malformed and covered by a layer of skin. SB may also cause bowel and bladder complications, and many children with SB have hydrocephalus (excessive accumulation of cerebrospinal fluid in the brain).

Treatment

There is no cure for SB because the nerve tissue cannot be replaced or repaired. Treatment for the variety of effects of SB may include surgery, medication, and physiotherapy. Many individuals with SB will need assistive devices such as braces, crutches, or wheelchairs. Ongoing therapy, medical care, and/or surgical treatments may be necessary to prevent and manage complications throughout the individual's life. Surgery to close the newborn's spinal opening is generally performed within 24 hours after birth to minimize the risk of infection and to preserve existing function in the spinal cord.

Prognosis

The prognosis for individuals with SB depends on the number and severity of abnormalities. Prognosis is poorest for those with complete paralysis, hydrocephalus, and other congenital defects. With proper care, most children with SB live well into adulthood.

Spina bifida is a condition that affects the spine and is usually apparent at birth. It is a type of neural tube defect (NTD).

Spina bifida can happen anywhere along the spine if the neural tube does not close all the way. When the neural tube doesn’t close all the way, the backbone that protects the spinal cord doesn’t form and close as it should. This often results in damage to the spinal cord and nerves.

Spina bifida might cause physical and intellectual disabilities that range from mild to severe. The severity depends on:

• The size and location of the opening in the spine.
• Whether part of the spinal cord and nerves are affected.

Types of Spina Bifida

The three most common types of spina bifida are:

Myelomeningocele (sounds like: my-low-ma-nin-jo-seal)
When people talk about spina bifida, most often they are referring to myelomeningocele. Myelomeningocele is the most serious type of spina bifida. With this condition, a sac of fluid comes through an opening in the baby’s back. Part of the spinal cord and nerves are in this sac and are damaged. This type of spina bifida causes moderate to severe disabilities, such as problems affecting how the person goes to the bathroom, loss of feeling in the person’s legs or feet, and not being able to move the legs.

Meningocele (sounds like: ma-nin-jo-seal)
Another type of spina bifida is meningocele. With meningocele a sac of fluid comes through an opening in the baby’s back. But, the spinal cord is not in this sac. There is usually little or no nerve damage. This type of spina bifida can cause minor disabilities.

Spina Bifida Occulta (sounds like: o-cult-tuh)
Spina bifida occulta is the mildest type of spina bifida. It is sometimes called “hidden” spina bifida. With it, there is a small gap in the spine, but no opening or sac on the back. The spinal cord and the nerves usually are normal. Many times, spina bifida occulta is not discovered until late childhood or adulthood. This type of spina bifida usually does not cause any disabilities.

Diagnosis

Spina bifida can be diagnosed during pregnancy or after the baby is born. Spina bifida occulta might not be diagnosed until late childhood or adulthood, or might never be diagnosed.

During Pregnancy

During pregnancy there are screening tests (prenatal tests) to check for spina bifida and other birth defects. Talk with your doctor about any questions or concerns you have about this prenatal testing.

• AFP – AFP stands for alpha-fetoprotein (sounds like: al-fa–fee-toe-pro-teen), a protein the unborn baby produces. This is a simple blood test that measures how much AFP has passed into the mother’s bloodstream from the baby. A high level of AFP might mean that the baby has spina bifida. An AFP test might be part of a test called the “triple screen” that looks for neural tube defects and other issues.
• Ultrasound – An ultrasound is a type of picture of the baby. In some cases, the doctor can see if the baby has spina bifida or find other reasons that there might be a high level of AFP. Frequently, spina bifida can be seen with this test.
• Amniocentesis (sounds like: am-knee-oh-sin-te-sus) – For this test, the doctor takes a small sample of the amniotic fluid surrounding the baby in the womb. Higher than average levels of AFP in the fluid might mean that the baby has spina bifida.

After the Baby Is Born

In some cases, spina bifida might not be diagnosed until after the baby is born.

Sometimes there is a hairy patch of skin or a dimple on the baby’s back that is first seen after the baby is born. A doctor can use an image scan, such as an, X-ray, MRI, or CT, to get a clearer view of the baby’s spine and the bones in the back.

Sometimes spina bifida is not diagnosed until after the baby is born because the mother did not receive prenatal care or an ultrasound did not show clear pictures of the affected part of the spine.

Treatments

Not all people born with spina bifida have the same needs, so treatment will be different for each person. Some people have problems that are more serious than others. People with myelomeningocele and meningocele will need more treatments than people with spina bifida occulta.

Causes and Prevention

We do not know all of the causes of spina bifida. The role that genetics and the environment play in causing spina bifida needs to be studied further.

However, we do know that there are ways for women to reduce the risk of having a baby with spina bifida both before and during her pregnancy.

If you are pregnant or could get pregnant, use the following tips to help prevent your baby from having spina bifida:

• Take 400 micrograms (mcg) of folic acid every day. If you have already had a pregnancy affected by spina bifida, you may need to take a higher dose of folic acid before pregnancy and during early pregnancy. Talk to your doctor to discuss what’s best for you.
• Talk to your doctor or pharmacist about any prescription and over-the-counter drugs, vitamins, and dietary or herbal supplements you are taking.
• If you have a medical condition―such as diabetes or obesity―be sure it is under control before you become pregnant.
• Avoid overheating your body, as might happen if you use a hot tub or sauna.
• Treat any fever you have right away with Tylenol® (or store brand acetaminophen).

Remember!

Spina bifida happens in the first few weeks of pregnancy, often before a woman knows she’s pregnant. Although folic acid is not a guarantee that a woman will have a healthy pregnancy, taking folic acid can help reduce a woman’s risk of having a pregnancy affected by spina bifida. Because half of all pregnancies in the United States are unplanned, it is important that all women who can become pregnant take 400 mcg of folic acid daily.

Living with Spina Bifida

Spina bifida can range from mild to severe. Some people may have little to no disability. Other people may be limited in the way they move or function. Some people may even be paralyzed or unable to walk or move parts of their body.

Even so, with the right care, most people affected by spina bifida lead full, productive lives.

The brain is a complex organ composed of gray parts and white matter, which can be hard to distinguish. Starting from an embryologic perspective allows you to understand more easily how the parts relate to each other. The embryonic nervous system begins as a very simple structure—essentially just a straight line, which then gets increasingly complex. Looking at the development of the nervous system with a couple of early snapshots makes it easier to understand the whole complex system.

Many structures that appear to be adjacent in the adult brain are not connected, and the connections that exist may seem arbitrary. But there is an underlying order to the system that comes from how different parts develop. By following the developmental pattern, it is possible to learn what the major regions of the nervous system are.

The Neural Tube

To begin, a sperm cell and an egg cell fuse to become a fertilized egg. The fertilized egg cell, or zygote, starts dividing to generate the cells that make up an entire organism. Sixteen days after fertilization, the developing embryo’s cells belong to one of three germ layers that give rise to the different tissues in the body. The endoderm, or inner tissue, is responsible for generating the lining tissues of various spaces within the body, such as the mucosae of the digestive and respiratory systems. The mesoderm, or middle tissue, gives rise to most of the muscle and connective tissues. Finally the ectoderm, or outer tissue, develops into the integumentary system (the skin) and the nervous system. It is probably not difficult to see that the outer tissue of the embryo becomes the outer covering of the body. But how is it responsible for the nervous system?

As the embryo develops, a portion of the ectoderm differentiates into a specialized region of neuroectoderm, which is the precursor for the tissue of the nervous system. Molecular signals induce cells in this region to differentiate into the neuroepithelium, forming a neural plate. The cells then begin to change shape, causing the tissue to buckle and fold inward (image). A neural groove forms, visible as a line along the dorsal surface of the embryo. The ridge-like edge on either side of the neural groove is referred as the neural fold. As the neural folds come together and converge, the underlying structure forms into a tube just beneath the ectoderm called the neural tube. Cells from the neural folds then separate from the ectoderm to form a cluster of cells referred to as the neural crest, which runs lateral to the neural tube. The neural crest migrates away from the nascent, or embryonic, central nervous system (CNS) that will form along the neural groove and develops into several parts of the peripheral nervous system (PNS), including the enteric nervous tissue. Many tissues that are not part of the nervous system also arise from the neural crest, such as craniofacial cartilage and bone, and melanocytes.

At this point, the early nervous system is a simple, hollow tube. It runs from the anterior end of the embryo to the posterior end. Beginning at 25 days, the anterior end develops into the brain, and the posterior portion becomes the spinal cord. This is the most basic arrangement of tissue in the nervous system, and it gives rise to the more complex structures by the fourth week of development.

Amyotrophic lateral sclerosis (ALS) is a nervous system disease that attacks nerve cells called neurons in your brain and spinal cord. These neurons transmit messages from your brain and spinal cord to your voluntary muscles - the ones you can control, like in your arms and legs. At first, this causes mild muscle problems. Some people notice

• Trouble walking or running
• Trouble writing
• Speech problems

Eventually, you lose your strength and cannot move. When muscles in your chest fail, you cannot breathe. A breathing machine can help, but most people with ALS die from respiratory failure.

The disease usually strikes between age 40 and 60. More men than women get it. No one knows what causes ALS. It can run in families, but usually it strikes at random. There is no cure. Medicines can relieve symptoms and, sometimes, prolong survival.