The nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. It is essentially the body's electrical wiring.
Structurally, the nervous system has two components: the central nervous system and the peripheral nervous system. According to the National Institutes of Health, the central nervous system is made up of the brain, spinal cord and nerves. The peripheral nervous system consists of sensory neurons, ganglia (clusters of neurons) and nerves that connect to one another and to the central nervous system.
Functionally, the nervous system has two main subdivisions: the somatic, or voluntary, component; and the autonomic, or involuntary, component. The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing, that work without conscious effort, according to Merck Manuals. The somatic system consists of nerves that connect the brain and spinal cord with muscles and sensory receptors in the skin.
Description of the nervous system
Nerves are cylindrical bundles of fibers that start at the brain and central cord and branch out to every other part of the body, according to the University of Michigan Medical School.
Neurons send signals to other cells through thin fibers called axons, which cause chemicals known as neurotransmitters to be released at junctions called synapses, the NIH noted. There are over 100 trillion neural connections in the average human brain, though the number and location can vary. For example, a new study published January 2018 in the journal Proceedings of the National Academy of Sciences found that out of the 160 participants studied, the brains of highly creative people have more connections among three specific regions of the brain than less creative thinkers.
"You have these three different systems that are all located in different parts of the brain, but they are all co-activated at once," said lead study author Roger Beaty, a postdoctoral fellow studying cognitive neuroscience at Harvard University. "People who are better able to co-activate them [came] up with more-creative responses."
A synapse gives a command to the cell and the entire communication process typically takes only a fraction of a millisecond. Signals travel along an alpha motor neuron in the spinal cord 268 mph (431 km/h); the fastest transmission in the human body, according to Discover magazine.
Sensory neurons react to physical stimuli such as light, sound and touch and send feedback to the central nervous system about the body's surrounding environment, according to the American Psychological Association. Motor neurons, located in the central nervous system or in peripheral ganglia, transmit signals to activate the muscles or glands. [Here's What You'd Look Like as Just a Nervous System]
Glial cells, derived from the Greek word for "glue," are specialized cells that support, protect or nourish nerve cells, according to the Oregon Institute of Health and Science University.
The brain's connections and thinking ability grew over thousands of years of evolution. For example, a virus bound its genetic code to the genome of four-limbed animals, and the code can still be found in humans' brains today, according to two papers published in the January 2018 journal Cell. This code packages up genetic information and sends it from nerve cells to other nearby nerve cells, a very important process in the brain. [An Ancient Virus May Be Responsible for Human Consciousness]
Diagnosing nervous system conditions
There are a number of tests and procedures to diagnose conditions involving the nervous system. In addition to the traditional X-ray, a specialized X-ray called a fluoroscopy examines the body in motion, such as blood flowing through arteries, according to the NIH.
Other standard neurological exams include an MRI (magnetic resonance imaging), CT scan, and an electroencephalogram (EEG), which records the brain's continuous electrical activity. Positron emission tomography (PET) is a procedure that measures cell or tissue metabolism and brain activity to detect tumors or diseased tissue or tumors, the NIH noted.
A spinal tap places a needle into the spinal canal to drain a small amount of cerebral spinal fluid that is tested for infection or other abnormalities, according to the NIH.
Diseases of the nervous system
"Of all the diseases of the nervous system, the most common difficulty that people have is pain, and much of that is nerve-related," according to Dr. Shai Gozani, founder and CEO of NeuroMetrix, a medical device company. "There are 100 million people who live with chronic pain."
According to the Mayo Clinic, patients with nerve disorders experience functional difficulties, which result in conditions such as:
- Epilepsy, in which abnormal electrical discharges from brain cells cause seizures
- Parkinson's disease, which is a progressive nerve disease that affects movement
- Multiple sclerosis (MS), in which the protective lining of the nerves is attacked by the body's immune system
- Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a motor neuron disease which weakens the muscles and progressively hampers physical function
- Huntington's disease, which is an inherited condition that cause the nerve cells in the brain to degenerate
- Alzheimer's disease, which covers a wide range of disorders that impacts mental functions, particularly memory.
Mayo Clinic also noted that the nervous system can also be affected by vascular disorders such as:
- Stroke, which occurs when there is bleeding on the brain or the blow flow to the brain is obstructed;
- Transient ischemic attack (TIA), which are mini-type strokes that last a shorter period of time but mimic stroke symptoms; and
- Subarachnoid hemorrhage, which is specifically bleeding in the space between your brain and the surrounding membrane that can be the result of a trauma or rupturing of a weak blood vessel;
Infections such as meningitis, encephalitis, polio, and epidural abscess can also affect the nervous system, the NIH noted.
Treatments vary from anti-inflammatory medications and pain medications such as opiates, to implanted nerve stimulators and wearable devices, Gozani said. "Many people also turn to herbal and holistic methods to reduce pain, such as acupuncture."
Study of the nervous system
The branch of medicine that studies and treats the nervous system is called neurology, and doctors who practice in this field of medicine are called neurologists. Once they have completed medical training, neurologists complete additional training for their specialty and are certified by the American Board of Psychiatry and Neurology (ABPN).
There are also physiatrists, who are physicians who work to rehabilitate patients who have experienced disease or injury to their nervous systems that impact their ability to function, according to the ABPN.
Neurosurgeons perform surgeries involving the nervous system and are certified by the American Association of Neurological Surgeons.
Editor’s Note: If you’d like more information on this topic, we recommend the following book:
You use your eyes to see, your ears to hear and your muscles to do the heavy lifting. Well, sort of. In fact, most body parts are far more complicated than that, while some seem to have no business being inside there at all.
Ready for Med School? Test Your Body Smarts
Additional reporting by Alina Bradford, Live Science contributor.
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