Nervous System Divisions
Nervous System is an organized network of nerve tissue in the body. It includes the central nervous system, the peripheral nervous system, and other nerve tissue.
Acting together, these divisions transmit signals between different parts of the body to coordinate the body’s voluntary and involuntary actions.
When a part of the nervous system is not functioning properly, which is usually caused by illness, injury, age, genetics, or other reasons, a person may experience difficulty with speaking, learning, swallowing, breathing, moving, memory, senses, mood, or other physical or psychological functions.
Central Nervous System
Includes the brain and the spinal cord.
Peripheral Nervous System
- Consists of nerves and neurons that are located outside of the brain and spinal cord.
- Connects the central nervous system to the limbs and organs, and relays messages back and forth from the brain and the extremities.
Subdivisions of the Central Nervous System
The body’s main control center, the brain is the major organ of the central nervous system and is located inside the skull.
The brain controls all the functions of the body, such as speaking, learning, behavior, swallowing, breathing, moving, memory, abstract thought, language usage, senses, mood, and much more.
A soft bundle of nerves that are attached to the base of the brain and go down to the lower back through the spinal canal. The spinal canal is protected by the bones of the spine, which are known as vertebrae.
Signals between the brain and the nerve roots travel up and down the spinal cord to enable the brain and body to communicate. The vertebrae have disks that provide cushioning and flexibility to the spine and spinal cord.
Subdivisions of the Peripheral Nervous System
Autonomic Nervous System
Also known as the visceral nervous system or involuntary nervous system. A division of the peripheral nervous system that influences the function of internal organs.
Its functions are primarily unconscious, yet some do work along with the conscious mind. The unconscious autonomic nervous system controls such functions as heart rate, digestion, respiration rate, salivation, perspiration, the diameter of the pupils, discharge of urine, sexual arousal, and reflex actions such as coughing, sneezing, swallowing, and vomiting.
Breathing is an example whereby the conscious and the unconscious autonomic nervous system may work together. The autonomic nervous system has two divisions:
Sympathetic Nervous System
It is responsible for stimulating activities such as causing the body to become active and responsive, especially as it relates to the fight or flight response.
Parasympathetic Nervous System
It is responsible for rest activities and controls the body’s rest and digest or feed and breed response. This response is activated when the body is at rest, especially after eating, digestion, urination, and defecation.
A large bundle of nerve fibres connect the left and right cerebral hemispheres of the brain.
The forward part of the brain, and also the largest part, is the central location for memory, thought, and reason, as well as the central location for hearing and sight.
Located below the cerebrum, the cerebellum is responsible for controlling muscular coordination, equilibrium, and balance.
The part of the brain is the enlarged upper end of the spinal cord. It is responsible for controlling breathing, circulation, sneezing, swallowing, and other involuntary activities.
Brain Weight and Energy Requirements
The energy requirements of the brain are very high. The adult brain weighs approximately 3.3 pounds, which is about 2.7 percent of our total body weight.
About 20 percent of the heart’s output (at rest) is necessary to keep the brain functioning. The brain requires more oxygen for its tissue cells than any other tissue cells of the body. In fact, the oxygen needs are so great that if there is no circulation to the brain for only ten seconds, a person will become unconscious.
Structure of a Typical Neuron
- Nerve Tissue
- Nodes of Ranvier
- Schwann Cells
Although the nervous system is very complex, there are only two main types of cells in nerve tissue:
- The actual nerve cell is the neuron. It is the “conducting” cell that transmits impulses and the structural unit of the nervous system.
- The other type of cell is neuroglia or glial cell.
The word neuroglia means “nerve glue.” These cells are nonconductive and provide a support system for the neurons. They are a special type of connective tissue for the nervous system.
Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic.
Amitotic means that if a neuron is destroyed, it cannot be replaced because neurons do not go through mitosis (the part of the cell cycle in which chromosomes in a nucleus are separated into two identical sets of chromosomes, and each set ends up in its own nucleus.)
Each neuron has three basic parts:
- One or more dendrites
- A single axon
The cell body.
The branched projections of a neuron that act to propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron, from which the dendrites project.
A long, slender projection of a nerve cell, or neuron, that typically conducts electrical impulses away from the neuron’s cell body. The function of the axon is to transmit information to different neurons, muscles, and glands.
There are two types of axons occurring in the peripheral nervous system and the central nervous system: unmyelinated and myelinated axons.
Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system.
Nodes of Ranvier
Gaps in the myelin sheath at evenly spaced intervals along myelinated nerve fibers. Myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction.
Demyelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis.
A type of neuroglia, the main functions of which are to provide support and insulation to axons in the central nervous system of some vertebrates. Oligodendrocytes do this by creating the myelin sheath. Equivalent to the function performed by Schwann cells in the peripheral nervous system.
They are the principal glia of the peripheral nervous system. Glial cells function to support neurons.
Brain Wave and Electrical Activity Testing
An EEG test is performed to detect abnormal brain waves. It also detects the electrical activity of the brain. This test requires the use of electrodes that are pasted onto the scalp.
These electrodes are small metal discs with thin wires. EEG results are printed out as a graph on a computer, as well as amplified and recorded for evaluation by a doctor.
Anatomy of the Spinal Cord
- Vertebral Column
- Intervertebral Disc
- Facet Joints
- Vertebral Arch
- Intervertebral Foramen (Neural Foramen)
- Ligamentum Flavum
- Cauda Equina
A major part of the central nervous system that extends from the base of the brain down to the lower back and is encased by the vertebral column.
Consists of nerve cells and bundles of nerves that are covered by three thin layers of protective tissue called membranes.
The cord connects the brain to all parts of the body via thirty-one pairs of nerves that branch out from the cord and leave the spine between vertebrae (backbones).
All of the bones, muscles, tendons and other tissues reach from the base of the skull to the tailbone. The spine encloses the spinal cord and the fluid surrounding the spinal cord. Also called the backbone, spinal column, or vertebral column.
Vertebral Column protects the spinal cord from injury and encloses the spinal cord and the fluid surrounding it.
The pads of cartilage are filled with a gel-like substance that lies between vertebrae and acts as shock absorbers.
Connect the vertebrae to each other and permit backward motion.
The elastic bands of tissue that support the spine by preventing the vertebrae from slipping out of line as the spine moves.
These are the narrow stem-like structures on the vertebrae that form the walls of the front part of the vertebral arch.
This is a thin membrane that produces fluid to lubricate the facet joints and allow them to move easily.
A circle of bone around the spinal canal through which the spinal cord passes. Composed of a floor at the back of the vertebra, walls (the pedicles), and a ceiling where two laminae join.
Intervertebral Foramen (or Neural Foramen)
An opening between the vertebrae through which nerves leave the spine and extend to other parts of the body.
A part of the vertebra at the back portion of the vertebral arch forms the roof of the canal through which the spinal cord and nerve roots pass.
A large ligament that is often involved in spinal stenosis. It runs as a continuous band from lamina to lamina in the spine.
A sack of nerve roots that continues from the lumbar region, where the spinal cord ends and continues down to provide a neurologic function to the lower part of the body. It resembles a horse’s tail.
Eyes and Nervous System
The eyes are a component of the central nervous system. The images seen through the eyes are transmitted by the nervous system to the brain.
The eye is designed to focus an image onto the retina, which is located at the back of the eye, and the nerve that carries the image from the retina to the brain is the optic nerve.
Components of Eye Anatomy
- Optic Nerve
- Anterior Chamber
- Aqueous Fluid (Aqueous Humor)
- Blind Spot
- Central Retinal Artery
- Central Retinal Vein
- Ciliary Muscles
- Ciliary Processes
- Cones (Cone Cells)
- Lacrimal Gland
- Optic Cup
- Optic Disc (Optic Nerve Head)
- Retinal Pigment Epithelium
- Rods (Rod Cells)
- Schlemm’s Canal
- Trabecular Meshwork
- Uvea (Uveal Tract)
- Posterior Chamber
- Vitreous Humor
The outer, transparent, dome-like structure covers the iris, pupil, and anterior chamber. It is part of the eye’s focusing system.
The coloured ring of tissue is suspended behind the cornea and immediately in front of the lens. It regulates the amount of light entering the eye by adjusting the size of the pupil.
The transparent, double-convex structure is suspended between the aqueous humour and vitreous humour. It helps to focus light on the retina.
Pupil is the adjustable opening at the centre of the iris that allows varying amounts of light to enter the eye.
The light-sensitive layer of tissue that lines the back of the eyeball. It sends visual messages through the optic nerve to the brain
Macula is the small, sensitive area of the central retina that provides a vision for fine work and reading.
Optic nerve is the bundle of over one million nerve fibres that carry visual messages from the retina to the brain.
The space in front of the iris and behind the cornea.
Aqueous Fluid (Aqueous Humor)
The clear, watery fluid that flows between and nourishes the lens and the cornea. It is secreted by the ciliary processes.
A small area of the retina where the optic nerve enters the eye. It occurs normally in all eyes.
Central Retinal Artery
Central retinal artery is the blood vessel that carries blood into the eye and supplies nutrition to the retina.
Central Retinal Vein
The blood vessel that carries blood from the retina.
The layer is filled with blood vessels that nourish the retina. It is part of the uvea.
Ciliary muscles are the muscles that relax the zonules to enable the lens to change shape for focusing.
The extensions or projections of the ciliary body that secrete aqueous humor.
Cones (Cone Cells)
One type of specialized light-sensitive cells (photoreceptors) in the retina that provide sharp central vision and color vision.
The thin, moist tissue (membrane) that lines the inner surfaces of the eyelids and the outer surface of the sclera.
Tiny yellow or white deposits in the retina or optic nerve head.
The central part of the macula that provides the sharpest vision.
The interior lining of the eyeball, including the retina, optic disc, and macula. It is the portion of the inner eye that can be seen during an eye examination by looking through the pupil.
The small almond-shaped structure that produces tears and is located just above the outer corner of the eye.
The white, cup-like area in the center of the optic disc.
Optic Disc (Optic Nerve Head)
The circular area (disc) where the optic nerve connects to the retina.
Retinal Pigment Epithelium (RPE)
The pigment cell layer that nourishes the retinal cells. It is located just outside the retina and attached to the choroid.
Rods (Rod Cells)
One type of specialized light-sensitive cells (photoreceptors) in the retina that provide side vision and the ability to see objects in dim light (night vision).
The passageway for the aqueous fluid to leave the eye.
The tough, white, outer layer (coat) of the eyeball. Along with the cornea, it protects the entire eyeball.
The spongy, mesh-like tissue near the front of the eye that allows the aqueous fluid (humor) to flow to Schlemm’s canal then out of the eye through ocular veins.
Uvea (Uveal Tract)
The middle coat of the eyeball, consisting of the choroid in the back of the eye and the ciliary body and iris in the front of the eye.
The space between the back of the iris and the front face of the vitreous. It is filled with aqueous fluid.
The transparent, colorless mass of gel that lies behind the lens and in front of the retina and fills the center of the eyeball.
The fibers that hold the lens suspended in position and enable it to change shape during accommodation.
The middle, thickest layer of tissue in the cornea.
The inner layer of cells on the inside surface of the cornea.
The outermost layer of cells of the cornea and the eye’s first defense against infection.
- Binocular Vision
- Contrast Sensitivity
- Intraocular Pressure (IOP)
- Peripheral Vision
- Visual Acuity
- Visual Field
- Refractive Power
The ability of the eye to change its focus from distant to near objects. This process is achieved by the lens changing its shape.
The blending of the separate images seen by each eye into a single image. It allows images to be seen with depth.
The ability to perceive differences between an object and its background.
Intraocular Pressure (IOP)
- Pressure of the fluid inside the eye.
- Normal IOP varies among individuals.
The ability to see objects and movement outside of the direct line of vision. Also called side vision.
The ability to distinguish details and shapes of objects. It is also called central vision.
The entire area that can be seen when the eye is forward, including peripheral vision.
The clearness or sharpness of vision.
The ability of the eye to bend light as light passes through it.
Nervous System Injuries and Conditions
- Grand Mal Seizure
- Tonic Phase
- Clonic Phase
- Postictal Phase
The sudden occurrence of abnormal electrical activity in the brain. A seizure can affect sensation, behavior, movement, or consciousness.
In a mild seizure, the victim may experience tingling, twitching, hallucinations, familiarity (déjà vu), or fear. In a more severe seizure, the victim may experience unconsciousness.
Seizures can result from many causes that affect brain function, including:
- Neurological conditions
- Medical conditions such as an infection, metabolic abnormalities, alcohol withdrawal, poison, and high fever
- Decreased oxygen supply to the brain
- Head injuries
- Brain irritation
- Brain hemorrhage
A disorder where the victim experiences repeated and unpredictable seizures. Unlike most other seizures or convulsions that can be traced to a particular cause, the cause of epileptic seizures is unknown.
Grand Mal Seizure
A type of seizure where the victim may experience a warning sensation, and then possibly cry out, and then collapse into unconsciousness and experience violent muscle contractions. A grand mal seizure has four phases:
The victim experiences sensations of taste, smell, or sound that are unusual and that serve to alert him that he is about to have a seizure.
2. Tonic Phase
The victim loses consciousness and holds his breath, which causes him to appear cyanotic (having a bluish coloration of the skin). Also the victim’s unconscious body becomes rigid with arms and legs fully extended.
3. Clonic Phase
The victim experiences alternating muscle contractions and muscle relaxation, which cause the body to jerk. Occasionally the victim may urinate or defecate.
4. Postictal Phase
The victim becomes comatose and the body goes limp. Eventually the victim returns to consciousness and experiences a sense of confusion, along with fatigue and headache.
Health Conditions of the Nervous System
- Spinal Stenosis
- Herniated Disk
Spinal stenosis is a narrowing of spaces in the spine that results in pressure on the spinal cord and/or nerve roots. This disorder usually involves the narrowing of one or more of the following three areas of the spine:
- The canal in the center of the vertebral column through which the spinal cord and nerve roots run
- The canals at the base or roots of nerves branching out from the spinal cord
- The openings between the vertebrae through which nerves leave the spine and go to other parts of the body
This narrowing may:
- Affect either a small or large area of the spine.
- Cause pressure to the lower part of the spinal cord, or to the nerve roots that branch out from that area, and possibly cause pain or numbness to the legs.
- Cause pressure to the upper part of the spinal cord, and possibly cause pain to the neck, shoulders, or even the legs.
- Include an array of symptoms that are not listed here.
- Spinal stenosis usually results from a gradual, degenerative aging process where either structural changes or inflammation can begin the process.
A painful condition that results when a disk located between two vertebrae of the spine bulges backward, usually compressing a nerve root and interfering with the function of that nerve.
Characterized by a pain that radiates down the sciatic nerve from a person’s back into the buttocks and down the legs to the feet.
Any disease that affects the spinal nerve roots. Symptoms include pain, numbness, tingling, and weakness. One possible cause of radiculopathy is a herniated disk. One possible symptom of radiculopathy is sciatica.
It is a disorder of bone growth that results in the most common type of dwarfism. It occurs in one in every 15,000 to one in 40,000 live births. Most cases appear as spontaneous mutations. This means that 2 parents without achondroplasia may give birth to a baby with the condition.
It can also be inherited. If a child gets the defective gene from 1 parent, the child will have the disorder. If 1 parent has achondroplasia, the infant has about a 50% chance of inheriting the disorder. If both parents have the condition, the infant’s chances of being affected increase to about 75%.
When one vertebra slips forward on another, which may result from a degenerative condition, or an accident, or, very rarely, may be acquired at birth.
When poor alignment of the spinal column causes a vertebra to slip forward onto the one below, it can place pressure on the spinal cord or nerve roots at that point.
Arthritis Types That May Affect the Spine
- Rheumatoid Arthritis
- Spinal Stenosis
If the degenerative process of osteoarthritis affects the facet joints and disks of the spine, the condition is sometimes referred to as spondylosis.
This condition may be accompanied by disk degeneration and an enlargement or overgrowth of bone that narrows the central and nerve root canals.
The portions of the vertebral column with the greatest mobility (e.g., the neck area) are often the areas most affected in people with rheumatoid arthritis.
Rheumatoid arthritis is associated with inflammation and enlargement of the soft tissues (synovium) of the joints.
Inflammation of the synovial membrane, which is the specialized connective tissue that lines the inner surface of capsules of synovial joints and tendon sheath.
Although rheumatoid arthritis is not a common cause of spinal steno-sis, the damage that results to the ligaments, bones, and joints begins as synovitis.
Acquired Causes of Spinal Conditions
- Tumors of the Spine
- Paget’s Disease
- Ossification of the Posterior Longitudinal Ligament
- Lumbar Radiculopathy
- Cervical Radiculopathy
- Thoracic Radiculopathy
Acquired conditions that cause spinal stenosis are the following:
Tumors of the Spine
Abnormal growths of soft tissue that may affect the spinal canal directly by inflammation or by growth of tissue into the canal. Tissue growth may lead to bone resorption (bone loss due to over activity of certain bone cells) or displacement of bone.
Accidents may either dislocate the spine and the spinal canal or cause burst fractures that produce fragments of bone that penetrate the canal.
A chronic disorder that typically results in enlarged and abnormal bones. Excessive bone breakdown and formation cause thick and fragile bone. As a result, bone pain, arthritis, noticeable bone structure changes, and fractures can occur.
The disease can affect any bone of the body, but is often found in the spine. The blood supply that feeds healthy nerve tissue may be diverted to the area of involved bone.
Also, structural problems of the involved vertebrae can cause narrowing of the spinal canal, producing a variety of neurological symptoms. Other developmental conditions may also result in spinal stenosis.
Ossification of the Posterior Longitudinal Ligament
A condition that occurs 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.
Ossification means “forming bone.” These deposits may press on the nerves in the spinal canal.
A pinched nerve in the spine resulting in discomfort and physical limitations. Caused by any disease that affects the spinal nerve roots.
When there is compression of the spinal nerve roots, the symptoms include pain, numbness, tingling, and weakness. One of several possible causes of radiculopathy is a herniated disk.
One of several possible symptoms of radiculopathy is sciatica, which is a condition that causes pain to radiate down from a person’s back, to the buttocks, and then down the legs and into the feet.
Radiculopathy is usually reversible with timely medical treatment. The types of radiculopathy specifically related to the areas of the spine where the nerves are compressed are the following:
Pressure on the nerve root in the lower back. Can cause sciatica or intense pain in the legs. Other symptoms include sexual dysfunction and incontinence. In more severe cases, it can cause paralysis.
Pressure on the nerve root in the neck. It can cause painful burning or tingling in the neck, shoulders, and arms.
Pinched nerves in the center area of the spine. This creates pain in the chest and torso area. Occasionally the symptoms can be misdiagnosed as shingles.
- Blind Spot
- Legal Blindness
- Low Vision
- Dry Eye Syndrome
- Ghost Image
- Eye Inflammation
- Refractive Errors
A clouding of the lens in the eye that affects vision, most cataracts are related to aging. Cataracts are very common in older people. By age eighty, more than half of all Americans either have a cataract or have had cataract surgery.
A cataract can occur in one eye or both eyes, but it cannot spread from one eye to the other.
When the surface of the cornea is not spherical, which causes a blurred image to be received at the retina.
A disorder characterized by an irregular corneal surface (cone shaped) resulting in blurred and distorted images.
Any gap in the visual field corresponding to an area of the retina where no visual cells are present. It is associated with eye disease.
When visual acuity in the better eye with corrective lenses is 20/200 or worse, or when visual field is limited to a twenty-degree diameter (tunnel vision) or less in the better eye.
A visual acuity of 20/200 requires that a person must be twenty feet or closer from an eye chart to see what a person with normal vision can see at two hundred feet.
Note: These criteria are used to determine eligibility for government disability benefits as of this writing, and do not necessarily indicate a person’s ability to function. Moreover, changes for government disability may occur at a later date.
Visual loss that cannot be corrected with eyeglasses or contact lenses and interferes with daily living activities.
The condition otherwise known as farsightedness. It is the ability to see distant objects more clearly than close objects. May be corrected with glasses or contact lenses.
The condition otherwise known as nearsightedness, myopia is the ability to see close objects more clearly than distant objects. May be corrected with glasses or contact lenses.
The gradual loss of the eye’s ability to change focus (accommodation) for seeing near objects caused by the lens becoming less elastic. Associated with aging and occurs in almost all people over age forty-five.
Dry Eye Syndrome
A common condition that occurs when the eyes do not produce enough tears to keep the eye moist and comfortable. Common symptoms of dry eye include pain, stinging, burning, scratchiness, and intermittent blurring of vision.
A fainter second image of an object being viewed.
Scatter from bright light that decreases vision.
Rings around lights due to optical imperfections within or in front of the eye.
The corneal clouding that causes the sensation of looking through smoke or fog.
The body’s reaction to eye trauma, infection, or entrance of a foreign substance. Often associated with pain, heat, redness, swelling, and/or loss of eye function.
Inflammation of the cornea.
Imperfections in the focusing power of the eye (e.g., hyperopia, myopia, and astigmatism).
- Fluorescein Angiography
- Snellen Visual Acuity Chart
To temporarily enlarge the pupil with special eye drops (mydriatic) to allow the eye care specialist to better view the inside of the eye.
A test to examine blood vessels in the retina, choroid, and iris. A special dye is injected into a vein in the arm and pictures are taken as the dye passes through blood vessels in the eye.
A test to determine the best eyeglasses or contact lenses to correct a refractive error (e.g., myopia, hyperopia, or astigmatism).
The standard to determine the fluid pressure inside the eye (intraocular pressure).
The measure of the total refractive errors of the eye, including nearsightedness, farsightedness, astigmatism, and other refractive errors that cannot be corrected with glasses or contacts.
The measurement of refractive error. A negative diopter value signifies an eye with myopia, and a positive diopter value signifies an eye with hyperopia.
Snellen Visual Acuity Chart
One of many charts used to measure vision.
Eye Surgery and Treatments
- Ablation Zone
- Laser Keratome
- All-Laser LASIK (Bladeless LASIK)
- Excimer Laser
- Photo-Refractive Keratectomy
- Radial Keratotomy
A surgical term that means to remove.
The area of tissue that is removed during laser surgery.
A laser device used to create a corneal flap.
All-Laser LASIK (Bladeless LASIK)
A procedure where a laser keratome device is used to cut a corneal flap for LASIK surgery.
An ultraviolet laser used in refractive surgery to remove corneal tissue.
The surgical removal of corneal tissue.
A surgical incision of the cornea.
The carving of the cornea to reshape it.
The acronym for light amplification by stimulated emission of radiation. A laser is an instrument that produces a powerful beam of light that can vaporize tissue.
The acronym for laser assisted in-situ keratomileusis, which refers to creating a flap in the cornea with a microkeratome and using a laser to reshape the underlying cornea.
The purposeful adjustment of one eye for near vision and the other eye for distance vision.
A mechanical surgical device that is affixed to the eye by use of a vacuum ring. When secured, a very sharp blade cuts a layer of the cornea at a predetermined depth.
A complication of refractive surgery where the achieved amount of correction is more than desired.
A complication of refractive surgery where the achieved amount of correction is less than desired.
Photo-Refractive Keratectomy (PRK)
A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping, and the use of a computer-controlled excimer laser to reshape the stroma.
Radial Keratotomy (RK)
A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea using radial cuts.