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149 Cards in this Set

  • Front
  • Back
neurons
nerve cells
central nervous system.
the brain and spinal cord
Central Nervous System (CNS)
Brain, Spinal Cord, Autonomic Nervous System (ANS)
Peripheral Nervous System (PNS)
Cranial Nerves, Spinal Nerves, Autonomic Nervous System (ANS). onsists of nerves that branch out from the central nervous system and connect it to other body parts. made up of two sets of nerves: The cranial nerves, & the spinal nerves.
conus medullaris
a cone-shaped ending
filum terminale
a band of connective tissue
cauda equina
Numerous nerves branch out from the conus medullaris, they look like a horse's tail (because there are so many).
autonomic nervous system (ANS)
a subdivision of both systems based upon function. The autonomic nervous system has nerves throughout the smooth muscles of the organs, blood vessels, cardiac muscles, and glands of the body. t functions without conscious control.
glial cells
supportive cells that form a connective tissue component in the nervous system
afferent
sensory neurons
efferent
motor neurons
myelin
a fatty white material
Dendrites
look like tree branches and function as receptors for signals and conduct electrical signals towards the nerve cell body
Axons
(nerve fibers) thin, chain-like extensions with a uniform appearance. They are the impulse generators and conductors that transmit nerve impulses away from the nerve cell body.
Cranial nerve 1: olfactory nerve
The sense of smell. There are numerous olfactory nerves that relay sensory impulses of smell from the membranes of the nasal cavity. Damage to these nerves results in a loss of smell.
Cranial nerve 2: optic nerve
Vision. This is a sensory-only nerve that conducts impulses from the retina of the eye. It originates in the back of the eyeball, and damage to these nerves results in blindness.
Cranial nerve 3: oculomotor nerve
This nerve controls all movement of the eyeballs. It is primarily a motor nerve. It comes out of the midbrain and serves four of the six eye muscles, as well as the muscle of the eyelid, the iris (which causes pupil dilation), and the ciliary body. Damage to these nerves can cause a droopy eyelid, constant dilated pupil, or the inability to rotate the eyeball.
Cranial nerve 4: trochlear nerve
This is a very small nerve, both sensory and motor, that innervates the superior oblique muscle of the eye.
Cranial nerve 5: trigeminal nerve
This is the largest of the cranial nerves. It is primarily sensory, but does send motor impulses to the muscles of mastication (chewing).
Cranial nerve 6: abducens nerve
This is a small nerve that provides both motor and sensory innervation to the lateral rectus eye muscle. Damage to this nerve causes the eyeball to be pulled medially, and makes it unable to move laterally at all.
Cranial nerve 7: facial nerve
This is a mixed nerve. The motor fibers innervate facial, scalp, and superficial neck muscles. The salivary glands are also innervated by this. The sensory fibers arise from the taste buds on the tongue. Damage to these nerves distorts taste perception, particularly with sweets, and causes the face to sag.
Cranial nerve 8: vestibulocochlear nerve
Hearing and equilibrium. This is also called the acoustic nerve. It is a sensory nerve that arises within the inner ear. The vestibular branch controls equilibrium and balance, while the cochlear portion controls hearing. Damage to these nerves can cause deafness or dizziness and inability to maintain balance.
Cranial nerve 9: glossopharyngeal nerve
This is a mixed nerve that innervates the tongue and pharynx. The sensory portion transmits impulses from the tonsils, salivary gland, and back third of the tongue. The motor fibers innervate the muscles of the pharynx that help with swallowing. Damage to these nerves would result in a loss of bitter and sour taste, or could cause difficulty swallowing
Cranial nerve 10: vagus nerve
This arises from the medulla and has both motor and sensory fibers that serve autonomic organs of the thoracic and abdominal cavities. It is associated with such functions as breathing, heartbeat, hunger pangs, and peristalsis. One of its branches supplies the larynx and controls speech. Damage to both of these nerves would result in death; damage to only one would affect speech and swallowing.
Cranial nerve 11: accessory nerve
This is a motor nerve only. It innervates the muscles of the pharyngeal region as well as the sternocleidomastoid and trapezius muscles. Damage to this nerve would make it difficult to rotate the head or shrug the shoulders.
Cranial nerve 12: hypoglossal nerve
This supplies motor fibers to the muscles of the tongue. Damage would cause difficulty speaking, swallowing, or sticking out the tongue.
nerves
a cord-like structure made up of a bundle of nerve fibers and neurons (nerve cells) running to various organs and tissues of the body
Neurology
the branch of medicine dealing with disorders of the nervous system
psychiatry
is that branch of medicine that deals with the study, treatment, and prevention of mental disorders
amnesia
Lack or loss of memory; the inability to remember past experiences.
aphasia
Defect or loss of the power of expression, especially speech, but also writing or signing, or of comprehending either spoken or written language due to an injury or disease of the brain.
apraxia
The loss of the ability to carry out familiar, purposeful movements in the absence of paralysis or another motor or sensory impairment.
asterixis
A coarse, slow, nonrhythmic movement, usually of the outstretched hands, but also other muscle groups when there is sustained contraction of them. This is also called "liver flap" because it is associated with hepatic comas; however, it is observed in other conditions as well.
ataxia
Failure of muscular coordination resulting in a reeling, wide-based gait (the manner or style of walking).
athetosis
A form of dyskinesia marked by ceaseless slow, sinuous, writhing movements, especially of the hands (picture a pianist just before he touches the keys, but slower) which is involuntary.
catatonia
A psychomotor disturbance usually (but not always) associated with schizophrenia. It can take on different forms and severity, including a decrease in reactivity to the environment, resistance to all instructions or attempts to be moved, maintenance of a rigid posture, excited, uncontrollable and purposeless motor activity, or the assumption of bizarre fixed postures.
causalgia
A burning pain due to injury of a peripheral nerve.
cephalalgia
A headache. There are many different types of headaches which occur for many different reasons. A few of these are stress headaches, cluster headaches, and one of the most severe, migraine headaches.
chorea
The ceaseless occurrence of a wide variety of rapid, highly complex, jerky, dyskinetic movements that appear to be well coordinated, but are performed involuntarily. There are several specific types of this.
concussion
A violent jar or shock to the brain or the condition which results from such an injury.
convulsion
A violent, involuntary contraction or series of contractions of the usually voluntary muscles. Sometimes used interchangeably with seizure.
delirium
An acute mental syndrome characterized by a reduced ability to maintain attention to external stimuli, disorganized thinking, rambling and incoherent speech, a reduced level of consciousness, incorrect sensory perceptions, disorientation to time, place, or person, and memory impairment. This is a reversible condition
delusion
A false belief that is firmly maintained despite irrefutable evidence to the contrary and in spite of members of the culture not sharing the belief. For example, an individual experiencing delusions might believe that he or she is a famous person.
dementia
An organic mental syndrome characterized by a loss of intellectual abilities, including impairment of memory, judgment, and abstract thinking, as well as changes in personality. This does not include functional lapses due to depression or clouding of consciousness as in delirium. It is caused by a large number of conditions, some reversible and some progressive, which cause widespread cerebral damage. The most common cause is Alzheimer disease.
dissociation
A process whereby specific internal mental contents, such as memories, ideas, feelings, and perceptions, are lost to conscious awareness and become unavailable to voluntary recall. This is a defense mechanism where such mental processes are separated from the rest of a person's mental activity in order to avoid emotional distress.
dysarthria
Slurring and inappropriate phrasing during speech, as well as the inability to control speech volume due to disturbances of muscular control. This is a result of central or peripheral nerve damage.
dysdiadochokinesia
The inability to perform rapid alternating movements.
dysmetria
Inability to control muscular range of motion.
dysphasia
Impairment of speech, specifically lacking coordination and ability to arrange words in their proper order.
dysphoria
Disquiet, restlessness, malaise.
fasciculations
Brief, small, irregular twitches of muscle visible through the skin, caused by a single motor filament.
hallucination
A sense perception without a source in the external world. The perception of an object or sound when no such object or sound exists. These can involve all of the senses, but the most common are audio and visual hallucinations.
hypersomnolence
Sleep disorders consisting of the need for excessive amounts of sleep or causing extreme drowsiness when awake. This can be either psychogenic in origin, have an organic cause involving the nervous system, or be related to medication or drug use.
hypesthesia
Abnormally decreased sensitivity, particularly to touch.Abnormally decreased sensitivity, particularly to touch.
hypotonia
Diminished tone of the skeletal muscles.
ideation
The formation of a mental concept or image.
suicidal ideation
Formulating thoughts about harming or killing oneself.
homicidal ideation
Formulating thoughts about harming or killing someone else.
insomnia
The inability to sleep.
malaise
A vague feeling of bodily discomfort and fatigue.
myoclonus
A brief, lightning-like contraction of a muscle, a portion of a muscle, or a group of muscles. This occurs in normal healthy people as they fall asleep. A hiccup is a diaphragmatic myoclonus.
paralysis
Loss or impairment of motor function or sensation in a body part.
paranoia
The term used to describe behavior characterized by systematic delusions of persecution, delusions of grandeur, or a combination of the two.
paraphilia
In general this is a gross impairment in the capacity for sexual activity between adult human partners. There are different types of paraphilia, and they are much more predominant in males. The major paraphilias include: fetishism, transvestism, pedophilia, exhibitionism, voyeurism, masochism, and sadism.
fetishism
The use of physical objects as the preferred method of producing sexual excitement (such as shoes).
transvestism
The dressing by heterosexual males in female clothing.
pedophilia
A preference for sexual activity with prepubescent children.
exhibitionism
Repetitive acts of exposing the genitals to an unsuspecting stranger for sexual excitation.
voyeurism
Looking at unsuspecting people who are naked, undressing, or engaging in sexual activity for the purpose of sexual excitation.
masochism
The intentional participation in an activity in which the individual is physically harmed or the individual's life is threatened in order to produce sexual excitement. Examples would be humiliation, bondage, or whipping.
sadism
The inflicting of physical or psychologic harm or suffering on a sexual partner as a method of stimulating sexual excitement or orgasm.
paresis
Slight or incomplete paralysis.
paresthesia
An abnormal touch sensation such as burning, pricking, or feeling that something is crawling over your skin when no external stimulus is present.
Phalen maneuver
A test in a physical examination to determine the presence of carpal tunnel syndrome.
proprioception
A sense or perception, usually at a subconscious level, of the movements and position of the body and especially its limbs, independent of vision; this sense is gained primarily from input from sensory nerve terminals in muscles and tendons (muscle spindles) and the fibrous capsule of joints combined with input from the vestibular apparatus.
Romberg test
a test performed by having the patient stand with feet together or walk with his/her eyes closed to determine the presence of clumsiness in movements or gait.
scotoma
An area of lost or depressed vision within the visual field, generally surrounded by normal vision. This is also used in psychiatry as mental scotoma or a figurative blind spot, wherein the patient has no insight into his or her problems.
seizure
Literally this means a sudden attack or recurrence of a disease. It is often used interchangeably with the term convulsion. There are different types of seizures.
tonic-clonic
Characterized by loss of consciousness and alternate muscular contraction and relaxation in rapid succession. The contraction and relaxation is called clonus; tonic is the restoration of normal muscle tone. (Also called a grand mal seizure.)
grand mal
Also known as a tonic-clonic seizure (pronounced like "mall").
absence seizure
Brief generalized seizures, manifested by a 10–30 second loss of consciousness with eye or muscle flutterings either with or without the loss of muscle tone, but without clonic activity. (Also called a petit mal seizure.)
petit mal
Also known as absence seizures.
somnolence
Sleepiness.Sleepiness.
stupor
A lower level of consciousness. The patient responds only to vigorous stimulation. In psychiatry it is used to describe a disorder marked by reduced responsiveness.
tic
An involuntary, compulsive, repetitive movement, usually involving the face or shoulders. This is not to be confused with a tick, which is a blood-sucking parasite. (Know anyone like that?)
Tinel sign
A tingling sensation in the distal end of a limb when percussion is made over the site of a nerve. This is used to determine the presence of carpal tunnel syndrome.
tremor
An involuntary trembling or quivering.
vertigo
An illusory sense that either the environment or one's own body is revolving. It is mistakenly used synonymously with dizziness.
Alzheimer disease
A progressive degenerative disease of the brain of unknown etiology, with atrophy of the cerebral cortex including plaques and tangles of neuron particles. It is manifested by memory loss, personality changes and eventually dementia. It occurs twice as often in women as in men.
Arnold-Chiari malformation
A congenital deformity that occurs when portions of the medulla oblongata and cerebellum protrude into the spinal canal.
arteriovenous malformation
This is a congenital deformity of tangled, dilated blood vessels where arterial contents flow directly into veins. This produces a consistently enlarging vasculature, which causes neurologic abnormalities by compressing neural tissue, the spinal cord, or interrupting blood supply to the brain.
astrocytoma
A tumor composed of neuroglial cells. It is the most common type of primary brain tumor and is also found throughout the central nervous system. There are different classifications and grades of these tumors.
Bell palsy
A functional disorder of cranial nerve VII resulting in paralysis of one side of the facial muscles and distortion of taste. This is usually due to a lesion on the nerve.
bipolar disorder
A mood/personality disorder where both depressive and excited/elated periods occur. It usually begins with depression, and at least one time during the illness there is an elated period.
carpal tunnel syndrome
A disorder caused by compression of the median nerve in the carpal tunnel; it is characterized by pain and burning or tingling paresthesias in the fingers and hand, sometimes radiating up the arm.
cerebral palsy
Often used synonymously with a stroke. CVA is a nonspecific term for ischemic (cutting off the blood supply) or hemorrhagic (bleeding) lesions that affect the brain. Technically, a stroke is an ischemic-type lesion.
dysthymia
A mood disorder characterized by depressed feelings (being sad, feeling down in the dumps) with a loss of interest or pleasure from one's usual activities, which persists for two or more years. This differs from major depression because the symptoms typically last longer (sometimes ongoing for many, many years) and are not as severe as they are in depression.
cerebrovascular accident (CVA)
Often used synonymously with a stroke. CVA is a nonspecific term for ischemic (cutting off the blood supply) or hemorrhagic (bleeding) lesions that affect the brain. Technically, a stroke is an ischemic-type lesion.
encephalitis
Inflammation of the brain.
encephalomyelitis
Inflammation of the brain and spinal cord.
encephalopathy
Any degenerative disease of the brain.
epilepsy
A recurrent paroxysmal disorder of cerebral function characterized by sudden brief attacks of altered consciousness, motor activity, sensory phenomena, or inappropriate behavior. One episode is generally called a seizure. Any recurrent seizure pattern may be called epilepsy.
glioma
A tumor composed of tissue which represents neuroglia. The term is sometimes used to describe any tumor of the brain or spinal cord.
Guillain-Barre syndrome
An acute, usually rapidly progressive form of inflammatory polyneuropathy (usually lesions affecting many peripheral nerves simultaneously) characterized by muscle weakness and mild sensory loss. This usually occurs between five days and three weeks after an infectious disorder, surgery, or an immunization.
herniated nucleus pulposus
A herniated, ruptured, or prolapsed intervertebral disc. Spinal vertebrae are separated by cartilage discs consisting of an outer anulus fibrosus and an inner nucleus pulposus. Degenerative changes can cause the nucleus to rupture or protrude through the anulus fibrosus, where it irritates the nerve root and causes back pain. (Note: annulus is an acceptable alternative spelling.)
hydrocephalus
A condition marked by dilatation of the cerebral ventricles, usually secondary to obstruction in the pathways for cerebrospinal fluid, with a subsequent accumulation of cerebrospinal fluid within the skull. This is typically characterized by an enlarged head, prominence of the forehead, brain atrophy, mental deterioration, and/or convulsions.
Lou Gehrig disease
(Also called amyotrophic lateral sclerosis or ALS). A motor neuron disease of unknown cause characterized by muscular weakness and atrophy, cramps, visible fasciculations, spasticity, hyperactive reflexes, dysarthria and dysphagia. This is more common in men than women. 50% of patients die within three years of onset; 20% live five years, and 10% live for ten years.
amyotrophic lateral sclerosis (ALS)
nother name for Lou Gehrig disease.
meningitis
Inflammation of the meninges.
meningocele
A hernia protrusion of the meninges through a bony defect.
microcephaly
Smallness of the head, usually associated with mental retardation.
multiple sclerosis
A disease in which there are foci of demyelination throughout the white matter of the central nervous system, sometimes extending into the gray matter. Symptoms of these lesions include weakness, incoordination, paresthesias, speech disturbances, and visual complaints. The disease is usually prolonged.
Munchausen syndrome
The repeated fabrication of illness, usually acute, dramatic, and very convincing, by a person who wanders from hospital to hospital for treatment. Patients are often able to mimic serious disorders with great skill. There is a variation of this illness called Munchausen by proxy wherein the affected person uses a child as the patient, falsifying history and even injuring the child or using drugs, adding blood or bacterial components to urine specimens, and other means to simulate a disease.
myasthenia gravis
A disease characterized by episodic muscle weakness, primarily in muscles innervated by the cranial nerves.
myelomeningocele
A hernia protrusion of the spinal cord and its meninges through a defect in the vertebral canal.
narcolepsy
Recurrent, uncontrollable, brief episodes of sleep. Also associated with sudden loss of muscle tone, sleep paralysis, or hypnogogic hallucinations.
neurofibromatosis
A familial condition characterized by developmental changes in the nervous system, muscles, bones, and skin with the formation of multiple soft tumors distributed over the entire body.
paraplegia
Paralysis of the legs and lower part of the body.
Parkinson disease
A slowly progressive and degenerative central nervous system disorder with four characteristic features: slowness and poverty of movement, muscular rigidity, resting tremor, and postural instability.
peripheral neuropathy
Any traumatic, toxic, inflammatory, or degenerative changes in the peripheral nerves. Results in sensory loss, muscle weakness and atrophy, decreased deep tendon reflexes, and vasomotor symptoms occurring either individually or in any combination. This can afflict a single nerve or many nerves simultaneously (polyneuropathy).
personality disorders
These are disordered patterns of behavior characterized by relatively fixed, inflexible, and stylistic reactions to stress. These disorders are rigid and not adaptive and they damage social, interpersonal, and work relationships. These disorders can be classified into several different types. Following is a list of some of the most common personality disorders.
antisocial
Also called psychopathic or sociopathic personality. Individuals with these disorders act out their conflicts and ignore normal rules of social behavior. They are impulsive, irresponsible, amoral, and unable to forego immediate gratification. They cannot form sustained relationships with others but use charm to get their way. They cannot tolerate frustration, and opposition will likely elicit hostility, aggression, or serious violence. They show little to no remorse or guilt and rationalize and blame others for their behavior.
avoidant
These individuals are extremely sensitive to rejection, and they fear beginning relationships but have a strong desire for affection and acceptance. They appear shy and timid and are distressed by their inability to relate to others.
borderline
People with this disorder are unstable in many areas, including self-image, mood, behavior, and relationships. They are characterized by frequent mood shifts, impulsivity, inappropriate and uncontrolled intense anger. They are often extremists—the world is either black or white, hated or loved, but never neutral.
cyclothymic
These individuals fluctuate in their moods between high-spiritedness and gloom and pessimism, each sustained for weeks or longer. Their mood changes are often rhythmic and predictable and can be set off by trivial causes or no external causes at all.
dependent
Individuals suffering from this disorder surrender responsibility for their lives to others and allow the needs of those they are dependent upon to take precedence over their own needs. They lack self-confidence and initiative, and they are extremely uncomfortable with being alone for more than brief periods of time.
histrionic (hysterical)
People suffering from this disorder seem very egocentric. Winning the esteem and admiration of others is important to them, so they have attention-seeking and theatrical behavior. Emotional immaturity is evident with exaggerated childish responses to any wounding of their vanity. Their behavior is inconsistent because they adopt whatever conduct will place them in a favorable light. In their relationships they have an insatiable need for affection and exhibit sexually aggressive behavior with an underlying wish for nonsexual affection.
narcissistic
This disorder is marked by an exaggerated sense of self importance and fantasies of unlimited success. Individuals suffering from this disorder also tend to be preoccupied with envy. They constantly seek attention, tend to exploit others, and are overly sensitive to failure or criticism. They also tend to have multiple somatic complaints.
obsessive-compulsive
People suffering from this are conscientious and have high aspirations but tend to be perfectionists and cannot gain satisfaction from their achievements. They are reliable, dependable, orderly, and methodical, but they are also completely inflexible and cannot adapt to a change in circumstances. They are cautious, have anxiety over responsibility (which they take very seriously), and pay attention to every detail. They have difficulty with interpersonal relationships because they lose total control over their feelings and must rely on others.
paranoid
Individuals project their own conflicts and hostilities onto others. They are markedly sensitive in relationships and tend to find hostile and malevolent intentions behind trivial and even kindly acts by others. Their suspicions often lead to aggressive feelings or behavior. They have a sense of superiority and often belittle others.
passive-aggressive
Those with this personality type are characterized by helplessness, clinging dependency, and procrastination. The passivity is designed to gain attention, to avoid responsibility, or to control others. The behavior is often obstinacy, inefficiency, and sullenness, often disguised under apparent compliance. They tend to provoke and engage in arguments, especially with those in authority.
schizoid
Individuals with this disorder are withdrawn, solitary, introverted, emotionally cold, and distant. They are absorbed in their own thoughts and feelings and greatly fear intimacy of any kind with others. They tend to daydream and prefer speculation to action. Fantasy is a common coping mechanism.
post-traumatic stress disorder (PTSD)
A neurotic disorder which occurs following exposure to an overwhelming environmental stress (very common in war veterans, for example). Symptoms include consistently reliving the experience, a numbing emotional responsiveness, and a general dysphoria. (This is also acceptably presented as posttraumatic.)
psychosis
A mental disorder characterized by gross impairment of reality; it includes delusions, hallucinations, markedly incoherent speech, disorganized and agitated behavior. This term is also used in a general sense to refer to mental disorders where the patient's mental impairment interferes with his/her ability to deal with regular daily life.
quadriplegia
Paralysis of all four limbs.
schizoaffective disorder
A mental disorder that is characterized by both schizophrenia and mood disturbances.
schizophrenia
A group of mental disorders which are chronic, impair function, and are characterized by psychotic symptoms involving impairment of thought, perception, feelings, and behavior. There are six specific criteria that should be present for a diagnosis of schizophrenia:
1) Certain psychotic symptoms, delusions, hallucinations, formal thought disorder, 2) Deterioration from a previous level of functioning, 3) Continuous signs of the illness for at least six months, 4) Generally, an onset before age 45, 5) Symptoms specifically not due to mood (affective) disorders, 6) Symptoms not due to an organic mental disorder or mental retardation.
spina bifida
Defective closure of the bony encasement of the spinal column.
spina bifida cystica
If the cord and meninges protrude through the defect, it is called spina bifida cystica.
spina bifida occulta
If the cord and meninges do not protrude through, it is called spina bifida occulta.
spondylolisthesis
Forward displacement of one vertebra over another. The most common vertebrae affected are the fifth lumbar (over the sacrum) and the fourth lumbar (over the fifth lumbar).
spondylosis
Immobility and consolidation of a vertebral joint, or a general term for degenerative changes of a vertebral joint due to osteoarthritis.
subdural empyema
A collection of pus between the dura and the arachnoid membranes. This is most often a complication of sinus infection, but can also be due to ear infection, cranial trauma or surgery, and bacteremia.
syringomyelia
A slowly progressive syndrome in which fluid-filled neuroglial cavities form within the spinal cord. These may extend up into the medulla oblongata or down into the thoracic region. Symptoms include neurological deficits, including muscular weakness and atrophy with dissociated sensory loss (for example, loss of pain and temperature sensation but with preservation of the sense of touch). Thoracic scoliosis is often present.
syrinx
A fluid-filled neuroglial cavity. It also means a tube, pipe, or fistula.
Tourette syndrome
Tourette syndrome (or Gilles de la Tourette) is an autosomal multiple tic disorder that begins in childhood with simple facial and vocal tics, and progresses to multiple, complex jerking movements that can occur in any part of the body. In adolescence, the condition may worsen. The patient may grunt, snort, and shout involuntarily. It is more common in males by a ratio of 3:1.
transient ischemic attack (TIA)
Neurologic abnormalities that occur suddenly, are brief (usually lasting only minutes), and are caused by dysfunction in the arterial distribution to the brain. These often occur as a precursor to a stroke.
trigeminal neuralgia
A disorder of the trigeminal (5th) cranial nerve which causes bouts of severe pain lasting seconds to minutes in the distribution of one or more of the nerve's sensory divisions, but most commonly in the mandibular and/or maxillary portion.
Wallerian degeneration
A series of changes that take place in a severed peripheral nerve fiber. This affects a portion of the neuron, which swells, becomes granular, and breaks up into irregular fragments which eventually disappear. The myelin sheath (if there is one) also fragments and forms irregular globules before disappearing. Because it affects the peripheral nervous system, it is possible to have some regeneration of the damaged nerve fibers.