The Nervous System PDF

The Nervous System What Are the Divisions of the Nervous System? The nervous system is divided into two main parts: 1. The central nervous system, or CNS, includes the brain and the spinal cord. 2. The peripheral nervous system, or PNS, includes the cranial nerves exitin...

The Nervous System What Are the Divisions of the Nervous System? The nervous system is divided into two main parts: 1. The central nervous system, or CNS, includes the brain and the spinal cord. 2. The peripheral nervous system, or PNS, includes the cranial nerves exiting from the brain and the peripheral nerves exiting from the spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The peripheral nervous system consists of the: Somatic nervous system, which controls skeletal muscles Autonomic nervous system, which controls cardiac and smooth muscles Enteric nervous system, which controls the glands the autonomic and enteric nervous systems consist of the Sympathetic nervous system, which uses energy or speeds up or increases body functions Parasympathetic nervous system, which conserves energy or slows down or decreases body functions, returning them to normal Remember: The somatic nervous system is under conscious control, and the autonomic nervous system is under unconscious control. The autonomic nervous system is like our “automatic nervous system.” The sympathetic nervous system is used for “fight or flight” situations. The parasympathetic nervous system is used for “rest and digest” or “feeding and breeding.” Histology of the Nervous System What Types of Cells Form the Nervous System? There are two main types of cells that make up the nervous tissue: 1. Neurons. Neurons elicit the electrical impulse. 2. Neuroglia. Neuroglia support the neurons. 1 What Types of Neuroglia Are There? There are six types of neuroglia cells: 1. Astrocytes. The astrocytes are responsible for creating a barrier that prevents infectious organisms from entering the brain from the blood. 2. Oligodendrocytes. The oligodendrocytes form the myelin sheath in the central nervous system. 3. Microglia cells. Microglia cells engulf foreign materials and organisms in the process called phagocytosis. 4. Ependymal cells. Ependymal cells line the ventricles of the brain. These cells have cilia, which circulate the cerebrospinal fluid (CSF). 5. Neurolemmocytes. Neurolemmocytes are also called Schwann cells. These cells produce the myelin sheath in the peripheral nervous system. 6. Satellite cells. These cells support the cell bodies of neurons in a ganglion. What Is a Ganglion? A ganglion is a group of neuron cell bodies that form what looks like a swelling or lump within the nerve. What Are the Main Types of Neurons? There are three main types of neurons: 1. Multipolar neurons. These neurons are located mostly in the brain and spinal cord. 2. Bipolar neurons. These neurons are associated with special senses, which include sight, hearing, smelling, tasting, and equilibrium. 3. Unipolar neurons. These neurons are associated with general senses. These senses include crude touch, vibration, pain, cold/hot temperature, pressure, and soft touch. What Is the Difference between a Neuron That Is Myelinated and One That Is Unmyelinated? Myelin is a lipoprotein insulator that surrounds the neuron. Oligodendrocytes form this insulator in the CNS, and Schwann cells or neurolemmocytes form this insulator in the PNS. Myelinated neurons look white and are said to be white 2 matter, whereas unmyelinated neurons are said to be gray matter. Myelinated neurons have a faster conduction time then those that are unmyelinated. What Are Some Common Diseases and Disorders That Can Affect Nerves? 1. Neuralgia. Neuralgia is simply nerve pain. What are the signs and symptoms associated with neuralgia? Signs and symptoms associated with nerve pain are A. Numbness or tingling, which is called paresthesia B. A hot or burning sensation, which is called causalgia C. Shooting or radiating pain 2. Neuritis. Neuritis is the inflammation of a nerve. What are the signs and symptoms associated with neuritis? The signs and symptoms of nerve inflammation are neuralgia or loss of function. 3. Neuropathy. A neuropathy is any condition that can cause dysfunction or weakness to peripheral nerves. This dysfunction can be caused by a trauma, degeneration, nerve impingement, or a subluxation. What Are Examples of Disorders or Diseases That Can Affect the Myelin Sheath? Some disorders that can affect the myelin sheath are Tay-Sachs disease, multiple sclerosis, and Guillain-Barré syndrome. Tay-Sachs disease. Tay-Sachs disease is a progressive genetic disease in which there is an accumulation of lipids (lipidosis) around and within the myelin sheath. What are the signs and symptoms associated with Tay-Sachs disease? Tay-Sachs disease presents with paralysis, blindness, and mental retardation, and death usually occurs by the age of four. Multiple sclerosis. Multiple sclerosis, or MS, is a progressive disease in which there is a demyelination of neurons in the brain and spinal cord. Plaquing can be seen in the area of demyelination. The causes of MS are unknown, although genetics may be a factor, since a higher incidence within families has been observed. Also, an autoimmune component may play a role because of 3 elevated IgG antibodies and T lymphocytes. Multiple sclerosis can occur in men but is more common in women between the ages of 20 and 40. What are the signs and symptoms associated with multiple sclerosis? Multiple sclerosis presents with numbness in the extremities, thorax, and face; optic pain and/or blindness in one eye; hand and leg pain; muscle weakness and joint stiffness; abnormal gait; and dizziness. Guillain-Barré syndrome. Guillain-Barré syndrome is a serious, potentially fatal immune disorder in which the body’s immune system attacks peripheral nerves and causes inflammation and demyelination of these nerves. Guillain- Barré syndrome can affect both men and women, commonly between the ages of 25 and 50, although it can occur at any age. This disease occurs in three phases: 1. Acute 2. Plateau 3. Recovery The recovery phase can last three years as remyelination and axon growth occur. The cause is unknown, although vaccines, viruses, pregnancy, Hodgkin’s disease, lupus, and bacterial infections such as Campylobacter jejuni may start the disease process. What are the signs and symptoms associated with Guillain-Barré syndrome? Guillain-Barré syndrome presents with muscle paresthesia, weakness, paralysis, myalgia, difficulty breathing, and abnormal heart rhythms. The Brain What Protects the Brain? The brain is protected by the meninges. What Are the Meninges? The meninges are three layers of connective tissue that protect the brain and spinal cord. 4 These three layers, from superficial to deep, are 1. Dura mater 2. Arachnoid mater 3. Pia mater The cerebrospinal fluid is located in the space between the arachnoid mater and the pia mater. This region is called the subarachnoid space. The brain has what are called four ventricles through which cerebrospinal fluid circulates. There are two lateral ventricles, a third ventricle, and a fourth ventricle. An area called the choroid plexus forms the cerebrospinal fluid from blood plasma. What Are the Parts of the Brain? The parts of the brain include the 1. Brain stem 2. Diencephalon 3. Cerebellum 4. Cerebrum What Is the Brain Stem? The brain stem is the most inferior portion of the brain and is attached to the spinal cord. It has four parts: 1. Medulla oblongata 2. Pons 3. Midbrain 4. Red nucleus 1. Medulla oblongata. The medulla oblongata is the most inferior portion of the brain stem and extends from the pons to the spinal cord. What is the function of the medulla oblongata? Functions. The medulla oblongata is a relay station between sensory and motor impulses. It regulates our vital reflexes, such as heart rate, respiratory rate, and blood vessel diameter. The medulla oblongata also controls our nonvital reflexes, such as swallowing, sneezing, coughing, hiccupping, and regurgitation. The motor tracts located in the medulla oblongata carry impulses to skeletal muscles. 5 2. Pons. The pons bridges many parts of the brain. The pons contains two respiratory centers. The apneustic and pneumotaxic centers, together with the medulla oblongata, help control breathing. What is the function of the pons? Function. The apneustic center increases the inflow of air by prolonging inhalation. The pneumotaxic center increases the amount of air that is exhaled. 3. Midbrain. The midbrain extends from the hypothalamus to the diencephalon to the pons. It connects the third and fourth ventricles and relays sensory impulses from the spinal cord to the thalamus and motor impulses from the cerebral cortex to the pons. What is the function of the midbrain? Function. The midbrain is responsible for the visual and auditory reflexes. It contains the Superior colliculi, which coordinate the movements of the eyeballs and head in response to visual stimuli Inferior colliculi, which coordinate the movement of the head and trunk in response to visual stimuli Substantia nigra, which produces and secretes dopamine for subconscious fine motor control 4. Red nucleus. The red nucleus works with the basal ganglion and the cerebellum to coordinate muscle movement. The red nucleus is an extrapyramidal tract. Extrapyramidal tracts coordinate nondominant movement. Pyramidal tracts coordinate controlled movement. What Is the Diencephalon? The diencephalon contains these parts: 1. Epithalamus 2. Thalamus 3. Hypothalamus 1. Epithalamus. The epithalamus contains the pineal gland. The pineal gland promotes sleepiness when it secretes the hormone melatonin in response to darkness at night. 6 2. Thalamus. The thalamus is the main region of the brain for relaying sensory impulses that reach the cerebrum from the spinal cord, brain stem, and cerebellum. The thalamus is also the location for the reticular activating system, which controls our levels of consciousness. This system is also located in the midbrain and helps control our sleep and wake cycles (circadian rhythms). 3. Hypothalamus. The hypothalamus is the link between the endocrine system and the nervous system. It is located inferior to the thalamus. The hypothalamus monitors and regulates our thirst centers, hunger centers, satiety centers (the feeling that your stomach is full), thermostat center, and pleasure centers. The hypothalamus also regulates behavioral and emotional patterns and contributes to the maintenance of our sleep and wake patterns. What Is the Cerebellum? The cerebellum makes up the posterior portion of the brain. It is located in back of the cerebrum. It contains an outer cortex composed of gray matter and an inner portion of white matter called the arbor vitae (tree of life). The cerebellum is responsible for our equilibrium, or balance, and our proprioception, or our sense of our body’s position in relation to space. The cerebellum is also responsible for motor information that maintains muscle tone and coordination. These tasks are involuntary. What Is the Cerebrum? The cerebrum is the largest portion of the brain. It has left and right hemispheres that are divided by a longitudinal fissure. These two hemispheres communicate with each other by a structure made up of around 200 million neurons called the corpus callosum. The cerebrum contains an outer cerebral cortex made up of gray matter. Inferior to the gray matter is white matter, which connects all parts of the brain. What Are the Basal Ganglia? The basal ganglia are gray matter that contain nuclei that are deep in the cerebrum. The basal ganglia control emotion, memory, muscle tone, and learned reflexes, such as writing, tying shoes, or brushing teeth. What Are the Lobes of the Cerebrum? The lobes of the cerebrum are named for the skull bones that protect them. The lobes of the cerebrum are 7  1. Frontal lobes. The frontal lobes contain the premotor area for voluntary muscle action. The left frontal lobe controls the right side of the body, and the right frontal lobe controls the left side of the body. 2. Parietal lobes. The parietal lobes are the area for general sensory perception. The right lobe is sensitive to the left side of the body, and the left lobe is sensitive to the right side of the body. 3. Temporal lobes. The temporal lobes have an auditory area and an olfactory area. Auditory area. The auditory area receives sensory impulses from the inner ear and interprets the sound for hearing. Olfactory area. The olfactory area receives sensory impulses from the nasal cavity and interprets the information as smell. 4. Occipital lobes. The occipital lobes contain the visual areas. The visual areas receive sensory information from the eye (retina) and interpret the information as sight. What Is Broca’s Area? Broca’s area is a region of the brain that is found in the left frontal lobe and is responsible for speech. What Is the Primary Somesthetic Area? The primary somesthetic area is located in the postcentral gyrus (a gyrus is a fold in the brain). This area is responsible for tactile sensation. What Is the Primary Gustatory Area? The primary gustatory area receives and interprets sensations of taste. What Is the Hippocampus? The hippocampus is the region of the brain that is responsible for our memory. What Are the Left and Right Hemispheres of the Brain Responsible For? Left hemisphere. The left hemisphere is responsible for logic, language, intellectual skills, math skills, scientific skills, and reasoning. Right hemisphere. The right hemisphere is responsible for artistic ability, musical ability, creative thinking, imagination, and pattern perception. 8  What Are Cranial Nerves, and What Is Their Function? Cranial nerves exit the brain and are responsible for sensory function, motor function, or both. Cranial nerves that are responsible for both sensory and motor information are said to be mixed. What Are Some Examples of Diseases or Disorders of Cranial Nerves? Two examples of diseases that can affect cranial nerves are trigeminal neuralgia and Bell’s palsy. 1. Trigeminal neuralgia. Trigeminal neuralgia is also called tic douloureux. Trigeminal neuralgia can be caused by degenerative changes or by a compression or impingement of the trigeminal nerve (C.N. V) as a result of trauma to the nerve. What are the signs and symptoms associated with trigeminal neuralgia? Trigeminal neuralgia presents with deep, sharp, stabbing pain along the nerve distribution. This pain can be extremely severe. 2. Bell’s palsy. Bell’s palsy is the inflammation of C.N. XII. This neuritis will cause temporary paralysis of one side of the face. The causes of Bell’s palsy include trauma to the nerve and an infection caused by a virus or bacterium. What are the signs and symptoms associated with Bell’s palsy? The signs and symptoms of Bell’s palsy present with a loss of function of the muscles of facial expression, tearing of the eye on the affected side, and drooling from the side of the mouth on the affected side. The individual may also experience headaches. Pathophysiology of the Brain What Are Some Diseases and Disorders That Can Affect the Brain? Diseases that can affect the brain include Alzheimer’s disease Amyotrophic lateral sclerosis Cerebral palsy Cerebrovascular accident (stroke) Epilepsy (seizures) Encephalitis 9  Huntington’s disease Parkinson’s disease What Is Alzheimer’s Disease? Alzheimer’s disease is a progressive degenerative disease that can occur in the cerebral cortex of the brain. The direct cause of Alzheimer’s disease is unknown, although the lack of certain neurotransmitters, such as acetylcholine, somatostatin, and norepinephrine, may be a factor. Also, tangled fibers in the neurons, degen- eration of neurons, trauma, family history, and genetic defects contribute to the development of Alzheimer’s disease. What are the signs and symptoms associated with Alzheimer’s disease? The signs and symptoms of Alzheimer’s disease include loss of memory, dementia, disorientation, confusion, irritability and restlessness, personality changes (mood swings, depression, anger, hostility), and deterioration of speech. What Is Amyotrophic Lateral Sclerosis? Amyotrophic lateral sclerosis, or ALS, is commonly called Lou Gehrig’s disease after the New York Yankees first baseman who died from the disease. This is a progressive degenerative disease that affects the motor neurons of the cranial and spinal nerves. The direct cause of ALS is unknown, although hereditary and environmental factors (such as viruses, meta- bolic imbalances, and immune disorders) may contribute to it. ALS occurs more commonly in men between the ages of 40 and 75. As the neurons deteriorate and eventually die, the skeletal muscles that they innervate weaken and eventually atrophy. This disease commonly results in death within six years of onset. What are the signs and symptoms associated with amyotrophic lateral sclerosis? The signs and symptoms associated with ALS include muscle spasms and cramps, fatigue, muscle weakness, slurred speech, difficulty chewing, difficulty swallowing, and, in the later stages, choking, muscle paralysis, and difficulty breathing. What Is Cerebral Palsy? Cerebral palsy is a nonprogressive disorder that affects the motor portion of the brain. This disease, although nonprogressive, is permanent. Cerebral palsy is caused by an infection, trauma, or other toxic conditions that damage the brain during or before birth. 10 What are the signs and symptoms associated with cerebral palsy? The signs and symptoms of cerebral palsy will present with an impairment of motor function (speech, walking), muscle spasms, and spastic paralysis. What Is a Cerebrovascular Accident? A cerebrovascular accident, or CVA, is commonly referred to as a stroke. A CVA results when the blood flow from one or more blood vessels in the brain is impaired, interrupted, or stopped. This lack of blood supply (ischemia) results in brain cell and tissue necrosis (cell death) and brain damage. The cause of a CVA can be either an occlusion or blockage in an artery, called an embolism; clotting in an unbroken blood vessel, called a thrombosis; or a rupture in an artery, causing a hemorrhage (bleeding). Contributing factors that can increase the risk of a CVA include a family history, hypertension, atherosclerosis, diabetes mellitus, cardiovascular disease, high cholesterol, poor diet, and smoking. What are the signs and symptoms associated with a cerebrovascular accident? The signs and symptoms associated with a CVA include numbness, paralysis of one side of the face and/or one side of the body (hemiparesis), slurred speech (dysphasia), lack of ability or inability to speak (aphasia), muscle weakness, loss of time or memory, mental confusion, impairments of sight, coma, and even death. What Is Epilepsy? Epilepsy is an abnormal brain condition that causes seizures. These seizures occur as a result of an abnormal increase in electrical impulses of neurons in the brain. Most epileptic cases and seizures are of unknown cause. Causes and contributing factors can include brain injuries, brain tumors or lesions, diabetes mellitus, chemical toxins, CVA, fevers, infections (meningitis, encephalitis), problems with normal metabolism, phenylketonuria, hypocalcemia, hyponatremia, hypoglycemia, and withdrawals from drug and alcohol addiction. What are the signs and symptoms associated with epilepsy? Signs and symptoms associated with epilepsy include nausea; visual disturbances; bad, abnormal, or unusual tastes and smells; abnormal levels or loss of consciousness; muscle tremors or convulsions; and loss of sensation. What Is Encephalitis? Encephalitis is an inflammation of the brain. This inflammation can cause seizures, delirium, cephalgia, dam- age to the brain, coma, and death as a result of the degenerative changes that occur within the brain. Encephalitis is caused by an infection. Viral infections are most common. 11 What are the signs and symptoms associated with encephalitis? Signs and symptoms associated with encephalitis include fever, cephalgia, lethargy, delirium, and seizures. What Is Huntington’s Disease? Huntington’s disease, or Huntington’s chorea, is a progressive, hereditary disorder in which signs and symptoms commonly begin between 35 and 50 years of age. This disorder will eventually lead to death. Before death, the individual becomes totally incapacitated, with severe choreiform movements (rapid contortions and rigidity of muscles). Huntington’s chorea is an autosomal dominant disorder affecting the front lobe of the cerebral cortex of the brain. It causes a decrease in the levels of the neurotransmitter GABA, resulting in severe nerve damage to the cortex. What are the signs and symptoms of Huntington’s disease? The signs and symptoms of Huntington’s chorea present with rigidity in the muscles of the face, trunk, and upper extremities; mental changes; depression; dystonia; ataxia; muscular writhing; choreic movement of muscles; difficulty swallowing; and dementia. What Is Parkinson’s Disease? Parkinson’s disease, or shaking palsy, is a slow, progressive degenerative disease that affects motor neurons. Parkinson’s disease tends to be more common in men between the ages of 40 and 65, although women and younger people can also have this disease. The cause of Parkinson’s disease is unknown, although the cells of the substantia nigra portion of the mid- brain fail to produce adequate amounts of the neurotransmitter dopamine. This may be the result of a brain tumor, carbon monoxide poisoning, drugs, or repeated chronic head trauma. What are the signs and symptoms associated with Parkinson’s disease? Signs and symptoms associated with Parkinson’s disease include resting tremors, slow movements, slowed speech, lack of coordination and balance, a shuffling gait, and a freezing, masklike facial expression. Pathophysiology of the Spinal Cord What Are Examples of Disorders and Diseases That Can Affect the Spinal Cord? 12 Examples of diseases and disorders that can affect the spinal cord are meningitis, poliomyelitis, and spina bifida. What Is Meningitis? Meningitis is the inflammation of the protective covering of the brain and spinal cord. This protective covering is called the meninges. Meningitis can be caused by either a virus or a bacterium, such as Staphylococcus aureus, Streptococcus pneumoniae, or Neisseria meningitides, that is transmitted through droplets from the respiratory system by coughing, sneezing, and exhaling. The organisms can enter the blood through the nasopharynx, then make their way to the cerebrospinal fluid. What are the signs and symptoms associated with meningitis? The signs and symptoms associated with meningitis include fever, sleepiness, headache, neck ache and stiffness, confusion, and vomiting. Viral meningitis normally gets better on its own in about two weeks. Bacterial meningitis, on the other hand, if not treated, can result in severe neurological malfunctions, such as paralysis and death. What Is Poliomyelitis? Poliomyelitis is a disease that causes inflammation of the gray matter of the spinal cord. This inflammation destroys motor neurons in the brain and spinal cord. Poliomyelitis is caused by infection with the polio virus. What are the signs and symptoms associated with poliomyelitis? The signs and symptoms associated with poliomyelitis include paralysis, flaccid paralysis, muscle atrophy, res- piratory failure, and infections of the digestive system and respiratory system. Death can occur as a result of respiratory failure. What is Spina Bifida? Spina bifida is a congenital defect in which the posterior arches of the vertebrae (L5 and/or LS1) fail to fuse. The posterior arch includes the spinous process and laminae. There are two different types of spina bifida: 1. Spina bifida occulta. Spina bifida occulta presents with no protrusion; just a small impression or dimple may be present in the region of the defect. No symptoms are present. 2. Spina bifida cystica. Spina bifida cystica presents with a sac or cystlike protrusion at the level of L5- S1. There are two types: Spina bifida cystica meningocele. In this condition, the meninges and cerebrospinal fluid protrude into the saclike protrusion. Spina bifida cystica meningomyelocele. In this condition, the spinal cord and/or the spinal nerves protrude into the saclike protrusion. 13 Spina bifida is a congenital abnormality and can be linked to low levels of folic acid (B vitamin) during pregnancy. What are the signs and symptoms associated with spina bifida cystica meningomyelocele? Spina bifida cystica meningomyelocele is the most serious and presents with severe neurological dysfunction, which includes partial or complete paralysis and loss of bowel and urinary bladder control The Special Senses The special senses include smell, taste, vision, and hearing. This section will review vision and hearing. What Are Some Common Diseases Associated with the Eye? Diseases that can affect the eye include Astigmatism Myopia Hyperopia Nyctalopia Conjunctivitis Strabismus Cataracts. Glaucoma What Is Astigmatism? Astigmatism is the abnormal bending of the light that enters the eye. It is caused by an abnormal curve in the lens of the eye. This causes the light entering the eye to bend irregularly. 14 What Is Myopia? Myopia is a condition in which an individual can see objects that are close but cannot see objects that are far away clearly. Myopia occurs when the eyeball has an elongated shape. What Is Hyperopia? Hyperopia is a condition in which an individual can see objects that are far away but cannot see objects that are close clearly. Hyperopia occurs when the eyeball has a shortened shape. What Is Nyctalopia? Nyctalopia is commonly referred to as night blindness. The individual is unable to see objects in the darkness of night. Nyctalopia can occur when there is a retinol or vitamin A deficiency or if there is a degeneration of the retina. (Nyctamblyopia is a decrease in vision at night with no visible structural changes in the eye.) What Is Conjunctivitis? Conjunctivitis is commonly called “pinkeye” and occurs when the protective mucous membrane that covers the eyeball, the conjunctiva, is inflamed. Conjunctivitis is very contagious and can be caused by bacterial infections or viral infections. What Is Strabismus? Strabismus is the deviation of one or both eyes. This deviation can be medial or lateral, and can converge or diverge. When one eye deviates medially, it is called convergent strabismus. When one eye deviates laterally, it is called divergent strabismus. Strabismus can be caused by a traumatic injury to the eye and/or brain, or it can be inherited. What Is Glaucoma? Glaucoma occurs when there is an increase in intraocular pressure. This is a potentially dangerous disorder. If left untreated, it can result in damage to the optic nerves, and blindness can occur. Glaucoma is caused by an excessive buildup of aqueous humor in the anterior chamber of the eye. This buildup can be the result of an obstruction or trauma. What Is a Cataract? A cataract is a cloudiness or fogginess in vision caused by a buildup of calcium within the lens or cornea of the eye. A cataract can be caused by degeneration of aging, trauma, exposure to sunlight, and certain medications. What Are Some Common Diseases Associated with the Ear? Some common diseases that are associated with the ear include Tinnitis Otitis media 15  Otitis interna Meniere’s disease What Is Tinnitus? Tinnitus is the term used to describe “ringing” in the ears. Tinnitus can be caused by an injury, an infection, oto- sclerosis (which is a type of conductive hearing loss resulting from a stapes that is immobile), and presbyacusis (which is caused by the deterioration of the auditory system that can occur during the normal aging process). What Is Otitis Media? Otitis media is a middle ear infection that causes pain, inflammation, and fever. It is more common in children and occurs as a result of an upper respiratory infection that has spread up the eustachian tubes into the middle ear. What Is Otitis Interna? Otitis interna is also called labyrinthitis and is an inner ear infection that results in inflammation, dizziness (vertigo), and nausea. Otitis interna occurs when there is an infection of the inner ear. What Is Meniere’s disease? Meniere’s disease most commonly occurs in males and affects the inner ear. The individual will experience dizziness (vertigo), a ringing sound in the ears (tinnitus), and nausea. This condition progresses to a loss of hearing. Meniere’s disease occurs when the endolymph (the fluid found in the inner ear) accumulates in the inner ear. 16