Mechanisms of Hearing and Mechanism of Movement in Human Anatomy

1. Discuss the structures/mechanisms of hearing

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The structures of the ear that are involved in hearing include the outer ear, the middle ear, and the inner ear. The outer ear or pinna, in addition to playing a protective role, also functions to collect sound. The sound waves enter the ear through the auditory meatus into the auditory canal (a tube that is ¾ in diameter and 2.5 cm in length). The auditory canal ends in the tympanic membrane (eardrum), which is a thin, semitransparent diaphragm that demarcates the middle ear entrance (Raichel, 2000.)

The middle ear is an air-filled cavity and contains three ossicles: the malleus, incus, and stapes. While the malleus is attached to the eardrum, the incus connects the malleus and stapes, and the stapes cover the oval window. Connecting the middle ear cavity with the pharynx is a 37 mm long tube called the eustachian tube. The Eustachian tube extends from the middle ear space to the upper part of the throat behind the soft palate. This tube is normally closed, and contraction of the palatal muscles during the processes of yawning, swallowing chewing, etc., opens the eustachian tube. This tube serves to equalize the pressure on each side of the eardrum (Raichel, 2000.)

Just below the oval window is the round window and the structures, which are present to the right of the oval and round window, are called the inner ear (labyrinth).

The principal parts of the inner ear are the vestibule, semicircular canals, and the cochlea. The semicircular canals are some interconnecting canals, which are comprised of membranes, sensory cells, nerves, and fluid (Raichel, 2000).

The main function of the semicircular canals is to give a sense of balance to the body. The cochlea is a sensory system, the function of which is to convert vibratory energy into electrical signals. These electrical signals are then conveyed to the brain for sound detection and interpretation. The cochlear duct is bounded by the basilar membrane, which consists of hair cells (sensory cells for hearing) (Raichel, 2000.)

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The mechanism of hearing involves the following process. The outer and middle ear converts sound pressure to vibrations.

The sound transducer mechanism (consisting of the tympanic membrane, and the three ossicles) is present in the middle ear.

The three ossicles are supported by ligaments and are moved by the involuntary movement of the muscles or by deflection of the tympanic membrane (Davis & Masten, 2003).

The vibrations of the eardrum are transmitted to the malleus, which in turn passes it to the incus, and finally, to the stapes which are embedded in the oval window. Ultimately, the vibrations are conveyed to the fluid of the inner ear. The sound-conducting mechanism amplifies sound by one main mechanism: the large surface area of the eardrum when compared to the small surface area of the base of the stapes (footplate) results in a hydraulic effect. The eardrum has about 25 times the surface area of the oval window. Thus, all the sound pressure, which collects on the eardrum is transmitted through the ossicular chain and is concentrated on the much smaller area of the oval window. All this creates a significant increase in pressure. Vibrations of the basilar membrane then stimulate the hair cells of the inner ear, which discharges small electrical impulses.

These electrical impulses are conveyed by the auditory nerves to the brain (Davis & Masten, 2003).

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A young, healthy adult’s ears respond to sound waves in the frequency range of 20 to 16,000 Hz. The most sensitive frequency range of the ear ranges from 2000 to 5000 Hz (Davis & Masten, 2003).

2. Discuss the major brain structures involved in hearing analysis/comprehension

Nerve fibers from the spiral ganglion enter the dorsal and ventral cochlear nuclei located in the upper part of the medulla. At this point, second-order neurons pass mainly to the opposite side of the brain stem to the superior olivary nucleus. From the superior olivary nucleus, the auditory pathway passes upwards and terminates in the nucleus of the lateral lemniscus. Many bypass this nucleus and pass it on to the inferior colliculus. From the inferior colliculus, the pathway passes to the medial geniculate nucleus, and from here the auditory pathway proceeds by the auditory radiation to the auditory cortex (Guyton 1986)

The primary auditory cortex lies mainly in area 41 in the anterior transverse temporal gyrus situated on the upper surface of the superior temporal gyrus. In addition, the primary auditory cortex also extends over the lateral border of the temporal lobe, most of the insular cortex, and into the lateral portion of the parietal operculum. The primary auditory cortex projects to area 42, which is the auditory association area in the posterior transverse temporal gyrus (Sircar, 2008.)

Wernicke’s area (area 22) is present in the rest of the superior temporal gyrus, behind areas 41 and 42. Wernicke’s area is involved in sound interpretation and comprehension of spoken language (Sircar, 2008.)

3. Explain for each major structure what problems in hearing and /or analysis/ comprehension would arise if they were damaged

Conduction deafness is caused by any disturbance in the conduction of sound from the outer ear to the cochlea, while any damage to the auditory nerve or hair cells in the cochlea can result in nerve deafness. Diseases of the cochlea or auditory nerve can also result in tinnitus, which is the sensation of buzzing or humming in the ears. With aging, there is a loss of hair cells and neurons, which leads to gradual hearing loss; a condition called presbycusis (Bear, Connors, Paradiso, 2006.)

Word deafness or verbal auditory agnosia is a severe comprehension deficit of spoken words, with relatively intact abilities in other areas of language.

This condition results from lesions close to the primary auditory cortex (Harrison & Owen, 2002.)

4. Discuss the structures/mechanism of movement

The skeletal muscle is involved in the movement of the human body. The skeletal muscles are attached to the skeleton and are also known as striated muscles (Fogiel, 1987.)

A bone or joint can move only when there is a pull on it by the muscles.

An individual skeletal muscle can be considered to be having a tendon, and a belly. The origin of a muscle is that part of it that is attached to a bone and does not move during muscular contraction (Fogiel, 1987.)

The muscle is inserted into a particular portion of a bone, which moves during muscular contraction (Fogiel, 1987.)

The contraction of skeletal muscles is controlled by the nervous system. Motor neurons have endings, which make a contact with the muscle fibers at the motor endplate. The motor endplate is a junction, which secretes the neurotransmitter called acetylcholine (Fogiel, 1987.)

Muscles merge into strong connective tissue called tendons that attach it to the bone. Ligaments are tendon-like tissues and are the connections between bones (Fogiel, 1987.)

Muscle fibers are of two types: slow-twitch and fast-twitch. Muscle fibers utilize a chemical substance called ATP (adenosine triphosphate) as their energy source. The energy for the synthesis of ATP is obtained from the splitting of sugar (Frost & Goodheart, 2002.)

Actin and myosin are chemicals present in filaments of contractile fibers within muscles. As the electrical signals sent by the brain through the motor nerves reach the muscles, acetylcholine is discharged on the muscle fiber. The actin and myosin filaments slide together. A complex combination called actinomyosin is formed, and the fiber contracts, thereby shortening the muscle. This shortening of the muscle pulls on the bones to which the muscle is attached at each end and moves it; this is the basic mechanism of human movement. Stabilization of the bones by muscular tension creates body posture (Fogiel, 1987.)

5. Discuss the major brain structures involved in the movement: their role and what problems might arise if there were damage to each.

In humans the motor system involved in movement includes: cortical and subcortical areas of grey matter, spinal tracts, cerebellum, and basal ganglia. In addition, a further influence on the motor system is provided by feedback from sensory system and cerebellar afferents (Waxman, 2002).

The motor cortex in the frontal lobes is divided into the primary motor area and the premotor area. The primary area controls individual muscles or small groups of muscles. The premotor area controls more complex muscle movements, usually involving groups of muscles performing some specific task (Guyton, 1986.)

Three of the basal ganglia nuclei (globus pallidus, caudate nucleus, and the putamen) lie deep in the cerebrum. The other two nuclei (subthalamic nucleus and substantia nigra) are located in the midbrain. The basal ganglia plays an important role in generating voluntary movement (Latash, 2008.)

The cerebellum is important in controlling the balance between agonist and antagonist muscle contractions during rapid changes in body positions. It has a “damping” function, i.e., it prevents overshoot of movements. It also controls ballistic movements like typing etc (Guyton, 1986.)

Motor disturbances include weakness (paresis), paralysis, abnormal movements, and abnormal reflexes. They can result from lesions of the motor pathways or from lesions in the muscles themselves. Myasthenia gravis is a disorder of the myoneural junction, with decreased acetylcholine resulting in weakness and fatigue (Waxman, 2002.)

The upper motor neuron conveys impulses from motor areas of cerebrum and subcortical brain stem to the anterior horn cells of the spinal cord. Lesions of the UMN (damage to cerebral cortex, internal capsule, brain stem, spinal cord) result in paralysis or paresis of the involved muscles, increased muscle tone (hypertonia), spasticity, muscle atrophy, and abnormal reflexes (Waxman, 2002.)

Damage to the basal ganglia leads to various diseases like Parkinson’s disease and Huntington’s disease. They are characterized by akinesia, bradykinesia, dyskinesia, athetosis, and chorea. Disease of the cerebellum is characterized by reduced muscle tone, and a loss of coordination of smooth movements (Waxman, 2002.)

6. What are the stages of sleep? Describe the pattern one observes, the activity that occurs during each stage, and what adverse effect a person might experience with significant alterations to each stage of sleep.

Every individual experiences the same basic stages of sleep, with individual variations in the length and depth of each stage, and the number of hours of sleep (Reinhart, 1996). There are 4 stages of sleep, which occurs every 90-100 minutes, and are called NREM (non-rapid eye movement) sleep. REM (rapid eye movement) sleep is another event in each cycle, which often occurs after sleep has begun its first cycle (Reinhart, 1996.)

Stage 1 is the time between wakefulness and the start of drowsiness. At this time, the body relaxes, the brain waves slow down, and there is a decrease in the body temperature, respirations, and pulse rate. Some people have muscle jerks, which might awaken them, but they quickly return to sleep. This stage, which lasts 1-10 minutes, is sometimes called the “alpha” sleep due to the type of waves seen on an EEG (Reinhart, 1996.)

In stage 2, the brain activity increases along with many short bursts of thoughts and memories. There is continued relaxation of the muscles, slowing down of body functions, with even and steady breathing. This stage lasts for 10 minutes. If a person is awakened during this stage, they might deny that they were asleep (Reinhart, 1996.)

In stage 3, there is continued relaxation with even slower respirations and a slower pulse rate. The body is totally at rest, and in this stage, it is often difficult to awaken the person. This stage lasts 5 minutes. If awakened, the person will not feel rested and might actually feel more tired. This stage is sometimes called “delta” sleep (Reinhart, 1996.)

Stage 4 is the deepest stage, lasting 30-45 minutes. The body is totally relaxed and the brain activity is slow. The person is very difficult to arouse. If a person were awakened in this stage, they would be very groggy (sleep inertia), sometimes incapacitated for 10-15 minutes and end with a disturbed sleep cycle (Reinhart, 1996.)

REM sleep is concurrent with the 4 stages of sleep and occurs within an hour after falling asleep. In this stage, there is fluctuation of pulse rate and blood pressure. While there is relaxation of larger muscles, the smaller muscles might twitch and there is rapid eye movement (Reinhart, 1996).

The heart rate and respiration usually become very irregular, and the EEG shows a desynchronized pattern of beta waves similar to that occurring during wakefulness. Therefore, this type of sleep is also called desynchronized sleep or paradoxical sleep because it is a paradox that a person can still be asleep despite marked activity in the brain (Guyton, 1986).

A person is said to dream during REM sleep. It is very difficult to arouse a person during this stage, and if awakened in this stage, the person might recall what they were dreaming about (Reinhart, 1996). With age, the period of wakening is more and the person tends to return to the stage 1 cycle and into REM sleep (Reinhart, 1996).

7. Discuss the major disorders of sleep

Disrupted or inadequate sleep is known as insomnia. Insomnia has three degrees: acute, transient, and chronic. The transient type is most common of the three and is not that significant. It lasts only for a few nights and a result of any type of excitement (Reinhart, 1996). Acute type of insomnia is more disturbing and is due to stress or illness. Acute insomnia usually disappears in a few days and the sleep cycles returns to normal as before. Chronic insomnia is the more serious type of the three. It is due to continuous sleep deprivation and could last for even months together. The most common cause is prolonged stress or illness (Reinhart, 1996).

Major sleep disorders can be considered under the following headings:

  • Primary sleep disorders does not have a known etiology. They are thought to be due to endogenous abnormalities in sleep-wake generating or timing mechanisms, and are complicated by conditioning factors. Primary sleep disorders are subdivided into dyssomnias (abnormalities in amount, quality, or timing of sleep) and parasomnias (abnormal behavioral or physiological events occurring in association with sleep, specific sleep stages or sleep-wake transitions) (American Psychiatric Association, 2000.)

Dyssomnias are primary disorders in which there is difficulty in initiating or maintaining sleep or of excessive sleepiness. There is a disturbance in the amount, quality, or timing of sleep. They include primary insomnia, primary hypersomnia, circadian rhythm sleep disorder, breathing related sleep disorder, narcolepsy, and dysomnia not otherwise specified (American Psychiatric Association, 2000.)

Primary insomnia is a sleep disorder where there is difficulty in initiating or maintaining sleep or the presence of a nonrestorative type of sleep. The condition lasts for at least 1 month. A prominent feature of primary hypersomnia is sleepiness during the daytime (American Psychiatric Association, 2000.)

Circadian rhythm sleep disorder could occur due to air travel or shift work. Narcolepsy has symptoms of prominent daytime sleepiness, cataplexy, sleep paralysis and sleep related hallucinations. Breathing-related sleep disorders like sleep apnea may involve a complaint of chronic insomnia and daytime impairment (American Psychiatric Association, 2000.)

  • Sleep disorder related to another metal disorder-is due to a diagnosable mental disorder (often a mood or anxiety disorder) (American Psychiatric Association, 2000.)
  • Sleep disorder due to general medical condition-results from the direct physiological effects of a general medical condition on the sleep-wake system (American Psychiatric Association, 2000.)
  •  Substance-induced sleep disorder-involves sleep disturbance due to the concurrent use, or recent discontinuation of use of a substance (including medications) (American Psychiatric Association, 2000.)

8. Discuss the circadian rhythms, the pineal gland, and melatonin. What implications do circadian rhythms have for people whose work requires them to continually rotate work shifts?

The pineal gland is located in the center of the brain, and is involved in the regulation of the action of the hypothalamus, pituitary and parathyroid glands and the pancreas; thus, it can be considered as the regulator of the endocrine system. Due to its ability to respond to changes in illumination and temperature, the pineal gland is considered to be the photo-thermo-endocrine transducer. By the action of its hormones melatonin and arginine vasotocin, the pineal gland has an inhibitory action on the hypothalamus. Whenever there is an increase in light intensity, the pineal gland responds by decreasing the secretion of its hormones. Thus, there is minimal inhibition of the secretion of hypothalamic regulating hormone in the daytime (Hinwood, 1992.)

During the night, the levels of melatonin and arginine vasotocin increase, and this decrease the hypothalamic secretions. By this action, the pineal gland creates a circadian rhythm, which is 24-hour cycle system and forms the ‘biological clock’ of the body (Hinwood, 1992.)

The retina also plays a role in the circadian rhythm. Special photoreceptors, which are present in the retina picks up light signals and directly transmits them to the suprachiasmatic nucleus of the brain. The eyes, thus, signals the pineal gland about the presence or absence of light by this neural pathway (Hinwood, 1992.)

One of the main functions of melatonin is to synchronize the body’s inherent circadian rhythm with the light-dark cycle. Some of the other proposed roles includes: inducing a natural sleep without the side effects that accompany hypnotic sedatives, inhibiting the hormones that stimulate reproductive activity, antioxidant property, slowing aging process by removing free radicals, and enhancing immunity (Sherwood, 2005.)

Normally, there is a synchrony between circadian processes and the sleep-wake cycle. As a result, during daytime, both alertness and arousal are high and performance is most efficient. However, during the night, alertness is the lowest, which allows for optimal sleep. In those people who work in shifts, there is a desynchronization of these processes with the sleep-wake cycle. As a result, these persons may not be able to adapt to working at night (Rom & Markowitz, 2006.)

During some night shifts, the worker will have reduced alertness and performance efficiency due to sleep deprivation and from asynchrony of other circadian rhythms. Although a normal amount of sleep is obtained during other night shifts, there will still be a loss of alertness and compromised performance because of a lack of adaptation of other circadian rhythms to night shifts (Rom & Markowitz, 2006.)

The consequence of such disturbance in circadian rhythms includes: sleep deprivation, higher risk for operational errors or accidents, increased negative mood and gastrointestinal symptoms like loss of appetite, indigestion, and ulcers (Rom & Markowitz, 2006; Rodahl, 2000.)

References

  1. American Psychiatric Association, 2000. Diagnostic and Statistical Manual of Mental Disorders: DSM-IV-TR. American Psychiatric Publication.
  2. Bear, MF, Connors, BW, Paradiso, MA, 2006. Neuroscience: Exploring the Brain. Lippincott Williams & Wilkins.
  3. Davis, ML, Masten, SJ, 2003. Principles of Environmental Engineering and Science. McGraw-Hill Professional.
  4. Fogiel, M, 1987. The Essentials of Biology. Research & Education Assoc.,
  5. Frost, R, Goodheart, GJ, 2002. Applied Kinesiology: A Training Manual and Reference Book of Basic Principals and Practices. North Atlantic Books.
  6. Guyton, AC, 1986. Textbook of Medical Physiology. 7th Ed. W.B. Saunders Company.
  7. Harrison, JE, Owen, AM, 2002. Cognitive Deficits in Brain Disorders. Informa Health Care, 2002.
  8. Hinwood, BG, 1992. A Textbook of Science for the Health Professions. Nelson Thornes.
  9. Latash, ML, 2008. Neurophysiological Basis of Movement. Human Kinetics.
  10. Raichel, DR, 2000. The Science and Applications of Acoustics. Springer.
  11. Reinhart, RO, 1996. Basic Flight Physiology. McGraw-Hill Professional.
  12. Rom, WN, Markowitz, SB, 2006. Environmental and Occupational Medicine. Lippincott Williams & Wilkins.
  13. Rodahl, K, 2000. The Physiology of Work. Taylor & Francis
  14. Sircar, S, 2008. Physiology. Thieme.
  15. Sherwood, L, 2005. Fundamentals of Physiology: A Human Perspective. Thomson Brooks/Cole
  16. Waxman, SG, 2002. Clinical Neuroanatomy. McGraw-Hill Professional.
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