The Respiratory System - Structure And Function The respiratory system is the system in the human body that enables us to breathe.

The act of breathing includes: inhaling and exhaling air in the body; the absorption of oxygen from the air in order to produce energy; the discharge of carbon dioxide, which is the byproduct of the process.

The parts of the respiratory system

The respiratory system is divided into two parts:

Upper respiratory tract:

This includes the nose, mouth, and the beginning of the trachea (the section that takes air in and lets it out).

Lower respiratory tract:

This includes the trachea, the bronchi, broncheoli and the lungs (the act of breathing takes place in this part of the system).

The organs of the lower respiratory tract are located in the chest cavity. They are delineated and protected by the ribcage, the chest bone (sternum), and the muscles between the ribs and the diaphragm (that constitute a muscular partition between the chest and the abdominal cavity).

The trachea – the tube connecting the throat to the bronchi.

The bronchi – the trachea divides into two bronchi (tubes). One leads to the left lung, the other to the right lung. Inside the lungs each of the bronchi divides into smaller bronchi.

The broncheoli - the bronchi branches off into smaller tubes called broncheoli which end in the pulmonary alveolus.

Pulmonary alveoli – tiny sacs (air sacs) delineated by a single-layer membrane with blood capillaries at the other end.

The exchange of gases takes place through the membrane of the pulmonary alveolus, which always contains air: oxygen (O2) is absorbed from the air into the blood capillaries and the action of the heart circulates it through all the tissues in the body. At the same time, carbon dioxide (CO2) is transmitted from the blood capillaries into the alveoli and then expelled through the bronchi and the upper respiratory tract.

The inner surface of the lungs where the exchange of gases takes place is very large, due to the structure of the air sacs of the alveoli.

The lungs – a pair of organs found in all vertebrates.

The structure of the lungs includes the bronchial tree – air tubes branching off from the bronchi into smaller and smaller air tubes, each one ending in a pulmonary alveolus.

The act of breathing

The act of breathing has two stages – inhalation and exhalation

Inhalation – the intake of air into the lungs through expansion of chest volume.

Exhalation – the expulsion of air from the lungs through contraction of chest volume.

Inhalation and exhalation involves muscles:

1. Rib muscles = the muscles between the ribs in the chest.2. Diaphragm muscle

Muscle movement – the diaphragm and rib muscles are constantly contracting and relaxing (approximately 16

times per minute), thus causing the chest cavity to increase and decrease.

During inhalation – the muscles contract:

Contraction of the diaphragm muscle – causes the diaphragm to flatten, thus enlarging the chest cavity.

Contraction of the rib muscles – causes the ribs to rise, thus increasing the chest volume.

The chest cavity expands, thus reducing air pressure and causing air to be passively drawn into the lungs. Air passes from the high pressure outside the lungs to the low pressure inside the lungs.

During exhalation – the muscles relax:

The muscles are no longer contracting, they are relaxed.

The diaphragm curves and rises, the ribs descend – and chest volume decreases.

The chest cavity contracts thus increasing air pressure and causing the air in the lungs to be expelled through the upper respiratory tract. Exhalation, too, is passive. Air passes from the high pressure in the lungs to the low pressure in the upper respiratory tract.

Inhalation and exhalation are involuntary and therefore their control requires an effort.

The act of breathing – Illustration & Animation

Source: Merck Manual

Changes in chest volume during inhalation and exhalation – note that it only shows the movement of the

diaphragm, not that of the rib muscles.

Source: Wikipedia

The respiratory system- Illustration

Source: Wikipedia

The respiratory airways include the respiratory apertures (mouth and nose), the trachea and a branching system of long, flexible tubes (bronchi) that branch of to shorter and narrower tubes (broncheoli) until they end in sacs called the pulmonary alveoli.

The lungs encompass the entire system of tubes branching out from the main bronchi to the alveoli.

Measuring the functioning of the lungs is a medical tool for diagnosing problems in the respiratory system.

Measurements of lung function

2. Air volume (in liters) – lung capacity

Maximum lung volume is known as TLC (total lung capacity). It can be obtained by maximum strenuous inhalation.

The maximum lung volume of a healthy adult is up to 5-6 liters. In children the maximum lung volume is up to 2-3 liters, depending on age. In infants it is up to 600-1000 milliliters.

Note! Differences in lung volume can only be caused by gender, age, and height.

Essential air volume is the maximum volume utilized by the lungs for inhalation, also known as VC (vital capacity).

Residual volume (RV) is the volume of air remaining in the lungs after strenuous exhalation when the lungs feel completely empty. Residual volume prevents the broncheoli and the alveoli from sticking together. Residual volume is approximately 1.5 liters (adults).

The differential between total lung capacity and residual volume is the maximal volume utilized by the lungs in order to breath. It is known as vital capacity (VC). In an adult, the VC is between 3.5 and 4.5 liters.

Tidal Volume or VT is the volume of air displaced between normal inspiration and expiration. In a healthy adult the tidal volume is approximately 500 milliliters.

2. Rate of airflow through the respiratory airways (into and out of the lungs).This measures the effectiveness of airflow.

3. Efficiency of diffusion of oxygen from the pulmonary alveoli into the blood (not dealt with in this unit).

Regulation of Respiration in HumansArticle Shared by <="" div="">

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In this article we will discuss about the regulation of respiration in humans.

Oxygen requirement by the body differs depending on the activity. It is lowest at rest and increases during routine activity and further increases in muscular exercise. Similarly production of carbon dioxide also is dependent on the rate of metabolic activity in the body.

Respiratory system has the responsibility of meeting needs of the body by altering the rate and depth of respiration in order to keep the pO2 and pCO2 at normal levels. ADVERTISEMENTS:

The regulation of respiration can be brought about by: 1. Neural mechanism.

2. Chemical influence.

3. Non-chemical influence.

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The chemical and non-chemical influence has to act through the neural mechanism only (Fig. 4.25).

I. Neural Mechanism of Respiration: Centers are present in brainstem. The brainstem centers are required for rhythmic respiration whether during asleep or awake. The cerebral cortical center is required for voluntary alterations in respiration.

Brainstem centers are present in the reticular formation of pons and medulla oblongata.

In the pons, the centers present are: i. Pneumotaxic

ii. Apneustic

In medulla oblongata, the centers present are: ADVERTISEMENTS:

i. Inspiratory (dorsomedial group of neurons)

ii. Expiratory (ventrolateral group of neurons)

There is a lot of interconnection between the various centers. The interplay of the different centers is essential for a proper regulation of respiration. The medullary centers are termed as basic centers, whereas the pontine centers are called regulatory centers. The pontine centers act through the medullary centers and bring about smooth rhythmic respiration.

From the medullary centers, which are also spontaneously active, the impulses are sent to spinal cord through the reticulospinal pathway, which ends on the anterior horn cells in spinal cord. Both the phrenic (C3-C5) and intercostal nerve (T1-T11) take origin from spinal cord and influence the activity of diaphragm and intercostals muscles, respectively.

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So if there is a complete transverse section of spinal cord at the level of: i. C2 segment person dies of respiratory paralysis.

ii. C6 person survives because the diaphragmatic respiration continues.

In a normal person, the inspiratory center (IC) appears to generate impulse on its own. During the course of the generation of impulse, it is presumed that the rate of impulse generation goes on increasing till it reaches a certain point and then there will be sudden cessation of impulse generation. Because of this, IC is known to act as a ramp generator.

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The impulses from the apneustic center have a regulatory influence on the inspiratory center. The apneustic center activity in turn is controlled by the impulses coming from the pneumotaxic center and through the vagus nerve from the stretch receptors of lungs.

When the influence by the vagus and pneumotaxic center over the apneustic center is lost, there will be prolonged inspiration and a sudden expiration. This type of breathing is known as apneustic breathing (Fig. 4.26).

Sequence of events during normal regulation of respiration by neural mechanism: ADVERTISEMENTS:

i. Onset and gradual increase in the number of impulses production in the inspiratory center because of the ramp generator.

ii. This leads to:

a. Impulses being sent from IC to spinal cord for stimulation of phrenic and intercostals nerves.

b. Reciprocal inhibition of expiratory center by IC.

c. Excitatory impulses from IC sent to pneumotaxic center through multisynaptic pathway.

iii. When inspiration is going on, there will be gradual inhibition of the apneustic center by the impulses coming from the pneumotaxic center and also from the afferent vagal fibers coming from the distended alveoli.

iv. Apneustic center influence over the IC ceases completely. Hence the activity of inspiratory center stops and leads to no inhibition influence over the expiratory center (EC). No more impulses from the inspiratory center to motor neurons in the spinal cord.

v. The muscles of inspiration start relaxing. This starts the process of expiration which normally lasts for about 3 sec.

vi. After this, once again the activity in the IC starts, leading to the next respiratory cycle.

Location of the respiratory centers in CNS for rhythmic respiration can be experimentally studied from the following observations: 1. If transection is done above pons, the rhythmic respiration continues as usual.

2. If a mid-pontine section is done along with bilateral vagotomy, there will be a prolonged inspiration followed by a sudden short expiration (apneustic type of breathing).

3. If transection is done between pons and medulla oblongata, though respiration continues on its own, it will be irregular. Sometimes it becomes shallow and sometimes it is deeper. This type of breathing is known as gasping.

4. If transection is done below medulla (at the beginning of the spinal cord), it leads to complete cessation of breathing.

So by the above studies, it can be concluded that the centers are present in brainstem. The pontine centers play role in smooth and rhythmic respiration.

Hering-Brueur reflex: Inflation of alveoli brings about cessation of inspiration and expiration commences.

The details are as under: i. Inflation of alveoli

ii. Leads of stimulation of stretch receptors present in the alveoli.

iii. Afferent impulses are carried by vagal fibers.

iv. Inhibit the activity of the respiratory center, cessation of inspiration.

v. Leads to relaxation of muscles of inspiration.

vi. Expiration commences.

This reflex is not very well seen in adults. The reflex probably helps to prevent over distension of the alveoli.

II. Chemical Influence on Respiration: This is brought about by the chemoreceptors.

They are called: i. Peripheral

ii. Central chemoreceptors.

Peripheral Chemoreceptors (Fig. 4.27):

a. Carotid bodies which are present at the branching of internal carotid artery.

b. Aortic bodies are present in the arch of aorta.

From the carotid bodies, the afferent impulses will be carried by the sinus nerve (Fig. 4.28) a branch of glossopharyngeal nerve and from the aortic bodies by the aortic nerve branch of vagus nerve.

The peripheral chemoreceptors respond to: i. Decrease in pO2 ii. Increase in H+ iii. Increase of pCO2 of blood. Details of the role of peripheral chemoreceptors in regulation of respiration are shown in Figs 4.29 to 4.33.

Central Chemoreceptors: They are present in the brainstem near the respiratory centers. They are more sensitive to hydrogen ions, but the hydrogen ion of blood cannot stimulate them because the blood brain-barrier is impermeable for the hydrogen ion to diffuse through. Hence, the increase in partial pressure of carbon dioxide forms the stimulus (Fig. 4.34).

Decreased pO2, increased pCO2 together (asphyxia) will have an additive effect on chemoreceptors. Hence there will be maximum respiratory response in such a situation (Fig. 4.35). Asphyxia occurs in conditions, like drowning or strangulation.

III. Non-Chemical Influence on Respiration: Non-chemical influence on respiratory centers pertains to impulses coming from: i. Baroreceptors

ii. Muscle spindles of respiratory muscles to control depth of respiration.

iii. Pain receptors

iv. Intracranial tension

v. Irritant receptors stimulation in lungs while coughing.

vi. Higher parts of CNS

vii. Irritation of nasal mucosa (sneezing)

viii. Mechanoreceptors in pharynx (deglutition).

ix. Receptors of muscles and joints.

Depending on the location of the receptors influencing the respiratory centers, there will be appropriate alterations in the respiration.

he nervous system includes both the Central nervous system and Peripheral nervous system. The Central nervous system is made up of the brain and spinal cord and The Peripheral nervous system is made up of the Somatic and the Autonomic nervous systems.

The Central Nervous System (CNS)The central nervous system is divided into two major parts: the brain and the spinal cord.

The BrainThe brain lies within the skull and is shaped like a mushroom.  The brain consists of four principal parts:

the brain stem

the cerebrum the cerebellum the diencephalon

The brain weighs approximately 1.3 to 1.4 kg. It has nerve cells called the neurons and supporting cells called the glia.

There are two types of matter in the brain:  grey matter and white matter.  Grey matter receives and stores impulses.  Cell bodies of neurons and neuroglia are in the grey matter.  White matter in the brain carries impulses to and from grey matter.  It consists of the nerve fibers (axons).

The hemispheres are further divided into four lobes:

Frontal lobe Temporal lobes Parietal lobe Occipital lobe

The CerebellumThis is located behind and below the cerebrum.

The DiencephalonThe diencephalon is also known as the fore brain stem. It includes the thalamus and hypothalamus. The thalamus is where sensory and other impulses go and coalesce.

The hypothalamus is a smaller part of the diencephalonOther Parts of the BrainOther parts of the brain include the midbrain and the pons:

the midbrain provides conduction pathways to and from higher and lower centers

the pons acts as a pathway to higher structures;  it contains conduction pathways between the medulla and higher brain centers

The Spinal CordThe spinal cord is along tube like structure which extends from the brain. The spinal cord is composed of a series of 31 segments.  A pair of spinal nerves comes out of each segment.  The region of the spinal cord from which a pair of spinal nerves originates is called the spinal segment.  Both motor and sensory nerves are located in the spinal cord.

The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. It lies within the vertebral column, the collection of bones (back bone).

Other Parts of the Central Nervous SystemThe meninges are three layers or membranes that cover the brain and the spinal cord.  The outermost layer is the dura mater.  The middle layer is the arachnoid, and the innermost layer is the pia mater. The meninges offer protection to the brain and the spinal cord by acting as a barrier against bacteria and other microorganisms.

The Cerebrospinal Fluid (CSF) circulates around the brain and spinal cord. It protects and nourishes the brain and spinal cord.

NeuronsThe neuron is the basic unit in the nervous system. It is a specialized conductor cell that receives and transmits electrochemical nerve impulses. A typical neuron has a cell body and long arms that conduct impulses from one body part to another body part.

here are three different parts of the neuron:

the cell body dendrites axon

Cell Body of a NeuronThe cell body is like any other cell with a nucleus or control center.

DendritesThe cell body has several highly branched, thick extensions that appear like cables and are called dendrites.  The exception is a sensory neuron that has a single, long dendrite instead of many dendrites.  Motor neurons have multiple thick dendrites. The dendrite's function is to carry a nerve impulse into the cell body.

AxonAn axon is a long, thin process that carries impulses away from the cell body to another neuron or tissue.  There is usually only one axon per neuron.

Myelin SheathThe neuron is covered with the Myelin Sheath or Schwann Cells. These are white segmented covering around axons and dendrites of many peripheral neurons. The covering is continuous along the axons or dendrites except at the point of termination and at the nodes of Ranvier.

The neurilemma is the layer of Schwann cells with a nucleus. Its function is to allow damaged nerves to regenerate.  Nerves in the brain and spinal cord do not have a neurilemma and, therefore cannot recover when damaged.

Types of NeuronNeurons in the body can be classified according to structure and function. According to structure neurons may be multipolar neurons, bipolar neurons, and unipolar neurons:

Multipolar neurons have one axon and several dendrites. These are common in the brain and spinal cord

Bipolar neurons have one axon and one dendrite.  These are seen in the retina of the eye, the inner ear, and the olfactory (smell) area.

Unipolar neurons have one process extending from the cell body. The one process divides with one part acting as an axon and the other part functioning as dendrite. These are seen in the spinal cord.

The Peripheral Nervous SystemThe Peripheral nervous system is made up of two parts:

Somatic nervous system Autonomic nervous system

Somatic Nervous SystemThe somatic nervous system consists of peripheral nerve fibers that pick up sensory information or sensations from the peripheral or distant organs (those away from the brain like limbs) and carry them to the central nervous system.

These also consist of motor nerve fibers that come out of the brain and take the messages for movement and necessary action to the skeletal muscles. For example, on touching a hot object the sensory nerves carry information about the heat to the brain, which in turn, via the motor nerves, tells the muscles of the hand to withdraw it immediately.

The whole process takes less than a second to happen. The cell body of the neuron that carries the information often lies within the brain or spinal cord and projects directly to a skeletal muscle.

Autonomic Nervous SystemAnother part of the nervous system is the Autonomic Nervous System. It has three parts:

the sympathetic nervous system the parasympathetic nervous system the enteric nervous system

This nervous system controls the nerves of the inner organs of the body on which humans have no conscious control. This includes the heartbeat, digestion, breathing (except conscious breathing) etc.

The nerves of the autonomic nervous system enervate the smooth involuntary muscles of the (internal organs) and glands and cause them to function and secrete their enzymes etc.

The nerves of the autonomic nervous system enervate the smooth involuntary muscles of the (internal organs) and glands and cause them to function and secrete their enzymes etc.

The Enteric nervous system is the third part of the autonomic nervous system. The enteric nervous system is a complex network of nerve fibers that innervate the organs within the abdomen like the gastrointestinal tract, pancreas, gall bladder etc. It contains nearly 100 million nerves.

Neurons in the Peripheral Nervous SystemThe smallest worker in the nervous system is the neuron. For each of the chain of impulses there is one preganglionic neuron, or one before the cell body or ganglion, that is like a central controlling body for numerous neurons going out peripherally.

The preganglionic neuron is located in either the brain or the spinal cord. In the autonomic nervous system this preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ.

In the somatic nervous system there is only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.

Human Eye: anatomy, structure and function

The eye is a sensory organ. It absorbs light rays from our environment and transforms them in such a way that the information in the brain can be processed further. The eye and brain form a unit that has developed together in the course of evolution (visual system). The process of processing is called "seeing","watching" or "looking". The visual impression is essentially generated from visual memory, in which only little new information from the eye is incorporated.

Human eye anatomy

Parts of the eye, structureThe human eye is a spherical body, also known as the eyeball. It lies in the eye socket and is "attached"to various muscles. With the help of the eye muscles, the eyeball can be turned in different directions ("look out of the eye wraps").

Cornea

The front side of the eye is the cornea. The cornea is transparent and consists of six layers. The ring-shaped transition from the cornea to the sclera is called limbus (from Latin for "border"). With the help of the stem cells located there, the cornea is permanently renewed. It is slightly thinner in the centre than the outer areas. This is especially important in the case of eye lasers, when a part of the cornea is removed in order to optimize the refractive power. The curvature of the cornea refracts the light by about 45 dioptres. The cornea is covered with tear fluid, which is formed in the tear glands and serves to supply and protect the eye.

Anterior and posterior chamber, intraocular fluid

Behind the cornea is the anterior eye chamber, which is filled with intraocular fluid. It contains nutrients to supplie the eye lens and cornea. In addition, immune factors are floating in intraocular fluid, which serve to render potentially damaging foreign bodies and germs harmless. It also helps to maintain a constant intraocular pressure. Behind the iris the posterior chamber of the eye begins. The intraocular fluid is produced and released at the ciliary processes. It then slowly flows through the pupil into the anterior chamber of the eye.

Iris

The iris is located in the center of the cornea. It consists of many fine muscle pathways that can contract or expand. The resulting round opening in the center is called the pupil. The darker it is, the more light is needed for vision - the pupil becomes correspondingly larger in darkness. In very bright light, the pupil is only small. The

iris is (except for so-called albinos) coloured by certain pigments (blue, brown, green, grey or corresponding mixed values).

Lens

Behind the pupil is the eye lens (Phakos). It is responsible for about 15 dioptres of the refractive power, but it can change its refractive power. Thanks to this ability (so-called accommodation), the eye can see sharply both near and far. The eye lens is a kind of liquid sphere. They can be compared to a water-filled balloon. In the middle - on the so-called lens equator - the lens is hung up on the zonula fibres, which originate from the ciliary muscle. The lens fluid solidifies over the decades. It is thus the cause of widespread presbyopia. In addition, protein structures in the lens of the eye increasingly clump together over time, clouding the image. This deterioration in visual quality is also known as cataract. Often, the old lens is removed during cataract surgery and an artificial lens is simply inserted - which ensures a clear view even in old age.

Ciliary muscle

The ciliary muscle is located behind the cornea in a ring-shaped shape inside the eye. It can actively influence the curvature of the eye lens. In a relaxed state, the lens is flat and drawn out - so you can see well in the distance. However, if the ciliary muscle tightens (contract tone), the diameter of the ring becomes smaller. The zonula fibres relax and the lens takes on a rather bulbous, spherical shape. This changes the refractive power of the lens so that you can see well in the vicinity. This process is called accommodation.

Vitreous chamber

The inner space of the eyeball is filled by the vitreous body. It consists of a gel-like clear liquid and is especially important for the stability of the eyeball: the liquid generates a pressure, the so-called intraocular pressure. This ensures that the surrounding layers do not peel off and collapse. In addition, without intraocular pressure, the eye would be much more sensitive to external pressure influences that affect the cornea from the outside. An abnormal intraocular pressure is the cause of many eye diseases, such as glaucoma.

The dioptric apparatusAll components through which the rays of light pass before they hit the retina have a dioptric effect. They are also called dioptric apparatus. The aqueous humor and the vitreous body are ideally almost transparent, so that there is hardly any measurable refraction of individual photons. The cornea and eye lens are responsible for the actual dioptric effect. The entire eye has a refractive power of approx. 59 dioptres (dpt), of which approx. 43 dpt (75%) is on the cornea and approx. 19 dpt (25%) on the eye lens (in a relaxed, non-accommodated state).

Sclera

The eyeball is surrounded by three layers. The outer shell is called sclera. The dermis has a whitish colour - in the front, open part of the eye you can see it well. It encloses the eyeball almost completely and protects the eye. The dermis is interrupted only in two places: in the front by the circular, transparent cornea (Cornea) and in the back of the eye by the optic nerve coming from the inside of the eye.

Choroid

Inside the protective sclera follows the choroid, which, as the name suggests, is permeated by numerous blood vessels and capillaries. The blood supplies the retina with nutrients and oxygen. The choroid is dark pigmented and ensures that unprocessed light is absorbed (instead of being reflected into the inside of the eye). The effect of the "red eyes on photos" is related to this: the flash is so intense that it brightens up the inner eye. The red blood vessels of the choroid are then visible in the photo.

Retina

The retina is located on the back / inside of the eye. It consists of different cell layers: the photoreceptors (sticks for chiaroscopy, cones for color vision) convert the light impulse into an electrical nerve impulse. The light information is bundled in so-called receptive fields, amplified and transmitted to the brain via the visual verve (visual pathway).The actual "visual process" then takes place on the retina. The retina consists of a number of different cell types with very different tasks. First of all, the so-called sensory cells are important. They transform the light into an electrical impulse. How this works is explained on the page "How does seeing work?" described. There are two types of vision cells:

the chopsticks (light-dark vision, active in twilight or darkness) the cones (responsible for colour vision)

Three different cone cells are required for color vision:

Pins for red-visibility (approx. 46 % of all pins)

Cones for green vision (approx. 46% of all cones) Cones for blue vision (approx. 8% of all cones)

The three cone types each react to light of different wavelengths. If a photon with a wavelength in the red region hits a red cone, then it "fires" an impulse at the following cells. The other two cone types remain inactive for a "red photon" (at least statistically seen). They react accordingly when photons arrive with their specific wavelength.If one of these cone types is not properly formed due to a genetic defect, there is a color vision impairment or color blindness. See red-green weakness. The genetic abnormalities of the eye's colour are:

Protanomaly: Red vision weakness Protanopia: Red blindness Deuteranomaly: Green vision weakness Deuteranopia: Green blindness Tritanopia: Blue blindness Tritanomaly: Blauseh weakness

Further processing on the retinaThe retina consists of a large number of other cells that process the electrical impulses sent by the visual cells. The visual information from adjacent regions is bundled, compared and enhanced in contrast. Roughly speaking, it can be said that only "new" and "relevant" information from the "image" is passed on to the brain. Many interesting optical illusions are based on the processing of visual information in the retina. This "filtering" of information is very effective and economical. Evolution has developed the eye in such a way that it consumes as little energy as possible. Logically, you don't have to see everything to survive, but only what is important.

Stereoscopic visionThe pre-structured optical information is then transmitted to the brain via the optic nerve. The information is collected from the left and right eye and forwarded together. This is where the so-called "stereoscopic vision"is created. The information from the left and right eye is slightly different, since the angle of incidence is slightly different. From this difference, the brain can infer something like space. The spatial visual impression is thus created from the different information of the two eyes. Correctly, however, one has to say that the learned information such as perspective, proportions

and so on have a greater share of the three-dimensional visual impression than stereoscopic vision.Finally, the information reaches the brain via the optic nerve - and here it is distributed over large areas that are stored to varying degrees (via linked synapses of the individual nerve cells). Ultimately, this "neural pattern" is what we know as a visual image of reality. This pattern has been constantly developed and modified since the first day of opening the eyes. It is a mixture of visual memory and new visual impressions. Seeing is a living, dynamic process...

The world upside downInteresting: the image is turned over by the lens or mirrored point-shaped. At the end, it is displayed on the head and inverted on the retina. But of course there is no need for "translation": the information on the retina is naturally processed and interpreted by the brain in such a way that we recognize the image of the world "correctly around".

The yellow spot: Makula and Fovea

The area where the light information is concentrated on the retina is the area of "sharpest vision". It is called macula (yellow spot). Here, the photoreceptors are particularly densely packed. Practically all refractive errors (short-sightedness, long-sightedness, astigmatism and presbyopia) are based on the fact that the bundled light rays are not focused exactly on the macula. But: All these defective vision problems can be compensated by visual aids, i. e. optically correcting lenses.

Blind spot

The nerve endings of the approximately 1 million ganglion cells must leave the retina in one place and lead as an optic nerve into the brain. This position is slightly below the macula and slightly inward (towards the nose). It is called a blind spot because there are no photoreceptors in the area - therefore, everything projected into the region cannot be seen. Everything depicted in this area is "invisible". Thus, in a small part of the field of vision, one is virtually blind.

Anatomy and Physiology of the Ear

Click Image to Enlarge

The ear is the organ of hearing and balance. The parts of the ear include:

External or outer ear, consisting of:o Pinna or auricle. This is the outside part of the ear.

o External auditory canal or tube. This is the tube that connects the outer ear to the inside or middle ear.

Tympanic membrane (eardrum). The tympanic membrane divides the external ear from the middle ear.

Middle ear (tympanic cavity), consisting of:o Ossicles. Three small bones that are connected and transmit the sound waves to

the inner ear. The bones are called:

Malleus

Incus

Stapeso Eustachian tube. A canal that links the middle ear with the back of the nose. The

eustachian tube helps to equalize the pressure in the middle ear. Equalized pressure

is needed for the proper transfer of sound waves. The eustachian tube is lined with mucous, just like the inside of the nose and throat.

Inner ear, consisting of:o Cochlea. This contains the nerves for hearing.

o Vestibule. This contains receptors for balance.

o Semicircular canals. This contains receptors for balance.

Hearing starts with the outer ear. When a sound is made outside the outer ear, the sound waves, or vibrations, travel down the external auditory canal and strike the eardrum (tympanic membrane). The eardrum vibrates. The vibrations are then passed to 3 tiny bones in the middle ear called the ossicles. The ossicles amplify the sound. They send the sound waves to the inner ear and into the fluid-filled hearing organ (cochlea).

Once the sound waves reach the inner ear, they are converted into electrical impulses. The auditory nerve sends these impulses to the brain. The brain then translates these electrical impulses as sound.