Posts Tagged ‘ fish ’

Comparative Nitrogen Excretion

Introduction

In animals all waste products must be excreted from the body in some manner. The urinary system is involved in the excretion of nitrogenous waste products such as urea, uric acid or ammonia. The excretion of nitrogenous waste from animals is important because if it is allowed to build up, it can prove toxic. We obtain nitrogen from protein in the diet, when protein is metabolised it produces amino groups which are able to form the highly toxic ammonia. Different classes and species of animals deal with toxic ammonia in different ways. A summary:

Compound Chemical Formula Toxicity Water Solubility Animals
Ammonia NH3 High High Aquatic Animals
Urea (NH2)2CO Moderate Moderate Terrestrial Animals
Uric Acid C5H4N4O3 Low Low Reptiles and Birds

Method of Excretion by Species:

Fish

Fish excrete nitrogenous waste as ammonia; this is unusual because ammonia is highly toxic therefore storage in the body can pose a risk. However, fish are able to cope due to their environment – the large volumes of water they reside in allow them to continuously excrete ammonia (without the need for storage) directly into the water, diluting the ammonia to non-toxic levels.

Fish deaminate (remove the amine group) amino acids (obtained in the diet by protein consumption) in the gills. The pure ammonia diffuses into the respiratory water which is leaving via the gills; it is therefore excreted at the site of production with minimal time in the body to reduce toxicity.

There are two extremes of environment which fish have to adapt to however – marine and freshwater. The difference in concentration of salt between these two environments has led to slightly different methods of dealing with nitrogenous waste. Osmoregulation of fish living in fresh water requires them to excrete large volumes of dilute urine. This is due to osmosis, which allows large volume of water to enter body fluid from the surrounding hypotonic freshwater.

The converse is true for marine fish, the hypertonic water means water leaves bodily fluid by passive osmosis, in an attempt to conserve water small amounts of concentrated urine are released.

Elasmobranches, such as sharks, primarily live in seawater and are able to produce urea as well as ammonia. This is important for osmoregulation, as said earlier the hypertonic seawater means water is able to leave bodily fluids by osmosis. By producing urea (which is less toxic than ammonia and therefore storable), elasmobranches are able to increase their osmolarity higher than the seawater and therefore take up water from the sea like freshwater species. This is due to the urea being retained in the blood and therefore increasing the osmolarity of the blood.

Fish still have kidneys however, despite nitrogen excretion being handled by the gills. Instead the kidneys are involved in the excretion of excess water and divalent ions, e.g. Mg2+ and SO42-.

Lungfish, which are able to live out of water, have another adaption. They excrete ammonia in the usual way when in the water, but during times of drought when the lungfish buries itself in the mud the production of toxic ammonia could prove fatal. So instead, when buried in the mud, the lungfish produces urea which can be accumulated and excreted into the environment at less toxic levels.

Amphibians

As with fish the type of nitrogenous excretion depends entirely on environment. Typically tadpoles use the same method as fish – ammonia excretion via the gills. However adult amphibians produce urea as they do not remain constantly in the water. The urea is stored, diluted in the bladder, but as amphibians may be prone to desiccation they have adapted the ability to reabsorb water from the urine when needed. This requires them to have relatively large bladders for storage.

Amphibians do not drink to obtain water; instead they are able to absorb water through a region of the skin called the ventral pelvic area.

Birds

All birds excrete a paste like substance called uric acid (which contains the waste nitrogen) independent of habitat. It initially begins as watery urine which travels down the ureters, from the kidney to the cloaca. The cloaca is a unified exit for the gastrointestinal, urinary and reproductive systems.

In the cloaca, the urine mixes with faecal matter from the digestive tract and as much water is removed as possible to produce a dry, crystalline paste containing uric acid. A small amount of watery urine may be excreted as well.

Reptiles

The method of nitrogen excretion depends on the habitat; aquatic reptiles (e.g. turtles) will mainly excrete ammonia (possibly urea) as with fish. Whilst reptiles living in drier conditions (e.g. lizards, snakes & tortoises) excrete uric acid – the low toxicity, crystalline paste. This helps to conserve water and during development in the egg, a build-up is not fatal. Uric acid, as with birds, is excreted from the cloaca.

The kidneys of reptiles do not have loops of Henle and so they are unable to produce concentrated urine also, whilst a large bladder is present in chelonians (turtles) snakes and some lizards do not have a bladder at all.

Mammals

All mammals produce primarily urea (sometime ammonia) which is excreted in urine. Mammals are able to osmoregulate and maximise water conservation by varying the concentration of urine depending on the hydration of the body. Mammals living in environments with plentiful access to fresh water will excrete large amounts of dilute urine. Mammals living in dry or marine environments will excrete small amounts of concentrated urine.

Osmoregulation in mammals is mainly controlled by the hormone ADH, its release (when a mammal is dehydrated) will result in the reduction of water loss and increased reabsorption of water in the kidneys by the loop of Henle.

Osmoregulation

Osmoregulation has been mentioned above in some cases, it can be associated with the excretion of nitrogenous waste because without correct osmoregulation, levels of ammonia can build within the body. A well osmoregulated animal will be taking on water and balancing ions well enough to ensure that nitrogen can be excreted and not build up.

The primary problem facing osmoregulation is the build-up of salt ions, this can occur with animals living in marine habitats.

  • Fish can actively excrete NaCl via gills
  • Some cartilaginous fish such as sharks have specialised salt-excreting glands in their rectum
  • Birds and reptiles have specialised salt-excreting glands on their heads
  • Marine mammals are able to excrete salt in urine efficiently
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Comparative Digestion

Introduction

The method of digestion which an animal uses depends on its diet i.e. carnivore, herbivore or omnivore. For example not all mammals are carnivores (e.g. dogs and cats), herbivorous mammals include rabbits, chinchillas, guinea pigs etc.

Generally, carnivores consume animal tissue which is similar to their own; therefore all the body needs to do is break down the tissue and absorb the different components which can then be used in the carnivores own body. Omnivores have very similar digestive systems to carnivores with the addition of a caecum.

Herbivores consume plant matter which is more difficult to break down than tissue. Therefore herbivores have evolved fermentation systems which contain specific microflora, the microflora breakdown the plant material releasing useful nutrients which the herbivore utilises.

Digestion by Diet:

Carnivores

Because meat is easily digested, the gastric system of carnivores is typically short and simple. They are monogastric meaning they have only one stomach (unlike a ruminants’ stomach which has four chambers). Due to the ease at which components required for growth are obtained from food, some carnivores have lost the ability to synthesis them (e.g. cats are unable to synthesis taurine).

The teeth of carnivores are sharp and strong, this makes it easy to rip and tear meat from bones of prey. When possible, the meat is broken down further by the teeth to ensure maximum surface area for digestion by enzymes in the stomach and small intestines. True carnivores do not have digestive enzymes in their saliva.

Due to the lack of salivary enzymes, food spends little time in the mouth of a carnivore, it is shortly swallowed and travels down the oesophagus. The oesophagus is a tube which runs from the pharynx (back of the oral cavity) to the stomach. The walls of the oesophagus are protected from damage by food by stratified squamous epithelium arranged in longitudinal folds, this also allows for expansion as the food travels down to the stomach. Food is passed down the oesophagus by peristalsis which is the contraction and relaxation of longitudinal and circular muscles, pushing food down to the stomach in wave like motion.

The next stop is the stomach, the stomach has multiple roles in digestion, including:

  • A reservoir for food
  • A sterilising chamber, due to the low pH (high acid content – HCL)
  • A churning chamber to mix food with digestive gastric juices a
  • The initial site of protein digestion, primarily by pepsin – secreted by the epithelial lining of the stomach

Food is moved to the next site of digestion, the small intestine, by peristalsis. The small intestine is a long and narrow ‘tube’ with a structure and epithelium that maximises surface area. This is important because the small intestine is the primary site of digestion by enzymes. Food continues to travel along the small intestine by peristalsis. The small intestine can be divided into the duodenum, jejunum and the ileum. The pancreatic duct connects the pancreas to the duodenum – the majority of the digestive enzymes enter the small intestine by this duct. To aid in lipid digestion, bile is secreted by the liver (stored in the gallbladder). Bile emulsifies lipids which gives them a larger surface area, increasing enzyme efficiency.

The small intestine joins to the large intestine, which consists of the caecum, colon and rectum. In carnivores the caecum has no function (as it is used in herbivores/omnivores as a site of bacterial fermentation of plant matter). The colon absorbs minimal nutrients from the ingested food; instead its primary role is the reabsorption of water, vitamins and electrolytes from the mixture of food, saliva and gastric & pancreatic juices passing through. This prevents excessive water loss and therefore dehydration. The remnants are excreted via the rectum and anal sphincters.

Herbivores

Herbivores only consume plant material which is very difficult to digest. No vertebrates make an enzyme capable of breaking down cellulose, the tough sugar that makes up plant cell walls which is unfortunate as its digestion yields glucose. As the diet includes large amounts of fibre the digestive tract of herbivores is comparatively much longer than carnivores, due to fibre being much more difficult to digest.

To overcome this herbivores have developed a symbiotic relationship with a population of microflora that inhabit a specialised region of the gut for fermentation e.g. the caecum or rumen of ruminants. The microflora population of the gut is able to breakdown cellulose and use the glucose for its own metabolic needs. As a waste product of this process, the microflora population releases volatile fatty acids (e.g. acetate, butyrate & propionate) which the herbivore utilises for energy. The production of these fatty acids is known as fermentation (fermentation also produces heat which keeps the animal warm).

There are two types of fermenting herbivores, those which ferment in the foregut and those which ferment in the hindgut. The difference between them is the site of fermentation and the organ used for fermentation; the attributes of the fermentation chamber remain the same however – Anaerobic, plenty of fluid, regulated pH, steady nitrogen supply and the correct temperature.

  • Foregut Fermentation – The majority of foregut fermenters are ruminants (including cow, sheep, goat, ox and deer) who ferment their food before it reaches the ‘true’ stomach. The stomach of a ruminant exists as four chambers which are the rumen, reticulum, omasum and abomasum (true stomach). Non-ruminant foregut fermenters (e.g. camels, llamas and whales) do not have the four distinct chambers; instead they simply have modifications to the gut before the true stomach which allows them to ferment. Ruminants digest food more efficiently than hindgut fermenters as they are able to consume food into the rumen – the site of fermentation, allow microbial digestion and then regurgitate the ‘cud’ and chew it some more. This means by the time the ingested food reaches the abomasum, all the extractable nutrients have been metabolised (some microflora from the rumen may also be digested in the abomasum which increases nutrient intake).
  • Hindgut Fermentation – Hindgut fermenters (e.g. e.g. elephant, horse, guinea pig, rabbit, herbivorous reptiles, e.g. tortoise and herbivorous birds) have a digestive system very similar to carnivores, except due to the large amounts of fibre and other difficult-to-digest components of the diet, the complete digestive tract is much longer. Hindgut fermenters also have a working, enlarged caecum which is the site of bacterial fermentation. The process of fermentation is the same as that of foregut fermenters, however as the caecum is located after the stomach and small intestine, the majority of food reaches the caecum undigested. Bacterial fermentation occurs in the caecum and colon allowing some volatile fatty acids to be absorbed, but then the digested food is excreted (along with the microflora). This is why some hindgut fermenters are seen eating their faeces – the food making up the faeces has been digested by the microflora making it of nutritional value. The ingestion of the faeces allows the restoration of the microflora population.

The foregut fermenter herbivores are a lot more efficient as the food is digested on the first pass through the digestive system. Unfortunately for hindgut fermenters digestion is more difficult; however they do have the ability to expel their microflora population which is useful during times such as hibernation.

Omnivores

Omnivores consume both meat and plant matter; they have a digestive system very similar to carnivores but also have a working caecum (not as well adapted as in herbivores). Due to this flexibility they are able to consume a wide diet, which has also prevented them losing the ability to synthesise certain products in the body (as in carnivores).

The process of digestion is extremely similar to carnivores, except a few minor adaptations which allow them to digested plant matter – although not as efficiently as herbivores.

Digestion by Species:

Many species have digestive systems very similar to those shown above; however there may be slight tweaks to the systems between the species, below are some examples.

Birds

Birds do not have teeth and so cannot chew; they are able to break up food however by using their beak. Only some species of bird (e.g. sparrow) are able to produce saliva with the amylase enzyme (for digestion of carbohydrates prior to the stomach).

When a bird swallows food, it passes down the oesophagus into a structure called the crop. The crop is primarily a storage area for food consumed by the bird, differing in size between species however certain adaptions in some species allow it to produce ‘crop milk’ which is rich in protein and fat and fed to the young during their first few days of life. Another adaptation found in one species so far, is that the crop acts as a foregut fermentation chamber.

The stomach of a bird exists as two parts, the proventriculus and the gizzard (ventriculus). As the bird is unable to chew, powerful muscles in the gizzard allow it to grind food up, the presence of grit in the gizzard aids this process.

From this point, the digestive system is similar to other species – small intestine, large intestine, etc.  until it reaches the end of the tract. At the end of the large intestine, the digestive tract opens into the cloaca which is simply a common exit, shared with the urinary and reproductive tracts. Shortly before the cloaca there is a pair of caecum or caeca, unlike the single caecum found in other species. (The caeca are rudimentary or absent in species such as hawks and parrots).

Rabbits

Rabbits are herbivorous hindgut fermenters able to rapidly pass food through their digestive system and quickly eliminate fibre. Due to this ability, he rabbit has remained small and agile, able to quickly escape predators.

Rabbits have a very typical herbivorous digestive system, the only differences are they are unable to vomit and have a large caecum.

One variation, which rabbits are well known for is that they consume their own faeces – coprophagia. This is due to them being hindgut fermenters and losing a fair amount of nutrients and vitamins in the faeces. The rabbit produces two forms of faeces:

  1. Hard, fibrous –No nutritional value
  2. Soft, caecotrophs – High protein content as well as vitamins B & K and volatile fatty acids. The caecotrophs are not digested/damaged in the stomach acid as a layer of mucus surrounds them and protects them, this allows all the nutrients and vitamins to be absorbed in the small intestine

Myomorphs (Rodents)

Myomorphs are omnivorous and have a very typical digestive system. Due to a structure between the oesophagus and cardiac region of the stomach however, it is almost impossible for them to regurgitate food.

Most of the rodents in this group lack a caecum or similar specific organ involved in the fermentation of cellulose, hamster however do have a foregut – similar to ruminants, which has a high pH and a large microflora population.

All myomorphs, like rabbits show some degree of coprophagia for the same reasons – to consume the vitamins and nutrients lost in the faecal pellets.

Sciuromorphs (Chipmunks)

Also an omnivore and has a very similar digestive system to myomorphs.

Hystricomorph (Guinea Pigs/Chinchilla)

The guinea pig is an herbivorous hindgut fermenter, which a large caecum for its body size – containing up to 65% of the total contents of the digestive tract at one time. As will myomorphs, guinea pigs exhibit coprophagia.

Chinchillas are very similar to guinea pigs, they have evolved however to survive on the nutritionally poor yet highly fibrous grasses of the Andes, this means over indulgence of highly nutritious treats (captive kept chinchillas) can cause fatal constipation or diarrhoea.

Chelonians (Turtles, Tortoises & Terrapins)

Chelonians lack teeth so have to use their horny, beak like structure to cut up food. The small intestine of chelonians is relatively short when compared to mammals. Like birds, reptiles have a common exit from the body – the cloaca (urinary, digestive and reproductive systems exit from here).

Snakes

Snakes are carnivorous and possess many teeth which are regularly replaced. Due to the shape of the snake (elongated) the digestive system remains the same, but all the organs are also elongated appropriately.

Lizards

The diets of lizards vary greatly, so the digestive system adapts accordingly, from herbivorous to insectivorous. Variations occur in the efficiency of the caecum (herbivorous/omnivorous lizards).

Fish

Predatory, carnivorous fish have ‘throat teeth’ located just before their oesophagus used for catching and holding prey. Structure of the digestive system are more tube shaped than in other species and can vary in length greatly, depending on the diet of the fish.

Special Senses – Taste

Introduction

The tongue is the organ of taste. Perception of taste is also known as gustation and in a similar manner to smell it depends on chemoreceptors to detect specific chemicals from the environment. Specialised epithelial cells (Taste cells) detect these chemicals (tastants). In aquatic animals there is no distinction between olfaction and gustation. Insects use sensory hairs on their feet to taste. Taste probably evolved as a means for animals to detect whether what they were about to consume should be ingested or rejected.

In mammals, taste cells are clustered into taste buds which are scattered around the tongue and mouth. These taste buds are located very slightly beneath the surface of the tongue, a taste pore allows tastants to access the taste bud.

Taste buds are further localised onto raised clusters visible on the tongue, these are known as papillae. Differently shaped papillae are located at different sites on the tongue.

Taste Transduction

Mammals are able to detect the four conventional tastes (sweet, sour, bitter & salty) as well as ‘umami’, which is considered a savoury taste.

The sensory receptor cells (taste cells) which make up the taste buds contain smaller receptors in their plasma membranes. These receptors are specific to one of the above tastes.

When a tastant arrives at one of these receptors, it binds to it, resulting in a secondary messenger cascade. The receptor is coupled with a G-protein and the binding of a tastant activates the G-protein, beginning the secondary messenger process within the sensory receptor cell.

The G-protein eventually activates phospholipase C which in turn generates a secondary messenger known as IP3. IP3’s role in this secondary messenger system is to open IP3 gated calcium channels on the endoplasmic reticulum of the cell.

Opening of these channels causes Ca2+ to leave the endoplasmic reticulum and enter into the cytoplasm of the cell. Ca2+ also acts as a secondary messenger; it causes sodium channels to open allowing an influx of Na+ ions.

The sudden influx of Na+ ions causes the sensory receptor cell (taste cell) to depolarise, this depolarisation results in the local release of neurotransmitters (as taste cells do not have axons). This excites local sensory neurones which generate action potentials in response.

The axons of these neurones bundle together to form part of the facial nerve (VII), the glossopharyngeal nerve (IX) or the vagus nerve (X). Which one depends on the origin of the axon, anterior taste is carried by the facial nerve (VII) and posterior taste is carried by the glossopharyngeal nerve (IX). The throat/ epiglottis are dealt with by the vagus nerve (X).

All three cranial nerves eventually reach the gustatory nucleus of the ‘nucleus of the solitary tract’ in the medulla. From here the information can be sent to one of three places, each of which solicits a different response:

  • The gustatory cortex (via the thalamus) – Here taste is integrated and perceived as a taste.
  • The brainstem – From here reflexes associated with taste may be activated e.g. salivation
  • The Limbic system (especially the hypothalamus) – Emotional responses to taste (e.g. pleasure) can be elicited from here.

Special Senses – Hearing

Introduction

The ear is the organ of hearing (and balance). Specially adapted receptor cells located in the inner ear are able to react to sound waves. The ear can be divided into three main parts, these are:

  • External ear
  • Middle ear
  • Inner ear

The external ear can be further broken down into:

  • The pinna – This is the funnel shaped cartilage on the external surface of the ear, it helps to guide sound waves into the ear drum. It leads directly to the external auditory canal/ external auditory meatus.
  • The external auditory meatus – Acts as a canal for the sound waves, carrying them to the tympanic membrane (ear drum).

The middle ear can be further broken down into:

  • The tympanic cavity – An air filled cavity, lined with a ciliated columnar mucous membrane which opens out into the Eustachian tube.
  • The tympanic membrane – Commonly known as the ear drum, they provide a boundary between the external and middle ear. The membrane is thin which allows it to convey sound waves (vibrations) to the auditory ossicles.
  • The auditory ossicles – These are the three small, flexible bones in the middle ear which are responsible for transmitting sound waves to the inner ear. The individual bones are named below (including common names):
    • Malleus (hammer)
    • Incus (anvil)
    • Stapes (stirrup)

The inner ear can be further broken down into:

  • The bony labyrinth – Consisting of the bony cochlea, bony vestibule and the bony semicircular canals. They contain perilymph which flows around the outside of the membranous labyrinth.
    • Oval Window – A thin membrane which links the bony labyrinth to the middle ear, the stapes transmits vibrations to the oval window, these vibrations are carried to the cochlea.
    • Round Window – Allows the release of pressure formed by sound waves transmitted to the cochlea by the oval window. Also dissipates sound waves, to prevent the prolonged sensation of a sound due to sound waves reverberating in the ear.
    • The membranous labyrinth – A system of interlinking tubes, containing endolymph. Specialised sensory receptors designed to respond to sound waves lie within. The  membranous labyrinth also consists of three parts:
      • Membranous cochlea – A spiral shaped tube filled with endolymph that contains sensory receptor cells which detect sound. At the end of each receptor is a sensory hair, the nerve fibres from these hairs go on to contribute towards the vestibulocochlear nerve (VII).
      • Membranous vestibule – Connects with the cochlea and semicircular canals, it contains specialised sensory receptors involved in balance when still.
      • Membranous semicircular canals – Three endolymph filled canals containing sensory receptors involved in balance when moving.

The Cochlea

The cochlea (the spiral, snail shaped structure of the inner ear) is responsible for detecting sound and allowing it to be perceived with volume and frequency. The cochlea has two large perilymph filled canals, an upper and a lower canal, separated by a small endolymph filled cochlea duct which runs almost the entire length of the cochlea. The upper canal is known as the vestibular canal and the lower canal is known as the tympanic canal.

The floor of the endolymph filled cochlea duct (basilar membrane) houses the organ of Corti. This organ contains hair cells which project into the cochlea duct. The hair cells act as mechanoreceptors and detect sound wave vibrations. The hairs are able to do this because sound waves entering the cochlea cause the basilar membrane of the cochlea duct to vibrate. As a result, the hairs bend and because they are mechanoreceptors, they are able to initiate action potentials by depolarisation.

Each hair cell contains a bundle of ‘hairs’ which project from its surface. Vibration of the basilar membrane moves the hairs, causing them to bend against the surrounding fluid and the tectorial membrane. The tectorial membrane is a membrane which lies just above the hairs of the hair cell and remains fixed in that position.

The process of perceiving sound is as follows; sound waves which have entered the ear eventually reach the stapes. The stapes vibrates on the oval window which produces a pressure wave in the perilymph of the upper vestibular canal. This pressure wave travels along the vestibular canal to the ‘apex’ (tip) of the cochlea duct. The cochlea duct ends just before the apex, allowing the pressure wave to travel back the way it came to the round window except by means of the tympanic (lower) canal. The energy of these pressure waves causes the basilar membrane of the cochlea duct to vibrate, stimulating the hair cells.

Species Difference

  • Fish & Other Aquatic Animals
    • Fish have no outer ears, only inner ears with no direct fluid connection to their environment. This is due to the easier detection of sound pressure waves in a water environment.
    • Instead, fish have neuromast receptors running along their lateral lines each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula.
    • The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, suggesting that the lateral line and the inner ear share a common origin.
    • Amphibians/ Reptiles
      • Amphibians/ reptiles have a more familiar hearing system when compared to mammals, although again they have no outer ear they do have the middle and inner ear.
      • They have an exposed tympanic membrane, from here the pathway continues very similar to mammals, via the small bones into fluid filled channels in the inner ear.

Perception of Frequency

The ear is able to distinguish between different frequencies of sound. Similar to wavelengths of electromagnetic scale (e.g. visible light), sound waves also have different variations in frequencies. In terms of audible sounds, the lowest (infra) sound detectable is 10Hz (1Hz being one vibration a second). This is an infrasound meaning it is not detectable by humans. Elephants however are able to hear sounds at this very low frequency. On the opposite end of the scale (ultrasound), bats are able to hear sounds with as high a frequency as 100kHz. To put things in to perspective, a humans hearing range is 20Hz – 20kHz.

The ear is able to detect these different frequencies due to an adaptation by the cochlea. The basilar membrane of the cochlea duct varies in stiffness and thickness. The result is that different frequencies of pressure waves cause different parts of the basilar membrane to vibrate; this therefore stimulates different hair cells. High pitch sounds (e.g. 18kHz) are detected near the stiff, thin base of the basilar membrane (near the oval window) and low pitch frequencies (e.g. 1kHz) are detected near the loose, thick end of the basilar membrane (near the apex of the cochlea duct).

Because of the way the basilar membrane vibrates, nerves which transmit the raw sound wave information to the brain each have their own characteristic frequency – the frequency at which they generate action potentials most frequently. These afferent nerves are located along the length of the cochlea – nerves located near the apex will have a characteristic frequency (the frequency at which action potentials are generated most readily) which is low (e.g. 1kHz) whereas nerves located near the oval window will have much higher characteristic frequencies.

There are two problems with this however. It is difficult for the basilar membrane to vibrate at very low frequencies e.g. 10Hz, if the basilar membrane is not vibrating then the hairs do not detect movement and the nerves are not stimulated – no sound. On the opposite end of the scale, very high frequencies cause vibrations so fast the neurones are not able to generate action potentials fast enough to keep up.

A process called phase-locking helps to detect the lower frequencies. This is where groups of neurones instead of generating action potentials as normal, they generate them at the frequency of the sound wave. For example, a 60Hz sound wave will cause a group of neurones to fire 60 times in a second.

At higher frequencies, when the nerves simply cannot fire fast to phase lock (e.g. 100kHz) the brain is able to simply detect that the very thinnest part of the basilar membrane is vibrating and interpret this as a sound. This is known as tonotopy.

Perception of Volume

Volume is also determined by the hair cell mechanoreceptors. In terms of waves, a louder sound has greater amplitude (height) but the same frequency (cycles per second). This is mimicked along the basilar membrane of the cochlea, louder sounds cause greater vibration of the membrane – making it rise and fall with greater height.

The greater displacement of the membrane causes the hairs of the hair cells to brush against the tectorial membrane more vigorously. This generates more action potentials in the sensory neurones as the brain perceives a louder sound.

Transduction of Sound Wave Signals

As you know, sound waves are ultimately detected by the hairs of the mechanoreceptor hair cells. The bending of the hairs must be converted into the generation of action potentials by sensory neurones which synapse with the hair cells before the information can be processed in the brain.

When a sound wave causes a certain part of the basilar membrane to vibrate (location depending on its frequency) the hairs bend against the tectorial membrane. As they bend mechanoreceptors cause ion channels in the hair cells to open/close depending on which direction they bend.

The initial direction of bending (stimulus applied) opens the ion channels causing an influx of ions and depolarisation of the mechanoreceptor. The depolarisation causes an increase in the amount of neurotransmitter released across the synapse with sensory neurone. This in turn increases the frequency at which the sensory neurones generate action potentials.

As the hairs bend the other way (stimulus removed), the ion channels close. The closure of the ion channels causes the hair cell to hyperpolarise. This reduces the amount of neurotransmitter released by the cell and the synapsing sensory neurone reduces the frequency at which action potentials are generated.

Auditory Pathway

The auditory pathway shows how sound waves are eventually processed by the brain. So far we have discussed from the sound entering the cochlea (via the external auditory canal) to hair cells of the organ of Corti.

From the hair cells, the majority of information travels along large, fast, myelinated nerves. The axons of these nerves join together and form the auditory nerve (part of the vestibulocochlear nerve [VII]).

The auditory nerve initially travels through the cochlear nuclei; one nucleus located both dorsally and laterally. Information from either ear remains separate at this stage.

The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the inferior/caudal colliculus of the midbrain.

The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. It is here that sound location data from different areas becomes integrated, allowing sound direction/ distance to be interpreted.

Two ways in which sound location is distinguished include:

  • Time delay – If a sound arrives at one ear first, this is recognised in the inferior colliculus.
  • Volume Differences – Subtle differences in the volume of a sound at each ear also help to place a sound.

The Medial Geniculate Nucleus is part of the auditory thalamus and is what finally connects the inferior colliculus to the auditory cortex. It is involved in the tonotopic organisation of sound information, as well as the detection of relative intensity and duration of a sound.

The primary auditory cortex is the region of the brain that is responsible for processing of auditory information. It is located on the temporal lobe, and performs the basics of hearing; pitch and volume.

The auditory cortex is involved in tasks such as identifying and segregating auditory “objects” and identifying the location of a sound in space.

Comparative Animal Respiration

Introduction

Respiration is the process of obtaining sufficient oxygen from an organism’s environment to support its cellular metabolic requirements. Respiration is not just the mechanical process of breathing – the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction. It is also the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and ATP (energy).

In a typical scenario, organisms find greater difficulty obtaining oxygen (O2) than dealing with the waste gas product of respiration – carbon dioxide (CO2). This is because CO2 is more readily dissolved in plasma and so can be excreted faster than oxygen is obtained.

For oxygen to enter body fluid/plasma it must first dissolve in a fluid layer. There must be a thin layer between the fluid layer and body fluid/plasma.

This comparative article will look at the way different species all tackle the problem of obtaining oxygen for aerobic respiration, starting with insects, then amphibians, fish, mammals and finally birds.

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Insect Respiration

Insects are able to obtain all the oxygen they need for cellular metabolism without lungs. Instead insects have a hard exoskeleton which contains valve like openings called spiracles. Typically there is one pair of spiracles per body segment. Air flow is regulated by small muscles that operate flap-like valves within each spiracle which contract to close the spiracle, or relax to open it.

Oxygen travels along the spiracles which branch off into a network of tracheal tubes, further branching into tracheoles that terminate as sacs. These sacs provide the thin, moist interface necessary for gaseous exchange.

Oxygen transported this way is delivered directly to the tissues that require it, meaning it is not transported via the circulatory system. Because of this the circulatory system of insects is greatly reduced (as compared to a mammal for example).

Oxygen diffuses into cell cytoplasm along a concentration gradient (having first dissolved in the moist tracheole surface); in a similar manner CO2 diffuses out of the cytoplasm to leave the insect via the spiracles.

Because of the inefficiency of this method of respiration, insects must retain small body sizes. As body size or activity of an insect increases, it may be able to increase oxygen intake by manipulating the spiracle and abdominal muscles to increase ventilation. If activity decreases, due to the toxic nature of elevated oxygen levels, the insect can close spiracles to prevent increased oxygen intake.

Amphibian Respiration

Amphibians have lungs which they use to respire, but they are also able to obtain oxygen through their skin. As expected with a gaseous exchange surface, the skin is thin, moist and well vascularised. Oxygen is therefore able to dissolve in the moist layer of the skin and diffuse directly into the blood. When submerged beneath the water surface, amphibians obtain all their oxygen through their skin.

This does however pose a problem, because the skin is a very thin barrier (due to it having evolved as a ‘respiratory organ’) it provides very little in the way of protection when compared to mammalian skin for example (which has evolved as a protective barrier). This restricts amphibians to generally aquatic/damp habitats where less stress is posed on the skin.

As mentioned above, amphibians also have a pair of saclike lungs, although they have a limited surface area and are not nearly as efficient as mammalian lungs. Without ribs or a diaphragm (and the lack of involvement of the chest muscles in ventilation) amphibians must use a force pump mechanism to ventilate the lungs.

The force pump mechanism of breathing is very energy expensive, the mechanism is as follows. With the mouth and nostrils closed, the amphibian will lower the floor of the oral cavity (resulting in the distinctive expansion of the throat) resulting in a reduction of pressure. The nostrils are then opened allowing air to rush into the mouth (as pressure equalises). The air in the mouth is then forced into the lungs by further contraction of the oral cavity floor.

Fish Respiration

Fish live in predominantly aquatic environments however there are exceptions, such as the lung fish which are able to utilise lungs to obtain oxygen. Aquatic fish however use a set of respiratory organs known as gills.

Gills are highly vascularised with a large surface area, short diffusion distance and an always moist surface. Their structure consists of:

  • Four gill arches (either side of the head) or two gill arches in cartilaginous fish
  • Each gill arch then has two rows of gill filaments. There are many gill filaments per column.
  • On the upper and lower sides of each gill filament there are many lamellae. The blood flow within the lamellae is counter-current meaning the deoxygenated blood flow direction is opposite to the oxygenated water flow direction. This mechanism maintains a concentration gradient thus increasing the efficiency of the respiration process. Cartilaginous fish do not have a counter current flow system as they lack bones which are needed to have the opened out gill that bony fish have.

In order to pass the oxygenated water over the gills fish have two main methods, either ram ventilate (swim through the water with an open mouth forcing water to pass over the gills, stopping would cause the fish to drown) or use a similar method to amphibians – a double force pump. Both of these methods are unfortunately energy expensive.

The mechanism of the double force pump method is as follows. The fish utilises the expansion and contraction of two cavities, the buccal cavity (mouth) and the opercular cavity (gills):

  • Water is sucked into the buccal cavity by expanding both the buccal and opercular cavities. The operculum (the bony flap on the outside of the gills which is used to protect them) remains closed to prevent water escaping.
  • By contracting the buccal cavity water is now forced over the gills into the opercular cavity. The mouth and operculum remain closed to keep the cavities sealed.
  • With the mouth still closed, the operculum opens and the opercular cavity now contracts forcing the deoxygenated water out.

The exposure of the gills to water poses certain problems concerning salt movement. In freshwater salt is able to easily diffuse out of the fish into the hypotonic water. To maintain a healthy salt concentration within their body, they:

  • Do not drink freshwater (which would dilute the body salt levels)
  • Excrete a mucus over their body surface to minimise water and salt movement
  • Excrete large volumes of diluted urine
  • Obtain more salt from their diet and absorb as much as possible through the gills

Marine fish on the other hand, have the completely opposite problem. Water is continuously lost from the fish and salt is continually being taken up. To maintain healthy levels of water and salt marine fish:

  • Excrete a small volume of isotonic urine (The kidneys lack loops of Henle meaning they are unable to concentrate their urine)
  • Chloride is actively pumped out of the fish by specialised chloride cells in the epithelium of the gill, whilst sodium diffuses out between epithelial cells along the electrical gradient
  • Marine, cartilaginous fish have a specialised salt excreting gland in their rectum
  • They are also able to increase the osmolarity of their tissue fluids enough so that they actually extract water from seawater by osmosis
  • To increase their water levels, they must drink the seawater and excrete the salt via the specialised rectum glands – this is energy expensive however.

Mammalian Respiration

Compared to other species, mammalian respiration is highly efficient; there is a very large surface area within the lungs which is maximised by the bubble like structure of the alveoli. The lungs also benefit from very thin membranes between the moist layer within the alveoli and the blood. The blood supply to the lungs is very great.

Mammals have a sealed thoracic cavity, which is sealed by the diaphragm. In conjunction with the ribs, the two sets of muscles are able to control breathing.  Normally, the diaphragm’s relaxed position recoils (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). This process works in conjunction with the intercostal muscles connected to the rib cage. Contraction of these muscles lifts the rib cage, thus aiding in increasing the thoracic volume. Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. The intercostal muscles simultaneously relax, further decreasing the volume of the lungs. This increased pressure forces air out of the lungs. Conversely, contraction of the diaphragm increases the volume of the (partially empty) lungs, decreasing the pressure inside, which creates a partial vacuum. Environmental air then follows its pressure gradient down to fill the lungs. The ventilation system of mammals is basically a suction pump.

Avian Respiration

Just like mammals, birds have ribs, although they lack a diaphragm to seal the thoracic cavity. The thoracic and abdominal cavities are thus not separated and this single large body cavity is known as the coelom.

The lungs of the bird are connected to the wall of the coelom by connective tissue and are unable to enlarge themselves like mammalian lungs. Instead air is moved in and out of the bird by expanding the coelom; this enlarges air sacs connected to the coelom causing air to pass through the lungs and into the air sacs.

A complete avian respiratory cycle involves two inspirations and two expirations (unlike mammals which involve only one). Whereas mammals have lungs in which air is inspired and then exhaled, you can think of birds as having 3 sets of lungs. They have anterior (cranial) air sacs, posterior (caudal) air sacs and the lungs themselves. Air is shifted around these 3 sets of respiratory organs during inhalation and exhalation, the exact mechanism is below:

  1. During the first inspiration the air stream is divided into two as the trachea divides. The air does not go directly to the lung, but instead travels to the caudal (posterior) air sacs. A small amount of air will pass through the caudal air sacs to the lung.
  2. During the first expiration, the air is moved from the posterior air sacs into the lungs. Blood capillaries flow through the lungs and this is where the oxygen and carbon dioxide are exchanged.
  3. When the bird inspires the second time, the air moves to the cranial air sacs. (Whilst more air is inhaled into the caudal air sacs)
  4. On the second expiration, the air moves out of the cranial air sacs, into the trachea and out of the nostrils. (Whilst the air inspired during the second inspiration moves through the lungs).

Notice how efficient this is, before the initial breathe has even been exhaled, more air has been brought into the respiratory system.