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The Body in Equilibrium: Components of Homeostasis

The Body in Equilibrium

Negative Feedback:
Dynamic Equilibrium:
Set Points:
Negative feedback:
Body Temperature:
Blood Glucose:
Factors, Temperature:
Regulating Body Temperature:
The Skin: Effecter:
The Skin: Receptor:
The Hypothalamus: Control Centre:
Temperature Extremes:
Heat Exhaustion/Stroke:
Osmotic Conditions:
Blood Plasma:
IVF Therapy:
Kidney Tubule:
Homeostasis can be defined as the tendency of an organism or cell to regulate its internal conditions. It also helps to maintain the bodily functions, regardless of outside conditions. The organism or cell keeps homeostasis by observing its conditions and responding appropriately when these conditions deviate from their optimal state. In humans, the regulation of body temperature involves devices such as sweating when the core temperature becomes excessive or shivering to produce heat, (YourDictionary, 2019).
Negative Feedback:
Homeostasis is in place to provide a steady internal environment for set procedures to happen in the body. Every process or reaction has a necessary ultimate environment where it works best. This is known as “the set point”. Although, impacts, such as an external influence, for instance an increase in external temperature, can cause a change for the ideal set point, then the body will correct this change, this corrective change process is called negative feedback, (Biology Dictionary, 2019).
An example of negative feedback is body heat regulation, the hypothalamus responds to variations in the core temperature and then responds in the best way. This means If the temperature drops too low, the body will begin to shiver in an attempt to bring the temperature back up, on the other hand, if the body is too warm, the body will sweat in an attempt to cool down through evaporation, (YourDictionary, 2019).
Dynamic Equilibrium:
Dynamic equilibrium is when the body makes fine alterations within the negative feedback loop to keep the body at set point within a ‘safe limit’ (not making drastic changes the would deviate too much from the set point of the body), (Biology Dictionary, 2019).
In your body, glucose is in dynamic equilibrium. Even though glucose has stages of high and low concentration, it remains stable. If the level of glucose in the body falls from the dynamic equilibrium and cannot replace the glucose it needs, the body would die in the end, (Biology Dictionary, 2019).
Set Points:
On the right is a photo showing three of the body’s most important set points, these being the pH of the body’s blood, the amount of sugar in the blood and lastly the core temperature of the body, (Biology Reference, 2019).
The bloods perfect pH is within a very thin range of 7.35 to 7.45. If the bloods pH goes below 7.35 it is called acidosis. If the pH goes above 7.45 is alkalosis. Either state can be life-threatening. The body can live just a few hours with a blood pH level that is below 7.0 or above 7.7. If the bloods pH goes below 6.8 or above 8.0 this is rapidly fatal (Saladin,2001).
Glucose levels in the blood are typically measured in milligrams per decilitre (mg/dl), with the normal range being between 70 to 110 mg/dl. If the glucose level starts to leave this range, the amount of insulin and glucagon produced by the pancreas will be adjusted to bring glucose levels back to the normal range. When the system is working correctly, there is always some insulin and glucagon being produced by the pancreas that is trying to find a balance between glucose release into the blood, and glucose uptake into cells to maintain the set point in the blood (Cannon, 1932).
The core body temperature is typically around 37.2 to 37.6 degrees Celsius and fluctuates by 1 degree over a twenty-four-hour period. If the core temperature goes lower than 33 degrees Celsius the person will possibly die of hypothermia. On the other hand, if the core temperature goes above 42 degrees Celsius, the person will likely die from hyperthermia, (Blessing, 1997).

Figure 1 Picture sourced by Physiologyweb, 2015.
This is a graph that shows the normal range for the body’s core temperature as well as its ideal temperature. As the graph shows, the body is never at set point for long, this is because the body is constantly correcting its core temperature and sending signals to the hypothalamus to maintain the temperature. However, by the time the body corrects the temperature it would be either to hot or cold and would need to re-correct it, this means it is constantly changing, (Physiologyweb, 2015).
Negative feedback:
This is a negative feedback loop (right). A negative feedback loop is made up of three essential parts: the receptor, the control centre and the effector, (Shaffer,2014).
The receptor checks situations that happen in the body. Receptors see changes in function and start the body’s homeostatic response. They are linked to a control centre that combines the information it collects from the receptors, (Shaffer,2014).
The brain is the primary control centre in most of the homeostatic systems. When the control centre obtains the data from the receptors regarding abnormalities from its set point, it sends signals to the effectors (via the nerves) to restore the body back to set point conditions, (Shaffer,2014).
Effectors are numerous structures such as muscles and organs that collect signals from the control centre, (Shaffer,2014).
Body Temperature:
As shown in the diagram to the left, this is another negative feedback loop specific to the body temperature. Here the stimuli is the external temperature, this is the temperature of the surrounding areas i.e standing outside in the sun or being sat indoors. The stimuli then sends a message to the sensor (control centre) where it decides what it needs to do to either cool down the body or heat it up. Once it has decided it send another signal to the effectors that trigger the response. As demonstrated in the diagram if the body becomes too warm it will begin to sweat in an attempt to cool down by evaporation, on the other hand, if the body becomes too cold, the effectors will tell the muscles to contract making them shiver to try and heat the body up by friction. This process is called thermoregulation, (BioNinja, 2019).
Blood Glucose:
When a meal is being digested, especially a meal high in carbohydrate foods for example bread, or pasta, the amount of glucose in your blood rises, which is sensed by the nervous system, which then sends a signal to the specialised cell in the pancreas which, in turn, release the hormone, insulin. This is important predominantly for the brain and the muscles, as it is an energy source for cellular respiration. If the body doesn’t consume food over a long period of time or exercises for a long period of time, the glucose level in the blood could fall hazardously low, again sensed by the nervous system, which send a signal to different cell in the pancreas to cause the release of glucagon. It is vital that the glucose in your blood is at a controlled level so that it does not rise too high or fall too low. This control is set by the pancreas; located behind the stomach, the pancreas makes enzymes that is used in digestive system as well as hormones to control the blood glucose level in the blood. The pancreas continuously monitors and controls the blood sugar levels by using insulin and glucagon. When the blood sugar level starts to increase after a meal, the pancreas releases insulin. Insulin lets glucose be taken to the cells of the body where it is used. Glucagon is the hormone that is produced when there is a fall in blood glucose, it increases the release of glucagon from the pancreas to encourage glucose production, (Luman, 2019).
Diabetes: Diabetes is caused when the body doesn’t produce any insulin (type 1) or a failure to respond to the insulin produced (type 2). This means instead of the glucose going into cells, it continues to stay in the blood stream making the blood hyperglycaemic, which has effects on the body.
On effect it has is nerves may not work as normal because a slightly high blood sugar level can, over time, harm some of the nerves in the body. This is a difficulty diabetics face, this is called peripheral neuropathy of diabetes. The nerves that take messages of feeling and pain from the feet are usually affected. If sensation is lost in parts of the feet, the person may not know if they have sustained any damage to the feet. For example, if the person was to tread on something sharp or develop a blister they would be unaware; this means that they are also more prone to problems such as minor cuts and bruises as well as this they cannot feel pain so well from the foot, they do not protect these small wounds by not walking on them or covering them, possibly due to them not knowing they have them, therefore, they can quickly become worse and develop into ulcers.
Many diabetics also experience narrowing of blood vessels going to the feet, this is called peripheral arterial disease. This is caused by fatty deposits called atheroma that can build up on the inside lining of arteries, this is sometimes called furring of the arteries. Due to this build up It can reduce blood flow to certain parts of the body. The arteries in the legs are normally affected. This can cause poor circulation to the feet, which can cause the above issue, as well skin with a poor blood supply does not heal properly normal and is more likely to be damaged. Therefore, if a minor cut or injury occurs, it may take longer to heal, this means it is likely to become worse and developing into an ulcer. Again, if the person also has reduced sensation and cannot feel the wound, this would also cause it to become worse, (WebMD, 2019).
Factors, Temperature: Environmental:
External temperature is one of the factors that cause a person’s body to heat up. If the air is hotter than the body temperature (37 oC) the body will gain heat and if the air is cooler than the body, it will lose heat to the environment. The body loses or gains 12% of the heat exchange from the air temperature in contact with the skin, (Safe, 2019).
The rays of the sun that are in contact with a person’s skin also adds to the body’s heat. In the sun a person feels warmer due to the radiant heat is heating their skin which in turn heats their blood then transfers it to the core of their bodies. It is cooler in the shade as the radiant heat on the body is reduced. Radiant heat can also come from other sources like ovens or any hot surface. This is why people become warm in kitchens as the external heat is heating the skin and blood. The body loses or gains 60% of the heat exchange from radiant heat gain or loss, (Safe, 2019).
Physical activity can alter the body’s normal temperature. The response to physical activity means there is an increase in metabolic rate, this results in a rise of heat production within the body. Like above, blood flow to the surface of the skin results in heat losses through sweating and radiation. The body’s temperature regulation throughout exercise is not as it should be, a rise in body temperature can happen, this could result in heat exhaustion or heatstroke, (Safe, 2019).
Regulating Body Temperature: The Skin: Effecter:
The hairs on the skin also help to control body temperature, this is one type of effecter. They lie flat when the body is to warm, and they rise when the body is cold, this is because the hairs trap a layer of air above the skin, which helps to insulate the skin against heat loss, (Dermacare Direct, 2016).
Goose Bumps or also known as piloerection, or the pilomotor reflex. The bumps we get are encouraged by cold and they are basically just a temporary change in the skin. They are caused by a nerve release from the sympathetic nervous system, which is an involuntary as well as the nerve discharges that raise the hair follicles in our skin. It is the elevation of the hair that causes the goose bump, and which then increases the height of our hairs and the amount of air we trap to increases the insulation, if the body needs it, (Dermacare Direct, 2016).
The Skin: Receptor:
The peripheral thermoreceptors are located in the skin these are known as the receptors. They monitor the temperature in their immediate area. There are two types: cold receptors – they increase their release rate as their temperature declines, with a maximum firing rate at about 25oC. On the other hand, warm receptors begin firing at about 30oC and fire maximally at about 44oC, becoming inactive again at about 50oC. Afferent nerves connect the sensors and transmit the temperature information to the hypothalamic regulatory centre in the brain, In the hypothalamus the actual temperature is compared with the body’s set point. If there are any differences, they are adjusted so the temperature can be brought back to set point, (Dermacare Direct, 2016).
The Hypothalamus: Control Centre:
The hypothalamus is the part of the brain which monitors the body’s temperature. It defines the set point for temperature which is around 370C. It monitors abnormalities the body may experience that can cause it from moving away from the set point. It receives information from temperature-sensitive receptors in the skin and circulatory system about the body temperature from the shell to the core. The hypothalamus responds to this information by distributing nerve impulses to effectors such as the skin and blood vessels, to respond to deviations in the set point, this is so it can maintain the correct body temperature, for instance, if the body become too cold, the hair erector muscles would then contract, this raises the hairs on the skin and traps a layer of air next to the skin for insulation, (WiseGeek, 2019).
Temperature Extremes: Hypothermia:
Hypothermia normally occurs when a person’s body temperature drops below 35oC, the normal body temperature is around 37oC. This can rapidly become life threatening and should be treated as a medical emergency. It’s typically caused by being in a cold environment and can be caused by a mixture of factors, such as being outdoors in cold conditions for a long time, living in a poorly heated house or falling into cold water, (Lallanilla, 2013)
Paradoxical undressing is a condition where people start undressing at a time when you really should not be undressing. As the body starts to slowly lose more and more heat, the body induces vasoconstriction (moving veins away from the skin to avoid the blood becoming cold) as a way to deal with this, which is the reflexive contraction of blood vessels. Eventually your muscles (which are needed to induce vasoconstriction) just give up out absolute exhaustion and fail, the blood the body has been trying to keep close to the vital organs starts to rush to the body’s extremities. Here people start to experience a kind of “hot flush”. The already extremely confused person experiencing hypothermia then proceeds to take all of their clothes off as a way to deal with this sudden burst of heat, when this happens there is no reversing this and within the hour the person will die as their body has shut down and now became too cold, (Lallanilla, 2013)
Hyperthermia normally occurs when a person’s body temperature rises above 38oC, the normal body temperature is around 37oC. This can rapidly become life threatening and should be treated as a medical emergency if it rises above 40oC. It’s typically caused by being in a hot environment and can be caused by a mixture of factors, such as being outdoors in overly hot conditions for a long time, being ill or over exercising, (NHSinform, 2019)
If pathogens should enter the body, then macrophages, which would be fighting the invaders, secret chemicals called pyrogens. These chemicals then tell the hypothalamus to raise the body temperature, this redefine the set point, this is in an attempt to can disable many bacteria that can’t live above normal body temperature, meaning raising the temperature kills the pathogens. The body does this as a last resort as at the same time as killing the pathogens off it is denaturing the body’s cells, (NHSinform, 2019).
Heat Exhaustion/Stroke:
The first symptoms of heat illness occur as the body temperature climbs above normal, and can include but not limited to headaches, Vomiting and feeling fatigued. These early symptoms that people sometimes call heat exhaustion. If steps are not taken to reduce body temperature now, heat exhaustion can worsen and become heat stroke, (NHS, 2018).
Heat stroke is a serious and possibly life-threatening. This is when the body’s temperature rises to above 41oC. People have been known to develop neurological changes, such as mental confusion or unconsciousness. At these high temperatures, body proteins and the membranes around the cells in the body, especially in the brain, begin to be denature or breakdown. The extreme heat can affect internal organs, causing failure of the heart muscle cells and blood vessels as well as damage to the internal organs, and possibly death, (NHS, 2018).
There are two types of heat stroke:
Exertional heat stroke which occur when someone is very active in a hot environment, such as playing sports on a hot summer day or participating in military training activities.
Non-exertional heat stroke tends to happen in people who have a reduced ability to regulate their own body temperature, such as older people, very young children or people with chronic illnesses. High heat in the surrounding environment, without a lot of activity, can be enough to cause heat stroke in these people.
Heat stroke can come on suddenly, but warning symptoms often appear first. Some physical symptoms include: abdominal cramps, nausea, weakness and/or heavy sweating or a lack of sweat, on the other hand some mental symptoms include: odd or bizarre behaviour, hallucinations and seizures, (NHS, 2018).
Osmotic Conditions: Osmosis is a form of diffusion, this is the procedure of diffusion of water across a semipermeable membrane, our cells are made from this membrane. If two solutions of different concentration are separated by a semipermeable membrane, the solvent (water) will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. In the body, water molecules are free to pass across our cell membranes in both directions, either in or out. This does not require any energy because of this it’s classed as a type of passive transport. Osmosis is vital in our bodies because it regulates, the hydration of the cells, the entry of nutrients into cells as well as the outflow of waste from our cells, (Lumen, 2019).
Dehydration may be triggered by restricted water intake, excessive water loss, or both. The most common reason for dehydration is failure to drink liquids. The lack of water is far more serious than the lack of food. The average person loses approximately 2.5 percent of total body water per day in urine and in expired air as well as some in the digestive track. If, in addition to this loss, the loss through sweat is greatly increased. Dehydration may result in shock and death within only a few hours. When swallowing is problematic in very ill people, or when people cannot act to a sense of thirst because of age or illness, the failure to reimburse for the daily loss of body water will result rapidly in dehydration and its penalties. Large volumes of water also may be lost from the body by vomiting or diarrhoea. This will start to make the blood thicker as it is getting rid of water, to stop this it starts to take the water from the cells making them hypertonic as the blood outside the cell has a higher concentration of salt due to the lack of water, (The Editors of Encyclopaedia Britannica, 2019).
It is possible to drink too much water. Overhydration could lead to water intoxication and hyponatremia. Ingesting too much water messes up the electrolyte balance in blood and tissues. Hyponatremia most often occurs when babies are given water instead of formula or formula that has been mixed with too much water, (Helmenstine, 2019).
When too much water passes into the body’s cells, the tissues swell with the extra fluid. The cells keep a specific concentration gradient, so excess water outside the cells (the serum), draws sodium from within the cells out into the serum in an attempt to restore the essential concentration. As more water gathers, the serum sodium concentration drops, this is known as hyponatremia. The other way cells try to recover the electrolyte balance is for water outside the cells to rush into the cells via osmosis. The movement of water across a semipermeable membrane from higher to lower concentration is called osmosis. Although electrolytes are more concentrated inside the cells than outside, the water outside the cells is more concentrated, since it contains fewer electrolytes. Water move across the cell membrane in an effort to balance out the concentration. Hypothetically, cells could swell to the point of bursting this is known as a hypotonic cell, (Helmenstine, 2019).
Blood Plasma: Hypertonic:
The opposite of hypertonic. Solutes outside the cell have a lower solute concentration and a higher water concentration while having an osmolarity outside the cell is lower than the osmolarity inside the cell.
Cells tend to gain water due to osmosis because the water travels from an area of higher water concentration (outside of the cell) to an area of lower water concentration (inside of the cell). When cells gain water, they risk rupturing, a process known as lysis, unless they can maintain their balance (through a selectively permeable membrane), (Silverstein,2015).
Cells often lose water when in a hypertonic solution. There is a greater concentration of solutes outside the cell than in the cell. Those outside the cell have less water than the solutes inside. The osmolarity outside the cell is higher than the osmolarity inside the cell. In osmosis, cells tend to lose water because the water travels from an area of low osmolarity (inside of the cell) to an area of high osmolarity (outside of the cell). When this happens, cells can become dehydrated and die unless they are properly hydrated. The ocean is a prime example of a hypertonic solution as the solutes (salts) outside of the cells are greater than inside of the cells. This is demonstrated when staying submerged in the ocean for a while and your body begins to dry out due to the solvent (ocean) having a greater number of solutes than the inside of your body, (Silverstein,2015).
Below are photos of what plasma cells would look like if they were hypotonic (Inside the cell was 75mg. Outside the cell was 25mg), hypertonic (Inside the cell was 25mg. Outside the cell was 75mg) and isotonic (the saturation was the same inside the cell and outside (there where 50mg inside and outside the cell)).
IVF Therapy: Intravenous fluid therapy is a common practice today. It is an efficient method of supplying fluids directly into the extra cellular fluids of the body specifically through the veins (venous system).
The purpose of the IV drips are to supply fluids when clients are unable to take in an acceptable volume of fluids by mouth, this could be for a number of things, one being nil by mouth in hospitals or patients who are vomiting and are unable to keep fluids down.
As well as water IV drips can also provide salts needed to maintain electrolyte balance. Electrolytes in the body were placed in such a way that it should maintain cellular equilibrium to achieve normal body functioning. If there will be an inequality to these electrolytes, it will result to a serious disorder or malfunctioning; that’s why IVF may come in a form of prevention as it is able to prevent electrolyte imbalance to occur.
They also provide glucose (dextrose), which is the main fuel for metabolism. Glucose is not an electrolyte. It is a form of sugar; when carbohydrates are metabolized, it turns into glucose which the cell absorbs for cellular energy production and consumption. These are not the only use for IVF therapy, but they are some of the main ones used every day in hospitals.
IV solutions may come in different forms; depending on its tonicity (the capability of the solute to cause water movement from one compartment to another) an IV may be: an isotonic, hypotonic or hypertonic solution.
Isotonic: These solutions that have a concentration of dissolved particles equal to that of Intercellular fluid. As there is no water displacement there is no effect on the cell. An example of an isotonic IV drip is D5W (5% dextrose in water) which contains no electrolytes. The purpose of this type of drip is to preserve fluid intake and restore blood volume. Nurses would use this drip on patients who are vomiting, have acute diarrhoea and/or fever, (Rosenthal, 2006).
Hypotonic: Hypotonic solutions have lesser tonicity than that of the Intracellular fluid (ICF) because it has a lower solute concentration. If the ICF becomes more concentrated, it pulls water from the extracellular fluid (ECF), therefore, makes the cell swell. A sample of Hypotonic Fluid would be 0.45% NaCl which is half strength normal saline fluid. It is used to provide ‘free’ water and treat cellular dehydration. ‘Free’ water is necessary to aid the kidneys in removal of solute via urine, (Weinstein, 2001).
Hypertonic: Hypertonic solutions have tonicity or solute concentration in ECF greater than that of the ICF. Therefore, it pulls fluids out of the ICF, consequently making the cells shrink. An example of a hypertonic fluid is D5 in 0.45% NaCl (5% Dextrose in half strength normal saline) which is a mix of an isotonic IV drip as well as a hypotonic IV drip; the purpose of hypertonic solution is, it draws fluids from the ICF causing cells to shrink and ECF to expand, (Kee, 2003).
Kidney Tubule: Below are a series of diagrams explaining the kidney tubule.

ADH: ADH or the Anti-Diuretic Hormone is also commonly known as vasopressin, is a hormone that regulates the concentration of the body’s urine. ADH is produced by the pituitary gland that is situated in the brain. The pituitary gland monitors the concentration of the blood plasma or more known as it’s osmolarity, (Lappin, 2019).
The release of anti-diuretic hormone from the pituitary gland into the bloodstream is measured by a number of factors. A reduction in blood volume results in a lowered blood pressure because of dehydration and if the person suffers from a haemorrhage, (Lappin, 2019).
This decrease in pressure is detected by sensors (osmoreceptors) in the heart and large blood vessels; the information is sent to the hypothalamus in the brain. The hypothalamus will then release ADH from the pituitary into the blood stream which will bind to the receptors on kidney cells in the collecting ducts, which, in this case, are the effectors, (Lappin, 2019).
Alcohol prevents anti-diuretic hormone release which causes an increase in urine production and dehydration, so people are told not to drink alcohol when it’s very hot, like people do on holiday as it stops the reuptake of water the body may need as ADH is not telling the body to retain water, (Bharadwaz,2018).
The collecting ducts are continuous with the nephron but are theoretically, not part of it. Each colleting duct collects filtrate from several nephrons. The collecting duct is lined with epithelium cells with receptors for ADH. When stimulated by ADH, these cells will open their aquaporin channel proteins in their cell membranes. This allows water to pass from the collecting duct and into the interstitial spaces between the cells in the kidneys, however this water is then recovered by the vasa recta by osmosis and water enters the blood stream again, meaning the body does not lose this vital water, (Bharadwaz,2018).
Picture sourced by Bharadwaz,2018.
As more water is now in the blood stream it makes it more dilute. This will be detected in the hypothalamus by osmoreceptors where they will cause the pituitary gland to stop secreting ADH into the bloodstream. If there is no ADH in the blood, the walls of the collecting duct remain totally impermeable to water. As the dilute urine passes down the collecting duct, no water can be reabsorbed into the blood by osmosis and so a large volume of dilute urine will be produced. This is what happens when people drink alcohol even if their body is desperately in need of the water, (Lappin, 2019).
Picture sourced by Lappin, 2019.

Bibliography: Amoeba Sisters, 2017, Homeostasis and Negative/Positive Feedback, (10/05/19)
Amy Wills, 2016, Symptoms of heat stroke and heat exhaustion – plus what is heat rash and prickly heat? (11/05/19).
Anne Marie Helmenstine, 2019, Can You Drink Too Much Water? Water Intoxication and Hyponatremia (11/05/19)
Biology Dictionary, 2019, Dynamic Equilibrium,
Biology Dictionary, 2019, Negative feedback, (10/05/19)
Biology Reference, 2019, Homeostasis, (10/05/19)
BioNinja, 2019, Feedback Loops, (10/05/19)
Blessing, William W. The Lower Brainstem and Bodily Homeostasis. New York: Oxford University Press, 1997. (10/05/19)
Cannon, Walter B. The Wisdom of the Body. New York: W. W. Norton, 1932. (10/05/19)
Cristin M. Rolf, 2018, Hypothermic Death in the Arctic State, (11/05/19)
Deborah C. Silverstein,2015, Crystalloids, Colloids, And Hemoglobin-Based Oxygen-Carrying Solutions, (11/05/19)
Dermacare Direct, 2016, The Skin Structure

The Muscle Arrangement and Functions of Cephalopod Tentacles

Structured models of movement involve the description of muscle specialization, which refers to the use of muscles to perform specific tasks such as locomotion or posture. Although the mechanics and arrangements of muscles in vertebrates have been extensively studied, there is a growing interest in exploring non-model organisms in the field of movement – specifically the cephalopod i.e. Loligo pealei. This paper explores the scaffolds and modulations of myofilament arrangements that give rise to unique movements and properties of the squid tentacles with the rate of change in dimensions and velocity occurring due to specific contractions of the different isoforms (or types) of myofilaments, including: (1) transverse muscle fibers (2) longitudinal muscle fibers and (3) oblique layers of muscle fibers. The dominant functional muscle types used in the different activities of the tentacles is explored in context of the underlying biomechanical functions of the myofilaments. The methodology used to obtain the results for the biomechanical and biochemical components to derive the current model of the cephalopod tentacle was described, drawing importance to the techniques parallel to the vertebrate models. Additionally, ontogeny of the developing sarcomeres from juvenile to mature tentacles, the functional implications of ultrastructural modifications, and the effect of ATPase activity on the peak contractile strength and velocity are discussed to further explore the current tentacle model. This paper combines the standard models of macro and ultra-structures, their movements, and its applications of force and velocity to examine, and best represent, the cephalopod tentacle.

Muscles are categorized as a collection of muscle fibers, as shown in fig. 1, which are composed of sarcomeres that have thin actin filaments that overlap and interact with the thick myosin filaments to produce contracted and relaxed lengths of the muscle, contributing to the change in velocity, force, and lengths of movements. All muscles operate under the conditions of the sliding filament principle (Squire J.M., 2016), where contraction occurs when bands of the sarcomere change in length.
Figure 1- Hierarchy of muscle structures present in vertebrates (Squire J.M., 2016)
However, these models are studied in the context of the skeletal system, where a frame is provided for the support of the muscular system. In the cephalopod tentacle, there is a more unique muscular arrangement that produces more diverse and impressive movements and functions. Most of the research regarding the functional morphology and muscle arrangement has been conducted in 1982 (Kier W.M., 1982), mapping out the base components of the squid, Loligo pealei’s tentacle as shown in fig. 2. The cephalopod appendages consist of five pairs of tentacles that are used for locomotion, attacking prey, feeding, copulation, and manipulating and exploring the environment. These appendages were initially observed to be composed of longitudinal and transverse striated muscle fibers that can produce skeletal independent movements to elongate, shorten, turn, and twist the tentacles. Moreover, the tentacles have also been observed to display regenerative abilities and with the cephalopods being able to seemingly recover from damage to loss of whole limbs (Bello G., 1995). Better understanding of the morphology and contractile properties of the tentacles will provide an extensive description of its impressive abilities. This is developed through the discussion of the recent, most accurate anatomical model of the squid tentacle, the ultrastructures of the muscles, the functional significance of oblique striations, and the support provided by the funnel retractor muscles, highlighted in the following sections.
Figure 2- Cross-section of a loliginid squid – Ax (axon), L (longitudinal), T (transverse) (Kier W.M., 1982)
Recently, research has shown that there are three main general groups of muscles present in the cephalopod tentacles that control the majority of the movements of the tentacles: Transverse muscles, Longitudinal muscles, and Oblique/Helical muscles (Kier W.M., 2016). As shown in fig.3, the core of the tentacle houses the artery and axial nerve core, which innervates and controls the appendage, covered by the transverse muscle mass. Surrounding the transverse muscle mass are groups of longitudinal muscle fibers, that run parallel to the axial length of the tentacle. Thin layers of helical (oblique) muscle fibers sheath the tentacle overlapping the circular muscles, which are the distal arrangements of the transverse muscles. Without a skeletal structure, the cephalopod tentacles rely on muscular hydrostat, which utilizes the incompressible properties of water and musculature for support (Chantler P.D., 1983). Bending movements of the tentacle are a product of the longitudinal and transverse muscle fibers contracting and relaxing, respectively, to overcome shortening of the arm while producing angular movements. Elongation of the tentacles occur when the transverse muscle mass contracts radially, reducing the circumferential volume of the muscle and therefore increasing the length of the muscle as the displacement of mass is productively increased. The cephalopod tentacle is also capable of bidirectional twisting when the helical (oblique) muscle layers, wrapped on top of the circular muscles, arranged at right-handed and left-handed helices contract and relax, depending on the direction of the turn.
Figure 3- Anatomical model of an oral tentacle, closest to the mouth – AN (axial nerve), AR (artery), TM (transverse), LM (longitudinal), CM (circular), HM (helical/oblique) (Kier W.M., 2016).
An issue that the tentacle faces because of its constant distortion of shape, length, and size, is the change in volume due to a lack of a rigid structure. The cephalopod tentacle utilizes its resistance to volume change in active bending of the arm (Smith et al. 1985). The orientation of the transverse muscle contractions change the medial-distal diameters of the tentacles, which allows it to retain a constant length while bending and twisting as shown in fig. 4.

Figure 4- Illustration of muscular hydrostat mechanism – Radial contractile properties give rise to motions with uniform length (Smith et al. 1985)
The relative contribution of the transverse and longitudinal muscle contractions varies according to the type of motion performed, the direction of the motion, and the amount of resistance experienced by longitudinal compression. Aside from the arrangements and varied contributive contractions of the tentacle muscles, ultrastructures provide an additional element of unique movements to the cephalopod.
These movements of the tentacles are possible, largely due to the ultra structure properties of the (Kier W.M. 2016) Transmission electron micrographs sections of the transverse muscle mass, shown in fig. 5, reveals the cross striation patterns with remarkably short sarcomeres and extended thick filaments present in the tentacles. In mammalian myofibrils, thick filaments can be approximately 2-6 µm whereas those in the cross striated transverse muscles, they are 0.8 µm. Having short myofilaments and sarcomeres makes denser muscular elements over a unit length of fiber, which influences the contractile speed of the tentacles. This specialization contributes to the fast active movements of the tentacles, specifically in catching prey and locomotive agility.
Figure 5- Transverse muscle mass with cross striations and short sarcomeres (Kier W.M. 2016)
This importance of ultrastructure development is emphasized in both the development of juvenile cephalopods and in the ATPase activity of the muscles. The change in length and resultant increase in contractile velocity of the cephalopod was observed in the ontogeny of its musculature (Thompson et al. 2006). As the ultrastructures develop in the juvenile, the transverse muscles present at the radial end of tentacles, called circular muscles, begin to express shorter lengths of thick filaments due to the selection of performance during growth and senescent gene expression. Having shorter thick filaments would mean having smaller distances between myosin cross bridges which leads to increased velocity of contraction. Moreover, even though ATPase activity in myosin on the sarcomere level controls the contractile properties in muscles, it largely had no influence on the tentacle contractile velocity (Shaffer