Reverse Osmosis:

Reverse osmosis is a separation technique where pressure is applied to a solution to force the solvent to move from an area of low concentration to one of high concentration, leaving behind the heavier solute particles.

It is called reverse osmosis because it goes against the natural process of osmosis, where solvent moves from an area of high concentration to one of low concentration.

This occurs in Bowman’s capsule in the kidneys when filtering waste from the blood. The afferent arteriole is larger than the efferent arteriole, which creates a region of high pressure in the glomerulus. This high pressure goes against osmotic pressure and forces solutes out of the solvent, resulting in reverse osmosis and the filtration of waste and excess fluid from the blood.

Dialysis:

Dialysis is the process where a machine called a dialyser performs the role of a kidney when the latter is unable to function properly. It removes toxins from the blood and filters them out when kidneys cannot do so. It is also used to remove drugs from the body.

One type of dialysis is called hemodialysis, where blood is filtered and cleaned outside the body before being supplied back in.

Procedure:

• To access both oxygenated and deoxygenated blood for filtration, an arteriovenous graft or fistula is created. The former is done by connecting an artery and a vein with a small plastic tube called a graft. In the case of the latter, they are connected directly surgically.
• Two needles are inserted into the graft or fistula, which are connected to tubes that connect to the dialyser.
• The impure blood from a person enters a dialyser, where it is passed through filtering fibres to filter out the waste and is mixed with a liquid called a dialysate. These filtering fibres are filled with the dialysate, and are tube-like in nature. These tubes serve as a semipermeable membrane, through which only water, electrolytes, and toxins are allowed to pass through into the dialysate through diffusion and osmosis.
• Certain medicines and drugs may be added into the dialysate to diffuse into the patient’s filtered blood, such as erythropoietin, which is typically produced by the kidneys.
• The cleaned blood is pumped back into the body, while the dirty dialysate fluid is pumped out of the machine. Clean dialysate fluid is constantly supplied back into the machine.

Pros:

• Flexibility: Hemodialysis can be done in a hospital setting or at home.

Cons:

• Dietary restrictions: Patients undergoing hemodialysis are recommended to avoid consuming excess fluids and certain foods.
• Restrictive: Hemodialysis needs to be regularly conducted for an average of three days a week, each session lasting four hours. This takes up a lot of the patient’s time.
• Increased risk of sepsis (blood poisoning): Due to the insertion of an external catheter into the bloodstream, there is risk of contamination and the entry of bacteria into the blood if proper sanitation is not ensured. Sepsis can be fatal.

A second type of dialysis is called peritoneal dialysis, where blood is filtered and cleaned within the body itself, particularly within the membranes of the abdomen called the peritoneum.

Peritoneal Dialysis:

Procedure:

• A catheter, or a small, flexible tube, is inserted into the abdominal membranes and fills the abdominal cavity with dialysate.
• The dialysate draws out harmful substances and excess fluids from the body.
• The used dialysate is filtered out of the body and into a disposable bag via the catheter.

Pros:

• Can be done at home due to not requiring being constantly hooked up to a dialysis machine, unlike hemodialysis.

Cons:

• Peritonitis, or the infection of the abdominal membrane: Due to an external device like the catheter being repeatedly inserted and removed into the peritoneum, there is a risk of contamination and infection.
• Weight gain: The dialysate fluid contains sugar molecules, which can be absorbed back into the body and increase caloric intake as a result. Without compensating for this by exercising or reducing caloric intake, this can lead to eventual weight gain.

Organ Transplantation:

What?

A medical procedure where a damaged organ or tissue in a patient, called the recipient, is removed and replaced with a healthy, functioning one from an organ donor.

Successful transplants have been conducted for multiple organs and body tissues, save for the brain and a few others. Some, like the kidneys and liver, are easier to transplant than others. The average human being is born with two kidneys (not me though, I have three), but can live normally even with only a single functioning kidney. As a result, kidney donations are extremely accessible and easy to set up.

Livers, on the other hand, everyone only gets one of. But the neat thing is that parts of the liver can regenerate, meaning that chunks of a person’s liver can be donated to a recipient with liver failure, with the donor recovering their original liver in a short span of time.

Kidney and liver organ transplants are some of the most common organ transplants worldwide as a result.

When?

These are usually conducted in life-threatening situations, such as genetic conditions like cystic fibrosis or being born with an organ defect.

Whom?

Organ donors:

• Must consent prior to registering themselves as donors.
• Must be 18 years or older if donating organs while alive, such as kidneys and liver.
• Must be brain dead if donating organs posthumously.
• Must have a compatible blood type. If blood types between donor and recipient are not compatible, then the recipient’s immune system may view the donor’s blood cells as antigens and generate an immune response, causing the former’s body to reject the donated organ. People with an O- blood type are universal donors and can donate to anyone, but can only receive from other O- blood type holders.
• Must be healthy. Though for some transplants, health isn’t a requirement, when donating organs like the liver, one must not be suffering from any conditions associated with the organ, as they would risk transferring the disease to the recipient.

Why?

The benefits of organ donation include:

• Donating organs means saving lives. In the U.S. alone, more than 103,000 people are waiting for organ donations, with almost 17 dying each day waiting to receive one.
• Donating your body to science allows medical schools and students to learn about human anatomy and physiology through hands-on experience.

…But?

Organ donations can go wrong in several ways, such as:

• Not receiving organs on time: Certain organs can only survive outside the body for a short period of time, meaning that if the surgery is done too late, the donation as a whole is rendered useless.
• Weakened immune system due to immunosuppressants: Immunosuppressants are drugs given to organ recipients prior to the surgery where they receive their new organ to weaken the immune system and prevent it from generating an immune response. This is due to the fact that the immune system may view the foreign organ as a harmful antigen and may attack it, causing the recipient’s body to reject the organ and not integrate it into the body. However, a weakened immune system results in the patient being more susceptible to infections by pathogens while recovering from the transplant.
• Religious beliefs preventing organ donation: Certain religions may frown upon and altogether ban those practicing them from being organ donors, reducing the availability of donors further.
• Social pressure: Organ donors may feel pressured by friends, family, or loved ones to whom they may be donating an organ, resulting in a case of dubious consent.
• Organ trafficking: The practice of removing organs illegally from dead or living persons, often regardless of whether consent was given or not, and selling said organs, which is another illegal act.

However…

Newer, modern alternatives to organ transplants are emerging. One example is 3D organ printing, or bioprinting.

What?

This technology uses real cells in combination with 3d printers to essentially “3d print” new organs and artificially create them. This can either be done by directly 3d-printing pieces of tissue and organs, or by 3d-printing a mold or structure of said organ in which cells can be injected to replicate.

The cells in question can be taken from donors or from the patient themselves.

Homeostasis

What is homeostasis?

Homeostasis is the maintenance of internal physical and chemical conditions within a normal range in response to external or environmental changes or stimuli. The nervous system and the endocrine system are both responsible for homeostasis.

Homeostasis is something we can only do because we are endotherms, a.k.a warm-blooded animals that can control our own body temperature. Well, us mammals along with the birds. Reptiles and amphibians, a.k.a “cold-blooded” animals, are ectotherms, whose body temperature changes based on their surroundings and is out of their control. That’s why reptiles spend so much time basking in the sun, to warm themselves up.

The three key components of homeostasis, which together make up the body’s control system responsible for maintaining homeostasis, are:

Receptors:

These are cells or structures responsible for sensing stimuli and relaying the sensation to the coordination centres. They can be classified based on the location of the stimuli they receive as either exteroceptors or interoceptors.

They can also be classified based on the type of stimulus they receive:

Coordination centres:

These organs are responsible for receiving the stimulus sent from the receptors and sending an appropriate input to the effectors to respond to the stimulus. The brain and spinal cord are the typical coordination centres.

Effectors:

These organs receive inputs from the coordination systems and respond to the stimulus accordingly. These are typically muscles or glands.

Ex:- Blood sugar levels:

Receptors: Liver.
The liver detects either a rise or fall in blood sugar levels beyond the normal range and sends a stimulus to the pancreas.

Coordination centres: Pancreas.
The pancreas receives the input from the liver and acts accordingly.

Effectors: Pancreas.
In the case of a blood sugar crash, the pancreas secretes glucagon, an enzyme which signals to the liver to break down stored glycogen into glucose to enter the bloodstream.

In the case of a blood sugar spike, the pancreas secretes insulin, an enzyme which signals to the liver to store excess glucose in the bloodstream as glycogen.

To summarize:

To connect the receptors, coordination centers, and effectors, the nervous and endocrine system both come into play.

homeostasis is generally made at:

• A body temperature of around 37 degrees Celsius/96.5 degrees Fahrenheit.
• A blood pH of around 7.3.
• A blood sugar level between 80-110 milligrams/decilitre.
• A body made up of 70% water, and blood that is 90% water by volume.
• And finally, enough carbon dioxide, but not too much! Otherwise, cells will begin to oxidize. This creates oxidative stress, which makes your cells quite literally break down.

The Nervous System:

Neurons:

Neurons, or nerve cells, are of three types:

Reaction Time:

Reaction time is the time between the introduction of a stimulus and the start of a reflex, and is a measure of how readily an individual responds to a stimulus. It can be measured through the simple falling ruler experiment done with yourself and a friend.

1. Hold the ruler by the end so that the “zero” marking is aligned with the thumb and index finger of the other person.
2. Release the ruler.
3. Using a stopwatch, measure the time taken for your friend to catch the ruler.
4. Using the final position of your friend’s fingers, calculate the distance covered by the ruler while falling till it was caught.
5. Divide the distance by the time, and you get the reaction time in meters per second!

Reaction time can be influenced by factors such as alcohol or drugs, age, type of stimulus, distractions, and fatigue.

Reactions are voluntary, while reflexes are involuntary. Reflex arcs are pathways followed by nerve impulses in reaction to a stimulus. An example of a reflex is the knee-jerk reflex. Voluntary actions are controlled by the brain, while involuntary actions are controlled by the spinal cord, which serves as a bridge between receptors and the brain, and the brain and effectors as well.

Don’t Be So Impulse(ive)!

A nerve impulse is an electrical signal that propagates through neurons to convey a message or signal in the nervous system. They can travel up to 120 meters per second through nerve fibres!

…Nerve fibres?

Well, you didn’t think it just stopped at nerves, didn’t you?

Nerves themselves

Nerves themselves are made of anywhere between just a few to a million nerve fibres, also known as fascicles, all bundled together by a covering called the perineurium. The fibres themselves have a width between 5-20 micrometers, as wide as the blood capillaries in your body!

Structure:

As far as cells go, they’re pretty simple. Nothing more than a cell membrane with cytoplasm inside. However, this “simple” structure is the backbone for their functioning.

Function:

Within both the cytoplasm inside the membrane and the fluid outside, there are positively and negatively charged ions. However, their distribution in these respective regions is imbalanced. And an imbalance of charges creates a potential difference, also known as voltage! Now, typically, this voltage stays at -70 mV.

However, when an impulse occurs, channels in the membrane open. These allow sodium ions from the outside to diffuse inside, causing the voltage to rise to +40 mV. Then, different channels allow potassium ions from the inside to diffuse outside, causing the voltage to fall back down to -70 mV.

Finally, pumps in the membrane move the sodium and potassium ions to their initial positions in preparation for another response.

Nerve impulses are…

• All or Nothing: When a nerve impulse occurs, the voltage either rises all the way to 40 mV, or it doesn’t occur at all.
• One-Way: Nerve impulses can’t go back the way they came. They can only move in one direction.
• Domino Effect: Adding on to the fact that they can’t go back, nerve impulses can’t travel over the gaps called synapses. Instead, neurotransmitters bridge the gap and trigger an impulse in the next nerve fibre, and so on and so forth, creating a domino effect.

…Neurotransmitters?

These are chemicals that are released at synapses to bridge the gap between neurons and stimulate an impulse in the next neuron, because charges can’t cross the gap! There are many types of neurotransmitters, but they can roughly be divided into excitatory and inhibitory neurotransmitters.

Excitatory neurotransmitters cause the next nerve cell’s voltage within the membrane to rise, triggering an impulse! They “excite” the neuron, hence their name. Too much of this, however, called overexcitation, can cause panic attacks and anxiety, like if there’s too much norepinephrine in your head, which excessively triggers the fight-or-flight response.

Inhibitory neurotransmitters cause the next neuron’s voltage to drop even further, meaning an even larger amount of excitatory neurotransmitter is required to stimulate an impulse. They “inhibit” impulses, hence their name. Once again, though, if there’s too much of this, sedation can occur, making you feel sleepy and inactive.

Reaction time

The interval between the presentation of a stimulus (visual, auditory, or tactile) and the initiation of a voluntary response is what reaction time is defined as. It is measures the speed of nervous system processing, involving perception, mental processing, and motor execution.

Factors influencing reaction time:

Brain Anatomy

The brain and spinal cord make up the central nervous system, while the nerves that connect the brain and spinal cord to the other parts of the body are part of the peripheral nervous system.

The brain itself can be divided into the cerebrum, which is the largest part, the thalamus, and the hypothalamus. The cerebrum is divided into two hemispheres: the left and right hemispheres, which control opposite sides of the body. The left controls the right side of the body, and vice-versa.

The cerebrum is divided into four lobes: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

The frontal lobe is responsible for thinking, including planning, problem solving, and speech.

The parietal lobe is responsible for regulating and processing sensory inputs from receptors.

The occipital lobe is in charge of visual stimuli and sight.

The temporal lobe works with the ears to process auditory stimuli.

The cerebellum, which is located beneath the cerebrum, is responsible for voluntary actions and maintaining the body’s balance and equilibrium. Finally, the brainstem controls involuntary actions.

Positive and Negative Feedback Loops

Negative feedback loops are more common and involve the reversal of the original stimulus. These occur when bodily conditions change from the ideal range. They typically involve the process of decreasing then increasing levels to stay within the normal range. This can be observed throughout a variety of bodily processes, such as:

Blood Pressure

Changes in blood pressure are picked up by baroreceptors in the walls of the aorta and carotid arteries, which supply blood to the brain. These detect the stretch of the walls in response to blood volume per unit of time.

If blood pressure is low, impulses are sent to the brain, which triggers the release of aldosterone and antidiuretic hormone (ADH) from the kidneys. Aldosterone signals the kidneys to retain sodium, causing water retention as well due to osmosis. ADH signals the kidneys to release less urine. The increased fluid and narrowing of blood vessels (vasoconstriction) help raise blood pressure.

If blood pressure rises, blood vessels dilate (vasodilation) and heart rate decreases to lower it.

Body Temperature

External conditions can cause body temperature to drop or rise. Thermoreceptors sense these changes and relay the information to the brain, which compares it to the body’s normal temperature range.

If temperature is too low, vasoconstriction occurs to prevent heat loss, and shivering happens, which is rapid muscle contraction and relaxation producing heat. Body hairs stand on end (goosebumps) to trap heat.

If temperature rises, vasodilation occurs, and sweating helps cool the body via evaporation. Sweat glands, which originate from the epidermis and extend into the dermis, produce sweat made of water, salt, and dissolved urea.

Water Levels (Osmoregulation)

Changes in internal water levels are picked up by osmoreceptors, which relay this information via impulses to the hypothalamus. The hypothalamus stimulates the posterior pituitary gland to release ADH (AntiDiuretic Hormone), which affects how permeable the renal tubules in the kidneys are to water.

If the body is dehydrated, the cells in the collecting ducts become more permeable, allowing water to diffuse back into the bloodstream. More ADH is secreted to conserve water. More water is reabsorbed back into the blood, and the urine produced is more concentrated, lower in volume, and darker yellow in color.

If the body has excess water, the collecting duct cells become less permeable, causing most water to be flushed out as urine. Less water is reabsorbed, and less ADH is released. Urine is larger in volume, more dilute, and lighter in color or even colorless.

The formal term for internal water levels is blood water potential, which measures how much water is in the blood. Too much can cause high blood pressure; too little leads to dehydration.

Positive Feedback Loops

Positive feedback loops are less common and involve the amplification of the original stimulus. They encourage continued changes in the same direction and can be harmful if conditions exceed equilibrium, but are useful in certain processes.

Blood Clotting

Blood clots form through a series of enzyme activations. One enzyme, thrombin, activates both the next and previous enzymes in the sequence, creating a positive feedback loop. Thrombin levels continue to rise until the clot is fully formed.

Childbirth

During labour, the baby’s head stretches the cervix. This stimulus causes the hypothalamus to signal the pituitary gland to release oxytocin, which causes uterine contractions. Contractions further stretch the cervix, creating a positive feedback loop until the baby is born.

In this process: Receptor: cervix, senses the stimulus of the baby’s head.
Control center: brain/pituitary gland, produces oxytocin.
Effector: uterus, contracts and widens the pelvic region.
Oxytocin levels continue rising until after delivery, then reset to normal.

Endocrine System

Anterior Pituitary Gland

The anterior pituitary gland secretes several important hormones:

• Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to release cortisol.
• Follicle stimulating hormone (FSH): Stimulates egg development in ovarian follicles in females and triggers ovulation, and stimulates sperm cell development in testes of males. FSH release is controlled by GnRH from the hypothalamus.
• Luteinizing hormone (LH): Triggers sex hormone release in both males and females. In females, signals ovaries to produce estrogen and progesterone, and release a mature egg. LH release is controlled by GnRH from the hypothalamus.

Posterior Pituitary Gland

The posterior pituitary gland does not synthesize hormones but stores those produced by the hypothalamus:

• Antidiuretic hormone (ADH): Stimulates kidneys to reabsorb water when the body is dehydrated, resulting in more concentrated urine. Alcohol inhibits ADH, leading to diluted urine and dehydration.
• Oxytocin: Released during physical labour, such as childbirth.

Pineal Gland

A small gland in the brain that secretes melatonin, regulating the sleep cycle by reducing heart rate and other body processes. High light intensity detected by photoreceptors inhibits melatonin, while low light encourages its production.

Thyroid Gland

Located at the base of the neck. It secretes:

• Thyroxine: Regulates metabolism of carbohydrates, fats, and proteins. Iodine deficiency can cause goitre (swelling at the base of the neck).
• Calcitonin: Reduces calcium levels in the blood, counteracting parathyroid hormone.

Parathyroid Glands

Located next to the thyroid gland. They secrete parathyroid hormone (PTH), which increases calcium levels and counteracts calcitonin.

Thymus

Produces thymosin, which stimulates production of T-cells, a type of white blood cell.

Adrenal Glands

Two glands located on top of the kidneys. They secrete adrenaline and cortisone.

Adrenaline Effects:

• Increases heart rate, boosting oxygen supply to skeletal muscles.
• Stimulates diaphragm and intercostal muscles to enhance gas exchange in lungs.
• Diverts blood to skeletal muscles and away from digestive system and skin.
• Stimulates glycogenolysis, breaking down glycogen into glucose.

The adrenal cortex (largest part) secretes:

• Aldosterone: Regulates blood pressure by increasing sodium and fluid retention.
• Cortisol: Released during stress, part of the fight-or-flight response, increases blood glucose.
• Androgens: Trigger puberty and development of male and female sex characteristics.

Adrenal Medulla

The adrenal medulla is the inner part of the adrenal gland. It secretes:

• Adrenaline (epinephrine)
• Noradrenaline (norepinephrine): Released from both the adrenal gland and the brain, it functions as both a hormone and a neurotransmitter. Works with adrenaline in the fight-or-flight response by:
  • Constraining blood vessels to increase blood pressure
  • Increasing blood glucose levels
  • Enhancing attention and focus

Pancreas

The pancreas regulates blood glucose levels via specialized cells:

• Beta cells: Secrete insulin, which lowers blood sugar by signaling the liver to store glucose as glycogen. A deficiency in insulin leads to diabetes.
• Alpha cells: Secrete glucagon, which raises blood sugar by signaling the liver to release stored glucose.

Ovaries

These are typically only found in those with XX chromosomes, or females. They produce oestrogen, which is responsible for the development of secondary sexual characteristics in females during puberty. During the menstrual cycle, oestrogen thickens the lining of the uterine walls to prepare for ovulation. It also triggers the release of luteinizing hormone (LH).

Testes

These are sex organs typically only found in those with XY chromosomes, or males. They produce testosterone, which is responsible for the development of secondary sexual characteristics in males during puberty, as well as triggering sperm production.

Plant Hormones and Behaviours

Plant movements can either be tropic (relating to the growth of the plant) or nastic (irrelevant to the growth of the plant).

Nastic movements include:

• The touch-me-not plant shutting its leaves when receiving the stimulus of being touched.
• The Venus flytrap shutting its leaves when an insect lands on them and gets trapped.

Tropic movements can either be hydrotropic (movement towards water), phototrophic (movement towards light) or geotropic (movement towards soil).

As plants lack a nervous system, they rely on hormones for regulating growth and other activities. However

Auxins in the shoots

Auxins avoid light. Thus, when exposed to light, auxins migrate to the part of the shoot that is in the shade. As a result, the part of the shoot in the shade begins to grow at a faster rate than the part of the shoot exposed to the light. This causes the plant to bend towards the light.

Auxins are also greatly influenced by gravity. In a shoot growing horizontally, auxins will accumulate at the lower side, causing it to grow faster. As a result, the plant curves upwards.

Auxins in the roots

In the roots, auxins inhibit growth. As roots grow out, gravity causes auxins to become concentrated on the bottom side of the root. Thus, the bottom side of the root grows slower than the upper side, which grows down due to gravity. Thus, roots exhibit geotropism.

Commercial uses of auxin

• Weed killer/herbicide: Too much of a good thing can be a bad thing, and that is especially true for auxins. Excess auxin within a plant stimulates the release of the enzyme ethylene, which inhibits the growth of roots and shoots and makes the plant die.
• Furthermore, auxin only affects dicotyledons, and not monocots or grasses. Thus, farmers who grow monocot crops like rice spray herbicide containing auxins in their fields. This causes any dicot weeds to grow too fast and die, while allowing the desirable monocot crops to continue growing.
• However, once sprayed in a field, the spread of auxins can’t be controlled, and may negatively impact the surrounding environment if it leaches out.
• Cloning plants: Farmers can take a cutting of a desirable plant and dip the tip of the cutting in a solution of auxins, which causes the rapid development of roots and the simple creation of clones. A similar process is used in laboratory settings.

Gibberellin

This is a plant hormone responsible for two main functions within a plant: stem elongation and seed germination.

Stem elongation:

Gibberellins are produced near the nodes of where new leaves are growing. They are secreted at a low concentration to encourage cells in the stem of the plant to elongate. As a result, the plant increases in height.

Plants with a lack of gibberellin are considered dwarf plants, while those with excess amounts grow very tall.

Furthermore, they also promote the elongation and overall increase in size of fruit, as well as the flowering of plants.

Seed germination:

Gibberellins trigger the hydrolysis of starch in the endosperm of the seed into glucose, which allows the plant embryo to begin germination.

Commercial uses of gibberellin:

Aesthetic purposes: Gibberellin can be harvested as a solution that is then sprayed on plants to increase growth, promote flowering, and increase the size of fruits.

Cytokinins

Coming from the word “cytokinesis”, which is a stage in cell division where the cytoplasm is split in two as the two daughter cells are produced, these hormones aid in cell division.

Abscisic acid

Plant hormones that regulate the plant’s response to stress, and act throughout the plant in a variety of ways:

Seeds:

A buildup of abscisic acid in the seed of a plant prevents the seed from germinating and prolongs its dormant phase. This typically occurs when the seed is in unfavourable conditions to germinate. However, once conditions become less harsh, abscisic acid finally ceases to inhibit gibberellins and allows seed dormancy to end.

Roots:

If the plant detects low water levels in the surrounding soil, the roots produce abscisic acid. This stimulates the elongation of roots and increases the root’s ability to conduct water through itself. It also inhibits lateral root growth and focuses only on making roots grow deeper.

Leaves:

Due to low water levels, abscisic acid from the roots travels to the leaves, and forces water out of guard cells through reverse osmosis, causing them to shrivel up and close the stomata, preventing further water loss.

Furthermore, during autumn, abscisic acid causes trees to shed their leaves. As temperatures drop, groundwater in the soil becomes frozen, preventing the tree from utilizing it. Thus, it is unable to replace the water lost through transpiration, hence it sheds its leaves.

Furthermore, leaves are unable to support the weight of snow in winter, resulting in the breaking of branches that have a larger load of snow piled on top. Thus, leaves are shed.

Buds:

Abscisic acid is produced in undeveloped, or terminal flower buds. This causes the buds to enter a period of dormancy and not flower until conditions are favorable. It also results in the conversion of the leaves that accompany the bud into bud scales, which are harder and protect the bud against mechanical stress.

Pavlov and Classical Conditioning

Ivan Pavlov was a Russian scientist whose discovery of the psychological phenomenon of classical conditioning arose from his experiment with dogs. He observed that the dogs would salivate when they were given food, as well as in the presence of the lab technician who usually gave them food.

Intrigued, he designed an experiment where he would ring a bell while presenting the dogs with food, and measure their saliva output. While initially, there was no response to the bell alone, the pairing of the bell with the food led the dogs to associate the noise with salivation.

Pavlov began to observe that the dogs produced a similar output of saliva when he began to ring the bell alone. The main principle behind Pavlov’s experiment is this: a stimulus that triggers a natural biological response is paired with a new stimulus that triggers the same reaction.

Unconditioned Stimulus (Food)

The food was an unconditioned stimulus, which is a stimulus that triggers a natural biological response. In this case, the stimulus of the food activates receptors in the eyes (rods and cones) and the nose (olfactory receptors), which send electrical impulses to the control centre, the brain. The brain interprets the stimulus and sends impulses to the effectors, in this case the salivary glands, which execute the natural biological response of salivation.

The natural behavioural response of salivation when presented with food is called an unconditioned stimulus, because it occurs naturally.

Unconditioned stimulus → Unconditioned response
Food → Salivation

Neutral Stimulus (Bell)

The sound of the bell ringing was initially a neutral stimulus. This means that while the stimulus was picked up by the dog through auditory receptors in the ears and was relayed to the control centre via electrical impulses, it did not trigger any natural biological responses.

Neutral stimulus → No response
Bell ringing → No response

Conditioning Process

Over time, as the bell was rung along with giving food to the dogs, synaptic connections were formed and strengthened between the neurons that relayed the auditory stimulus to the brain, and the neurons that stimulated the salivary glands to release saliva.

Food → Salivation
Bell ringing → No response
Food + Bell ringing → Salivation

Eventually, the bell ringing became a conditioned stimulus, meaning through external conditioning it now mimics an unconditioned stimulus and triggers a conditioned biological response.

Conditioned Stimulus and Conditioned Response

Neutral stimulus → Conditioned stimulus

The dogs then began to salivate at the sound of the bell ringing alone, due to the formation of a neural pathway where the auditory stimulus relayed to the brain via electrical impulses resulted in the brain sending signals to the effectors (salivary glands) to release saliva.

This makes salivation a conditioned response, as it is a natural biological response that now occurs in response to a conditioned stimulus.

Conditioned stimulus → Conditioned response
Bell ringing → Salivation

Extinction

But this isn’t a cheat code for infinite dog saliva– not that you should want infinite dog saliva in the first place– because of extinction.

If a conditioned stimulus is repeatedly presented without the unconditioned stimulus that triggers the same response, eventually, the association between the conditioned stimulus and the conditioned response will begin to fade.

Bell ringing (🛇 Food) x 10 → No salivation

Meaning that if the bell is rung in front of the dog enough times without giving it food, then the dog will stop associating the noise with salivating. This is called extinction.

Limits of Classical Conditioning

Furthermore, classical conditioning can only trigger pre-existing natural biological responses with new stimuli, and can’t create new behavior. So Pavlov could make his dogs drool when he rang a bell, but he couldn’t make them do backflips.

Additionally, the conditioned response is not an exact replica of the unconditioned response. Pavlov found that the chemical composition of the saliva released when he rang the bell was slightly different from the saliva released at the sight of food alone.

Brief Notes on Eye, Tongue, and Ear Anatomy

The eye, ears, and tongue are all sense organs, which are specialized structures in the human body that allow the individual to receive and respond to specific types of stimuli in the environment.

The Eye

This sense organ is responsible for vision and visual stimuli. The parts of the eye are as follows:

Cornea: A transparent, curved structure located in the front of the eye. It allows light to enter the eye, and refracts all light entering the eye so it is focused on the lens and retina. It also protects the iris, pupil, and lens.

Iris: A colored structure located behind the cornea. It is responsible for controlling the amount of light entering the eye by adjusting the size of the pupil. The colour of the iris also determines eye colour.

The iris reflex is the involuntary contraction and relaxation of the iris in response to a light stimulus. If the eye is exposed to high-intensity lighting, the iris contracts and constricts the pupil to reduce the amount of light entering the eye and prevent damage to the retina. If the eye is exposed to low-intensity lighting, the iris relaxes and dilates the pupil to increase the amount of light entering the eye which allows the eye to see better in dim lighting.

Pupil: An aperture through which light enters the eye whose size is controlled by the iris. There are also two sets of muscles in the pupil, the circular muscles and the radial muscles. These are smooth muscles, meaning the action they perform is involuntary! I mean, when was the last time you saw someone control their pupils?

To dilate the pupils, the radial muscles contract while the circular muscles relax. To constrict the pupils, the circular muscles contract while radial muscles relax. Thus, these guys also form an agonist-antagonist muscle pair!

Ciliary bodies: Muscles that control the width and thickness of the lens by contracting and relaxing, regulating the degree to which light entering the eye is refracted.

Aqueous humour: A transparent fluid in the eye located between the iris and the cornea. It exerts pressure on the eye, supplies nutrients, and refracts light.

Lens: A transparent, biconcave structure which refracts light entering the eye. Its thickness and width are controlled by the ciliary bodies.

Vitreous humour: Transparent fluid in the eye located behind the lens and in front of the retina. It exerts pressure on the eye to maintain its shape and delivers nutrients to the retina.

Retina and fovea: The retina is a thin layer of light-sensitive tissue at the back of the eye. It contains photoreceptors, which are types of receptors sensitive to visible light.

Photoreceptors: There are two types of photoreceptors: the rods and the cones. Rods are sensitive to dim lighting conditions and help in night vision. However, they are not sensitive to colored light, which is why everything appears grey in dim lighting or darkness. Cones are sensitive to high-intensity lighting and colored light, but are useless at night.

The fovea is a point in the retina which has a large concentration of cones. Light that is projected onto the fovea creates the sharpest image.

Optic nerve and the “blind spot”: The optic nerve is a bundle of nerve cells that transmits electrical impulses from the photoreceptors to the brain where they are interpreted as images. It also regulates the iris reflex.

The blind spot is where the optic nerve cuts into the retina where there are no photoreceptors. Any light that is reflected onto the blind spot is not picked up by photoreceptors, so the brain automatically fills in the image in the blind spot based on information from the surroundings.

The movement of light into the eye is as follows:

Light → Cornea → Aqueous humour → Pupil → Lens → Vitreous humour → Retina → Fovea → Photoreceptors → Optic Nerve → Brain