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For us to sense the world around us, our sensory organs receive stimuli. The messages those organs produce go through a process known as transduction. This means that the signals are transformed into impulses that travel through the thalamus to the appropriate cortices. Our bodies are constantly sending signals to the brain, but if it were to register every sensation, it would be overwhelming. Imagine feeling, seeing, hearing, touching, etc.. everything. This doesn’t happen due to the combination of sensory adaptation (the decrease in responsiveness to stimuli due to constant stimulation) and sensory habituation (our perception of sensation is partially due to how focused we are on them.) Now that I mention it, you likely will start to feel the socks on your feet, even though you hadn’t noticed that before, as you weren’t focused on the sensation. This connects to the cocktail-party phenomenon which is the idea that as your focus changes, you ignore certain stimuli, for example, at a party, if you’re focused on your friend talking, and someone yells your name across the room, your focus may involuntarily shift to whoever is yelling at you, causing you not to hear what your friend is saying.

ENERGY SENSES

Vision

Step 1: Gathering Light: Light reflects off of surfaces and hits our eyes. The colour we perceive that surface to depend on a few things; one, the light intensity which is to say, how much energy is in the light that hits the eye. The second is the light wavelength, which determines the hue that we see. (Or if we even see it at all- wavelengths longer than visible light such as infrared waves, microwaves and radio waves or shorter than visible light such as UV waves and X-rays aren’t registered by the eye) A mix of all the visible hues of light produces white. When an object appears a certain colour, it has absorbed all other colours and reflected the colour we see it as. A green plant, for example, would not be able to survive in the green light, as it would reflect the green, causing it to starve. 

Step 2: Within the Eye: Reflected light enters the eye through the cornea which is a clear protective covering over the eye. The light travels through a hole known as the pupil. The iris (what make your eyes a certain colour) is a collection of muscles which work to constrict the pupil in extremely light situations, and expand the pupil in dark places- this is known as accommodation. The curved lens focuses the light like a camera focuses on an image. The image is flipped upside down as it travels through the lens. This now focused, inverted image project itself on the retina which works as a screen. On this screen are specialised neurons which are activated by light.

Step 3: Transduction: The light needs to be translated into neural impulses to be registered by the brain. There are two kinds of neurons which do that job. Cones, which are activated by colour and rods, which respond to black and white. Rods outnumber cones, and are especially useful in dark situations when colour cannot be registered at all; this is why in the dark, it is nearly impossible to figure out what colour an object is. There just isn’t enough coloured light being reflected for the cones to be activated properly. Cones are concentrated in the centre of the retina. At the very centre of the retina is the fovea; where the highest concentration of cones exists. Peripheral vision depends on cones, while your centred of vision depends on cones (in light, of course). Once enough rods and cones fire, they activate the next layer of bipolar cells; ganglion cells, whose axons make up the optic nerve, sending impulses to the lateral geniculate nucleus (LGN) of the thalamus. Messages are relayed to the visual cortex in the occipital lobes. The optic nerve has no rods or cones and is known as your blind spot (different to the driving blind spot). There are two parts to the optic nerve. Impulses from the right side of both retinas go to the right hemisphere, while impulses from the left side of both retinas go to the left hemisphere; it is that separation of the optic nerve that allows this to take place. The spot where the optic nerves cross each other is the optic chiasm.

Here’s a fun experiment to show where the blind spot is. (Here is the original link for anyone curious: https://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/- it has a few more interesting experiments to try as well)

Sit with your nose between the cross and black circle. Cover your left eye, and look at the cross with your right eye. Move towards the computer while still using your right eye. Eventually, the black circle will disappear and reappear, showing you your blind spot. Do the same with your left eye, except now focus on the black circle. Weird right?? (Science rules!) 

Step 4: In the Brain: There is still a lot of debate as to how our visual cortex actually works to register vision. Some say the visual cortex is where sensation ends and perception begins. Some say interpretation begins at the retina. Some say it begins at the thalamus. Scientists David Hubel and Torsten Wiesel were responsible for finding that groups of neurons in the visual cortex respond to different types of visual images. There are feature detectors for vertical lines, curves, motion, and so on.

Theories of Colour Vision

A lot is still unknown about how we see colour. The oldest theory is the Young-Helmholtz Trichromatic theory which hypothesised that we have 3 different typed of cones which detect, red, blue and green. They are activated in different combinations to produce the spectrum of colour we see. There are issues with this theory; namely afterimages and colour blindness. That’s where the opponent-process theory of colour vision comes in. (Not to be confused with the opponent-process theory of emotion. I’ll get to that in a later post). This theory states that sensory receptors are arranged in pairs: red/green, yellow/blue, and black/white. When one sensor is stimulated, its pair is inhibited. This explains after images. Try this illusion out: Stare at this image for 30 seconds, and then look at a white surface. If the flag doesn’t immediately show up try blinking a few times. 

What’s happening is that you’ve exhausted your cones, and so the opponents will fire. This is why dichromatic colour blindness also tends to be consistent as to which two colours are affected such as red-green colour blindness, or yellow-blue colour blindness

Hearing

While both vision and hearing are considered energy senses, they are very different stimuli. Sound waves are vibrations in the air, instead of electromagnetic waves used to produce images. The vibrations are captured by our ears and transduced into neural impulses which allow the brain to register and interpret the sound. Sound waves have amplitude and frequency. Amplitude is the height of the wave and determines how loud the sound will be. Frequency is the length of the waves and determines the pitch of the sound. High-frequency waves are densely packed together, while low-frequency waves are spaced apart. 

The process of hearing works much like a Rube Goldberg machine, where one thing moving activates the next part of the sequence. The pinna (outer ear) collects sound, which travels down the auditory canal. The sound reaches the tympanic membrane (the eardrum) which vibrates as sound hits it. Attached to the tympanic membrane are the ossicles; the smallest bones in your body. The tympanic membrane is connected to the hammer/malleus, which is connected to the anvil/incus, which is connected to the stirrup/stapes, which all transmit the vibration to the oval window, a similar membrane to the eardrum. The windows membrane is connected to the cochlea, a snail-like structure filled with perilymph, and endolymph. The vibration of the oval window causes these fluids to move. At the base of the cochlea is the basilar membrane, lined with hair cells connected to the Organ of Corti, which are neurons activated by the moving hair cells. The Organ of Corti fires neural impulses and those impulses travel to the brain via the auditory nerve.

Pitch Theories

Place Theory: Place theory states that hair cells in the cochlea respond to different frequencies based on their location. Some bend to high pitches, and others to low pitches.

Frequency Theory: Research has shown that while place theory is accurate for high pitches, but not for low tones, who are sensed by the rate at which the cells fire. We sense pitch because hair cells fire at different rates (frequencies) in the cochlea. 

Deafness

For each step in the hearing process, there are just as many ways those steps can go wrong. Conduction deafness refers to a problem conducting sound to the cochlea (so any point in the ear canal, to the eardrum, ossicles, or oval window). Sensorineural deafness occurs when the hair cells in the cochlea are damaged. Often this is caused by overstimulation, causing a high pitched ringing. Prolonged exposure to that much stimulation can severely damage the hair cells which do not regenerate. 

Touch

There is one more type of energy sense and it is just as different as the other two. Our skin contains millions of different types of receptors. Some (nociceptors) respond to pain, some (thermoreceptors) respond to temperature. Some (Messiner’s Corpuscles) respond to plain old touch. Some (Pacinian Corpuscles) respond to pressure, all of which can be found all over the skin. The relationship between these receptors is not well understood. What we do know is that the brain interprets the amount of indentation (temperature change) and the intensity of touch. Where nerve endings fire also determines where the touch is coming from. Certain parts of our body have more concentrated amounts of receptors. 

For example, in my physiology class, we did an interesting experiment where a friend took a paper clip and unfolded it until it had 2 ends. The friend would poke me on different parts of my skin with either 1 or 2 of the ends and see if I could guess how many ends I was being poked with. On the fingertip, I guessed with 100% accuracy, while on my shoulder, I was guessing the whole time. 

Gate-control theory explains how pain is experienced. Some pain messages are more important than others. When a priority message is sent, the “gate” swings open for it and swings shut for lower priority messages. This is why scratching an itch gets rid of the itch. The scratching is a higher priority message, and so the itch is temporarily ignored in order to prioritise the itching. That’s also why you shouldn’t scratch an itch- you’re not doing yourself any good. Endorphins also work to shut this gate, working like morphine to shut off any pain. 

CHEMICAL SENSES

Gustation/Taste

Fun fact before I start: I remember that taste is gestation because of Gusteau from Ratatouille who was named after the French word for taste. When you eat food, the tastebuds are not responding to energy, but rather the chemicals on the tongue. These tastebuds are located on papillae, which are bumps on the tongue. There are 5 different kinds of tastebuds scattered throughout the tongue which respond to Sweet, Salty, Bitter, and Umami (savoury). The more densely packed the taste buds, the more intense the taste is. The tongue is not unique to taste; you’ll know this if you’ve ever tried to eat a jalapeno because spicy food stimulates nociceptors, which is why the experience is agonising for some people. Weaker nociceptors increase a person’s spice tolerance, which is why certain cultures do better eating spicy foods; they’ve adapted to the pain.

Smell/Olfaction

Olfaction is created by chemicals entering the nose and being absorbed by receptors on the mucous membrane. A lot is not known about these receptor cells- some researchers estimate there are as many as 100 different types. These cells are linked to the olfactory bulb which gathers messages from olfactory receptor cells and relays them to the brain. The nerve fibres from the olfactory bulb connect to the brain differently than all the other senses; instead of going to the thalamus, the message is sent to the amygdala and then the hippocampus. This could be why smells can trigger memories much more strongly than other senses can.

BODY POSITION SENSES

Let's get something out of the way: you do not have 5 senses. How many you have depends on who you ask, but a prominent one is the sense of position.

Vestibular Sense

Your vestibular sense helps you determine where your body is in space. The semicircular canals in the inner ear are responsible for this. The canals are tubes partially filled endolymph. When the head moves, the fluid moves too. The hair cells activate, sending signals to the brain telling it that the head has moved. Dizziness is caused when the brain receives several conflicting signals in a short period of time, caused by rapid movement, or when you’re in a boat, and the eyes register a static view, while the inner ear is registering all kinds of movement.

Kinesthetic Sense

While vestibular sense tracks the overall orientation of the body, kinesthetic sense tracks the position and orientation of specific body parts. Receptors in muscles and joints send information to the brain, helping the brain track the body.

PERCEPTION

Perception is the process of understanding and interpreting sensations. Psychophysics is a branch of psychology which studies the interaction between sensations and how we experience them.

Thresholds

Senses have limits. The absolute threshold is the smallest amount of stimulus we can detect. There are several videos online helping people track the absolute threshold of pitch that they can detect by playing higher and higher frequencies. The proper definition of the absolute threshold is the minimal amount of stimulus we can detect 50% of the time as things such as other sensory inputs can make studying the absolute threshold extremely difficult. Stimuli below the absolute threshold are referred to as subliminal.

Another important threshold is the difference threshold. This is also referred to as the just-noticeable difference and is rather self-explanatory. It is the smallest amount of change in a stimulus needed for us to detect a change. This threshold is determined by Weber’s Law, discovered by psychophysicist Ernst Weber (it is sometimes called the Weber-Fechner law.) The change needed is proportional to the original intensity of the stimulus. The more intense a stimulus, the more it will need to change before we notice a difference. Each sense varies according to a constant (which differs between the senses). The constant for hearing is 5%- when listening to a 50-decibel tone, it would need to reach 52.5 decibels to notice a difference. 

Perceptual Theories

Signal Detection Theory: This theory investigates the effects of distractions and interference we normally experience in our lives. This area of research revolves around attempting to predict what we will perceive. It takes into account how motivated we are to perceive specific stimuli and our expectation. These are known as response criteria or receiver operating characteristics. A false positive is when we think we are perceiving a non-present stimulus. While a false negative is the failure to perceive a present stimulus.

Top-Down Processing: When we use top-down processing, we perceive by filling in gaps what we sense. For example, when reading this sentence: I _m t_king p_ych_ _ gy, you can pretty easily fill in the blanks based on the context of what you read before, and the letters I did give you. In other words, top-down processing uses background knowledge to fill in gaps. Our schemata are mental representations of how we think the world should be. They can create a perceptual set or a predisposition to experience something a specific way.  https://www.youtube.com/watch?v=Bafuyg98no4- Watch this video about backmasking (”hidden” messages in songs when played backwards.) Try looking away when the song starts playing and then reading the supposed message. I know personally, when I did this, I couldn’t understand the hidden message until I read what I was supposed to be hearing, and then I could hear it clearly. Several parents became worried about this phenomenon, likely because they had schemata of the music their kids were listening to as dangerous. 

Bottom-Up Processing: This is also known as feature analysis, where only the features of the object are used to build a perception. This is an automatic process, however, is much slower than top-down processing. As a result, it is less prone to error.

Principles of Visual Perception

There are numerous rules for visual perception. One of the first decisions the brain makes is the figure-ground relationship. This is self-explanatory; what is the figure and what is the background? A famous illusion connected to this principle is the vase face illusion. Do you see the background as black or white? Do you see the figure as the vase or the face? 

Gestalt Rules

Gestalt psychologists at the beginning of the twentieth century noted several principles which explain how we perceive groups of objects, and theorised that we perceive images as groups rather than individuals. (Hence the name “Gestalt”) Here is a great diagram from https://www.verywellmind.com/gestalt-laws-of-perceptual-organization-2795835 illustrating those principles.

Proximity: Objects that are close together are more likely to be perceived as a part of the same group

Similarity: Objects that look similar to each other are likely to be perceived as part of one group

Continuity: Objects creating a continuous form are likely to be perceived as part of the same group

Closure: Similar to the sentence connected to top-down processing. Objects making a recognisable image are likely to be viewed as part of the same group

Law of Pragnanz: You are more likely to perceive and interpret complex images as the simplest form possible

Law of Common Region: Objects grouped together within the same region of space tend to be viewed as their own group.

Constancy

Our ability to maintain a constant perception of an object despite a constantly changing world is known as constancy. Here are a few types.

Perceived Motion

Our brains can detect how fast images move across the retinas and use our own motion to determine the speed the object is moving. This causes some interesting phenomena where the brain thinks a still object is moving. For example, you may be familiar with the stroboscopic effect. This is what causes perceived motion in flipbooks. There is also the phi phenomenon. Movie Marquees use this phenomenon by turning on and off lights sequentially, creating the appearance of one moving light.

Depth Cues

Depth perception is what allows us to perceive the world in 3-dimensions rather than 2. Eleanor Gibson performed an interesting experiment known as the visual cliff experiment, which found that crawling toddlers can perceive depth. In this experiment, the infant is placed onto one side of a glass table, creating the appearance of a cliff. The infant would not crawl onto the glass. There are two categories of cues that we use; monocular (needing one eye) and binocular (needing two)

Monocular Cues

Artists tend to use monocular cues to create the illusion of depth in their art. A wonderful artist on deviant art created a really helpful diagram, illustrating each of these ideas. https://www.deviantart.com/akenon/art/Monocular-Cues-34065283

Linear Perspective- In order to draw a faraway house, artists tend to draw two lines which converge at a focal point, taking advantage of the linear perspective cue. 

Relative Size Cue- When drawing two similarly sized cats, one far away and one nearby, an artist will draw the faraway cat as significantly smaller, creating the illusion of distance.

Interposition Cue- If the artist draws a cat that is partially obscured by a tree, we will view the tree as closer than the cat.

Texture Gradient- Another way artists create the illusion of distance is by making close up objects more detailed than those faraway

Shadowing- By using shading in an image, an artist can imply a light source, further implying depth in the image. 

Binocular Cues

Binocular Disparity/Retinal Disparity- Each eye sees an object at a slightly different angle. The farther away an object is, the more similarly the eyes will perceive it, the closer it is, the more retinal disparity there will be. Notice in this diagram how the trees don’t seem to move, but the finger moves drastically?

Convergence- As an object comes closer to our face, the eyes must move towards each other to keep it in focus. The more the eyes converge, the closet the object is. 

How Culture Affects Perception

One of the more interesting (in my opinion) parts of psychology is exploring how culture affects us. Some of the perceptual rules that for years psychologists believed to be innate are actually learnt. Cultures which don’t use monocular depth cues in their art don’t see depth in pictures using these cues. It also affects how optical illusions affect our brains. The famous Muller-Lyer illusion often does not affect cultures that don’t use right angles and corners in architecture. 

 This is a brief overview of some specific human organ systems and processes. It will be pretty long.

Digestion

Digestion is a process where large food molecules are broken down and absorbed for the body to use. Fats are broken down into glycerol and fatty acids, polysaccharides like starch are broken down into monosaccharides, nucleic acids are broken into nucleotides, and protein is broken into amino acids. Vitamins and minerals are already small enough to be absorbed.

The digestive canal is known as the alimentary canal and is around 30 ft long spanning from the mouth to the anus, and made of smooth (involuntary) muscle which moves food along via peristalsis. The autonomic nervous system controls digestion.

Mouth

The mouth marks the beginning of both chemical and mechanical digestion. The enzyme salivary amylase begins to break starch down, meanwhile, the tongue and teeth work to mechanically grind food down into pieces small enough to be swallowed. Animals teeth is a good indicator of what diet it has. Humans have three kinds of teeth; incisors for cutting, canines for tearing, and molars for grinding, indicating that we are omnivores.

Oesophagus

The oesophagus is only the tube that moves food further down the alimentary canal, and into the stomach. The oesophagus and trachea (windpipe) are connected, so a flap of cartilage at the back of the pharynx (throat) called the epiglottis closes down.

Stomach

Like in the mouth, both chemical and mechanical digestion occurs here. The stomach is where protein digestion starts. With its thick muscular walls, it begins to churn food and secretes gastric acid, made of hydrochloric acid and enzymes that digest proteins. Hydrochloric acid eats away at the muscle and activates pepsinogens which digest the proteins. Meanwhile, the protein rennin breaks down the protein commonly found in milk. The environment in the stomach is extremely acidic, especially in comparison with the rest of the body. The human body remains at a neutral 7-7.5 range, while inside the stomach, the pH drops to around 2-3. The cardiac sphincter at the top of the stomach keeps acidified foods, and gastric acid from escaping into the oesophagus, while the pyloric sphincter at the bottom keeps food in the stomach until it is ready to be digested. 

Ulcers in the stomach, oesophagus, and duodenum (the first section of the small intestine) is caused by excessive acid.

Small Intestine

The small intestine finalises digestion. In fact, digestion finishes in the duodenum which makes up only 10 inches of the 6-metre long small intestine. Its pH is slightly basic, at 8. The enzymes in the intestine are amylases, proteases, lipases, and nucleases. The pancreas secretes pancreatic amylases into the small intestine to aid in starch digestion. Peptidases, like trypsin and chymotrypsin, continue to break down proteins. 

The small intestine is lined with fingerlike projections called villi which absorb nutrients and increase the surface area of the small intestine. Each villus contains capillaries which absorb amino acids, vitamins, and monosaccharides directly into the bloodstream and a lacteal which absorbs fatty acids and glycerol into the lymphatic system. Villi have their own projections known as microvilli.

Liver and gallbladder

The liver produces bile which emulsifies fats. Its pH is 11 and neutralises chyme (the acidified food that has come out of the stomach). The bile is sent to the gallbladder which stores bile until it is needed in the small intestine.

The liver also breaks down and recycles red blood cells, detoxifies the blood (removes drugs and alcohols) produces cholesterol which provides structure to cell membranes and produces our nitrogenous waste, urea via protein metabolism.

Pancreas

Along with producing the hormone insulin, the pancreas plays an essential role in the digestive system. It produces enzymes like amylases, peptidases, lipases, and secretes them into the small intestine. It also produces sodium bicarbonate which helps neutralise the chyme from the stomach.

Large Intestine

Once food has reached the large intestine, digestion is complete. Instead, egestion, vitamin production, and reabsorption of water occur here. Egestion is the removal of undigested waste. Bacterial symbionts who live in the large intestine produce vitamins B, K, and folic acid. When reabsorption of water goes wrong, it leads to constipation or diarrhoea. 

Rectum

The rectum completes egestion. Here, faeces are stored until it is released through the anus.

Gas Exchange

Air enters through the nasal cavity, where it is moistened, warmed, and filtered. After that, air passes through the larynx, into the trachea, into the bronchi, and to the bronchioles, which end with microscopic air sacs called alveoli.

Unlike earthworms, humans have an internal respiratory surface., The rib cage expands, causing the diaphragm to contract, and move down, causing the chest cavity to expand, and the internal pressure to decrease. Air is drawn into the lungs because of the negative pressure, as pressure moves from areas of low pressure to areas of high pressure.

The medulla in the brain stem sets the breathing rhythm by monitoring CO2 levels, and pH levels in the blood. When the blood’s pH drops to below 7.4, autonomic nerves in the medulla increase the breathing rate.

Transport of Oxygen and Carbon Dioxide

Oxygen is carried through the blood by the protein haemoglobin, which combines with oxygen to form oxyhaemoglobin. CO2, the byproduct in cellular respiration is dissolved into the blood. CO2 is carried in the plasma as the reversible blood-buffering carbonic acid-bicarbonate ion system which keeps the blood pH levels at 7.4.

Circulation

The human circulatory system is a closed circulatory system with arteries, veins, and capillaries. Arteries carry blood away from the heart, and are under enormous pressure, as their walls are made of a thick layer of elastic, smooth muscle. Veins carry blood back to the heart and are not under as much pressure. You can tell the difference between veins and arteries, mainly because veins do not have thick muscular walls. They have valves to prevent backflow and are found within skeletal muscles which propel blood upward and back to the heart. Capillaries allow for the diffusion of nutrients and wastes between cells and blood. The walls are one cell thick, so blood has to travel single file through it.

Blood

Blood is made of thrombocytes (platelets), erythrocytes (red blood cells), and leukocytes (white blood cells) suspended in a liquid matrix called the plasma. The average human body contains 4 to 6 litres of blood.

Thrombosis

Thrombosis is also known as blood clotting. It begins when clotting factors from thrombocytes and damaged tissues are released. Anticlotting factors circulate in the plasma to prevent the development of a thrombus where it is not needed. Serum is plasma without those clotting factors.

Components of Blood

The plasma, along with clotting factors contains hormones, antibodies, dissolved gases, nutrients, and wastes. It is 90% water. Erythrocytes contain haemoglobin and oxygen. They do not have a nucleus and have a lifespan of around 120 days. They are made in the bone marrow and recycled in the liver. Leukocytes are responsible for fighting infection. They too are formed in the bone marrow. When you get pus from infection, that is actually the dead leukocytes and bacteria. (War is hell). Finally, thrombocytes, which as mentioned above, form thromboses. They are actually cell fragments formed in the bone marrow from megakaryocytes,

The Heart

The heart is a muscle found beneath the sternum. On average, it beats 70 times per minute and pumps 5 litres of blood per minute. It has two atria that receive blood, and two ventricles which pump blood. 

The hearts natural pacemaker is called the sinoatrial (SA) node. Electrical impulses travel through the cardiac and body tissues to the skin, where they are able to be detected by an electrocardiogram (EKG). Blood pressure is the lowest in the veins, and highest in the arteries. When you get a blood pressure rating, like 120/80 (the average for resting adults), the high number is the systolic pressure, measuring when the ventricles contract and the low number is the diastolic number, measuring when the heart relaxes.

When you see a diagram of the heart, it is flipped, as you are looking straight at it.

Pathway of Blood 

Blood enters the heart from the superior and inferior vena cava. From there it travels to the:

Right atrium

Right atrioventricular (AV) valve, also known as the tricuspid valve

Right ventricle

Pulmonary semilunar valve

Pulmonary artery

Lungs

Pulmonary vein

Left atrium

Left atrioventricular (AV) valve, or bicuspid/mitral valve

Left ventricle

Aortic semilunar valve

Aorta

The cells in the body

Back to the vena cava

Blood circulates through the coronary circulation (the heart), renal circulation (the kidneys), and hepatic circulation (the liver)

Endocrine System

Animals have two regulatory systems that release chemicals: The endocrine system, and the nervous system. Together they work to maintain homeostasis. The endocrine system secretes hormones, and the nervous system releases neurotransmitters.

Hormones are produced in ductless (endocrine) glands and move through the blood to their target cell, tissue, or organ. They produce either an immediate short-lived response or a dramatic long-term development of a whole organism, for example, adrenaline (the fight or flight hormone) vs. ecdysone (the hormone that leads to metamorphosis) 

Tropic hormones are hormones that stimulate other glands to release other hormones and have far-reaching effects. For example, the anterior pituitary releases the thyroid-stimulating hormone (TSH), which causes the thyroids to release thyroxin. Other chemical messengers reach their target by different means, such as the pheromones found in a dogs urine.

The Hypothalamus

The hypothalamus is a part of the brain, and plays a key role, as it is the bridge between the nervous system and the endocrine system. During times of stress, it sends electrical signals to the adrenal glands, however, it also produces oxytocin and antidiuretic hormone which are stored in the pituitary gland. It also contains the body’s thermostat and regulates hunger and thirst. Some other glands include:

The Anterior Pituitary- releases growth hormone, which stimulates bone growth, the luteinizing hormone, which stimulates the growth of ovaries and testes, the thyroid-stimulating hormone, which stimulates the thyroid gland, the adrenocorticotropic (ACTH) hormone, which stimulates the adrenal cortex to secrete glucocorticoids, and the follicle-stimulating hormone (FSH) which stimulates the gonads to produce sperm and ova.

Posterior Pituitary- releases oxytocin, which stimulates the contraction of the uterus and mammary glands, and the Antidiuretic hormone which promotes the retention of water

Thyroid- releases thyroxin which controls the metabolic rate, and calcitonin, which lowers blood calcium levels.

Parathyroid- releases parathormone, which raises blood calcium levels

Adrenal Cortex- releases glucocorticoids which raises blood sugar levels

Adrenal Medulla- releases epinephrine (adrenaline) and norepinephrine which raises blood sugar level by increasing the rate of glycogen break down.

Pancreas- releases insulin which lowers blood glucose levels, and glucagon, which raises blood glucose levels.

Thymus- releases thymosin which stimulates T-lymphocytes during the immune response.

Pineal- releases melatonin which is involved in biorhythms (helps induce sleep)

Ovaries- release estrogen which stimulates the uterine lining, promotes the development and maintenance of secondary female sex characteristics and progesterone which promotes uterine lining growth

Testes- release androgens which support sperm production and promote secondary sex characteristics in men

Cell Response 

There are two hormone types: steroidal or nonsteroidal, also known as polypeptide hormones. They stimulate their target cells in different ways.

Since steroid hormones are lipids, they can diffuse directly through the cell membrane, and bind to a receptor within the cell. 

Protein or polypeptide hormones cannot dissolve through the cell membrane, so they have to bind to a receptor on the surface of the cell. When the hormone binds to a receptor, it triggers a second messenger such as c-AMP which coverts the chemical signal into a response.

Feedback Mechanisms

Feedback mechanisms are self-regulating mechanisms that increase or decrease an action or the level of a particular substance.

Positive Feedback

Positive feedback enhances a response that already exists. A perfect example is during childbirth, when the pressure of the baby's head against the uteruses opening of the uterus stimulates more uterine contractions, leading to increased pressure. The feedback loop ends when the baby is born.

Negative Feedback

Negative feedback maintains homeostasis. It releases or decreases the number of hormones, minerals, etc in the body. For example, when thyroxin levels in the body are too low, the hypothalamus stimulates the anterior pituitary to release TSH, which causes the thyroid to release more thyroxin.

Nervous System

The nervous system is made of two parts: the central nervous system, made of the brain and spinal cord, and the peripheral nervous system, made of the nerves outside of the CNS. The peripheral nervous system is further divided from there.

Peripheral Nervous System

Sensory- Conveys information from sensory receptors or nerve endings

Motor- Stimulates voluntary and involuntary muscles. Consists of two systems:

Somatic System- controls the voluntary muscles

Autonomic System- controls the involuntary muscles

Sympathetic: 

Fight-or-flight response

Increases heart and breathing rate

The liver converts glycogen into glucose

Bronchi in the lungs dilate to increase gas exchange

Adrenaline increases blood glucose levels

Parasympathetic:

Opposes the sympathetic system

Calms the body

Decreases heart and breathing rate

Enhances digestion

The Neuron

The neurons are the cells that make up the nervous system. It contains a cell body, dendrites to receive electrical impulses, an axon where the impulse travels down to the synapse where neurotransmitters are released through the synaptic cleft. Most axons are wrapped in a myelin sheath which protects the axon and increases the speed of neural impulses.

The Reflex Arc

The reflex arc is the simplest nerve response. When you touch your finger to a hot stove, the reflex arc has you pull your finger away before your brain has time to register what is happening. As your finger touches the stove, the sensory neurons on it are triggered. They send an impulse to interneurons within the spinal cord. These interneurons send an impulse to motor neurons, which move the impulse towards the muscles in the finger, jerking it away.

How a Neuron Functions

All cells have a membrane potential which is a difference in electrical charge between the cytoplasm (negative charge) and the extracellular fluid (positive charge). This difference is between -50mV and -100mV. When this number is negative, it means that the inside of the cell is negative in relation to the outside.

Resting Potential

An unstimulated neuron is polarised and has a membrane potential of around -70mV. The sodium-potassium pump maintains this polarisation by pumping ions that leak inward. A stimulus must be strong enough to overcome the resting threshold or resting potential. The larger the membrane potential, the stronger the stimulus be to cause the nerve to fire.

Action Potential

An action potential is also known as an impulse and is generated in a neuron’s axon. When a stimulus overcomes the threshold, the permeability of the membrane changes. Sodium channels open, and sodium floods into the cell, against the concentration gradient. Potassium channels then open, and potassium floods out of the cell. This movement of ions is known as a wave of depolarisation and reverses the polarity of the membrane. This is the action potential. It is localised and is short-lived. The sodium-potassium pump returns the sodium and potassium where they belong. This period of repolarisation is known as the refractory period. During this time, the neuron cannot respond to another impulse.

The Synapse

As the electrical impulse reaches the synapse, it stimulates the vesicles at the terminal branch of the neuron. These vesicles contain chemical neurotransmitters. When the synaptic membrane is depolarised, Ca++ ions rush into the terminal branch, causing the vesicles to fuse with the presynaptic membrane, and release the neurotransmitters via exocytosis.

The Eye and The Ear

Eye

Cones- photoreceptors in the retina that distinguish between different colours

Cornea- tough, clear covering that protects the eye and allows light to pass through

Humour- fluids that maintain the shape of the eyeball

Iris- coloured part of the eye that controls how much light enters the eye

Lens- focuses light onto the retina

Pupil- a small opening in the middle of the iris

Retina- converts light into nerve impulses that are carried to the brain

Rods- photoreceptors in the retina that are sensitive, but cannot distinguish between different colours

Ear

Auditory Canal- ear canal, where sound enters

Cochlea- fluid-filled part of the ear, sends nerve impulses to the brain

Ear bones- hammer, anvil, and stirrup, transmit vibrations from eardrum to oval window

Eustachian tube- equalises pressure between environment and inner ear

Oval window- sends waves of pressure to the cochlea

Semicircular canals- fluid-filled, helps you maintain balance

Tympanum- eardrum, vibrates as sound hits it

Excretion

Excretion is the removal of metabolic wastes, including CO2, and nitrogenous wastes. The organs of excretion are the skin, lungs, liver, and kidneys. The skin excretes sweat with water and salts, including urea, the lungs excrete water vapour and CO2 from the Krebs cycle, the liver does not excrete things from the body, however, it deaminates amino acids and produces urea, and the kidneys excrete water and urea.

The Kidneys

Depending on how dehydrated the body is, the kidney adjusts the concentration of urine. Kidneys get their blood through the renal artery, and filter around 1,500 litres of blood per day, and produce, on average, 1.5 litres of urine. The kidney responds to the bodies water needs by the Antidiuretic hormone, released by the anterior pituitary, which targets the collecting tube of the nephron. It controls how much water is reabsorbed by the kidneys.

The Nephron

The nephron is the basic functional unit of the kidney. It consists of a cluster of capillaries, known as the glomerulus, which sits within a cuplike structure called Bowman’s capsule, a long narrow tube called the tubule, and the loop of Henle. Each kidney contains around 1 million nephrons. The nephron does its job in four stages: filtration, secretion, reabsorption, and excretion.

Filtration

Filtration depends on diffusion. It is passive and nonselective. The filtrate is made of anything small enough to diffuse through the glomerulus and into the bowman’s capsule. This includes glucose, salts, vitamins, urea etc. From Bowman’s capsule, the filtrate travels into the loop of Henle and then the collecting duct or tubule. From there, the filtrate trickles into the ureter, and the urinary bladder for storage, then to the urethra, and out of the body,

Secretion

Secretion is the active, selective uptake of molecules that did not filter into the Bowman’s capsule, in the nephron’s tubules.

Reabsorption

Reabsorption is the process by which most of the water and solutes (glucose, amino acids, and vitamins) that entered the tubule during filtration and returned to the capillaries. The longer the loop of Henle, the greater the reabsorption of water.

Excretion

Excretion is the removal of metabolic wastes. Everything that moves into the collecting tubule is excreted from the body.

Muscles

There are 3 types of muscles; smooth, cardiac, and skeletal

Smooth or involuntary muscle makes up the walls of blood vessels and the digestive tract. It is not striated and is controlled by the autonomic nervous system

Skeletal or voluntary muscles are large, striated, and multinucleate. They work in pairs, contracting and relaxing.

Cardiac muscle is muscle found in the heart and is also striated. It is able to generate its own action potential, so each cell can beat on their own.

The Sliding Filament Theory

Within the cytoplasm of skeletal muscle cells are fibres called myofibrils. They run parallel to the length of the cell. They contain thick and thin filaments. These filaments contain actin proteins. Each filament is composed of myosin proteins. Muscles contract when thick and thin filaments slide over each other.

The Immune System

Our bodies are designed to fight the numerous microscopic organisms designed to hurt us. These microscopic organisms are called pathogens. In this way, our body is almost like a fortress, lined with booby traps, walls, and guards ready to fight any intruders who get in.

First Line of Defence

The first line of defence is nonspecific. They are barriers that keep pathogens out.

We have skin, the mucous membrane that traps microbes, cilia in the respiratory system that sweep mucus lined with microbes out, and stomach acid that kills any pathogens that get through the nose and mouth.

Second Line of Defence

Microbes that get through the barriers have another layer of nonspecific defence to get through. There are three types. 

There is the inflammatory response that causes redness, swelling, and soreness and warmth, increasing the blood supply, nutrients, and white blood cells. Histamine is a chemical that induces the inflammatory response. Along with that, sometimes fever is used to make it harder for microbes to survive.

There are phagocytes, which eat microbes. One type of white blood cell, called macrophage extend pseudopods and engulf microbes.

Finally, there are interferons, which are chemicals that block against viral infection.

Third Line of Defence-Specific Defence

The third line of defence relies on lymphocytes, another kind of white blood cell. There are B lymphocytes and T lymphocytes. These are made in the bone marrow, and circulate the blood, lymph, and lymphatic tissue: the spleen, lymph nodes, tonsils, and adenoids. They recognise antigens (anything that triggers an immune response)

B lymphocytes produce antibodies against a specific antigen. This is called a humoral response.

T lymphocytes fight pathogens in what is called cell-mediated response.

Antibodies

Antibodies are designed to fit only one particular antigen. They neutralise antigens by binding to them and forming an antigen-antibody complex that a phagocyte will eat.

Clonal Selection

Clonal selection is an essential mechanism in the development of immunity. Antigens that enter the body bind to specific T or B lymphocytes. When a lymphocyte has been selected, it becomes very metabolically active, proliferates, and differentiates into plasma and memory cells.

Plasma cells fight antigens immediately in the primary immune response. They are not long-lived.

Memory cells live the lifetime of the person. Memory cells circulate through our blood that is specific to the diseases of which they fought, whether they were diseases we had or diseases we were vaccinated for. The ability of the immune system to generate a secondary immune response is called immunological memory.

Types of Immunity

Passive immunity is temporary. Antibodies are borrowed and do not survive long. For example, maternal antibodies pass through the placenta to the fetus, or from the breast milk to the baby.

Active immunity is permanent. Antibodies are made on their own.

ABO Blood Types

ABO antibodies circulate in the plasma of the blood and bind with ABO antigens if an improper transfusion takes place. To prevent this, a persons blood is cross-matched to insure their transfusion will be compatible.

Blood type O has no blood cell antigens that can be clumped, so it is the universal donor. Blood type AB has no antibodies to clump the donor's blood, so it is the universal receiver.

AIDS

AIDS stands for acquired immune deficiency syndrome. It is caused when the HIV virus (human immunodeficiency virus) attacks T cells. It is a retrovirus, transcribing itself in reverse, making its own DNA. When the white blood cell count drops, the victim is highly susceptible to infections they normally would have been able to fight off. 

Allergies, Antibiotics, Vaccines, and Autoimmune Diseases

Allergies are hypersensitive immune responses to substances called allergens. They cause an excessive amount of histamine to be released, causing blood vessels to dilate. In extreme cases, the victim will go into anaphylactic shock, where their throat closes up and they are no longer able to breathe.

Antibiotics are medicines that are able to fight specifically bacteria or fungi. The overprescription of antibiotics leads to strains of resistant bacteria, due to natural selection.

Vaccines train the immune system to generate antibodies for certain viruses, as once you have a virus, there is no way to get rid of it. All you can do is reduce the viral load. Most viruses go dormant, however for the few that can ruin a life, vaccines are essential.