Asexual and Sexual Reproduction

Animals can reproduce by means of asexual or sexual reproduction that best suit their population, the environment, or other prerequisites. In this article, we will discuss the different mechanisms regarding the asexual and sexual reproduction of invertebrates and vertebrates.

Mechanisms of asexual reproduction

Invertebrates carry out the several simplest forms of asexual reproduction.

Budding: new individuals arise from the outgrowth of existing ones; in any case of budding, you can observe the trait “connected individuals,” because the offspring and parents are connected together.

Fission: the splitting and separation of a parent organism into two individuals of about the same size; e.g. binary fission in bacteria.

Fragmentation and regeneration: as the name confirms, this type of reproduction is a two-step process; appears in annelids, sponges, corals, cnidarians, and tunicates.

Parthenogenesis: happens when an egg develops without being fertilized; it can be observed in both invertebrates and vertebrate (much rare). Among invertebrates, this can occur in species of bees, wasps, and ants. Male bees, for instance, are fertile haploid adults that are not fertilized, while female bees are fertile diploid adults. Among vertebrates, such as the Komodo dragon and hammerhead shark, females can produce genetically-identical offspring as a rare response to low population density.

Variations of sexual reproduction

Normally, among vertebrates, as in humans, sexual reproduction involves simply the mating between a male and a female individual. However, for many sexual animals, finding a mate can be difficult, and this is the reason for the emergence of deviations of sexual reproduction.

Hermaphroditism: characterized by an individual having both the male and female reproductive system; this type of adaption makes the finding of a mate easier in some vertebrates (any two individuals can mate).Sometimes, self-fertilization is possible.

Sex reversal: individuals can transform between male and female; this is especially useful for sedentary animals like oysters and corals, because by transforming into males during the time of ovulation, more gametes (sperm) is released into the environment so chances of fertilization can greatly increase.

Parthenogenesis of bees.

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Axonal Transport

Recall that in neurons, peptide transmitters such as glycine and GABA are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals. But how is this process carried out? In this article, you will learn the specifics of axonal transport, which is responsible for the transport of substances across a neuron.

Preface

Axonal transport relies on microtubules that run along the neural axon and serve as ‘tracks’ for motor proteins like kinesin and dynein. They are capable of binding to and transporting mitochondria, cytoskeletal polymers, and synaptic vesicles containing neurotransmitters. Axonal transport is categorized by fast or slow, anterograde (away from the cell body) or retrograde (to the cell body).

Fast and slow transport

Motor proteins carrying vesicular cargoes move relatively fast compare with those transporting cytoskeletal proteins or organelles such as mitochondria.

Through fluorescence microscopy, it is discovered that slow transport is not indeed slower than fast transport. Instead, slow transport processes with a mechanism called "Stop and Go," in which motor proteins constantly pauses during the transport, making the overall transit rate much slower.

Anterograde transport

Anterograde transport moves molecules/organelles outward from the cell body to the axonal terminal or cell membrane. Kinesin is responsible for both the fast and slow transport of cargo during an anterograde transport. It is good to remember that kinesin is an ATPase and is also involved in the separation of chromosomes during mitosis and meiosis. Lipids, proteins, and substances packed in vesicles that cannot be synthesized in the axon or the axonal terminal, as well as organelles like mitochondria, uses anterograde transport to travel across the neuron.

Retrograde transport

Retrograde transport, on the other hand, carries molecules/organelles away from the axonal terminal towards the cell body. Utilizing dynein as the motor protein, retrograde transport returns used synaptic vesicles, deteriorated organelles, and carries signals from the synapse back to the cell body.

A view of axonal transport.

Reference:

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.

Summary of Neurotransmitters

Neurotransmitters are molecules responsible for the carrying of messages (signals) between neurons. It involves various areas, including biology, psychology, and chemistry, making it an especially crucial topic. Today, we will discuss the major neurotransmitters used by the human body and also their significances.

Acetylcholine

Acetylcholine is a transmitter used by all motor axons, autonomic preganglionic neurons, postganglionic parasympathetic fibers, and some cells of the motor cortex and basal ganglia. It is the chief neurotransmitter of the parasympathetic nervous system, which contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows the heart rate (vasodilator).

*preganglionic neurons connect the CNS to the ganglia (groups of neuron cell bodies in the peripheral nervous system), and postganglionic neurons connect the ganglia to the effector organ. The terms autonomic, parasympathetic, motor cortex (the region of the cerebral cortex involved with voluntary movements), and basal ganglia (a group of subcortical nuclei in the brains of vertebrates) will be discussed in later articles.

In the central nervous system, acetylcholine appears to have multiple roles, such as thought, memory, and learning; it is in abnormally short supply in the brains of persons with Alzheimer's disease.

Biogenic Amines

Biogenic amines include epinephrine, norepinephrine, dopamine, histamine, and serotonin. The first three belong to the category catecholamine (neurotransmitters containing the catechol, benzene with two side-by-side hydroxyl groups). Histamine and serotonin have otherwise composition.

The catecholamines are all derived from the amino acid tyrosine that is catalyzed by the enzyme tyrosine hydroxylase into DOPA. Information about this detailed process will be explained in another article regarding adrenergic transmission [click to understand more].

Dopamine is produced by the action of the enzyme DOPA carboxylase on DOPA. It is present in several brain regions. Dopamine coming from the substantia nigra into the corpus striatum represents the major dopamine activity, and it plays an essential role in the coordination of body movements. Dopamine is also believed to be involved in motivation, reward, and reinforcement.

In Parkinson's disease, the dopaminergic neurons of the substantia nigra degenerate, leading to characteristic motor dysfunction.

Norepinephrine is synthesized from dopamine by the enzyme dopamine β-hydroxylase. It is the primary transmitter for postganglionic sympathetic neurons, which is released into the blood from the adrenal medulla. In the brain, norepinephrine is synthesized by the locus coeruleus, a nucleus (cluster of neurons in the CNS) in the pon, where it influences sleep and wakefulness, attention, and feeding behavior.

Epinephrine is produced from norepinephrine by the enzyme phenylethanolamine-N-methyltransferase (PNMT). It is released by chromaffin cells (neuroendocrine cells) of the adrenal medulla and the rostral ventromedial medulla (RVM), a group of neurons in the medulla oblongata. Epinephrine plays an important role in the fight-or-flight response, as well as regulating visceral functions (e.g. respiration)

Histamine is present in neurons of the hypothalamus. In the brain, histamine mediates arousal and attention, similar to that of ACh and norepinephrine. It is also released from mast cells in response to allergic reactions or tissue damage.

Serotonin is found in high concentrations in the raphe region of the pons and upper brainstem and has a wide projection to the forebrain. It involves a wide range of behaviors, including sleep and wakefulness, cognition, the feeling of happiness, etc. (unnecessary to really know its specific effect)

Reuptake by the presynaptic membrane is a major factor in terminating transmitter action of the biogenic amines.

Amino Acids

Amino acids include glycine,y-aminobutyric acid (GABA), glutamate, and aspartate.

Glycine is an inhibitory transmitter in spinal interneurons. GABA is an inhibitory transmitter of the central nervous system. Both of them generate IPSPs via ligand-gated Cl- channels. Glycine is produced by the mitochondria, whereas GABA is synthesized from glutamate.

Glutamate and aspartate are excitatory transmitters of the CNS that generate EPSPs, and are products of the Krebs cycle. Both glutamate and aspartate are produced in the mitochondria

While non-peptide transmitters are synthesized in nerve terminals, peptide transmitters are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals [click to understand more]. Both glutamate and aspartate are produced in the mitochondria

Reuptake by presynaptic membranes is a major factor terminating the transmitter action of the amino acids.

Nitric Oxide (NO)

Unlike other transmitters, NO is neither packaged in vesicles nor released by exocytosis. As a gas, it readily diffuses across cell membranes to adjacent target tissue once being synthesized. NO is an inhibitory transmitter in the central and enteric nervous systems. In smooth muscles, NO is responsible for vasodilation and increasing blood flow (endothelial-derived relaxing factor).

List of neurotransmitters and their locations.

Reference:

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.

The Editors of Encyclopaedia Britannica. “Acetylcholine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 26 Dec. 2019, www.britannica.com/science/acetylcholine.

The Human Excretory System

The excretory systems are central to homeostasis because they dispose of metabolic wastes and control body fluid composition through the regulation of solute movement between internal fluids and the external environment. Today we will overlook the basic structures of the human excretory system.

Excretory organs

Kidneys: a pair of organs that functions in both osmoregulation and excretion.

Ureter: the duct in which urine produced by each kidney exits and drains into the urinary bladder.

Urinary bladder: a sac where urine is collected from the ureters.

Urethra: the tube in which urine is expelled from the urinary bladder during urination; empties to the outside near the vagina in females and through the penis in males.

Sphincter muscles near the junction of the urethra and bladder regulate urination.

Kidney structure

Renal cortex: the granular outer section of a dissected kidney; contains the glomeruli and convoluted tubules of nephrons.

Renal medulla: the smooth, striated inner section of a dissected kidney; contains the loops of Henle and the collecting tubules of nephrons.

In both the renal cortex and medulla, blood is supplied by a renal artery and drained by a renal vein. Within them lie tightly packed excretory tubules and blood vessels. It is these tubules that carry out the process of filtration of blood entering the kidney. Fluids in the filtrate are reabsorbed into the surrounding blood vessels and exit the kidney in the renal vein.

Renal pelvis: an inner cavity formed by the ureter as it enters the kidney; urine collects into the pelvis from the collecting tubules and exits the kidney via the ureter.

Types of nephron

Nephrons: the functional units of a kidney; exist between and across the renal cortex and medulla.

Cortical nephrons: nephrons that only reach a short distance into the medulla; represents 85% of total nephrons in a human kidney.

Juxtamedullary nephrons: nephrons that extend deep into the medulla; are essential for the generation of concentrated urine (hyperosmotic) due to its long loop of Henle.

Nephron

Glomerulus: a ball of capillaries that is surrounded by the Bowman’s capsule; blood pressure forces fluid from the blood in the glomerulus into the lumen of the Bowman’s capsule.

Bowman’s capsule: a cup-shaped swelling at the end of tubule; passes filtrate collected from the glomerulus to the proximal tubule.

Proximal tubule: the section of the tubule in which reabsorption of ions, water, and nutrients takes place.

Loop of Henle: functionally divided into two sections, the descending limb and the ascending limb. The descending limb consists mainly of aquaporin and is responsible for the removal of water. The ascending limb is impermeable to water and further divided into two regions, the thin and thick segment. In the thin segment, NaCl diffuses out of the permeable tubule into the interstitial fluid, whereas in the thick segment NaCl is actively transported.

Distal tubule: plays a key role in regulating the K⁺ and NaCl concentration of body fluids, and contributes to the regulation of pH by controlling the secretion of H⁺ and reabsorption of HCO3^-.

Collecting duct: processes the filtrate into urine and carries it to the renal pelvis; hormonal control of the permeability of the collecting duct ultimately determines the concentration of the urine.

Blood enters the nephron through the afferent arteriole and leaves through the efferent arteriole. The capillaries of this arteriole form the branched peritubular capillaries, which surround the proximal and distal tubule. Other branches extend downward and form the vasa recta, surrounding the loop of Henle.

An overview of the human excretory system

Nephron organization

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

The Editors of Encyclopaedia Britannica. “Kidney.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 23 Jan. 2019, www.britannica.com/science/kidney#ref287943.

Supplements on the Action Potential

Conduction velocity is an important topic when considering an action potential because the rate at which the axons within nerves conduct action potentials governs how rapidly an animal can react to a certain stimulus. When studying through the information, make sure to connect the factors of conduction velocity with means of evolutionary adaption so you can get a better idea of the reason behind the structural differences of various types of neurons.

Conduction Velocity of the Action Potential

Main Factors:

Cell diameter: The greater the cell diameter, the greater the conduction velocity.

Myelin: The greater the myelination, the greater the conduction velocity

• Large myelinated fibers = fast conduction
• Small unmyelinated fibers = slow conduction

Myelin sheaths are produced by glia: oligodendrocytes in the CNS and Schwann cells in the PNS. They wrap many layers of lipid membranes that act as excellent insulators.

In myelinated axons, voltage-gated sodium channels are restricted to gaps in the myelin sheath called nodes of Ranvier, as well as the contact between the extracellular fluid and the axonal membrane. As a result, action potentials are not generated in the regions between the nodes; instead, the current generated by an action potential at a node travels within the axon all the way to the next node. This mechanism for propagating action potentials is called saltatory conduction. The action potential appears to be jumping from node to node along the axon.

The major selective advantage of myelination is its space efficiency. Recall that both cell diameter and myelination can increase the conduction velocity of an action potential. However, a myelinated axon 20 μm in diameter has a conduction speed faster than an unmyelinated axon of diameter 40 times greater. Hence, more myelinated axons can be packed into the space occupied by just one giant axon.

Propagation of action potentials in a myelinated axon

Reference: 

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Neuromuscular Transmission

Neuromuscular (Cholinergic) transmission is a type of synaptic transmission that occurs in the neuromuscular junction, the synapse between the axons of a motor neuron and a skeletal fiber. It relies on the binding of acetylcholine (ACh) released from presynaptic nerve terminals to acetylcholine receptors on the postsynaptic membrane. This process is very important because it enables a motor neuron to transmit a signal to the muscle fiber, causing muscle contraction.

Sequence:

1.      Action potential travels down the axon, ends in the presynaptic motor axon terminal, and opens voltage-gated calcium channels.

2.      Increase in Ca²⁺ permeability of the axon terminal causes an influx of extracellular Ca²⁺ into the axon terminal.

3.      The rise in intracellular free Ca²⁺causes the release of acetylcholine from synaptic vesicles into the synaptic cleft.

4.      Acetylcholine diffuses to the postjunctional membrane (represents a major time component).

5.      Acetylcholine binds to cholinergic receptors (ligand channels) on the postjunctional membrane, leading to the opening of the channels. The channels will remain open until the acetylcholine is removed.

6.      Opening of ligand-dependent channels results in an increased conductance to Na⁺ and K⁺. Because of the greater net force on sodium, an influx of sodium dominates.

7.      Influx of Na⁺ causes local depolarization of the postjunctional membrane. This depolarization is referred to as the end-plate potential (or EPP). The more acetylcholine that is released, the greater the depolarization (the greater the end-plate potential). Because the skeletal muscle membrane in the synaptic region does not have voltage-gated sodium channels, the action potential cannot be initiated in this region.

8.      The EPP spreads, causing depolarization of areas of muscle membrane adjacent to the end plate, where voltage-gated sodium channels are present. Their opening causes the initiation of an action potential that spreads across the surface of the skeletal muscle cell.

Single quanta of acetylcholine are released randomly under resting conditions. Each produces a small depolarization of the postsynaptic membrane, called a miniature end-plate potential (MEPP). MEPPs do not generate action potentials.

Neuromuscular transmission

Acetylcholine synthesis and choline recycling

Acetylcholine is an acetyl ester of choline. Its synthesis takes place in the cytoplasm and is catalyzed by choline acetyltransferase. Acetylcholine is then taken up into synaptic vesicles by an active vesicular transport mechanism. Acetylcholinesterase, which is weakly associated with the postsynaptic membrane and is located within the synaptic cleft, terminates the action of the transmitter via hydrolysis to acetate and choline. The active reuptake of choline from the extracellular fluid into the nerve terminal recycles the choline.

Acetylcholine synthesis and choline recycling

Reference:

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Water

Water is the molecule that sustains all kinds of life forms. Its structure allows it to interact with other molecules and water molecules itself, resulting in many unique emergent properties that help make Earth suitable for life.

Polar covalent bond

Water is a polar molecule, meaning that its overall charge is unevenly distributed. In an H₂O molecule, the oxygen has two regions of partial negative charge (δ⁻) and the hydrogen has a partial positive charge (δ⁺) due to the difference in electronegativity between the two elements. Oxygen is more electronegative than hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen, leading to the formation of polar covalent bonds as a result in the unequal sharing of electrons.

The partial charges of water molecules make it possible for it to form hydrogen bonds with one another, through intermolecular interaction between the hydrogen of one molecule and the oxygen of another. Hydrogen bonds, as you will later see, contribute to a variety of important properties in which water possesses.

Cohesive behavior

Hydrogen bonds hold water molecules together, making it more structured than most other liquids. Several phenomena arise as a result of this behavior, such as cohesion, adhesion, and surface tension.

Cohesion: the attraction between different substances through hydrogen bonds, in this case, water molecules and other substances. Think of a straw. Without cohesion, you would not be able to suck up any water because the water molecules are unable to cling onto the surface of the straw. This is cohesion.

Adhesion: the clinging of one substance to another, in this case, water molecules to water molecules. Let’s take the straw as an example again. Without adhesion, you would again be unable to suck up any water, because the water will fall down the straw due to gravity since the molecules below are not attached to the ones above.

Surface tension: a measure of how difficult it is to stretch or break the surface of a liquid. Water has an unusually high surface tension due to its hydrogen bonding, and this can be observed in a water droplet, which has the shape of a dome.

Moderation of temperature

Water moderates air temperature by absorbing heat from the air that is warmer and releasing the stored heat to air that is cooler. Several characteristics of water allow it to do so effectively, and that is its unusually high specific heat and heat of vaporization.

Specific heat: defined as the amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C. The specific number and calculations of water’s specific heat (1 cal/g．°C) is of little importance here, and you only need to know that compared with most other substances, water has an unusually high specific heat, making it less likely to change temperature than other liquids when absorbing or losing a given amount of heat. This is due to its hydrogen bonds: a lot of heat are spend on disrupting water’s hydrogen bonds before the water molecules can begin moving faster, and when the temperature drops slightly, many additional hydrogen bonds are formed, releasing heat into its surroundings.

The heat of vaporization: the quantity of heat a liquid must absorb for 1 g of it to be converted from liquid to gas. For the same reason, that water has a high specific heat, it also has a high heat of vaporization relative to other liquids. This characteristic of water allows it to moderate Earth’s climate, through evaporative cooling and circulation of air currents across the globe.

Ice v. liquid water

Water is one of the few substances that are less dense as a solid than as a liquid. In other words, ice floats on liquid water. Water expands when solidify because its hydrogen bonds form crystalline lattices when freezing, making ice about 10% less dense than liquid water, at 4 °C. This property has many values, including the sustentation of polar ice caps and its ecosystems.

Water as a solvent

Water is a very versatile solvent, due to its characteristic of being a polar molecule. The partial charges of water molecules, both positive and negative, can surround solute ions and form a sphere of water molecules called the hydration shell. That is, a compound is identified as “water-soluble” as long as water is able to establish a hydration shell around it. This characteristic is very vital since many different kinds of polar compounds are dissolved in the water of such biological fluids like blood, the sap of plants, and the liquid within all cells.

Water as a polar molecule

Reference: 

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Action Potential

When a neuron responds to a stimulus, a sudden change in the voltage (stimulated by the stimulus) across the dendrites and the cell body causes specialized channels called voltage-gated ion channels to open on the surface of axons, triggering an action potential.

An action potential is defined as a rapid membrane depolarization that changes the normal resting negative potential to a positive potential follow by a repolarization back to the normal negative membrane potential.

Involved Membrane Channels

Ungated potassium channel: always open; maintains K⁺ efflux

Voltage-gated sodium channel: closed under resting conditions, quickly opens and closes when detecting nearby membrane depolarization; once closes, will not respond to a second stimulus until the cell almost completely repolarizes. This channel is required for the depolarization phase (influx of Na⁺) of an action potential, and preventing the opening of these channels, which halts depolarization, will prevent the development of an action potential.

Voltage-gated potassium channel: As is the case for the voltage-gated sodium channel, membrane depolarization is the signal that causes it to open. However, it opens more slowly than the sodium channel, and thus its opening peaks later during the action potential. It provides a rapid repolarization phase, so preventing its opening slows repolarization.

Threshold and Subthreshold

When the neuron is depolarized to a level called the threshold, it fires an action potential. Subthreshold potentials of all types are referred to as electrotonic potentials (graded potentials).

Subthreshold potential v. Action potential:

Proportional to stimulus strength (graded) │   independent of stimulus strength (all or none)

Not propagated but decremental with distance │   propagated unchanged in magnitude

Exhibits summation │   summation not possible

Depolarization phase

Initial depolarization: voltage-gated sodium channels open (opens fast, close fast). Membrane conductance to sodium increases, rapid Na⁺ influx, depolarizing the membrane close to the sodium equilibrium potential (+65 mV).

Sodium channels are opening throughout depolarization, and peak sodium conductance is not reached until just before the peak of the action potential. Even though peak sodium conductance represents a situation with a large number of open sodium channels, influx is minimal because the membrane potential is close to the sodium ion equilibrium potential (low electric force; mentioned in Resting Potential).

Repolarization phase

Early repolarization: the voltage-gated sodium channels rapidly close, eliminating Na⁺ influx. Meanwhile, the voltage-gated potassium channels are still opening (they are slower, remember?), increasing potassium conductance beyond the value under resting conditions. This leads to rapid potassium ion efflux that repolarizes the cell.

Peak potassium conductance does not occur until about mid-repolarization. At this point, even though the force on the potassium ions is less than at the beginning of repolarization, there is greater efflux because of the much greater conductance. If the voltage-gated potassium channels do not open during repolarization, the cell will still repolarize through the ungated potassium channels. However, the process will be slower.

The original gradients are reestablished via the Na/K-ATPase pump.

Refractory periods

Absolute refractory period: during this period, no matter how strong the stimulus is, a second action potential cannot be induced. Therefore, its length determines the maximum frequency of action potentials.

Relative refractory period: during this period, a greater than normal stimulus is required to induce a second action potential.

Breakdown of an action potential.

Axon Action Potential and Changes in Conductance

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Resting Potential

In neurons, as in other cells, ions are unequally distributed between the interior of cells and the surrounding fluid, resulting in a negatively charged environment in the cell relative to the outside. For a resting neuron, the resting potential is about -70 millivolts (mV).

Involved Membrane Channels

Potassium ions (K⁺) and sodium ions (Na⁺) play an essential role in the formation of the resting potential. During a resting potential, the concentration of K⁺ is higher inside the cell, while the concentration of Na⁺ is higher outside. This concentration gradient is well-established by important membrane channels.

Sodium-potassium pump: maintain the Na⁺ and K⁺ gradients; uses the energy of ATP hydrolysis to actively transport Na⁺ out of the cell and K⁺ into the cell, in a ratio of three to two, respectively. The pump acts slowly, producing a small net charge, so a neuron relies on ion channels to create a drastic gradient.

Potassium channel: also called the potassium leak channels, these channels permanently open and are crucial to the establishing of a resting potential. A neuron has many potassium channels, so the large efflux (outflow) of K⁺results in a negatively charged environment inside the cell.

Sodium channel: also called the sodium leak channels, these channels are also permanently open, just like the potassium channels. However, there is way less sodium channels than there are potassium channels, so Na⁺ cannot readily pass through the membrane, resulting in a positively charged environment outside the cell.

Electrochemical Equilibrium

Two vectors determine the state of equilibrium of a neuron-the concentration force and the electrical force. When both vectors are in balance, the neuron is said to be in equilibrium. Considering an electrochemical equilibrium requires imagination and logic; when dealing with questions of cells whose ions are not in equilibrium, keep in mind that you should deal each ion separately, balancing one force before considering the other.

When a neuron reaches equilibrium, you can try using the Nernst equation to calculate the equilibrium potential of individual ions.

                            [ion]outside   
Eion = 62 mV( log ――――――――  )

                       [ion]inside

For instance, plugging the K⁺ concentration (extracellular concentration: 5 mM, intracellular concentration: 140 mM) into the Nernst equation reveals that the equilibrium potential for K⁺ is -90 mV, and Na⁺ (extracellular concentration: 150 mM, intracellular concentration: 15 mM) is +62 mV.

Because neither K⁺ nor Na⁺ is at equilibrium in a resting neuron, there is a net flow of each ion across the membrane. As long as the resting potential remains, the K⁺ and Na⁺ current, as well as the

ion concentrations, will hold steady, until an action potential is induced. And that will be another story to begin with.

Membrane channels and pumps generate the resting potential.

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Vertebrate Innate Immunity

In animals, there are two types of immunity: innate immunity is common among all animals, and adaptive immunity is found only in vertebrates. Innate immunity offers a primary defense in all animals and sets the stage for adaptive immunity in vertebrates.

Innate immunity includes barrier defenses and molecular recognition that relies on a small set of receptor proteins that bind to molecules or structures that are absent from animal bodies but common to a group of viruses, bacteria, or other pathogens.

Barrier Defense

Mammals have mucous membranes and skin as their barrier defenses. The mucous membranes line the respiratory, digestive, urinary, and reproductive tracts, producing mucus, a viscous fluid that traps pathogens and other particles. In airways, ciliated epithelial cells sweep mucus and entrapped material upward, preventing infection in the lungs. Saliva, tears, and mucous secretions which contain lysozyme bathe various exposed epithelia, wash away intruding bacteria and fungi, and destroys the cell walls of susceptible bacteria. The gastric juice (pH 2) as well as skin secretions such as oil and sweat (pH 3-5) also help prevents the growth of many bacteria.

Cellular Innate Defense

Mammals rely on recognition protein called Toll-like receptor (TLR) to detect, devour, and destroy invading pathogens. Each TLR protein binds to fragments of molecules characteristic of a set of pathogens. Upon recognizing the pathogen, TLR proteins produce signals that initiate responses tuned to the invading microorganism.

The two main types of phagocytic cells in the mammalian body are neutrophils and macrophages.

Neutrophils: circulate in the blood, get attracted by signals from infected tissues and then engulf and destroy the infecting pathogen.

Macrophages: larger phagocytic cells; migrate throughout the body or reside permanently in organs and tissues where they are likely to encounter pathogens, engulf, then destroy or present antigens.

Some other notable cells that are involved in innate defense include dendritic cells, eosinophils, and natural killer cells.

Dendritic cells: mainly populate tissues that contact the environment, such as skin; like macrophages, they are antigen-presenting cells (APC), engulfing pathogens and presenting the antigens.

Eosinophils: often found in tissues underlying an epithelium; defend against multicellular invaders such as parasitic worms by discharging destructive enzymes.

Natural killer cells: circulate throughout the body, detect the abnormal array of surface proteins characteristic of some virus-infected and cancerous cells, and release chemicals that lead to cell death.

Peptide and Protein Production

Pathogen recognition triggers the production and release of a variety of peptides and proteins that attack pathogens or impede their reproduction.

Antimicrobial peptides: damages broad groups of pathogens by disrupting membrane integrity.

Interferons: proteins that provide innate defense by interfering with viral infections; can be secreted by virus-infected cells to limit cell-to-cell infection, or secreted by some white blood cells to help activate macrophages.

Complement system: consists of roughly 30 proteins in blood plasma; circulate in an inactive state and are activated by substances on the surface of many pathogens, leads to lysis of invading cells.

Inflammatory Response

Characterized by 1. rubor (redness), 2. calor (heat), 3. tumor (swelling), 4. dolor (pain), and 5. function laesa (loss of function).

It is a set of events triggered by signaling molecules released upon injury or infection.

Step1: Activated macrophages discharge cytokines, signaling molecules that recruit neutrophils to the site of injury or infection. At the same time, mast cells, immune cells found in connective tissues, release the signaling molecule histamine at sites of damage.

Step2: Local blood supply increases, capillaries widen and become more permeable, allowing the delivery of antimicrobial peptides to the site. Activated complement proteins promote further release of histamine, attracting more phagocytic cells. Pus, a fluid rich in white blood cells, dead pathogens, and debris from damaged tissue, accumulates.

Step3: Phagocytic cells such as neutrophils digest pathogens and cell debris at the site and the tissue heals.

The three basic inflammations:

Local Inflammation

Systemic Inflammation (throughout the body)

Chronic Inflammation (ongoing)

Toll-like Receptor (TLR)

Antigen-Presenting Cells (APC)

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

The Anatomy of a Neuron

Although the morphology of various types of neurons differs in some respects, they all contain four distinct regions with differing functions: the cell body, the dendrites, the axon, and the axon terminals. 

Cell Body

The cell body, or soma, contains the nucleus and is the site of the synthesis of virtually all neuronal proteins and membranes. Some proteins are synthesized in dendrites, but no proteins are made in axons and axon terminals, which do not contain ribosomes. Proteins and membranes that are required for renewal of the axon and nerve termini are synthesized in the cell body and assembled there into membranous vesicles or multiprotein particles by a process called anterograde transport. Transported along microtubules down the length of the axon to the terminals, these vesicles are inserted into the plasma membrane or other organelles. Axonal microtubules are also the tracks along which damaged membranes and organelles move up the axon toward the cell body; this process is called retrograde transport [1]. Lysosomes, where such material is degraded, are found only in the cell body.

In the CNS, neurons have extremely long dendrites with complex branches. This allows them to form synapses with and receive signals from a large number of other neurons. On the other hand, motor neurons and sensory neurons have long axons that can relay signals to and from the CNS.

Dendrite

Most neurons have multiple dendrites, which extend outward from the cell body and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them in the direction of the cell body. 

Axon

Almost every neuron has a single axon. Axons are specialized for the conduction of action potential away from the cell body toward the axon terminus generated as a result of a sudden change in voltage. In a typical neuron, the action potential duration is about a millisecond; when an action potential is not triggered, a neuron is in a resting state. 

Axon Terminal

An action potential is actively conducted down the axon into the axon terminals, small branches of the axon that form the synapses, or connections, with other cells.

Axon Hillock

Axon hillock is the junction (joining) of the cell body and the axon. It is also the site where an action potential originates.

Myelin Sheath

Myelin is a lipid-rich (fatty) substance that surrounds the axons of some nerve cells that act as insulation to increase the rate at which signals/information (encoded as electrical impulses) is passed along the axon. The myelin membranes originate from and are a part of the Schwann cells in the PNS and the oligodendroglial cells in the CNS. 

General Features of a Neuron

Reference: 

Overview of Neuron Structure and Function. Khan Academy, https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/overview-of-neuron-structure-and-function. Accessed 10 Jan. 2020.
Section 21.1 Overview of Neuron Structure and Function. Molecular Cell Biology. 4th Edition, NCBI, https://www.ncbi.nlm.nih.gov/books/NBK21535/. Accessed 10 Jan. 2020.
Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Vertebrate Circulatory Systems

The vertebrate circulatory systems are often being referred to as the cardiovascular system.

Arteries, veins, and capillaries are the three main types of blood vessels.

Arteries: carry blood from the heart to organs throughout the body; branch into arterioles.

Capillaries: microscopic vessels with very thin, porous walls, often form networks called capillary bed; dissolved gases and other chemicals are exchanged by diffusion between the blood and the interstitial fluid across the tissue cells here.

Veins: vessels that carry blood back to the heart; capillaries converge into venules and then into veins.

Single Circulation

In sharks, rays, and bony fishes, blood travels through the body and returns to its starting point in a single circuit (loop), an arrangement called single circulation. 

These animals have a heart that consists of two chambers: an atrium and a ventricle. Blood that leaves the heart passes through two capillary beds before returning to the heart, and when it passes through the gill capillaries (the first capillary bed encountered in the circuit), blood pressure drops substantially, limiting the rate of blood flow in the rest of the animal's body. However, as fish swims, the contraction and relaxation of its muscles help accelerate the pace of its circulation.

Double Circulation: Amphibians

Double circulation provides a vigorous flow of blood to the brain, muscle, and other organs because the heart repressurizes the blood after it passes through the capillary beds of the lungs or skin.

Frogs and other amphibians have a heart with three chambers-two atria and one ventricle. The ventricle, incompletely divided by a valve, diverts most of the oxygen-rich blood from the right atria into the systemic circuit and most of the oxygen-poor blood from the right atrium into the pulmocutaneous circuit. When an amphibian is underwater, it can shut off blood flow to the lungs and circulates blood through its ventricles, enhancing its time of submerging.

Double Circulation: Mammals and Birds

The hearts of mammals and birds have four chambers-two atria and two ventricles. Due to the total separation of its ventricles, birds and mammals cannot vary blood flow to the lungs without varying blood flow throughout the body in parallel.

The four-chambered heart enables a large input of fuel and oxygen and output of waste compare with the circulatory systems found in other animals, as endotherms need much more energy than ectotherms do. This four-chambered heart system arose independently in the distinct ancestors of birds and mammals,  reflecting convergent evolution.

Double Circulation: Reptiles

In the three-chambered heart of turtles, snakes, and lizards, an incomplete septum partially divides the single ventricle into right and left chambers. Their pulmonary and systemic circuits are also connected, which allows blood flow and exchange even when the animal is underwater.

Vertebrate Circulatory Systems 

Reference:

Campbell, et al. (2017). Biology: A Global Approach (11th ed.)