mtracy's blog

Gnathostomes are a group of organisms which have true jaw structures, rather than a simple mouth. It is believed that these jaw structures developed from modified gill arches in fish. The first arch, the premandibular arch, is thought to have moved upward, forming the beginnings of the braincase as a plate underneath the brain itself. The second gill arch, the Mandibular arch, bends forward into two seperate sections. The top, which would eventually form the upper jaw, is called palatoquadrate cartilage. The bottom, forming the lower jaw, is mandibular cartilage. Eventually these cartilaginous structures would ossify in some organisms, giving them boney jaws. The third gill arch, the hyoid arch, forms a structure which supports the jaws themselves, providing a foundation to rest or directly connecting to the jaws. This also serves the purpose of anchoring the jaw to the braincase. Of course this anchoring method differs, or is entirely absent depending on the form of jaw suspension that develops in a particular organism.

Chondrichtyes are a class of cartilaginous fish and contains two subclasses; the Holocephali and the Elasmobronchii. There are about 40 extant species of Holocephali, one example being the ratfish. The main defining feature of the Holocephali is that they have a holostylic jaw suspension. This means that the upper jaw is fused with the braincase. Holocephali are also oviparous and will lay 1-2 eggs at a time. Males will have claspers as well as a frontal tenaculum.

Elasmobrochii includes fish such as sharks, rays and skates. The defining characteristics of Elasmobrochii are their plated gills. These may be on the side of their heads as with sharks, or located ventrally in the rays and skates. This distinction seperates rays and skates into a subgroup called Batoids. Elasmobrachi have placoid scales, that are similar to teeth like structures. In fact, they have a pulp cavity, an enamel like substance that coats them and are even made of dentin, as teeth are. Furthermore, rather than teeth plates, these have polyphydont dentitions.

                There a numerous methods of reproduction. For instance, many organisms reproduce through a method known as ovipary. This simply means that the organism lays their eggs. The eggs may be fertilized before or after laying. A secondy method is known is vivipary. In organisms that reproduce by vivipary, they will hold their eggs until hatching. Vivipary comes in two forms, Lecithotropy and Matrotrophy, which is defined by the method of nutrients given to the embryo. During Lecithotrophy, the embryo is sustained by lecithin found in the yolk of the egg. Matrotrophy adds additional methods of sustaining the embryo. These methods include: Oophagy, Adelphophagy, Placentation, Epithliotrophy, and Dermatotrophy. During Oophagy, the mother will produce additional eggs, which may or may not be fertilized, which the current embryo will feed on. Adelphophagy is very similar in that an embryo will feed on other embryos within the mother. Placentation simply connects the embryo to the mother’s circulatory system, allowing it to gain nutrients from the mother more directly. During Epithliotrophy, the mother produces a lining within her womb, which the embryo may ingest for further nutrients. This too is similar to Dermatotrophy, in which the embryo will ingest parts of the mother’s outer skin for nutrients. These methods will allow for an offspring to be larger and thus more viable, however it requires a substantial energy sacrifice for the mother.

Neurons have three parts to them, the soma, dendrites, and axons. The soma of a nueron is its main body, containing the cells organelles. Radiating out from the soma is the dendrites, where neurons receive signals from other neurons. Dendrites may receive either excitatory or inhibitory signals This transmission of signals is usually through the passage of neurotransmitter between the synapses of dendrites and axons. However, sometimes neurons also pass direct electrical signals through strcutrues called gap junctions. Axons are what the neuron will send a signal through. At the end of the axon is the axon terminal, where neurotransmitters are stored and ready for release. Axons almost always reach to another neurons dendrites, however there are cases where they reach to somas and even sometimes other axons.

In order for a neuron to release neurotransmitter from its axon terminal, it must produce an action potential. The action potential is caused by receiving enough of an exitatory signal at its dendrites. These signals are called graded potentials and will deteriorate over time. If enough graded potentials sum, the cell will release an action potential down its axon and release neurotransmitter to the next neuron. Action potentials are always the same strength, there is no variability to them like with graded potentials. Once they exist, they exist and travel down the axon. Of course while traveling some of the charge may leak out of the axon itself. This is prevented by having thicker axons or by the presence of a myelin sheath over the axon.

The phylum of Chordata encompasses a vast and diverse group of organisms. In order for an organism to be classified as a chordate, it must possess all five characteristics of a chordate at some stage in its lifetime; though many chordate traits are lost with further development. The first of these unifying characteristics is the presence of the notochord. This is a rod which runs the length of the organisms body to provide a ridged yet flexible support structure. Chordates will also possess a dorsal hallow nerve chord. As the name suggests, this is a hallow chord which runs the length of the body and sits dorsal to the notochord. The third trait all chordates have is some sort of iodine fixing gland. In many species this may be an endostyle. For humans this is our thyroid gland. Another trait is the presence possess pharyngeal gill slits, which are located along the pharynx. Lastly, all chordates will possess a post-anal tail at some point in their lifetime.

There are billions of neurons in the human body which work together and govern our thoughts, sensations and movement. Neurons, or any cell for that matter, have the ability to hold a charge in their membrane. The charge itself is caused by the passage of positively and negatively charged ions through ion channels and pumps in the membrane itself. In the human body the main positive charged ions you will find are sodium (Na), potassium (K), and calcium (Ca). Largely chloride (Cl) will be the only negatively charged ion effecting neurons.

Depolarization and the flow of these ions is governed by both the concentration and electrical gradients. At rest, the average neuron will be at about -70mV. This charge is governed by the equilibrium potential of the Na+ and K+ ions flowing between the membrane and the exterior of the cell. The concentration of K+ will be greatest inside the cell and low outside he cell. In contrast, Na+ will have a high concentration outside the cell and a low concentration inside the cell. Therefore when say, a K+ channel opens, large amounts of K+ will leave the cell, pushing its electrical potential more negative and hyperpolarize it. When an Na+ channel opens, large amounts of Na+ will rush into the cell, pushing its electrical potential to be more positive and depolarizing it.

Transportation:

At 6:45am I left my home and began my drive to Umass. Due to heavy traffic, I thought it would have been a good idea to take a detour to avoid said traffic. However, this likely made my commute longer than it would have been otherwise. Unfortunately there was heavy traffic along this detour as well. During my commute someone else had tried to avoid the traffic by using the incorrect lanes and they had gotten pulled over. At 7:50 I arrived on campus and parked in lot 12. From lot 12, I walked to my first course in the ILC. I walked from the ILC to the library. At around 10:00 I had a brief walk around campus. Furthermore, I walked to statistics in Morill 1 at around 12:30.

There are a number of ways atoms can interact non-covalently. These interactions can be ionic, dipole or van der Waals. All three of these are electrostatic in nature and are classified by their magnitude and duration. For example, ionic interactions are permanent and full charges while van der Waals interactions are temporary and partial charges. Dipole interactions are a mixture of permanent, temporary, partial and full charges, however. Non-covalent interactions can be dipole-dipole, ion-ion, ion-dipole and hydrogen bonds.

Covalent bonds are stronger than all types of non-covalent bonds. Covalent bonds occur in both polar and non-polar variations. In the human body, the only non-polar covalent bonds will be seen between carbon-carbon bonds and carbon-hydrogen bonds. Of the non-covalent interactions, ionic are the strongest, while van der Waals are the weakest. It is important to note that many weak interactions can sum to form a powerful interaction. In fact, this is crucial to the structure of many proteins in the human body. While covalent bonds form the general linear structure of a molecule, the non-covalent are largely responsible for keeping the three-dimensional shape and structure of the molecule stable. Many interprotein interactions occur due to these non-covalent interactions as well.

The inside of our body is mostly aqueous. It is therefore important to know how molecules interact with water, as well as the properties of water itself. Water is a polar molecule as its large electronegative oxygen pulls electrons away from its smaller hydrogen. This is actually what gives water many interesting properties. Due to its polarity, water tends to adhere to other water molecules. Polarity also governs what will dissolve in water. A general rule of thumb is that like dissolves like. Therefore other polar molecules will dissolve in water and are thus hydrophilic. Take salt for instance, NaCl. The Na is positively charged and the Cl is negatively charged, making it polar. When salt is poured into water, it dissolves as water forms a hydration shell around each atom. However, if a non-polar molecule is poured into water it will not dissolve. Rather, it will clump together. This is what is called a hydrophobic molecule. An example of this can be seen when pouring oil into water. The oil clumps together, even if poured into separate locations in the water. This adherence of oil clumping together isn't because of any sort of attraction between the oil molecules. Instead, this is because of entropy and energetic favorability. When oil is poured in the water, the water must form a shell around it. If there are multiple clumps of oil, more water has to form shells around those clumps as well, therefore decreasing entropy. When all the oil clumps adhere together, forming one large oil clump, the entropy is at its highest and therefore the oil has the tendency to stick together.

The Cyclostomata are split into Myxinodiea and Petromyzontiformes. The Myxinodiea, or hagfish, are boneless fish that live exclusively in marine environments. Some live along the continental shelf, while others live in the deeper regions. Hagfish lack true teeth and jaws. Rather they have a keratinized teeth and tongue. This keratinized acts in a pully-like fashion, where one muscle contracts to pull it one way, then relaxes and another muscle contracts to pull it the opposite way. This causes a sliding motion which the hagfish uses to scrape away at other organisms skins and tear holes into them. Hagfish are also isosmotic, and thus have unusual kidney function as they generally do not need to do much in the way of regulation. Their eyes are small, and are largely just used for detecting light and not true eyes for visual processing. While they have no bones, they do possess a cartilage plate underneath their brain. Additionally hagfish have a lateral line system which allows them to detect the movement of water around them, or if they themselves are moving.

Petromyzontiformes, or lamprey, are similar to hagfish. Like the hagfish, lamprey’s do not have any bones and a nostril on the top of their head. However unlike the hagfish, lamprey have true eyes and one pineal eye used for light detection. Additionally they have a circular mouth which is used to suck on and attach to either prey or various objects, such as rocks. Lamprey are parasitic and survive by sucking the fluid out of other fish. As adults, lamprey live in a marine environment, though they migrate back to freshwater to spawn. Lamprey larva are known as ammocoets and live as filter feeders. These ammocoets tend to pile up into structures known as redds. Surprisingly this larval stage is the primary lifestage for lamprey. Ammocoetes live for seven to eight years before maturing into an adult lamprey. At this time the lamprey will migrate to a marine environment.

Vertebrata are a subphylum of Chordates. These, of course, have all the characteristics of any chordate, but have a number of other characteristics unique to their group. These additional traits include the presence of a braincase and a tripartite brain, with cranial nerves. During development the ectoderm pinches off to form the neural crest and cells migrate throughout the body to form a variety of structures. These have the ability to form teeth, pigment cells, bone, muscles and more. Rather than simple filter feeding, muscles now control the action of taking in water for both breathing and feeding. Likewise, muscles now control the digestive tract and move the nutrients along through peristalsis. The pituitary gland has been split into both adenohypophyis and neurohypophysis hormonal control. Furthermore, the optic tectum is present in vertebrates, providing better visual information processing.

A specific class of Vertebrata is called the Cyclostomata, which may be further divided into Myxinoidea, the hagfish, and Petromyzontida, lampreys. Hagfish have no bones and their skeleton is mostly made of cartilage. However, they do have vestiges of a hemal spine, ventral to their notochord. While they do not have true eyes, they do have eyespots used to detect light. Hagfish have a single nostril on the top of their head and a single semicircular ear canal. Their skin is covered in a number of mucus pores, which produce large amounts of mucus and slime when the fish is agitated. These fish are mainly considered scavengers, though may be known to eat smaller marine worms. Their mouths contain keratinized teeth which they use to pierce prey with and burrow inside or by sucking the nutrients out.