Term of the Day

Osteoid: The organic component of bone matrix, accounting for approximately 1/3 of the matrix. It is made up of collagen fibers, glycoproteins, and proteoglycans all secreted by the osteoblasts. These substances contribute to bone structure and give bone both flexibility and tensile strength.

Term of the Day

Papillary Dermis: Thin, superficial layer of the dermis composed of loose connective tissue and thrown into nipple-like projections called dermal papillae, which interdigitate with the overlying epidermis. The papillae contain capillaries, free nerve endings (which act as pain receptors) and Meissner's corpuscles (which act as touch receptors). On the palms and soles, the papillae lie on top of larger mounds called dermal ridges which, in turn, produce corresponding epidermal ridges. The latter give us our fingerprints.

Term of the Day

Proteoglycans: A component of the ground substance of connective tissue. They consist of a protein core, to which negatively charged polysaccharides called glycosaminoglycans (CAGs) are attached. The CAGs form an intertwining network that traps water and forms a substance that varies from a viscous fluid to a semisolid gel. The higher the CAG content, the stiffer the ground substance is. Chondroitin sulfate, keratan sulfate, and hyalouronic acid are three of the most prominent glycosaminoglycans.

Term of the Day

Mesenchyme (Mesenchymal Tissue): In embryology, the first true tissue formed from the mesoderm germ layer. It is a type of connective tissue composed of star-shaped mesenchymal cells surrounded by a matrix of fluid ground substance containing fine fibers. Mesenchyme eventually differentiates into all other connective tissues.

Term of the Day

Anatomical Position: A common reference point from which the positions of body parts and their relationships to surrounding structures are described. In anatomical position, the body is erect with feet together and toes pointing forward; arms are at the sides with the palms facing forward and thumbs pointing away from the body. When describing the relative location of structures, the body is always referred to as if it were in this position.

Term of the Day

Rami Communicantes: Short nerve fibers that connect an intercostal nerve to the sympathetic trunk. The white ramus communicans travels from the nerve to the sympathetic ganglion, and the gray ramus communicans travels from the ganglion to the intercostal nerve.

Term of the Day

Esophageal Hiatus: An oval-shaped aperture in the right crus of the diaphragm at about the level of the T10 vertebra through which the espohagus passes from the thoracic cavity into the abdominal cavity. The opening also allows passage of the vagal trunks and the esophageal branches of the left gastric vessels.

Term of the Day

Zona Pellucida: Extracellular matrix layer that surrounds the secondary oocyte/ovum. It is composed of glycoproteins and induces the sperm to undergo the acrosome reaction.

Term of the Day

Denticulate Ligaments: Saw-toothed ligaments that suspend the spinal cord in the dural sac on each side from the foramen magnum to the area between the T12 and L1 nerve roots. The ligaments themselves are extensions of the pia mater located midway between the dorsal and ventral nerve roots. They attach to the dura by 21 toothlike processes.

Term of the Day

Internal Anal Sphincter: An involuntary sphincter surrounding the superior 2/3 of the anal canal. It is a thickening of the circular smooth muscle layer of the large intestine and is innervated by the pelvic splanchnic nerves. The sphincter responds to the pressure of feces in the rectal ampulla by relaxing.

Concept of the Week : The Motor Unit

The cell bodies of motor neurons that innervate skeletal muscles are located in the ventral horn of the grey matter of the spinal cord. Axons of these nerves leave via the ventral root and travel to the appropriate skeletal muscle.

A single motor neuron can form synapses with many muscle fibers, due to a multiplicity of axon collaterals. An individual motor neuron and all the muscle fibers it innervates is called a motor unit, the functional unit of the neuromuscular system.

Motor units can vary in size. The smallest consist of a single motor neuron that innervates just a few muscle cells. Such an arrangement is typical in situations where very precise control of fine movements is necessary (e.g. finger or eye movements). A large motor unit may innervate hundreds of muscle cells. Such units are responsible for gross movements such as leg or trunk movements. The smaller the motor unit, the greater precision of fine motor skills.

Term of the Day

Cor Pulmonale: Right ventricular enlargement (hypertrophy, dilation, or both) secondary to pulmonary hypertension. Increased afterload (pressure in the pulmonary trunk) leads to chronic pressure overload in the right ventricle, thus increasing the work it must do. The right ventricle ultimately fails when pulmonary artery pressure equals systemic blood pressure.

Concept of the Week: The Frank-Starling Law of the Heart

The Frank-Starling Law of the Heart:

Named for physiologists Otto Frank and Ernest Starling, this property of the heart concerns its intrinsic ability to regulate stroke volume (output) in response to changes in venous return (input). There is a directly proportional relationship between end diastolic volume (EDV) and stroke volume (SV), within limits. That is, if venous return increases, end diastolic volume will increase, causing an increase in stroke volume. Essentially, the Frank-Starling Law states that, within defined limits, the heart will pump whatever volume of blood it receives! Simple, right?

The tricky bit is in understanding how this all works. As EDV increases, the ventricles become more full, and the cardiac muscle cells in the walls of the ventricle become increasingly stretched. Any muscle cell has an optimal length for contraction, but in a normal resting heart, the muscle fibers are shorter than their optimal length. Thus, as EDV increases, the cardiac muscle cells get stretched closer and closer to their optimal length for contraction. This results in more forceful contractions and the ejection of a greater stroke volume.

What's really neat about this law is that it regulates each ventricle independently of the other and adjusts strength of contraction on a beat-by-beat basis. The end result is that each ventricle pumps out the same volume that it receives, preventing blood from pooling in either the pulmonary or systemic circuit! How cool is that?

Term of the Day

Splanchnic Nerve: A preganglionic autonomic nerve of the abdominopelvic viscera. Preganglionic, sympathetic fibers that pass through the paravertebral (sympathetic chain) ganglia without synapsing form splanchnic nerves. Instead, they synapse in a prevertebral (visceral) ganglion such as the celiac ganglion, the superior mesenteric ganglion, or the inferior mesenteric ganglion (e.g. thoracic -- greater, lesser, least -- splanchnic nn, lumbar splanchnic nn, and sacral splanchnic nn). Pelvic splanchnic nerves are parasympathetic and formed by branches of the ventral rami of S2, S3, and S4. Their ganglia are at or near the target organ.

P.S. Have a very Happy New Year!

Term of the Day

Alkaline Reflux Gastritis: inflammation of the stomach caused by reflux of bile and pancreatic secretions (proteolytic enzymes and bicarbonate-rich fluid) that disrupt the mucosal barrier of the stomach. It is one of the risk factors associated with gastrectomy or pyloroplasty. Symptoms include nausea, bilious vomiting, and epigastric pain that worsens after eating and is not relieved by antacids.

Concept of the Week: Everybody Do the Chloride Shift!

The majority of carbon dioxide is transported in the blood as bicarbonate ion (60-70%). The chemical reaction that forms the bicarbonate takes place within the erythrocytes (red blood cells) and is catalyzed by the enzyme carbonic anhydrase. Carbon dioxide reacts with water in the cytosol to form carbonic acid, which then dissociates into a negatively charged bicarbonate ion and a positively charged hydrogen ion. The bicarbonate ions quickly diffuse out into the blood plasma. This, however, would result in a net loss of negative charge within the RBC if the process were not counterbalanced. The chloride shift balances the loss of bicarbonate as negatively charged chloride ions diffuse into the RBC from the plasma. Thus, the loss of negatively charged bicarbonate is offset by an equal acquisition of negatively charged chloride, allowing the RBC to maintain its normal membrane potential. What happens to the hydrogen ion? It binds to hemoglobin, which acts as a buffer to prevent intracellular pH from dropping too low. It drops just low enough to trigger the Bohr effect, which reduces the affinity of hemoglobin for oxygen and facilitates oxygen release to the tissues.

Welcome to the Anatomy and Physiology Student's Forum!

I'm Scott Boulding, and I have been a professional Anatomy & Physiology, Pathophysiology, and Biology teacher for some years now. The idea for this blog was born of my own students' struggles with and attempts to master the large volume of challenging material these courses represent. One of them aptly described the learning process as akin to "taking a sip of water out of a fire hose!" So, I decided to create an online forum in which they could share ideas, ask questions, provide useful links, exchange study tips, and so on. Soon realizing that such a resource could benefit other students from the high school through the grad school levels, I decided to make this blog available to the general public. If you are taking Anatomy and Physiology, Human Biology, Gross Anatomy, Human Physiology, or Pathophysiology, this blog is for you. If you are a teacher of any of these courses, you are also welcome to provide comments, feedback, and answers to student questions. Everyone with an interest in these subjects is welcome here!

In addition to the questions and comments threads, I will also be posting key terms and their definitions daily (think of it as your Anatomy and Physiology Word-a-Day calendar), as well as discussing and explaining a different A&P concept each week. Feel free to add to the discussion, expand upon the information, ask questions -- whatever will be useful to you as you try to learn the material! From time to time, I will also throw in some short quizzes to help you test your knowledge and understanding

If you feel like you need more intensive tutoring, you can also find me at www.globalscholar.com or at http://imlive.com/waitingroom1ms.asp?hostnick=DocentisMaximus. If you're in the Tampa Bay area and want some live, one-on-one tutoring, just drop me an email (sboulding@tampabay.rr.com), and I'll be happy to arrange a meeting that is convenient for both of us.

In the meantime, enjoy the blog; I hope you find it helpful. As with anything, you'll get out of it what you put into it -- trite, I know, but true, nevertheless. Good luck with your studies!

Scott Boulding