Term of the Day

Colles' Fracture: Fracture of the distal radius with dorsal displacement or angulation of the distal fragment. Typically results from forceful trauma, commonly from falling on outstretched hands. The ulna is unaffected, since it doesn't form part of the wrist joint.

Term of The Day

Middle Cranial Fossa: Basin-like depression inside the skull formed by the sphenoid bone and petrous portion of the temporal bone. The temporal lobe of the brain is supported by this fossa. Illustrated in pink and yellow in the diagram to the right.

Concept of the Week

The Structural and Functional Classification of Joints

Structurally, bones are classified according to the nature of the material that joins the bones together:

1. Fibrous Joints: The bone ends are united by collagenous fibers.

2. Cartilaginous Joints: The bone ends are united by cartilage (either fibrocartilage or hyaline cartilage).

3. Synovial Joints: The bone ends are covered with articular cartilage and enclosed within a joint capsule lined with synovial membrane.


Functionally, joints are classified according to the amount of movement permitted:

1. Synarthroses: Completely immovable joints (e.g. sutures)

2. Amphiarthroses: Slightly movable joints (e.g. interosseous membrane conncting radius to ulna)

2. Diarthroses: Joints that are freely movable in one or more planes of space (e.g. knee joint)

Questions, Comments, Concerns, Feedback

Use this thread to ask questions, make general comments on topics unrelated to this week's featured concept and key terms, or just leave feedback regarding how the blog might be improved. Check the bottom of the page for this week's crossword puzzle on the parts of the cell.

Term of the Day

Comminuted Fracture: A break in which the bone fragments into many pieces. Especially common in elderly people suffering from osteoporosis.

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.

Announcement

For those of you who think I'm a one-trick pony -- all science and medicine -- I also excel at English Composition and Literary Criticism. If you're having trouble with high school or college writing assignments, you might want to visit my new website, A-Plus Custom Essays and Research Papers (apluspapers.weebly.com). I offer a wide variety of services to fit every budget!

Term of the Day

Marfan's Syndrome: Genetic disorder of connective tissue. Abnormal connective tissue results from a defect in the protein fibrillin, which helps provide scaffolding and structural support between cells. Signs and symptoms include: excessive height, abnormally loose joints, long spiderlike fingers, poor vision, and a weakened heart and blood vessels due to inadequate connective tissue reinforcement. The trait is dominant and carried by the FBN1 gene. For you punk rockers out there, Joey Ramone was diagnosed with Marfan's.

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.

Concept of the Week

The Mitotic Spindle

During prophase of mitosis, cytoskeletal microtubules disassemble. As the centriole pairs migrate away from each other, they organize the assembly of a new system of microtubules called the mitotic spindle. Lengthening of these microtubules pushes the centrioles farther and farther apart, ultimately pushing them to opposite poles of the cell.

Then the centrioles cause asters (also composed of microtubules) to radiate outward from the ends of the spindle and anchor it to the plasma membrane. While all this is taking place, some of the spindle fibers attach to protein complexes called kinetochores on the centromere of each pair of sister chromatids. These fibers are referred to as kinetochore microtubules. Those spindle fibers which do not attach to chromosomes are called polar or nonkinetochore microtubules.

The kinetochore tubules pull on the chromosomes until they ultimately line up at the equator of the spindle to form the metaphase plate. During anaphase, the kinetochore fibers shorten, pulling each chromosome towart the end of the cell it faces. The polar microtubules slide past each other (due to the action of kinesin motor proteins), which forces the 2 poles of the cell further apart such that the entire cell elongates.

During telophase, the spindle finally breaks down and disappears.

Term of the Day

Ligands: Signaling molecules that bind specifically to plasma membrane receptors (e.g. neurotransmitters, hormones, paracrines, etc.). Binding of the ligand changes the shape of the receptor which, in turn, may act as an enzyme or an ion channel or be coupled to one of these by a G protein.

Term of the Day

Telomere Clock: A theory concerning cellular aging and death. The telomere clock is presumed to determine the maximum number of times a given cell can divide. Telomeres are strings of nucleotides at the very ends of chromosomes, presumably to keep the chromosome from fraying or fusing with other chromosomes. In humans, the telomere base sequence TTAGGG repeats itself over a thousand times. The sequence, itself, does not code for anything, but every time the cell divides, up to 100 bases are lost from the telomere, which gets shorter. The theory is that when telomeres shorten to a critical point, then a signal to cease further mitosis is given. Credence to this theory was given by the 1994 discovery of the telomerase enzyme, which protects telomeres from degrading. Interestingly, telomerase is found in almost all cancer cells (which undergo uncontrollable mitosis), but not in other cell types.

Term of the Day

Stress Proteins: Originally called heat-shock proteins (hsp), these proteins are produced in large quantities by body cells in response to a variety of traumatizing stimuli -- high fever, oxygen deprivation, etc. In fact, stress proteins seem to be synthesized under conditions that would tend to denature proteins. Stress proteins guide the folding process during which proteins achieve their unique three dimensional shapes (and therefore acquire their ability to function).

Term of the Day

Isotope: Different atoms of an element that have the same number of protons but different numbers of neutrons. Stable isotopes do not change their nuclear structure over time. Certain isotopes called radioactive isotopes are unstable because their nuclei decay to form a simpler and thus more stable configuration.

Term of the Day

Metabolism: Broadly defined, metabolism refers to the sum total of all chemical reactions that take place in body cells. Some of these reactions involve the breaking down of larger molecules into smaller ones (catabolic reactions), synthesizing more complex molecules and structures from simpler subunits (anabolic reactions), and using nutrient molecules and oxygen to produce energy in the form of ATP (cellular respiration).

Concept of the Week

Homeostasis: This term refers to the body's ability to maintain relatively stable internal conditions in the face of continual change it its environment. There are a great many variables that need to be controlled and kept within optimal ranges in order for the trillions of cells that make up our bodies to continue functioning properly. Adequate blood levels of nutrients and gases must be maintained; heart activity and blood pressure must constantly be monitored and adjusted; wastes must not be allowed to accumulate in body tissues; body temperature must be maintained; electrolyte levels must be tightly controlled; blood pH must be kept within narrow limits; and on and on. And, under normal conditions, the body does an excellent job of monitoring and controlling all of these factors and more!

The body is said to be in homeostasis when its needs are being met and it is functioning smoothly. This is an extraordinarily complex task, and every single organ system has its own particular role to play in maintaining the constancy of the internal environment. Although the literal translation of homeostasis is "unchanging," do not fall into the trap of assuming that these important variables are maintained at a rigid, constant level. Homeostasis implies a dynamic state of equilibrium. That is to say, internal conditions do fluctuate, but are only allowed to do so within a relatively narrow range.

Homeostasis is so crucial, that most disease can be regarded as a disturbance of homeostasis (homeostatic imbalance).

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

Rhombencephalon: In embryology, one of the three primary brain vesicles. Also known as the hindbrain, it gives rise to the metencephalon and myelencephalon (secondary brain vesicles), and ultimately to the pons, cerebellum, and medulla oblongata.

Term of The Day

Hemoglobin F: Unique type of hemoglobin formed during fetal development. It has a higher affinity for oxygen than adult hemoglobin and is made of 2 alpha and 2 gamma polypeptide chains, rather than the 2 alpha and 2 beta chains of adult hemoglobin. After birth, erythrocytes carrying Hemoglobin F are destroyed by the baby's liver and are replaced by erythrocytes carrying hemoglobin A.

Term of the Day

Duchenne Muscular Dystrophy (DMD): Inherited, sex-linked, recessive, muscle-destroying disease. Females are carriers of the abnormal gene, but the disease almost exclusively strikes males. The gene results in the lack of an important protein called dystrophin, which is crucial for maintaining the structural integrity of the sarcolemma. DMD is typically diagnosed between ages 2 and 6 as previously normal children become clumsy and begin to fall more frequently. The degenerative process progresses from the extremities to the head and trunk. Death comes when the diaphragm and other inspiratory muscles are damaged. Patients rarely live beyond their 20s and die of respiratory failure.

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

Alopecia: absence of hair from where it usually grows; baldness. May be hereditary (e.g. male pattern baldness) or secondary to disease, injury, or chemical exposure (e.g. chemotherapy agents).

Term of the Day

Anaphylaxis: Type of immediate hypersensitivity triggered when allergens cross-link to IgE antibodies attached to mast cells or basophils, resulting in systemic release of inflammatory chemicals.

Concept of the Week

Inflammation: Usually a localized response to tissue damage or injury. It represents the body's attempt to defend itself. The hallmark signs of inflammation include redness, heat, edema, and pain.

1. Injured cells, mast cells, basophils, etc. release histamine and other inflammatory chemicals that increase blood flow to the affected area, leading to erythema of the skin and a feeling of warmth in the affected area.

2. Histamine and other chemicals increase capillary permeability, allowing antibodies and other immune chemicals to leave the capillaries and enter the tissue spaces. This causes localized edema, which, in combination with chemicals released from damaged cells (especially prostaglandins), causes pain.

3. Neutrophils and other leukocytes migrate out of the capillaries and into affected tissue spaces via diapedesis. Neutrophils and macrophages are attracted to the area of damage via positive chemotaxis by chemicals released from damaged cells, activated complement proteins, lymphokines, and other chemical signals.

Term of the Day

Alveolar Ventilation: The portion of tidal volume that actually reaches the alveoli of the lungs. During exhalation, part of this gas remains in the conducting airways and re-enters the alveoli with the next inhalation.

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.

Health Alert

15 January 2009: the Kellog Co. announced that its peanut butter crackers may be tainted with salmonella and consumers should not eat them. The alert affects the Austin and Keebler brands. the problem is the peanut butter produced by the Peanut Corporation of America, which has been linked to an outbreak of salmonella affecting over 430 people in 43 states.

Term of the Day

Pyelonephritis: Infection and inflammation of the renal pelvis and interstitium. It is usually bacterial in nature (E. coli, Proteus, or Pseudomonas being the usual culprits), but viruses and fungi can also be causative organisms. It is more common in women, and urinary obstruction and vesicoureteral reflux are risk factors. One or both kidneys may be involved.

Health News: Poor Sleep Prior to Rhinovirus Exposure Leads to Reduced Resistance to the Common Cold

Poorer sleep efficiency and shorter sleep duration in the weeks before exposure to a rhinovirus are linked to greater susceptibility to the common cold, according to the results of a study reported in the January 12 issue of the Archives of Internal Medicine.

The study sample consisted of 153 healthy men and women volunteers aged 21 to 55. For 14 days, subjects reported their sleep duration and sleep efficiency, defined as percentage of time in bed actually asleep for the previous night and whether they felt rested. The investigators calculated average scores for each sleep variable during the 14-day baseline. Participants were then quarantined, inoculated with nasal drops containing a rhinovirus, and monitored for the development of a clinical cold on the day before and for 5 days after rhinovirus exposure. Clinical cold was defined as infection in the presence of objective signs of illness.

Participants who reported less than 7 hours of sleep were nearly 3 times more likely to get a cold than those with 8 hours or more of sleep, suggesting a correlation with average sleep duration. Participants with less than 92% sleep efficiency were 5.5 times more likely to get a cold than those with 98% or more efficiency, also suggesting a correlation with sleep efficiency.

So, your mother was right -- you need to get plenty of sleep!

Arch Intern Med. 2009;169:62-67.

Term of the Day

Postconcussive Syndrome: A cluster of symptoms that may persist for weeks or months following a cerebral concussion, including: headache, anxiety, irritability, insomnia, depression, impaired concentration, forgetfulness, and fatigability.

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.

Case Study -- A Simple Case of Pneumonia?

O.K. ladies and gents, time to play Dr. House! We'll start with a relatively simple case of pneumonia...

Patient A presents to the emergency room with symptoms of pneumonia, including cough, dyspnea, and hemoptysis. X-ray confirms the presence of diffuse infiltrates in the right lung, and on physical examination, hypoventilation and hypotension are noted. The history reveals that, prior to the pneumonia, the patient had not experienced any respiratory symptoms of any kind. The patient did report, however, that he had been experiencing a mild, stabbing chest pain 10-15 seconds after swallowing food or liquids, and often found it difficult to swallow. He reported that if he "bore down" as if he were defecating, swallowing was usually easier. The problem was worse when he had to eat in a hurry, and he said food sometimes "went down the wrong pipe" producing coughing fits. He had been experiencing this for months, and had tried managing his own symptoms using Maalox, without any real success -- the problem seemed to be getting worse. He reported a particularly bad episode of regurgitation and coughing the day before his breathing problems started.

What kind of pneumonia does A have? Is pneumonia the real underlying problem here? If not, what is/are the likely diagnosis(es)? What test would you order next? What "sign" would you be looking for on this test?

Term of the Day

Dehydroepiandrosterone (DHEA): A weak androgen (male sex hormone) secreted by the zona reticularis of the adrenal cortex in both genders. This steroid hormone can be converted into testosterone in the peripheral tissues.

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.

Term of the Day

Blood-Testes Barrier: This refers to the ability of the sustentacular (Sertoli) cells of the seminiferous tubules to prevent potentially toxic or harmful substances from entering or leaving the tubules. Many proteins, ions, hormones, and drugs are blocked from access to the developing spermatids. This works the other way, too. The barrier prevents the sperm in the tubules from diffusing into the bloodstream. This is a good thing, because sperm are immunogenic. That is, they are not recognized by cells of the immune system, so that if they entered the blood, they would be treated as foreign antigens and antibodies against them would be formed. This could lead to sterility.

Pathophysiology News: Mobile Phones and Meningioma

Here is some good news for those of you worried about a possible connection between cell phone use and intracranial malignancies. In an international study published in the International Journal of Epidemiology, researchers tracked 1209 meningioma cases and 3299 population-based control subjects. Data were collected regarding regularity of mobile phone use, duration of use, cumulative number of hours of use, cumulative number of calls, and several other indicators of cell phone use patterns.

When the researchers tried to correlate these data with meningioma risk, they found no connection. In fact, the risk of meningioma among regular cell phone users was actually lower than that for never or non-regular users! Furthermore, meningioma risk did not increase with years of mobile phone use, cumulative hours of use, or cumulative number of calls. These findings were similar regardless of age, gender, or telephone network type (analog vs. digital).

So, one more health scare bites the dust! You cell phone junkies can rest a little easier now.

Term of the Day

Portal Confluence: The area where the superior and inferior mesenteric veins converge and join the splenic vein to form the hepatic portal vein. It is readily visible on ultrasound.

Term of the Day

Hilton's Law: Joints (articulations) have an extensive nerve supply. These articular nerves are actually branches of the nerves that supply the overlying skin and the muscles that move the joint. Hilton's Law essentially says that the nerves that supply a joint also supply the muscles that move the joint and the skin covering their attachments.

Term of the Day

Furuncle: The medical term for a "boil" -- an inflammation of the hair follicles. It may develop from pre-existing folliculitis that spreads into surrounding dermis. The causative microorganism is usually S. aureus.

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

Acsites: Accumulation of fluid in the peritoneal cavity, which reduces the amount of fluid available for normal physiologic functions. Cirrhosis of the liver is the most common cause, but other diseases associated with ascites include: heart failure, constrictive pericarditis, abdominal malignancies, nephrotic syndrome, and malnutrition.

Term of the Day

Epiploic Foramen: Also called the Foramen of Winslow. Lying immediately posterior the free margin of the lesser omentum, this opening forms the mouth of the omental bursa (lesser sac) -- a blind-ended peritoneal recess situated posterior to the stomach and lesser omentum. The epiploic foramen is the only communication between the omental bursa and the general peritoneal cavity.

Term of the Day

Introns: Nucleotide sequences that have no known cellular function (i.e they don't "code" for anything). Introns account for much of the DNA in the genomes of eukaryotes, and are often found within a gene, interrupting its nucleotide sequence. Prevailing thought holds that introns are remnants of sequences that were active and important earlier in evolutionary history.

Term of the Day

Hypersomnia Sleep Apnea (HSA) Syndrome: Periodic breathing and episodes of apnea (absence of air flow for at least 10 seconds during sleep) associated with stage I and II and REM sleep. Decreased arterial oxygen saturation leads to arousal, thus interrupting the sleep cycle and leading to sleep and REM deprivation. HSA syndrome is mainly the result of upper airway obstruction occurring during sleep. Patients may experience hundreds of episodes of apnea per night, which leads to extreme daytime sleepiness. Snorting and thrashing about during sleep are common, and some patients may experience sleepwalking or enuresis.

Pathophysiology News -- Vitamin Supplements and Cancer Prevention

Bad news for those of you counting on vitamin supplements (especially antioxidants) to reduce your cancer risk. A new study to be released in the 7 January 2009 issue of the Journal of the National Cancer Institute indicates that vitamin C, vitamin E, and beta carotene do not help prevent cancer or reduce cancer mortality. The study followed 7627 women who were cancer-free at the outset of the experiment. Control subjects were given placebos; one experimental group was given 500 mg of vitamin C daily; another experimental group was given 600 IU of vitamin E every other day; and a third experimental group took 50 mg of beta carotene every other day. The average duration of treatment was 9.4 years.

Six hundred twenty-four women went on to develop invasive cancer, with 176 of them dying. None of the antioxidants performed any better than the placebo in reducing total cancer incidence. Neither was there any statistically significant difference among the 4 groups in cancer mortality. Combined use of all three antioxidants was also found to have no effect.