MSK Physeo

Endochondral Ossification Pathway Steps

Begins with hyaline cartilage model surrounded by perichondrium --> Mesenchymal cells within perichondrium differentiate into osteoblasts → forms bony collar (periosteum) → nutrition to chondrocytes, via diffusion, is blocked → chondrocytes within model release calcium and die → porous holes formed --> Periosteal bud penetrates bone collar providing blood supply to inner portion of cartilage model → osteoblasts travel from periosteum to cartilage model → spongy bone replaces cartilage in diaphysis (primary ossification center) --> Osteoclasts remove central portion of spongy bone along diaphysis → medullary cavity formed --> Osteoblasts in periosteum continue to lay down bone → compact bone beneath periosteum --> Secondary ossifcaton centers form after birth at epiphyses → formation of spongy bone filled with hematopoietc stem cells → continued release of osteoclasts, erythrocytes and leukocytes

Succinylcholine: CU? MOA? AE?

CU = temporary paralysis (helpful when intubating/mechanically ventilating a pt) MOA: binds and activates Nm (nicotinic) Ach receptors found on skeletal muscle --> Na+ enters cell, K+ leaves cell. This muscle becomes completely depolarized and prevents additional stimulation AE = hyperkalemia (efflux of K+ from cell can lead to hyperkalemia)

Give the pathway from Ca2+ release from the SR --> relaxation of the muscle after contraction (just steps no explanation)

Ca2+ binds troponin C --> tropomyosin + troponin move off actin/undergo conformational change --> myosin is free and contains ADP + phosphate --> ADP phosphate complex is released --> myosin undergoes conformational change --> actin is pulled inward --> myosin binds with actin --> muscle is fully contracted. New ATP molecule binds to myosin-actin crossbridge --> myosin head detaches from actin --> ATP is hydrolyzed --> ADP + phosphate + energy are produced --> energy causes myosin head to be cocked --> cocked myosin head that pull actin when tropomyosin is moved again.

Role of intracellular Ca2+ in neurotransmission

Ca2+ binds troponin C, and when this happens --> tropomyosin moves away from actin.

Innervation of GTOs

*GTO is innervated by type 1b sensory axons*, which *send stretch related information to an interneuron within the spinal cord.* The interneuron is an inhibitory neuron. This is because it *inhibits that alpha motor neuron*, which *normally is responsible for causing muscle contraction*.

How does the metaphysis help with bone elongation?

*Hyaline cartilage* will remain in metaphysis and is a source of new bone growth. So, in the metaphysis we have *chondrocytes continually making new cartilage* and the *osteoblasts in the diaphysis will ossify the old cartilage*. And this will allow the bone to elongate. So, osteoblasts in diaphysis ossify old cartilage --> bone elongation.

Sarcomere Bands

A band I Band H band

What is a motor unit?

A motor unit is an alpha motor neuron and the muscle fiber it innervates.

A man is involved in an extremely intense arm wrestle when his arm suddenly involves relaxes. Explain the physiologic pathway that caused his arm to suddenly relax.

An intense arm wrestle could transmit significant tension to the GTO. Activation of the GTO could cause sensory information to travel to the spinal cord, through type 1b sensory axons. These synapse on the inhibitory interneuron, which inhibits the alpha motor neuron, ultimately resulting in forced relaxation of the corresponding muscle. So the intense arm wrestle activated the GTO, causing this individuals armed to suddenly and involuntarily relaxed.

What is responsible for sending the information that we want to move to the muscles?

Anytime we want to move, the primary motor cortex in the frontal lobe processes this thought and sends signals down to the corresponding muscles. A specific pathway, called the corticospinal tract is responsible for sending this important information from the primary motor cortex to the muscles.

How does exercise promote angiogenesis?

As we can see from the myoglobin curve, a decrease in partial pressure of oxygen (due to active consumption of oxygen by the muscle tissues) results in a partial desaturation of myoglobin. If the dotted line on the right represents, let's say, 90% saturation, then the dotted line on the left would represent let's say 70% saturation. This sudden drop in oxygen sends a signal to the body telling it that the muscle needs more oxygen. So, the body attempts to increase the vascular supply to the muscle. The body achieves this by increasing the release of vascular endothelial growth factor or VEGF. So, the sudden drop in oxygen content is responsible for the increased VEGF which promotes angiogenesis. Exercise results in decreased oxygen within the tissue, this results in increased VEGF, which results in increased angiogenesis.

The skeletal muscle of a knockout mouse is able to contract despite total depletion of intracellular skeletal muscle calcium concentrations. What protein defect most likely explains the finding?

From the question stem, we know that the mouse's skeletal muscle can contract without calcium. In order to get this question right, you need to be familiar with the role of intracellular calcium in neurotransmission. Calcium normally binds to troponin C and when this happens, tropomyosin moves away from actin. If calcium is not present and the skeletal muscle can still contract, then we can infer that the knockout mouse must be lacking the protein tropomyosin. If tropomyosin is not present, then in theory, myosin should be able to bind to actin, without the need of calcium first binding to troponin and moving tropomyosin out of the way. So a lack of tropomyosin means the muscle could contract without calcium.

Golgi tendon organs (GTOs): What is it? Innervation of GTOs

Golgi tendon organs or GTOs are an *important part of the negative feedback mechanism*, whereby an *excessively stretched muscle can cause forced relaxation*.

Hydroxyapatite provide the _____ for the bones

Hardness

Where does hematopoiesis in utero take place? in an adult?

Hematopoiesis in utero occurs in the yolk sac, the liver and the spleen. As an adult, it occurs in the bone marrow.

How does IL-1 affect osteoclast activity?

Interestingly IL-1 is also called the osteoclast activating factor, which is a fitting name because it functions similar to RANKL in that it *stimulates these osteoclasts precursors to* combine, *form*ing a multinucleated *osteoclast*. In other words, if a patient is super ill, with systemic inflammation for a long time, eventually osteoclast can begin to erode the bone, causing osteoporosis.

What's the difference between Lambert-Eaton and myasthenia gravis?

Lambert-Eaton and myasthenia gravis are both autoimmune disorders that disrupt neurotransmission. These two disorders can be hard to remember. I like to use the letter L in Lambert-Eaton, to remind me that L comes before M in the alphabet. The letter M helps remind me of myasthenia gravis. So if L comes before M then *Lambert-Eaton* must be caused by *disruption of the presynaptic cleft*. Whereas, *myasthenia gravis* must be caused by *disruption of the post synaptic cleft*. So L comes before M and the presynaptic cleft comes before the postsynaptic cleft.

What causes Lambert-Eaton syndrome? What is the pathophysiology behind the symptoms?

Lambert-Eaton syndrome is caused by antibodies that bind to and disrupt the presynaptic calcium channels. From the previous question we learned that, when this calcium channel opens, calcium enters the cell and induces acetylcholine release. When acetylcholine enters the synaptic cleft, it can bind to the acetylcholine receptors in the muscle. This is ultimately responsible for causing muscle contraction. Because Lambert-Eaton syndrome blocks these channels, we can deduce that less acetylcholine will be released and the skeletal muscle will not be stimulated as much. This is why these patients have muscle weakness. The muscle weakness gets better with use, because *over time the alpha motor neuron is stimulated more and more, until enough acetylcholine is able to be released* into the synaptic cleft and cause muscle contraction.

Osteoclast Formation Pathway: start from bone marrow and go to maturation of osteoclasts (+what can prevent maturation). (Just the pathway, no details)

M-CSF stimulates stem cells in bone marrow --> monocytes/macrophages/osteoclast precursors (just macrophage from here on out) --> RANKL binds to RANK receptor on macrophage --> stimulates NF-kB --> mature osteoclast forms. OPG binds RANKL --> RANKL cannot bind macrophage and activate it --> macrophage will NOT become osteoclast. OPG counteracts RANKL activity to decrease osteoclast activity.

A 55 year old female patient has not had a menstrual period in 5 years. Her clinician informs her that she has an increased risk for osteoporosis. Why?

Menopause will cause decreased levels of estrogen. Estrogen acts on osteoblasts to increase their release of OPG. OPG normally inhibits osteoclasts. So this patient has decreased estrogen, which means there will be decreased stimulation of the osteoblasts to secrete OPG. So there will be decreased OPG and this means that osteoclast activity would actually be increased and this would lead to increased resorption leading to osteoporosis, meaning porous bones.

Two experimental mice are conditioned on two separate treadmills over a several month period. Mouse A runs at a speed of 2 f/s for 5 minutes several times throughout the day. Mouse B runs at a speed of 0.5 f/s for long durations once a day. How will a biopsy of the muscle fibers of mouse A likely compare to that of mouse B?

Mouse A is undergoing short bursts of high-intensity training and mouse B is undergoing longer, but easier sustained training. From the previous slide we learned slow twitch muscle fibers are primarily involved in long lasting sustained force. So slow twitch is involved in long and sustained exercise. We also learned that fast twitch fibers are primarily involved in short forceful movements. So fast twitch is involved in short forceful exercise. With this in mind, mouse A will likely have an abundance of fast twitch muscle fibers and mouse B will likely have an abundance of slow twitch muscle fibers.

What is myasthenia gravis? What is the NMJ? Why does myasthenia gravis result in progressive weakening of muscles with repetitive views?

Myasthenia gravis is an autoimmune disorder, where antibodies bind and disrupt the acetylcholine receptors at the neuromuscular junction. The neuromuscular junction is exactly what it sounds like: the region between the alpha motor neuron and the muscle. So again, antibodies bind and disrupt the acetylcholine receptors at the neuromuscular junction. The *dysfunctional acetylcholine receptors* results in *excessive endocytosis of the receptors*, aka the *Ach receptors move into the cell*. And this means that when the presynaptic neuron releases acetylcholine, there are less receptors available to stimulate. This means initially, the acetylcholine binds some few receptors that are available and results in muscle contraction. However, as more acetylcholine is released into the neuromuscular junction, there aren't any additional acetylcholine receptors available, so the muscle is no longer able to contract, which results in progressive weakening of the muscles with repetitive use.

What is myoglobin and how does it help slow twitch fibers?

Myoglobin is a protein similar to hemoglobin, which *carries oxygen*. However, myoglobin is more abundant in the muscle and is able to *pull oxygen from the blood* and *into the muscle* tissue. The abundance of myoglobin allows the muscle to pull in a *significant volume of oxygen*, which is *necessary for the production of ATP*.

What causes shortening of the sarcomere? what does this cause?

Myosin head binds to actin, causing shortening of the sarcomere. This causes muscle contraction.

A 10 year old male patient is hit in the upper left arm with a baseball bat. Radiograph shows a fracture at the metaphysis. If the damage to the metaphysis was irreversible, what would be the long term consequence of this fracture?

The metaphysis is the growth plate and it's made of hyaline cartilage and chondrocytes. And this is the source of new bone growth, as the child matures. If irreversible damage occurred to this location, the metaphysis in other words the growth plate, then appropriate bone growth new bone would fail to occur. Therefore, with permanent damage to this growth plate, this patient as an adult, may have one arm that is shorter than the other.

What is the metathesis?

The metathesis lies between the epiphysis and the diathesis. The metaphysis is also called the epiphyseal end plate.

Muscle anatomy: from macroscopic to microscopic

The muscle contains many fascicles and each fascicle contains many muscle fibers or muscle cells. Each muscle fiber, contains many myofibrils. The myofibrils are comprised of many sarcomeres. A sarcomere is the structural unit of skeletal muscle. Sarcomeres contain thin filaments and thick filaments. So, from macroscopic to microscopic, we have: Muscle → fascicle → muscle fiber (muscle cell)→ myofibril → sarcomere --> thin and thick filaments

What underlying molecular abnormality is responsible for the stiffening of the muscles seen in rigor mortis?

The muscles stiffen because the individual is no longer producing ATP and the *lack of ATP prevents* the *myosin heads from detaching from actin*. So a muscle relaxation is unable to occur. Aka, the binding of ATP is necessary for the myosin to come off of the actin, allowing the muscle to relax. So what molecular abnormality is responsible for rigor mortis? A lack of ATP.

How are osteoblasts made?

The periosteum contains mesenchymal cells, which can differentiate into osteoblasts when needed.

How does the SR store calcium?

There is a protein within SR, called *Calsequesterin*, which binds to and stores calcium.

A Band

This is the dark band. It's dark because it contains overlapping myosin and actin (At the top of the pic is a recreated image of an electron microscopy and the A region is dark, whereas the I region is light.)

I Band

This is the light band. It's light because it only contains the thin filament actin.

A 66 year old with a 50 pack-year smoking history presents with generalized muscle weakness. He states that it is worse in the morning but gets better throughout the day. What is the underlying explanation for the muscle weakness?

This pt has a history of smoking, which should make you think of lung cancer. Because this patient has muscle weakness, that gets better throughout the day, so worse in the morning but it gets better throughout the day, we can infer that the patient must have small cell lung cancer, resulting in a paraneoplastic syndrome called Lambert-Eaton syndrome.

Wave summation

Wave summation refers to the idea that, when a muscle fiber is stimulated more frequently, the strength or force of contraction increases. On the bottom of the figure we can see action potentials. This is showing us how frequently the muscle fiber is stimulated. On the top, we can see how much force, aka force of contraction, is generated by the muscle fiber. This is represented by the Myogram. When a muscle fiber is stimulated with a single action potential, so that is represented right as the first little hump in the action potential line in the far left of the graph, it briefly contracts and then relaxes. So contraction-relaxation. This is called a twitch. However, when several action potentials occur in succession, the muscle fiber contracts with more force. So as you can see, we have two successive action potentials and then four successive action potentials. And each time there is contraction and then relaxation, contraction, relaxation. So these are called summation of twitches. When many action potentials occur for a sustained period of time, the muscle fiber undergoes maximum contraction and produces the maximum force. So this is shown on the far right ride of the graph, many action potentials cause sustained contraction. This is called tetanus. So, we've looked at this concept with a single muscle fiber, but it can also be applied to entire muscles.

How would the H band and I band change during muscle contraction?

When the sarcomere contracts, the I band becomes shorter because the myosin heads on *both sides* of the I band pull the I band towards the center of the myosin (aka toward the Z line). The H band is the region of the sarcomere that only contains thick filaments. Because the actin moves closer towards the M line, during contraction, the H band must shorten. So the H band shortens during muscle contraction. So, in summary, both the I band and the H band, would shorten during muscle contraction.

I Band length on sarcomere contraction

When the sarcomere contracts, the I band becomes shorter because the myosin heads on both sides of the I band, pull the I band towards the center of the myosin. As a result the actin smoothly slides, bringing the thick filaments closer to each other, which ultimately results in shortening of the I band.

How are fast twitch fibers able to achieve fast powerful movements?

While fast twitch fibers have less mitochondria and myoglobin, compared to slow twitch fibers, they are able to achieve fast powerful movements, because of their ability to *rapidly metabolize ATP*. This occurs via *anaerobic glycogenolysis*, as the muscle supply of glycogen is rapidly metabolized.

thin filaments are composed of __________

actin

What is malignant hyperthermia?

autosomal dominant disorder that results in mutations in the voltage-sensitive ryanodine receptors

What is ossification?

bone development

Why would an activating mutation in the FGFR3 gene cause short bones?

chondrocytes are vital to the elongation of long bones at the metaphysis. Under normal conditions, the chondrocytes will lay down new hyaline cartilage and the osteoblasts will ossify some cartilage. And this process will repeat again and again, as the bone grows longer and longer. Now also recall that the FGFR3 gene codes for a receptor that inhibits chondrocytes. Now without continued growth of chondrocytes, the osteoblasts would ossify old hyaline cartilage, but the chondrocytes would fail to produce new hyaline cartilage as normal. This would mean, that the osteoblasts would essentially ossify all the cartilage and the bones would remain short and failed to grow. And this condition is actually called achondroplasia.

A 67 year old female is being intubated for a hip replacement surgery. The anesthesiologist suddenly notices that she develops hyperkalemia. What drug was likely used to assist with the intubation that resulted in the hyperkalemia?

*Hyperkalemia* during surgery should make you think of the drug *succinylcholine*. Succinylcholine is a drug used to induce temporary paralysis. This is especially helpful when intubating or mechanically ventilating a patient. *Succinylcholine* works by *binding to and activating the nicotinic acetylcholine receptor on the skeletal muscle*. The muscle becomes completely depolarized for a sustained period of time and ultimately prevents additional stimulation. Because succinylcholine activates the nicotinic acetylcholine receptor, it causes sodium to enter the cell and potassium to exit the cell. The sustained efflux of potassium is responsible for the hyperkalemia and is an important side effect to be aware of when using succinylcholine.

What do osteoblasts do?

1) Osteoblasts will lay down new bone which is composed of *hydroxyapatite*. --Hydroxyapatite is composed of calcium and phosphate. 2) Osteoblasts will also lay down *type 1 collagen*. --Some students like to remember that bone has type 1 collagen by remembering that long bones are the shape of the number 1.

2 Types of muscle fibers

1) Slow twitch (type I) 2) Fast twitch (type II)

How do myosin and actin interact at a molecular level to cause muscle contraction? Give the pathway from Ca2+ release from the SR --> relaxation of the muscle after contraction

Calcium is released from the sarcoplasmic reticulum, binds to troponin C and moves tropomyosin, so that myosin heads can bind to actin. So, basically, when calcium binds to troponin it causes tropomyosin and troponin to move off of the actin. The troponin and tropomyosin undergo a conformational change, which allows myosin to bind to actin. At this point the myosin heads can bind to actin. Ca2+ binds troponin C --> tropomyosin + troponin move off actin/undergo conformational change --> myosin binds actin --> muscle movement Notice how the myosin head contains ADP and phosphate. As the ADP phosphate complex is released, the myosin head undergoes a conformational change, which pulls the actin inward. At this point, the sarcomere and the muscle is fully contracted and cannot relax until a new ATP molecule binds to the myosin actin crossbridge. When ATP binds to the myosin actin cross bridge, this results in detachment of the myosin from the actin. So the myosin heads come off. Finally, the ATP is hydrolyzed, resulting in the production of ADP and phosphate, plus energy. So the energy released during this process allows the myosin head to be cocked so that it can pull the actin, once tropomyosin is moved out of the way.

What drug can be useful in treating malignant hyperthermia? How?

Dantrolene. Dantrolene is a ryanodine receptor antagonist. So it blocks this channel. By blocking the ryanodine receptor, less calcium is released into the cell, which ultimately decreases muscle contraction.

What drugs can cause malignant hyperthermia?

Drugs = 1) succinylcholine inhaled anesthetics such as: 2)desflurane 3) isoflurane How?

Normal physiologic oxygen delivery to muscle tissue

During exercise, myoglobin saturation decreases, and this makes sense. If you think back to the oxygen dissociation curve the process here is the same. On the y-axis we have oxygen saturation and on the x-axis we have the partial pressure of oxygen, O2 pressure. The myoglobin dissociation curve looks something like this. The tissue is so active that the cells are rapidly consuming oxygen. This means that the partial pressure of oxygen decreases, so it will move toward the left on the x axis.

2 types of ossification

Endochondral ossification (hyaline cartilage --> endochondrial ossification) Intramembranous ossification (fibrous membrane --> intramembranous ossification)

What upregulates OPG activity? What does that lead to?

Estrogen! Estrogen will up regulate osteoprotegerin secretion, which will decrease the ability of RANKL to bind and stimulate those osteoclasts. So it would make sense that a postmenopausal woman would have increased osteoclasts and increased bone resorption as a result of the decreased estrogen.

Explain the entire pathway of neurotransmission from the corticospinal tract to muscle contraction

Eventually, axons from the corticospinal tract, synapse on the alpha motor neuron, which is found in the ventral horn of the spinal cord. The top of the picture (where corticospinal is synapsing on the alpha motor neuron) represents the ventral horn of the spinal cord, where the cortical spinal tract synapses on the alpha motor neuron, which sends motor information to the muscle. As you can see from the figure, the axons of the corticospinal tract release acetylcholine. Ach can then bind to Ach receptors on the postsynaptic cleft of the alpha motor neuron. When this happens, the protein channel opens and allows sodium to enter the cell and potassium to leave the cell. As sodium enters the cell, it causes depolarization. Depolarization eventually reaches the axon hillock, which causes voltage-sensitive sodium channels to open. This causes more sodium to enter the cell, causing additional depolarization. And then this triggers voltage-sensitive calcium channels to open, allowing Ca2+ to enter the cell. Intracellular Ca2+ is necessary for the release of neurotransmitters (in the picture, the NT released is Ach). So Ach is packaged into vesicles and then released into the synaptic cleft. Once in a synaptic cleft, acetylcholine binds to the acetylcholine receptor on skeletal muscle tissue. So this region of the skeletal muscle where the Ach binds to its receptor, is called the *motor end plate*. Binding of acetylcholine to the motor causes the ligand gated channel to open, which results in sodium influx and potassium efflux. This results in depolarization. The change in voltage is sensed by the voltage sensitive sodium channels in the skeletal muscle, causing additional sodium to enter the cell. As the cell continues to depolarize, the L-type calcium channels open. Notice how the L-type calcium channels are found in these invaginations. This part of the cell is called the T-tubule. As calcium enters the cell, it causes a conformational change in the L-type calcium channel. This conformational change results in a mechanical interaction with the ryanodine receptor present on the sarcoplasmic reticulum, which allows calcium, within the sarcoplasmic reticulum to be released. (Release of calcium from the SR is not directly dependent upon calcium from the L-type calcium channel. Rather, it's directly dependent upon the mechanical interaction of the L-type calcium channel with the ryanodine receptor.) Calcium release from the sarcoplasmic reticulum, results in increased cytosolic calcium, which can then bind to troponin C. When calcium binds to troponin C, it causes tropomyosin to move away from the actin. At this point, the myosin combines to actin, and causes muscle contraction. Finally, the sarcoplasmic reticulum (SR) contains a calcium ATPase pump called serca and this actively pumps intracellular calcium into the SR. (This process is important because it keeps intracellular calcium concentrations low, which facilitates muscular relaxation.)

How does the FGFR3 mutation cause achondroplasia?

FGFR3 inhibits chondrocytes in the metathesis from producing more hyaline cartilage. Normally this just makes it so bones aren't abnormally long. If the FGFR receptor is hyperactive, as is seen in the activating mutation that causes achondroplasia, then it doesn't really matter how much FGF you have, the chondrocytes will be inhibited and there will be a failure of growth. FGFR3 is hyperactive in achondroplasia which is a type of dwarfism. So you can imagine that if FGFR3 is hyperactive, FGFR3 being the thing that can inhibit the metaphysis, then with hyperactivity the *metaphysis will fail to produce new cartilage*. In other words, the growth plate will fail to produce new cartilage.

what inhibits the chondrocytes in the metathesis? How? what disease presents will this inhibition of chondrocytes?

FGFR3. FGFR3 is a protein on the chondrocytes that will inhibit the chondrocytes in the metathesis. Chondrocytes have these little receptors called FGFR4 or FGF receptors. when these FGF receptors are acted upon by FGFs then they will be inhibited, preventing them from producing more hyaline cartilage. Now it is normal to have some FGFR3 stimulation from FGFs, this is a normal regulatory mechanism to prevent the bones from becoming abnormally long.

Fast twitch (type II) muscle fibers: What type of movement are they primarily involved in?

Fast twitch fibers are primarily involved in *short and forceful movements*.

Type 1 collagen provides the _____ for the bones.

Flexibility

What two compensatory mechanisms allow the biceps muscle to increase the force of contraction during heavy exercise?

For simplicity, let's say the biceps muscle has five motor units. An alpha motor neuron will be stimulated with increased frequency during heavy exercise, which would ultimately cause tetanus of the corresponding muscle fiber. So increased frequency results in tetanus, which results in increased force. Additionally most, if not all five of the motor units, would be activated and would all reach tetanus. So increased recruitment of motor units results in increased force. Okay, so what are the two compensatory mechanisms? Increased frequency and increased recruitment of motor units.

Osteoclast Formation Pathway: start from bone marrow and go to maturation of osteoclasts (+what can prevent maturation).

Hematopoietic stem cells reside within the bone marrow. Macrophage colony stimulating factor will stimulate these cells to develop into osteoclast precursors, monocytes or macrophages. For simplicity, we will just call these macrophages. Then the RANKL will stimulate those cells by binding to the rank receptor and it will stimulate the nuclear factor kappa B (NF-kB). This will allow for the fusion of precursors and maturation of osteoclasts. In other words, these precursors will combine and form a multinucleated osteoslasts. Now there is another hormone involved and it's called osteoprotegerin or OPG. OPG can bind to the RANKL, so that it will not be able to bind to the osteoclast precursors and activate them, helping them to become osteoclasts. In other words, OPG will counteract RANKL and decrease osteoclast activity.

What type of cartilage is in the metathesis?

Hyaline cartilage

If a patient is super ill, with systemic inflammation for a long time, what can happen to bone? why?

If a patient is super ill, with systemic inflammation for a long time, eventually osteoclast can begin to erode the bone, causing osteoporosis. Inflammation --> increases IL-1 release --> IL-1 stimulates osteoclast precursor to become --> mature osteoclast --> erodes bone --> osteoporosis

What happens if there is a defect in osteoblasts laying down type 1 collagen?

If there is a defect in osteoblasts laying down type 1 collagen, then a patient can suffer from osteogenesis imperfecta.

How does inflammation play a role in osteoclast activity?

If there is long-standing inflammation, there will be increased release IL-1.

What happens to osteoclast actvity in a patient with *hyper*parathyroidism? How?

In a patient with hyperparathyroidism, there will be increased release of PTH, and the PTH would act on the osteoblasts to increase their release of RANKL. With increased RANKL, there will be increased formation of multinucleated osteoclasts. Which means there will be increased bone resorption.

What happens to osteoclast actvity in a patient with low calcium? How?

In a patient with low calcium, the parathyroid gland would sense this and increase its release of PTH in order to activate osteoclasts. And once the osteoclasts are activated, there will be increased bone resorption. Which ultimately means there will be increased calcium and phosphate in the blood, because that is what hydroxyapatite is made of. So the point is that in a patient with low calcium, bone resorption and increased activity of osteoclasts can be very helpful, because it can increase calcium level in the blood.

Factors contributing to bone density

In general terms, think of bone density as being determined by the osteoclast to osteoblast activity ratio. 1) Age: with age, there will be increased bone loss. This means that the ratio would be high. 2) Gender: gender plays an important role -- women are more prone to bone loss, and menopause is a significant part of this. 3) Ethnicity/Race: african americans tend to have increased bone density 4) Genetics: genetics as with many other functions in the body, can play a role in bone density

A 47-year-old male is administered an inhaled anesthetic and suddenly develops intense muscle contractions. What molecular abnormality explains the sudden change?

In some patients, inhaled anesthetics such as desflurane and isoflurine can cause malignant hyperthermia. Malignant hyperthermia is an autosomal dominant disorder, that results in mutations in the voltage-sensitive ryanodine receptors. So malignant hyperthermia is caused by mutations in voltage-sensitive ryanodine receptors. When inhaled anesthetics or succinylcholine are administered, they can cause excessive release of calcium from the sarcoplasmic reticulum into the cytosol. In this case, the patient was given an inhaled anesthetic. The excess intracellular calcium can bind to troponin C, which ultimately results in severe muscle contractions. So mutations in the ryanodine receptor explain the sudden and intense muscle contractions.

Endochondral ossification

In the first type of ossification, there is a hyaline cartilage model and this allows for endochondral ossification. This occurs in *long bones* -- ribs, tibia, fibula, femus, radius. In other words, long bones are those that look long and skinny. Endochondral ossification begins with a hyaline cartilage model and it's surrounded by perichondrium. As you may remember from the cards above, the bone is surrounded by periosteum, meaning a round bone. Now this is perichondrium, so it's peri (peri = around) chondrium (chondrium = cartilage). Within the perichondrium, there are mesenchymal cells and these will differentiate into osteoblasts. And these osteoblasts will form a bony collar, because they will start laying down bone and this is where we get periosteum. And as a result of this bony collar forming, the nutrition to the chondrocytes will actually be blocked. Because the chondrocytes receive their nutrition through diffusion. Hyaline cartilage doesn't have a blood supply going through it, so it doesn't receive nutrients from the blood. Instead it receives it through diffusion and as a result of the bony collar, diffusion is totally blocked. And so what will happen as a result is the chondrocytes within the model will release calcium and they will die. And this will create porous holes.

Intramembranous ossification

In the second type of ossification, everything starts with a fibrous membrane and then intramembranous ossification can occur. Intramembranous ossification occurs in flat bones. For example, the skull and pelvic bones. Just think of any bones that appear flat or not long and skinny, like the long bones. So this starts with dense irregular connective tissue with mesenchymal cells. These mesenchymal cells, will differentiate into osteoblasts. And as a side note mesenchymal cells were also in endochondral ossification located in the perichondrium, later called the periosteum. So in intramembranous ossification, mesenchymal cells will likewise differentiate into osteoblasts, which can then create spongy bone. And this spongy bone will be in the center and then compact bone will be on the outside of the bone and just as with endochondral ossification, periosteum will surround the compact bone.

Increased production of RANKL would ____(increase or decrease) osteoclast activity. Increased production of OPG would ___(increase or decrease) osteoclast activity.

Increased production of RANKL would *increase* osteoclast activity increased production of OPG would *decrease* osteoclast activity

A patient is unable to produce adequate levels of OPG. What will happen to the bones as a result? Name this disease + it's characteristics.

Normally, RANKL will upregulate osteoclast activity. Also recall that OPG will inhibit RANKL by directly blocking it. So this patient has low levels of OPG and this will result in higher levels of RANKL, which will result in increased osteoclasts. With *excessive osteoclast activity*, a *bone would become more porous* and experience *osteoporosis*. This condition of low OPG, resulting in osteoporosis, is called *Paget's disease*. So, Paget's disease doesn't stop there. In fact, *osteoblasts* will attempt to compensate for the increased osteoclast activity and *lay down new bone in a rapid and disorganized manner*. And this will result in *bones that look deformed*. It's important to recognize that *decreased levels of osteoprotegerin* can negatively impact the patient overall, leaving them with *osteoporosis and Paget's disease*.

A Band length on sarcomere contraction

Notice how the A band is the region spanning the total length of the thick filaments. This means, when the sarcomere contracts, the A band contains more actin, making it appear darker, but the length of the A band hasn't changed.

What upregulates osteoblasts?

Osteoblasts are upregulated by mechanical stress. Mechanical stress includes any *weight-bearing exercise*.

How do osteoblasts make new bone?

Osteoblasts make new bone by laying down *type 1 collagen* and *hydroxyapatite*, which is calcium and phosphate.

With decreased osteoprotegerin levels, what would happen to the level of free RANKL?

Osteoblasts secrete both, rank ligand and OPG. OPG normally binds to RANKL, preventing it from interacting with osteoclasts. Our patient has decreased levels of OPG, which means that there will be an increase in the level of free RANKL, because less RANKL will be bound by OPG, and this answers our question. Low OPG means high free RANKL.

What are osteoclasts derived from? What do osteoclasts do?

Osteoclasts are derived from macrophages or monocyte precursors in the bone marrow. The job of osteoclasts is to resorb bone.

What inhibits osteoclasts?

Osteoclasts are inhibited by: 1) osteoprotegerin (OPG) 2) estrogen

What stimulates osteoclasts?

Osteoclasts are stimulated by: 1) macrophage colony stimulating factor (M-CSF) 2) RANK Ligand (RANKL) 3) interleukin 1 (IL-1) 4) PTH.

where do OPG and RANKL come from?

Osteoprotegerin and RANKL, both come from the same source: osteoblasts. It's almost as if osteoblasts secrete the poison and the antidote at the same time.

How does PTH play a role in osteoblast/osteoclast function?

PTH acts on osteoblasts to increase RANKL release --> increased osteoclast maturation --> more bone reabsorption

What is the periosteum?

Periosteum = the outside of a bone.

Sacromere anatomy + how does it cause muscle contraction

Sarcomere is comprised of thick filaments and thin filaments. The thick filaments are comprised of the protein myosin. The thin filaments are comprised of the protein actin. Myosin heads bind to actin, causing shortening of the sarcomere, which is ultimately responsible for muscle contraction. There are different bands of the sarcomere: A band: I use the letter A in A band to help remind me that it is the dark band, because the word dark has the letter A in it. The A band is dark because it *contains overlapping myosin and actin*. (At the top of the pic is a recreated image of an electron microscopy and the A region is dark, whereas the I region is light.) I Band: I use the letter I to help remind me that the I band is the light band because the word light has the letter I in it. The i band is light because it *only contains the thin filament actin*. When the sarcomere contracts, the I band becomes shorter because the myosin heads on both sides of the I band, pull the I band towards the center of the myosin. As a result the actin smoothly slides, bringing the thick filaments closer to each other, which ultimately results in shortening of the I band. Notice how the A band is the region spanning the total length of the thick filaments. This means, when the sarcomere contracts, the A band contains more actin, making it appear darker, but the length of the A band hasn't changed.

Explain the steps of what happens in the secondary endochondral ossification center to make bone

Secondary ossification centers form after birth at epiphyses → formation of spongy bone filled with hematopoietic stem cells → continued release of osteoclasts, erythrocytes and leukocytes ----- The secondary ossification centers form after birth at the epiphysis. And as with the primary ossification center in the diaphysis, the epiphysis will create spongy bone and this will be filled with hematopoietic stem cells. This will allow for the continued release of osteoclasts, as well as urethra sites and leukocytes and remember up to this point, osteoclast came from the other hematopoietic centers, like the yolk sac, the liver and the spleen. But now we're actually forming the bone marrow. Because now the bone actually has these hematopoietic stem cells.

How are slow twitch fibers about to be involved in long lasting sustained force?

Slow twitch muscle fibers are able to achieve this because they are highly concentrated in *mitochondria* and *myoglobin*.

Slow twitch (type I) muscle fibers: What type of movement are they primarily involved in?

Slow twitch muscle fibers are primarily involved in *long lasting sustained force*.

Parts of the bone in endochondral ossification + what each part produces what part elongates and how?

So as we mentioned before up to this point, we have the medullary cavity (MC), and then we have spongy bone, on the ends and it's filled with hematopoietic stem cells. And now we have the epiphysis on the end of course the diathesis, in the center, and finally we have the metaphysis. And the metaphysis can also be called the growth plate or the epiphyseal plate. And recall that the metaphysis still has hyaline cartilage. And the chondrocytes here will create new cartilage. And the osteoblasts right here, on the diaphysis side will ossify cartilage. So this cartilage will then become bone. So now with the chondrocytes creating new cartilage, and the osteoblasts ossifying old cartilage, the metaphysis will remain the same size. However, the diaphysis will elongate.

Explain the steps of what happens in the primary endochondral ossification center to make bone

So here is a bone model and it's made of hyaline cartilage, which I will color it in purple. It is important to recognize that cartilage is made by chondrocytes and this model is full of chondrocytes, which I will draw with these little green circles. The outermost portion contains connective tissue, forming what is called a perichondrium and peri simply means around and chondrium refers to the cartilage. To keep it simple, all you need to know is that the perichondrium contains mesenchymal stem cells. These stem cells, within the perichondrium, can differentiate into osteoblasts. Osteoblasts will begin to secrete hydroxyapatite and type 1 collagen, forming bone. So bone will begin to be laid down in the blue area. And this creates the bony collar and as discussed on the previous slide, hyaline cartilage is avascular. So it received its nutrition via diffusion, but now with this bony collar, nutrients cannot reach the chondrocytes within. This causes death to the chondrocytes. So we'll signify their death with these red X's. And as we said before, once they die, they will leave behind calcium and porous holes. So the next step in endochondral ossification is to understand that a periosteal bud will penetrate the bone collar and this will actually provide blood supply to the inner portion of the cartilage model. So before the hyaline cartilage was avascular and now it receives blood. And what will happen as this periosteal bud penetrates the bone collar is, it will carry with it osteoblasts from the periosteum and it will take it within the cartilage. And so what will happen is the osteoblasts will begin to lay down spongy bone and this will replace the cartilage in the diaphysis, that portion of the bone, which has the bony collar. Now these osteoblasts laying down spongy bone in the diaphysis, is called the primary ossification center. So up to this point, we have a bony collar and then chondrocytes, which died as a result of the bony collar and not getting nutrients and they left behind porous holes. And now there is a signal, due to the death of the chondrocytes for a periosteal bud, to penetrate this bony collar. And this will bring blood, hence the red. And notice that I said periosteum instead of perichondrium, because this is now periosteum. And that is because this portion of the mesenchymal connective tissue is now adjacent to bone, not to cartilage (osteum = bone). This periosteal bud will bring with it osteoblasts, from this mesenchymal (outside) layer. And of course they will begin to secrete type 1 collagen, and hydroxyapatite. And so here, the osteoblasts are creating spongy bone. After the periosteal bud bring in the osteoblasts to create spongy bone, osteoclasts can then travel through the blood, from wherever hematopoiesis is taking place in the fetus. Hematopoiesis in utero occurs in the yolk sac, the liver and the spleen. As an adult, it occurs in the bone marrow. The point is that these osteoclasts will enter the periosteal bud and infiltrate where this spongy bone is. And these osteoclasts will begin to degrade the inner portion of the bone, creating a medullary cavity. The next step in endochondral ossification is that the osteoblasts in the periosteum will continue to lay down bone and this will create compact bone beneath the periosteum. So now we've discussed the primary ossification center.

Full Pathway of GTOs + End Result

So in summary, when a muscle contracts excessively, the tension that is generated is transmitted to the tendon, which causes activation of the GTO. The GTO sends sensory information to the spinal cord via type 1b sensory axons. These then synapse on and activate the inhibitory interneuron, which ultimately inhibits the alpha motor neuron. The net effect of activation of the GTO is an involuntary inhibition of muscle contraction. So it causes forced muscle relaxation.

Osteoblasts in an 8 year old male secrete type 1 collagen and hydroxypatite as normal. However, the collagen is defective. What may occur in the bones as a result of the defective type 1 collagen?

So the mineralization in this patient is normal, which is the hydroxyapatite. Therefore, the patient would have very hard bones. However the type 1 collagen forming the matrix throughout the bone would be very weak. And therefore, the bones would be very inflexible. In other words, the bones would be very brittle. Brittle bones would be very prone to fracture. So in fact, many patients with defective type 1 collagen, break their bones very frequently to the point that some clinicians are suspicious of child abuse. This disease is known as osteogenesis imperfecta, OI.

A new experimental drug is known to inhibit voltage-sensitive calcium channels in the alpha motor neuron. How will this drug likely alter skeletal muscle activity?

So the question is referring to these channels right here. Remember, when this calcium channel opens, calcium enters the cell and induces acetylcholine release. When acetylcholine enters the synaptic cleft, it binds to acetylcholine receptors in the muscle, right here. This is ultimately responsible for causing muscle contraction. Because this new experimental drug blocks these calcium channels, we can deduce that less acetylcholine will be released, and the skeletal muscle will not be stimulated as much. So, how will this drug likely alter skeletal muscle activity? It will decrease skeletal muscle activity.

secondary endochondral ossification center detailed explanation

So to summarize up to this point, we have a periosteum and it covers the whole bone, because at this point the osteoblasts have had a chance to totally create compact bone. And outside the compact bone we have periosteum and now we have the medullary cavity, I'll just write MC and then at the ends we have the secondary ossification centers. And these will also have a periosteal bud. So now blood will reach the ends of the bone, bringing osteoblasts, I will just write blast for short, and they will also bring hematopoietic stem cells. And I will just write these in green. And of course, this will occur in both epiphysis, or both periosteum buds. And as before, the osteoblasts will create spongy bone and unlike with the primary ossification center, in which the osteoclasts entered and removed the inner portion, and removed the spongy bone, creating the medullary cavity, these secondary ossification centers will not lose their spongy bone. In fact, the spongy bone will remain and all of the osteoclast precursors and all of the hematopoietic stem cells, will take up residence here. This spongy bone, filled with all these hematopoietic stem cells, in green, is now called the bone marrow. And of course, these stem cells will include osteoclasts. So now, we have a mature bone. We have the diaphysis here (middle of bone), and if we erase these labels we can show the epiphysis (ends of bone) on either end.

What are T-tubules? What about the distribution of T-tubules helps it in its function?

T-tubules are *deep invaginations of the muscle cell membrane* or sarcolemma, that *extend into the muscle fibers*. In the pic, notice how the muscle muscle branches into smaller and smaller components, until eventually we see the muscle fiber right here. The muscle fiber is comprised of many myofibrils. From the image, we can see this blue stuff that wraps all around the muscle cells and invaginates within, like this. So these are the T-tubules. This is important because the *expansive distribution* of T-tubules *allows* the *depolarizing signal to reach all of the myofibrils* at the *same time*, which means the muscle fiber can *contract uniformly*.

Where is the GTO located?

The GTO is located between the muscle-tendon junction. (The tendon and the muscle)

H Band

The H band is the region of the sarcomere that only contains thick filaments.

What region of the sarcomere only contains thin filaments?

The I band is the only region of the sarcomere that contains thin filaments. Like I mentioned earlier, it's important to remember each component of the sarcomere. For step one, it's most high yield to know how the different regions of the sarcomere are altered when the muscle contracts and relaxes.

The _____ to _____ ratio determines the activity of osteoclasts.

The OPG to RANKL ratio determines the activity of osteoclasts.

how does an abundance of mitochondria help slow twitch fibers?

The abundance of mitochondria allows for the *continuous production and supply of ATP*

Pathway of neurotransmission from the corticospinal tract to muscle contraction (just the steps no extra fluff)

corticospinal tract axons syanpse @ alpha motor neuron in ventral horn of spinal cord --> axons of corticospinal tract release Ach --> Ach binds to Ach receptors on postsynaptic cleft of alpha motor neuron --> protein channel opens and Na+ enters cell while K+ leaves cell --> Na+ entering cell leads to depolarization --> depolarization reaches the axon hillock --> voltage-sensitive Na+ channels open --> more Na+ enters cells --> more depolarization --> voltage-sensitive Ca2+ channels open --> Ca2+ enters cell --> increase in intracellular Ca2+ allows NT (Ach) to be packaged into vesicles and released. Ach binds to Ach receptor on motor end plate on skeletal muscle --> ligand-gated channel opens --> Na+ influx and K+ efflux --> depolarization --> change in voltage. change in voltage is sensed by voltage sensitive Na+ channels in skeletal muscle --> more Na+ enters cells --> cell depolarizes more --> L-type Ca2+ channels, found in invaginations called T-tubules, open --> Ca2+ enters cell --> conformational change in L-type Ca2+ channel --> mechanical interaction b/w L-type Ca2+ channel w/ ryanodine receptor on SR --> Ca2+ within SR is released --> increased cytosolic Ca2+ --> Ca2+ binds to troponin C --> tropomyosin moves away from actin --> myosin combines w/ actin --> muscle contraction Serca (a Ca2+ ATPase pump in the SR) actively pumps intracellular Ca2+ into SR --> intracellular Ca2+ are low --> muscle relaxation

Where do osteoblasts come from?

mesenchymal stem cells in the periosteum (not the bone marrow)

thick filaments are composed of ______

myosin

What do osteoblasts express, that can be detected in the serum, that aids them in forming bone?

osteoblasts express this BAP, aka bone specific alkaline phosphatase, which aids them in forming bone. Alk phos can be found in other organs such as the liver. So to distinguish clinically, you should know that osteoblasts secrete a bone specific ALP, aka BAP.

How can succinylcholine/inhaled anesthetics cause malignant hyperthermia?

succinylcholine/inhaled anesthetics administered --> excessive release of Ca2+ from SR into cytosol --> excess intracellular Ca2+ binds to troponin C --> severe muscle contractions