week three

What is the bone organization?

A bone is an organ, containing tissue types, which contain cells. Bone > osteon > lamella (circular bands of tissue) with haversion canal > collagen fiber > collagen fibril > collagen molecules. Bones are 33% protein, mostly collagen. They are 67% inorganic; calcium, phosphates. The bone serves as a storehouse; 99% of body's calcium is in the bones.

What is a chemical synapse?

A chemical synapse is the functional junction where an impulse is transmitted from one neuron to another without the neurons actually touching, or to a target organ. The chemical NT diffuses across the synaptic cleft (in synaptic vesicles) toward the dendrites of other neurons.

What is a synaptic (graded) potential?

A graded potential is a local electrical signal created through local depolarization in a neuron. It occurs at the dendrites, and is what happens when the cell gets a chemical signal from a neuron.

Compare an action potential's speed and efficiency on a non-myelinated vs myelinated axon. Compare the distribution of voltage-gated channels on myelinated vs non-myelinated axons.

A myelinated axon allows a signal to travel more quickly because the nodes of Ranvier in between the myelin have ion channels that allow the signal to refresh itself. When the signal reaches these channels, it jumps between the nodes. However, unmyelinated actions are slower and less efficient because they have these voltage gated channels along the entire axon instead of just at the nodes.

What is a neuron?

A neuron is a specialized cell that transmits messages from one part of the body to another. It lacks centrioles, so it can't divide. When neurons are dead, they're dead. A nuclei is a cluster of neurons.

What are the three types of molecules in a nucleotide?

A nucleotide has a sugar backbone, phosphate group, and nitrogen base pairs. The base pairs have five varieties: G, C, A, T, and U.

Why is a resting membrane said to be"polarized"? What is a typical resting membrane potential?

A resting membrane is polarized because it has more positive neurons sitting on its outside surface than inside surface. A typical resting membrane potential is -70 mV. It is the leaky K+ channels that create this charge, as there end up being more Na+ ions outside than K+ ions inside, making the intracellular space negative and extracellular space positive. The neuron will remain inactive as long as this continues to be true.

Why are action potentials "non-decremental" with distance?

Action potentials are the same size/amplitude as they travel throughout the axon, and they refresh themselves along the way. They go quickly through myelinated spaces, then jump past nodes as Na+ rushes out of local channels there. Meanwhile, the ball keeps channels from reopening in the refractory period, so the AP can't travel backwards and only has net movement in one direction. The AP is an all or none response. It cannot travel partway and die along the axon, like a graded potential might. It is either conducted along the entire axon, or it doesn't happen at all.

What is the relationship between Alzheimer's and protein misfolding?

Alzheimer's is associated with amyloid plaques, which develop when proteins misfold and aggregate together. The plaques (along with tau protein) block the microtubule highways through which NTs travel from the cell body to the axon terminals. This messes with the cell's ability to transmit NT, meaning the signal can't continue to the next cell. This is the thinking behind a lot of neural diseases, including Parkinson's. When folding goes wrong, it generally is picked up by the lysosome & proteinase. However, when the lysosome is dysfunctional due to a cause like hyperglycemia (building up free radicals), this cannot occur and misfolded proteins accumulate.

What is an osteoblast?

An osteoblast is a cell that secretes bone. They lay down bone around themselves. When the calcium level in the blood is too high, Calcitonin stimulates osteoblasts to deposit calcium in the bone.

What is an osteoclast?

An osteoclast is a cell that removes bone. When the calcium level in the blood is too low, PTH stimulates osteoclasts to break down bone matrix and release calcium ions into the blood.

What is a ligand-gated channel?

Another molecule binds with a protein channel to open it and allow something (like Calcium) to pass through the membrane of the cell, moving high to low. Not using ATP, just facilitated diffusion. Neurotransmitters open these after crossing synapse. The ligand can be many types of things, but not the thing moving through the channels.

How do the PNS glial cells called Schwann cells myelinate axons?

Axons in the PNS are myelinated by Schwann cells. Many of these cells wrap themselves around the axon in a jelly-roll fashion, meaning there are many phospholipid layers to support the axon. Initially, the membrane coil is loose, but Schwann cell cytoplasm gradually is squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes called the myelin sheath encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. Because the sheath is formed by many individual Schwann cells, it has gaps/indentations called nodes of Ranvier at regular intervals. Unmyelinated axons have a little bit of protection from Schwann cells, but not quite as much as insulated ones. It is more like axons are just kind of pushed into the Schwann cell together. one Schwann > many axons vs many Schwanns > one axon

What were the results of the astronaut bone density study?

Bone volume in astronauts dropped significantly in comparison to ground control. Gravity does not stress the bones by weighing them down. Beyond a lack of gravity, a severely sedentary lifestyle can also remove bone volume. In a graph like this, the star signals significance, and the standard error bars don't overlap.

What are the functions of bone?

Bones are responsible for support, protection, muscle signaling, blood production, and storage (fat, calcium, phosphorus).

How do bones grow?

Bones grow by mineralizing a cartilage scaffold. In children, osteoblast activity outnumbers osteoclast activity, as children are growing a lot of bones. This process utilizes stem cells. When differentiating, a new stem cell stays with the population, but the other newly differentiated cell becomes one of the types needed to create bone.

How do bones remodel?

Bones remodel by removing bone, then adding bone. Osteoclasts dissolve, osteoblasts add, and so on. This means that the bones in the body have different ages.

How do bones repair?

Bones repair by adding bone, then removing bone. When they first break, a hematoma forms. This causes the bone to build up slightly bigger than before at first. When it is healing, osteoclasts remove the extra bone.

What creates this blood brain barrier?

Capillaries in the body are generally very leaky, but brain capillaries have very tight junctions. These are cell membranes basically stuck together. Further, glial/support cells called astrocytes place their long "feet" up against the capillaries, holding them together even more. The phospholipid bilayer makes it so that oxygen and lipid soluble molecules are allowed through. Glucose and some essential amino acids can come through with help.

What are the benefits of cell division?

Cells must be replaced a lot. Cell division produces more cells for growth and repair processes.

What is the central dogma?

DNA > RNA > proteins. Transcription is the synthesis of an RNA copy of DNA. Translation is the synthesis of a protein from the information on the RNA copy. mRNA complementary bases match DNA, then move to the ribosome where mRNA bases are matched with amino acids.

How are nucleotides arranged within DNA and RNA?

DNA is a double helix nucleotide chain with covalent bonds in the backbone and hydrogen bonds between nitrogen base pairs. Binding between the bases is very specific: G to C, and T to A. RNA is only a single nucleotide strand. Its bases are G, C, A, and U.

Why is DNA not lipid soluble?

DNA is not lipid soluble due to its sugar phosphate backbone. For something to be lipid soluble, it must be non polar. However, DNA is hydrophilic and would be soluble in polar solutions instead, like water.

Explain the advantages of a "transcription" step (DNA to RNA) before "translation" (RNA to Protein).

DNA is rather like a coded message by itself. Its information is not useful to the body unless it is decoded. Also, most ribosomes are in the cytoplasm, not the nucleus, and most DNA never leaves the nucleus during interphase. DNA therefore requires not only a decoder, but also a trusted messenger to carry the instructions for building proteins to the ribosomes. Further, with this process, cells are constantly making proteins, billions at a time. If it only used DNA without a copy, it could only make one product. But with many copies, many proteins can be made.

Why doesn't DNA leave the nucleus?

DNA must stay protected within the nucleus. There are other organelles within the cytoplasm. If DNA moved there, it could be exposed to some dangerous stuff, like free radicals.

Describe the structure & function of dendrites, axons, synaptic terminals.

Dendrites are neuron processes that convey incoming messages toward the cell body. Depending on the neuron type, there may be hundreds of branching dendrites. Axons are neuron processes that generate nerve impulses and conduct them away from the cell body. There is only one axon per neuron, which arises from a cone-like region of the body called the axon hillock. Axon terminals contain hundreds of tiny vesicles that contain chemicals called neurotransmitters. When impulses reach axon terminals, they stimulate the release of NTs into the extracellular space between neurons, or between a neuron and its target cell.

Describe three factors that can increase the permeability of the BBB.

Factors that raise BBB permeability include diabetes mellitus (high glucose), hypertension (high BP), hypertonic fluids, and chronic inflammatory pain.

How does the Golgi apparatus contribute to protein synthesis?

Folding proteins are transported here from the rough ER. In the Golgi, there is some secondary processing that occurs. Enzymes will cut pieces off of proteins to change their shape and the way they fold until it finally leads to a functional protein. More processing, modifying, snipping off parts.

What role do sex hormones & growth hormones have on bone density?

Growth hormone is a general metabolic hormone. However, its major effects are directed to the growth of the skeletal muscles and long bones of the body. Estrogen helps women maintain bone density by suppressing osteoclast activity. Because of this, menopause leads the level of osteoclast activity to be raised.

What are risk factors for osteoporosis amongst women?

Having a very low bone mass predisposes women to earlier osteoporosis. Exercise helps with this. Early menopause also predisposes women to early osteoporosis.

What would happen if a resting cell became less permeable to potassium?

If a resting cell became less permeable to potassium, meaning its leakage channels closed, it would depolarize. This is because not only is K+ trapped inside the cell, but Na+ ions rush in when their v-gated channels open, so over time the cell becomes even more positive and depolarized.

What can happen if misfolded proteins build up within cells?

If the proteasome and lysosome are unable to catch misfolded proteins, aggregates can build up and lead to certain diseases, such as Alzheimer's, Parkinson's, Huntington's, and ALS. Lysosome dysfunction is also associated with hyperglycemia and diabetes.

Describe the conditions on a resting neuron's membrane: distribution of (Na+, K+, Cl- & proteins-) inside & outside of the neuron, and charge distribution across the membrane.

In a resting neuron, there is a high concentration of Na+ outside the cell and low inside. There is also a high concentration of K+ inside and low outside. This is always going on, and contributes to the resting membrane potential. Also, negative proteins (A-) are highly concentrated inside the cell, and Cl- outside. However, K+ leakage channels allow K+ ions to leak out at very slow rates regularly through passive transport, as K+ is going from high to low concentration. Since ions have charge, this affects the charge of the membrane. As K+ ions move out, the intracellular space gets more negative, and the extracellular space gets more positive.

What are the two phases of the cell cycle?`

Interphase (the long phase) and cell division.

What happens to the Ca2+ you eat if your vitamin D levels are very low?

Low Vitamin D levels tend to lead to the body not being able to take in enough calcium, thus leading to fragile bones. This can cause diseases, like rickets. Rickets is a disease of children in which the bones fail to calcify. As a result, the bones soften, and the weight-bearing bones of the legs become bowed.

How does our behavior & environment alter chromatin without altering the DNA?

Methylation. Histones can be wide apart or close together. In their relaxed state, there are less methyl groups and more acetyl groups, making the DNA easier to read. In their packed state, there is more methylation and less acetyl groups, and DNA is inaccessible. This is affected by the body and its our environment, such as through changes in blood sugar, temperature, dehydration.

What is myelin?

Myelin is fatty white insulation for axons.

What are neurotransmitters and where are they made?

NTs are the chemicals that are transmitted between neurons. Some NTs are proteins, ions, etc. They are housed in vesicles when in the axon terminals. NT molecules are made in the cell body.

What is a nerve?

Nerves carry the axons of many neurons. They are wound together and all traveling in the same general direction.

Why are nerves & neurons surrounded by connective tissues in the PNS?

Neurons are wound together into a fascicle, which is next to even more fascicles in a nerve surrounded by connective tissue. There is an artery and vain in the nerve to supply oxygen so that nerves can functionally do their jobs.

Explain why, in general, neurons send signals in 1 direction only.

Neurons only have receptors in one place and NT storage vesicles moving one way. Axons can only move signals away from the cell body, and dendrites can only convey chemical messages to the cell body.

How does an osteoclast remove bone? How does its shape enhance its function?

Osteoclasts move across the surface of bone, eroding it away. They get very close to bone, then transport hydrogen ions into the extracellular space surrounding the bone surface. This is basically them concentrating an acidic environment against bone. Because osteoclasts move hydrogen ions against their concentration gradient, this occurs through active transport. The ruffled membrane of osteoclasts increases their SA, allowing more transport across the membrane through more transporters. Thus, this leads to more hydrogen ions released, and bone is degraded more rapidly.

How are embryonic stem cells different from adult?

Pluripotent stem cells, found in embryos, can turn into any kind of cell. However, multipotent stem cells, found in adults, can only turn into the same or a similar type of cell (for ex, another type of blood cell). These are the type of stem cells found in adult bodies.

What 2 cellular structures are designed to break down misfolded proteins?

Protein misfolding happens a lot. However, the proteasome and lysosome work to destroy misfolded proteins.

Summarize the major events in protein synthesis: transcription & translation.

Protein synthesis involves two major phases: Transcription is when complementary mRNA is made using the information in the DNA gene with the help of an enzyme. Translation is when the information carried in mRNA molecules is decoded and translated from nucleic acids into proteins by tRNA. The acids are joined together in peptide bonds.

What does repair and growth of cells require?

Protein synthesis.

What is the structure of proteins?

Proteins are made up of amino acids. The primary structure uses covalent bonds, the secondary uses hydrogen bonds, and the tertiary and quaternary structures use weaker hydrogen and hydrophobic bonds.

What happens if a protein is folded incorrectly? Can the protein still function?

Proteins cannot function if they are folded incorrectly (or broken down like through denaturation) because their functions depend on their specific three-dimensional shapes.

Where do proteins go after synthesis?

Proteins may go inside the cell (back to nucleus for instance), close by the cell, or far away in the body.

What are the final destinations of proteins processed by the rough ER and Golgi apparatus?

Proteins that travel just through the rough ER end up in other areas of the cell. However, proteins that travel through the Golgi end up outside the cell, such as with insulin.

What is the ribosome?

Ribosomes are tiny, bilobed dark bodies made of proteins and one variety of RNA called ribosomal RNA. Ribosomes are the actual sites of protein synthesis in the cell. Ribosomes that float freely in the cytoplasm manufacture proteins that function inside the cell, while others attach to membranes like the rough ER, which produces proteins that function outside the cell.

What are sense organs?

Sense organs include the skin, nose, eyes, and anything else that senses the outside world.

What types of neurons can be present in a nerve?

Sensory neurons are neurons which carry impulses from sensory receptors to the CNS. They keep us informed about what is happening both inside and outside the body. Motor neurons carry impulses from the CNS to the viscera and/or muscles and glands. Interneurons connect the sensory and motor neurons in neural pathways.

What role do bones play in the body's ability to move?

Skeletal muscles, which are attached to bones by tendons, use the bones as levers to move the body and its parts. Any time you use your muscles, it puts stress on bones, which causes them to grow. Bones get micro cracks, and then repair builds them up further and makes them stronger than they were before.

What is the role of Na+-K+ pumps in the membrane of cells?

Sodium potassium pumps utilize around 40% of the body's energy. They are used to bring Na+ ions out and K+ ions in after the normal ion concentration inside and outside the cell has been disturbed. They are used after repolarization and are absolutely necessary in the action potential process, as they use active transport to move ions from a low gradient to a high gradient.

Where can you find stem cells in an adult?

Stem cells are found in the body in places of high cell turnover, like in the stomach, where the very low acidic pH can damage epithelial cells.

How are stem cells different from fully differentiated cells?

Stem cells divide to produce one stem cell, replacing itself in the stem cell population pool, and one differentiated cell that is specialized to do its specific job. Self renewal maintains the stem cell pool. Differentiation replaces dead or damaged cells through a person's lifetime. Multipotent stem cells are used to do this in adults.

How does the body get Vitamin D?

Sunlight contributes to the conversion of cholesterol to vitamin D. It can also be found in foods, such as milk and bread. The skin and liver are both involved in making active vitamin D, which then acts on the kidney and small intestine by stimulating the active transport of dietary calcium ions across cell membranes in those organs.

What are spatial and temporal summation?

Temporal summation is when a signal might not reach threshold by itself, but if sent multiple times, it will reach threshold and trigger an AP. Spatial summation is when the signal can't reach threshold alone, and requires a group of signals from multiple sources to work together to produce an AP.

What does the blood-brain-barrier (BBB) do?

The BBB protects the brain from damage and does not let blood borne substances like ions through, which would normally affect things like osmolarity. The brain is very dependent on a consistent internal environment; without it, uncontrolled neural activity might result. Signaling is reliant on ions; if more ions come in, they will affect these signals.

What is the CNS and what is the PNS?

The CNS refers to the brain and spinal cord. It interprets oncoming sensory information and sends out instructions accordingly. The PNS refers to everything else. It carries impulses to and from the CNS. Essentially: the PNS transmits sensory signals to the CNS, which sends motor signals to other parts of the body.

How is a graded potential different from an action potential?

The GP is not as strong as an action potential, and will die out with long distances/only travel partway down an axon. It is what occurs when the incoming signal is not powerful or large enough, and an AP is not triggered.

Why are graded potentials important?

The GP is what activates the neuron to initiate and transmit the action potential if the stimulus is strong enough and there is a big enough influx of sodium ions into the cell. It is when the decision is made as to whether or not there should be an AP.

What occurs in mitosis?

The cell divides/reproduces itself. Essentially division of the nucleus into two daughter nuclei.

What occurs in the interphase?

The cell grows and carries on with its usual metabolic activities. This is the longer phase of the cell cycle, during which the cell is very active and preparing for division.

What are the functions of microtubules in a neuron?

The chemical NT diffuses across the synaptic cleft (in synaptic vesicles) toward the dendrites of other neurons. The vesicles move through the cell body to the terminals through microtubule pathways. They are then released through active transport & exocytosis.

How are neurotransmitters broken down or removed from their receptor sites on a dendrite (ie. how does the signal stop?)

The electrical changes prompted by NT binding are very brief because the NT is quickly removed from the synaptic cleft, either by diffusing away, reuptake into the axon terminal, or by enzymatic breakdown. This limits the effect of each nerve impulse.

What are the result of the gymnastics study relating to bone density?

The gymnastics group has more bone content than the control group, and the men have more bone content than women. This is likely because their arm bones are under more stress/impact and thus grow stronger. Kids who play soccer would probably have slightly less strong bones because they are not doing weight bearing exercise with their arms; however, activity in general still contributes to bone strength.

What happens to the leftover bits of proteins that are modified by the rough ER and Golgi?

The lysosome picks up leftover bits and breaks them back down into amino acids to recycle them for later proteins.

What is the motor division of the PNS?

The motor division carries motor signals from the CNS to effector muscles and glands.

Describe the events in the in an action potential (what happens in the depolarization vs repolarization phases).

The neuron starts out at rest. Then, something happens somewhere in the cell that starts to allow positive ions to come in. When enough have come in for the signal to reach threshold/cause enough charge change/depolarization, the voltage gated channels are triggered to open. Then, Na+ rushes in, creating huge amount of positive charge on the inside of the membrane until the ball blocks the channel. However, the K+ channel is still open, and K+ ions are going outside the cell. As positive charge starts to leave, inside of cell becomes more negative again, creating repolarization. There is always an undershoot in which the inside becomes a little more negative before returning to original gradient, and the Na+-K+ pump is responsible for returning resting membrane potential. The Na+ channel can open and close quickly whereas the K+ channel has a huge structure that must move through the membrane and opens more slowly. This means we can get a huge depolarization quickly, while K+ more slowly moves the cell back to its resting state.

Where is the nucleus of a neuron found?

The nucleus of a neuron is found in the cell body.

Describe 1 role of histone proteins associated with DNA.

The packing or relaxing of histone proteins affects the way that DNA is read.

How does the rough ER contribute to protein synthesis?

The rough ER is studded with ribosomes. Some proteins require a membrane bound specialized environment to fold, and can't fold just in the cytoplasm. The rough ER is also highly folded itself, giving it a high rate of production of protein strands. The amino acids begin to fold here, then are transported over to the Golgi. Minimal processing, folding.

What kind of signal is transmitted down an axon?

The signal traveling down an axon is electrical, but the neurotransmitters that cross the synapse are chemical.

What are the somatic and autonomic divisions of the motor division of the PNS?

The somatic division concerns voluntary movement, and the autonomic division concerns involuntary movement.

What are the sympathetic and parasympathetic divisions of the autonomic division of the motor division of the PNS?

The sympathetic division concerns the fight or flight response, and the parasympathetic division concerns rest.

Why is a reflex a negative feedback loop?

This is a negative feedback loop because something happens quickly in response to a stimulus, but that response generally doesn't increase the stimulus again by itself. Instead, the body goes back into balance.

What is a mechanically-gated channel?

This is when something causes the shape of the membrane to change, pulling the channel open. Pressure is applied, then pressure is removed and the gate closes. The pressure could be high BP in the capillary, touch to the skin, hair cells in the ear bending, etc.

What is hyperpolarization?

This is when something negative enters that takes the cell further away from threshold by making the inside of the membrane more negative. This means that the next time a signal comes, it will be harder to reach threshold and become depolarized because more positive ions must come into the cell. In other words, it causes inhibition. An example is when GABA causes Cl- ions to enter a cell.

What is chromatin?

When a cell is not dividing, its DNA is carefully wound around proteins called histones, winding them together to make them more compact. This forms a loose network of "beads on a string" called chromatin that is scattered throughout the nucleus.

How does the homeostasis of bone regulation/calcium work?

This process involves the release of Calcitonin at high levels of calcium in the blood, and the release of PTH at low levels of calcium in the blood. The release of Calcitonin stimulates bones to take in calcium (through the help of osteoblasts), and kidneys to urinate more calcium out. The release of PTH stimulates bones to release calcium (through the help of osteoclasts), kidneys to urinate less calcium out, and the intestines to absorb more calcium (through the consumption of vitamin D).

What is a voltage-gated channel?

This type of channel opens when a depolarization event occurs, meaning the charges on the membrane reverse, and the inside of the cell becomes positive while the outside becomes relatively negative. This is because there is a voltage sensing (positively charged) section of the channel, which is attracted to the negatively charged part of the membrane. At rest, this means the positive sections of the channel are attracted to the negative inside of the membrane, which keeps the channel closed.

Which event in protein synthesis occurs within the nucleus & which event occurs in the cytoplasm?

Transcription occurs in the nucleus and translation occurs in the cytoplasm.

How is DNA copied?

Unzipping the hydrogen bonds between nitrogen base pairs exposes the information for copying by mRNA. However, this process does not break the covalent bonds of the backbone. After the process is done, the hydrogen bonds can be put back together again.

What is Vitamin D?

Vitamin D is a fat-soluble vitamin produced in the skin on exposure to UV radiation. It is necessary for normal bone growth and function.

Did Vitamin D + Ca2+ supplements in older women affect bone density?

Vitamin D supplements lead to no change in the risk of total fracture incidence in women of avg. age over 59 years. The graph point is not far enough from the 1.0 line, which would indicate no association between the factor and risk. This is because the supplements are giving the bones the opportunity for calcium, but not actually producing change because there is no exercise. Without exercise, the bones' demand for calcium is not changing. With added strength training exercise, it is found that there is a significant increase in bone mineral content. Low-bearing weight creates the opportunity for bones to grow.

How does the folding of insulin work?

When insulin is first produced by a ribosome, it has 110 amino acids in its primary structure. After it goes through the rough ER, the signal peptide is cut off, leaving it as 86 amino acids. It then gets in a vesicle and is transported to the Golgi, at which point further packaging and modification occurs. The C peptide is cleaved off, leaving active insulin with 35 amino acids. Essentially, for insulin to be created, it must be made, folded, and processed before it is released. The end product is a functional protein.

What is an osteocyte?

When osteoblasts get trapped in a newly formed bone matrix, they become mature bone cells, osteocytes.

How does the Na+ voltage gated channel work?

When the cell is at rest (negative inside, positive outside) the channel is closed. However, when depolarization is triggered, the charge of the membrane changes, the channel senses that change as the positive section is attracted to the now negative outside part of the membrane, and the channel opens to allow Na+ ions to follow their concentration gradient rush into the cell. At some point, due to random molecular motion, the little ball comes in to block the channel, which causes the refractory period. When the gradient is at rest again, the channel will close.

How does the K+ voltage gated channel work?

When the cell is at rest, the K+ v gated channels are closed because the positive sections are attracted to the negative inside of the membrane. After the charge of the membrane changes, the positive sections become attracted to the negative outside of the membrane, and the channel opens, allowing K+ ions to follow their concentration gradient and diffuse out of the cell.

Why do women tend to have a lower bone density than men?

Women already have different body proportions than men that lead to less bone mass. However, estrogen helps to maintain the health and normal density of a woman's skeleton. Around menopause, bone density drops off a little more in women than men.

Do men have a risk for osteoporosis/decrease in bone density?

Yes. Women's bone density decreases more, but men still suffer a drop off due to aging. Osteoporosis affects half of women over 65 and some 20 percent of men over the age of 70.

Where is RNA made?

mRNA, tRNA, and rRNA are all made in the nucleus. They are taken out of it to help with protein synthesis. mRNA helps with both steps, but tRNA and rRNA work at the ribosome.

Describe these parts of a reflex arc & put them in correct order: efferent/motor neuron, interneuron, stimulus, response, sensory/ afferent neuron, & receptor.

stimulus > receptor (which reacts to the stimulus) > sensory neuron > interneuron > motor neuron > effector (muscle or gland being stimulated) > response Reflexes bypass the brain and go to the spinal cord. Reflexes are like one way streets (once it begins it always goes in the same direction). They occur over neural pathways called reflex arcs and involve both CNS and PNS structures. Somatic reflexes include those that stimulate the skeletal muscles. Autonomic reflexes include those that regulate the activity of the smooth muscles, the heart, and glands.