All posts by drrenhartung

Origin, belly, and insertion

These terms origin, belly, and insertion have to do with both the anatomy of a given muscle and with movement of that same muscle.  I like to think of the brachialis muscle during an arm curl as a good example for these terms.

Origin:  This refers to one attachment point of the muscle (usually to bone).  You can think of the origin of the muscle as its anchor point, when the muscle pulls we think of this end as the end that does not move during the given movement…  During an arm curl the origin of the brachialis muscle is the humerus.

Belly:  This is the thickets portion of the muscle along its length (this term just has to do with anatomy).

Insertion:  This refers to another attachment point of the muscle (again, usually to bone), in this case the muscle is attached to the part of the body that the muscle is moving.  In the brachialis and arm curl example the insertion is on the ulna (specifically the coronoid process and ulnar tuborisity).  As we perform an arm curl it is the unla (and the rest of the lower arm) that is being moved as the brachialis contracts.

I’ll be coming back here to add some images that will helpfully help clarify what these terms mean 🙂

Muscle attachments

Skeletal muscles are attached to bones in two major ways:  Indirect attachment and direct or fleshy attachment.

Indirect attachment:  In this form of attachment there is a tendon between the muscle and the bone being pulled on.  Tendons are considered to be part of the muscle by the way.  Tendons are made of dense regular connective tissue (lots of collagen fibers all running in the same direction kind of like an untwisted rope) and they have lots of tensile strength (the weakest normal tendons can still withstand around 7000 pounds per square inch of force).  The muscle’s other connective tissues (endomysium, perimysium, and epimysium) all converge to form the tendon and the tendon connects strongly to the bone with collagen fibers of the tendon connective to the periosteum of the bone or to the periosteum and directly to the bone itself.  A good example of an indirect connection that you can feel on your arm right now is the attachment of the biceps brachii and brachialis muscles connection to the bones of your forearm, you can feel the muscle in the middle of your upper arm and follow it down to the tendons that insert into your radius and ulna.

Direct attachment or Fleshy attachment:  In this form of muscle attachment there is no visible tendon, the muscle appears to  connect directly to the bone.  Between the muscle and the bone however the connection is basically the same as that of a tendon.  The connective tissues of the muscle come together into a very short span of dense connective tissue that immediately inserts into the periosteum of the bone involved.  The deltoid muscles at your shoulders are attached to your clavicle and scapula via fleshy attachment, and your pectoralis muscles are attached at your sternum also via fleshy attachment.


Connective tissues of muscle and muscle compartments

There are at least three different connective tissues that surround the muscle and connect the muscle cells to each other and to other structures (such as bones), these tissues are important for the structure/shape of the muscle and for conducting the force of contraction to other structures (usually bones).  These connective tissues include endomysium, perimysium, epimysium, tendon, and fascia.

Here is a brief description of each of the connective tissues of muscle….

Endomysium:  This connective tissue surrounds individual muscle fibers (muscle cells).   Endomysium is made mostly of elastic fibers and it contains capillary blood vessels and lymphatic vessels, and also nerves.  Endomysium considered wispy and delicate, but I suspect that this is simply because it is so thin, for it’s size it has to be tough.

Perimysium:  This connective tissue surrounds groups of muscle cells called fascicles.  The arrangement of these fascicles has an effect on the strength and coordination capabilities of each given muscle.

Epimysium:  This connective tissue surrounds the outside of the entire muscle.  Epimysium is made of dense irregular connective tissue, producing a tough outer covering for the muscle.  Epimysium functions to contain the muscle and to separates muscles from each other so that they can contract independently.

Tendon:  All of the above connective tissues come together and connect to the tendon at the end of a muscle (muscles don’t always have tendons by the way).  Tendons are made of dense regular connective tissue (all of the collagen fibers are running in the same direction) giving them very high tensile strength (so they can conduct a lot of pulling force without getting damaged).  The end of a tendon is connected to a bone (connected deeply to the periosteum layer of the bone), and this bone will be pulled on when the muscle contracts.

Fascia:  Dense connective tissue sheets that surround multiple muscles creating muscle compartments.

Fascia and Muscle Compartments:

Groups of skeletal muscles are surrounded by fascia creating muscle compartments, within these compartments are specific muscles and the blood vessels and nerves that service those muscles (for simplicity I tend to think of these fascial compartments like a plastic bag that snugly surrounds the muscles and has holes for the blood vessels and nerves–  the bag does not stretch much, it is a snug compartment) .  Example fascial compartment:  The front of your thigh contains the anterior compartment of the thigh, this compartment is surrounded by a layer of dense connective tissue and it contains the quadriceps group along with a few other muscles (the sartorius and the articularis genus).  This anterior compartment of the thigh is one of three in the thigh.  There are also compartments in the lower legs, and in your arms.  So, why do we care?  Compartment syndrome!!!

Compartment Syndrome:

Acute Compartment syndrome is a very painful and dangerous medical emergency that can occur after trauma to a compartment (I have seen two cases of acute compartment syndrome, one was a vehicle accident where the person’s thigh was hit by his steering wheel and the other was the result of a horse kicking a woman in her thigh).  The trauma to the muscles in the compartment can cause them to swell due to inflammation and may cause bleeding into the compartment.  Again, these are very snug compartments so there is little room for bleeding or swelling and they can quickly become pressurized, if the pressure in the compartment is too high blood flow into the compartment decreases and all of the muscles and nerves in the compartment experience ischemia (low blood flow).  This quickly becomes very painful (a lot more painful that we might expect from the visible injuries), and the person also often experiences parasthesia (pins and needles, similar to what we feel when a limb is returning from “falling asleep”).  If this goes untreated the muscles in the compartment can die from lack of blood flow.  The treatment is to surgically cut open the compartment and relieve the pressure.

Chronic compartment syndrome is not emergent and is caused usually by repeated excessive exercise (most commonly seen in runners).  Basically, whenever we exercise a muscle it swells due to the dilation of blood vessels inside the muscle, add that to the normal swelling due to low grade inflammation that occurs when we push a muscle to its limits and the pressure can become high enough to cause ischemia inside a compartment and a lot of pain.  Usually when this happens the person stops exercising and the pain decreases.  Prevention and treatment are to allow the muscles to completely recover after pushing them to their limits before pushing them to their limits again (if you have really pushed them give them a couple of days to recover before pushing them again).

Muscular System

Some of the muscles of the body… click for larger image

The muscular system includes all muscle tissues of the body.  A few specific organs of the muscular system include the biceps brachii muscles, the pectoralis muscles, and the rectus abdominis muscle.

In terms of the general physiology of muscle…  All muscle tissues have one thing in common, they all contract when stimulated (the cells get shorter and pull on whatever they are attached to); in this way muscle converts chemical energy (ATP) into kinetic energy (movement).  The major job of all muscle is to create movement of various body parts, including movement inside the body.  We use muscle that are attached to bones to move our limbs and to breath, the muscle tissue in our hearts moves our blood, and smooth muscle in the walls of our digestive organs move food along our digestive tracts.  Muscles are also important for helping maintain body temperature as they generate heat when they contract.  Muscle is also important for opening and closing passages within the body and between the body and the outside world (sphincters between organs as well as the mouth and other openings).

Side note on body temperature control: In broad terms body temperature homeostasis is maintained by the interaction of four body systems: The muscular system generates heat, the cardiovascular system moves that heat throughout the body, the integumentary system releases heat as needed and the nervous system adjusts the activity of the other systems (through various negative feedback mechanisms) in order to maintain a relatively constant body temperature.

There are three different muscle tissue types, each with different cellular anatomy and physiology.

Below is a table comparing and contrasting the three forms of muscle tissue…

Type of Muscle TissueSkeletal MuscleCardiac MuscleSmooth Muscle
Type of Muscle TissueSkeletal MuscleCardiac MuscleSmooth Muscle
Cell ShapeLong tubesShort branched tubesFusiform shape (tapering at both ends), thinnest of the three.
Nucleimultiple nuclei outer edges of cellsingle nucleussingle nucleus
Striations?striatedstriatednot striated
Location in bodyMost attached to bonesOnly in heartfound throughout the body, usually wrapped around tubed organs
Voluntary or Involuntary?voluntaryinvoluntaryinvoluntary
Autorhythmic?not autorhythmic, only stimulated by nervous system.autorhythmicsome are autorhythmic
Innervation requirementsmust be innervated. Cells will atrophy and eventually die without innervationCan function without innervation, though innervation helps control heart ratesome require innervation and some do not
All have in commonall three contract when stimulated and use the same basic mechanism to create contraction

More for muscular system:

Muscular System in the lab

Connective tissues of muscle and muscle compartments

Muscle attachments (indirect vs fleshy)

Origin, belly, and insertion

Gross functions of muscle (prime mover, synergists, etc.)

Carpal tunnel syndrome

Muscle innervation and its clinical significance


Muscle injuries

Anatomy of a skeletal muscle cells

Myofilaments and the sarcomere

The neuromuscular junction

Motor units and recruitment

Excitation contraction coupling

Sliding filament theory

Length tension relationship

Twitch, summation, and tetanus

Isometric and isotonic contractions

Muscle metabolism

Classes of muscle fibers

Joints (Articulations)

A joint or articulation is a connection between two bones.  Joints can have a large range of motion (a healthy shoulder joint that lets you swing your arm around) or very immobile (the sutures between the bones of the skull allow no movement).    Joints are classified by both structure and by range of motion.  I’ll go through the structural categories first.

Structural Classification of Joints:

In terms of structural categories there are synostosis (bony) joints, fibrous joints, cartilaginous joints, and synovial joints.

synostosis (bony) joints:  The two bones are connected by bone.  This type of connection between bones can occur normally during development (Example:  in a baby the hips are made up of three separate bones that will fuse together to form a complete hip bone) and as part of normal aging (the skull plates tend to fuse together as we get older), but synostosis can also occur as part of a disorder (sometimes a joint that should be mobile can fuse after injury or due to arthritis)— this type of abnormal synostosis is often called ankylosis.

fibrous joints:  In fibrous joints two bones are joined by dense connective tissue (lots of collagen fibers).  This type of joint allows very little to no movement.  Examples include the joints between the skull plates called sutures and the joints between your teeth and jaw bones (called gomphosis joints).

cartilagenous joints:  The bones in a cartilagenous joint are joined by cartilage.  Examples of cartilagenous joints include the joints between vertebrae in the spinal column and the joint between the pubic part of the hip bones (called symphysis pubis).

synovial joints:  Synovial joints are the most complex joints.  In a synovial joint the bones are connected to each other by dense connective tissues (including a fibrous capsule and ligaments) and there is a synovial cavity between the two bones that is filled with synovial fluid.  The surfaces of the two bones are covered with cartilage (usually hyaline cartilage).

Range of Motion Classification of Joints:

In terms of range of motion the categories of joints consist of synarthrosis joints, amphiarthrosis joints, and synovial joints.

Synarthrosis joints:  Synarthrosis joints allow very little to no movement between the bones involved.  Bony joints are synarthrosis joints along with most fibrous joints and some cartilagenous joints.

Amphiarthrosis joints:  Amphiarthrosis joints allow a little bit of movement.  Most amphiarthrosis joints are cartilagenous joints.

Synovial joints:  Synovial joints allow for the greatest range of motion.  The most mobile of the synovial joints is the shoulder joint.

More on Synovial Joints:

Synovial joints have their own categories based on structure and range of motion.  These different types of synovial joints are:  ball and socket joints, condyloid joints, hinge joints, pivot joints, and plane joints.  Understanding the structure of these joint types allows you to really understand the ways in which each allows movement to happen.

A ball and socket joint consists of one long bone having a ball like head that is inside of a rounded socket in the other bone.  This kind of joint has the largest range of motion.  Two examples of ball and socket joints are the hip joint and the shoulder joint.  The shoulder joint has the greatest range of motion, allowing us to swing our arms around in a large circle.  The hip joint has a large range of motion too, but it needs to be stronger in order to hold the body’s entire weight and the structure needed to give that strength limits the hips range of motion compared to the shoulder.

Hinge joints can only move on one plane (your knee can only bend one way–  if it bends sideways you know there is problem).  Examples of hinge joints include the knee, the joints between the finger bones (but not the base of the fingers), and the elbow joint (you can move your lower arm sideways, but that movement is happening as you rotate your shoulder).

Condyloid joint‘s have an oval shape to the articular surfaces.  This allows a large motion through one plane (like a hinge joint), and a bit of movement through another.  We find condyloid joints connecting the base of our fingers.  The condyloid joint at the base of our fingers allows us to open or close our hands into a fist (large motion possible in that plane) and it allow is to wag our fingers —  move them sideways a bit.

A pivot joint only allows one type of movement, it allows a bone to twist or rotate.  One example of a pivot joint is between the radius and ulna near the elbow.  This particular joint is called the radioulnar joint and it allows the radius to twist alongside the ulna so that we can twist our lower arm/hand–  think of the way your hand twists when using a screwdriver.  There is another pivot joint in your neck, this one is between the first two vertebrae and it allows your head to rotate in a silent “No” movement.

In a saddle joint the two bones meet with surfaces that are kind of like saddles in shape.  This allows movement similar to the condyloid joint but with a wider range of motion.  The saddle joint at the base of the thumb is important for allowing us to have opposable thumbs.

In a plane joint the articular surfaces of the two bones are flat in shape allowing the two bones to slide over one another.  Plane joints have relatively tight joint capsules, so the movement of each individual joint is quite limited.  However, in places like the wrist multiple gliding joints work together to allow for a larger range of motion.


Bone development and growth

Early in development a human embryo’s skeleton develops first as hyaline cartilage and sheets of dense irregular connective tissue, later these get converted into bone.  The hyaline cartilage that makes up much of the skeleton undergoes a process called endochondral ossification.  Intermembranous ossification involves the formation of bone in an area without cartilage, usually an area containing dense irregular connective tissue.  I’ll discuss what happens in a long bone as an example of what occurs during endochondral ossification.

In the hyaline cartilage of an embryonic long bone a primary ossification center develops in the middle of the bone (along the diaphysis), this area is changed from cartilage into bone on the outside as the perichondrium (membrane that surrounds cartilage tissue) transforms into periosteum (outer membrane of a bone) and osteoblasts that develop there start forming bone matrix, this creates a collar of bone around the outside of the diaphysis of the long bone.  At almost the same time, deep inside the same area of the bone, the cells in the cartilage enlarge and start secreting substances that encourage bone formation.  Osteoblasts then start forming bone matrix and as the area of bone tissue increases osteoclast cells start to break down the center area of bone matrix ultimately creating a medullary cavity.

Secondary ossification centers develop in the epiphyses (the ends) of the long bone and turn the inside of the epiphyses into bone.  Once the primary ossification center reaches a secondary ossification center a boarder of hyaline cartilage known as an epiphyseal plate or growth plate forms between the two, these growth plates are responsible for the long bone growing longer.  The growth plates are thin lines of hyaline cartilage that continually add bone matrix away from the epiphysis as the bone grows.  The growth plates will remain active until adulthood is reached and then they will turn completely into bone.  Here is an image of a growth plate under a microscope…

Growth plate 40X magnification. The wavy line running from left to right across the image is the growth plate. It is surrounded by spongy bone.

Appositional growth of a long bone refers to the bone getting wider as it grows, this occurs as osteoblast cells add bone matrix to the outside of the bone (under the periosteum) while osteoclast cells remove bone matrix inside the bone.  This basic process continues until the bone is at its adult width.

It is important to know that although our bones stop growing when we reach adult size they never stop remodeling themselves.  Bone tissue is a dynamic ever changing tissue that is continuously renewing itself and changing based on calcium needs of the body, stresses placed on the bones by muscles and gravity, and other factors.

Intermembranous ossification refers to the formation of bone without cartilage present.  The skull plates, the clavicles, and other bones of the fetal skeleton develop using the process of intermembranous ossification.  The process starts with mesenchymal stem cells dividing and grouping together in string like formations.  The mesenchymal cells then differentiating into osteoblasts and start to secrete bone matrix (collagen proteins and calcium phosphate) which hardens.  Some of the osteoblasts become trapped in the matrix and become osteocytes.  In this way the stringy collections of mesenchymal cells becomes spicules of bone tissue.  These spicules grow and merge with each other to form compact bone on the outer surfaces of the bone (cortical bone) or stay as spicules inside the bone (spongy bone).

Intermembranous ossification is important for the formation of especially flat bones, but it is also important during the healing of a fractured bone.


Axial Bones vs. Appendicular bones

Axial Bones are simply all of the bones that are found in the head, neck, chest, and spine.  In other words all of the skull bones, facial bones, all of the ribs, the sternum, all of the vertebrae, the sacrum, and the coccyx.  Bones that are not axial include those in the arms and legs and bones that connect the arms and legs to the axial bones (scapulas, clavicles, and hips).  In the image below all of the axial bones have been shaded yellow…

Axial bones are in yellow, all other bones here are girdle bones or appendicular bones

Bones that are in the arms and legs are called appendicular bones, while bones that connect the arms and legs are “girdle” bones.  The pectoral girdle consists of four bones that connect the arms to the body (right and left clavicles and scapulas), and the pelvic girdle consists of the right and left hip bones (also known as os coxae or just coxal bones).

Blood calcium homeostasis

Major points covered here:  Importance of blood calcium levels, mechanisms used to control blood calcium levels, hormones involved in controlling blood calcium levels, and some things that happen of things go wrong with blood calcium levels (pathophysiology of blood calcium levels).

Proper blood calcium levels are very important to our health.  If calcium levels are not right it can effect our skeletal muscles, our heart’s rhythm, blood clotting, and more.  So, our bodies use three major mechanisms in order to maintain blood calcium homeostasis (keep blood calcium levels in a healthy range)…

  1. Calcium absorption —  basically we eat foods that contain calcium and our blood stream then absorbs calcium from our digestive tract.
  2. Calcium storage and release–  calcium can be moved from the blood stream and stored in bone tissue (bone mineralization), and calcium can be released from bone tissue and back into the blood when needed (resorption of bone).
  3. Calcium excretion —  calcium can be excreted from the blood stream by our kidneys and released from the body in urine.

Your body uses hormones to control each of these three major mechanisms.  Here are each of the major hormones involved and what they do…

  • Vitamin D (calcitriol):  This vitamin acts as a hormone, it increases blood calcium levels by stimulating more absorption of calcium from the digestive tract, decreasing calcium excretion into urine, and encouraging calcium release from bone tissue.  This vitamin/hormone is also an interesting molecule because our bodies can make their own calcitriol with some help from sunlight.  UV radiation from the sun hitting our skin allows the first chemistry to happen on the way to making this hormone, the molecule is then modified by the liver and finally by the kidneys before we finally have active calcitriol.
  • Calcitonin:  This hormone is released form the thyroid gland and it lowers blood calcium levels by stimulating the storage of calcium into bone tissue (“bone mineralization” or “bone deposition”), and by slightly increasing the excretion of calcium into urine by the kidneys.
  • Parathyroid hormone (PTH):  Parathyroid hormone is made by the parathyroid glands and it raises blood calcium levels by encouraging the release of calcium from the bone tissue and into the blood (resorption of bone), and by decreasing excretion of calcium into the urine by the kidneys (encourages the kidneys to reabsorb calcium, keeping more calcium in the blood).

So, what can happen if things go wrong with blood calcium levels?

Rickets:  Rickets is a weakening of bone tissue that usually happens if a child does not ingest or produce enough vitamin D.  Remember that calcitriol/vitamin D is important for increasing blood calcium levels, especially encouraging the absorption of calcium from the gut.  If the child is not absorbing enough calcium they will not have enough calcium available to build strong bones.  The child’s bones well simply be weaker as a result, this can lead to fractures or simply to the bending of bones that should be strong enough to handle normal stress without bending (having bowed legs as the femurs bend is somewhat common in rickets).   This can also happen in adults by the way, but in adults we call it osteomalacia.

Hypocalcemia:  Low blood calcium levels.

Some possible causes:  too little vitamin D, diarrhea, thyroid tumors, pregnancy and/or lactation, thyroid tumors, and more…

Signs and symptoms:

  • Oral and facial paresthesias (pins and needles feeling– kind of like the feeling you get after your arm or leg has “fallen asleep” as the feeling comes back you can have uncomfortable paresthesias).
  • Hyperactive skeletal muscles–  hyperactive reflexes, tetany, and other muscle spasms.  One form of tentany that is somewhat common in hypocalcemia is carpopedal spasms in which the hands and feet are effected with muscle spasms that are classical in appearance (do a google image search for carpopedal spasm and you will see that classic appearance).  These spasms can be painful.
  • Cardiac arrhythmia:  Usually include a decrease in the heart’s rate and strength of contraction and can be as severe as a deadly ventricular tachycardia known as torsade de pointes.
  • Petechia (tiny bruises):  bruising can occur because calcium is needed for blood clotting.  If calcium levels are too low the blood does not clot and any tiny break in a capillary can lead to patechia forming.  Larger bruises and excessive bleeding are also possible.

Hypercalcemia:  High blood calcium levels

Some possible causes:  Kidney failure, parathyroid tumor, high bone tissue turnover rate (usually due to another condition such as hyperthyroidism or due to being immobile for a long period), and more…

Signs and symptoms:  This depends on how quickly the calcium levels go up.  If they go up relatively slowly the body seams to compensate pretty well for the change and the person may not have any signs or symptoms.  However, if the increase in calcium is quick they may have…

  • Cardiac arrhythmia including an increase in heart rate and strength of contraction and other possible changes (like a shortened Q-T interval).
  • Depression of skeletal muscle activity–  hypoactive reflexes for example.
  • Formation of calcium stones:  These could be renal stones (in kidneys or bladder) or biliary stones (in gallbladder).
  • Gastric symptoms —  abdominal pain and possible vomiting
  • Increased urination

So, what is the take home here?  Blood calcium levels are important and they have to stay within certain limits for us to stay healthy.  Therefore, our bodies have important hormone controlled mechanisms to help maintain blood calcium homeostasis, and if these systems don’t work right blood calcium levels can move outside of homeostatic range causing us health problems that can be serious.

Skeletal System–Bone histology

Histology of Compact Bone

Compact bone is found in the outer layers of most bones and the diaphysis of long bones.

I’ll start with a ground section of compact bone (in a ground section the bone still contains its hardened calcium matrix–  so sections must be ground thin enough for histology rather than simply cutting them).

Compact bone (bone ground 40X magnification)
Compact bone (ground bone 100X magnification)


Compact bone (bone ground 450X magnification)


Compact bone labeled (ground bone 450X magnification)

Next let’s look at some images of decalcified bone.  Here the bone tissue has been treated with acid to remove the hardened calcium matrix, making it soft enough for sectioning.

transverse section of a small long bone (40X magnification)
transverse section of a small long bone labeled (40X magnification)
transverse section of a small decalcified long bone (100X magnification)
decalcified bone (450X magnification)

Note in the above image that decalcified bone looks a bit different from ground bone, but the major features (osteons, central canals, lamellae, etc.) are still visible.

Spongy bone Histology

Spongy bone 100X magnification


In the above image we see decalcified spongy bone.  The lighter stained tissue here is bone, but in spongy bone the bone tissue is arranged with spaces between the bone tissue, that are filled with red bone marrow (stains a darker purple).  The spaces allow spongy bone to be lighter than compact bone.  The pieces of bone in spongy bone are also arranged so that they keep the bone strong…  kind of like the cross beams and pillars in a steel building keep the building strong even though the building has lots of open spaces in the inside.

At high magnification we can see osteocytes inside of lacunae.

Spongy bone 450X magnification