38: The Musculoskeletal System - Biology

38: The Musculoskeletal System - Biology

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38: The Musculoskeletal System

How Muscles Work

Muscles are one of those things that most of us take completely for granted, but they are incredibly important for two key reasons:

  • Muscles are the "engine" that your body uses to propel itself. Although they work differently than a car engine or an electric motor, muscles do the same thing -- they turn energy into motion.
  • It would be impossible for you to do anything without your muscles. Absolutely everything that you conceive of with your brain is expressed as muscular motion. The only ways for you to express an idea are with the muscles of your larynx, mouth and tongue (spoken words), with the muscles of your fingers (written words or "talking with your hands") or with the skeletal muscles (body language, dancing, running, building or fighting, to name a few).

­Because muscles are so crucial to any animal, they are incredibly sophisticated. They are efficient at turning fuel into motion, they are long-lasting, they are self-healing and they are able to grow stronger with practice. They do everything from allowing you to walk to keeping your blood flowing!

­When most people think of "muscles," they think about the muscles that we can see. For example, most of us know about the biceps muscles in our arms. But there are three unique kinds of muscle in any mammal's body:

  • Skeletal muscle is the type of muscle that we can see and feel. When a body builder works out to increase muscle mass, skeletal muscle is what is being exercised. Skeletal muscles attach to the skeleton and come in pairs -- one muscle to move the bone in one direction and another to move it back the other way. These muscles usually contract voluntarily, meaning that you think about contracting them and your nervous system tells them to do so. They can do a short, single contraction (twitch) or a long, sustained contraction (tetanus).
  • Smooth muscle is found in your digestive system, blood vessels, bladder, airways and, in a female, the uterus. Smooth muscle has the ability to stretch and maintain tension for long periods of time. It contracts involuntarily, meaning that you do not have to think about contracting it because your nervous system controls it automatically. For example, your stomach and intestines do their muscular thing all day long, and, for the most part, you never know what's going on in there.
  • Cardiac muscle is found only in your heart, and its big features are endurance and consistency. It can stretch in a limited way, like smooth muscle, and contract with the force of a skeletal muscle. It is a twitch muscle only and contracts involuntarily.

In this article, we will look at the different types of muscles in your body and the amazing technology that allows them to work so well. From here on, we will focus on skeletal muscle. The basic molecular processes are the same in all three types.

Skeletal muscle is also called striated muscle, because when it is viewed under polarized light or stained with an indicator, you can see alternating stripes of light and dark.

Skeletal muscle has a complex structure that is essential to how it contracts. We will tease apart a skeletal muscle, starting with the largest structures and working our way to the smaller ones.

The basic action of any muscle is contraction. For example, when you think about moving your arm using your biceps muscle, your brain sends a signal down a nerve cell telling your biceps muscle to contract. The amount of force that the muscle creates varies -- the muscle can contract a little or a lot depending on the signal that the nerve sends. All that any muscle can do is create contraction force.

A muscle is a bundle of many cells called fibers. You can think of muscle fibers as long cylinders, and compared to other cells in your body, muscle fibers are quite big. They are from about 1 to 40 microns long and 10 to 100 microns in diameter. For comparison, a strand of hair is about 100 microns in diameter, and a typical cell in your body is about 10 microns in diameter.

A muscle fiber contains many myofibrils, which are cylinders of muscle proteins. These proteins allow a muscle cell to contract. Myofibrils contain two types of filaments that run along the long axis of the fiber, and these filaments are arranged in hexagonal patterns. There are thick and thin filaments. Each thick filament is surrounded by six thin filaments.

Thick and thin filaments are attached to another structure called the Z-disk or Z-line, which runs perpendicular to the long axis of the fiber (the myofibril that runs from one Z-line to another is called a sarcomere). Running vertically down the Z-line is a small tube called the transverse or T-tubule, which is actually part of the cell membrane that extends deep inside the fiber. Inside the fiber, stretching along the long axis between T-tubules, is a membrane system called the sarcoplasmic reticulum, which stores and releases the calcium ions that trigger muscle contraction.

During contraction, the thin filaments slide past the thick filaments, shortening the sarcomere.

The thick and thin filaments do the actual work of a muscle, and the way they do this is pretty cool. Thick filaments are made of a protein called myosin. At the molecular level, a thick filament is a shaft of myosin molecules arranged in a cylinder. Thin filaments are made of another protein called actin. The thin filaments look like two strands of pearls twisted around each other.

During contraction, the myosin thick filaments grab on to the actin thin filaments by forming crossbridges. The thick filaments pull the thin filaments past them, making the sarcomere shorter. In a muscle fiber, the signal for contraction is synchronized over the entire fiber so that all of the myofibrils that make up the sarcomere shorten simultaneously.

There are two structures in the grooves of each thin filament that enable the thin filaments to slide along the thick ones: a long, rod-like protein called tropomyosin and a shorter, bead-like protein complex called troponin. Troponin and tropomyosin are the molecular switches that control the interaction of actin and myosin during contraction.

While the sliding of filaments explains how the muscle shortens, it does not explain how the muscle creates the force required for shortening. To understand how this force is created, let's think about how you pull something up with a rope:

  1. Grab the rope with both hands, arms extended.
  2. Loosen your grip with one hand, let's say the left hand, and maintain your grip with the right.
  3. With your right hand holding the rope, change your right arm's shape to shorten its reach and pull the rope toward you.
  4. Grab the rope with your extended left hand and release your right hand's grip.
  5. Change your left arm's shape to shorten it and pull the rope, returning your right arm to its original extended position so it can grab the rope.
  6. Repeat steps 2 through 5, alternating arms, until you finish.

Muscles create force by cycling myosin crossbridges.

To understand how muscle creates force, let's apply the rope example.

Myosin molecules are golf-club shaped. For our example, the myosin clubhead (along with the crossbridge it forms) is your arm, and the actin filament is the rope:

  1. During contraction, the myosin molecule forms a chemical bond with an actin molecule on the thin filament (gripping the rope). This chemical bond is the crossbridge. For clarity, only one cross-bridge is shown in the figure above (focusing on one arm).
  2. Initially, the crossbridge is extended (your arm extending) with adenosine diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin.
  3. As soon as the crossbridge is formed, the myosin head bends (your arm shortening), thereby creating force and sliding the actin filament past the myosin (pulling the rope). This process is called the power stroke. During the power stroke, myosin releases the ADP and Pi.
  4. Once ADP and Pi are released, a molecule of adenosine triphosphate (ATP) binds to the myosin. When the ATP binds, the myosin releases the actin molecule (letting go of the rope).
  5. When the actin is released, the ATP molecule gets split into ADP and Pi by the myosin. The energy from the ATP resets the myosin head to its original position (re-extending your arm).
  6. The process is repeated. The actions of the myosin molecules are not synchronized -- at any given moment, some myosins are attaching to the actin filament (gripping the rope), others are creating force (pulling the rope) and others are releasing the actin filament (releasing the rope).

The contractions of all muscles are triggered by electrical impulses, whether transmitted by nerve cells, created internally (as with a pacemaker) or applied externally (as with an electrical-shock stimulus).

How Bones Work

Hulton Archive/Getty Images
A diagram showing back and side views of the human skeleton, circa 1900

The human body is an incredible machine. It runs so well most of the time that we don't have to pay much attention to any of the life-sustaining systems that are in motion around the clock, humming along without our mindful involvement.

Right now, your body is performing vital and complicated tasks nearly too numerous to comprehend. Fortunately, our bodies don't demand our comprehension in order to pump the heart, oxygenate blood, regulate hormone production, interpret sensory data and carry out every other process that keeps our biological boats afloat.

In this article, we'll discuss one of the systems that makes life possible: the skeletal system.

Bones prevent you from puddling on the floor in the form of a jellyfish, but what else do they do? Bones rebuild themselves, they produce blood cells, they protect our brains and our organs, they provide a giant system of levers that allow us to move ourselves around, and bones also help maintain a steady amount of calcium in our bodies.

And, even if you never make your mark on the world (or in the history books), your bones will stick around long after you have otherwise vanished to declare to the world: "These skeletal remains once supported skin and tissue and organs! This person once existed!" And as the construction crew that unearthed your bones reels back in horror, every life choice you ever made will seem -- if but for a moment before you are shoveled into a Dumpster -- very much worthwhile.

Before we leave behind our skeletal remains to freak out future generations, we should first learn some basics about bones: What are bones made of? What happens when they break? And just how many of them do you have, anyway?

There are 206 bones in the adult body. Bone is a honeycomblike grid of calcium salts located around a network of protein fibers. These protein fibers are called collagen.

When you patch a hole in a piece of drywall, you usually cover it with tape that has a gummy fibrous grid, and then cover that with wall compound mortar. Bone is made in much the same way. Collagen fibers are gummed together by a kind of shock-absorbing glue [source: University of California-Santa Barbara]. Then, all of this is covered and surrounded by calcium phosphate, which hardens everything into place. Not only do bones make use calcium for strength, they also keep some stored in reserve. When other parts of the body need a calcium boost, the bones release the needed amount into the bloodstream.

There are two different types of bone tissue: cortical bone (the outer layer) and cancellous bone (the inner layer). Cortical bone, also known as compact bone, provides external protection for the inner layer against external force. It makes up 80 percent of bone mass and is dense, strong and rigid [source: Hollister].

Cortical bone is covered by a fibrous membrane called the periosteum. Think of the periosteum as a utility vest that fits over the bone -- it has brackets and places for muscles and tendons to attach. The periosteum contains capillaries that are responsible for keeping the bone nourished with blood.

In the case of long bones such as the femur (the upper leg bone), the periosteum covers the central portion of the bone but -- like a sleeveless vest -- stops short of the cartilage tissue that resides on both ends of the bone (we'll discuss this cartilage in a later section).

Cancellous bone, also known as trabecular or spongy bone, is the inner layer of bone and is much less dense than cortical bone. It's formed by trabeculae, which are needlelike structures that create a meshwork. However, instead of a network of bone structure with periodic gaps, cancellous bone is more like a network of connecting spaces with periodic structure. The latticework of tiny chambers is filled either with bone marrow or connective tissue. Within these marrow-filled spaces is where new blood cells are produced.

Though cancellous bone only makes up about 20 percent of the body's bone mass, it plays important roles in body function. It provides structural stability and acts as a kind of shock absorber inside the bone, but without adding too much to the overall weight of the body.

In the next section, we'll learn more about bone marrow.

Inside the cavities of cancellous bone is soft, fatty tissue comprised of an irregular network of blood vessels and cell types. This is called bone marrow. There are two types of marrow: red and yellow.

Red marrow contains stem cells, unspecialized cells that can grow into different types of specialized cells. They're responsible for replenishing and replacing cells in the body that have been damaged or lost. (For the whole story on stem cells, give How Stem Cells Work a read.) There are two types of stem cells found in red marrow:

    Hematopoietic stem cells (HSCs). This type of stem cell is responsible for creating billions of new blood cells daily, at a rate of about 8 million every second [source: Houston Museum of Natural Science]. HSCs create every type of blood cell: red blood cells (which carry oxygen throughout the body), white blood cells (which fight infections and kill bacteria) and platelets (which help your blood clot). Marrow stem cells can even produce more marrow stem cells. HSCs can leave the marrow and enter the bloodstream, where the ratio of blood cells to stem cells is about 100,000-to-1 [source: National Institutes of Health].

Yellow marrow is mostly fat, and as we age, it can be found in places where red marrow once resided -- some of the bones in our arms, legs, fingers and toes, for instance. If the body needs more blood cells, yellow marrow can transform back into red marrow and produce them. Some bones have a lot more red marrow than others -- the pelvic bone, the spine's vertebrae and our ribs are all rich with it. The body also stores iron in bone marrow.

Bone marrow can become diseased. Myeloproliferative disorders (MPDs) cause the overproduction of immature cells from the marrow. Disorders such as aplastic anemia and myelodysplastic syndromes (MDS) hinder the marrow's ability to produce enough blood cells.

Several marrow diseases can be treated through stem cell transplants, which introduce healthy stem cells to the patient's body to replace the diseased cells. The traditional way to transplant these stem cells is to extract bone marrow from the donor's hip bone with a syringe and introduce the material into the recipient's body. You don't have to actually experience someone penetrating the process to imagine how unpleasant it is. Increasingly, doctors are harvesting the marrow stem cells from the bloodstream instead, resulting in better stem cell samples for the recipient and less pain and discomfort for the donor.

In the next section, we'll examine some of the bones that help prevent your brain and lungs from sliding down into your socks -- the axial bones.

Bones can be broadly divided into two categories: axial and appendicular. In this section, we'll take a look at the axialbones, so named because they form the axis of the body. Axial bones are associated with the central nervous system and protect delicate organs such as your heart and brain.

    The cranium. Though that coconut on top of your neck feels like one big unit with a jaw attached, the cranium is actually comprised of 22 interlocking cranial and facial bones. These cranial plates and oddly shaped bones are held together by joints, though these joints (quite wisely) don't allow for movement (except for the mandible, or jawbone). Deep in your ear is the smallest bone in your body, the stirrup. It's about the size of a grain of rice.

The spine. Your spine (also known as the vertebral column) is comprised of 33 specialized bones called vertebrae. These vertebrae provide form for the rest of the body and protect the spinal cord. Starting from the head and moving downward, the first seven vertebrae are cervical vertebrae, which keep your beautiful cranium from rolling down the street every time you come to a sudden stop. They also allow you to nod "yes" or shake your head "no." Next are the 12 thoracic vertebrae, forming the back of your rib cage. Below the thoracic vertebrae are the lumbar vertebrae, which bear much of the body's load. Most back muscles are connected to these workhorses. Below these is the sacrum, which actually begins in childhood as five different vertebrae, but over time fuses into one unit. Below this is another single unit that begins life in several pieces, the coccyx (tailbone).

In the next section, we'll learn about the bones that serve more than they protect: appendicular bones.

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Start Quiz: Biology 38 The Musculoskeletal System MCQ Quiz

This NASA image is a composite of several satellite-based views of Earth. To make the whole-Earth image, NASA scientists combine observations of different parts of the planet. (credit: NASA/GSFC/NOAA/USGS)

Viewed from space, Earth offers no clues about the diversity of life forms that reside there. The first forms of life on Earth are thought to have been microorganisms that existed for billions of years in the ocean before plants and animals appeared. The mammals, birds, and flowers so familiar to us are all relatively recent, originating 130 to 200 million years ago. Humans have inhabited this planet for only the last 2.5 million years, and only in the last 200,000 years have humans started looking like we do today.

Chapter 38: The Musculoskeletal System MCQ Multiple Choices Questions Quiz Test Bank

38.1 Types of Skeletal Systems

38.3 Joints and Skeletal Movement

38.4 Muscle Contraction and Locomotion

Name: Biology 38 The Musculoskeletal System MCQ
Download URL: Download MCQ Quiz PDF eBook
Book Size: 16 Pages
Copyright Date: 2015
Language: English US
Categories: Educational Materials

Question: The cells responsible for bone resorption are ________.

Question: The Haversian canal:

is arranged as rods or plates

contains the bone's blood vessels and nerve fibers

is responsible for the lengthwise growth of long bones

synthesizes and secretes matrix

Question: Compact bone is composed of ________.

Question: Which of these is a facial bone?

Question: The forearm consists of the:

Question: The pectoral girdle consists of the:

Question: The epiphyseal plate:

is arranged as rods or plates

contains the bone's blood vessels and nerve fibers

is responsible for the lengthwise growth of long bones

synthesizes and secretes bone matrix

Question: The movement of bone away from the midline of the body is called ________.

Question: Synchondroses and symphyses are:

Question: Which of the following is not a characteristic of the synovial fluid?

regulation of water balance in the joint

protection of articular cartilage

Question: All of the following are groups of vertebrae except ________, which is a curvature.

Manual of Human Embryology by Franz Keibel and Franklin P. Mall (1910)

Upper Limb

Table Of Ossification Of The Bones Of The Superior Extremity
Bone Centres Time of appearance of centre Union of primary and secondary centres remarks.
Clavicle Diaphysis 6th week There are two centres in the shaft, a medial and a lateral. These blend on the 45th day (Mall). Shaft and epiphysis unite between the 20th and 25th years.
Sternal epiphysis 18th to 20th year
Scapula Primary centres: The chief centre appears near the lateral angle. The subcoracoid centre appears at the base of the coracoid process and also gives rise to a part of the superior margin of the glenoid fossa. The coracoid process joins the body about the age of puberty. The acromial epiphysis centres (two or three in number) fuse with one another soon after their appearance and with the spine between the 22nd and 25th years (Quain) 20th year (Wilms). The subcoracoid and the epiphysis of the coracoid process, the glenoid fossa, the inferior angle, and the vertebral margin join between the 18th and 24th years in the order mentioned (Sappey).
1. That of the body, the spine, and the base of the glenoid cavity. 8th week (Mall) 1
2. Goraooid process 1st year
3. Subcoracoid 10th to 12th year
Acromial epiphyses 15th to 18th year
Epiphysis of the inferior angle. 16 to 18th year
Epiphyses of the vertebral border. 18th to 20th year
Epiphyses of upper surface of coracoid. 16th to 18th year.
Epiphysis of surface of glenoid fossa. 16th to 18th year.
Humerus Diaphysis 6th to 7th week (Mall) The epiphyses of the head, the tuberculum majus and the tuberculum minus (the last is inconstant) unite with one another in 4th-6th year and with the shaft in 20th-25th year. The epiphyses of the capitulum, lateral epicondyle, and trochlea unite with one another and then in the 16th-17th year join the shaft. The epiphysis of the medial epicondyle joins the shaft in the 18th year.
Head 1st to 2d year
Tuberculum majus 2d to 3d year
Tuberculum minus 3d to 5th year
Capitulum 2d to 3d year
Epioondylus med 5th to 8th year
Lateral margin of trochlea 11th to 12th year
Epicondylus lat 12th to 14th year
Radius Diaphysis 7th week (Mall) The superior epiphysis and shaft unite between the 17th and 20th years. The inferior epiphysis and shaft about the 21st year (Pryor) M 21st year, F 21st-25th year (Sappey). Sometimes an epiphysis is found m the tuberosity (R. and K.) and in the styloid process (Sappey).
Carpal end F 8th month - M 15th month (Pryor)
Humeral end 6th-7th year
Ulna Diaphysis 7th week The centre for the shaft of the ulna arises a few days later than that for the radius. The proximal epiphysis is united to the shaft about the 17th year the inferior epiphysis between the 18th and 20th years F 20th - 21st years, M 21st - 24th years (Sappey). There is sometimes an epiphysis in the styloid process (Sohwegel) and in the tip of the olecranon process (Sappey).
Carpal end F 6th-7th year - M 7th-8th year (Pryor)
Humeral end 10th year
Carpus Os capitatum F 3d-6th month M 4th-10th month The navicular sometimes has two centres of ossification (Serres. Rambaud and Renault). Serres and Pryor have described two centres of ossification in the lunatum. Debierre has described two centres in the pisiform, one in a girl of eleven, the other in a boy of twelve. The OS hamatum may have a special centre for the hamular process. Pryor has found two centres in the triquetrum. Pryor (1908), describes the centres of ossification of the carpal bones as assuming shapes characteristic of each bone at an early period.
Os hamatum F 5th-10th month M 6th-12th month
Os triquetrum F 2d-3d year M about 3 years
Os lunatum F 3rd-4th year M about 4 years
Os naviculare F at 4 years, or early in 5th year M about 5 years
Os mult. maj. F 4th-5th year M 5th-6th year
Osmult. min. F 4th-5th year M 6th-6th year
Os pisiforme F 9th-10th year M 12th-3th year
Metacarpals Diaphyses 9th week (Mall) The centres for the shafts of the second and third metacarpals are the first to appear. There may be a distal epiphysis for the first metacarpal and a proximal epiphysis for the second. Pryor (1906). found the distal epiphysis of the first metacarpal in about 6 per cent, of cases. It is a family characteristic. It arises before the 4th year and unites later. Pryor found the proximal epiphysis of the second metacarpal in six out of two hundred families. It unites with the shaft between the 4th and 6th-7th year sometimes, however, not until the 14th year. In the seal and some other animals all the metacarpals have proximal and distal epiphyses (Quain). The epiphyses join the shafts between the 15th and 20th years. There may bean independent epiphysis for the styloid process of the 5th metacarpal. The epiphysis of the metacarpal of the index finger appears first. This is followed by those of the 3d, 4th, 5th, and 1st digits.
Proximal epiphysis of the first metacarpal 3d year
Distal epiphyses of the metacarpals 2d year
Phalanges Diaphyses 9th week (Mall)
First row Proximal epiphyses 1st-3rd year (Pryor) The shafts of the phalanges of the second and third fingers are the first to show centres of ossification. The phalanges of the little finger are the last, the epiphysis in the middle finger is the first to appear. This is followed by those of the 4th, 2d, 5th, and 1st digits.
Middle row Diaphyses 11th-12th week (Mall) The centres in the shafts of this row are the last to appear. The epiphysis of the phalanx of the middle finger is the first to appear. This is followed by those of the ring, index, and little finger (Pryor).
Proximal epiphyses 2nd-3rd year
Terminal row Diaphyses 7th-8th week The terminal phalanx of the thumb is the first to show a centre of ossification in the shaft. This is the first centre of ossification in the hand. It is developed in connective tissue while the centres of the other phalanges are developed in cartilage (Mall). The epiphysis of the ungual phalanx of the thumb is followed by those of the middle, ring, index, and little fingers. The fusion of the epiphyses of the phalanges with the diaphyses takes place in the 18th-20th year.
Proximal epiphyses 2nd-3rd year
Sesamoid bones Ossification begins generally in the 13th - 14th years, and may not take place until after middle life (Thilenius). For table of relative frequency in the embryo and adult see p. 385.
Days and weeks refer to the prenatal, years to the postnatal period. M = male F = female.

According to Poirier, Traite d'Anatomie, p. 138, two centres appear in the eighth week, and unite in the third month to form a centre of ossification for the body of the scapula.

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