LECTURE OUTLINE CHAPTER 9



ANATOMY LECTURE OUTLINE SECTION 2

Skeletal Muscle Tissue and Muscle Organization

Skeletal muscle is almost always attached to the skeleton and must cross at least one joint to have a detailed body action. The term origin of a skeletal muscle refers to the more fixed or stable structure (bony landmark) and the insertion, is the less fixed or more moveable end. In the appendicular skeleton, the origin is more medial and the insertion is more distal. Most often muscles act as though they are all concentrated at the point of insertion and not where the bulk of the muscle is seen.

The muscle mass will tend to lie "upstream" to the body parts moved. Such that the muscles which move the upper arm actually originate from the axial skeleton (chest and upper back); those moving the leg originate on the thigh, those moving the finger lie on the forearm, and so on.

Lecture Outline

I. Functions of Skeletal Muscular System

1. Movement of Body - by pulling on bones.

2. Posture maintenance and Protection of other tissues.

3. Heat production – from use, due to high rate of ATP metabolized.

4. Guard orifices – in the form of sphincters.

II. Structural Arrangement Skeletal Muscle Cells

Microanatomy of Skeletal Muscle

The muscle cell are also called muscle fibers Straw and coffee stirrers can also be wrapped in small bundles with plastic wrap (endomysium), these in turn wrapped again as fascicles (perimysium), and these bundles wrapped still again as epimysium. Red, white and blue threads or strings can be included to represent arteries, nerves and veins. Connective tissue nomenclature follows the established pattern of the prefixes epi- upon, peri- around, and endo- within or inside and the root suffix -mysium for muscle. All of these layers of connective tissue collectively form cord-like tendons or sheet-like aponeuroses when there is not muscle fiber enclosed. These structures most often attach to bone and sometimes soft tissue.

Examination of Skeletal Muscle starting form whole muscle that is visible with the unaided eye.

1. Muscle wrapped in epimysium (whole muscle)

2. Fascicles wrapped in perimysium (creates the muscle patterns seen in gross anatomy)

3. Muscle fibers (cells) wrapped in endomysium (long, cylindrical and multinucleated)

4. Myofibrils – the protein structures inside the muscle cell (microscopic)

5. Myofilaments - the two main structural elements of muscle.

i. myosin – thick myofilament

ii. actin – thin myofilament

III. Muscle Control and Motor Units

A. Motor Unit is the number of muscle fibers (cells) innervated by one neuron. This may be very small consisting of only a few fibers or large consisting of many hundreds of fibers.

B. Muscle Tone - continuous contraction of some motor units to maintain some tension in the muscle.

C. Hypertrophy - synthesis of more myofilaments by demand (not more muscle cells).

D. Atrophy - the loss of myofilaments from disuse (muscle mass becoming smaller).

IV. Types of Skeletal Muscle Fibers

A. Fast Twitch Fibers - fast acting, with high energy requirements.

B. Slow Twitch Fibers - contain more myoglobin so contraction, though slower, can be sustained.

C. Intermediate Twitch Fibers - have attributes of both, exercise (or lack thereof) can change muscle from one class into another (within genetic limits)

V. Classification of Skeletal Muscles by Organization of Muscle Fascicles

Fascicular Arrangements of Skeletal Muscle - All skeletal muscle is made up of fascicles (bundles of muscle fibers) and fascicle arrangements can vary considerably, resulting in muscles with different shapes and functional capabilities. Below are the most common patterns of muscle fascicle arrangements.

1. Circular – arranged in a circular pattern - usually act as valves or sphincters.

2. Parallel - fibers run parallel the entire length of the muscle, permit maximum shortening.

3. Convergent - fibers converge on a narrow insertion from multiple directions of a board origin, permits range of directional motions.

4. Pennate - short fibers attach at angles to a central tendon, provide less shortening but are powerful.

Circular

The fascicular pattern is circular when the fascicles are arranged in concentric rings. Muscles with this arrangement surround external body openings, which they close by contracting. The general term used for these kinds of muscles is “sphincter”. Examples include the orbicularis oris of the mouth and orbicularis oculi of the eyes.

Parallel

In a parallel arrangement, there are two categories: a) Strap-like and b) Fusiform.

a) In Strap-like muscles the length of the fascicles run parallel to the long axis of the muscle and the width of the origin is basically the same as the width of the insertion, so it resembles a strap, a great example of this type of muscle the sartorius muscle of the thigh. It is called the tailors muscle and it is also the longest muscle in the human body.

b) In Fusiform muscles the ends are tapered like a cigar and are often called spindle-shaped with an extended belly. A great example is the biceps brachii muscle of the arm.

Convergent

A convergent muscle has a broad origin, and its fascicles converge toward a narrow insertion area. Such a muscle is triangular or fan shaped like the pectoralis major muscle of the anterior thorax, and the piriformis of the gluteal muscles.

Pennate Muscles

In a pennate pattern, the fascicles are short and they attach to a central tendon that runs the length of the muscle. They often attach obliquely (at an angle). Pennate muscles come in three forms:

Unipennate, in which the fascicles insert into only one side of the tendon, as in the extensor digitorum of the forearm, and extensor digitorum longus muscle of the leg.

Bipennate, in which the fascicles insert into the tendon from opposite sides so the muscle “grain” resembles a feather. The rectus femoris of the thigh is bipennate.

Multipennate, which looks like many feathers side by side, with all their quills inserted into one large tendon. The deltoid muscle, which forms the roundness of the shoulder is multipennate.

The arrangement of a muscle’s fascicles determines its range of motion and its power. Because skeletal muscle fibers may shorten to about 70% of their resting length when they contract, the longer and the more nearly parallel the muscle fibers are to a muscle’s long axis, the more the muscle can shorten. Muscles with parallel fascicle arrangements shorten the most, but are not usually very powerful. Muscle power depends more on the total number of muscle fibers (cells) in the muscle: The greater the number of muscle fibers, the greater the power. The stocky bipennate and multipennate muscles, which pack the most fibers, shorten very little but are extremely powerful.

VI. Actions - names for specific muscles change with motion desired

A. Prime Mover (Agonist) - produces desired action.

B. Synergist - assists prime mover.

C. Antagonist- opposes action of prime mover, restores original position.

VII. Names of Muscles

A. Direction of Fibers - rectus, oblique, transverse, etc.

B. Size - magnus, minor, longus, etc.

C. Position - abdominis, femoris, pectoralis, dorsalis, subscapularis, etc.

D. Action - adductor, flexor, tensor, extensor, supinator, etc.

E. Origin and/or insertion - sternocleidomastoid, stylohyoid, glossopharyngeal, ileocostalis, zygomaticus, etc.

F. Specific features (imagined or otherwise) - serratus, sartorius, semitendinosus, buccinators.

The Eight Part Code for the Naming of Muscles

Now let's talk about how muscles are named. Surprisingly, virtually all muscles are named based on one or more of the following eight criteria. Learn these eight criteria - and their Latin (or sometimes Greek and even French) equivalents - and you can locate in the body virtually any muscle that someone names. And thus, unless you're an anatomist, you don't have to memorize all 799 muscles (approximately) to be anatomically conversant. Here are the basic criteria:

• Size

• Shape

• Orientation of the muscle fibers

• Mechanical action of the muscles

• Number of origins of a muscle

• The points of origin and insertion

• Name of the muscle function

• The muscle's location

Size

When it comes to size, muscles are large or small, short or long, or wide. The largest muscle in a related group of muscles is often referred to as maximus or magnus. An example that you're familiar with is the gluteus maximus. Gluteus is Latin for your rear end - or more politely, your buttock. Thus, gluteus maximus identifies the largest muscle in your butt-the one you feel when you squeeze the cheeks of your butt together. Another example is the adductor magnus, which is the large muscle running down the inner thigh that pulls the leg back in from the side. You can feel this muscle if you balance against a table, putting your hand against the inside of the opposite thigh and then resisting as that hand tries to push the leg out to the side.

Minimus, not surprisingly, refers to the smallest of a group. Thus gluteus minimus identifies the smaller butt muscle located underneath its maximus big brother. Longus, as you might suspect, refers to the longest of a group--as in the adductor longus, which is thinner than the adductor magnus and runs essentially parallel to it. Brevis identifies the shortest of a group. The adductor brevis runs across the thigh to assist in pulling the thigh in towards your body's midline as opposed to down the length of your inner thigh as do the adductor magnus and minimus. In Latin, the word "latus" means "side." Thus latissimus identifies the largest muscle "in width" in a group. Latissimus dorsi is the name of the large muscles that run from under your arms, across your "sides," and then across the middle of your back. Bodybuilders refer to these as their "lats."

Shape

There are really only four shapes that you need to concern yourself with when it comes to naming muscles: trapezoids, triangles, saw tooth, and flat.

A trapezoid is like a rectangle, but with only two sides that are parallel as opposed to all four. For simplicity you can think of it as an oddly shaped rectangle. The trapezius is the large distorted rectangle that sits on the upper portion of the back and is rotated 45% so that one corner attaches to each shoulder, one at the top of the neck, and the fourth corner to the middle of the back. I actually think it looks more diamond shaped, but I didn't discover it, so I didn't get to name it.

The fourth letter in the Greek alphabet is delta, which is drawn in the shape of a triangle Δ. Deltoid, then, refers to the triangular shaped muscle that sits on the top of the shoulder. The Latin verb for saw is "serrare." In anatomy, then, serratus means saw-toothed in shape. Serratus anterior is the name of the muscle that holds your scapula (shoulder blade) to your ribs. It gets its saw-like appearance from the pattern it makes is it attaches to each individual rib.

The French word for flat is "plat." Think of the word plateau. The platysma muscle is the broad, thin, flat muscle that has its origin across the middle of your chest, runs up across the front and sides of your neck and ends at your chin. Its primary role is to hold the other muscles of the neck in place and help with facial expressions. It comes into play when you tense your jaw and neck.

Orientation of the Muscle Fibers

Orientation refers to how the muscle fibers line up to the midline of the body. This makes it pretty simple. They can only be parallel, perpendicular, or diagonal to that line.

Rectus is the word used to identify those muscles whose fibers run parallel to the midline. The Latin word for "right" is "rectus." Rectus abdominis, for example, identifies the six-pack abdominal muscle that runs down the front of the stomach--with its fibers running parallel to the spine. I have to admit, getting parallel out of rectus is a bit of a stretch. Its original root, as I mentioned, is in the Latin word for "right or true." Think of the word rectitude which identifies the quality of being virtuous or straight. Well, from that you get "rectus" as identifying a muscle that runs straight and true -- or dare I say: parallel. Don't blame me. Nobody ever said all doctors were Latin scholars. These are the famous "six-pack" muscles. They get that name because, when well developed, the muscle gets segmented by bands of cartilage that give the muscle the appearance of having six separate sections.

Oblique comes from the old French and means "at an angle." Thus we have both the external and internal obliques of the abdomen, which run one on top of the other, with their fibers running at an angle to the midline and perpendicular to each other. These are the muscles just to the outside of your six-pack, and these are the muscles you work when you do sit ups and touch each elbow to the opposite leg.

Transverse means crosswise or perpendicular to the midline of the body. Thus we have the transversus abdominis. From the name, you can pretty much guess that they're located in the abdomen and which direction their fibers run. In fact, these muscles are located in the sides of the abdomen, underneath the obliques. And the fibers do indeed run across the body. And it's these muscles you feel when you stretch from side to side.

Mechanical Action of the Muscle

Here we're talking about what the muscle does. Does it flex, extend, turn up, turn something down, lift it up, lower it, rotate around a joint, move body parts away from a midline or pull them back towards the midline, make an area rigid, or close an opening? The action that a muscle performs is often used in naming that muscle.

Flexor muscles decrease the angle at a joint. The flexor pollicis longus is the muscle in the forearm that pulls on the thumb and bends or flexes it inward toward the palm. We already know that longus means it's the longest muscle in its group -- and, in fact, it runs the full length of the forearm from the elbow to the thumb. And "pollicis" is Latin for thumb. Thus, the name tells us that it's the long muscle that flexes the thumb.

Extensors are the muscles that counter flexors. They increase the angle at a joint. The extensor pollicis longus, therefore, is the long muscle in the forearm that straightens out the thumb once it's been bent inward.

Pronators turn limbs so that they face downwards or backwards. If you hold your arm out in front of you, palm up, it's the pronator teres muscle that allows you to turn the arm at the elbow so that the palm is now facing downwards. The Latin word "pronus" means "face down" - as in lying prone. And "teres" is Latin for "rounded or cylindrical," which refers to the shape of the muscle.

The counter to a pronator is a supinator. The supinator, for example, is the muscle in your forearm that turns your palm back facing up after you've pronated it down. Supinator comes from the Latin word supinum, which means "lying on your back."

Levators, as is obvious from the word, are muscles that lift things up. The levator scapulae is a muscle that pulls the scapula up, as in the action of shrugging the shoulders. Depressors have the opposite action of levators; they pull things downward or open. The depressor anguli oris is a muscle found at each corner of the mouth and pulls them down to make a frowny face.

Abductor muscles move bones away from a midline in the body. The term is used both generically to describe the action of any muscle that moves away from the midline (the gluteus medius, for example, is an abductor in that its action is to pull the thigh out from the midline) and as part of the formal name of a handful of muscles such as the abductor pollicis brevis, which pulls the thumb away from the palm.

Adductors move the bones back towards the midline as we saw with both the adductor longus and the adductor brevis, which are located in the inner thigh and that we looked at previously. Tensor muscles make things rigid. The tensor fascia lata muscle in the leg tightens and gets rigid to support the knee. Sphincters close openings, as does the anal sphincter.

Number of Points of Origin

A small number of muscles are named after the number of points of attachment they have at their points of origin. These would be: The biceps brachii in your arm that has two points of origin. Brachii is Latin for branches - thus, your biceps muscle has two branches at its origin. The triceps brachii in your arm has three points of origin. And the quadriceps in your leg has four points of origin.

Origin and Insertion

An even smaller number of muscles are named after the parts of the body where they start and end -- their origin and insertion. The sternocleidomastoid muscle, for example, originates from both the sternum and clavicle (breastbone and collarbone) and inserts into the mastoid bone (just below the ear).

Named by Function

And a small number of muscles are named after their function.

The risorius is a facial muscle that is crucial for the expressions of smiling and laughter. Humans are the only animals that have a well-developed risorius; this comes from the Latin word "risus," which means "laugh."

Masseter comes from the Greek word for "chew," and that's its function in the human jaw. The sartorius muscle (from the Latin word for "tailor") runs from the outer hip, across the thigh, and ends at the inner knee. This muscle pulls the leg up at the knee while simultaneously turning it inward. It is used to cross the legs in the manner of an old time tailor sitting on the floor and sewing hides together, hence the name.

Location

And finally, some muscles are named by where they are found in the body. The temporalis muscle is named after the temporal bone (your temple) on top of which it is located. Likewise, the zygomatic bone is your cheekbone, and the zygomaticus muscles are located over your cheekbone.

VIII. Lever Systems

The operation of most skeletal muscles involves leverage – using a lever to move an object. A lever is a rigid bar that moves on a fixed point called the fulcrum, when a force is applied to it.

The applied force, or effort, is used to move a resistance, or load. In the human body, the joints are fulcrums, and the bones act as levers. Muscle contraction provides the effort that is applied at the muscle’s insertion point on the bone. The load is the bone itself, along with overlying tissues and anything else you are trying to move with that lever.

Levers - fulcrum is the joint(s) crossed, force is muscle insertion, resistance is whatever is moved.

1st Class Lever - fulcrum between force and resistance, reverses direction of motions, may have M.A of < or > 1, depending on placement of fulcrum.

2nd Class Lever - resistance is between fulcrum and force, M.A. of > 1, few examples in body.

3rd Class Lever - force applied between fulcrum and resistance, M.A. < 1, most common in body, amplify speed and movement.

Levers: Power Versus Speed

A lever allows a given effort to move a heavier load, or to move a load farther and faster, than it otherwise could. If the load, which is usually referred to as the resistance, is close to the fulcrum and the effort arm is applied far from the fulcrum, a small effort exerted over a relatively large distance can move a large load over a small distance. Such a lever is said to operate at a mechanical advantage and is commonly called a power lever – as it is conferring an advantage of force to the body movement. For example, a person can lift a car with a power lever or jack. The car moves up only a small distance with each downward push of the jack handle, but relatively little muscle effort is needed.

If, on the other hand, the resistance is far from the fulcrum and the effort arm is applied near the fulcrum, the force exerted by the muscle must be greater than the load to be moved or supported. This lever system is a speed lever and operates at a mechanical disadvantage. Speed levers are useful because they allow a load to be moved rapidly or a large distance with a wide range of motion. Using a shovel is an example. As you can see, small differences in the site of a muscle’s insertion can translate into large differences in the amount of force a muscle must generate to move a given load or resistance.

Regardless of type, all levers follow the same basic principle:

• Effort farther than load from fulcrum = lever operates at a mechanical advantage

• Effort nearer than load to fulcrum = lever operates at a mechanical disadvantage

Classes of Levers

Depending on the relative position of the three elements – Effort, Fulcrum, and the Resistance or load – a lever belongs to one of three classes.

1st Class Lever

In a first-class lever, the Fulcrum (pivot) is somewhere in between where the Effort arm is applied and the end that bears the load is at the other end of the lever. Some first-class levers in the body operate at a mechanical advantage (for strength), but others, operate at a mechanical disadvantage (for speed and distance). The arrangement looks like a seesaws. First-class leverage also occurs when you lift your head off your chest.

In a second-class lever, the Effort arm is applied at one end of the lever and the fulcrum is located at the other, with the Resistance or the load in between them in the middle. A wheelbarrow demonstrates this type of lever system, with the pivoting fulcrum at one end. The weight in the middle and pulling at the opposite end of the fulcrum. Second-class levers are uncommon in the body, but the best example is the action of plantarflexion, more commonly known as standing on your toes. All second-class levers in the body work at a mechanical advantage because the muscle insertion is always farther from the fulcrum than the load. Second-class levers are levers of strength, but speed and range of motion are sacrificed for that strength.

In a third-class lever, the effort is applied between the load and the fulcrum. These levers are speedy and always operate at a mechanical disadvantage – think of tweezers or a drawbridge. This is the most common lever arrangement in the human body. Most skeletal muscles of the body act in third-class lever systems. An example is the activity of the biceps muscle of the arm, lifting the distal forearm and anything carried in the hand. Third-class lever systems permit a muscle to be inserted very close to the joint across which movement occurs, which allows rapid, extensive movements (as in throwing) with relatively little shortening of the muscle. Muscles involved in third-class levers tend to be thicker and more powerful.

Differences in the positioning of the three elements of the lever systems (F, E and R) modify muscle activity with respect to speed of contraction, range of movement, and the weight of the load that can be lifted. In lever systems that operate at a mechanical disadvantage (speed levers), force is lost but speed and range of movement are gained. Systems that operate at a mechanical advantage (power levers) are slower, more stable, and used when strength is priority.

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