LESSON 5 Physiology and Actions of Muscles.

LESSON ASSIGNMENT Paragraphs 5-1 through 5-24.

LESSON OBJECTIVES After completing this lesson, you should be able to:

5-1. Match elements of muscle function with their descriptions.

5-2. Given a list of statements about muscle function, select the false statement.

5-3. Given incomplete statements about muscle function, complete the statements.

SUGGESTION After completing the assignment, complete the exercises at the end of this lesson. These exercises will help you to achieve the lesson objectives.

The term muscle (like the term bone) is used with two distinctly different meanings. In one case, the term is used to designate tissues. In the other case, the term refers to individual, discrete organs of the body. However, the structure and actions of tissues are often quite different in detail from the structure and actions of organs.

a. Tissues of the body are collections of like cells performing a common function. Muscle tissues are specialized to produce tension by contraction. In fact, they function solely by contraction.

b. As a by-product, muscle tissues also produce heat. (Shivering is a state in which the muscles of the body are primarily concerned with producing heat. Shivering involves contractions that are not synchronized and therefore do not produce motion.)

c. As used in muscle physiology, the term contraction is not necessarily

synonymous with the term shortening. Rather, contraction means the production of tension through the interaction of the muscle tissues.

There are three types of muscle tissue:

a. Smooth.  Smooth muscle tissue consists of elongated cellular elements. It is found mainly in the walls of visceral organs and blood vessels.

b. Striated.  Striated muscle tissue is composed of fibers. These fibers represent the fusion of many cells into a single functioning fiber (syncytium). Under the microscope, these fibers appear to have a transverse pattern of light and dark banding.

c. Cardiac.  Cardiac muscle tissue is also composed of banded fibers. However, its fibers have a branched character. Cardiac muscle tissue is found only in the wall of the heart.

The striated muscle fiber is a syncytium (para 5-3b).

a. The fiber, as a whole, is surrounded by a membrane known as the sarcolemma. The sarcolemma has specialized invaginations that enter the interior of the fiber at right angles to the sarcolemma. These are called transverse tubules (T-tubules). The T-tubules connect with the extracellular space and allow interstitial fluid to flow in and through the striated muscle fiber.

b. The fiber is filled with a type of intracellular fluid called sarcoplasm.

SARCO = flesh

c. Within the sarcoplasm is a tubular system called the sarcoplasmic reticulum that stores calcium, which is necessary for the muscle activation and contraction.

d. Myofilaments are found in the sarcoplasm.

MYO = muscle

FIL = thread

Myofilaments are long complexes of protein molecules, either actin or myosin. Thus, there are two main types of myofilaments: actin and myosin. The myosin filaments are thicker and have appendages known as myosin "bridges." The myosin filaments are surrounded by the thinner actin filaments.

e. Great numbers of well-developed mitochondria (the "powerhouse" elements of cells) are found in striated muscle fibers.

a. "Sliding Filament" Theory.  The current consensus of opinion of how a striated muscle fiber contracts is known as the "sliding filament" theory (Figure 5-1). This theory emphasizes the role of the myosin bridges. Energy is provided by the mitochondria in the form of ATP. With this energy, the myosin bridges swing and draw the actin filaments over the myosin filaments. The length of the striated muscle fiber is thus shortened.

b. "All-or-None" Phenomenon.  When stimulated to contract by a nervous impulse, a striated muscle fiber contracts totally or not at all. This is the "all-or-none" phenomenon. The striated muscle fiber has a threshold of stimulation. Below this threshold, the fiber will not act. When stimulated at or above this threshold, the fiber will contract totally every time.

Figure 5-1. Schematic diagram of the "sliding filament" theory.

c. Length-Tension Curve.  The contraction of a striated muscle fiber produces tension (force). The amount of this tension varies with the length of the fiber at each moment of contraction. This tension is greatest when the fiber is at its resting length. The tension is proportionately less when the fiber is shorter or longer than its resting length. These variations in tension according to differences in fiber length may be plotted. The resulting curve is called the length-tension curve of the striated muscle fiber.

An organ is a collection of tissues that together perform a particular function. The individual muscles of the body are individual organs. Their overall function is to produce effects by the production of tensions. An individual muscle of the body is made up of muscle tissues, fibrous connective tissues (FCT), and the muscular NAVL.

Several kinds of muscles have striated muscle fibers as their muscle tissue. These include the:

a. Skeletal Muscles.  The skeletal muscles are attached to the bones of the skeleton. Since they cross joints, they produce motion at these joints.

b. Branchiomeric Muscles.  The branchiomeric muscles are those associated with the jaws, pharynx, palate, and larynx.

c. Extraocular Muscles.  The extraocular muscles are within the orbit. They are attached to and move the eyeball.

d. Integumentary Muscles.  The integumentary muscles are developed in association with the deep surface of the skin (integument proper). A prime example of the integumentary muscles consists of the facial (mimetic) muscles.

The individual skeletal muscle is composed primarily of striated muscle fibers and FCT fibers.

a. Striated Muscle Fibers.  There are two types of striated muscle fibers--fast (white) and slow (red).

(1) Fast (white).  The fast striated muscle fibers can contract rapidly and strongly but only for a short time.

(2) Slow (red).  The slow striated muscle fibers tend to contract more slowly but for a sustained duration. The red color of slow striated muscle fibers is because of myoglobin protein. This protein has the capacity to store oxygen within the sarcoplasm. Thus, oxygen is available for the production of energy during the contraction.

b. FCT Fibers.

(1) Endomysium.  The endomysium is a meshwork of FCT that surrounds each striated muscle fiber individually.

(2) Perimysium.  A group of these striated muscle fibers is bound together in a bundle (fascicle) by an FCT envelope known as the perimysium.

(3) Epimysium.  The entire muscle is bound within an FCT sheath called the epimysium.

When the FCT fibers are relatively cold, they are stiffer and more liable to break. As the FCT fibers become warmer, they also become more elastic. Thus, warm-up exercises are always strongly suggested before engaging in vigorous activity.

A skeletal muscle generally has two major subdivisions.

a. Fleshy Belly.  The main portion is the fleshy belly, where muscle tissue is located.

b. FCT Attachments.  At the ends of the belly, the FCT continue and form some sort of attachment to the bones.

(1) In the case of many skeletal muscles, this attachment is a discrete cord of dense FCT known as a tendon.

(2) If the tendon is broad and flat rather than cord-like, we call it an aponeurosis.

(3) Often, we cannot see the tendon-like structure of attachment. Rather, the fleshy belly seems to be attached directly to the surface of the bone. Such an attachment is called a fleshy attachment. However, in reality, the FCT still forms the actual attachment to the bone.

(4) Muscle soreness is often the result of the tearing of the FCT attachment to a bone.

Skeletal muscles are categorized according to the manner in which the muscle fibers are oriented to the tendons of attachment.

a. In some muscles, the fibers are quite long and parallel and extend the length of the muscle (from attachment to attachment). This type of skeletal muscle is referred to as a ribbon or strap muscle.

b. In other muscles, the striated muscle fibers are oriented obliquely between the two tendons of attachments. Such muscles are said to have a quadrilateral structure.

c. If the striated muscle fibers appear to be attached to one tendon in a feather-like arrangement, the muscle structure is known as pennate.

(1) If all of the fibers are on one side of the tendon, the muscle structure is unipennate.

(2) If the fibers are on two sides, the muscle structure is bipennate.

(3) If the feather-like arrangement is branched, the muscle structure is multipennate.

Thus, ribbon muscles have long fibers. On the other hand, pennate (especially multipennate) muscles have great numbers of short fibers. These different structures of skeletal muscles affect both a muscle's relative strength and its distance of contraction.

a. Relative Strength.  The strength of a skeletal muscle is proportional to the cross-sectional area of its fibers. Therefore, a multipennate muscle is generally much stronger than a ribbon muscle.

b. Distance of Contraction.  On the other hand, the longer the fibers of a muscle, the greater will be its distance of contraction. As a very loose rule of thumb, a skeletal muscle can contract to three-fifths of its resting length. The ribbon muscles (such as the rectus abdominis M., which flexes the trunk) have long distances of contraction. The multipennate muscles have the least distance of contraction (but are very strong and stable).

a. Length-Tension Curve.  In paragraph 5-5c, we described the length-tension curve for a striated muscle fiber. A length-tension curve can also be constructed for a whole skeletal muscle. However, the FCT fibers of the skeletal muscle provide an additional component to the tension produced by the muscle fibers. As the muscle is extended beyond its resting length, the tension produced by the FCT fibers becomes greater and greater. Thus, the tension produced by a whole skeletal muscle increases greatly with increased length.

b. Fatigue.  Oxygen is used by the mitochondria of the muscle to produce energy in the form of ATP.

(1) As a muscle is used, its oxygen supply becomes depleted. Naturally, this depletion occurs more quickly in white striated muscle fibers than it does in red striated muscle fibers. With continued exercise, however, the oxygen becomes depleted in both types of fibers.

(2) However, ATP can still be formed, but much less efficiently, in a sequence which is anaerobic (without oxygen). In this anaerobic sequence, the glucose is only partially decomposed. The ultimate product of the anaerobic sequence is lactic acid.

(3) Lactic acid accumulates in the sarcoplasm of the muscle fibers. As this occurs, the muscle becomes stiffer and is no longer able to function well. This condition is called fatigue. An oxygen debt has been built up during the anaerobic production of ATP. This debt must be paid (the muscle must become replenished with oxygen) before the muscle will be able to function properly again.

c. Tonus.  Tonus is a state of semicontraction of the musculature of the body. The degree of tonus varies considerably with the state of health and exercise of the individual. Tonus serves to remove the slack from the skeletal muscles so they can act immediately when called upon. Also, at the joints, tonus serves to keep the opposing surfaces of the bones close together. This helps to prevent injury to the articular cartilages during muscular contractions.

After a skeletal muscle is injured, the wound area undergoes a specific series of changes.

a. Special body cells collect in the area and remove dead and dying tissue. At the margins of the wound, the healthy striated muscle fibers dedifferentiate (lose their special character and become more simple in structure).

b. If damaged tissue and foreign materials have been properly removed (debridement) and if the edges of the live muscle tissue are closely fitted to each other, the regenerating muscle fibers will actually join and produce a whole muscle again.

c. If a great amount of muscle tissue is missing, a defect will remain in the muscle. Some physicians have developed the "minced muscle" technique, used to replace these defects.

The skeletal and muscular systems of the body work together to produce motions and locomotion of the body. All of these actions are mechanical in nature. They utilize the various mechanics as studied in physics.

a. Vectors.  The various forces produced by contracting muscles have specific direction and magnitude. As such, these vectors or forces when plotted are represented by arrows whose length corresponds to the magnitude of the force and whose direction corresponds to the direction of the force.

b. Lever Systems.  The majority of the motions are of the rotary type and occur around an axis or fulcrum. These motions follow the physics of lever systems. The third class of lever (Figure 5-2) is the most common.

Figure 5-2. Types of lever systems.

c. Simple Pulley Systems.  Another common mechanism of the human body is the simple pulley system. Here, the direction of force can be at an angle to the muscle. This is achieved by having the muscle's tendon go around a bony eminence in the same way as a rope goes around a single pulley.

d. Pendulums.  During locomotion, the body uses several pendulums in the swinging of the upper and lower limbs.

The skeletomuscular unit (Figure 5-3) is a working concept of muscle and skeleton producing motion. The components of an S-M unit are: bones, a joint, and skeletal muscle(s).

Figure 5-3. The skeletomuscular unit.

a. Bones. Bones act as levers and as attachment sites for skeletal muscles.

b. Joint (Articulation). The joint is the center, fulcrum, point, or axis of motion.

c. Skeletal Muscle(s). Skeletal muscles apply the forces for motion. Any given motion utilizes a group of muscles working together.

During a given rotary motion, a skeletal muscle may have one of several different roles to play. During the motion, a muscle may change from one role to another.

a. Prime Mover.  Of a group of muscles acting upon a moving part, the one producing the strongest and most direct force is in the prime mover role. Its force is in the direction of the motion being produced.

b. Synergist.  When another skeletal muscle produces an added force in the same general direction as the prime mover, it is referred to as a synergist.

c. Neutralizer.  The muscles moving a part are often arranged so that they tend to move the part at a small angle from the intended direction. In such cases, an additional muscle, the neutralizer, is present to counteract and correct the direction of pull.

d. Antagonist.  Muscles whose lines of pull are opposite to the direction of motion are referred to as antagonists. Antagonists are extremely important for making a smooth, coordinated motion. They tend to adjust the actual direction, speed, and distance of the motion. Without proper antagonists, the motions of the body parts become uncontrolled and flailing. When the motion is completed, the antagonist contracts and returns the part moved to its original position.

e. Stabilizer.  A stabilizer is a skeletal muscle that ensures that the joint being moved is properly maintained.

f. Fixator.  When one joint is moved, the other joints of the body must be kept immobile so that the desired motion can take place normally. The skeletal muscles that hold these other joints immobile are called fixators.

Most skeletal muscles are not directly aligned with the desired motions of the joints. This means that they are potentially able to produce secondary motions at these joints. This potential secondary role of a muscle is very important to medical personnel for two reasons:

a. First, during evaluation of a patient's muscular system, a muscle may only appear to be working properly. In fact, it may not be functioning. Its action may have been taken over by another muscle acting in its secondary role.

b. Next, one may know that a muscle is no longer functioning properly. In such a case, it may be possible to design exercises to develop the secondary role of another muscle so that it will perform the action of the first muscle as a part of a rehabilitation program.

Besides moving the body parts around joints, skeletal muscles also perform other purposes in the human body.

a. Some muscles are specially designed to maintain the erect posture of the human body.

b. Breathing is the process by which air is moved into and out of the lungs. The skeletal muscles of the rib cage and the abdominal cavity produce the various muscular actions of breathing.

c. The interior pressures of the trunk must be increased by the muscles of the trunk wall for two purposes:

(1) Evacuation of substances from the body.

(2) Stabilization so that the trunk can act as a base for work of the upper members.

d. Skeletal muscles can also produce a more or less continuous contraction to immobilize an area of the body. This occurs around painful areas, such as inflamed joints or fractures. This muscular response is called splinting.

a. Atrophy. Whether by choice or as a result of injury or illness, a skeletal muscle may not be used. Without use, the striated muscle tissue tends to be lost. The general process in which muscle or another type of tissue decreases is called atrophy. Where the muscle tissue has been, there is an invasion of FCT and fat.

A = without

TROPHY = growth

b. Hypertrophy.  When a muscle is exercised to capacity, the muscle responds by increasing in mass. The increased mass results from an increase in the diameter of the individual muscle fibers. The number of muscle fibers does not increase. This general process is called hypertrophy.

c. Types of Exercises.

(1) Isometric.  An activity in which a muscle produces tension without a change in length is called an isometric exercise.

ISO = same

METRIC = measurement

For example, if you clasp your hands together and pull without actually moving them, you are participating in an isometric exercise. It has been shown that isometric exercises build muscle strength rapidly. On the other hand, the skeletomuscular system may still lack range of motion.

(2) Isotonic. In isotonic exercises, the active muscles change in length. As the prime mover decreases in length, the antagonistic muscle increases in length. However, both muscles are producing tension.

Generally, skeletal muscle tissue contracts in response to a signal from the nervous system. The skeletal muscles of one side of the body are controlled by the opposite side of the brain. Thus, injury to the left side of the brain tends to result in paralysis of the right side of the body.

The nerves of the body extend from the CNS to the individual muscles. Going to the individual skeletal muscle is the neurovascular bundle. This contains the NAVL for that muscle within a common FCT sheath. The point where this bundle enters the muscle is called the motor point. In a clinic or laboratory, this is the last point where a stimulus can be applied to make the whole muscle contract.

The central nervous system (CNS) receives information from the individual skeletal muscle. It also sends commands for action to the muscle.

a. General Sensations.  The usual general sensations of pain, temperature, pressure, etc., are included in the input from the skeletal muscle.

b. Stretch Receptors.  Associated with the individual skeletal muscle are two sense organs which analyze the degree of tension or stretch of the muscle as a whole.

(1) The stretch reflex. The muscle spindle is located within the substance of the fleshy belly of the muscle. The muscle spindle is very sensitive to the length of the muscle. It continuously sends information about the specific length of the muscle.

(2) The Golgi tendon organ reflex. Another stretch receptor is the Golgi tendon organ. As its name implies, it is located in the tendon of the muscle. When it informs the CNS that the stretch is excessive, the CNS commands the muscle to relax.

a. Motor Homunculus.  The various parts of the body are represented in the substance of the brain. If one plots the sequence and the amount of tissue devoted to each part of the body, one comes up with a caricature of the human being. This caricature (distorted image) is referred to as the motor homunculus.

b. Pyramidal/Extrapyramidal Motor Systems.  Various collections of neurons and their processes carry commands for actions to the individual skeletal muscles. Originating in the brain, these neurons and processes pass through the brain stem into the spinal cord. In general, they are grouped into the pyramidal motor system and extra pyramidal motor system.

(1) Since the pyramidal motor system is subject to volitional control, it can be used for testing during medical examinations.

(2) The extra pyramidal motor system is more automatic. For the most part, control in this system is non-volitional.

c. Modulation of Commands.  Several areas of the brain act as coordinators and modulators of the muscle activity of the body. These areas include the cerebellum and the basal ganglia. The sequential patterns of action to produce an overall motion appear to be programmed in the brain, particularly the cerebellum.

d. Motor Neurons.  The individual motor neuron has its cell body in the brainstem or spinal cord. The axon of the motor neuron passes out of the CNS to become a part of the nerves going to the individual skeletal muscles.

e. Motor Units.  In the skeletal muscle, the individual motor neuron (axon) has a terminal branching so that it contacts several striated muscle fibers. The actual number of striated muscle fibers contacted (innervated) by a single motor neuron are together known as a motor unit.

(1) When its motor units are small (involving fewer muscle fibers), a muscle can produce very fine actions. The extra ocular muscles are an example of this.

(2) With larger motor units, the muscle action is coarse.

(3) A variable number of motor units may be called into action at a given moment. The number recruited is the number needed for the required action.

f. Neuromuscular Junctions.  At the end of each branch of the terminal branching of the motor neuron, is an enlargement known as the bouton.

(1) The bouton has a specific relationship with the sarcolemma of the striated muscle fiber. There is no actual physical contact. Instead, there is a little space known as the synaptic cleft.

(2) Across this space to the striated muscle fiber, the command message is carried in the form of a special chemical acetylcholine (ACh). Once the message has been transferred, the ACh is degraded to no longer function.

(3) Nerve gases and organic phosphate insecticides produce their effects by interfering with the transmission or reception of messages across the synaptic cleft.

g. Axial Versus Appendicular Muscular Control.  The musculature of the body can be thought of in two categories: axial and appendicular.

(1) The axial musculature includes the skeletal muscles of the trunk and the upper and lower girdle regions.

(2) The appendicular musculature includes the skeletal muscles of the upper and lower limbs beyond the girdles.

(3) These two categories are important because they are controlled by the nervous system in different ways. Also, they react quite differently to various physiological and nervous situations.

(4) The muscles that operate the hands tend to be very specifically and highly controlled by the nervous system.