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Muscular System
Chapter 8
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2. Learning Objectives Lesson 8.1: Muscular System
List, locate in the body, and compare the structure and function of the three major types of muscle tissue.
Discuss the microscopic structure and function of a skeletal muscle, including sarcomere and motor unit.
Discuss how a muscle is stimulated and compare the major types of skeletal muscle contractions.
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3. Learning Objectives Lesson 8.1: Muscular System (Cont.)
List and explain the primary effects of exercise on the structure and function of skeletal muscles.
List and explain the most common types of movement produced by skeletal muscles.
Name, identify on a model or diagram, and give the function of the major muscles of the body discussed in this chapter.
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Introduction
Muscular tissue enables the body and its parts to move
Three types of muscle tissue exist in body (see Chapter 4)
Movement caused by muscle cells (called fibers)
Shortening or contracting
Muscle movement occurs when chemical energy (obtained from food) is converted into mechanical energy
If you weigh 120 pounds, about 50 pounds of your weight comes from your skeletal muscles.
Movements frequently constitute a major ability to adjust to changing conditions, and, thus, our ability to survive.
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Muscle Tissue
Types of muscle tissue (Figure 8-1)
Skeletal muscle: Also called striated or voluntary muscle
Microscope reveals crosswise stripes or striations
Contractions can be voluntarily controlled
Cardiac muscle: Composes bulk of heart
Cardiac muscle fibers are branched
Has dark bands called intercalated disks
Cardiac muscle fiber interconnections allow heart to contract efficiently as a unit
Each fine thread of a muscle is called a muscle fiber.
Cardiac muscle tissue demonstrates the principle that “structure fits function.”
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Muscle Tissue
Figure 8-1: A shows skeletal muscle, B shows cardiac muscle, and C shows smooth muscle.
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Muscle Tissue (Cont.)
Types of muscle tissue
Smooth muscle, or involuntary muscle; also called nonstriated or visceral muscle
Lacks cross stripes or striations when seen under a microscope; appears smooth
Found in walls of hollow structures, such as digestive tract, blood vessels, and so on
Contractions not under voluntary control
Function
All muscle fibers specialize in contraction (shortening)
Contractions of smooth muscles are highly regulated, which promotes food passage through the digestive tract.
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8. Structure of Skeletal Muscle
Muscle organs: Mainly striated muscle fibers and connective tissue
Connective tissue forms “wrappers” around each muscle fiber, around fascicles (groups) of muscle fibers, and around the entire muscle; fascia surrounds muscle organs and nearby structures
Most skeletal muscles extend from one bone across a joint to another bone
Fibrous connective tissue wraps around each individual muscle fiber.
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9. Structure of Skeletal Muscle (Cont.)
Muscle organs
Regions of a skeletal muscle (Figure 8-2)
Origin: Attachment to the bone that remains relatively stationary or fixed when movement at the joint occurs
Insertion: Point of attachment to the bone that moves when a muscle contracts
Body: Main part of the muscle
Muscles attach to bone by tendons
Strong cords or sheets of fibrous connective tissue that extend from the muscle organ
Some tendons enclosed in synovial-lined tubes (tendon sheaths) and are lubricated by synovial fluid
Tendon sheaths enclose some tendons.
Tendons facilitate body movement.
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10. Structure of Skeletal Muscle (Cont.)
Muscle organs
Bursae
Small synovial-lined sacs containing a small amount of synovial fluid; located between some tendons and underlying bones
Bursae help facilitate body movement.
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11. Attachments of a Skeletal Muscle
Figure 8-2: A muscle originates at a relatively stable part of the skeleton (origin) and inserts at the skeletal part that is moved when the muscle contracts (insertion).
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12. Structure of Skeletal Muscle (Cont.)
Muscle fibers
Structure of muscle fibers (Figure 8-3) and contraction of muscle fibers
Contractile cells are called muscle fibers; connective tissue holds muscle fibers in parallel groupings
Fibers of the cytoskeleton form cylinders that contain thick myofilaments (containing myosin) and thin myofilaments (containing mainly actin)
Basic functional (contractile) unit called sarcomere
Sarcomeres separated from each other by dark bands called Z lines
Sliding filament model explains mechanism of contraction
Thick and thin myofilaments slide past each other to contract
Contraction requires calcium and energy-rich adenosine triphosphate (ATP) molecules (Figure 8-4)
When the muscle relaxes, the sarcomeres can return to resting length, and the filaments resume their resting positions.
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13. Structure of Skeletal Muscle
Figure 8-3, A: Each muscle organ has many muscle fibers.
Figure 8-3, B: During contraction, the thin filaments are pulled toward the center of the sarcomere.
Figure 8-3, C: Electron micrograph showing the overlapping thick and thin filaments within each sarcomere, creating a pattern of dark striations in the muscle.
B: Courtesy Dr. HE Huxley.
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14. Muscle Contraction Mechanism
Figure 8-4: A nerve impulse travels to a muscle fiber through a motor neuron.
The impulse triggers the release of calcium ions.
The Ca++ ions bind to thin filaments and permit action to react with myosin.
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15. Functions of Skeletal Muscle
Movement
Muscles produce movement by pulling on bones as a muscle contracts
The insertion bone is pulled closer to the origin bone
Movement occurs at the joint between the origin and the insertion
Groups of muscles usually contract to produce a single movement
Prime mover: Mainly responsible for producing a given movement
Synergist muscles: Help the prime mover produce a given movement
Antagonist muscles: Oppose the action of a prime mover in a given movement
Voluntary muscular movement is normally smooth and free of jerks and tremors.
Muscles can also produce tension as they extend.
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16. Functions of Skeletal Muscle (Cont.)
Posture
A continuous, low-strength muscle contraction called tonic contraction (muscle tone) enables us to maintain body position
Only a few of a muscle's fibers shorten at one time
Produce no movement of body parts
Maintain muscle tone called posture
Good posture favors best body functioning
Skeletal muscle tone maintains good posture by counteracting the pull of gravity
Gravity tends to pull the head and trunk down and forward.
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17. Functions of Skeletal Muscle (Cont.)
Heat production
Survival depends on the body’s ability to maintain a constant body temperature
Fever: An elevated body temperature; often a sign of illness
Hypothermia: A reduced body temperature
Contraction of muscle fibers produces most of the heat required to maintain normal body temperature
Energy required to produce a muscle contraction is obtained from adenosine triphosphate (ATP).
Sometimes the generation of ATP during heavy muscle use can produce too much heat.
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18. Functions of Skeletal Muscle (Cont.)
Fatigue
Reduced strength of muscle contraction
Caused by repeated muscle stimulation without adequate periods of rest
Repeated muscular contraction depletes cellular ATP stores and outstrips the ability of the blood supply to replenish oxygen and nutrients
Contraction in the absence of adequate oxygen produces lactic acid, which contributes to muscle soreness
During exercise, the stored ATP required for muscle contraction becomes depleted.
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19. Functions of Skeletal Muscle (Cont.)
Fatigue
Oxygen debt: Used to describe the metabolic effort required to burn excess lactic acid that may accumulate during prolonged periods of exercise
Labored breathing after strenuous exercise is required to “pay the debt”
This increased metabolism helps restore energy and oxygen reserves to pre-exercise levels
The technical name for oxygen debt used by exercise physiologists is excess post-exercise oxygen consumption (EPOC).
If repeated stimulation occurs, not only does the strength of the contraction continue to decrease, but eventually the muscle loses its ability to contract.
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20. Role of Other Body Systems in Movement
Muscle functioning depends on the functioning of many other parts of the body
Most muscles cause movements by pulling on bones across moveable joints
Respiratory, circulatory, nervous, muscular, and skeletal systems play essential roles in producing normal movements
Multiple sclerosis, brain hemorrhage, and spinal cord injury are examples of how pathological conditions in other body organ systems can dramatically affect movement
Muscles do not function alone.
Skeletal system disorders, especially arthritis, have disabling effects on body movement.
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Motor Unit
Stimulation of a muscle by a nerve impulse is required before a muscle can shorten and produce movement
A motor neuron is the specialized nerve that transmits an impulse to a muscle, causing contraction
Neuromuscular junction (NMJ): Point of contact between a nerve ending and the muscle fiber
Motor unit: Combination of a motor neuron with the muscle fibers it controls (Figure 8-5)
Signal chemicals called neurotransmitters are released by the motor neuron.
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Motor Unit
Figure 8-5: A motor unit consists of one motor neuron and the muscle fibers supplied by its branches.
B: Courtesy Dr. Paul C. Letourneau, Department of Anatomy, Medical School, University of Minnesota, MN.
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Muscle Stimulus
A muscle will contract only if an applied stimulus reaches a certain level of intensity
Threshold stimulus: Minimal level of stimulation required to cause a muscle fiber to contract
Once stimulated by a threshold stimulus, a muscle fiber will contract completely, a response called “all or none”
When a muscle fiber is subjected to a threshold stimulus, it contracts completely.
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24. Muscle Stimulus (Cont.)
Different muscle fibers in a muscle are controlled by different motor units having different threshold-stimulus levels
Although individual muscle fibers always respond “all or none” to a threshold stimulus, the muscle as a whole does not
Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force
A muscle is composed of many muscle fibers that are controlled by different motor units.
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25. Types of Skeletal Muscle Contraction
Twitch and tetanic contractions
Twitch contractions are laboratory phenomena and not normal muscle activity; they are a single contraction of muscle fibers caused by a single threshold stimulus
Tetanic contractions are sustained muscular contractions caused by stimuli hitting a muscle in rapid succession
In addition to the tonic contraction of muscle that maintains muscle tone and posture, other types of concentration occur: twitch, tetanic, isotonic, and isometric.
Twitch is a quick, jerky response to a stimulus.
In tetanic contractions, contractions “melt” together to produce a sustained contraction (tetanus).
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26. Types of Skeletal Muscle Contraction (Cont.)
Isotonic contraction (Figure 8-6, A)
Produces movement at a joint because the muscle changes length
Concentric contraction: The muscle shortens at the insertion end of the muscle to move toward the point of origin
Eccentric contraction: The muscle lengthens under tension, thus moving the insertion away from the origin
Most types of body movements (walking, running, and so on) are produced by isotonic contractions
There are two types of isotonic contraction: concentric contraction and eccentric contraction.
Walking, running, breathing, lifting, twisting, and most body movements are examples of isotonic contraction.
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27. Types of Skeletal Muscle Contraction (Cont.)
Isometric contraction (Figure 8-6, B)
Contractions that do not produce movement; the muscle as a whole does not shorten
Although no movement occurs, tension within the muscle increases
Repeated isometric contractions make muscles grow larger and stronger.
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28. Types of Muscle Contraction
Figure 8-6, A: In isotonic contraction the muscle changes length, producing movement.
Figure 8-6, B: In isometric contraction the muscle pulls forcefully against a load but does not shorten.
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29. Effects of Exercise on Skeletal Muscle
Exercise, if regular and properly practiced, improves muscle tone and posture, results in more efficient heart and lung functioning, and reduces fatigue
Muscles change in relation to the amount of work they normally do
Prolonged inactivity causes disuse atrophy
Regular exercise increases muscle size, called hypertrophy
Skeletal muscles undergo changes that correspond to the amount of work that they normally do.
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30. Effects of Exercise on Skeletal Muscle (Cont.)
Strength training is exercise involving contraction of muscles against heavy resistance
Strength training increases the number of myofilaments in each muscle fiber, and as a result, the total mass of the muscle increases
Strength training does not increase the number of muscle fibers
Muscle hypertrophy can be enhanced by strength training (e.g., isometric exercises; weight lifting).
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31. Effects of Exercise on Skeletal Muscle (Cont.)
Endurance training is exercise that increases a muscle’s ability to sustain moderate exercise over a long period; it is sometimes called aerobic training
Endurance training allows more efficient delivery of oxygen and nutrients to a muscle via increased blood flow
Endurance training does not usually result in muscle hypertrophy
Endurance training (aerobic training) increases a muscle’s ability to sustain moderate exercise over a long period.
Aerobic training also causes an increase in the number of mitochondria in muscle fibers. This allows production of more ATP as a rapid energy source.
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32. Movements Produced by Skeletal Muscle Contractions
Angular movements
Flexion: Decreases an angle
Extension: Increases an angle
Abduction: Away from the midline
Adduction: Toward the midline
Circular movements
Rotation: Around an axis
Circumduction: Move distal end of a part in a circle
Supination and pronation: Hand positions that result from twisting the forearm
Most flexions are commonly described as bending.
Extension movements are opposite of flexions.
Abduction moves a part away from the midline of the body.
Adduction moves a part toward the midline.
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33. Movements Produced by Skeletal Muscle Contractions (Cont.)
Special movements
Dorsiflexion and plantar flexion: Ankle movements (upward and downward foot movement)
Inversion and eversion: Ankle movements (sideways)
In dorsiflexion the dorsum, or top, of the foot is elevated with the toes pointing upward.
In plantar flexion the bottom of the foot is directed downward so that you are in effect standing on your toes.
Inversion moves turn the ankle so that the bottom of the foot faces toward the midline of the body.
Eversion turns the ankle in the opposite direction so that the bottom of the foot faces toward the side of the body.
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34. Flexion and Extension of the Elbow
Figure 8-7, A and B: When the elbow is flexed, the biceps brachii contracts while the triceps brachii relaxes.
Figure 8-7, B and C: When the elbow is extended, the biceps brachii relaxes while the triceps brachii contracts.
A and C: Courtesy Rolin Graphics.
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35. Flexion and Extension of the Knee
Figure 8-8, A and B: When the knee flexes, the muscles of the hamstring group contract while the quadriceps femoris group relax.
Figure 8-8, B and C: When the knee extends, the hamstring muscles relax while the quadriceps femoris group contracts.
A and C: Courtesy Rolin Graphics.
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36. Examples of Body Movements
Figure 8-9 shows examples of body movements.
A is flexion and extension.
B is adduction and abduction.
C is rotation.
D is circumduction.
E is pronation and supination.
F is dorsiflexion and plantar flexion.
G is inversion.
H is eversion.
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37. Muscle Groups According to Function
As you study the illustrations on the previous slides, you should attempt to group them together according to function.
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38. Skeletal Muscle Groups
Muscles move bones, and the bones that they move are their insertion bones (Figure 8-10)
Muscles of the head and neck (Figure 8-11)
Facial muscles
Orbicularis oculi
Orbicularis oris
Zygomaticus
Muscles of mastication
Masseter
Temporal
Sternocleidomastoid: Flexes head
Trapezius: Elevates shoulders and extends head
Muscles of facial expression allow us to communicate many different emotions nonverbally.
The orbicularis oris (kissing muscle) puckers the lips.
The zygomaticus (smiling muscle) elevates corner of the mouth.
The masseter elevates the mandible.
The temporal assists in closing the jaw.
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39. General Overview of the Body Musculature
Figure 8-10 gives a general overview of the body musculature.
A is an anterior view; B is a posterior view.
See Table 8-2 for an overview of all the principal muscles in the body.
Courtesy John V. Hagen.
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40. Muscles of the Head and Neck
Figure 8-11: Muscles that produce most facial expressions surround the eyes, nose, and mouth.
Contraction of the frontal muscle allows you to raise your eyebrows.
The two sternocleidomastoid muscles are located on the anterior surface of the neck.
Courtesy John V. Hagen.
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41. Skeletal Muscle Groups (Cont.)
Muscles that move the upper extremities
Pectoralis major: Flexes arm
Latissimus dorsi: Extends arm
Deltoid: Abducts arm
Biceps brachii: Flexes forearm
Triceps brachii: Extends forearm
Both the pectoralis major and latissimus dorsi originate from structures over the lower back.
The biceps brachii is a two-headed muscle that serves as the primary flexor for the forearm.
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42. Skeletal Muscle Groups (Cont.)
Muscles of the trunk (Figure 8-12)
Abdominal muscles
Rectus abdominis
External oblique
Internal oblique
Transversus abdominis
Respiratory muscles
Intercostal muscles
Diaphragm
The outermost layer of the anterolateral abdominal wall is the external oblique.
The rectus abdominis muscle runs down the midline of the abdomen from the thorax to the pubis.
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Muscles of the Trunk
Figure 8-12 shows the muscles of the trunk.
A shows the anterior view, which shows superficial muscles.
B shows the anterior view, which shows the deeper muscles.
Courtesy John V. Hagen.
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44. Skeletal Muscle Groups (Cont.)
Muscles that move the lower extremities
Iliopsoas: Flexes thigh
Gluteus maximus: Extends thigh
Adductor muscles: Adduct thighs
Hamstring muscles: Flex leg
Semimembranosus
Semitendinosus
Biceps femoris
The iliopsoas originates from deep within the pelvis and lower vertebrae.
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45. Skeletal Muscle Groups (Cont.)
Muscles that move the lower extremities
Quadriceps femoris group: Extend lower leg
Rectus femoris
Vastus muscles
Tibialis anterior: Dorsiflexes foot
Gastrocnemius: Plantar flexes foot
Peroneus group: Flex foot
The peroneus group is a group of three muscles found along the side of the leg and is also known as the fibularis group.
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Questions?
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