The Muscular System. - composed of mostly skeletal muscle tissue, nervous tissue, blood and connective tissue

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1 The Muscular System Every action the body takes utilizes a muscular activity. Some of the muscles of the body are under voluntary control (skeletal muscles), and by using these muscle, you are able to respond to situations that, therefore, lead to survival. Some muscles are under involuntary control (smooth muscles). These muscles respond to situations automatically, which also lead to survival. The smooth muscles typically are associated with the internal organs of the body, whereas the skeletal muscles are associated with the muscles attached to the skeleton (such as the arms and legs). There is one tissue that is made of (cardiac muscle), and that is the heart. The muscle of the heart contracts involuntary but has a rhythmic pattern. Muscle Structure (Skeletal) see (fig. 9.2 p. 288) - composed of mostly skeletal muscle tissue, nervous tissue, blood and connective tissue Individual skeletal muscle is separated from adjacent muscles by a connective tissue called fascia. The main body of each muscle is made up of several bundles of skeletal muscle fibers called fascicles. (fasciculi, fasciculus - sing.) Each fasciculus is made up itself of muscle fibers (muscle cells). A muscle fiber is composed of many nuclei, mitochondria within the cytoplasm (sarcoplasm) which is surrounded by the muscle cell membrane (sarcolemma). Also within the sarcoplasm are numerous, parallel threadlike structures called myofibrils. Myofibrils play a critical role in muscle contraction. Myofibrils contain 2 kinds of protein molecules: myosin (thick filament) and actin (thin filament). The alternating pattern of actin and myosin (thin and thick bands) produce the light and dark striations characteristic of many muscles. These striations form a repeating pattern of units called sarcomeres. (see figs 9.4/ p. 289) & (see fig 9.6 )

2 How do muscles work? In order to understand how muscles contract, one must understand a little of the microscopic structure of muscles and how the protein molecules interact. It is found that the myofibrils are arranged in a specific manner. Figure 1 shows this arrangement. The thick, dark lines represent myosin. The thin lines are actin. Notice that actin and myosin overlap each other in certain areas. Area 1 is called the Z line. Area 2 is called the I band. The I band consists of actin, space, actin, space, actin. Area 3 is called the A band. The A band consists of overlapping actin and myosin. It consists of actin, myosin, actin, myosin, actin. Area 4 is called the H band. It consists of space, myosin, space, myosin and so on. The span between one Z line and the next Z line is called a sarcomere. Figure 2 shows two sarcomeres. Each sarcomere is a functional unit of muscle. Researchers noticed that when a muscle was stimulated to contract, the empty space of the I band got shorter. They began to study to determine what caused the I band to get shorter. It was found that the myosin myofilament also consisted of structures called cross-bridges. The cross-bridges extended ( when stimulated ) and attached to actin. Once they attached to actin, they pivoted, thereby causing actin myofilaments to move towards each other. The myosin myofilaments are stationary, so only the actin moves. Because of the movement of the actin, this process is called the sliding filament theory. Figure 2 and Fig p. 293 shows the cross-bridges associated with the myosin filaments. Those cross-bridges stretch and attach to the actin myofilaments. Figure 3 shows the cross-bridges already attached to actin and in the pivoted position.

3 Fig. 2: Muscle sarcomeres showing cross-bridges Fig. 3: Muscle sarcomeres showing cross-bridge attachment and pivot Notice that the I band got shorter, and the H band got shorter. The area of the overlap of actin and myosin increased. The Z lines are closer together. The shortening of muscle sarcomeres is muscle contraction. When a muscle relaxes, the cross-bridges turn loose and the actin myofilaments slide back to their original position, as seen in Figure 1. Located on the actin filament are binding sites for the cross-bridges to attach. These binding sites are originally blocked by two other protein filaments. One of these proteins is called troponin and the other is called tropomyosin. These two are typically referred to as the troponin / tropomyosin complex. This complex must move out of the way to allow cross-bridges to bind to actin. It has been found that calcium ions cause the troponin / tropomyosin molecules to move out of the way, thus

4 exposing the binding sites so the cross-bridges can attach. A cell organelle in the muscle cell called the sarcoplasmic reticulum stores calcium ions. A nerve impulse ultimately causes the sarcoplasmic reticulum to release the calcium ions. The calcium ions then cause the troponin / tropomyosin complex to move out of the way to allow crossbridges to bind to actin. Once the cross-bridges have attached to actin, ATP is used to cause the cross-bridges to pivot and then the muscle contracts. Smooth and Cardiac Muscle The contraction mechanisms of smooth and cardiac muscles are essentially the same as those of skeletal muscle previously discussed. But, there are some differences noted below. (Table 9.2 p.304) Smooth Muscle -muscle cells are shorter -single, central nuclei -thin actin / myosin filaments, so don t have striations -found in iris of eye and blood vessels (multiunit smooth muscle) -those found in walls of organs, like stomach, intestines, urinary bladder and uterus are (visceral smooth muscle). -when one fiber is stimulated, it excites other nearby fibers that in turn stimulate others -some visceral smooth muscles also show rhythmicity, which is a pattern of spontaneous repeated contractions -these characteristics allow for the wavelike motion called peristalsis seen in the intestines and ureters of kidneys -smooth muscle contractions lack the protein troponin but, use a different protein instead -hormones affect smooth muscle by stimulating and inhibiting contraction -smooth muscle is slower to contract and relax than skeletal, but they can contract longer with the same amount of energy (ATP) -smooth muscles can also change length without changing tautness (tightness), so the muscles of the stomach and intestines can stretch and still hold the pressure inside constant

5 Cardiac Muscle -only found in the heart -made of striated cells joined end to end -each muscle cell has a single nucleus with many actin and myosin filaments like that seen in skeletal muscle -contracts longer than skeletal muscles -opposing ends of cardiac muscle cells are connected by cross-bands called intercalated disks (Fig p. 304) -cardiac muscles contract as a unit which is referred to as syncytium, so the heart muscle responds in an all-or-none manner. This allows for patterns of contraction and relaxation causing the rhythmic contraction of the heart *Know p 308 fig 9.23/ 9.24* Remember that one end of a muscle is connected to an immovable or fixed part while the other end is connected to a moveable part on the other side of a joint. immovable end = origin (see fig 9.20, p 306) movable end = insertion When a muscle contracts, its insertion is pulled toward its origin. The head of a muscle is the part nearest its origin. Some muscles have more than one origin or insertion. For example, the biceps brachii of the arm has two origins, hence bi = 2, ceps =heads -----> two heads. One head attaches to the coracoid process of the scapula and the other attaches to the glenoid cavity of the scapula. Its insertion by a single tendon is on the radial tuberosity of the radius. Skeletal muscles always function in groups: To abduct the arm requires contracting the deltoid muscle which is said to be the prime mover or (agonist). A prime mover is the main muscle responsible for producing the action. But, while the prime mover is acting, other nearby muscles also contract to help the action of the prime mover be more effective. These muscles that contract and help the prime mover are called synergists.

6 Other muscles however, act as antagonists (against) to the prime movers. They resist a prime movers action and cause movement in the opposite direction. For example: the antagonist of the prime mover that raises the upper limb can lower the upper limb or the antagonist of the prime mover that bends the upper limb can straighten it. The nervous system controls these actions which allow the muscular system to function. There are over 700 muscles in the body (both superficial and deep) from the tiny muscles that wrinkle the forehead to the muscles of the thigh. Muscles make up over half of a person s body mass. The names of muscles often describe them. A name may indicated a muscle s size, shape, location, action, number of attachments, or even the direction of its fibers as in the following examples: pectoralis major: a large muscle (major) located in the pectoral (chest) region deltoid: shaped like a delta or triangle extensor digitorum: extends the digits (fingers/ toes) biceps brachii: a muscle with two heads (biceps) or points of origin, located in the brachium or arm sternocleidomastoid: attaches to the sternum, clavicle, and mastoid process external oblique: located near the outside with fibers that run obliquely (slanting direction)

7 The following tables list some of the common muscles in various body regions as well as their general location. These are the muscles that you are required to know. Face Muscles Frontalis Occipitalis Temporalis Orbicularis oculi Orbicularis oris Zygomaticus Masseter Buccinator Platysma Medial & Lateral Pterygoid Description of Location Located on the forehead Located on the occipital bone Located on the temporal bone Encircles the eye Encircles the mouth (kissing) Extends from the corner of the mouth to the zygomatic bone Located on the ramus of the mandible Wall of cheek (aids in blowing air-- trumpeter muscle) Thin muscles extending from chest to mandible Extend from the sphenoid and maxillary bones Arm Muscles Biceps brachii Triceps brachii Palmaris longus Extensor digitorum Pronator teres Supinator Brachioradialis Deltoid Description of Location Located on the anterior upper arm Located on the posterior upper arm Located on the lower arm and extends to the center of the palm Located on the lower arm, extends to the center of the back of the hand Located in the antecubital region; it pronates the hand by crossing the radius over the ulna Located in the cubital region; it supinates the hand by making the lower arm bones parallel to each other Located lateral to the elbow on the lower arm Located on the shoulder and attaches to the upper arm

8 Leg Muscles Sartorius Rectus femoris Vastus lateralis Vastus intermedius Vastus medialis Biceps femoris Semitendinosus Gracilis Tibialis anterior Gastrocnemius Description of Location Angles across the anterior side of thigh Located in the center of the anterior thigh Located lateral to the rectus femoris Located posterior to the rectus femoris (under the rectus femoris) Located medial to the rectus femoris Located on the posterior thigh. It is the lateral of two parallel muscles on the posterior thigh Located on the posterior thigh. It is the medial of two parallel muscles on the posterior thigh The most medial muscle of the thigh Located on the anterior lower leg Located on the posterior lower leg Torso Muscles Pectoralis major Trapezius Rectus abdominis Latissmus dorsi External oblique Gluteus maximus Sternocleidomastoid Levator ani Sphincter urethrae Description of Location Located on the chest Located on the upper back Vertical muscles of the abdomen Located on the lower back Located lateral to the rectus abdominis Located in the gluteal region Extends from the manibrium (sternum) to the mastoid process of the skull Located in the pelvic and pubic regions Located in the pelvic and pubic regions