Tendinous Trapezius

Striations

Present

Absent

Present

Nucleus

Multiple nuclei

Single nucleus

Single nucleus

Special features

Transverse tubule system is well developed

Lacks transverse tubules

Transverse tubule system is well developed; intercalated disks separate cells

Mode of control

Voluntary

Involuntary

Involuntary

Contraction

Contracts and relaxes relatively

Contracts and relaxes relatively

Network of fibers contracts as a

characteristics

rapidly

slowly; some types self-exciting; rhythmic

unit; self-exciting; rhythmic; remains refractory until contraction ends

Skeletal Muscle Actions

Skeletal muscles generate a great variety of body movements. The action of each muscle mostly depends upon the kind of joint it is associated with and the way the muscle is attached on either side of that joint.

Origin and Insertion

Recall from chapter 8 (page 276) that one end of a skeletal muscle is usually fastened to a relatively immovable or fixed part, and the other end is connected to a movable part on the other side of a joint. The immovable end is called the origin of the muscle, and the movable end is called its insertion. When a muscle contracts, its insertion is pulled toward its origin (fig. 9.20). The head of a muscle is the part nearest its origin.

Some muscles have more than one origin or insertion. The biceps brachii in the arm, for example, has two origins. This is reflected in its name biceps, meaning "two heads." As figure 9.20 shows, one head of the muscle is attached to the coracoid process of the scapula, and the other head arises from a tubercle above the glenoid cavity of the scapula. The muscle extends along the anterior surface of the humerus and is inserted by a single tendon on the radial tuberosity of the radius. When the biceps brachii contracts, its insertion is pulled toward its origin, and the elbow bends.

Interaction of Skeletal Muscles

Skeletal muscles almost always function in groups. As a result, when a particular body part moves, a person must do more than contract a single muscle; instead, after learning to make a particular movement, the person wills

Trapezius Insertion

Figure

The biceps brachii has two heads that originate on the scapula. This muscle is inserted on the radius by a single tendon.

Figure

The biceps brachii has two heads that originate on the scapula. This muscle is inserted on the radius by a single tendon.

the movement to occur, and the nervous system stimulates the appropriate group of muscles.

By carefully observing body movements, it is possible to determine the roles of particular muscles. For instance, abduction of the arm requires contracting the deltoid muscle, which is said to be the prime mover. A prime mover, or agonist, is the muscle primarily responsible for producing an action. However, while a prime mover is acting, certain nearby muscles also contract.

When a deltoid muscle contracts, nearby muscles help hold the shoulder steady and in this way make the action of the prime mover more effective. Muscles that contract and assist a prime mover are called synergists (sin'er-jists).

Still other muscles act as antagonists (an-tag'o-nists) to prime movers. These muscles can resist a prime mover's action and cause movement in the opposite direction—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. If both a prime mover and its antagonist contract simultaneously, the structure they act upon remains rigid. Similarly, smooth body movements depend upon the antagonists' relaxing and giving way to the prime movers whenever the prime movers contract. Once again, the nervous system controls these complex actions, as described in chapter 11 (p. 425).

The movements termed "flexion" and "extension" describe changes in the angle between bones that meet at a joint. For example, flexion of the elbow joint refers to a movement of the forearm that decreases the angle at the elbow joint. Alternatively, one could say that flexion at the elbow results from the action of the biceps brachii on the radius of the forearm.

Since students often find it helpful to think of movements in terms of the specific actions of the muscles involved, we may also describe flexion and extension in these terms. Thus, the action of the biceps brachii may be described as "flexion of the forearm at the elbow" and the action of the quadriceps group as "extension of the leg at the knee." We believe that this occasional departure from strict anatomical terminology eases understanding and learning.

Distinguish between the origin and the insertion of a muscle.

Define prime mover.

What is the function of a synergist? An antagonist?

Major Skeletal Muscles

This section concerns the locations, actions, origins, and insertions of some of the major skeletal muscles. The tables that summarize the information concerning groups of these muscles also include the names of nerves that supply the individual muscles within each group. Chapter 11 (pp. 429-436) presents the origins and pathways of these nerves.

Figures 9.21 and 9.22 show the locations of superficial skeletal muscles—that is, those near the surface. No tice that the names of muscles often describe them. A name may indicate a muscle's size, shape, location, action, number of attachments, or the direction of its fibers, as in the following examples:

pectoralis major A muscle of large size (major) located in the pectoral region (chest). deltoid Shaped like a delta or triangle. extensor digitorum Extends the digits (fingers or toes). biceps brachii A muscle with two heads (biceps), or points of origin, located in the brachium or arm. sternocleidomastoid Attached to the sternum, clavicle, and mastoid process. external oblique Located near the outside, with fibers that run obliquely or in a slanting direction.

Muscles of Facial Expression

A number of small muscles beneath the skin of the face and scalp enable us to communicate feelings through facial expression. Many of these muscles are located around the eyes and mouth, and they make possible such expressions as surprise, sadness, anger, fear, disgust, and pain. As a group, the muscles of facial expression connect the bones of the skull to connective tissue in regions of the overlying skin. Figure 9.23 and reference plate 61 show these muscles, and table 9.3 lists them. The muscles of facial expression include the following:

Epicranius Buccinator

Orbicularis oculi Zygomaticus

Orbicularis oris Platysma

The epicranius (ep"i-kra'ne-us) covers the upper part of the cranium and consists of two muscular parts— the frontalis (frun-ta'lis), which lies over the frontal bone, and the occipitalis (ok-sip"i-ta'lis), which lies over the occipital bone. These muscles are united by a broad, tendinous membrane called the epicranial aponeurosis, which covers the cranium like a cap. Contraction of the epicranius raises the eyebrows and horizontally wrinkles the skin of the forehead, as when a person expresses surprise. Headaches often result from sustained contraction of this muscle.

The orbicularis oculi (or-bik'u-la-rus ok'u-li) is a ringlike band of muscle, called a sphincter muscle, that surrounds the eye. It lies in the subcutaneous tissue of the eyelid and closes or blinks the eye. At the same time, it compresses the nearby tear gland, or lacrimal gland, aiding the flow of tears over the surface of the eye. Contraction of the orbicularis oculi also causes the folds, or crow's feet, that radiate laterally from the corner of the eye.

The orbicularis oris (or-bik'u-la-rus o'ris) is a sphincter muscle that encircles the mouth. It lies between the skin and the mucous membranes of the lips, extending upward to the nose and downward to the region between the lower lip and chin. The orbicularis oris is sometimes called the kissing muscle because it closes and puckers the lips.

Trapezius

Latissimus dorsi

Serratus anterior

External oblique

Rectus abdominis

Rectus femoris Adductor longus

Vastus lateralis

Peroneus longus

Latissimus Dorsi

Gracilis

Gastrocnemius

Soleus

Vastus medialis Semitendinosus

Semimembranosus

Trapezius

Serratus anterior

Rectus abdominis

Sartorius

Extensor digitorum longus

Tibialis anterior

Figure 9.21

Anterior view of superficial skeletal muscles.

Tensor fasciae latae

Sartorius

Rectus femoris Adductor longus

Vastus lateralis

Peroneus longus

Extensor digitorum longus

Tibialis anterior

Frontalis

Orbicularis oculi

Zygomaticus

Masseter Orbicularis oris

Sternocleidomastoid

Deltoid

Pectoralis major

Brachialis Biceps brachii

Brachioradialis

Gracilis

Gastrocnemius

Soleus

Brachialis

Temporalis Occipitalis

Sternocleidomastoid

Trapezius

Deltoid

Teres minor

Teres major

Triceps brachii

Brachioradialis

Biceps femoris

Vastus medialis Semitendinosus

Semimembranosus

Gastrocnemius

Calcaneal tendon

Infraspinatus Rhomboideus

Gluteus medius

Brachioradialis

Biceps femoris

Gastrocnemius

Calcaneal tendon

Tendinous Trapezius

Infraspinatus Rhomboideus

Gluteus medius

Gracilis

Vastus lateralis Sartorius

Soleus

Peroneus longus

Figure 9.22

Posterior view of superficial skeletal muscles.

Gracilis

Vastus lateralis Sartorius

Soleus

Peroneus longus

Figure 9.21

Anterior view of superficial skeletal muscles.

Figure 9.22

Posterior view of superficial skeletal muscles.

Epicranius

Frontalis

Occipitalis

Masseter

Sternocleidomastoid

Frontalis

Occipitalis

Masseter

Sternocleidomastoid

Skeletal Muscles Posterior Scalp

Temporalis

Orbicularis oculi Zygomaticus

Buccinator Orbicularis oris

Platysma

■ Epicranial aponeurosis

Temporalis

Orbicularis oculi Zygomaticus

Buccinator Orbicularis oris

Platysma

Temporalis Information

Temporalis

Temporalis

Lateral pterygoid

Medial pterygoid

Buccinator

Lateral pterygoid

Medial pterygoid

Buccinator

Lateral Medial Pterygoid

Figure 9.23

(a) Muscles of facial expression and mastication; isolated views of (b) the temporalis and buccinator muscles and (c) the lateral and medial pterygoid muscles.

The buccinator (buk'si-na"tor) is located in the wall of the cheek. Its fibers are directed forward from the bones of the jaws to the angle of the mouth, and when they contract, the cheek is compressed inward. This action helps hold food in contact with the teeth when a person is chewing. The buccinator also aids in blowing air out of the mouth, and for this reason, it is sometimes called the trumpeter muscle.

The zygomaticus (zi"go-mat'ik-us) extends from the zygomatic arch downward to the corner of the mouth. When it contracts, the corner of the mouth is drawn upward, as in smiling or laughing.

The platysma (plah-tiz'mah) is a thin, sheetlike muscle whose fibers extend from the chest upward over the neck to the face. It pulls the angle of the mouth downward, as in pouting. The platysma also helps lower

KBIbi^ Muscles of

31 Days To Bigger Arms

31 Days To Bigger Arms

You can have significantly bigger arms in only 31 days. How much bigger? That depends on a lot of factors. You werent able to select your parents so youre stuck with your genetic potential to build muscles. You may have a good potential or you may be like may of the rest of us who have averages Potential. Download this great free ebook and start learns how to build your muscles up.

Get My Free Ebook


Responses

  • prisca
    Where is the platysma and orbicularis oculi located?
    5 years ago
  • BESSIE
    What are the origin and insertion of the platysma muscle, and what is the effect of its contracting?
    5 years ago
  • zahra
    Where is the platysma muscle located?
    5 years ago
  • evelyn
    Where is the buccinator muscle located?
    2 years ago

Post a comment