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MUSCLES


Learning Objectives

  1. Identify primary muscle functions.
  2.  Identify muscle characteristics.
  3.  Identify types of muscle tissue.
  4.  Identify important functional muscles.

Muscles are responsible for many different types of body movements. The action of the muscle is determined mainly by the kind of joint it is associated with and the way the muscle is attached to the joint. At one end of some muscles are long white tendons that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the origin. The more flexible attachments, especially attachments to a movable bone, are termed insertions. Muscles seldom act alone; they usually work in groups held together by sheets of a white fibrous tissue called fascia. Muscles make up about one-half of the total body weight. Their main functions are threefold:

  1. Providing movement including internal functions such as peristalsis (rhythmic waves of muscular contraction within the intestines)
  2. Maintaining body posture through muscle tone, as in the muscles of the head, neck and shoulders, which keep the head up
  3. Providing heat through chemical changes that take place during muscle activity, such as exercise that warms the body

In addition, muscles are involved in such essential bodily functions as respiration, blood circulation, digestion, and other functions such as speaking and seeing

MUSCLE CONTRACTION

Muscle tissue has a highly developed ability to contract. Contractibility enables a muscle to become shorter or thicker, and this ability, along with interaction with other muscles, produces movement of internal and external body parts. Muscle contraction in a tissue or organ produces motion and provides power and speed for body activity. A contracting muscle is referred to as a prime mover. A muscle that is relaxing while a prime mover is contracting is called the antagonist.

STIMULUS FOR CONTRACTION

All muscles respond to stimulus. This property is called excitability or irritability. The mechanical muscular action of shortening or thickening (also called contraction) is activated by a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system.

CONTRACTION AND RECOVERY

The chemical action of muscle fibers consists of two stages, contraction and recovery. In the contraction stage, two protein substances (actin and myosin) react to provide energy through the breakdown of glycogen into lactic acid. In the recovery stage, oxygen reacts with lactic acid to release carbon dioxide and water.

MUSCLE FATIGUE

When a muscle contracts, it produces chemical waste products (carbon dioxide, lactic acid, and acid phosphate) which make the muscle more irritable. If contraction is continued, the muscle will cramp and refuse to move. This condition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to allow the blood to carry away the waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used.

TONICITY

Tonicity, or muscular tone, is a continual state of partial contraction that gives the muscle firmness. Isometric muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a person tenses his or her muscles against an immovable object. Isotonic muscle contraction occurs when the muscle is stimulated. The muscle shortens and movement occurs. An example would be lifting an object.

EXTENSIBILITY AND ELASTICITY

Muscles are also capable of stretching when force is applied (extensibility) and regaining their original form when that force is removed (elasticity).

MAINTENANCE OF MUSCLE TISSUE

During exercise, massage, or ordinary activities, the blood supply of muscles is increased. This additional blood brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and tone. The importance of exercise for normal muscle activity is clear, but excessive muscle strain is damaging. For example, if a gasoline motor stands idle, it eventually becomes rusty and useless. Similarly, a muscle cell that does not work atrophies, becoming weak and decreasing in size. On the other hand, a motor that is never allowed to stop and is forced to run too fast or to do too much heavy work soon wears out so that it cannot be repaired. In the same way, a muscle cell that is forced to work too hard without proper rest will be damaged beyond repair.

When a muscle dies, it becomes solid and rigid and no longer reacts. This stiffening, which occurs from 10 minutes to several hours after death, is called rigor mortis.

MUSCLE TISSUES

There are three types of muscle tissue: skeletal, smooth, and cardiac. Each is designed to perform a specific function.

Skeletal

Skeletal, or striated, muscle tissues are attached to the bones and give shape to the body. They are responsible for allowing body movement. This type of muscle is sometimes referred to as striated because of the striped appearance of the muscle fibers under a microscope (Figs. 1 and 2). They are also called voluntary muscles because they are under the control of a person’s conscious will.

Smooth

Smooth, or non-striated, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as well as in the duct glands and in the skin. Under a microscope, the smooth muscle fiber lacks the striped appearance of other muscle tissue. This tissue is also called involuntary muscle because it is not under conscious control.

Cardiac

The cardiac muscle tissue forms the bulk of the walls and septa (or partitions) of the heart, as well as the origins of the large blood vessels. The fibers of the cardiac muscle differ from those of the skeletal and smooth muscles in that they are shorter and branch into a complicated network (Fig. 2). The cardiac muscle has the most abundant blood supply of any muscle in the body, receiving twice the blood flow of the highly vascular skeletal muscles. Cardiac muscles contract to pump blood out of the heart and through the cardiovascular system. Interference with the blood supply to the heart can result in a heart attack.

MAJOR SKELETAL MUSCLES

In the following section, the location, actions, origins, and insertions of some of the major skeletal muscles are covered. In Figures 1 and 2 the superficial skeletal muscles are illustrated. Also note, the names of some of the muscles provides clues to their location, shape, and number of attachments.

Figure 1.—Posterior View of Superficial Skeletal Muscles

Figure 2.—Anterior View of Superficial Skeletal Muscles

MUSCLES OF THE HEAD

The muscles of the head can be classified as two groups, muscles of facial expression and muscles of mastication. How muscles work and function depends on the action of each muscle (movement), the type of joint it is associated with, and the way the muscle is attached on either side of the joint. Muscles are usually attached to two places: one end being joined to an immovable or fixed portion, and the other end being joined to a movable portion on the other side of a joint. The immovable portion is called the origin of the muscle, and the movable portion is called the insertion. When muscles of the head contract, the insertion end is pulled toward the origin.

MUSCLES OF FACIAL EXPRESSION

The muscles underneath the skin of the face are responsible for helping communicate feelings through facial expression. The muscles of the mouth express surprise, sadness, anger, fear, and pain. Table 1 lists the muscles of facial expression and Figure 1 illustrates these muscles.

MUSCLES OF MASTICATION

Mastication is defined as the process of chewing food in preparation for swallowing and digestion. Four pairs of muscles in the mandible make chewing movements possible. These muscles can be grouped into two different functions. The first group includes three pairs of muscles that elevate the mandible to close the mouth as in biting down. The last group includes one pair that can depress the mandible (open the mouth), make grinding actions side to side, and can make the mandible go forward in a protruding motion. Table 2 lists the muscles of mastication and Figure 1 illustrates these muscles.

Table 1.—Muslces of Facial Expression

 

Table 2.—Muscles of Mastication

CHEEKS

The cheeks are the side walls of the mouth. They are made up of layers of skin, a moist inner lining called mucosa, fat tissue, and certain muscles. The buccinator muscle of the cheeks prevents food from escaping the chewing action of the teeth.

LIPS

The lips are covered externally by skin and internally by the same mucous membranes that line the oral cavity. They form the anterior border of the mouth. The area of the external lips where the red mucous membrane ends and normal outside skin of the face begins is known as the vermilion border. Figure 3 illustrates the anatomy of the lips.

Figure 3.—Anatomy of the Lips

The lips are very sensitive and act as sensory receptors, allowing food and liquids to be placed in the mouth but guarding the oral cavity against the ingestion of excessively hot or cold substances. They also provide a seal for the mouth to keep food and saliva from escaping. The lips help to maintain the position of the teeth and are very important in speech.

TONGUE

The tongue (Fig. 4) is a vascular, thick solid mass of voluntary muscle surrounded by a mucous membrane (epithelium tissue). Located on the underneath side of the tongue is the lingual frenulum, which anchors the tongue in the midline to the floor of the mouth. The tip of the tongue is free moving and can readily change size, shape, and position.

Figure 4.—Dorsal Aspect of Tongue (left), Anatomy Floor of Mouth (right)

Surface (Dorsal Aspect)

On the surface of the tongue are rough projections called papillae. They provide the tongue with friction in handling food and also act as taste buds.

Taste Buds

The four types of taste sensations are sweet, sour, bitter, and salty-all resulting from stimulation of the taste buds. Most are located on the tongue and the roof of the mouth. Figure 5 illustrates taste buds of the tongue.

Figure 5.—Taste Buds of the Tongue

Tongue and Digestion

The tongue is an important muscle in the chewing process. It crushes food against the palate; it deposits food between the chewing surfaces of the teeth for mastication; it transfers food from one area of the mouth to another; it mixes food with saliva, which assists in the digestive process; assists in swallowing; and cleans the mouth of residue.

MYLOHYOID AND GENIOHYOID

The mylohyoid muscles anatomically and functionally form the floor of the mouth (Fig. 4 ). They elevate the tongue and depress the mandible. Their origin is the mandible and insertion is the upper border of the hyoid bone. The geniohyoid muscles are found next to each other, on each side of the midline, directly on top of the mylohyoid muscle, and have the same origin and function as the mylohyoid muscle.

PALATE

The palate (Fig. 6) forms the roof of the mouth and is divided into two sections:

  • Hard palate: This section is formed by the palatine process of the maxillary bones and is located in the anterior portion of the roof of the mouth. It has irregular ridges or folds behind the central incisors called rugae.
  • Soft palate: This section forms a soft muscular arch in the posterior part of the palate. The uvula is located on the back portion of the soft palate. When swallowing, the uvula is drawn upward and backward by the muscles of the soft palate. This process blocks the opening between the nasal cavity and pharynx, not allowing food to enter the nasal cavity. The soft palate must function properly to allow good speech quality.

Figure 6.—Anatomy of the Palate

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

Located in the posterior part of the mouth, on both sides of the tongue, are two masses of lymphatic tissue called the palatine tonsils. They assist the body to protect against infections.

TEETH

The teeth are located in the alveolar process of the maxillae and the mandible. They serve important functions of tearing and masticating food, assisting in swallowing, speaking, and in appearance. The health of the teeth affects the health of the entire body.

SALIVARY GLANDS

The functions of the three major salivary glands are to keep the lining of the mouth moist and to bond with food particles creating a lubricant effect that assists in the swallowing process of food. They act as a cleaning agent to wash away food particles that accumulate in the mouth and on the teeth. Figure 7 illustrates the salivary glands.

The salivary glands produce two to three pints of saliva daily, which greatly aids in the digestion process. Enzymes are present in saliva; they act on food, and start the breakdown process. In dentistry, knowing exactly where the saliva glands and ducts (openings) are located is important in keeping the mouth dry during certain dental procedures. Table 3 lists the three major salivary glands.

 

 Figure 7.—Salivary Glands

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

 


Table 3.—Major Salivary Glands

MASTICATION AND DEGLUTITION

The mastication process includes the biting and tearing of food into manageable pieces. This usually involves using the incisors and cuspid teeth. The grinding of food is usually performed by the molars and premolars. During the mastication process, food is moistened and mixed with saliva.

Deglutition is the swallowing of food and involves a complex and coordinated process. It is divided into three phases; the first phase of swallowing is voluntarily; phases two and three are involuntary.

  • Phase One: The collection and swallowing of masticated food.
  • Phase Two: Passage of food through the pharynx into the beginning of the esophagus.
  •  Phase Three: The passage of food into the stomach.

Temporalis

The temporalis muscle is a fan-shaped muscle located on the side of the skull, above and in front of the ear. This muscle's fibers assist in raising the jaw and pass downward beneath the zygomatic arch to the mandible (Fig. 1). The temporalis muscle's origin is the temporal bone. It is inserted in the coronoid process (a prominence of bone) of the mandible.

Sternocleidomastoid

The sternocleidomastoid muscles are located on both sides of the neck. Acting individually, these muscles rotate the head left or right (Figs. 1 and 2). Acting together, they bend the head forward toward the chest. The sternocleidomastoid muscle originates in the sternum and clavicle and is inserted in the mastoid process of the temporal bone. When this muscle becomes damaged, the result is a common condition known as a “stiff neck.”

Trapezius

The trapezius muscles are a broad, trapezium-shaped pair of muscles on the upper back, which raise or lower the shoulders (Figs. 1 and 2). They cover approximately onethird of the back. They originate in a large area which includes the 12 thoracic vertebrae, the seventh cervical vertebra, and the occipital bone. They have their insertion in the clavicle and scapula.

Pectoralis Major

The pectoralis major is the large triangular muscle that forms the prominent chest muscle (Fig. 1). It rotates the arm inward, pulls a raised arm down toward the chest, and draws the arm across the chest. It originates in the clavicle, sternum, and cartilages of the true ribs, and the external oblique muscle. Its insertion is in the greater tubercle of the humerus.

Figure 1.—Posterior View of Superficial Skeletal Muscles

Figure 2.—Anterior View of Superficial Skeletal Muscles

Note: Figures 1 and 2 are repeated here for your convenience.

Deltoid

 The deltoid muscle raises the arm and has its origin in the clavicle and the spine of the scapula (Figs. 1 and 2). Its insertion is on the lateral side of the humerus. It fits like a cap over the shoulder and is a frequent site of intramuscular injections.

Biceps Brachii

The biceps brachii is the prominent muscle on the anterior surface of the upper arm (Fig. 1). Its origin is in the outer edge of the glenoid cavity, and its insertion is in the tuberosity of the radius. This muscle rotates the forearm outward (supination) and, with the aid of the brachial muscle, flexes the forearm at the elbow.

Triceps Brachii

The triceps brachii is the primary extensor of the forearm (the antagonist of the biceps brachii) (Fig.2). It originates at two points on the humerus and one on the scapula. These three heads join to form the large muscle on the posterior surface of the upper arm. The point of insertion is the olecranon process of the ulna.

Latissimus Dorsi

The latissimus dorsi is a broad, flat muscle that covers approximately one-third of the back on each side (Figs.1 and 2. It rotates the arm inward and draws the arm down and back. It originates from the upper thoracic vertebrae to the sacrum and the posterior portion of the crest of the ilium. Its fibers converge to form a flat tendon that has its insertion in the humerus.

Gluteus

 The gluteus (maximus, medius, and minimus-not shown), are the large muscles of the buttocks, which extend and laterally rotate the thigh, as well as abduct and medially rotate it (Fig.2). They arise from the ilium, the posterior surface of the lower sacrum, and the side of the coccyx. Their points of insertion include the greater trochanter and the gluteal tuberosity of the femur. The gluteus maximus is the site of choice for intramuscular injections.

Figure 1.—Posterior View of Superficial Skeletal Muscles

Figure 2.—Anterior View of Superficial Skeletal Muscles

Note: Figures 1 and 2 are repeated here for your convenience.

Quadriceps

The quadriceps is a group of four muscles that make up the anterior portion of the thigh. The four muscles of this group are the rectus femoris that originates at the ilium; and the vastus (v.) lateralis, v. medialis, v. intermedius (not shown), that originate along the femur (Fig. 1). All four are inserted into the tuberosity of the tibia through a tendon passing over the knee joint. The quadriceps serves as a strong extensor of the leg at the knee and flexes the thigh. Additionally located in the quadriceps area is the adductor longus that adducts, rotates, and flexes the thigh.

Biceps Femoris

The biceps femoris (often called the hamstring muscle) originates at the tuberosity of the ischium (the lowest portion of the innominate or coxal bone, part of the pelvic girdle) and the middle third of the femur (Fig. 2). It is inserted on the head of the fibula and the lateral condyle of the tibia. It acts, along with other related muscles, to flex the leg at the knee and to extend the thigh at the hip joint.

Gracilis

The gracilis is a long slender muscle located on the inner aspect of the thigh (Figs. 1 and 2). It adducts the thigh, and flexes and medially rotates the leg. Its origin is in the symphysis pubis, and its insertion is in the medial surface of the tibia, below the condyle.

Sartorius

The sartorius is the longest muscle in the body. It extends diagonally across the front of the thigh from its origin at the ilium, down to its insertion near the tuberosity of the tibia (Fig. 2). Its function is to flex the thigh and rotate it laterally, and to flex the leg and rotate it slightly medially.

Gastrocnemius and Soleus

The gastrocnemius and soleus (together commonly called the calf muscles) extend the foot at the ankle (Figs. 1 and 2). The gastrocnemius originates at two points on the femur; the soleus originates at the head of the fibula and the medial border of the tibia. Both are inserted in a common tendon called the calcaneus, or Achilles tendon.

Tibialis Anterior

The tibialis anterior originates at the upper half of the tibia and inserts at the first metatarsal and cuneiform bones. It flexes the foot.

Diaphragm

The diaphragm (not shown) is an internal (as opposed to superficial) muscle that forms the floor of the thoracic cavity and the ceiling of the abdominal cavity. It is the primary muscle of respiration, modifying the size of the thorax and abdomen vertically. It has three openings for the passage of nerves and blood vessels.


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David L. Heiserman, Editor

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Revised: June 06, 2015