Study hint:Refer to pages 31 through 35 in your class syllabus. Study the muscles in chapter 10 that are mentioned in the Syllabus. You do not have to memorize the origins, insertions and actions of these muscles except as noted in the lecture. Memorize all of the muscle origins, insertions, and actions listed in the lab handouts packet. These are the muscles you will dissect in lab. Color in these muscles. Coloring muscles helps you study the direction of the muscle fibers.
In an Anatomy course, our primary objective is to understand the arrangement of individual muscles in the body. One approach to this study is a regional approach, that is, we will study the muscles by discussing each region of the body as we consider the Muscular System.
The face includes two very special groups of muscles: the muscles of facial expression and the muscles of mastication.
Muscles of Facial Expression |
| These unusual muscles insert into the skin. They are superficial, and their muscle actions account for the movements of the face we refer to as smiling, frowning, surprise, etc. These muscles are supplied by cranial nerve VII which is called the facial nerve. If the facial nerve on one side of the face is damaged, that side of the face becomes paralyzed (an example is a condition called Bell's palsy). |
Orbicularis oculi is the muscle that completely surrounds the orbit. Muscle fibers of this muscle are found in both the upper and lower eyelid, giving the muscle a circular profile. Muscles with a circular arrangement of their muscle fibers are called sphincters. Can you imagine what happens when these circular muscle fibers contract?
Orbicularis oculi is very important in the "blink" reflex.Ladies, exercise your face! Don't criticize my first efforts at multimedia online :( |
| Buccinator, sometimes called the bugler's muscle, is a muscle of the cheek. Blow on an imaginary straw to puff up your cheeks; then palpate buccinator. This muscle assists in chewing (mastication). It holds food between the gums and the cheeks while chewing. One sign of damage to the facial nerve is "drolling" because this muscle becomes paralyzed on one side of the face. |
Muscles of Mastication | |
| This powerful group of muscles occupy the usual skeletal muscle position in the body wall--that is, they are deep to the superficial fascia and are surrounded by their own deep fascia or epimysium. They control movements of the mandible, especially those that are important in "mastication" (look up this word in any dictionary if you don't know its meaning). :) | The powerful, masseter muscle can be palpated by clenching the jaw while moving your fingers along the ramus of the mandible. There are a total of 4 muscles of mastication. Masseter, and two of the other muscles of this group, close the mouth. Only one muscle of this group opens the mouth. Wouldn't it be nice if everyone knew this and acted accordingly?!? (It might keep one's foot out of one's mouth; not you, just someone.) |
They are supplied by cranial nerve V which is the trigeminal nerve. A person with a sunken hollow on one side of his face has sustained damage to the motor axons of the trigeminal nerve. This condition is not called Trigeminal neuralgia. (Trigeminal neuralgia is caused by a supersensitivity in the sensory axons of this nerve, such that people feel excruciating pain in part of their face from very minor stimulation.) | |
To introduce this region, please view two very large muscles: trapezius posteriorly and sternocleidomastoid anteriorly. In the drawings in your lab handouts, trace the total extent of trapezius from the occipital bone to the thoracic vertebrae. Its trapezoid shape gave this muscle the name trapezius.
Trapezius is an excellent example of a muscle that performs several different actions--even opposing actions--depending upon which muscle fibers are active. For example, trace out the insertion of trapezius along the spine of the scapula. The upper fibers elevate the scapula; whereas, inferior fibers inserting into the scapula from below, depress the scapula.
Let's apply our original definitions of origin and insertion to trapezius. Since the clavicle and spine of the scapula are muscle insertions, these bones should move toward the occipital bone origin during muscle contraction. This is true during elevation of the scapula by the upper fibers of trapezius. However, with the scapula fixed, the origin shifts to the scapula. These same fibers extend the head when the occiput moves posteriorly. :-o
Sternocleidomastoid is a muscle named for its origins (sternum and clavicle) and insertion (mastoid process of the temporal bone). With both muscles contracting, this muscle flexes the neck. If, on the other hand, only the right SCM contracts, the mastoid process is brought toward the shoulder and the face looks upward and toward the left. (This is a good time to go to the mirror and try this movement.)
As a group, muscles that attach to the ribs help in breathing, as either their primary or a secondary action. This statement applies to the four muscles listed in your syllabus. Pectoralis minor's primary action is to depress and protract the scapula. However, if a person keeps the scaupla stationary, this contracting muscle elevates its origin--ribs 3 through 5--and contributes to the expansion of the rib cage. The fact that accessory muscles can aid breathing during paralysis of the primary, respiratory muscles is important clinically. Apply the same concept to serratus anterior.
Color code the figure at the top of page 32 of your Syllabus, a drawing of the muscles that lie between the ribs. Color the muscle fibers of the external intercostal and internal intercostal muscles as you notice the direction of the respective muscle fibers. Put your hands in your pockets--the direction of the fingers parallels the direction of the muscle fibers of the external intercostal muscle. The internal intercostal muscle fibers are oriented in the opposite direction.
Remember that the intercostal space extends from the lateral border of the sternum posteriorly to the vertebral column. The external intercostal muscle is superficial in this space. It extends, bilaterally, from the lateral aspect of the vertebral column to approximately the midclavicular line, where it continues as the external intercostal membrane to the lateral border of the sternum.(Read this sentence again!)
The internal intercostal muscle belly lies deep to the external. Beginning anteromedially, fibers of the internal intercostal muscle can be seen through the external intercostal membrane. Laterally, at approximately the midclavicular line, the internal intercostal muscle lies deep to, and is covered by, the external intercostal muscle belly. The internal intercostal muscle extends posteriorly to continue this layer of the thoracic wall.
Anatomy instructors love to pin the internal intercostal muscle just lateral to the sternum in lab exams. Students groan at the two possible questions that can be asked.
Question 1. Identify muscle.
Question 2. Identify structure.
Muscles attaching to the scapula form quite an extensive group. The deltoid muscle consists of anterior, lateral and posterior fibers, all curving towards their insertion on the deltoid tuberosity of the humerus. This is another example of a muscle with multiple actions, depending upon which fibers are contracting.
Get clearly in mind that teres major is not a rotator cuff muscle. The tendons of rotator cuff muscles reinforce the fibrous capsule of the shoulder joint in all directions except inferiorly. Subscapularis anteriorly in the subscapular fossa, supraspinatus superiorly in the supraspinous fossa, infraspinatus posteriorly in the infraspinous fossa and teres minor also posteriorly, form the rotator cuff. The inferior aspect of the joint is subject to dislocation because muscle tendinous reinforcement is absent.
The muscle layering pattern we just discussed for the thoracic region also continues into the abdominal region. Muscle fibers of the external abdominal oblique muscle run parallel to the direction of your fingers when you put your hands in your pockets. Internal abdominal oblique muscle fibers run the opposite direction. The deepest muscle layer consists of muscle fibers running almost transversely (horizontally) called the transversus abdominis muscle. The tendons of these three muscles combine at the midclavicular line in a complicated fashion to form the anterior and posterior layers of an aponeurosis or large broad tendon called the rectus sheath. Students in the cadaver dissection course incise (cut) the rectus sheath to reveal the straight rectus muscle and its tendinous inscriptions. These regions appear white in the black and white drawing on page 33 of your Syllabus. Interestingly, the cat's rectus muscle is not segmented; that is, its muscle is not interrupted by tendinous inscriptions.
The embryological development of the muscles of the upper and lower extremities is an important clinical consideration. This pattern of development is reinforced by the arrangement of the superficial fascial. The fascia sends out extensions called septa which divide the spaces of the limbs into muscle groups with the same primary actions. These compartments also include the vessels and nerves. The muscle compartments of the upper limb are supplied by branches of the brachial plexus; those of the lower limb by branches of the lumbosacral plexus. You will study both plexuses during our discussion of the nervous system. Injury to a limb can cause bleeding that is confined to one compartment--a condition called compartment syndrome.
Study hint:This section marks the only time you must study the names of individual, peripheral nerves. However, this information is so important clinically that I have included it in this discussion.
| Organization of the Brachium | |||
| Anterior Compartment |
Flexor Muscles | Musculocutaneous Nerve | Coracobrachialis Biceps brachii Brachialis |
| Posterior Compartment |
Extensor Muscles | Radial Nerve | Triceps brachii |
| Organization of the Antebrachium | |||
| Anterior Compartment |
Flexor Muscles (plus Pronator) | Median and Ulnar Nerves | Pronator teres Flexor carpi radialis Palmaris longus Flexor carpi ulnaris Flexor digitorum superficialis Flexor digitorum profundum |
| Posterior Compartment |
Extensor Muscles (plus Supinator) | Radial Nerve | Brachioradialis Extensor carpi radialis longus Extensor digitorum Extensor carpi ulnaris Supinator |
| Organization of the Thigh | |||
| Anterior Compartment |
Flexor Muscles at the Hip | Femoral Nerve | Sartorius Quadriceps femoris (Know the 4 parts.) |
| Anteromedial Compartment |
Adductor Muscles | Obturator Nerve | Adductor longus Adductor brevis Adductor magnus, anterior portion Gracilus |
| Posterior Compartment |
Extensor Muscles at the Hip, Flexors at the Knee | Sciatic Nerve | Hamstrings (Know the 4 parts.) |
| Organization of the Leg | |||
| Anterior Compartment |
Dorsiflexors of the Foot | Deep Peroneal Nerve | Tibialis anterior Extensor digitorum longus |
| Lateral Compartment |
Evertors of the Foot (These muscles arise from the fibula.) | Superficial Peroneal Nerve | Peroneus longus Peroneus brevis |
| Posterior Compartment |
Plantar flexors of the Foot | Tibial Nerve | Gastrocnemius Soleus Tibialis posterior Flexor digitorum longus |
The muscles of the gluteal region arranged in dissectible regions that we call superficial, intermediate and deep layers. This layering is similar to that in the rest of the body; it is not reinforced by "sheets of fascia". These muscles act at the hip joint. Follow the information in your lab handouts packet and study only gluteus maximus and gluteus medius in detail. Then study the rotators of the femur as a group.
However, one of these smaller muscles deserves special mention. Piriformis is a landmark of the gluteal region. It originates from the sacrum, and passes through the greater sciatic foramen to insert on the greater trochanter of the femur. The very large, sciatic nerve passes inferior to piriformis. Spasms of this muscle can compress the sciatic nerve and cause sciatica, a very painful condition! (Just ask your classmate, Rita King. :(
That's it for today's lecture. :))