Muscular System

  • machines” of the body
  • Muscle make up nearly half the body’s mass. The essential function of muscle is contraction or shortening. This unique characteristic sets muscle apart from other tissues in the body. All body movements depend on the muscles. Thus, muscles can be viewed as the “machines” of the body.


Functions of the muscles
  1. Produces movement. All movements of the human body are result of muscular contraction.
  2. Maintaining posture. The skeletal muscles in the body maintain posture.
  3. Stabilizing joints. Presence of muscle tendons reinforces and stabilizes joints that have poorly fitting articulating surfaces.
  4. Generating heat. Heat is a by-product of muscle activity. This heat is essential in maintaining normal body temperature.
Types of muscles
Skeletal muscles
  • Also called: voluntary muscle, striated muscle
  • This type of muscle attaches to the body’s skeleton. Because of their attachment to the bony part of the body smoother contours of the body are formed. Skeletal muscle fibers are cigar-shaped, multi-nucleate cells and are the largest of the muscle fiber types. This is the only muscle type that can be controlled consciously, thus it is a voluntary muscle. Since its fibers appear to be striped it is known as striated muscle.

types of muscles

Summary of the characteristic of Skeletal Muscles:

  1. Subject to conscious control.
  2. Multinucleated.
  3. Cigar-shaped.
  4. Attaches to the body’s skeleton
  5. Soft and fragile.
  6. Its tissue can rapidly contract and with great force.
  7. Tires easily.
  8. Contraction is abrupt and rapid.

What is the reason why skeletal muscles are not ripped apart as they exert effort?

  • Skeletal muscles do not rip apart after exerting effort because thousands of skeletal muscle fibers are bundled together by connective tissues which are responsible for providing strength and support to the muscle as a whole.

Structure or Parts of Skeletal Muscle

  1. Endomysium – a delicate connective tissue sheath that encloses each skeletal muscle fiber.
  2. Perimysium – a coarse fibrous membrane that wraps the sheathed muscle fibers.
  3. Fascicle – bundle of fibers formed from group of sheathed muscle fibers wrapped by perimysium.
  4. Epimysium – a tough overcoat of connective tissue that bounds together fascicles. This is the connective tissue that covers the entire muscle.
  5. Tendons – these are cordlike structures that are formed from epimysia. These are composed of mostly collagenic fibers that can cross rough bony projections. Aside from anchoring muscles, tendons are very important in providing durability.
  6. Aponeuroses – these are sheet like structures that attaches muscles indirectly to bones, cartilages or connective tissue coverings of each other.
Smooth Muscles
  • Also called: visceral muscles, non-striated muscles, involuntary muscles
  • Smooth muscles, unlike skeletal muscles, have no striations. It is controlled involuntarily, meaning to say individuals cannot consciously regulate it. If skeletal muscles are found in the bones, smooth muscles are found on the walls of hollow visceral organs such as the stomach, urinary bladder and respiratory passages. The main function of smooth muscles is to propel substances along a definite tract or pathway within the body. These muscles have only one nucleus and are spindle-shaped.

Summary of the characteristics of smooth muscles

  1. Involuntary control.
  2. Found on the walls of the hollow visceral organs.
  3. Have no striations.
  4. Propels substances along a fixed tract inside the body.
  5. Spindle-shaped.
  6. Have a single nucleus.
  7. Arranged in layers or sheets. Most often, it is arranged in two layers, one circular the other longitudinal.
  8. Alternately contracts and relaxes to change the shape and size of an organ.
  9. Contraction of these muscles is slow and sustained.
Cardiac Muscle
  • Unlike the other two types of muscles mentioned above, cardiac muscle is only found in one place in the body – the heart. The function of the heart as the pump – propelling blood into the blood vessels and to all tissues of the body, is carried out because of the presence of cardiac muscle. This muscle has similarities to skeletal muscles as it is striated and smooth muscles as it is involuntary and cannot be controlled consciously. Cardiac muscle fibers are branching cells joined together by special junctions called intercalated discs.

Summary of the characteristics of cardiac muscle

  1. Only found in the heart.
  2. Striated.
  3. Involuntary control.
  4. Cushioned by small amounts of soft connective tissue.
  5. Arranged in spiral or figure 8-shaped bundles.
  6. Contracts at a steady rate set by the heart’s pacemaker.
Types of Muscles in the Body
Characteristic Skeletal Smooth Cardiac
Location in the body Attached to bones Located in the walls of the hollow visceral organs with the exception of the heart Walls of the heart
Muscle fiber shape and appearance Single, cigar shaped, very long, cylindrical, striated, multinucleated cells Singe, no striations, fusiform, cells have single nucleus Branching cell chains, uninucleate, with striations, intercalated discs
Regulation of contraction Voluntary Involuntary Involuntary
Speed of contraction Slow to fast Very slow Slow
Rhythmic contraction No Yes, in some Yes
Gross anatomy of Skeletal Muscles
Muscles in the head and neck

Facial Muscles

  1. Frontalis – covers the frontal bone. This muscle runs from the cranial aponeurosis to the skin of the eyebrows where it inserts. Frontalis muscle allows a person to raise his or her eyebrows and wrinkle ones forehead.head
  2. Orbicularis Oculi – the muscle fibers of this muscle run in circles around the eyes. It is the presence of this muscle that an individual is able to close his or her eyes, squint, blink and wink.
  3. Orbicularis Oris – also called the kissing muscle. This circular muscle runs around the mouth. This is responsible for closing and protruding ones mouth.
  4. Buccinator – this muscle runs across the cheek and inserts into orbicularis oris. This muscle plays a vital role during chewing. It is listed as a chewing muscle as it compresses the cheek to hold the food between the teeth during chewing. When a person is whistling and blowing a trumpet this muscle flattens the cheek.
  5. Zygomaticus – this muscle is referred to as the “smiling muscle” as it raises the corners of the mouth upward.
  6. Chewing muscles – aside from buccinators which is described above, chewing muscles include the masseter and the temporalis muscle. Masseter is the muscle covering the lower jaw and is responsible for closing the jaw when chewing by elevating the mandible. The temporalis muscle, a fan-shaped muscle, inserts into the mandible and acts as a synergist of the masseter in closing the jaw.
  7. Neck muscles – muscles in the neck are the platysma and sternocleidomastoid. Platysma is the sheet like muscle that covers the neck anterolaterally. Its action is to pull the corners of the mouth inferiorly which produces downward sag of the mouth. The other muscle of the neck, the sternocleidomastoid, is found on each side of the neck. The sternocleidomastoid muscles are two-headed muscles. Of the two heads of each muscle, one arises from the sternum and the other arises from the clavicle. When the sternocleidomastoid muscles contract together, the neck flexes. If only one muscle of the sternocleidomastoid muscle contracts, the head is rotated toward the opposite side.
Trunk Muscles

Anterior Muscles

  1. Pectoralis major – this muscle is a large fan-shaped muscle covering the upper part of the chest. The adduction and the flexion of the axilla is the action of this muscle. It forms the anterior wall of the axilla.
  2. Intercostals muscles – these muscles are located deep between the ribs. The external intercostals muscles help raise the rib cage for breathing air in, making it a vital structure in breathing. The internal intercostals ribs, on the other hand, help air to move out the lungs when an individual exhales forcibly.
  3. Muscles of the abdominal girdle – muscles of the abdominal girdle are subdivided into two categories – the anterior and posterior abdominal muscles. The anterior muscles serve as reinforcement of the body trunk.  Both of these muscles are suitable for containing and protecting the abdominal contents.

Anterior Abdominal Muscles

  • Rectus abdominis muscle – this paired strap-like muscle is the most superficial muscle of the abdomen. The MAIN FUNCTION of the rectus abdominis muscle is to flex the vertebral column. During defecation and childbirth, this muscle compresses the abdominal contents. Aside from that, it is also involved in forced breathing.
  • External Oblique muscle – this paired muscle makes up the lateral walls of the abdomen. The MAIN FUCNTION of this muscle is to flex the vertebral column like the rectus abdominis muscle. However, they also rotate the trunk and bend it laterally.
  • Internal oblique muscle – this paired muscle serves the same function as that of the external oblique muscles.


Posterior Muscles

  1. Trapezius muscles – these muscles are the most superficial muscles of the posterior and upper trunk. They serve as the antagonists of the sternocleidomastoids and they can elevate, depress, adduct ad stabilize the scapula. When seen together, the trapezius muscle forms a diamond or kite-shaped muscle mass.
  2. Latissimus Dorsi muscle – this is a large and flat pair of muscle that covers the entire lower back. Latissimus dorsi muscle extends and adducts the humerus. This muscle plays a vital role in bringing down the arm in a power stroke when swimming or striking a blow.
  3. Erector Spinae muscle – this group of muscle is a prime movers of back extension. Aside from acting as powerful back extensors, erector spinae muscle also helps control the action of bending over at the waist. When an injury to back structure occurs, these muscles go into spasms which are a common source of lower back pain. Each erector spinae muscle is composed of three muscle columns that cover the entire length of the vertebral column. The three muscle columns are longissimus, iliocostalis and spinalis.
  4. Deltoid muscles – these are fleshy and triangle-shaped muscles that form the round shape of the shoulders. They are bulky and have been the most common and most favorite injection site. The deltoid muscles are the prime movers of arm abduction.
Muscles of the Upper Limb

Muscles of the upper limbs are divided into three groups.

  1. First group – muscles arising from the shoulder girdle and cross the shoulder joint to insert into the humerus. These muscles move the arm.
  2. Second group – muscles that encloses the humerus and insert on the forearm bones. These muscles cause movement at the elbow joint.
  3. Third group – includes muscles of the forearm that cause their movement.
Muscles of the Humerus that Act on the forearm
  1. Biceps Brachii – this muscle bulges when the elbow is flexed. It is a powerful prime mover for the flexion of the forearm and acts to supinate the forearm.
  2. Brachialis – this muscle plays an essential in elbow flexion. It lies deep to the biceps muscle.
  3. Brachioradialis – this is a fairly weak muscle. It arises on the humerus and inserts into the distal forearm.
  4. Triceps brachii – this is the only muscle fleshing out the posterior humerus. It is the powerful prime mover of elbow extension. Other term for this muscle is the “boxer’s mucle” as it can deliver a straight arm knock-out punch. This muscle is also the antagonist of the biceps brachii muscle.
Muscles of the Lower Limb

Characteristic of the muscles of the lower limb:

  1. These muscles cause movement at the hip, knee and foot joints.
  2. They are one of the largest and strongest muscles in the body.
  3. These muscles play a vital role in walking and balancing the body.
Muscles causing movement at the hip joint
  1. Gluteus Maximus Muscle – this is a superficial hip muscle that shapes the buttocks. Gluteus maximus muscle is a very powerful hip extensor that is responsible in bringing the thigh to a straight line.
  2. Gluteus medius Muscle – this is a hip abductor muscle. Gluteus medius muscle is very important in stabilizing the pelvis when an individual is walking. When more than 5 ml of medication is administered intramuscularly, the gluteus medius muscle is used. However, the medial part of each buttock overlies the large sciatic nerve, hence; this area must be used carefully. Because of this reason, the fleshy gluteus maximus would be a better choice. In cases, where the gluteus medius is used for IM injection, the nurse should divide the buttocks into four equal parts mentally. The upper outer quadrant is a very safe site for an IM injection.
  3. Iliopsoas – this muscle is composed of two muscles – the iliacus and psoas major. Iliopsoas is a prime mover of hip flexion and it also acts to keep the upper body from falling backward when a person is standing erect.
  4. Adductor muscles – as their name indicate, these muscles adduct or press the thighs together. However, since gravity does most of the work for them, these muscles tend to become flabby very easily.
Muscles causing movement at the knee joint
  1. Hamstring group – these muscles form the muscle mass of the posterior thigh. This group is composed of three muscles – the biceps femoris, semimembranosus and semitendinosus muscles.
  2. Sartorius – this thin and strap-like muscle is not very significant because it is a very weak flexor of the thigh. However, this muscle is the most superficial muscle of the thigh. The Sartorius muscle is often referred to as the tailor’s muscle because it acts as a synergist to bring about the cross-legged position in which old-time tailor’s is often shown.
  3. Quadriceps group – four muscles compose this muscle group. Namely, the rectus femoris and the three vastus muscles. These muscles flesh out the anterior thigh. Quadriceps group acts to extend the knee powerfully.
Muscles causing movement at the ankle and foot
  1. Tibialis anterior – this is a superficial muscle of the anterior leg that acts to dorsiflex and invert the foot.
  2. Extensor digitorum longus – this muscle inserts into the phalanges of toes 2 to 5. It is a prime mover of toe extension and a dorsiflexor of the foot.
  3. Fibularis muscles – there are three fibularis muscles namely, the longus, brevis and tertius. These muscles are found on the lateral part of the leg and working as a group, they are responsible for the plantar flexion and the eversion of the foot.
  4. Gastrocnemius – this muscle forms the curved half of the posterior leg. It is a prime mover for plantar flexion of the foot.
  5. Soleus – this muscle lies deep to gastrocnemius. It has no effect on knee movement but it is a string plantar flexor of the foot.


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