Muscular System – Classifications, Major Muscles of the Body

Muscular System

  • Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called myofibrils. Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion. Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body. In general, muscles play an active role in all the movements of the body. Muscles are of three types, skeletal, smooth, and cardiac.

Classifications of Muscular System

  • Skeletal muscle: Skeletal muscle tissue is closely attached to skeletal bones. In a typical muscle such as the biceps, striated (striped) skeletal muscle fibres are bundled together in a parallel fashion. A sheath of tough connective tissue encloses several bundles of muscle fibres.
  • Smooth muscle: The smooth muscle fibres taper at both ends (fusiform) and do not show striations. Cell junctions hold them together and they are bundled together in a connective tissue sheath. The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue. Smooth muscles are ‘involuntary’ as their functioning cannot be directly controlled. We usually are not able to make it contract merely by thinking about it as we can do with skeletal muscles.
  • Cardiac muscle: Cardiac muscle tissue is a contractile tissue present only in the heart. Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together. Communication junctions (intercalated discs) at some fusion points allow the cells to contract as a unit, i.e., when one cell receives a signal to contract, its neighbours are also stimulated to contract.
Muscular System

Major Muscles of the Muscular System

  • The name of the muscle may also be helpful, and again, many of the terms are ones you have already learned. Some examples: “abdominis” refers to an abdominal muscle, “femoris” to a thigh muscle, “brachii” to a muscle of the upper arm, “oculi” to an eye muscle, and so on. Other parts of muscle names may be words such as “longus” or “maximus” that tell you about size, or “flexor” that tells you about function.
Muscular System

Overview of the Superficial Body Musculature: Red is muscle; white is connective tissue, such as tendons, aponeuroses, and retinacula. Image Source: openstax.org

Muscular System

Overview of the Superficial Body Musculature

Head and Neck Muscular System

  • Three general groups of muscles are found in the head and neck: those that move the head or neck, the muscles of facial expression, and the muscles for chewing. The muscles that turn or bend the head, such as the sternocleidomastoids (flexion) and the pair of splenius capitis muscles (extension), are anchored to the skull and to the clavicle and sternum anteriorly or the vertebrae posteriorly. The muscles for smiling or frowning or raising our eyebrows in disbelief are anchored to the bones of the head or to the under surface of the skin of the face. The masseter is an important chewing muscle in that it raises the mandible (closes the jaw).
Muscular System

Muscles of Facial Expression and Mastication: (a) Lateral view of facial muscles as seen from the right side. (b) Drawing and photograph of the anterior view of facial muscles. 

Trunk Muscular System

  • The muscles of the trunk cannot be described with one or two general functions. Some form the wall of the trunk and bend the trunk, such as the rectus abdominis (flexion) and the sacrospinalis group. The trapezius (both together form the shape of a trapezoid) is a large muscle that can raise (shrug) the shoulder or pull it back, and can help extend the head. Other muscles found on the trunk help move the arm at the shoulder. The pectoralis major is a large muscle of the chest that pulls the arm across the chest (flexion and adduction). 
  • On the posterior side of the trunk, the latissimus dorsi pulls the arm downward and behind the back (extension and adduction). These muscles have their origins on the bones of the trunk, the sternum, the or vertebrae, which are strong, stable anchors. Another set of muscles forms the pelvic floor, where the muscles support the pelvic organs and assist with urination and defecation. Yet another category is the muscles that are concerned with breathing. These are the intercostal muscles between the ribs and the diaphragm that separates the thoracic and abdominal cavities.
Muscular System

Muscles of the Thorax: (a) Anterior view shows a few selected intercostal muscles and the diaphragm. (b) Lateral view shows the external and internal intercostals.

Shoulder and Arm Muscular System

  • The triangular deltoid muscle covers the point of the shoulder like a cap, and can pull the humerus to the side (abduction), forward (flexion), or backward (extension). You already know the functions of the biceps brachii and triceps brachii, the muscles that form the bulk of the upper arm. Other muscles partially in the upper arm help bend the elbow (flexion). The muscles that form the bulk of the forearm are the flexors and extensors of the hand and fingers. You can demonstrate this yourself by clasping the middle of your right forearm with your left hand, then moving your right hand at the wrist and closing and opening a fist; you can both feel and see the hand and finger muscles at work.

Muscles of the Shoulder: (a) Posterior view of the neck and upper shoulder. The left side shows the superficial muscles. On the right, the superficial muscles are removed to show the deep muscles. (b) Posterior view of the thoracic region, with the trapezius and deltoid muscles removed. (c) Anterior view of the thoracic region.

Hip and Leg Muscular System

  • The hip muscles that move the thigh are anchored to the pelvic bone and cross the hip joint to the femur. Among these are the gluteus maximus (extension), gluteus medius (abduction), and iliopsoas (flexion). The muscles that form the thigh include the quadri-ceps group anteriorly and the hamstring group posteriorly. For most people, the quadriceps is stronger than the hamstrings, which is why athletes more often have a “pulled hamstring” rather than a “pulled quadriceps.” Movement of the knee joint depends on thigh muscles and lower leg muscles. Movement of the foot depends on lower leg muscles such as the gastrocnemius (dorsiflexion or flexion) and the tibialis anterior (plantar flexion or extension).

Superficial Muscles of the Leg: (a–c) Anterior, posterior, and lateral views. (d) Surface anatomy.

Muscles of the Hip and Thigh: (a) Anterior view. The vastus intermedius is labeled to allow for a complete listing of the quadriceps femoris muscles, but the muscle lies deep to the rectus femoris and cannot be seen in the figure. (b) Posterior view of the hip muscles. (c) Posterior view of the thigh muscles.

Further Readings