5.3: Physiology of the Muscular SystemMuscle tissue is found in three distinct types in the body: skeletal, smooth, and cardiac. Skeletalmuscle is under voluntary control.Voluntary controlmeans a conscious decision is made to movethis type of muscle.Skeletal muscletissue allows for conscious movement of the body and limbs.Smooth muscle and cardiac muscle are underinvoluntary control, meaning contraction of thismuscle happens without a conscious decision.Smooth muscleis found within the internal organs ofthe body, such as the digestive tract and blood vessels.Cardiac muscleis only found within theheart. Under the microscope, skeletal and cardiac muscle appear to bestriatedor striped inappearance, while smooth muscle is free of striations.06:06
Figure 5.34 Histological view of the three different types of muscle tissue:Cardiac (top),skeletal (middle), and smooth (bottom).Figure 5.35 Histology of skeletal muscle tissue.Note the visible striations (striped appearance).
Figure 5.36 Histology of smooth muscle tissue.Note there are not visible striations.Skeletal muscles, which make up over 40% of the body's weight, are attached to the skeleton bytendons, made of fibrous connective tissue.Tendonsconnect muscle to bone, whileligamentsconnect bone tissue to bone.When muscles contract, they become shorter. Muscles can only pull; they cannot push. Skeletalmuscles must work in antagonistic pairs because muscles are only able to pull in the direction of theirfiber orientation. If one muscle of anantagonistic pairbends the joint and brings the limb toward thebody (theflexor), the other one straightens the joint and extends the limb (theextensor), as shownin the figure below. The following is a review of action terminology (see Module 1) with examples ofantagonistic pairs found within the muscular system:Action Terms -Body MovementFlexion- closing of a joint,“bending”Extension- opening of a joint,“straightening”Antagonistic pair example:Flexor -biceps brachiiExtensor -triceps brachii