Through evolutionary history, the trend has been for muscles to become more complex. The need for this increasing complexity was the change in the type of locomotion, respiration, and feeding as vertebrates moved from their aquatic environment to a terrestrial environment. As the muscles transitioned, some remained relatively simple while others became complex to sustain the type of appendicular structure, cranial movement, and locomotion used on land.
The main function of muscles is movement. There are three types of muscles: cardiac, smooth, and skeletal. Muscles are derived from the myotomes of somites. The types of muscles we have labeled are all skeletal. Skeletal muscles are striated, voluntary, multinucleated, and neurogenic. These are composed of tonic fibers characterized by slow contraction and muscle tone. These are common in amphibians and reptiles.
Each muscle has a site of origin and insertion on the skeletal system. Many times the name of the muscle indicates the origin and insertion site. There are different ways that each muscle can be attached, including pinnate and fusiform. There are also different ways that muscles can respond to a force applied. In an isometric response, the muscle contracts but does not shorten and the tension increases. However, no movement is made. In an isotonic response, the muscle contracts and shortens and the tension is decreased; movement does occur.
Finally, muscles move in antagonistic pairs that aid in movement. These pairs are extensor/flexor, adductor/abductor, protractor/retractor, rotator/suppinator, and sphincter-dilator/constrictor.
The musculature is divided into two regions, the cranial and postcranial. The cranial consists of the muscles from the pectoral girdle forward, the epibranchial, hypobranchial, and branchiomeric. The postcranial consists of muscle from the pectoral girdle back, the axial and appendicular.
The axial muscles consist of epaxial and hypaxial muscles. The epaxial muscles are dorsal and support the body. They also move the head and tail. They extend from the skull to the tip of the tail. These muscles aid in stride length. The hypaxial muscles are on the ventral side and are dominant in tetrapods. They aid in respiration. Metamerism is the segmented appearance created by the epaxial and hypaxial muscles. The trend is for a reduction in this feature.
The appendicular muscles are muscles that are attached to thee girdles. There are two types, extrinsic and intrinsic. Extrinsic originated on the axial skeleton and inserts on the girdles or skeleton of the limbs. Intrinsic originates on the girdle as proximal muscles and inserts on distal parts.
Branchial is divided into two groups, epibranchial and hypobranchial. The epibranchial muscles are above the gills and originate from epaxial and hypaxial myotomes. The hypobranchial are below the gills. They originate from hypaxial myotomes and extend from the pectoral girdle to the lower jaw.
These are derived from the cephalic hypomeres. They consist of the first gill arch which is the mandibular muscles, the second gill arch which is the hyoid, and the third and fourth branchial.
There are three more important details that you should keep in mind when studying the muscles of Necturus. First, since the head is a specific region, a muscular sling is needed to keep the girdles in place. The muscles that are involved in joining the girdles to the trunk are the extrinsic appendicular muscles. Second, the branchiomeric muscles of the skull are primitive with less room to expand, which reduces bite strength. Finally, there is a trend for the muscles of the shoulder to become larger and spread more over the back. Each change that occurred was enabling the species for life on land.
-- Visceral Arch I
-- Visceral Arch II
-- Visceral Arch III - VII