Vertebrate muscles:

• striated vs. smooth
• voluntary vs. involuntary
• skeletal vs. non-skeletal

Skeletal muscle (left) & Smooth muscle (right)

Skeletal muscle = muscles attached to the skeleton that are striated & voluntary

Non-skeletal muscle = muscles not attached to the skeleton; most are smooth & involuntary
 

Vertebrate Muscles:

1 - Skeletal, striated, voluntary muscles
• axial
• body wall & tail
• hypobranchial & tongue
• extrinsic eyeball muscles
• appendicular
• branchiomeric (homologous to the branchial/ pharyngeal muscles from fishes to mammals, striated muscles, innervated by cranial nerves)
• integumentary
2 - Non-skeletal, smooth, chiefly involuntary muscles
• muscles of tubes, vessels, & hollow organs
• intrinsic eyeball muscles
• erectors of feathers & hair
3 - Cardiac muscle

4 - Electric organs

Skeletal muscles have muscular & tendinous portions:

• Muscle - consists of skeletal muscle cells (which, in turn, consist of myofibrils and myofilaments)
• Tendons - extensions of a muscle's tough connective tissue sheath (fascia & epimysium) that anchor a muscle to its origin & insertion

• Origin = site of attachment that is relatively fixed
• Insertion = site of attachment that is normally displaced by contraction of the muscle

Used with permission of John W. Kimball

Names of skeletal muscles are based on:

• direction of fibers (e.g., oblique)
• location or position (e.g., superficial)
• number of divisions (e.g., triceps)
• shape (e.g., deltoid)
• origin and/or insertion (e.g., iliocostalis)
• action (e.g., levator scapulae)
• size (e.g., major)
• or some combination of these

Homologies:

Human muscles were named several hundred years ago, & many of these names are still used. Based on similarities of origins & insertions, these names were subsequently used for the, apparently, corresponding muscles of other vertebrates. However, origins & insertions are not reliable criteria for determining homology because natural selection has sometimes favored 'shifts' in muscle position. More reliable criteria for determining homologies are:
• embryonic origin
• nerve supply

Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html

Axial Muscles:

• include the skeletal muscles of the trunk & tail
• extend forward beneath the pharynx as hypobranchial muscles & muscles of the tongue
• are present in orbits as extrinsic eyeball muscles (check slide 27 in this powerpoint presentation)
• are metameric (most evident in fish and aquatic amphibians where the axial muscles are used in locomotion; in other tetrapods, metamerism is obscured due to presence of paired appendages responsible for locomotion on land)
• are segmental because of their embryonic origin; arise from segmental mesodermal somites

Source: http://www.uta.edu/biology/restricted/3452mus.htm

Axial musculature of an aquatic salamander, Necturus maculosus. The layers of lateral hypaxial musculature are exposed from superficial to deep in the cranial to caudal direction. The number of external oblique layers varies between one and two in this species (the figured specimen exhibits two). Abbreviations: oes, M. obliquus externus superficialis; oep, M. obliquus externus profundus; oi, M. obliquus internus; ta, M. transversus abdominis (Brainerd and Simons 2000).

Trunk & tail muscles of fish:

Axial musculature consists of a series of segments (myomeres) separated by myosepta

• Myosepta serve as origins & insertions for segmented muscles
• Myomeres are divided into dorsal & ventral masses by a horizontal septum that extends between the transverse processes of the vertebrae:
• Epaxials = above the septum
• Hypaxials = below the septum
• Middorsal & midventral septa separate the myomeres of the 2 sides of the body. The midventral septum is called the LINEA ALBA.

Trunk & tail muscles of tetrapods

• Tetrapods, like fish, have epaxial & hypaxial masses, & these retain some evidence of metamerism even in the highest tetrapods.
• Modifications:
1 - epaxials are elongated bundles that extend through many body segments & that are located below the expanded appendicular muscles required to operate the limbs

2 - hypaxials of the abdomen have no myosepta & form broad sheets of muscle

3 - hypaxials are oriented into oblique, rectus, & transverse bundles

• lie along vertebral column dorsal to transverse processes & lateral to neural arches
• extend from base of the skull to tip of the tail
• Urodeles & some lizards - epaxials are obviously metameric & are referred to as the dorsalis trunci (see salamander above)
• Higher tetrapods - superficial epaxial bundles form long muscles that extend over many body segments; deep bundles are still segmented
Longest bundles:
1- longissimus group
• lies on transverse processes of vertebrae; includes the longest epaxial bundles
• subdivisions include:
• longissimus dorsi
• longissimus cervicis
• longissimus capitis
2 - iliocostalis group
• lateral to longissimus & spinalis
• arises on ilium & inserts on dorsal ends of ribs or uncinate processes
3 - spinalis group
• lies close to neural arches
• connects spinous processes or transverse processes with those several vertebrae anteriorly
Shortest bundles - intervertebrals
• remain segmented
• connect processes (spinous, transverse, & zygapophyses) of adjacent vertebrae

Hypaxials of tetrapods:

1 - Muscles of lateral body wall:
• oblique (external & internal), transverse, & rectus muscles
2 - Muscles that form longitudinal bands in roof of body cavity (subvertebral muscles)

Oblique & transverse muscles:

• Early amphibians & reptiles
• ribs developed in myosepta along entire length of the trunk
• urodeles still have myosepta the length of the trunk, but ribs no longer form in all of them
• Modern amniotes
• myosepta & ribs are restricted to the thorax (so abdominal muscles are not obviously segmented)
• hypaxials form 3 layers: external oblique, internal oblique, & transverse (in the thorax region: external & internal intercostals, which play an important role in respiration, & transverse muscle)

1 - External intercostal muscles, 2 - Internal intercostal muscles, 3 - Ribs, 4 - Intercartilaginous muscles,
5 - Sternum, 6 - Subcostal muscles, & 7 - Vertebral column
Source: http://www.sci.port.ac.uk/rad/anatomy/07/006.htm

Rectus muscles:

• weakly developed in most fish; 'stronger' in tetrapods
• support ventral body wall & aid in arching the back
• in mammals - rectus abdominis (typically extends from the anterior end of the sternum to the pelvic girdle)

Subvertebral muscles:

• underneath & against transverse processes of vertebrae
• includes the psoas &  iliacus in the lumbar region & the longus colli in the neck
• less developed in the thorax & none in the tail

FUNCTION OF EPAXIALS OF TETRAPODS:

1 - short epaxials perform same function as in fish (side-to-side movements of vertebral column)

2 - short & long bundles arch & support the vertebral column

3 - most anterior bundles = attach to & move the skull

FUNCTION OF HYPAXIALS OF TETRAPODS:

1 - Aquatic urodeles = used chiefly for swimming

 Diagram showing the onset and duration of lateral hypaxial muscle EMG activity relative to maximum body bending in two Ambystoma tigrinum during swimming. Values are means ± S.D. for alpha-burst (filled bars) and beta-burst (open bars) onset and duration times. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis. *Denotes the side of the body on which the electrodes were implanted (Bennett et al. 2001).

2 - Terrestrial urodeles = assist in locomotion

3 - Other tetrapods = reduced in volume compared to fish (because of shift in mode of locomotion); now support contents of abdomen, assist in respiration (especially intercostal muscles), & assist epaxials in bending vertebral column (rectus muscles)

Hypobranchial & tongue muscles:

• Fish
• hypobranchials extend forward from pectoral girdle & insert on mandible, hyoid, & gill cartilages
• hypobranchials strengthen floor of pharynx & assist branchiomeric muscles in elevating floor of mouth, lowering jaw, & extending gill pouches
• Tetrapods
• hypobranchials stabilize & move hyoid apparatus & larynx
• the tongue of amniotes is a 'sac' anchored to hyoid skeleton & filled with hypobranchial muscle

The neck muscles ending in "hyoid" are associated with the hyoid apparatus, whereas those
beginning or ending with "thyro" are attached to the larynx. These muscles are hypobranchia
and function in movement of the hyoid apparatus, larynx and/or floor of the mouth.

Appendicular muscles - move fins or limbs

• Extrinsic - originate on axial skeleton or fascia or trunk & insert on girdles or limbs
• Intrinsic - originate on girdle or proximal skeletal elements of appendage & insert on more distal elements

 
Fish - appendicular muscles serve mostly as stabilizers; intrinsic muscles are limited in number & undifferentiated

Tetrapods - appendicular muscles are much more complicated than in fish
• greater leverage required for locomotion on land
• jointed appendages (as opposed to fins) require complex muscles

Extrinsic appendicular musculature

• Dorsal group of the forelimbs, e.g., trapezius and latissimus dorsi, arise on:

1 - fascia of trunk in lower tetrapods

2 - skull, vertebral column, & ribs to a point well behind the scapula in higher tetrapods & converge

on the girdle & limb

• Ventral group, e.g., pectoralis, arises on sternum & coracoid, & converge on limb

RESULT = pectoral girdle & limb are joined to trunk by extrinsic appendicular muscles as illustrated in this diagram:

The 'muscular sling' of tetrapods. Appendicular muscles of the forelimbs suspend the anterior body of tetrapods from the shoulders. Some of these muscles are axial muscles (rhomboideus & serratus ventralis), some are branchial muscles (trapezius), & some arise from the forelimb musculature itself (pectoralis).

The pelvic girdle requires no such muscular anchoring because it is attached directly to the vertebral column. As a result, the volume of extrinsic muscle is relatively small in posterior limbs.

Extrinsic appendicular muscles:

1 - most develop from hypaxial blastemas in the body wall

2 - referred to as secondary appendicular muscles because it was not their original function to operate appendages

3 - chief extrinsic muscles of forelimbs of tetrapods include: scapular deltoid, latissimus dorsi, rhomboideus, serratus ventralis, & pectorals

Intrinsic appendicular muscles:

1 - form from blastemas within the limb bud

2 - called primary appendicular muscles

Appendicular muscles:

Amphibians - much more complex than in fish

Reptiles - more numerous & diverse than in amphibians; better support of body & increased mobility of distal segments of the limbs

Birds - intrinsic musculature is reduced; pectoralis (downstroke muscle) & supracoracoideus (upstroke muscle) are enlarged



Mammals - similar to reptiles but more diverse

Branchiomeric muscles:

1 - associated with the pharyngeal arches

2 - series of skeletal & smooth muscles

3 - adductors, constrictors, & levators operate jaws plus successive gill arches

Muscles of the Mandibular Arch:
• Squalus & other fish - operate the jaws (adductor mandibulae & intermandibularis)
• Tetrapods
• muscles of 1st arch still operate jaws
• adductors of mandible:
• masseter & temporalis (see diagram below)
• pterygoid
• digastric


Muscles of the Hyoid Arch:

• move hyoid arch
• aid in hearing (stapedial muscle)
• assist in moving lower jaw (e.g., digastric)


Muscles of 3rd & successive arches:

• Squalus - constrictors above & below gill chambers plus levators (including the cucullaris) that compress & expand the gill pouches
• Bony fish - muscles reduced; operculum plays important role in respiration
• Tetrapods - muscles further reduced; primary muscles include:
• stylopharyngeus (Arch III) - used for swallowing
• intrinsic muscles of the larynx or 'voicebox' (remaining arches)
• cucullaris - gives rise to trapezius, cleidomastoid, & sternocleidomastoid muscles of amniotes

Integumentary muscles:

Extrinsic integumentary muscles (e.g., platysma)
• originate (usually) on the skeleton & insert on the underside of the dermis
• striated
• move skin of amniotes
Intrinsic integumentary muscles (arrector pili muscles)
• entirely within the dermis
• found in birds & mammals
• mostly smooth muscles

Electric organs:

1 - consist of a number of electric discs (up to 20,000) piled in either vertical or horizontal columns

2 - each disc (electroplax) is a large coin-shaped cell

3 - evolved several times in a variety of fish (good example of convergent evolution)

Functions of electric organs:

1 - defense

2 - communication

3 - locating prey (electrolocation)

Related links:

Muscles and Electric Organs

Musculature of Chordate Taxa

Literature cited:

Bennett, W. O., R. S. Simons, and E. L. Brainerd. 2001. Twisting and Bending: The Functional Role of Salamander Lateral Hypaxial Musculature During Locomotion. Journal of Experimental Biology 204:1979-1989.

Brainerd, E. L. and R. S. Simons. 2000. Morphology and Function of Lateral Hypaxial Musculature in Salamanders. American Zoologist 2000 40: 77-86

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