Muscular System All portions are derived from mesoderm Muscle tissue is capable of contraction, so many bodily activities are carried out by the muscles. All muscle cells are elongate, therefore they are termed fibers. Muscle fibers lie in parallel arrays with the longitudinal axis of the muscle. 3 Fiber Types exist, categorized based on their histology and physiology.

Muscle Fiber Types Skeletal = striated, voluntary, multinucleate, peripherally located nuclei. Forms bulk of the body musculature. Cardiac = striated, involuntary, uninucleate, centrally located nucleus, branching (bifurcated) fibers, intercalated discs. Found in the heart and dorsal aorta as it exits the heart. Smooth = non-striated, involuntary, single centrally located nucleus. Found in the walls of the gut, blood vessels, urinary and genital ducts.

Figure 10.2 – Skeletal Muscle

Figure 10.3 – Cardiac Muscle

N = central nucleus; arrowhead = branching of fibers; I = intercalated disc

Figure 10.4 – Smooth Muscle

Cross-section Smooth Muscle Longitudinal -section

Skeletal Muscle Fiber Subtypes Red Fibers = High concentration of myoglobin (involved in oxygen uptake from the blood), high numbers of mitochondria, aerobic, slow-twitch, fatigue-resistant White Fibers = Lower myoglobin concentration and lower numbers of mitochondria, glycolytic, fast- twitch, fatigue-rapidly Intermediate Fibers = intermediate myoglobin concentration and relatively high numbers of mitochondria, fast-twitch, oxidative-glycolytic, fatigue-resistant Different muscles have different ratios of these fiber types.

Fig 10.7 – Skeletal muscle fiber subtypes Stained for Myosin ATPase Stained for NADH dehydrogenase Fig 10.7 – Skeletal muscle fiber subtypes S = slow-twitch, red muscle fiber FG = fast-twitch, white muscle fiber FOG = fast-twitch, intermediate muscle fiber

Classification of Muscles Based on embryonic origin and innervation Epaxial (dorsal) Axial Hyaxial (ventral) Somatic Cranial (eyes & branchiomeric) Cranial (hypobranchial)     Appendicular Cardiac (Secondarily striated) Visceral Smooth

Classification of Muscles Primitive Condition in vertebrates was 2 discrete sets of musculature: Somatic = muscles of the “outer tube” of the body. Visceral = muscles connected mainly with the gut tube (= “visceral animal”) or blood vessels (heart). Somatic Musculature derived from myotome region of the somite (epimere) Innervated by somatic motor neurons. Visceral Musculature is derived from hypomere (lateral plate mesoderm) or from mesenchyme Heart forms where lateral plate mesoderm meets at the midline Innervated by visceral motor fibers of the Autonomic N.S. Somite = segmented; Hypomere = unsegmented, with coelom in the middle.

Classification of Muscles Axial (somites) Cranial Branchiomeric and eye muscles (somitomeres = modified somites anterior to ear region) Hypobranchial (somites – ventral myotomes) Appendicular (somites) Cardiac (splanchnopleure) Smooth (splanchnopleure)

Fig 10.22 – Embryonic origin of cranial muscles in a shark embryo.

General Trends in Vert Musculature Muscles become more divided in Tetrapods as movements become more complex. Increased division in musculature leads to finer, more precise movements As legs become more important, appendicular musculature increases in size and volume. Muscle shapes also change. Axial musculature is less developed in volume, but it is more subdivided.

Axial Musculature In Amphioxus and embryonic cyclostomes (ammocoetes) somites form in complete, unbroken series from front part of the head to the trunk. In vertebrates, this series is interrupted by the expansion of the braincase in the ear region. Anteriorly only 3-4 somites persist – these develop into muscles that move the eyeball (rectus muscles, oblique muscles, retractors and levators) and muscles associated with pharynx (branchiomeric musculature) Posteriorly somites form the trunk musculature and muscles of the neck region. This musculature consists of successive segments called myomeres.

Somites (myomeres) 24 h chick Amphioxus

Axial Musculature Most primitive (ancestral) condition = single myomere (derived from a single somite) per body segment. No epaxial (dorsal) - hypaxial (ventral) division This is the apparent condition for Jaymoytius (fossil ostracoderm) and for Amphioxus Cyclostomes  each myomere overlaps adjacent myomere, but still no epaxial-hypaxial separation. Jawed Fish  myomeres show complicated folding, overlapping several to many body segments. Results in easier lateral undulations as overlap increases innervation and results in better coordination Separation of epaxial (dorsal) from hypaxial (ventral) divisions by a horizontal septum Tetrapods  myomeres cover many body segments as different muscles. Virtually complete loss of segmentation in advanced Tetrapods (birds, mammals)

Evolutionary trends in axial musculature in vertebrates. Note the increasing subdivision and reduction in segmentation in the advanced Tetrapod condition.

Cranial Musculature Extrinsic eye muscles = move eye within orbits Derived from 3-4 somitomeres anterior to ear region Jaw Musculature derived from two distinct embryonic sources, each with separate innervation Hypobranchial Musculature = derived from myotome region of trunk somites Ventral tips of myotomes grow forward and into pharyx region along sides of pharyngeal arches Innervated by nerves from cervical region of spinal cord (somatomotor nerves)

Cranial Musculature Jaw Musculature, Part 2 Branchiomeric Musculature = derived from somitomeres in pharyngeal region and anterior to ear region Formerly thought to be derived embryonically from splanchnopleure (surrounding gut tube) Innervated by cranial nerves (somatomotor fibers)

Fig 10.22 – Embryonic origin of cranial muscles in a shark embryo.

Pharyngeal Musculature Primitive Fish Condition = all pharyngeal (gill) arches have the following components: Constrictors = above and below gill slit; act to change size of gill slits Levators = attached to dorsal ends of gill arches; act to raise dorsal part of arch Adductors = pull dorsal and ventral halves together Interarcuals = bend upper end of arches backward

Pharyngeal Musculature In Tetrapods, much of branchiomeric and hypobranchial musculature is no longer associated with the digestive tract (pharynx), but: Assumes other orientations and functions Facial muscles Jaw muscles Part of shoulder musculature Or becomes lost Branchiomeric muscles are all supplied via cranial nerves (somatomotor fibers). In contrast, hypobranchial musculature is innervated via spinal nerves (also somatomotor fibers).

Fig 10.29 – Branchiomeric and shoulder musculature Branchiomeric musculature is associated with gill operation in sharks, becomes associated with facial, jaw and shoulder musculature in Tetrapods.

Appendicular Musculature Associated with limbs and limb girdles Derived from general myotomic musculature of the trunk Limb musculature originates from myomeres in lower vertebrates (e.g., sharks), as paired finger-like processes extend from ventral ends of myomere to become mesenchyme of embryonic fins. Tetrapod condition: Muscle mass develops as condensations from mesenchyme of limb bud, rather than from myomere processes. Limb bud mesenchyme originates from somites, so they also have embryonic derivation from myotome. Contributions from axial and branchiomeric musculature occur in pectoral musculature.

Shark Embryo: Note the finger-like processes extending from the ventral portions of the myomere. These will enter the fin bud to form the appendicular musculature. In Tetrapods, the appendicular musculature develops from limb bud mesenchyme (originates from myotome), but also receives contributions from axial and branchiomeric musculature.

Appendicular Musculature Fish Condition = two opposing muscle masses covering dorsal and ventral surfaces from girdle to base of fin. Dorsal muscles elevate fin Ventral muscles depress fin Appendicular muscles used for steering purposes; axial musculature powers locomotion movements during swimming Forelimb and hindlimb musculature similar in fishes

Appendicular Musculature Musculature of fore- and hindlimbs differs in Tetrapods Pectoral Girdle and forelimb musculature from 4 sources that form muscular “sling” that acts to support anterior region of body Dorsal limb Ventral limb Axial (levator scapulae, rhomboideus, serratus) Branchiomeric (trapezius and mastoid groups) Pelvic girdle solidly fused to vertebral column so no muscular sling present there

Fig 10.28 – Muscular “sling” supporting anterior portion of the body in association with the pectoral girdle in Tetrapods.

Appendicular Musculature General Evolutionary Trend … Appendicular musculature is of small importance and volume in fish (operates fins), but becomes dominant in the Tetrapods, where limbs are responsible for locomotion. Not only do appendicular muscles increase in size and volume, but complexity is vastly greater (allows much finer movements).

Evolutionary trends in appendicular musculature in vertebrates. Note the increasing size and complexity of the appendicular musculature in the advanced Tetrapod condition.

Cardiac Muscle Development Heart tube forms where vitelline veins fuse into subintestinal vein, near anterior end of subintestinal vein. Dorsal regions of lateral plate mesoderm grow to meet and surround heart tube. From this the heart muscle develops. Cardiac muscle is secondarily striated and innervated by Autononic NS (Vagus nerve). Heart muscle develops from this lateral plate mesoderm surrounding heart tube. Primordial epicardium thickens  will become outer coat of heart (epicardium) and muscular layer (myocardium). Endocardial cells form tubes continuous with vitelline veins and the developing ventral aorta. Lateral plate mesoderm grows to surround heart tube. Paired endocardial tubes fuse to form a single tube  lining of the heart. Subsequent folding of the heart tube and formation of medial septa lead to formation of the four chambers of the heart.

Muscle Terminology and Function Muscles never attach to skeletal elements directly Always via tendon, ligament or aponeurosis (= flat sheet of connective tissue). Muscle generally attaches to skeletal elements at either (both) end. Origin = most stable attachment Insertion = attachment at opposite end (generally more mobile end)

Muscle Classification by Action Extensor = opens a joint Flexor = closes a joint Adductor = draws segment toward midline of body Abductor = draws segment away from midline of body Pronator (Supinator) = rotates distal part of a limb to prone (supine) position Prone = face downward, palms forward Supine = face upward, palms rotated rotated forward [upward]} Rotator = twists a limb segment Levator = raises a structure Depressor = lowers a structure Constrictor (Sphincter) = surrounds orifices to close them Dilators = opens orifices

Muscle Homologies Several criteria exist for establishing muscle homologies, although each criterion has its uncertainties. Similar function between muscles suggests homology despite differences in origins or insertions Similar nervous innervation suggests homology, even if functions differ Common embryonic pattern of development

Fig 10.19 – Criteria for muscle homologies. Similar function despite differences in origin or insertion Jaw depressor muscles have different innervations, suggesting absence of homology Limb musculature in shark and Tetrapods both derived from myotome during development, suggesting homology

Muscle Homologies – Remembering the general evolutionary trends in the different musculatures will help in understanding homologies. Axial = decreased size but increased subdivision and reduction in segmentation in advanced vertebrates Appendicular = increase in size, volume and complexity as they become muscles of locomotion Branchiomeric and Hypobranchial = lose association with gills to become facial, jaw or shoulder musculature; or are lost.