Muscular System

• Function– Locomotion– Posture– Protection– Heat production

• Vertebrate muscles: • striated vs. smooth • voluntary vs. involuntary • skeletal vs. non-skeletal •

Skeletal muscle (left) & Smooth muscle (right)

Muscle Classification

• Skeletal Muscle– Attached to skeleton– Striated– Muscle fiber = Muscle cell• Multinucleate• Myofibrils are striated cylinders within myofiber

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

• 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

•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

Skeletal Muscle

• Myofilaments are proteins that result in contraction within the myofibrils–Actin – thin & has a receptor site for myosin–Myosin – thick & has a receptor site for

actin and ATP–Contraction – Myosin heads attach to actin

and with ATP perform a Power Stroke

Skeletal Muscle

•Sarcomere•The distance from Z line to Z line

•The basic unit of contraction•Sarcomere gets smaller as Power Stroke

occurs

Key Points

• Why is the sarcomere the functional unit of contraction?

• Why does the power stroke result in contraction?

• What would happen to contraction if you ran out of ATP?

Motor Neurons

• Skeletal muscle cannot contract without stimulation from a motor neuron

• Motor Unit = The motor neuron plus the myofiber(s) it innervates

Key Points

• Why would a spinal cord injury result in paralysis?

Somatic Muscles

• All of the body’s skeletal muscles except the branchiomeric muscles

• Voluntary• Body wall & Appendage muscles– Trunk and Tail– Hypobranchial– Tongue– Extrinsic Eyeball

Somatic Muscles

• Myotome derivatives primarily• Some from hypomere

Key Points

• What is a myotome?

Somatic Muscles

• Orient the body in the environment

Somatic Muscles

• Red Fibers– More blood supply for aerobic metabolism– Myoglobin for oxygen storage– Fatigue resistant– Fish for cruising long distances, tetrapods for

posture

Somatic Muscles

• White fibers– Less blood supply; geared for anaerobic

metabolism– Fatiguable– Fish for spurts of swimming– Tetrapods for sprints

Key Points

• Why is the breast meat of the goose dark, but the breast meat of the chicken is white?

Cardiac Muscle

• Striated with intercalated disks• Involuntary• Lateral plate mesoderm (hypomere) in origin

Smooth Muscle

• Involuntary• Lateral plate mesoderm in origin• Regulates internal environment• Innervated by Autonomic Nervous System• Found in the wall of tubes and hollow organs• Intrinsic Eye muscles• Erectors of feathers and hairs

Key Points

• Besides those mentioned, give a specific example of where might you find smooth muscle?

Gross features of skeletal muscle

• Origin, insertion• Tendon• Aponeurosis• Fascia

Muscle shapes

Skeletal Muscle Actions

• Flex/Extend• Adduct/Abduct• Levator/Depressor• Protract/Retract• Constrictor/Dilator• Rotator

Skeletal Muscle Actions

• Supinator/Pronator• Tensor (taut)

Skeletal Muscle Actions

• Agonist – primary mover• Antagonist – opposes primary mover• Synergist – helps primary mover

Development & Phylogeny

• Position• Embryology• Nerve supply

Development

• Dorsal Mesoderm – Epimere – Somite–Myotome– Sclerotome & Dermatome

• Lateral plate Mesoderm – Hypomere– Somatic – body wall muscles– Splanchnic – smooth muscle of viscera

AXIAL MUSCLES

• Trunk• Tail• Hypobranchial• Tongue• Extrinsic Eye

• 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

•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

• 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

• Epaxials of tetrapods: • lie along vertebral column dorsal to transverse

processes & lateral to neural arches • extend from base of the skull to tip of the tail

•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)

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)

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

Branchiomeric muscles: 1 - associated with the pharyngeal arches 2 - series of skeletal & smooth muscles 3 - adductors, constrictors, & levators operate jaws plus successive gill arches

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

Axial Muscles

•Metamerism as in myomeres

Axial Muscles

•Agnathans•Simple

•Segments (myomeres)•Myotome derivatives

Axial Muscles – Jawed Fish

• Horizontal or Lateral Septum• Epaxial Muscles– From myotomes in embryology– Innervated from dorsal rami of spinal nerves– Extend spine & some lateral bending– Extrinsic eye muscles (innervated by cranial

nerves)– Epibranchial muscles

Axial Muscles – Jawed Fish

•Hypaxial Muscles•From Myotomes

–Innervated by ventral rami of spinal nerves–Ventroflex and lateral bending

Hypaxial Muscles – Jawed fish

•Hypobranchial muscles•Located on floor of pharynx, pectoral girdle to

jaw•Are hypaxial muscles that migrated forward

•Function in respiration & feeding•E.g. Coracomandibularis, Coracohyoid

Axial Muscles - Tetrapods

•Epaxial trunk muscles•Dorsal muscles from skull to tail

•Dorsalis trunci in amphibians•Longissimus – long dominant spine extensor in

amniotes•Iliocostalis – most lateral epaxial spine muscle

important in reptiles

Epaxial Muscles

•See Vertebrate Muscles page

Epaxial Muscles in Tetrapods

• Trends • Decreased except in neck• Fewer myosepta

Axial Muscles - Tetrapods

• Hypaxial Muscles• Tend to form sling-like sheets• Lateral muscles support & compress body wall• Obliques• Transversus• Intercostals in amniotes only

Hypaxial Muscles - Tetrapods

• Rectus abdominis – ventroflexes and compresses abdomen

• Diaphragm – unique to mammals for breathing

Hypaxial Muscles in tetrapods

•See Vertebrate Muscles page

Hypobranchial and Tongue Muscles - tetrapods

• Function – stabilizes hyoid and larynx• E.g. geniohyoid, sternohyoid, sternothyroid,

thyrohyoid

Hypobranchial & Tongue muscles in tetrapods

•Tongue muscles•Lingu-; Gloss-•Anchors to hyoid•E.g. lingualis, styloglossus

Hypobranchial & Tongue muscles in Tetrapods

• See Vertebrate Muscles page

Extrinsic Eye muscles in tetrapods

• Voluntary• Obliques – rotates eye along its transverse

axis• Rectus – up, down, left, right• Retractor in some

Extrinsic Eye muscles

• See Vertebrate Muscles Page