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Lecture 12 MusclesKK Chapter 10 (with bits from Chapter 9), H&G Chapter 10

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Muscle Functions Locomotion - Muscles cause movement by actively

shortening. In skeletal muscles this results in bendingof the trunk or of a joint in an appendage.Lengthening is passive, so muscles generally work inopposition.

Tonus - muscles determine posture by resisting theforce of gravity.

Involuntary internal movements, e.g., heart beat, gutcontractions.

Generation of heat (endotherns) or electricity (insome fishes).

Body form - muscles along with the skeleton largelydetermine the outline of the body

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Skeletal (from myotomes and mesenchyme)- Axial (eyeball, trunk, tail)

- Appendicular (limbs)

- Branchial (pharynx, jaws)

- Hypobranchial (throat, tongue)

Visceral (from splanchnic hypomere)

- Heart

- Smooth muscles of the skin, gut, organs

General Categories of Muscles

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Origin of

Muscles

KK 10.20, H&G 10.7

Skeletal muscles arise

from the myotomes and

from the somatic (outer)

layer of the hypomere via

mesenchyme.

Visceral muscles arise

from the splanchnic

(inner) layer of the

hypomere.

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Structure of Skeletal MusclesConnective tissue

(tendons) attaches

muscle to bone,

surrounds the muscle

(epimysium), and

surrounds the fascicles

of muscle fibres or

cells (perimysium).

Individual fibres =

muscle cells.

Myofibrils are

subcellular.

KK 10.2, H&G 10.2

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Striated Muscle Cells

Skeletal muscle contains striatedmuscle cells. They are striated

because myosin molecules in

adjacent myofibrils are aligned.

These cells are large, up to several

cm long, multinucleate, and haveperipheral nuclei. Their contractions

are fast and voluntary.

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Cardiac Muscle Cells

Cardiac muscle cells arebranched and striated, withintercalated disks betweenthe cells. Their rate ofcontraction is fast and theirrhythm is controlled by theautonomic nervous system.The cells are inherentlycontractile.

KK 10.3, H&G 10.1

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Smooth Muscle Cells

Smooth muscle cells aresmall (

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Muscle Tissue Types

Smooth Muscle

Cardiac Muscle

Skeletal Muscle

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Types of Muscle Fibres in Skeletal Muscle

Muscle colour varies, from white to deep purply red/brown. This variation islargely caused by the amount of myoglobin, an iron-containing pigment

which stores oxygen. This variation can be seen within species (e.g.,trout, turkey) as well as among species.

Tonic Fibres - slow contraction, unable to propogate stimulus. These providetonus in appendages of non-mammals.

Twitch Fibres - contract quickly, briefly (unless repeatedly stimulated),entirely. The stimulus is propagated along their entire length.

Within the twitch fibres, there are slow-twitch fibres that contract slowly, but

fatigue slowly. They have high myoglobin content (red), and performslow or isotonic contractions in mammals. Fast-twitch fibres can bewhite or red, depending on whether they are required for sustainedaction.

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KK10.24,

H&G 10.8

Embryonic myotomes give rise directly to

the axial musculature (myomeres) of fish.Myomeres are separated by myosepta, and

separated into epaxial and hypaxial

components by the horizontal septum.

Extrinsic eye muscles are part of the axial

musculature.

Axial Musculature of Fish

KK 10.23

Shark

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The Other Skeletal Muscles

H&G 10.10, 10.11

KK 10.29a

branchial

hypobranchial axial

appendicular

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Fish swim with side-to-

side undulations using the

trunk musculature. The

same motions could havebeen used for terrestrial

locomotion during the

transition to land

locomotion.

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Early tetrapods

also use side-

to-side

undulations

using the trunk

musculature

during

locomotion aswell as

stepping

motions of the

limbs.KK 8.25

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Changes to Trunk

Muscles in Tetrapods

Tetrapod trunk muscles are reduced and

more dorso-ventral in orientation. Epaxial

muscles are dorsal in position, and

hypaxial muscles are lateral and ventral.

Tonus in dorsal muscles counteracts the

tendency of the trunk to sag. There are

muscles under the ribs, which is never the

case in fish.

teleost

salamander

lizard

KK 10.26

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In many mammals that are good runners, movement of the trunk is important,

as it is in fish. But now the movement of the trunk is dorso-ventral rather

than lateral.

KK 9.42, H&G 24.3

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Cetacea (whales and dolphins), descendants of terrestrial running mammals

who returned to the water, swim with dorso-ventral undulations of the body.

KK 9.30, H&G 24.3

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The giraffe is an extreme

example of a common trait of

mammals that are good

runners. The distal part of thelimb is slender with little in the

way of muscles. Movement of

the distal parts of limbs is by

muscles far away, attached by

long tendons. Birds are similar.

KK 10.5

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Branchial and Hypobranchial Muscles

These operate the gills and

jaws in fishes in fish.

KK 10.37

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Facial muscles in mammalsIn mammals, the

branchial musculaturenot only operates the

jaws, but also assists in

communication.