Muscles

MusclesMusclesComparative Vertebrate AnatomyComparative Vertebrate Anatomy

Highlight on Human MusclesHighlight on Human Muscles

Lectured by Bien Nillos, MDLectured by Bien Nillos, MD

from Latin from Latin musculus,musculus, diminutive diminutive of of musmus "mouse" "mouse"

a contractile tissue of animals and is derived from a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move Muscle cells contain contractile filaments that move past each other and change the size of the cell. They past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth are classified as skeletal, cardiac, or smooth muscles. Their function is to produce force and cause muscles. Their function is to produce force and cause motion. Muscles can cause either locomotion of the motion. Muscles can cause either locomotion of the organism itself or movement of internal organs. organism itself or movement of internal organs.

►All muscles derive from paraxial mesodermAll muscles derive from paraxial mesoderm►The paraxial mesoderm is divided along The paraxial mesoderm is divided along

the embryo's length into somites, the embryo's length into somites, corresponding to the segmentation of the corresponding to the segmentation of the body (most obviously seen in the vertebral body (most obviously seen in the vertebral column)column)

►Each somite has 3 divisions, sclerotome Each somite has 3 divisions, sclerotome (which forms vertebrae), dermatome (which forms vertebrae), dermatome (which forms skin), and myotome (which (which forms skin), and myotome (which forms muscle)forms muscle)

► The myotome is divided into two sections, the The myotome is divided into two sections, the epimere and hypomere, which form epaxial and epimere and hypomere, which form epaxial and hypaxial muscles, respectivelyhypaxial muscles, respectively

► Epaxial muscles in humans are only the erector Epaxial muscles in humans are only the erector spinae and small intervertebral muscles, and spinae and small intervertebral muscles, and are innervated by the dorsal rami of the spinal are innervated by the dorsal rami of the spinal nervesnerves

► All other muscles, including limb muscles, are All other muscles, including limb muscles, are hypaxial muscles, formed from the hypomere, hypaxial muscles, formed from the hypomere, and inervated by the ventral rami of the spinal and inervated by the ventral rami of the spinal nervesnerves

Types of MusclesTypes of Muscles

Skeletal MuscleSkeletal Muscle

► is anchored by tendons (or by aponeuroses at a is anchored by tendons (or by aponeuroses at a few places) to bone and is used to effect few places) to bone and is used to effect skeletal movement such as locomotion and in skeletal movement such as locomotion and in maintaining posture. Though this postural maintaining posture. Though this postural control is generally maintained as a control is generally maintained as a subconscious reflex, the muscles responsible subconscious reflex, the muscles responsible react to conscious control like non-postural react to conscious control like non-postural muscles. An average adult male is made up of muscles. An average adult male is made up of 42% of skeletal muscle and an average adult 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of female is made up of 36% (as a percentage of body mass)body mass)

►Smooth muscle or "involuntary muscle" Smooth muscle or "involuntary muscle" is found within the walls of organs and is found within the walls of organs and structures such as the esophagus, structures such as the esophagus, stomach, intestines, bronchi, uterus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and urethra, bladder, blood vessels, and the arrector pili in the skin (in which it the arrector pili in the skin (in which it controls erection of body hair). Unlike controls erection of body hair). Unlike skeletal muscle, smooth muscle is not skeletal muscle, smooth muscle is not under conscious control.under conscious control.

► Cardiac muscle is also an "involuntary muscle" Cardiac muscle is also an "involuntary muscle" but is more akin in structure to skeletal but is more akin in structure to skeletal muscle, and is found only in the heart.muscle, and is found only in the heart.

► Cardiac and skeletal muscles are "striated" in Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed that they contain sarcomeres and are packed into highly regular arrangements of bundles; into highly regular arrangements of bundles; smooth muscle has neither. While skeletal smooth muscle has neither. While skeletal muscles are arranged in regular, parallel muscles are arranged in regular, parallel bundles, cardiac muscle connects at bundles, cardiac muscle connects at branching, irregular angles (called branching, irregular angles (called intercalated discs). intercalated discs).

Skeletal Muscle, TypesSkeletal Muscle, Types

►Type I, slow oxidative, slow twitch, or Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries "red" muscle is dense with capillaries and is rich in mitochondria and and is rich in mitochondria and myoglobin, giving the muscle tissue its myoglobin, giving the muscle tissue its characteristic red color. It can carry characteristic red color. It can carry more oxygen and sustain aerobic more oxygen and sustain aerobic activity.activity.

► Type II, fast twitch muscle, has three major kinds Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speedthat are, in order of increasing contractile speed Type IIa, which, like slow muscle, is aerobic, rich in Type IIa, which, like slow muscle, is aerobic, rich in

mitochondria and capillaries and appears red.mitochondria and capillaries and appears red. Type IIx (also known as type IId), which is less dense Type IIx (also known as type IId), which is less dense

in mitochondria and myoglobin. This is the fastest in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful activity before muscle contraction becomes painful

Type IIb, which is anaerobic, glycolytic, "white" muscle Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of major fast muscle type, explaining the pale color of their flesh.their flesh.

►A muscle is a bundle of many cells called A muscle is a bundle of many cells called fibers. You can think of muscle fibers as fibers. You can think of muscle fibers as long cylinders, and compared to other long cylinders, and compared to other cells in your body, muscle fibers are quite cells in your body, muscle fibers are quite big. They are from about 1 to 40 microns big. They are from about 1 to 40 microns long and 10 to 100 microns in diameter. long and 10 to 100 microns in diameter. For comparison, a strand of hair is about For comparison, a strand of hair is about 100 microns in diameter, and a typical 100 microns in diameter, and a typical cell in your body is about 10 microns in cell in your body is about 10 microns in diameter.diameter.

►Origin = site of attachment that is Origin = site of attachment that is relatively fixedrelatively fixed

► Insertion = site of attachment that is Insertion = site of attachment that is normally displaced by contraction of normally displaced by contraction of the musclethe muscle

►Names of skeletal muscles are based Names of skeletal muscles are based on: on: direction of fibers (e.g., oblique)direction of fibers (e.g., oblique) location or position (e.g., superficial)location or position (e.g., superficial) number of divisions (e.g., triceps)number of divisions (e.g., triceps) shape (e.g., deltoid)shape (e.g., deltoid) origin and/or insertion (e.g., iliocostalis)origin and/or insertion (e.g., iliocostalis) action (e.g., levator scapulae)action (e.g., levator scapulae) size (e.g., major)size (e.g., major) or some combination of theseor some combination of these

Axial MusclesAxial Muscles

► include the skeletal muscles of the trunk & tailinclude the skeletal muscles of the trunk & tail► extend forward beneath the pharynx as extend forward beneath the pharynx as

hypobranchial muscles & muscles of the tonguehypobranchial muscles & muscles of the tongue► are present in orbits as extrinsic eyeball muscles are present in orbits as extrinsic eyeball muscles

(check slide 27 in this powerpoint presentation)(check slide 27 in this powerpoint presentation)► are metameric (most evident in fish and aquatic are metameric (most evident in fish and aquatic

amphibians where the axial muscles are used in amphibians where the axial muscles are used in locomotion; in other tetrapods, metamerism is locomotion; in other tetrapods, metamerism is obscured due to presence of paired appendages obscured due to presence of paired appendages responsible for locomotion on land)responsible for locomotion on land)

► are segmental because of their embryonic origin; are segmental because of their embryonic origin; arise from segmental mesodermal somitesarise from segmental mesodermal somites

► Tetrapods, like fish, have epaxial & hypaxial Tetrapods, like fish, have epaxial & hypaxial masses, & these retain some evidence of masses, & these retain some evidence of metamerism even in the highest tetrapods.metamerism even in the highest tetrapods.

►Modifications:Modifications:1 - epaxials are elongated bundles that extend 1 - epaxials are elongated bundles that extend

through many body segments & that are through many body segments & that are located below the expanded appendicular located below the expanded appendicular muscles required to operate the limbs muscles required to operate the limbs

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

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

Epaxials of TetrapodsEpaxials of Tetrapods

► lie along vertebral column dorsal to lie along vertebral column dorsal to transverse processes & lateral to transverse processes & lateral to neural archesneural arches

► extend from base of the skull to tip of extend from base of the skull to tip of the tailthe tail

►Urodeles & some lizards - epaxials are Urodeles & some lizards - epaxials are obviously metameric & are referred to obviously metameric & are referred to as the dorsalis truncias the dorsalis trunci

►Higher tetrapods - superficial epaxial Higher tetrapods - superficial epaxial bundles form long muscles that extend bundles form long muscles that extend over many body segments; deep over many body segments; deep bundles are still segmentedbundles are still segmented

► 1- longissimus group 1- longissimus group lies on transverse processes of vertebrae; lies on transverse processes of vertebrae;

includes the longest epaxial bundlesincludes the longest epaxial bundles subdivisions include: longissimus dorsi, subdivisions include: longissimus dorsi,

longissimus cervicis, longissimus capitislongissimus cervicis, longissimus capitis► 2 - iliocostalis group 2 - iliocostalis group

lateral to longissimus & spinalislateral to longissimus & spinalis arises on ilium & inserts on dorsal ends of ribs or arises on ilium & inserts on dorsal ends of ribs or

uncinate processesuncinate processes► 3 - spinalis group3 - spinalis group

lies close to neural archeslies close to neural arches connects spinous processes or transverse connects spinous processes or transverse

processes with those several vertebrae anteriorlyprocesses with those several vertebrae anteriorly

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