MUSCLE TISSUEMUSCLE TISSUE

Muscle tissueMuscle tissue facilitates movement of the animal facilitates movement of the animal by contraction of individual muscle cells (referred by contraction of individual muscle cells (referred to as to as muscle fibersmuscle fibers).).

Three types of muscle fibers occur in animals:Three types of muscle fibers occur in animals: skeletal (striated),skeletal (striated), smooth andsmooth and cardiac cardiac

TYPES OF MUSCLE TISSUETYPES OF MUSCLE TISSUE

Muscle occurs in three distinct types:Muscle occurs in three distinct types:

SkeletalSkeletal muscle tissue muscle tissue attaches to bones (via tendons) attaches to bones (via tendons) for voluntary movement: for voluntary movement: This is the type of muscle This is the type of muscle that you eat if you eat meat.that you eat if you eat meat.

SmoothSmooth muscle tissue muscle tissue contains spindle shaped cells contains spindle shaped cells with a single nucleus: it lines the gut, blood vessels and with a single nucleus: it lines the gut, blood vessels and glands its operation is involuntary.glands its operation is involuntary.

CardiacCardiac (heart) muscle (heart) muscle is composed of short, striated is composed of short, striated cells that can function in units due to the contracting signal cells that can function in units due to the contracting signal that passes from cell to cell by way of the gap junctions.that passes from cell to cell by way of the gap junctions.

SkeletalSkeletal muscle tissue muscle tissue attaches to bones (via tendons) for voluntary attaches to bones (via tendons) for voluntary movementmovement

Cellular organization of skeletal Cellular organization of skeletal muscle fibersmuscle fibers

Unlike most tissues, Unlike most tissues, skeletal muscleskeletal muscle does not consist of does not consist of individual cells.  Rather it is formed from huge, striated individual cells.  Rather it is formed from huge, striated (actin and myosin) multinucleated, long cells ((actin and myosin) multinucleated, long cells (fibers)fibers), , which are often bundled together to form a which are often bundled together to form a “muscle“muscle”.”.

The muscle fibers develop by fusion of many individual The muscle fibers develop by fusion of many individual embryonic cells called embryonic cells called myoblastsmyoblasts..

Each individual skeletal muscle fiber extends over much of Each individual skeletal muscle fiber extends over much of the length of the muscle in which it resides (up to many the length of the muscle in which it resides (up to many centimeters), with a uniform diameter that is typically centimeters), with a uniform diameter that is typically around 50 µm.around 50 µm.

Microscopic features of the muscle Microscopic features of the muscle fiber fiber The unique contractile cytoplasm of muscle fibers is called The unique contractile cytoplasm of muscle fibers is called

sarcoplasmsarcoplasm ( (sarcosarco = flesh). = flesh). The contractile machinery is concentrated into The contractile machinery is concentrated into myofibrilsmyofibrils ( (myomyo = =

muscle),muscle), long narrow structures (1-2 µm in diameter) which extend the long narrow structures (1-2 µm in diameter) which extend the length of the fiber and form the bulk of the sarcoplasm. length of the fiber and form the bulk of the sarcoplasm.

itochondria (once called itochondria (once called sarcosomessarcosomes) and) and sarcoplasmic reticulumsarcoplasmic reticulum (a highly specialized form of endoplasmic (a highly specialized form of endoplasmic

reticulum) surround each myofibril.reticulum) surround each myofibril. The plasma membrane of muscle fibers is sometimes called the The plasma membrane of muscle fibers is sometimes called the

sarcolemmasarcolemma.  .   The many nuclei within each fiber are displaced from the center of the The many nuclei within each fiber are displaced from the center of the

fiber by the specialized sarcoplasm and are located adjacent to the fiber by the specialized sarcoplasm and are located adjacent to the sarcolemma (except in special cases, like the intrafusal fibers of sarcolemma (except in special cases, like the intrafusal fibers of muscle spindles, and regenerated fibers).  muscle spindles, and regenerated fibers).  

Skeletal MuscleSkeletal MuscleIdentification:Identification: Teased or l.s. section shows distinct, Teased or l.s. section shows distinct, very large, straight fibers. Looks more like hair than very large, straight fibers. Looks more like hair than any other tissue. Also has distinct striations. any other tissue. Also has distinct striations.

Features to Know:Features to Know: striations striations (1) composed of dark A-(1) composed of dark A-bands and light I-bands; bands and light I-bands; nuclei (2) pushed to edge of nuclei (2) pushed to edge of fiber; sarcolemna (plasma fiber; sarcolemna (plasma membrane surrounding membrane surrounding fiber). fiber). Where Located:Where Located: skeletal skeletal muscles; under voluntary muscles; under voluntary control.control.

Skeletal Muscle - also called Skeletal Muscle - also called volutnary or striated muscle.volutnary or striated muscle.The striations (stripes) are due to a very orginized pattern The striations (stripes) are due to a very orginized pattern of proteins found within the cells. of proteins found within the cells.

The black arrow is The black arrow is pointing to a pointing to a nucleus.nucleus.

the entire length of the entire length of the fiber may include the fiber may include hundreds nuclei.   hundreds nuclei.  

Striations (Striations (regularly spaced bandsregularly spaced bands))

Each Each myofibrilmyofibril is marked by is marked by striationsstriations (alternating light (alternating light and dark bands)and dark bands) within the fibers along its length. Dark within the fibers along its length. Dark and light bands are composed of myosin and actin, and light bands are composed of myosin and actin, respectively.respectively.

In relaxed muscle, a single repeat of this pattern is about 2 In relaxed muscle, a single repeat of this pattern is about 2 µm long (the length varies with degree of contraction) and µm long (the length varies with degree of contraction) and is called a is called a sarcomeresarcomere..

Because the striations on adjacent myofibrils are usually Because the striations on adjacent myofibrils are usually aligned, the entire muscle fiber appears uniformly striated.  aligned, the entire muscle fiber appears uniformly striated.  

SarcomereSarcomere In each sarcomere, the broad band which appears dark in standard In each sarcomere, the broad band which appears dark in standard

histological procedures is called the histological procedures is called the A-bandA-band.  This band indicates the .  This band indicates the location of location of thick filamentsthick filaments ( (myosinmyosin); it is darkest where thick and thin ); it is darkest where thick and thin filaments overlap.   filaments overlap.  

The broad light band between the dark bands is the The broad light band between the dark bands is the I-bandI-band.  The I-.  The I-band indicates the location where band indicates the location where thin filamentsthin filaments (actin(actin) extend ) extend beyond the thick filaments.   beyond the thick filaments.  

A distinct dark line running down the middle of the I-band is the A distinct dark line running down the middle of the I-band is the Z-lineZ-line, , where thin filaments are attached end to end.  Where thick filaments where thin filaments are attached end to end.  Where thick filaments are attached end-to-end in the center of the A-band is the are attached end-to-end in the center of the A-band is the M-lineM-line.   .  

When muscle is stretched, an When muscle is stretched, an H-bandH-band appears along the middle of the appears along the middle of the A-band, between the free ends of the thin filaments.   A-band, between the free ends of the thin filaments.  

From Blue Histology (Copyright Lutz Slomianka 1998-2004) The image should be animated, if you watch patiently.)

Skeletal Muscle fibers, Cross Skeletal Muscle fibers, Cross SectionSection

Any particular Any particular cross section cross section of a muscle of a muscle fiber may fiber may reveal one or a reveal one or a few nuclei, or few nuclei, or none, but the none, but the entire length of entire length of the fiber may the fiber may include include hundreds.   hundreds.  

The structural organization ofThe structural organization of skeletal skeletal muscle cross section,muscle cross section, slideslide at 100Xat 100X Individual muscle fibers (4) are bundled into fascicles (5). Surrounding Individual muscle fibers (4) are bundled into fascicles (5). Surrounding individual fibers, fascicles, and the entire muscle are layers of connective individual fibers, fascicles, and the entire muscle are layers of connective

tissue (1-3).tissue (1-3).

Features to Know:Features to Know: connective tissue sheaths: connective tissue sheaths: epimysium (1), perimysium epimysium (1), perimysium (2), & endomysium (3); (2), & endomysium (3); muscle fiber with nuclei (6) muscle fiber with nuclei (6) along edges; fascicle (5). along edges; fascicle (5). You may seeYou may see

Skeletal Muscle cross section Skeletal Muscle cross section at 400xat 400x

Smooth Muscle - also called involuntary or visceral Smooth Muscle - also called involuntary or visceral muscle.muscle.Smooth muscleSmooth muscle is stronger than skeletal muscle. If you ever had is stronger than skeletal muscle. If you ever had stomach cramps you know how strong smooth muscle can be - a better stomach cramps you know how strong smooth muscle can be - a better example is labor (for those of you who have experienced or seen child example is labor (for those of you who have experienced or seen child birth) - the uterus has a large quantity of smooth muscle that works to birth) - the uterus has a large quantity of smooth muscle that works to "birth" a baby."birth" a baby.

Muscle cells are packed tightly Muscle cells are packed tightly together (no gaps between cells) together (no gaps between cells) and usually not distinct.and usually not distinct.

These cells function in involuntary These cells function in involuntary movements and/or movements and/or autonomicautonomic responses (such as breathing, responses (such as breathing, secretion, ejaculation, birth, and secretion, ejaculation, birth, and certain reflexes). These fibers are certain reflexes). These fibers are components of structures in the components of structures in the digestive systemdigestive system, reproductive , reproductive tract, and blood vessels.tract, and blood vessels.

Cellular organization of Cellular organization of Smooth muscleSmooth muscle

fibersfibers Smooth muscleSmooth muscle fibers lack the banding, although actin and fibers lack the banding, although actin and myosin still occur.myosin still occur.Smooth muscle fibers are spindle shaped cells that form Smooth muscle fibers are spindle shaped cells that form masses. masses.

Nuclei may or Nuclei may or may not be may not be visible. Note lack visible. Note lack of striations. of striations.

Smooth muscle Smooth muscle fibers fibers are are components of components of structures in the wall of the structures in the wall of the digestive systemdigestive system,, Inner Circular, Outer Longitudinal of smooth Inner Circular, Outer Longitudinal of smooth muscle fibersmuscle fibers

Smooth muscle fibers are components of Smooth muscle fibers are components of structures in the blood vessels.structures in the blood vessels.

Smooth Muscle in Arterial WallSmooth Muscle in Arterial Wall

CardiacCardiac (heart) muscle (heart) muscle is is composed of short, striated cellscomposed of short, striated cells

Note faint striations Note faint striations across fibers. Fibers across fibers. Fibers distinct, typically with distinct, typically with numerous small numerous small gaps between them. gaps between them.

Features to Know:Features to Know: nuclei (1), nuclei (1), intercalated disk (2). intercalated disk (2).

Like skeletal muscle, cartiac muscle is Like skeletal muscle, cartiac muscle is striated,striated, but cardiac muscle also contains structures called but cardiac muscle also contains structures called intercalated disks (blue arrow) that work to produce the intercalated disks (blue arrow) that work to produce the rhythmic contractions of the heart. Intercalated disks are rhythmic contractions of the heart. Intercalated disks are found only in cardiac muscle.found only in cardiac muscle.

And fibers in And fibers in cardiac cardiac muscle are muscle are much smaller much smaller & branched.& branched.