Muscle Histology

Assoc. Prof Dr. karim Al-Jashamy IMS 2009

Muscular TissueMuscle tissue is composed of cells which have the

ability to contract (shorten) and relax (lengthen).Muscle tissue is highly vascular zed and dense

tissue.Muscle tissue can not regenerate itself once it has

been destroyed.Muscle cells are called muscle fibers. There are three types of muscle tissues:

a. Striated or skeletal muscle

b. Cardiac muscle or myocardial tissue

c. Smooth or visceral muscle

Muscular Tissue Types Striated or skeletal muscle: This muscle is composed of

long, cylindrical, parallel fibers. There are bands or striations present which run across the

width of the fiber. These fibers are multinucleated (more than one nucleus present per cell).

This type of muscle tissue is under conscious or voluntary control.

This muscle has the greatest strength of contraction but tires most rapidly. It functions are locomotion and facial expressions.

Muscular Tissue Types Cardiac muscle: Cardiac muscle fibers are branched at the end

with striations present, usually only one nucleus is present per cell.

The branches of each fiber come into contact at specialized junctions called intercalated discs.

It is involuntarily controlled by the autonomic nervous system and hormones. Its contractions are short, but the muscle fibers are constantly contracting.

Its function is to create the pump which propels the blood throughout the body.

Muscular Tissue Types Smooth or visceral muscle: These cells are spindle-shaped

(tapered on the ends) and lack striations.

They have one nucleus present. They produce weaker contractions, however they contract for extended periods of time.

They are involuntarily controlled by the autonomic nervous system or hormones.

They are found in the digestive organs, arteries and veins, the trachea and bronchiole tubes, and the urogenital tract. Their contractions propels or moves substances or objects from one location to another.

Functions of muscle tissue Movement

Maintenance of

posture

Joint stabilization

Heat generation

Special functional characteristics of muscle

Contractility

Only one action: to shorten

Shortening generates pulling force

Excitability

Nerve fibers cause electrical impulse to travel

Extensibility

Stretch with contraction of an opposing muscle

Elasticity

Recoils passively after being stretched

Types of Muscle Tissue

Skeletal•Attach to and move skeleton

•40% of body weight

•Fibers = multinucleate cells (embryonic cells fuse)

•Cells with obvious striations

•Contractions are voluntary

Cardiac: only in the wall of

the heart

•Cells are striated

•Contractions are

involuntary (not

voluntary)

Smooth: walls of hollow organs

•Lack striations

•Contractions are involuntary (not voluntary)

Similarities…

Their cells are called fibers because they are elongated

Contraction depends on myofilaments

Actin

Myosin

Plasma membrane is called sarcolemma

Sarcos = flesh

Lemma = sheath

Skeletal muscle

Epimysium:

surrounds

whole muscle

Perimysium

is around

fascicle

Endomysium is around each

muscle fiber

Skeletal Muscle Each muscle: one nerve, one artery,

one vein Branch repeatedly

Attachments One bone to another Cross at least one movable joint Origin: the less movable attachment Insertion: is pulled toward the origin

Usually one bone moves while the other remains fixed

In muscles of the limb, origin lies proximal to the insertion (by convention)

Note: origin and insertion may switch depending on body position and movement produced

Attachments continuedMany muscles span two or more joints Called biarticular or multijoint muscles Cause movements at two joints

Direct or “fleshy” attachments Attachments so short that muscle appears to attach directly

to bone

Indirect: connective tissue extends well beyond the muscle (more common) Tendon: cordlike (most muscles have tendons) Aponeurosis: flat sheet Raised bone markings where tendons meet bones Tubercles, trochanters, crests, etc.

Some sites showing animations of muscle contraction

http://entochem.tamu.edu/MuscleStrucContractswf/index.html

http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/muscles/muscles.html

Skeletal muscle

Fibers (each is one cell) have striations

Myofibrils are organelles of the cell: these are made up of filaments

Sarcomere Basic unit of contraction Myofibrils are long

rows of repeating sarcomeres

Boundaries: Z discs (or lines)

This big

cylinder is a

fiber: 1 cell -an organelle

Myofibrils

Made of three types of filaments (or myofilaments):

Thick (myosin)

Thin (actin)

Elastic (titin)

______actin

_____________myosintitin_____

Sliding Filament Model

__relaxed sarcomere__ _partly contracted_

fully contracted

“A” band constant

because it is

caused by

myosin, which

doesn’t change

length

Sarcomere

shortens because

actin pulled

towards its middle

by myosin cross

bridgesTitin resists overstretching

EM (electron microscope): parts of 2 myofibrils

Sarcoplasmic reticulum is smooth ER Tubules surround myofibrils

Cross-channels called “terminal cisternae”

Store Ca++ and release when muscle stimulated to contract

To thin filaments triggering sliding filament mechanism of contraction

T tubules are continuous with sarcolemma, therefore whole muscle (deep parts as well) contracts simultaneously

Neuromuscular Junction

Motor neurons innervate muscle

fibers

Motor end plate is where they

meet

Neurotransmitters are released

by nerve signal: this initiates

calcium ion release and muscle

contraction

Motor Unit: a motor neuron and all the muscle fibers it innervates (these all

contract together)

•Average is 150, but range is four to several hundred muscle fibers in a motor

unit

•The finer movement, the fewer muscle fibers /motor unit

•The fibers are spread throughout the muscle, so stimulation of a single motor

unit causes a weak contraction of the entire muscle

Types of skeletal muscle fibers

Fast, slow and intermediate

Whether or not they predominantly use oxygen to produce ATP (the energy molecule used in muscle contraction) Oxidative – aerobic (use oxygen) Glycolytic – make ATP by glycolysis (break down of sugars without

oxygen=anaerobic)

Fast fibers: “white fibers” – large, predominantly anaerobic, fatigue rapidly (rely on glycogen reserves); most of the skeletal muscle fibers are fast

Slow fibers: “red fibers” – half the diameter, 3X slower, but can continue contracting; aerobic, more mitochondria, myoglobin

Intermediate: in between

A skeletal muscle contracts when its motor units are stimulated

Amount of tension depends on1. the frequency of stimulation2. the number of motor units involved

Single, momentary contraction is called a muscle twitch

All or none principle: each muscle fiber either contracts completely or not at all

Amount of force: depends on how many motor units are activated

Muscle tone Even at rest, some motor units are active: tense the

muscle even though not causing movement: “resting tone”

Muscle hypertrophy Weight training (repeated intense workouts): increases

diameter and strength of “fast” muscle fibers by increasing production of Mitochondria Actin and myosin protein Myofilaments containing these contractile proteins The myofibril organelles these myofilaments form

Fibers enlarge (hypertrophy) as number and size of myofibrils increase[Muscle fibers (=muscle cells) don’t increase in number but increase in diameter producing large muscles]

Endurance training (aerobic): doesn’t produce hypertrophy

Muscle atrophy: loss of tone and mass from lack of stimulation Muscle becomes smaller and weaker

Note on terminology: in general, increased size is hypertrophy; increased number of cells is hyperplasia

Tendon Anatomy Very strong, stronger than muscle for size As strong as bone with a failing point similar to steel! Can transmit force through ability to glide Passive component of the musculotendinous unit in

light of their incredible influence on the foot.

Tendon Histology 30% Collagen, 2% Elastin,

68% Water Bulk is supplied by reticulin 70% Type I collagen

Ligament Histology 33% Composition: 90% Type I

collagen,

Elastin, Glycosaminoglycans.

67% Water

Anatomy of the Tendon Tropocollagen – the most basic molecular unit of

tendon

3 Coverings:

Endotenon- fascicles are surrounded by this areolarCT, contains BV, L, N, and FB.

Epitenon- Fascicles bound together by this 1-2 cell fibroblastic & synovial layer

Paratenon- loose areolar layer continuous with the epitenon & perimysium, straight.

Anatomy of the Tendon

Tendon/Synovial Sheath: acts like a pulley when tendon has an angled course.

Peritenon- term applied collectively to all CT structures associated with a tendon incl para-, meso-, epi-, and endotenon.

Tendon Anatomy

Tendon Anatomy

Tendon Circulation 3 Sources:

a) Small amount from the central blood vessels originating in the muscle.

b) Some from vessels of the bone and periosteum near the tendon’s insertion.

c) Majority comes from small vessels in the paratenon or through the mesotenon. If absent then carried thru the vincula. Synovial fluid also nourishes the tendon.

Tendon Sheath Anatomy

Tendon Innervation Afferent supply only Source in musculotendinous

junction and external local nerves.

Golgi tendon organs: monitor increases in tension rather than length.

Tendon Attachment to Bone Attach at 90° angles to bone

in 4 layers:

1. Tendon collagen fibers

2. Fibrocartilage

3. Bone

4. Sharpey’s fibers – originate in bone and end in perisoteum.