Skeletal Muscle TissueSkeletal Muscle Tissue

Human AnatomyHuman AnatomySonya Schuh-Huerta, Ph.D.Sonya Schuh-Huerta, Ph.D.

Muscle

• Muscle = Latin word for “little mouse”

• Muscle is the primary tissue in the: – Heart (cardiac muscle)– Walls of hollow organs (smooth muscle)

• Skeletal muscle– All of your voluntary muscles

Skeletal Muscle: Basic Function

Voluntary movement

Locomotion

Manipulation of environment

Facial expression

Maintenance of posture

Joint stabilization

Heat generation• Muscle contractions produce heat helps maintain normal body temperature

Coach Laura

Muscle Tissue: Unique Features

Contractility

Excitability

Extensibility

Elasticity

• Skeletal muscle tissue:– Packaged into skeletal muscles– 640 muscles in human body– Makes up ~40% of body weight– 15% more dense than fat– Cells are termed muscle

fibers & are striated!

Skeletal Muscle Tissue

Coach Sean

Skeletal Muscle

• Each muscle is an organ! – Consists mostly of muscle tissue– Skeletal muscle also contains:

• Connective tissue• Blood vessels• Nerves

Basic Features of a Skeletal Muscle

• Connective tissue & fascicles– Sheaths of connective tissue bind a skeletal

muscle & its fibers together• Epimysium = dense regular connective tissue

surrounding entire muscle• Perimysium = surrounds each fascicle

(group of muscle fibers)• Endomysium = a fine sheath of connective tissue

wrapping each muscle cell

Basic Features of a Skeletal Muscle

• Connective tissue sheaths are continuous with tendons– When muscle fibers contract, pull is exerted

on all layers of connective tissue & tendon– Sheaths provide elasticity & carry blood

vessels & nerves

Skeletal Muscle: Levels of Organization

Bone

Muscle fiber(1 cell)

Fascicle (wrapped byperimysium)

Epimysium

Tendon Muscle fiberin middle of a fascicle

Muscle Endomysium(between individual muscle fibers)

Basic Features of a Skeletal Muscle

• Nerves & blood vessels:– Each skeletal muscle supplied by branches of

• 1 nerve• 1 artery• 1 or more veins

Capillary Network Surrounding Skeletal Muscle Fibers

Basic Features of a Skeletal Muscle

• Nerves & blood vessels:– Nerves & vessels branch repeatedly– Smallest nerve branches serve:

• Individual muscle fibers!!!• Neuromuscular junction signals the muscle to

contract (also called motor endplate)

Basic Features of a Skeletal Muscle

• Muscle attachments– Most skeletal muscles run from one bone to

another– One bone will move, other bone remains fixed

• Origin = less movable attachment (directly attached to the bone)

• Insertion = more movable attachment (attached to bone by a tendon)

Originby directattachment

Muscle contracting

Insertion byindirect attachment

Brachialis

Tendon

Muscle Attachments

Muscle Attachments

– Muscles attach to origins & insertions by CT• Fleshy attachments (direct) CT fibers are short• Indirect attachments CT forms a tendon or

aponeurosis

– Bone markings present where tendons meet bones

• Tubercles, trochanters, & crests…

Muscle At the Microscopic Level

Photomicrograph: Skeletal muscle at 300x

Nuclei

Striations

Muscle Fiber (cell)

Huge, long cylindrical cells 10–100 m in diameter many centimeters long

Multinucleated cells formed by fusion of many embryonic cells

Striations visible

Microscopic Structure of the Muscle FiberReview

Nucleus

Sarcolemma

Myofilament

I band I bandA band

Thin (actin)filament

Thick (myosin)filament

M lineSarcomere

Myofibril

Microscopic Structure of the Muscle Fiber

=> “Bag of pretzels”

Mechanism of Contraction

• 2 major types of contraction– Concentric contraction muscle shortens

to do work • Most common type of contraction

– Eccentric contraction muscle generates force as it lengthens

• Muscle acts as a “brake” to resist gravity• “Down” portion of a pushup is an example

– Explains concentric contraction• Myosin heads attach to thin filaments at both ends

of a sarcomere– Then pull thin filaments toward the center of sarcomere

• Thin & thick filaments do NOT shorten, the sarcomere shortens!

– Initiated by release of Ca2+ from the SR!– Powered by ATP!

The Sliding Filament Mechanism of Contraction

MovementThin (actin)

filament

Thick (myosin)filament

Myosinhead

The Sliding Filament Mechanism of Contraction

Ca2+

ATP

The Sliding Filament Mechanism of Contraction

Sliding Filament Mechanism

• Contraction changes the striation pattern– Fully relaxed thin filaments partially overlap

thick filaments– Contraction thin filaments completely

overlap thick filaments, & Z discs move closer together

• Sarcomere shortens• I bands shorten, H zone disappears• A band remains same length

The Molecular Components

Ca2+ & ATP

myosin binds

ATP ADP & Pi

powerstroke

new ATP binds

release & recovery stroke

myosin binds again

Sliding Filament Mechanism in Action! How your muscles contract…

animation by www.encognitive.com

Functional Anatomy of Skeletal Muscle

• Muscle extension – Muscle is stretched by a movement opposite

of that which contracts it

• Muscle fiber length & force of contraction– Greatest force produced when a fiber starts

out slightly stretched– Myosin heads can pull along the entire length

of the thin filaments do more work & generate greater contraction!

The Role of Titin

• Titin a spring-like molecule (protein) in sarcomeres (= strong like spider’s silk)

– Resists overstretching– Holds thick filaments in place– Unfolds when muscle is stretched

Innervation of Skeletal Muscle

• Motor neurons innervate skeletal muscle tissue– Neuromuscular junction the point where

nerve ending & muscle fiber meet– Axon terminals ends of axons

• Store neurotransmitters (Acetylcholine)

– Synaptic cleft space between axon terminal & sarcolemma

The Neuromuscular Junction

Nucleus

Nerveimpulse

Myelinated axonof motor neuron

Axon terminal of neuromuscular junction

Sarcolemma ofthe muscle fiber

1 Nerve impulse stimulates the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft.

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

Muscle fiber

Synapticcleft

SarcoplasmicReticulum

Ca2+

t tubule

Neuron

The Neuromuscular Junction

Ach binds to its receptor & causes Na+ to flow into the cell & the membrane to depolarize.This triggers massive Ca2+ to be released from the SR!

2

Motor UnitsSpinal cord

Motor neuroncell body

Muscle

Branching axonto motor unit

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motor neuronaxon

Axon terminals atneuromuscular junctions

(a) Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.

(b) Branching axon terminals form neuromuscular junctions, one per muscle fiber (photomicrograph 110).

Quiz – What is this?

Types of Skeletal Muscle Fibers

• Skeletal muscle fibers are categorized according to 2 characteristics– How they manufacture energy (ATP)– How quickly they contract!

• Oxidative fibers produce ATP aerobically

• Glycolytic fibers produce ATP anaerobically by glycolysis

Types of Skeletal Muscle Fibers

• Skeletal muscle fibers are divided into 3 classes:

• Slow oxidative fibers– Red slow oxidative fibers

• Fast glycolytic fibers– White fast glycolytic fibers

• Fast oxidative fibers– Intermediate fibers

Types of Skeletal Muscle Fibers

• Slow oxidative fibers (slow twitch)

– Red color due to abundant myoglobin – Obtain energy from aerobic metabolic reactions– Contain a large number of mitochondria– Richly supplied with capillaries– Contract slowly & resistant to fatigue– Fibers are small in diameter

Dark meat?

=> O2 store

Types of Skeletal Muscle Fibers

• Fast glycolytic fibers (fast twitch)

– Contain little myoglobin & few mitochondria – About twice the diameter of slow-oxidative fibers– Contain more myofilaments & generate more power!!!– Depend on anaerobic pathways (glycolysis)– Contract rapidly & tire quickly

White meat?

Types of Skeletal Muscle Fibers

• Fast oxidative fibers– Have an intermediate diameter– Contract quickly like fast glycolytic fibers– Are oxygen-dependent (aerobic pathways)– Have high myoglobin content & rich supply of

capillaries– Somewhat fatigue-resistant– More powerful than slow oxidative fibers

Physiology of Skeletal Muscles

• Predominately powered by

oxidation of fats & carbs

• Fast twitch muscles operate with little O2 (anaerobic) to break down glucose & produce ATP (energy) for quick powerful bursts of contraction, but tire quickly…

–By-product is lactic acid (“Feel the burn!”)

–Lactic acid build-up (lactic acid threshold or blood lactate accumulation)

Disorders of Muscle Tissue

• Muscle tissues experience few disorders– Heart muscle is exception (many problems)– Skeletal muscle

• Remarkably resistant to infection!

– Smooth muscle• Problems stem from external irritants (things we

breathe in & eat)

Disorders of Muscle Tissue

• Muscular dystrophy– A group of inherited muscle destroying

diseases• Affected muscles enlarge with fat & connective

tissue • Muscle fibers & muscles degenerate – can lead to

paralysis & death• Types of muscular dystrophy

– Duchenne muscular dystrophy – Myotonic dystrophy– Others….

Disorders of Muscle Tissue

• Myofascial pain syndrome– Pain is caused by tightened bands of muscle

fibers

• Fibromyalgia– A mysterious chronic-pain syndrome– Affects mostly women– Symptoms fatigue, sleep abnormalities,

severe musculoskeletal pain, & headache

Muscle Tissue Throughout Life

• Muscle tissue develops from myoblasts– Myoblasts fuse to form skeletal muscle fibers– Skeletal muscles contract by the 7th week of

development

Embryonic mesoderm cells undergo cell division (to increase number) and enlarge.

Embryonicmesoderm cells

MyoblastsMyotube(immaturemultinucleatemuscle fiber)

Satellite cell

Mature skeletalmuscle fiber

Several myoblasts fuse together to form a myotube.

Myotube matures into skeletal muscle fiber.

1 2 3

Muscle Tissue Throughout Life

• Satellite cells– Surround skeletal muscle fibers– Resemble undifferentiated myoblasts– Fuse into existing muscle fibers to help them

grow

• Cardiac muscle– Pumps blood in embryo end of week 3

What happens with age?

• With increased age:– Amount of connective tissue

increases in muscles– Number of muscle cells decreases

• Loss of muscle mass with age:– Decrease in muscular strength is

50% by age 80 (sarcopenia = “muscle wasting”)

What happens with age?

• Can your body produce new muscle cells?

• NO. -Or very few.– Not many functional stem cells in muscle tissue

The Body’s Capacity for Regeneration:Adult Stem Cells

– Good to excellent:• Epithelial tissue, bone, dense irregular connective

tissue, blood, fat

– Moderate:• Smooth muscle, dense regular connective tissue

– Weak:• Skeletal muscle, cartilage

– None or almost none:• Cardiac muscle, nervous tissue

Human stem cells, S. Huerta

What can you do about this?

• Exercise is Key!!!-For maintaining healthy muscles,

bones, & joints well into old age!

USE IT OR LOSE IT…

Effects of Exercise on the Muscles

• So how do you gain more (or maintain) muscle? Especially, when you’re fighting an uphill battle with age?

• Strength training, lifting weights, & core exercises (3X week+)

• Healthy balanced diet (with protein!)

• Proper recovery time & rest

• This prevents loss of muscle cells, & increases the strength & size of the individual cells

How does this happen?

• With strength/weight training exercise:–Tiny microscopic tears occur

–An inflammation & healing

response triggers the growth

of more myofilaments &

myofibrils that pack within

the muscle cell

–The soreness that results:

Delayed Onset Muscle

Soreness (DOMS) The Good Pain!

–The effect Stronger, bigger muscles (that keep working!)

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What kind of exercise?• ALL KINDS – those who do multi-sports, cross-training, switching

between activities, develop greater muscle strength, overall fitness, & keep the muscles constantly adjusting & being challenged

• Also, have less injuries due to wear-and-tear of doing one repetitive sport

Questions…?

What’s Next?Lab: Skeletal musclesWed Lecture: Muscles of the Body Wed Lab: Finish Muscles & Review