Skeletal Muscle Tissue and Muscle Organization

Lecture Presentation by

Steven Bassett

Southeast Community College

Chapter 9

The Muscular

System

Skeletal Muscle

Tissue and Muscle

Organization

© 2015 Pearson Education, Inc.

Introduction

• Humans rely on muscles for:

• Many of our physiological processes

• Virtually all our dynamic interactions with the

environment

• Skeletal muscles consist of:

• Elongated cells called fibers (muscle fibers)

• These fibers contract along their longitudinal axis


Introduction

• There are three types of muscle tissue

• Skeletal muscle

• Pulls on skeletal bones

• Voluntary contraction

• Cardiac muscle

• Pushes blood through arteries and veins

• Rhythmic contractions

• Smooth muscle

• Pushes fluids and solids along the digestive tract,

for example

• Involuntary contraction


Introduction

• Muscle tissues share four basic properties

• Excitability

• The ability to respond to stimuli

• Contractility

• The ability to shorten and exert a pull or tension

• Extensibility

• The ability to continue to contract over a range of

resting lengths

• Elasticity

• The ability to rebound toward its original length


Functions of Skeletal Muscles

• Skeletal muscles perform the following functions:

• Produce skeletal movement

• Pull on tendons to move the bones

• Maintain posture and body position

• Stabilize the joints to aid in posture

• Support soft tissue

• Support the weight of the visceral organs


Functions of Skeletal Muscles

• Skeletal muscles perform the following

functions (continued):

• Regulate entering and exiting of material

• Voluntary control over swallowing, defecation, and

urination

• Maintain body temperature

• Some of the energy used for contraction is

converted to heat


Anatomy of Skeletal Muscles

• Gross anatomy is the study of:

• Overall organization of muscles

• Connective tissue associated with muscles

• Nerves associated with muscles

• Blood vessels associated with muscles

• Microscopic anatomy is the study of:

• Myofibrils

• Myofilaments

• Sarcomeres



• Gross Anatomy

• Connective tissue of muscle

• Epimysium: dense tissue that surrounds the entire

muscle

• Perimysium: dense tissue that divides the muscle

into parallel compartments of fascicles

• Endomysium: dense tissue that surrounds

individual muscle fibers


Figure 9.1 Structural Organization of Skeletal Muscle


Tendon

Perimysium

Muscle fascicle

Endomysium

Perimysium

Nerve

Muscle fibers

Blood vessels

SKELETAL MUSCLE

(organ)

Perimysium

Muscle fiber

Endomysium

MUSCLE FASCICLE(bundle of cells)

Epimysium

Blood vessels

and nerves

Endomysium

CapillaryMitochondria

SarcolemmaEndomysium

Myofibril

Myosatellite

cell

AxonNucleusSarcoplasm

MUSCLE FIBER

(cell)

Epimysium


• Connective Tissue of Muscle

• Tendons and aponeuroses

• Epimysium, perimysium, and endomysium

converge to form tendons

• Tendons connect a muscle to a bone

• Aponeuroses connect a muscle to a muscle



• Gross Anatomy

• Nerves and blood vessels

• Nerves innervate the muscle by penetrating the

epimysium

• There is a chemical communication between a

nerve and a muscle

• The chemical is released into the neuromuscular

synapse (neuromuscular junction)


Figure 9.2 Skeletal Muscle Innervation


Neuromuscular

synapse

Skeletal

muscle

fiber

Axon

Nerve

A neuromuscular synapse as seen

on a muscle fiber of this fascicleColorized SEM of a neuromuscular

synapse

LM x 230 SEM x 400

ba


• Gross Anatomy

• Nerves and blood vessels (continued)

• Blood vessels often parallel the nerves that

innervate the muscle

• They then branch to form coiled networks to

accommodate flexion and extension of the muscle



• Microanatomy of Skeletal Muscle Fibers

• Sarcolemma

• Membrane that surrounds the muscle cell

• Sarcoplasm

• The cytosol of the muscle cell

• Muscle fiber (same thing as a muscle cell)

• Can be 30–40 cm in length

• Multinucleate (each muscle cell has hundreds of

nuclei)

• Nuclei are located just deep to the sarcolemma


Figure 9.3ab The Formation and Structure of a Skeletal Muscle Fiber


Myoblasts

Muscle fibers develop

through the fusion of

mesodermal cells

called myoblasts.

Myosatellite cell

Development of a

skeletal muscle fiber.

Nuclei

Immature

muscle fiber

External appearance

and histological view.

a

b


• Myofibrils and Myofilaments

• The sarcoplasm contains myofibrils

• Myofibrils are responsible for the contraction of

muscles

• Myofibrils are attached to the sarcolemma at each

end of the muscle cell

• Surrounding each myofibril is the sarcoplasmic

reticulum



• Myofibrils and Myofilaments

• Myofibrils are made of myofilaments

• Actin

• Thin protein filaments

• Myosin

• Thick protein filaments


Figure 9.3b-d The Formation and Structure of a Skeletal Muscle Fiber


Nuclei

MUSCLE FIBERSarcoplasm

Myofibril

Sarcolemma

Terminal cisterna

Sarcolemma

Sarcoplasm

Myofibrils

Mitochondria

Sarcolemma

T tubulesSarcoplasmic

reticulum

Triad

Thick filament

Thin filament

Myofibril

External appearance

and histological view.

The external organization

of a muscle fiber.

Internal organization of a muscle fiber.

Note the relationships among myofibrils,

sarcoplasmic reticulum, mitochondria,

triads, and thick and thin filaments.

b

c

d


• Sarcomere Organization

• Myosin (thick filament)

• Actin (thin filament)

• Both are arranged in repeating units called

sarcomeres

• All the myofilaments are arranged parallel to the

long axis of the cell




• Sarcomere

• Main functioning unit of muscle fibers

• Approximately 10,000 per myofibril

• Consists of overlapping actin and myosin

• This overlapping creates the striations that give the

skeletal muscle its identifiable characteristic




• Each sarcomere consists of:

• Z line (Z disc)

• I band

• A band (overlapping A bands create striations)

• H band

• M line


Figure 9.4b Sarcomere Structure


I band A band

H band Z line Titin

Thick

filament

Thin

filamentZone of overlap M line

Sarcomere

I band A band

H band Z line

TEM x 64,000M line

Sarcomere

Zone of overlapZ line

b A corresponding view of a sarcomere in a myofibril in

the gastrocnemius muscle of the calf and a diagram

showing the various components of this sarcomere



• Skeletal muscles consist of muscle fascicles

• Muscle fascicles consist of muscle fibers

• Muscle fibers consist of myofibrils

• Myofibrils consist of sarcomeres

• Sarcomeres consist of myofilaments

• Myofilaments are made of actin and myosin


Figure 9.5 Levels of Functional Organization in a Skeletal Muscle Fiber


SKELETAL MUSCLE

MUSCLE FASCICLE

MUSCLE FIBER

Surrounded by:Epimysium

Contains:Muscle fascicles

Surrounded by:Perimysium

Contains:Muscle fibers

MYOFIBRIL

Surrounded by:Endomysium

Contains:Myofibrils

Surrounded by:Sarcoplasmic reticulum

Consists of:Sarcomeres (Z line to Z line)

SARCOMERE

I band A band

Contains:Thick filaments

Thin filaments

Z line M line

H band

Titin Z line


• Thin Filaments (Actin)

• Consists of:

• Twisted filaments of :

• F actin strands

• G actin globular molecules

• G actin molecules consist of an active site (binding

site)

• Tropomyosin: A protein that covers the binding

sites when the muscle is relaxed

• Troponin: Holds tropomyosin in position


Figure 9.6ab Thin and Thick Filaments


Actinin Z line Titin

Troponin Active site Nebulin Tropomyosin G actin molecules

F actin

strand

The attachmentof thin filamentsto the Z line

Sarcomere

H band

Myofibril

Z lineM line

The detailed structure of a thin filament showing

the organization of G actin, troponin, and

tropomyosin

a

b


• Thick Filaments (Myosin)

• Myosin filaments consist of an elongated tail and a

globular head (cross-bridges)

• Myosin is a stationary molecule. It is held in place

by:

• Protein forming the M line

• A core of titin connecting to the Z lines

• Myosin heads project toward the actin filaments


Figure 9.6cd Thin and Thick Filaments


Sarcomere

H band

Myofibril

Z lineM line

The structure of

thick filaments

c

M line

Titin

Myosin head

HingeMyosin tail

A single myosin molecule detailing the structure and

movement of the myosin head after cross-bridge

binding occurs

d

Muscle Contraction

• A contracting muscle shortens in length

• Contraction is caused by interactions between

thick and thin filaments within the sarcomere

• Contraction is triggered by the presence of

calcium ions

• Muscle contraction requires the presence of ATP

• When a muscle contracts, actin filaments slide

toward each other

• This sliding action is called the sliding filament

theory


Muscle Contraction

• The Sliding Filament Theory

• Upon contraction:

• The H band and I band get smaller

• The zone of overlap gets larger

• The Z lines move closer together

• The width of the A band remains constant

throughout the contraction


Figure 9.7 Sliding Filament Theory (1 of 11)


Resting SarcomereA resting sarcomere showing the locations of the

I band, A band, H band, M, and Z lines.

I band A band M line

Z line H band Z line

Resting myofibril

Contracted SarcomereAfter repeated cycles of “bind, pivot, detach, and reactivate”

the entire muscle completes its contraction.

Contracted myofibril

I band A band M line

Z line H band Z line

In a contracting sarcomere the A band stays the same width,

but the Z lines move closer together and the H band and the

I bands get smaller

Muscle Contraction

• The Neural Control of Muscle Fiber Contraction

• An impulse travels down the axon of a nerve

• Acetylcholine is released from the end of the

axon into the neuromuscular synapse

• This ultimately causes the sarcoplasmic reticulum

to release its stored calcium ions

• This begins the actual contraction of the muscle


Figure 9.8 The Neuromuscular Synapse


Arriving action

potential

Synaptic

cleft

ACh receptor

site

Sarcolemma of

motor end plate

AChE molecules

Junctional fold

Synaptic

vesicles

ACh

Glial cell

b Detailed view of a terminal, synaptic cleft,

and motor end plate. See also Figure 9.2.

Motor

neuron

Path of action

potentialAxon

Synaptic

terminal

Muscle FiberMyofibril

Motor end plate

Myofibril

Sarcolemma

Mitochondrion

A diagrammatic view of a

neuromuscular synapse.

a

Muscle Contraction

• Muscle Contraction: A Summary

• The nerve impulse ultimately causes the release

of a neurotransmitter (ACh), which comes in

contact with the sarcoplasmic reticulum

• This neurotransmitter causes the sarcoplasmic

reticulum to release its stored calcium ions

• Calcium ions bind to troponin




Contraction Cycle Begins

Active-Site Exposure

1

2

Ca2+

Actin

Ca2+

Tropomyosin

Activesite

The contraction cycle involves a series of

interrelated steps. The cycle begins with

electrical events in the sarcolemma that

trigger the release of calcium from the

terminal cisternae of the sarcoplasmic

reticulum (SR). These calcium ions enter

the zone of overlap.

Calcium ions bind to troponin in the

troponin– tropomyosin complex. The

tropomyosin molecule then rolls away

from the active sites on the actin

molecules of the thin filaments.

Muscle Contraction

• Muscle Contraction: A Summary (continued)

• The bound calcium ions cause the tropomyosin

molecule to roll so that it exposes the active sites

on actin

• The myosin heads now extend and bind to the

exposed active sites on actin

• Once the myosin heads bind to the active sites,

they pivot in the direction of the M line




Cross-Bridge Formation

Myosin Head Pivoting

3

4

Myosin head

Cross-bridgeOnce the active sites are exposed, the

myosin heads of adjacent thick

filaments bind to them, forming

cross-bridges.

After cross-bridge formation, energy is

released as the myosin heads pivot

toward the M line.

Muscle Contraction


• Upon pivoting of the myosin heads, the actin

filament slides toward the M line

• ATP binds to the myosin heads causing them to

release their attachment and return to their original

position to start over again




Cross-Bridge Detachment

Myosin Reactivation

ATP

ATP

ATP then binds to the myosin heads,

breaking the cross-bridges between the

myosin heads and the actin molecules.

ATP provides the energy to reactivate

the myosin heads and return them to

their original positions. Now the entire

cycle can be repeated as long as

calcium ion concentrations remain

elevated and ATP reserves are

sufficient.

5

6

Muscle Contraction


• Upon contraction:

• I bands get smaller

• H bands get smaller

• Z lines get closer together


Figure 9.7 Sliding Filament Theory


Figure 9.9 The Events in Muscle Contraction


STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION

Synaptic

terminal

Motor

end plate T tubule Sarcolemma

ACh released, binding

to receptors

Action

potential

reaches

T tubule

Sarcoplasmic

reticulum

releases Ca2+

Ca2+

Actin

Myosin

Active-site

exposure,

cross-bridge

formation

Contraction

begins

12

3

4

5

6

7

8

9

10

ACh removed by AChE

Sarcoplasmic

reticulum

recaptures Ca2+

Active sites

covered, no

cross-bridge

interaction

Contraction

ends

Relaxation occurs,

passive return to

resting length

Motor Units and Muscle Control

• Motor Units (Motor Neurons Controlling Muscle

Fibers)

• Precise control

• A motor neuron controlling two or three muscle

fibers

• Example: the control over the eye muscles

• Less precise control

• A motor neuron controlling perhaps 2000 muscle

fibers

• Example: the control over the leg muscles


Figure 9.10 The Arrangement of Motor Units in a Skeletal Muscle


Muscle fibers

Motor

nerve

Axons of

motor neurons


• Muscle tension depends on:

• The frequency of stimulation

• A typical example is a muscle twitch

• The number of motor units involved

• Complete contraction or no contraction at all (all or

none principle)

• The amount of force of contraction depends on the

number of motor units activated



• Muscle Tone

• The tension of a muscle when it is relaxed

• Stabilizes the position of bones and joints

• Example: the amount of muscle involvement that

results in normal body posture

• Muscle Spindles

• These are specialized muscle cells that are

monitored by sensory nerves to control muscle

tone



• Muscle Hypertrophy

• Enlargement of the muscle

• Exercise causes:

• An increase in the number of mitochondria

• An increase in the activity of muscle spindles

• An increase in the concentration of glycolytic

enzymes

• An increase in the glycogen reserves

• An increase in the number of myofibrils

• The net effect is an enlargement of the muscle



• Muscle Atrophy

• Discontinued use of a muscle

• Disuse causes:

• A decrease in muscle size

• A decrease in muscle tone

• Physical therapy helps to reduce the effects

of atrophy


Types of Skeletal Muscle Fibers

• Three Major Types of Muscle Fibers

• Fast fibers (white fibers)

• Associated with eye muscles (fast contractions)

• Intermediate fibers (pink fibers)

• Slow fibers (red fibers)

• Associated with leg muscles (slow contractions)


Figure 9.11a Types of Skeletal Muscle Fibers


Slow fibers

Fast fibers

Smaller diameter,

darker color due to

myoglobin; fatigue

resistant

Larger diameter,

paler color;

easily fatigued

LM x 170

LM x 170

Note the difference in the size of

slow muscle fibers (above) and

fast muscle fibers (below).

a


• Features of Fast Fibers

• Large in diameter

• Large glycogen reserves

• Relatively few mitochondria

• Muscles contract using anaerobic metabolism

• Fatigue easily

• Can contract in 0.01 second or less after

stimulation

• Produce powerful contractions



• Features of Slow Fibers

• Half the diameter of fast fibers

• Take three times longer to contract after

stimulation

• Can contract for extended periods of time

• Contain abundant myoglobin (creates the red

color)

• Muscles contract using aerobic metabolism

• Have a large network of capillaries



• Features of Intermediate Fibers

• Similar to fast fibers

• Have low myoglobin content

• Have high glycolytic enzyme concentration

• Contract using anaerobic metabolism

• Similar to slow fibers

• Have lots of mitochondria

• Have a greater capillary supply

• Resist fatigue


Table 9.1 Properties of Skeletal Muscle Fiber Types



• Distribution of Fast, Slow, and Intermediate

Fibers

• Fast fibers

• High density associated with eye and hand

muscles

• Sprinters have a high concentration of fast fibers

• Repeated intense workouts increase the fast fibers



• Distribution of Fast, Slow, and Intermediate

Fibers (continued)

• Slow and intermediate fibers

• None are associated with the eyes or hands

• Found in high density in the back and leg muscles

• Marathon runners have a high amount

• Training for long distance running increases the

proportion of intermediate fibers


Organization of Skeletal Muscle Fibers

• Muscles can be classified based on shape or

by the arrangement of the fibers

• Parallel muscle fibers

• Convergent muscle fibers

• Pennate muscle fibers

• Unipennate muscle fibers

• Bipennate muscle fibers

• Multipennate muscle fibers

• Circular muscle fibers



• Parallel Muscle Fibers

• Muscle fascicles are parallel to the longitudinal

axis

• Examples: biceps brachii and rectus abdominis


Figure 9.12ab Skeletal Muscle Fiber Organization


(h)

(d)

(g)

(a)

(b)

(e)

(c)

Parallel Muscles

Parallel muscle

(Biceps brachii muscle)

a b Parallel muscle with

tendinous bands

(Rectus abdominis

muscle)

Fascicle

Body

(belly)

Cross section

(f)


• Convergent Muscle Fibers

• Muscle fibers form a broad area but come

together at a common point

• Example: pectoralis major


Figure 9.12d Skeletal Muscle Fiber Organization


Convergent Muscles

d Convergent muscle

(Pectoralis muscles)

Tendon

Base of

muscle

Cross

section

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)


• Pennate Muscle Fibers (Unipennate)

• Muscle fibers form an oblique angle to the tendon

of the muscle

• An example is unipennate

• All the muscle fibers are on the same side of the

tendon

• Example: extensor digitorum


Figure 9.12e Skeletal Muscle Fiber Organization


Pennate Muscles

e Unipennate

muscle (Extensor

digitorum muscle)

(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Extended

tendon


• Pennate Muscle Fibers (Bipennate)


of the muscle

• An example is bipennate

• Muscle fibers are on both sides of the tendon

• Example: rectus femoris


Figure 9.12f Skeletal Muscle Fiber Organization


(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Pennate Muscles

f Bipennate

muscle

(Rectus femoris

muscle)


• Pennate Muscle Fibers (Multipennate)


of the muscle

• An example is multipennate

• The tendon branches within the muscle

• Example: deltoid muscle


Figure 9.12g Skeletal Muscle Fiber Organization


(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Pennate Muscles

g Multipennate muscle

(Deltoid muscle)

Tendons

Cross section


• Circular Muscle Fibers

• Muscle fibers form concentric rings

• Also known as sphincter muscles

• Examples: orbicularis oris and orbicularis oculi


Figure 9.12h Skeletal Muscle Fiber Organization


(h)

(d)

(g)

(a)

(b)

(e)

(c)

(f)

Circular Muscles

Circular muscleh

(Orbicularis oris muscle)

Contracted

Relaxed

Muscle Terminology

• Origins and Insertions

• Origin

• Point of muscle attachment that remains stationary

• Insertion

• Point of muscle attachment that is movable

• Actions

• The function of the muscle upon contraction


Muscle Terminology

• There are two methods of describing

muscle actions

• The first makes reference to the bone region the

muscle is associated with

• The biceps brachii muscle causes “flexion of the

forearm”

• The second makes reference to a specific joint the

muscle is associated with

• The biceps brachii muscle causes “flexion at the

elbow”


Muscle Terminology

• Muscles can be grouped according to

their primary actions into four types

• Prime movers (agonists)

• Responsible for producing a particular movement

• Antagonists

• Actions oppose the action of the agonist

• Synergists

• Assist the prime mover in performing an action

• Fixators

• Agonist and antagonist muscles contracting at the

same time to stabilize a joint


Muscle Terminology

• Prime Movers example:

• Biceps brachii – flexes the lower arm

• Antagonists example:

• Triceps brachii – extends the lower arm

• Synergists example:

• Latissimus dorsi and teres major – contract to

move the arm medially over the posterior body

• Fixators example:

• Flexor and extensor muscles contract at the same

time to stabilize an outstretched hand


Muscle Terminology

• Most muscle names provide clues to their

identification or location

• Muscles can be named for:

• Specific body regions or location

• Shape of the muscle

• Orientation of the muscle fibers

• Specific or unusual features

• Its origin and insertion points

• Primary function

• References to occupational or habitual action


Muscle Terminology

• Examples of muscle names related to:

• Specific body regions or locations

• Brachialis: associated with the brachium of the

arm

• Tibialis anterior: associated with the anterior tibia

• Shape of the muscle

• Trapezius: trapezoid shape

• Deltoid: triangular shape


Muscle Terminology


• Orientation of the muscle fibers

• Rectus femoris: straight muscle of the leg

• External oblique: muscle on outside that is

oriented with the fibers at an angle

• Specific or unusual features

• Biceps brachii: two origins

• Teres major: long, big, rounded muscle


Muscle Terminology


• Origin and insertion points

• Sternocleidomastoid: points of attachment are

sternum, clavicle, and mastoid process

• Genioglossus: points of attachment are chin and

tongue

• Primary functions

• Flexor carpi radialis: a muscle that is near the

radius and flexes the wrist

• Adductor longus: a long muscle that adducts the

leg


Muscle Terminology


• References to occupational or habitual actions

• Buccinator (means “trumpet player”): the

buccinator area moves when playing a trumpet

• Sartorius: derived from the Latin term (sartor),

which is in reference to “tailors.” Tailors used to

cross their legs to form a table when sewing

material


Levers and Pulleys: A Systems Design for Movement

• Most of the time, upon contraction, a muscle

causes action

• This action is applied to a lever (a bone)

• This lever moves on a fixed point called the

fulcrum (joint)

• The action of the lever is opposed by a force

acting in the opposite direction



• There are three classes of levers

• First class, second class, third class

• First class

• The fulcrum (joint) lies between the applied force

and the resistance force (opposed force)

• Example: tilting the head forward and backward


Figure 9.13 Levers and Pulleys (2 of 6)


First-Class Lever

In a first-class lever, the applied force and the

resistance are on opposite sides of the

fulcrum. This lever can change the amount of

force transmitted to the resistance and alter the

direction and speed of movement. There are

very few first-class levers in the human body.

R

F

AF

R

F

AF

Resistance Fulcrum Applied force

Movement

completed


• Classes of Levers

• Second class

• The resistance is located between the applied force

and the fulcrum (joint)

• Example: standing on your tiptoes




Second-Class Lever

In a second-class lever, the resistance lies

between the applied force and the fulcrum.

This arrangement

magnifies force at the

expense of distanceand speed; the direction

of movement remains

unchanged. There are

very few second-class

levers in the body.

AF

R

F

F

R

AF

Movement

completed


• Classes of Levers

• Third class

• The force is applied between the resistance and

fulcrum (joint)

• Example: flexing the lower arm




Third-Class Lever

In a third-class lever, which is the most

common lever in the body, the applied force

is between the resistance and the fulcrum.

This arrangement increases speed and

distance moved but requires a larger

applied force.

R

F

F

AF

R

Movement

completed


• Sometimes, a tendon may loop around a bony

projection

• This bony projection could be called a pulley

• Example: lateral malleolus and trochlea of the eye




The Lateral Malleolus

as an Anatomical Pulley

Plantar flexion of the foot

Pulley

Fibularis

longus

Lateral

malleolus

The lateral malleolus of the fibula is an

example of an anatomical pulley. The

tendon of insertion for the fibularis longus

muscle does not follow a direct path.

Instead, it curves around the posterior

margin of the lateral malleolus of the

fibula. This redirection of the contractile

force is essential to the normal function

of the fibularis longus in producing

plantar flexion at the ankle.



The Patella as an

Anatomical

Pulley

Pulley

Quadriceps muscles

Quadriceps tendon

Patella

Patellar

ligament

Extension

of the leg

The patella is another

example of an anatomical

pulley. The quadricepsfemoris is a group of four

muscles that form the anterior musculature of the

thigh. These four muscles attach to the patella by the

quadriceps tendon. The patella is, in turn, attached to the

tibial tuberosity by the patellar ligament. The quadriceps

femoris muscles produce extension at the knee by this

two-link system. The quadriceps tendon pulls on the

patella in one direction throughout the movement, but

the direction of force applied to the tibia by the patellar

ligament changes constantly as the movement proceeds.

Aging and the Muscular System

• Changes occur in muscles as we age

• Skeletal muscle fibers become smaller in diameter

• Due to a decrease in the number of myofibrils

• Contain less glycogen reserves

• Contain less myoglobin

• All of the above results in a decrease in strength

and endurance

• Muscles fatigue rapidly


Aging and the Muscular System

• Changes occur in muscles as we age (continued)

• There is a decrease in myosatellite cells

• There is an increase in fibrous connective tissue

• Due to the process of fibrosis

• The ability to recover from muscular injuries

decreases


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Skeletal Muscle Tissue and Muscle Organization

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