The Muscular System: Skeletal Muscle Tissue and Organizationcherylchowbiology.weebly.com/uploads/1/7/1/7/17179396/10._muscular... · The Muscular System: Skeletal Muscle Tissue

© 2012 Pearson Education, Inc.

9 The Muscular System:

Skeletal Muscle Tissue

and Organization


Introduction

• Humans rely on muscles for:

• Many of our physiological processes

• Virtually all our dynamic interactions with the

environment


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

• 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

• 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



• 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


Figure 9.1 Structural Organization of Skeletal Muscle

Epimysium

Muscle fascicle

Endomysium

Perimysium

Nerve

Muscle fibers

Blood vessels

SKELETAL MUSCLE (organ)

MUSCLE FASCICLE (bundle of cells)

Perimysium

Muscle fiber

Endomysium

Epimysium

Blood vessels

and nerves

Endomysium

Perimysium

Tendon

MUSCLE FIBER (cell)

Mitochondria

Sarcolemma

Myofibril

Axon Sarcoplasm

Capillary

Endomysium

Myosatellite

cell

Nucleus



• Gross Anatomy • Nerves and blood vessels

• Nerves innervate the muscle

• There is a chemical communication between a nerve and a muscle

• The nerve is “connected” to the muscle via the motor end plate • This is the

neuromuscular junction

• Blood vessels innervate the endomysium to supply oxygen and continuous removal of waste


Anatomy of Skeletal Muscles • Microanatomy of skeletal muscle fibers

• Sarcolemma

• Membrane that surrounds the muscle cell

• Sarcoplasm

• The cytosol of the muscle cell

• Muscle fiber = a muscle cell

• Can be 30–40 cm in length

• Multinucleated (each muscle cell has hundreds of nuclei)

• Nuclei are located just deep to the sarcolemma



• Myofibrils and Myofilaments • The sarcoplasm contains

myofibrils • Myofibrils are responsible

for the contraction of muscles

• Surrounding each myofibril is the sarcoplasmic reticulum

• Myofibrils are made of myofilaments • Actin

• Myosin



• Sarcomere

Organization

• Myosin (thick filament)

• Actin (thin filament)

• Both are arranged in

repeating contractile

units called

sarcomeres

• This overlapping

creates the striations



• Sarcomere

Organization

• Each sarcomere

consists of:

• Z line (Z disc)

• I band

• A band (overlapping A

bands create striations)

• H zone (band)

• M line



• Levels of Organization

• 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


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 9.10a The Neuromuscular Synapse

A diagrammatic view of a

neuromuscular synapse

Motor

neuron

Axon

Path of action potential

Neuromuscular synapse

Motor end plate

Myofibril


Figure 9.11 The Events in Muscle Contraction

Ca2

Actin

Myosin

Sarcolemma T tubule

Motor

end

plate

Synaptic

terminal

STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION

Triggered by calcium release Calcium is stored


Figure 9.12 The Arrangement of Motor Units in a Skeletal Muscle

Motor unit 1

Motor unit 2

Motor unit 3

KEY

SPINAL CORD

Muscle fibers

Axons of motor neurons

Motor nerve

The more motor units contracting,

the more force generated


Muscle Hypertrophy

• Exercise causes an

increase in:

• the number of

mitochondria

• 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

(hypertrophy)


Motor Units and Muscle Control

• 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 skeletal muscle

fibers: • Fast fibers (white fibers)

• Associated with eye muscles

• Ex. Sprinting muscles

• Intermediate fibers (pink fibers)

• Slow fibers (red fibers)

• Associated with leg muscles

• Ex. Long distance runners


Figure 9.13a Types of Skeletal Muscle Fibers

Note the difference in the size of

slow muscle fibers (above) and

fast muscle fibers (below).

LM 170

LM 170

Slow fibers Smaller diameter,

darker color due to

myoglobin; fatigue

resistant

Fast fibers Larger diameter,

paler color;

easily fatigued



• Features of fast fibers:

• Large in diameter

• Large glycogen reserves

• Relatively few mitochondria

• 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), stores oxygen

• Muscles contract using aerobic metabolism

• Have a more extensive blood supply



• Features of intermediate fibers:

• Contract faster then slow fibers but slower then

fast fibers

• Similar to fast fibers except:

• Have lots of mitochondria

• Have a greater blood supply

• Resist fatigue


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



• Convergent muscle

fibers

• Muscle fibers form a

broad area but come

together at a common

point

• Example: pectoralis

major



• Pennate muscle fibers

• 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



• An example is

bipennate

• Muscle fibers are on

both sides of the tendon

• Example: rectus

femoris



• An example is

multipennate

• The tendon branches

within the muscle

• Example: deltoid

muscle



• Circular muscle fibers

• Muscle fibers form

concentric rings

• Also known as

sphincter muscles

• Examples: orbicularis

oris and orbicularis

oculi


Muscle Terminology

• Origin, Insertion, and Action

• Origin

• Point of muscle attachment that remains stationary

• Insertion

• Point of muscle attachment that is movable

• Action

• The function of the muscle upon contraction


Muscle Terminology

• Muscles can be grouped according to their primary actions into four types:

• Prime movers (agonists)

• Responsible for producing a particular movement

• Ex. Biceps brachii – flexes the lower arm

• Antagonists

• Actions oppose the action of the agonist

• Ex. Triceps brachii – extends the lower arm

• Synergists

• Assist the prime mover in performing an action

• Ex. Latissimus dorsi and teres major – contract to move the arm medially over the posterior body

• Fixators

• Agonist and antagonist muscles contracting at the same time to stabilize a joint

• Ex. Flexor and extensor muscles contract at the same time to stabilize an outstretched hand



• 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

• 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


Muscle Terminology


• Origin and insertion points

• Sternocleidomastoid: points of attachment are sternum, clavicle, and mastoid process

• 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


Aging and the Muscular System

• Changes occur in muscles as we age

• Skeletal muscle fibers become smaller in diameter

• There is 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

• The ability to recover from muscular injuries decreases

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The Muscular System: Skeletal Muscle Tissue and Organizationcherylchowbiology.weebly.com/uploads/1/7/1/7/17179396/10._muscular... · The Muscular System: Skeletal Muscle Tissue