PART 4 MUSCULAR TISSUE
Unit 2 Covering, Support, and Movement
OVERVIEW OF MUSCLE TISSUE
Compare and contrast the three basic types of
muscle tissue
• List four special physical characteristics of
muscle tissue
• List four important functions of muscle tissue
•
Skeletal Muscle
Description Long, cylindrical, multinucleate
cells; obvious striations
Function
Voluntary movement;
locomotion; manipulation of
the environment; facial
expression; voluntary control
Location
In skeletal muscles attached to
bones or occasionally to skin
Cardiac Muscle
Description Branching, striated, generally
uninucleate cells that
interdigitate at specialized
junctions (intercalated disks)
Function
As it contracts, it propels
blood into the circulation;
involuntary control
Location
The walls of the heart
Smooth Muscle
Description Spindle-shaped cells with
central nuclei; no striations;
cells arranged closely to form
sheets
Function
Propels substances or objects
along internal passageways;
involuntary control
Location
Mostly in the walls of hollow
organs
Physical Characteristics of Muscle
Electrical
excitability
• The ability to receive and respond
to a stimulus
Contractility
• The ability to shorten forcefully
when adequately stimulated
Extensibility
• The ability to be stretched or
extended
Elasticity
• The ability to recoil after being
stretched
Muscle Functions
Producing movement
Maintaining posture and body position
Stabilizing joints
Generating heat
ANATOMY OF SKELETAL MUSCLE
Describe the gross structure of a skeletal muscle
•Describe the microscopic anatomy of a skeletal
muscle fiber
•Describe the sliding filament model of muscle
contraction
•
Skeletal Muscle Organs

A skeletal muscle is an organ because it is
made up of several kinds of tissues
 Skeletal
muscle fibers
 Blood vessels
 Nerve fibers
 Connective tissue
Nerve Supply of Muscle



Muscle has a rich
nerve supply
Each muscle fiber is
innervated by its own
nerve ending
Allows for precise
neural control
Blood Supply of Muscle

Muscle has a rich blood supply
 Contracting
muscle fibers use
huge amounts of energy and
require continuous delivery of
oxygen and nutrients

Capillaries are long and
winding

Accommodates for changes in
muscle length
Connective Tissue Sheaths



Individual muscle fibers are wrapped and held
together by connective tissue sheaths
Sheaths support each cell and reinforce the
muscle as a whole
The sheaths are:
 Epimysium
– surrounds whole muscle
 Perimysium – surrounds fascicles
 Endomysium – surrounds each fiber
Figure 10.1 Organization of Skeletal Muscle
Muscle Attachments



Most skeletal muscles span
joints and are attached to
bones in at least two places
When muscle contracts,
insertion moves towards origin
In limb muscles, origin
typically lies proximal to
insertion
Types of Muscle Attachments

Direct attachments
 muscle’s
epimysium is fused to
bone’s periosteum

Indirect attachments
 muscle’s
connective tissue sheaths
are attached to bone either as a
 ropelike
tendon
 sheetlike aponeurosis
Types of Muscle Attachments

Direct attachments
 muscle’s
epimysium is fused to
bone’s periosteum

Indirect attachments
 muscle’s
connective tissue sheaths
are attached to bone either as a
 ropelike
tendon
 sheetlike aponeurosis
Anatomy of a Skeletal Muscle Fiber

Sarcolemma
T

tubules
Sarcoplasm
 Glycosomes
 Myoglobin

Specialized organelles
Myofibrils
 Sarcoplasmic reticulum

Figure 10.2 Microscopic Organization of Skeletal Muscle
Myofibrils

Each muscle fiber contains thousands of
myofibrils


Account for about 80% of cell volume
Myofibrils are composed of even smaller
myofilaments
 Understanding
how myofilaments are organized is
key to understanding how muscle cells contract
Striations, Sarcomeres, and Myofilaments


Repeating light and dark bands are
evident along length of each myofibril
A bands are dark
 Lighter
midsection region called H zone
 H zone bisected by dark M line

I bands are light
 Bisected
by dark Z disk
Striations, Sarcomeres, and Myofilaments


Dark A bands and light I bands
are nearly perfectly aligned
with each other
Gives the cell as a whole its
striated appearance
Striations, Sarcomeres, and Myofilaments
Sarcomere
• Smallest contractile unit of a
muscle fiber
• Functional unit of skeletal muscle
• Region between two successive Z
disks
• Contains an A band flanked by
half an I band at each end
• Aligned end-to-end like boxcars
in a train
Striations, Sarcomeres, and Myofilaments

Sarcomere’s banding pattern is due to
myofilaments
 Thick
filaments extend entire length of A band
 Thin filaments extend across I band and partway
into A band
 Thick and thin filaments overlap at A band ends
Figure 10.3 Arrangement of Filaments Within a Sarcomere
Figure 10.3 Zones and Bands of a Sarcomere
Molecular Composition of Myofilaments

Thick filament composed of myosin protein


Rodlike tail with flexible hinge
2 globular heads


Orientation within A band


Actin and ATP binding sites; ATPase enzymes
Tails point towards M line
Heads face Z disk
Molecular Composition of Myofilaments

Thin filament composed of actin protein


Kidney-shaped with a myosin binding site
Thin filament composed of two strands of actin
molecules twisted together
One end is firmly attached to Z disk
 Other end extends into A band

Molecular Composition of Myofilaments

Tropomyosin
rod shaped protein
 spirals around thin filament
 helps stiffen and stabilize it
 Blocks the myosin binding sites on actin


Troponin
binds to actin
 binds to tropomyosin
 binds calcium ions
 Lifts tropomyosin off thin filament to expose myosin
binding sites

Figure 10.5 Structure of Thick and Thin Filaments
Sarcoplasmic Reticulum


Elaborate network of smooth
endoplasmic reticulum
wrapped around each
myofibril
Regulates intracellular levels
of ionic calcium
Stores calcium and releases it
on demand
 Provides the final “go” signal
for contraction

T Tubules


Specialization at each A
band-I band junction
Sarcolemma protrudes deep
into cell interior
 Forms
T tubule
 Comes into close contact with
sarcoplasmic reticulum
Sliding Filament Theory of Contraction

The theory:
During contraction, thin filaments slide
past thick filaments
 Occurs when muscle fibers are
stimulated by nerve

 Myosin
heads latch onto myosin-binding
sites of actin molecules and sliding
begins
 Filament
sliding shortens sarcomeres,
which shortens muscle fiber, which
shortens entire muscle
Sir Hugh Huxley
Figure 10.6 The Sliding Filament Mechanism
CONTRACTION OF A SKELETAL MUSCLE
FIBER
Explain how events at the neuromuscular junction
stimulate a skeletal muscle fiber to contract
• Describe how an action potential is generated
• Explain excitation-contraction coupling
• Describe cross bridge cycling
• Explain the length-tension relationship in a
skeletal muscle fiber
•
Overview of Skeletal Muscle Contraction

1.
2.
3.
For a skeletal muscle fiber to contract, 3 events
have to occur:
The fiber must be activated by a nerve
The fiber must generate and propagate an action
potential along its sarcolemma and down its T
tubules
Ca2+ must be released from the SR to trigger
contraction
The Neuromuscular junction


The synapse between a
somatic motor neuron
and a skeletal muscle
fiber
Neurotransmitter is
acetylcholine (ACh)
 Causes
a change in
membrane permeability,
which leads to a change in
membrane potential
The
Neuromuscular
Junction
Figure 10.10a
Figure 10.10b The Neuromuscular Junction
Recipe for Muscle Fiber Activation
Ingredients:
Directions:
Neuron
Skeletal
muscle cell
Acetylcholine
Ion channels
Na+
K+
Ca2+
1. Action potential arrives at motor
neuron’s axon terminal
2. Voltage-gated Ca2+ channels open
and Ca2+ enters the axon terminal
3. Ca2+ entry causes some synaptic
vesicles to release acetylcholine by
exocytosis
Recipe for Muscle Fiber Activation
Ingredients:
Directions:
Neuron
Skeletal
muscle cell
Ach
Ach receptors
Ion channels
Na+
K+
Ca2+
4. Ach diffuses across synaptic cleft
and binds to sarcolemma receptors
5. Ach binding opens ion channel Na+ diffuses into cell, producing a
local change in membrane potential
(depolarization)
6. Ach broken down by
acetylcholinesterase
Figure 10.10c The Neuromuscular Junction
Figure 10.10d The Neuromuscular Junction
Generation and Propagation of Action
Potential

Sarcolemma is polarized
 there
is a potential difference across
the membrane
 The resting membrane potential

Action potential
A
reversal of resting membrane
potential
 Released electrical energy
 Involves 2 steps
 Depolarization
 Repolarization
Recipe for Generating and
Propagating an Action Potential
Ingredients:
Directions:
Sarcolemma
Ach
Ach receptor
Ion channels
Na+
K+
Ca2+
1. Ach binds to sarcolemma receptors
a. Na+ channels open
b. Na+ diffuses into cell
c. Sarcolemma depolarizes
2. Depolarization in NMJ  depolarization
of adjacent sarcolemma segments
3. Areas of depolarization quickly
repolarize
a. K+ channels open
b. K+ diffuses into cell
c. Sarcolemma repolarizes
Phases of an Action Potential in Skeletal Muscle
Excitation – Contraction Coupling

Connects nervous excitation of a skeletal muscle fiber
to contraction
Recipe for Excitation – Contraction
Coupling
Ingredients:
Directions:
Sarcolemma
T tubules
Action Potential
SR
Voltage-gated
Ca2+channels
Ca2+
Troponin
Tropomyosin
1. Action potential propagates along
sarcolemma, into T tubules, towards SR
2. Voltage-gated Ca2+ channels in SR open
3. Ca2+ pours into cytosol; binds to troponin
4. Troponin moves tropomyosin away from
myosin-binding sites on actin - Myosin now
free to bind with actin - contraction cycle
begins
5. Contraction cycle continues until Ca2+
active transport pumps return Ca2+ to SR
Figure 10.8 Excitation – Contraction Coupling
Cross Bridge Cycling

The series of events during which myosin heads pull
thin filaments toward the sarcomere’s center
Figure 10.7 The Contraction Cycle
Figure 10.7 Summary of the Events of Contraction and
Relaxation in a Skeletal Muscle Fiber