Transcript Smooth muscle

Chapter 10
Muscle Tissue
Lecture Outline
INTRODUCTION
• Motion results from alternating contraction
(shortening) and relaxation of muscles; the
skeletal system provides leverage and a
supportive framework for this movement.
• The scientific study of muscles is known
as myology.
OVERVIEW OF MUSCLE
TISSUE
• Types of Muscle Tissue
• Skeletal muscle tissue is primarily
attached to bones. It is striated and
voluntary.
• Cardiac muscle tissue forms the wall of
the heart. It is striated and involuntary.
• Smooth (visceral) muscle tissue is located
in viscera. It is nonstraited (smooth) and
involuntary.
• Table 4.4 compares the different types of
Functions of Muscle Tissue
• Producing body movements
• Stabilizing body positions
• Regulating organ volumes
– bands of smooth muscle called sphincters
• Movement of substances within the body
– blood, lymph, urine, air, food and fluids, sperm
• Producing heat
– involuntary contractions of skeletal muscle
(shivering)
Properties of Muscle Tissue
• Excitability
– respond to chemicals released from nerve cells
• Conductivity
– ability to propagate electrical signals over
membrane
• Contractility
– ability to shorten and generate force
• Extensibility
– ability to be stretched without damaging the tissue
• Elasticity
Skeletal Muscle -- Connective Tissue
• Superficial fascia is loose connective tissue & fat
underlying the skin
• Deep fascia = dense irregular connective tissue
around muscle
• Connective tissue components of the muscle include
– epimysium = surrounds the whole muscle
– perimysium = surrounds bundles (fascicles) of 10100 muscle cells
– endomysium = separates individual muscle cells
• All these connective tissue layers extend
beyond the muscle belly to form the tendon
Connective Tissue Components
• Tendons and aponeuroses are extensions
of connective tissue beyond muscle cells
that attach muscle to bone or other
muscle.
• A tendon is a cord of dense connective
tissue that attaches a muscle to the
periosteum of a bone (Figure 11.22).
• An aponeurosis is a tendon that extends
as a broad, flat layer (Figure 11.4c).
Nerve and Blood Supply
• Each skeletal muscle is supplied by a nerve,
artery and two veins.
• Each motor neuron supplies multiple muscle
cells (neuromuscular junction)
• Each muscle cell is supplied by one motor
neuron terminal branch and is in contact with
one or two capillaries.
– nerve fibers & capillaries are found in the
endomysium between individual cells
Sarcolemma, T Tubules, and
Sarcoplasm
• Skeletal muscle consists of fibers (cells)
covered by a sarcolemma (Figure 10.3b).
– The fibers contain T tubules and sarcoplasm
– T tubules are tiny invaginations of the
sarcolemma that quickly spread the muscle
action potential to all parts of the muscle fiber.
• Sarcoplasm is the muscle cell cytoplasm
and contains a large amount of glycogen
for energy production and myoglobin for
oxygen storage.
Transverse Tubules
• T (transverse) tubules are invaginations of the
sarcolemma into the center of the cell
– filled with extracellular fluid
– carry muscle action potentials down into cell
• Mitochondria lie in rows throughout the cell
Myofibrils & Myofilaments
• Muscle fibers are filled with threads called
myofibrils separated by SR (sarcoplasmic
reticulum)
• The sarcoplasmic reticulum encircles each
myofibril. It is similar to smooth endoplasmic
reticulum in nonmuscle cells and in the
relaxed muscle stores calcium ions.
Sarcoplasmic Reticulum
(SR)
• System of tubular sacs similar to smooth
ER in nonmuscle cells
• Stores Ca+2 in a relaxed muscle
• Release of Ca+2 triggers muscle
•
Filaments and the
Sarcomere
Thick and thin
filaments overlap each
other in a pattern that creates striations
(light I bands and dark A bands)
• The I band region contains only thin
filaments.
• They are arranged in compartments called
sarcomeres, separated by Z discs.
• In the overlap region, six thin filaments
surround each thick filament
Sarcomere
• Figure 10.5 shows the relationships of the
zones, bands, and lines as seen in a
transmission electron micrograph.
• Exercise can result in torn sarcolemma,
damaged myofibrils, and disrupted Z discs
(Clinical Application).
The Proteins of Muscle
• Myofibrils are built of 3 kinds of protein
– contractile proteins
• myosin and actin
– regulatory proteins which turn contraction on
& off
• troponin and tropomyosin
– structural proteins which provide proper
alignment, elasticity and extensibility
• titin, myomesin, nebulin and dystrophin
The Proteins of Muscle -- Myosin
• Thick filaments are composed of myosin
– each molecule resembles two golf clubs twisted
together
– myosin heads (cross bridges) extend toward the
thin filaments
• Held in place by the M line proteins.
The Proteins of Muscle -- Actin
• Thin filaments are made of actin, troponin, &
tropomyosin
• The myosin-binding site on each actin
molecule is covered by tropomyosin in relaxed
muscle
• The thin filaments are held in place by Z lines.
From one Z line to the next is a sarcomere.
Sliding Filament Mechanism Of
Contraction
• Myosin cross
bridges
pull on thin
filaments
• Thin filaments
slide
inward
• Z Discs come
toward
each other
• Sarcomeres
How Does Contraction
Begin?
1. Nerve impulse reaches an axon terminal &
synaptic vesicles release acetylcholine (ACh)
2. ACh diffuses to receptors on the sarcolemma
& Na+ channels open and Na+ rushes into
the cell
3. A muscle action potential spreads over
sarcolemma and down into the transverse
tubules
4. SR releases Ca+2 into the sarcoplasm
5. Ca+2 binds to troponin & causes troponintropomyosin complex to move & reveal
Contraction Cycle
• Repeating sequence of events that cause
the thick & thin filaments to move past
each other.
• 4 steps to contraction cycle
– ATP hydrolysis
– attachment of myosin to actin to form
crossbridges
– power stroke
– detachment of myosin from actin
• Cycle keeps repeating as long as there is
ATP and Myosin
• Myosin heads are activated by ATP
• Activated heads attach to actin & pull
(power stroke)
• ADP is released. (ATP released P &
ADP & energy)
• Thin filaments slide past the thick
filaments
• ATP binds to myosin head & detaches it
from actin
Relaxation
• Acetylcholinesterase (AChE) breaks down
ACh within the synaptic cleft
• Muscle action potential ceases
• Ca+2 release channels close
• Active transport pumps Ca2+ back into
storage in the sarcoplasmic reticulum
• Calcium-binding protein (calsequestrin) helps
hold Ca+2 in SR (Ca+2 concentration 10,000
times higher than in cytosol)
• Tropomyosin-troponin complex recovers
binding site on the actin
Events Occurring After a Nerve Signal
1. Arrival of nerve impulse at nerve terminal causes
release of ACh from synaptic vesicles
2. ACh binds to receptors on muscle motor end plate
opening the gated ion channels so that Na+ can
rush into the muscle cell
3. Inside of muscle cell becomes more positive,
triggering a muscle action potential that travels over
the cell and down the T tubules
4. The release of Ca+2 from the SR is triggered and
the muscle cell will shorten & generate force
5. Acetylcholinesterase breaks down the ACh
attached to the receptors on the motor end plate so
the muscle action potential will cease and the
Pharmacology of the NMJ
• Botulinum toxin blocks release of
neurotransmitter at the NMJ so muscle
contraction can not occur
– bacteria found in improperly canned food
– death occurs from paralysis of the diaphragm
• Curare (plant poison from poison arrows)
– causes muscle paralysis by blocking the ACh
receptors
– used to relax muscle during surgery
• Neostigmine (anticholinesterase agent)
– blocks removal of ACh from receptors so
MUSCLE METABOLISM
• Creatine phosphate and ATP can power
maximal muscle contraction for about 15
seconds and is used for maximal short
bursts of energy (e.g., 100-meter dash)
(Figure 10.13a).
– Creatine phosphate is unique to muscle
fibers.
MUSCLE METABOLISM
• The partial catabolism of glucose to
generate ATP occurs in anaerobic cellular
respiration (Figure 10.13b). This system
can provide enough energy for about 3040 seconds of maximal muscle activity
(e.g., 300-meter race).
• Muscular activity lasting more than 30
seconds depends increasingly on aerobic
cellular respiration (reactions requiring
Muscle Fatigue
• Inability to contract after prolonged
activity
• Factors that contribute to fatigue
– central fatigue is feeling of tiredness and a
desire to stop (protective mechanism)
– insufficient release of acetylcholine from
motor neurons
– depletion of creatine phosphate
– decline of Ca+2 within the sarcoplasm
– insufficient oxygen or glycogen
The Motor Unit
• Motor unit = one somatic motor neuron & all the
skeletal muscle cells (fibers) it stimulates (10 cells to
2,000 cells)
– muscle fibers normally scattered throughout belly of
muscle
– One nerve cell supplies on average 150 muscle cells that
all contract in unison.
Motor Unit Recruitment
• Motor units in a whole muscle fire
asynchronously
– some fibers are active others are relaxed
– delays muscle fatigue so contraction can be
sustained
• Produces smooth muscular contraction
– not series of jerky movements
• Precise movements require smaller
contractions
– motor units must be smaller (less fibers/nerve)
Cardiac versus Skeletal
Muscle
• More sarcoplasm and mitochondria
• Larger transverse tubules located at Z
discs, rather than at A-l band junctions
• Less well-developed SR
• Limited intracellular Ca+2 reserves
– more Ca+2 enters cell from extracellular
fluid during contraction
• Prolonged delivery of Ca+2 to
sarcoplasm, produces a contraction that
last 10 -15 times longer than in skeletal
Physiology of Cardiac Muscle
• Autorhythmic cells
– contract without stimulation
• Contracts 75 times per min & needs lots of
O2
• Larger mitochondria generate ATP
aerobically
• Extended contraction is possible due to
slow Ca+2 delivery
– Ca+2 channels to the extracellular fluid stay
SMOOTH MUSCLE
• Smooth muscle tissue is nonstriated and
involuntary and is classified into two types:
visceral (single unit) smooth muscle
(Figure 10.18a) and multiunit smooth
muscle (Figure 10.18b).
– Visceral (single unit) smooth muscle is found
in the walls of hollow viscera and small blood
vessels; the fibers are arranged in a network
and function as a “single unit.”
Two Types of Smooth Muscle
• Visceral (single-unit)
– in the walls of hollow
viscera & small BV
– autorhythmic
– gap junctions cause
fibers to contract in
unison
• Multiunit
– individual fibers with
own motor neuron
ending
– found in large arteries,
large airways, arrector
Physiology of Smooth Muscle
• Contraction starts slowly & lasts longer
– no transverse tubules & very little SR
– Ca+2 must flows in from outside
• In smooth muscle, the regulator protein
that binds calcium ions in the cytosol is
calmodulin (in place of the role of
troponin in striated muscle);
– calmodulin activates the enzyme myosin
light chain kinase, which facilitates myosinactin binding and allows contraction to occur
at a relatively slow rate.
Smooth Muscle Tone
• The prolonged presence of calcium ions in
the cytosol of smooth muscle fibers
provides for smooth muscle tone, a state
of continued partial contraction.
• Smooth muscle fibers can stretch
considerably without developing tension;
this phenomenon is termed the stressrelaxation response.
• Useful for maintaining blood pressure or a
steady pressure on the contents of GI
tract
Atrophy and Hypertrophy
• Atrophy
– wasting away of muscles
– caused by disuse (disuse atrophy) or severing of
the nerve supply (denervation atrophy)
– the transition to connective tissue can not be
reversed
• Hypertrophy
– increase in the diameter of muscle fibers
– resulting from very forceful, repetitive muscular
activity and an increase in myofibrils, SR &
mitochondria
Muscle
Attachmen
t Sites:
Origin and
Insertion
• Skeletal muscles shorten & pull on the bones
they are attached to
• Origin is the bone that does not move when
muscle shortens (normally proximal)
• Insertion is the movable bone (some 2 joint
muscles)
HOW SKELETAL MUSCLES
ARE NAMED
• The names of most of the nearly 700
skeletal muscles are based on several
types of characteristics.
• These characteristics may be reflected in
the name of the muscle.
• The most important characteristics include
the direction in which the muscle fibers
run, the size, shape, action, numbers of
origins, and location of the muscle, and
the sites of origin and insertion of the
Muscles of Abdominal Wall
• 4 pairs of sheetlike muscles
– rectus abdominis = vertically oriented
– external & internal obliques and transverses
abdominis
• wrap around body to form anterior body wall
• form rectus sheath and linea alba
• Inguinal ligament from anterior superior iliac
spine to upper surface of body of pubis
• Inguinal canal = passageway from pelvis
through body wall musculature opening seen
as superficial inguinal ring
• Inguinal hernia = rupture or separation of
Compartment Syndrome
• Skeletal muscles in the limbs are
organized in units called compartments.
• In compartment syndrome, some external
or internal pressure constricts the
structures within a compartment, resulting
in damaged blood vessels and subsequent
reduction of the blood supply to the
structures within the compartment.
• Without intervention, nerves suffer
damage, and muscle develop scar tissue