Bio 322- Human Anatomy Todays topics Muscular system
Slide 2
Organization of muscles and the muscular system Muscle
function: 1.Movement Generate force to move bones walking, moving
body parts Generate force to move body contents blood, wastes,
food, childbirth, etc 2.Stability Muscle tone resists force of
gravity and helps stabilize some joints Posture 3.Communication
Speaking, writing, facial expressions, etc 4.Control of body
openings SPHINCTER muscles control openings of eyes, mouth,
digestive system, urethra, anus 5.Heat production - Muscles
generate a lot of heat when contracting (85% of body heat) Keeps
body around 98.6 degrees important for enzyme function! These
functions may be carried out by skeletal, smooth, or cardiac
muscle, but well mostly focus on skeletal muscle
Slide 3
Connective tissues of a muscle A muscle (biceps brachii,
pectoralis, etc..) is an organ made up of 1,000s of individual
muscle cells (MUSCLE FIBERS) that work together to generate force
Multiple layers of connective tissue allows for attachment to bone
and is important for organization of the muscle fibers Keeps muscle
fibers organized and packed tightly together Connective tissue
merges with tendons to attach to bone
Slide 4
Connective tissues of a muscle Innermost Thinnest Outermost
Thickest 1.ENDOMYSIUM : areolar connective tissue that surrounds
EACH muscle fiber Allows blood vessels and nerves access the muscle
fiber Muscle fibers are grouped together in bundles called
FASCICLES (maybe 10 fibers per fascicle) 2.PERIMYSIUM : Thicker
connective tissue that surrounds EACH FASCICLE Fascicles can be
seen with the naked eye as grains like on a piece of steak
3.EPIMYSIUM : Thicker connective tissue that surrounds the ENTIRE
MUSCLE Keeps fascicles bundled together 4.FASCIA : Connective
tissue that surrounds and separates muscles from other tissues DEEP
FASCIA separates one muscle from another (ex.: between biceps
brachii and brachialis) SUPERFICIAL FASCIA contains adipose tissue
and separates muscles from overlying skin (ex.: between biceps and
skin of arm) Blends into and is hard to differentiate from
epimysium
Slide 5
Connective tissues of a muscle Attachment of muscle to bones
1.DIRECT : connective tissue surrounding muscle (epimysium) fuses
directly with the periosteum of the bone 2.INDIRECT : Epimysium and
deep facia transitions into a tendon which then inserts into the
periosteum of a bone Very common, creates a physical gap between
muscle and bone 3.APONEUROSIS : Broad, sheet-like tendon that fuses
muscle to bone (abdominal muscles) Aponeurosis of external
oblique
Slide 6
Muscle anatomy terminology Most muscles of the body are
attached to different bones on either end. The contraction of the
muscle causes the movement of one of the bones ORIGIN the
stationary attachment point of a muscle to a bone this bone does
not move when muscle is contracted INSERTION : the moveable
attachment point of a muscle to a bone this bone moves when the
muscle contracts
Slide 7
Actions of groups of muscles Very often, two or more muscle act
together to produce movement at a joint Ex.: Biceps brachii and
brachialis both act to flex the forearm AGONIST : muscle that
produces the most force during a particular movement SYNERGIST :
muscle that aids the agonist in producing a given movement Usually
insertion is different than that of the agonist ANTAGONIST : muscle
that opposes the agonist creates movement in a joint that is in the
opposite direction of the agonist FIXATOR : a muscle whose function
is to prevent movement of ANOTHER bone Instrinsic muscles have an
origin and insertion in the SAME region (tongue, back muscles,
etc..) Extrinsic muscles have an origin and insertion in DIFFERENT
regions Flexor digitorum superficialis originates on humerus and
proximal radius (elbow and forearm) but inserts on the
phalanges
Slide 8
Skeletal muscle histology (more than one, located near plasma
membrane) (long and cylindrical) (thin C.T. that surrounds EACH
muscle fiber) (due to overlapping of actin and myosin) Muscle
fibers have several nuclei because they come from the fusion of
several MYOBLASTS Occurs during embryonic development
Undifferentiated myoblasts exist in adults as SATELLITE CELLS (a
form of stem cell)
Slide 9
Slide 10
Microscopic anatomy of a muscle fiber Like all cells, muscle
cells have a nucleus, plasma membrane, cytoplasm, etc In muscle
cells some structures have special names: Plasma membrane =
SARCOLEMMA Cytoplasm = SARCOPLASM Endoplasmic reticulum =
SARCOPLASMIC RETICULUM - VERY important to muscle fiber function
because it acts as the internal storage site of Ca 2+ (needed for
muscle contraction)
Slide 11
Microscopic anatomy of a muscle fiber The vast majority of
space inside a cell is occupied by the protein filaments that cause
the cell to contract bundles of filaments (called MYOFILAMENTS)
combine to form MYOFIBRILS Larger muscle fibers will contain more
myofibrils Other essential organelles (mitochondria, ER, etc) are
packed in between the myofibrils The sarcoplasmic reticulum forms a
loose network around each myofibril Dilations of the S.R. are known
as TERMINAL CISTERNAE (contain a great deal of Ca 2+ ) Myofilaments
(Thick/thin filaments) Myofibrils Muscle Fiber NOTE: this image
shows ONE muscle cell (fiber)
Slide 12
Microscopic anatomy of a muscle fiber TRANSVERSE TUBULES
(T-tubules) small tubes that are created by the infolding of the
sarcolemma The T-tubule combines with two terminal cisternae on
either side to form a TRIAD The T-tubule carries electrical signals
(nerve impulses) to the inside of the cell and triggers the release
of Ca 2+ from the SR and terminal cisternae Nerve impulse
Slide 13
Molecular structure of myofibrils Each MYOFIBRIL consists of a
bundle of long, parallel myofilaments : Thick filaments, thin
filaments, elastic filaments Thick Filaments Made up of hundreds of
protein molecules called MYOSIN shaped like a golf club Myosin
molecules are arranged end-to-end in a spiral fashion with the
heads facing outward Thin filaments Made up of long winding strands
of the protein ACTIN Actin molecule has an ACTIVE SITE to bind to
the myosin head When a muscle is relaxed, the active sites of actin
are covered up by two proteins called TROPOMYOSIN and TROPONIN
myosin cant bind to actin and cause contraction Elastic filaments
Smallest of the filaments made up of protein called TITIN A
stretchy filament that anchors the thick filaments in place
Slide 14
Molecular structure of myofibrils Myosin and actin are
considered CONTRACTILE PROTEINS since they are responsible for the
actual shortening of the muscle fiber (contraction) Tropomyosin and
troponin are considered REGULATORY PROTEINS since their job is to
regulate when and if a muscle cell will contract There are also a
number of accessory proteins that also play a role in muscle fiber
contraction DYSTROPHIN a HUGE protein that anchors actin filaments
to the endomysium Links the shortening muscle fiber to external CT
This protein is mutated and non-functional in the disease MUSCULAR
DYSTROPHY
Slide 15
Organization and association of sarcomere elements with linking
proteins, sarcolemma, and endomysium Linking proteins such as
dystrophin allows muscle cell to translate shortening of sarcomere
into shortening of muscle cell and shortening of muscle tissue
Slide 16
Organization of filaments in a myofibril The thick, thin, and
elastic filaments are arranged in a VERY specific, ordered fashion
that allows contraction to occur efficiently Z-DISK is a large
protein that the thin filaments and elastic filaments are attached
to Z-disk is also attached to sarcolemma helps translate
contraction of myofibrils to contraction of muscle fiber Thick
filaments are suspended between the thin filaments by elastic
filaments Two Z-disks with the associated thick and thin filaments
in between is called a SARCOMERE the functional unit of contraction
in a myofibril Contraction brings the Z-disks closer together,
shortening the length of the sarcomere = CONTRACTION!!
Slide 17
Organization of filaments in a myofibril Remember that skeletal
muscle contains STRIATIONS alternating bands of light and dark The
I-band (light band) of the sarcomere contains only elastic
filaments and thin filaments The A-band (dark band) contains
overlapping thin, thick, and elastic filaments LIGHT = I -band DARK
= A-band The H-band is the region in the middle where there are
only thick filaments since the thin filaments do not extend that
far only present in RELAXED sarcomere
Slide 18
Role of the nervous system in muscle contraction Specialized
neurons that trigger muscle contraction are known as MOTOR NEURONS
As the axon of a motor neuron approaches the muscle it branches out
into dozens of smaller fibers allows one nerve to stimulate MANY
different muscle fibers This arrangement helps coordinate the
timing of the contractions of the 1000s of muscle fibers found in a
muscle Although each nerve fiber can stimulate 100s of muscle
fibers, one muscle fiber can only be stimulated by ONE nerve fiber
prevents the muscle fiber from getting mixed signals
Slide 19
Role of the nervous system in muscle contraction When a motor
neuron sends an impulse, ALL of the muscle fibers associated with
it contract at the same time this group of muscle fibers is called
a MOTOR UNIT Large muscles may have 100s of different motor units
Strength of muscle contraction depends on number of motor units
stimulated The muscle fibers of a motor unit are often spread out
over a fairly large area The process of adding more motor units
during a strong contraction = RECRUITMENT Ability to vary # of
motor units (i.e., strength of contraction) allows for fine control
of small movements
Slide 20
Role of the nervous system in muscle contraction Having many
motor units is important for repeated or sustained muscle
contractions Eventually, some motor units get tired and others will
take over (like shift work) Not all motor units contain the same #
of muscle fibers Activation of smaller motor units allow for weaker
muscle contractions (better suited for fine movements) Activation
of larger motor units lead to strong contractions (better suited
for gross movements) Weak nerve impulse = fewer and smaller motor
units Stronger impulses = more, larger motor units
Slide 21
The neuromuscular junction How does a nerve tell a muscle when
and how to contract??? Neurons talk to muscle fibers via the
NUEROMUSCULAR JUNCTION Interaction between a motor neuron and a
muscle fiber is a SYNAPSE Nerves dont physically contact muscle
fibers separated by a very small gap called the SYNAPTIC CLEFT
Motor neurons communicate with muscle cells using the
neurotransmitter ACETYLCHOLINE (ACh)
Slide 22
The neuromuscular junction Motor nerves contain an large bulb
at their ends called a SYNAPTIC BULB This bulb contains vesicles
filled with ACh The sarcolemma of the muscle fiber contains 100s of
ACh receptors near the neuromuscular junction Arrival of a nerve
impulse at the synaptic bulb trigger the exocytosis of ACh ACh
travels across the synaptic cleft and binds to ACh receptors on the
sarcolemma The binding of ACh to receptors on the muscle fiber
initiates events that cause contraction After the ACh binds to
receptors on the muscle fiber it is rapidly degraded by
ACETYLCHOLINESTERASE
Slide 23
Excitation of cells of the muscular system Motor neurons and
muscle fibers are both known as ELECTRICALLY EXCITABLE The voltage
across the cell membrane changes when the cell is stimulated When
there is a charge difference across the cell membrane, the cell is
said to be POLARIZED Negative = iNside, pOsitive = Outside Lots of
Na+ outside; DNA, proteins, other anions inside The electrochemical
Na+ gradient (more outside) is maintained by the Na+/K+ ATPase
pushes Na+ outside against its conc. gradient DNA, protein,
phosphate Na+ _ _ _ _ _ _ _ K+
Slide 24
Excitation of cells of the muscular system Stimulation of nerve
or muscle cell leads to DEPOLARIZATION of the cell membrane
Stimulus causes Na+ ion channels to open Na+ rushes in (down conc.
gradient) For a BRIEF period the inside of the cell becomes
positively charged Depolarization causes Na+ channels to close and
K+ channels to open K+ rushes out (down its conc. gradient) Loss of
(+) charged K+ allows inside of cell to regain negative charge
Return of membrane charge ( inside, + outside) is called
REPOLARIZATION The change of the cell interior from () to (+) and
then back to (-) is known as an ACTION POTENTIAL DNA, protein,
phosphate Na+ _ _ _ _ _ _ K+ DNA, protein, phosphate Na+ _ _ _ _ _
_ K+ DNA, protein, phosphate Na+ _ _ _ _ _ _ _ K+ POLARIZED
REPOLARIZED DEPOLARIZED
Slide 25
Localization of an action potential When the membrane of a
nerve or muscle cell experiences an ACTION POTENTIAL (polarization,
depolarization, repolarization), this is a LOCAL occurrence Only a
small part of the muscle cell membrane is depolarized at a given
time Think of The Wave at a sports event NERVE OR MUSCLE FIBER
DEPOLARIZATION NERVE OR MUSCLE FIBER REPOLARIZATION Depolarization
of a region is IMMEDIATELY followed by repolarization Necessary to
allow a nerve or muscle cell to be restimulated Depolarization of
one region triggers the depolarization of a region next to it
Allows action potentials to be self-propagating
Slide 26
Excitation of skeletal muscle Nerve impulse arrives at synaptic
bulb Entrance of Ca 2+ into synaptic bulb triggers exocytosis of
ACh-containing vesicles ACh travels across synaptic cleft and binds
to ACh receptors on sarcolemma The region of the muscle fiber where
ACh binds is called the MOTOR END PLATE-the muscle part of the
neuromuscular junction Motor end plate
Slide 27
Excitation of skeletal muscle The ACh receptors actually play
two roles : ACh receptor AND ion channel LIGAND-GATED ION CHANNEL
opens up when ACh binds Binding of ACh (ligand) to its receptor
causes conformational change that opens up the ion channel Na+ ions
rush IN, causing voltage across membrane to reverse (depolarize)
Slower diffusion of K+ OUT of the cell begins repolarization This
LOCAL fluctuation of membrane voltage (at the motor end plate)
leads to the activation of VOLTAGE-GATED ion channels on the REST
of the muscle fiber
Slide 28
OUTSIDE (+) INSIDE (-) Ligand Na+ Location and function of
Ligand-gated -vs- Voltage gated ion channels Ligand Na+ Ligand
gated ion channels are ion channels that require a ligand in order
for them to open and cause depolarization These channels will only
be found at the motor end plate of a muscle cell When ACh binds to
the channel it opens up letting Na+ enter the cell Voltage gated
ion channels are ion channels that require a change in membrane
voltage to open and cause depolarization These channels sense
changes in membrane voltage (a nearby depolarization) that triggers
them to open up and let Na+ enter the cell Found over the rest of
the muscle cell (but not at motor end plate) These channels are
responsible for propagating the action potential over the surface
of a muscle cell Motor end plateThe rest of the muscle cell
Slide 29
Excitation/Contraction coupling This is the process by which we
translate the action potential on the sarcolemma to the contraction
of myofilaments inside the muscle cell Action potentials that leave
the motor end plate spread out over the entire surface of the
sarcolemma and travel down T-tubules dependent on voltage gated ion
channels This triggers the opening of voltage-gated Ca 2+ channels
in the SR Ca 2+ diffuses into sarcoplasm
Slide 30
Excitation/Contraction coupling Ca 2+ released from the SR
binds to troponin on the thin filaments and causes troponin to
change shape The troponin/tropomyosin complex shifts position
thereby exposing the active sites of the actin molecules The
exposure of the active sites on actin (thin filaments) makes them
available to bind to the heads of the myosin molecules (thick
filaments)
Slide 31
Contraction The enzyme MYOSIN ATPase hydrolyzes ATP, releasing
energy (ATPADP+P +ENERGY) This energy is used to activate the
myosin head The myosin head reaches out to bind to the active site
of actin In the 2 nd step, the myosin head binds to the exposed
active site of actin, creating a CROSS-BRIDGE
Slide 32
Contraction After binding to actin, the myosin head releases
the ADP+ P causing it to return to its original conformation This
pulls on the actin, causing the thin filament to slide past the
thick filament this brings the Z-disks closer together Binding of a
new molecule of ATP causes the myosin head to release the actin
molecule Hydrolysis of ATP then allows the myosin head to reach out
to another actin molecule farther down the thin filament This
process repeats over and over until the sarcomere is shortened by
about 40% Each ratcheting of the myosin head uses one ATP Each
myosin head ratchets 5X per second 1000s of myosin heads work
together to shorten each muscle fiber Contraction uses a HUGE
amount of ATP!!!
Slide 33
Relaxation Once the contraction is finished, the motor neuron
stops releasing ACh Remaining ACh bound to the receptors is
degraded by ACETYLCHOLINESTERASE This prevents action potentials
from being generated at the motor end plate Without stimulation, Ca
2+ is pumped back INTO the SR active transport (uses ATP) As Ca 2+
diffuses away from troponin, the tropomyosin molecules cover up the
active sites of actin again prevents myosin from binding Without
myosin binding, the thin filaments slide BACK over the thick
filaments and the sarcomere lengthens (returns to resting
length)
Slide 34
Summary 1.Arrival of action potential at synaptic bulb of motor
neuron causes release of ACh across synaptic cleft (process
requires Ca 2+ ) 2.Binding of ACh to sarcolemma at motor endplate
creates local action potential (dependent on the availability of
ACh from motor neuron) 3.Action potential at motor end plate
spreads out over sarcolemma and down T-tubule to SR (these action
potentials are generated by voltage gated ion channels and are
dependent on the initial ACh-dependent action potential) 4.Release
of Ca 2+ from SR allows active sites of actin to be bound by myosin
heads 5.Ratcheting of the myosin heads is dependent on the energy
released from ATP hydrolysis 6.Ratcheting of myosin causes thin
filaments to slide over the thick filaments brings Z- disks closer
to each other (sarcomere shortening) 7.End of motor neuron
stimulation causes Ca 2+ to be pumped back into SR (diffuses away
from troponin) active transport of Ca 2+ requires ATP 8.Myosin
releases actin and allows thin filaments to slide back over thick
filaments and return to resting position (sarcomere lengthens) Be
sure to understand and remember which steps in the entire process
require ATP, ACh, or Ca 2+ !!!!!!
Slide 35
Bio 322 Human Anatomy Todays topics Muscular system
Slide 36
Action potential and muscle physiology animations Action
potential at Neuromuscular junction Action potentials and muscle
contraction Crossbridge formation and contraction contraction of
the sarcomere
Slide 37
What factors are needed and when??? Ca 2+ ACh ATP ACh release:
Ca 2+ triggers exocytosis of ACh from synaptic bulb Crossbridge
formation: Ca 2+ binds to troponin causing tropomyosin to shift
positions and expose active sites of actin Action potential
generation: ACh binds to receptors on sarcolemma of motor end plate
causing Na+ channels to open and lead to depolarization of
sarcolemma Without this initial action potential there can be NO
MUSCLE CONTRACTION Maintenance of cell polarization: Na+/K+ ATPase
requires ATP to maintain Na+ gradient thereby keeping interior of
cell negatively charged Muscle fiber contraction: Myosin heads use
ATP to reach out and attach to active sites of actin Muscle fiber
relaxation: Ca 2+ pumps use active transport to pump Ca 2+ BACK
into the SR
Slide 38
Length-Tension relationship and muscle tone The amount of force
or tension a muscle can generate is related to how contracted or
stretched it was prior to stimulation Remember that muscle
contraction is dependent on the overlapping of thick and thin
filaments A B C If a muscle is overly stretched or contracted
before stimulation weak contraction A -OPTIMAL RESTING LENGTH the
thin filaments are just overlapping with all the myosin heads
maximal contraction B When the muscle is overly contracted the thin
filaments overlap the thick filaments so much that the thick
filaments are nearly butting up against the Z-disk no room for
additional sarcomere shortening C When the muscle is overly
stretched only a SMALL part of the thin filament overlaps with the
thick filaments myosin heads on thick filaments cant get a hold of
thin filaments
Slide 39
Muscle tone Even at rest, muscles maintain a certain degree of
contraction called MUSCLE TONE This allows the sarcomeres of the
muscle fibers to remain at their optimal length Helps ensure that
the muscle is ready for maximal contraction at all times Muscle
tone is important for posture, balance, and joint stability Muscles
of the trunk and legs keep us centered and upright WITHOUT
conscious effort Muscle tone in deltoid and biceps brachii helps
stabilize shoulder joint Stretch receptors within muscle tissue is
known as MUSCLE SPINDLE Muscle Spindle Motor neuron
Slide 40
Different types of muscle contractions Not every muscle
contraction results in shortening of the muscle length and movement
of a body part Muscle contraction technically refers to the
generation of force and tension within a muscle Isometric
contraction (constant length) muscle contracts (develops tension in
elastic components of muscle tissue) but DOES NOT SHORTEN in length
Isotonic contraction (constant force) muscle contracts (develops
tension) and may either lengthen or shorten depending on force of
contraction think of trying to lift a 5lb bar versus a 500lb bar
Both of these contractions are in play when we lift a weight
Slide 41
At this point muscle tension exceeds the load and the muscle is
allowed to shorten Isometric and Isotonic contractions Example :
Lifting a 50lb weight off of the floor Requires both isometric and
isotonic contraction After you grab the weight, you increase muscle
contraction (tension) but have not yet moved the weight from the
floor Tension increases, but muscle length stays the same
(ISOMETRIC CONTRACTION) Eventually you generate enough force
(tension) to overcome gravity and you lift the weight off the floor
Tension stays the same but the muscle shortens as you move the
weight (ISOTONIC CONTRACTION)
Slide 42
Classes of muscle fibers Skeletal muscle fibers can be divided
into 2 main categories SLOW TWITCH (oxidative) and FAST TWITCH
(glycolytic) Slow and fast twitch fibers differ in function and
metabolic needs, and are more suited for certain functions Nearly
all muscles contain a mixture of slow and fast twitch fibers -
difference is in proportion All of the muscle fibers from a given
motor unit contain ONLY fast or slow twitch not mixed Slow Twitch
muscle fibers These fibers exhibit a slower twitch (longer cycle of
contraction/relaxation) about 100 msec/twitch They possess lots of
mitochondria (site of ATP generation), myoglobin (O 2 stores in
muscle), and are well supplied with blood vessels Well-suited to
aerobic respiration Very resistant to physiological fatigue Muscles
with lots of slow twitch fibers have a dark red appearance red meat
Since these fibers do not fatigue easily, they are found in higher
abundance in muscles requiring long, sustained contractions calf
muscles (walking), back and trunk muscles (posture)
Slide 43
Fast twitch muscle fibers These fibers twitch more rapidly
about 7.5 sec/twitch These fibers contain less mitochondria, less
myoglobin, and fewer blood vessels, BUT MORE components required
for anaerobic fermentation and phosphagen system
(myokinase/creatine kinase) needed for immediate and short term
energy More susceptible to physiological fatigue Have a lighter
appearance than slow twitch white meat Sarcoplasmic reticulum is
more able to quickly release/absorb Ca 2+ - more rapid and forceful
contractions Since they rely on anaerobic fermentation, they
produce a lot of lactic acid causes fatigue Ability to produce
short, forceful contractions makes them suited for larger, less
used (relatively) muscles arm, leg muscles
Slide 44
Fast twitch Slow twitch White meat vs.- dark meat What can you
tell about the different muscles of a chicken???
Slide 45
Cardiac Muscle Striated like skeletal muscle (overlapping thick
and thin filaments) Intercalated disks and mechanical junctions
allow one cardiac muscle cell to stimulate its neighbors and remain
tightly connected unlike skeletal muscle Cardiomyocytes dont
necessarily need nervous system input (at least not the way
skeletal muscles do) Stimulation of specialized cells trigger
contraction of muscle cells throughout the heart (via intercalated
disks) Twitches are very slow compared to skeletal muscle (about
250 msec) allows the heart to contract and expel all the blood it
contains Relies almost exclusively on aerobic respiration very
resistant to fatigue (GOOD!) Lots of myoglobin, mitochondria,
glycogen stores (around nucleus) This also makes cardiac muscle
VERY susceptible to damage from disruptions of blood flow
Slide 46
Smooth muscle Tapered cells with NO STRIATIONS Cells DO contain
thick and thin filaments, but not arranged in away so that the cell
appears striated Thick and thin filaments attach to plasma membrane
via a complex internal cytoskeleton and protein masses called DENSE
BODIES Thin filaments attach to dense bodies and cytoskeleton
instead of Z-disks (skeletal muscle) Need Ca 2+ for contraction,
like all muscle Ca 2+ comes primarily from extracellular fluid, NOT
FROM SARCOPLASMIC RETICULUM Ca 2+ flows into cell via channels
(contraction) Ca 2+ is pumped out during relaxation
Slide 47
Stimulation, contraction, relaxation of smooth muscle Like
cardiac muscle, smooth muscle is involuntary and doesnt necessarily
need nerve stimulation to contract (but often does) Can also
contract in response to hormones, CO 2 levels, changes in pH,
stretch, etc Ca 2+ channels are opened in response to these factors
This is how some smooth muscle contraction is regulated in the
stomach, intestines, blood vessels, etc There are 2 main types of
smooth muscle: Multiunit smooth muscle requires innervation from
autonomic nervous system (involuntary). Nerve fibers innervate many
smooth muscle cells creating a motor unit (like skeletal muscle)
Single unit smooth muscle muscle cells communicate via gap
junctions. Nerve fiber releases ACh near one cell and the
stimulation is passed to other connected cells (like cardiac
cells)
Slide 48
Stimulation, contraction, relaxation of smooth muscle Although
smooth muscle contains thick and thin filaments and requires Ca 2+
for contraction, it contains no troponin as a regulatory protein
Smooth muscle contains a unique protein called calmodulin
Associated with THICK FILAMENTS When calmodulin binds Ca 2+ it
activates an enzyme that transfers phosphate group from ATP to
myosin head (PHOSPHORYLATION=adding phosphate to a protein). Myosin
then binds to actin and uses ANOTHER ATP to ratchet Process uses 2
ATP molecules as opposed to skeletal muscle (only 1 ATP) Regulation
occurs at thick filament, not at thin filament like skeletal muscle
When thick filaments ratchet they pull on thin filaments that are
attached to DENSE BODIES and cytoskeleton contraction of the muscle
cell
Slide 49
Stimulation, contraction, relaxation of smooth muscle As
compared to skeletal muscle, smooth muscle twitch is VERY SLOWLY
Pumping Ca 2+ into/out of the cell occurs much more slowly without
a SR (like skeletal muscle) Ratcheting of smooth muscle myosin
requires more steps and uses more ATP
Phosphorylation/dephosphorylation of myosin is slow For smooth
muscle to relax 2 things need to occur: 1.Removal of Ca 2+ -
inactivates calmodulin 2. Dephosphorylation of myosin However, in
some cases, smooth muscle can stay contracted even in the absence
of ATP allows some muscles to remain contracted without using
ATP