biol 3151: principles of animal...
TRANSCRIPT
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BIOL 3151:
Principles of Animal
Physiology
ANIMAL
PHYSIOLOGY
Dr. Tyler EvansEmail: [email protected]
Phone: 510-885-3475
Office Hours: F 8:30-11:30 or appointment
Website: http://evanslabcsueb.weebly.com/
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MUSCLE STRUCTURE AND REGULATION OF CONTRACTION
PREVIOUS LECTURE
• large forces generated during muscle contraction are the result of combining the
actions of many polymers of myosin
• polymers of myosin are
called THICK FILAMENTS
• thick filaments are
doubled headed,
meaning they have
clusters of the myosin
head at each end
• in muscle tissue, thick
filaments of myosin slide
along polymers of actin
called THIN FILAMENTS
textbook Fig 5.15 pg 212
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textbook Fig 5.17 pg 215
MUSCLE STRUCTURE AND REGULATION OF CONTRACTION
PREVIOUS LECTURE
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REGULATION OF MUSCLE CONTRACTION• in a typical muscle cell, intracellular Ca+2 is very low and binding sites on TnC are
empty.
• empty TnC interacts with TnI to block myosin from binding to actin
• when muscle is activated, intracellular Ca+2 spikes (100-fold) and binds to TnC
• binding of Ca+2 to TnC induces a change in conformation in TnI that exposes the
myosin binding site on actin
• because TnT is bound to tropomyosin the complex exposes the myosin
binding site by sliding down tropomyosin
textbook Fig 5.22 pg 220
PREVIOUS LECTURE
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PREVIOUS LECTUREEXCITATION OF VERTEBRATE MUSCLE
textbook Fig 5.24 pg 226
• in striated muscle the EFFECTIVE REFRACTORY PERIOD is short
• in cardiac cells the effective refractory period is much longer, as voltage-gated
Ca+2 channels to open longer
• allows time for the action potential to spread to cardiomyocytes and
ensures simultaneous contraction critical to heart function
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• we have seen how muscles are composed of proteins that can exist in many
different isoforms (e.g. myosin)
• muscles use different combinations of these isoforms to produce different
MUSCLE FIBER TYPES
MUSCLE DIVERSITY
TODAY’S LECTURE
e.g. WHITE or RED MUSCLE-based upon the concentration of MYOGLOBIN, an
oxygen binding protein found in muscle
e.g. FAST TWITCH or SLOW TWITCH-based on the speed of contraction
e.g. GLYCOLYTIC or OXIDIATIVE-based on metabolic specialization
e.g. TYPE I or TYPE II-based on presence of different myosin isoforms
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MUSCLE DIVERSITY
• many organisms use sound producing organs use muscles that are specialized
for high-frequency contractions to generate sound
SOUND PRODUCING ORGANS
• for example, the muscles of the shaker organ in a RATTLESNAKE tail can
contract 100 times per second (100 Hz)
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MUSCLE DIVERSITY
• many organisms use sound producing organs in combination with muscles that
are specialized for high-frequency contractions
SOUND PRODUCING ORGANS
• for example, the CICADA
makes its buzzing noises by
bending a region of its
exoskeleton called the
TYMBAL at a rate of about
200 times per second
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MUSCLE DIVERSITY
• many organisms use sound producing organs in combination with muscles that
are specialized for high-frequency contractions
SOUND PRODUCING ORGANS
• for example, the
TOADFISH produces a
high pitched whistle using
muscles that vibrate its
swim bladder at a rate of
more than 200 times per
second
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MUSCLE DIVERSITY
• the frequency that these SONIC MUSCLES contract is impressive considering
potential time consuming cellular events that could delay contraction, such as
refractory periods and formation of myosin-actin cross bridges
• surprisingly, the contractile machinery of sonic muscles is not that different from
skeletal muscle
So what makes sonic muscles able to contract and relax so
quickly?
SOUND PRODUCING ORGANS
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MUSCLE DIVERSITYSOUND PRODUCING ORGANS
• sonic muscles have a high concentration of SARCOPLASMIC RETICULUM, whose
function is to store and release Ca+2 using ion channels
• recall that striated muscle contracts when calcium (Ca+2) levels increase
within the myofibril and relax when Ca+2 levels return to resting levels
• in sonic muscles, the SARCOPLASMIC RETICULUM floods the cytoplasm of
muscle cells with Ca+2
• SARCOPLASMIC RETICULA are found
interspersed among myofibrils
textbook Fig 5.26 pg 228
1. ENHANCED ABILITY TO CYCLE CALCIUM
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• flooding muscle cells with Ca+2 is a great method for speeding up contraction,
but presents a problem for relaxation
• recall relaxation occurs when Ca+2 returns to resting levels
• So sound producing organs have special means to rapidly remove Ca+2 from
muscle cells:
a. sarcoplasmic reticulum are also able to rapidly uptake Ca+2
b. sonic muscles have high levels of a Ca+2 buffer called PARVALBUMIN
• parvalbumin binds free Ca+2 in cells and therefore accelerates relaxation
MUSCLE DIVERSITYSOUND PRODUCING ORGANS
1. ENHANCED ABILITY TO CYCLE CALCIUM
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MUSCLE DIVERSITYSOUND PRODUCING ORGANS
2. FAST CROSS-BRIDGE CYCLING• the myosin head must form a cross-bridge, undergo the powerstroke and then
detach for a muscle to contract
• the slowest step in this cycle is the detachment of the myosin head from actin
• in the toadfish, detachment rates of myosin in sonic muscles are about six times
faster than in fast-twitch skeletal muscle
• the molecular basis for this has not been established
• for example, the
TOADFISH produces a
high pitched whistle using
muscles that vibrate its
swim bladder at more
than 200 times per
second
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MUSCLE DIVERSITYSOUND PRODUCING ORGANS
3. SHORTEN SARCOMERE LENGTH BEYOND LIMITS SEEN IN
OTHER ANIMALS• special adaptations to muscle anatomy that allow muscles to shorten beyond
what is typically possible
• shortening of sarcomeres is important to achieving high frequency low force
contraction in sonic muscles
textbook Fig 5.19 pg 216
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MUSCLE DIVERSITYSOUND PRODUCING ORGANS
IF SARCOMERES ARE SHORT AND
CONTRACTION VELOCITY FAST,
WHAT IS THE LIKELY EFFECT ON
FORCE?
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MUSCLE DIVERSITYSOUND PRODUCING ORGANS
TRADE-OFFS TO SONIC MUSCLE DESIGN
• the muscle designs that enable high-frequency contractions also limit their
ability to generate force
• sound producing organs use elements that can be vibrated with relatively little
force.
• Sound producing organs are dedicated to sound production with no other
physiological functions
• it takes longer to
lift heavier
objects
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MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED
MUSCLES• in some cases, a muscle may undergo TRANS-DIFFERENTIATION, in which
muscle is diverted from its typical developmental pathway to create a tissue
with novel properties
e.g. HEATER ORGAN IN BILLFISH (includes marlin and swordfish)
• Billfish possess a trans-differentiated eye muscle that functions as a heater
organ
• by warming the optical sensory system, billfish can maintain visual function
even when pursuing prey in deep, cold waters
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MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED
MUSCLES
• all muscles produce some heat a by-
product of normal metabolism through
the chemical reactions that both
produce and hydrolyze ATP
• in muscles, movement of Ca+2 in or out
of muscle cells is an ATP dependent
process (requires energy)
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• billfish constantly cycle Ca+2 between
the sarcoplasmic reticulum and
interior of the cell. This activity
produces metabolic heat, but because
the heater organ contains few
myofibrils and the Ca+2 is cycled very
quickly no contraction is triggered
• to facilitate this process heater organs
have high numbers of sacroplasmic
reticula and mitochondria
MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED
MUSCLES
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MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED
MUSCLES
• a second type of trans-differentiated muscle is the ELECTRIC ORGAN, a modified
muscle tissue containing cells called ELECTROCYTES
• these cells produce an electric discharge that when large can stun prey
• charge can also be small and used in communication
• have evolved independently several times over evolutionary history
• the electric eel
uses large
electrical charges
to stun its prey
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MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED
MUSCLES• electric organs arise during embryonic development
• a cluster of muscle precursor cells called MYOBLASTS form near the site of the
electric organ
• cells in the central portion of this cluster lose their sarcomeres when they
become innervated by special ELECTROMOTOR NEURONS
• these cells eventually form the electrocytes of the electric organ
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MUSCLE DIVERSITYINVERTEBRATE MUSCLES
• as we have seen, some vertebrates can trigger very rapid contractions by quickly
cycling Ca+2 within cells
• however, the flight muscles of many insects can have contraction cycles that are
much faster: 250-1000 contractions per second!
• this frequency is much too high to be achieved by cycling Ca+2
• alternatively, insects use an alternative mechanism
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MUSCLE DIVERSITYINVERTEBRATE MUSCLES
• insect still activates flight muscles via a single neuronal stimulation
• however, a single action potential is followed by a long series of contraction and
relaxation cycles
• this type of muscle is called ASYNCHRONOUS FLIGHT MUSCLE because
contraction is not synchronized with the arrival of an action potential
• most insects use asynchronous flight muscles
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MUSCLE DIVERSITYINVERTEBRATE MUSCLES
• ASYNCHRONOUS FLIGHT MUSCLES
are able to contract and relax at
high frequency because it does not
require cycles of Ca+2
• rather than pulses of Ca+2,
concentrations remain high
throughout multiple contraction-
relaxation cycles
• insect flight muscles use a variant
of TnC which has only a single Ca+2
binding site
• rather than requiring new inputs of
Ca+2, this TnC binds and releases a
single Ca+2 molecule and its affinity
for Ca+2 is regulated by shape
changes of the flight muscle
textbook Fig 5.36 pg 241
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MUSCLE DIVERSITYINVERTEBRATE MUSCLES
• some invertebrates also possess muscles specialized for extended contraction
• Bivalve mollusks possess muscles capable of generating long duration
contractions while expending very little energy
e.g. adductor muscles in
California mussels
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MUSCLE DIVERSITYINVERTEBRATE MUSCLES
• mussel adductor muscles respond to acetylcholine and Ca+2 just like vertebrate
muscles.
• however, the trigger for relaxation in adductor muscles is not a decline in Ca+2 as
in vertebrates. Instead, another neurotransmitter called SEROTONIN causes the
muscle to relax independent of intracellular Ca+2 concentration
textbook Fig 5.37 pg 242
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LECTURE SUMMARY• many organisms use sound producing organs using muscles that are specialized
for high-frequency contractions
• The ability to contract and relax at high frequency is the result of three factors:
1. ENHANCED ABILITY TO CYCLE CALCIUM: sonic muscles have a high
concentration of sacroplasmic reticula and parvalbumin
2. FAST CROSS-BRIDGE CYCLING: in the toadfish, detachment rates of myosin in
sonic muscles is about six times faster than in fast-twitch skeletal muscle
3. ADAPTATIONS FOR SHORT SARCOMERES: special adaptations to muscle
anatomy allow these muscles to shorten beyond what is typically possible
• insects achieve rapid contraction during flight using ASYNCHRONOUS FLIGHT
MUSCLES that do not require cycles of Ca+2
• shelled organisms rely on a second neurotransmitter, SEROTONIN, to cause
adductor muscle relaxation and this allows for sustained contraction
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MIDTERM EXAM #1
STUDY! (AND GOOD LUCK)
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