BIOL 3151: Principles of Animal Physiologyevanslabcsueb.weebly.com/.../1/2/1/9/12193389/lecture_9.pdf · 2018-09-07 · MUSCLE DIVERSITY TODAY’S LECTURE e.g. WHITE or RED MUSCLE-based

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/

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


MUSCLE STRUCTURE AND REGULATION OF CONTRACTION

PREVIOUS LECTURE

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


PREVIOUS LECTURE

PREVIOUS LECTUREEXCITATION OF VERTEBRATE MUSCLE


• 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

• 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

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)

MUSCLE DIVERSITY

• many organisms use sound producing organs in combination with muscles that

are specialized for high-frequency contractions


• 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

MUSCLE DIVERSITY

• many organisms use sound producing organs in combination with muscles that

are specialized for high-frequency contractions


• 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

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?


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


1. ENHANCED ABILITY TO CYCLE CALCIUM

• 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


1. ENHANCED ABILITY TO CYCLE CALCIUM


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


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



IF SARCOMERES ARE SHORT AND

CONTRACTION VELOCITY FAST,

WHAT IS THE LIKELY EFFECT ON

FORCE?


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

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


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)

• 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


MUSCLES


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


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

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


• 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


• 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



• 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


• 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


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

MIDTERM EXAM #1

STUDY! (AND GOOD LUCK)

NEXT LECTURE

BIOL 3151: Principles of Animal Physiologyevanslabcsueb.weebly.com/.../1/2/1/9/12193389/lecture_9.pdf · 2018-09-07 · MUSCLE DIVERSITY TODAY’S LECTURE e.g. WHITE or RED MUSCLE-based

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