BIOL 3151: Principles of Animal Physiology

BIOL 3151: Principles of Animal

Physiology

ANIMAL PHYSIOLOGY

Dr. Tyler EvansEmail: [email protected]: 510-885-3475Office Hours: M 8:30-11:30 or appointmentWebsite: http://evanslabcsueb.weebly.com/

mailto:[email protected]

LAST LECTURE• large forces generated during muscle contraction are the result of combining the

actions of many polymers of myosin

MUSCLE STRUCTURE AND REGULATION OF CONTRACTION

• 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

Fig 5.17 pg 215

MUSCLE STRUCTURE AND REGULATION OF CONTRACTIONLAST LECTURE

LAST LECTURE


MUSCLE STRUCTURE AND REGULATION OF CONTRACTIONSEQUENCE OF EVENTS IN IN MUSCLE CONTRACTION• from action potential from brain causing the release of acetylcholine at the

neuromuscular junction• action potential spreading across SARCOLEMMA and triggering release of Ca+2 from

sarcoplasmic reticulum• interaction of troponin subunits with Ca+2 and actin and myosin

• 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


LAST LECTUREMUSCLE STRUCTURE AND REGULATION OF CONTRACTION

MUSCLE DIVERSITY: TYPES OF CONTRACTIONTODAY’S LECTURE

• TWITCH is the response of striated muscle to a single action potential• muscle twitch occurs in three phases:1. LATENT PERIOD-earliest part in muscle stimulation where there is no change in

length2. CONTRACTION PERIOD-muscle contracts and sarcomeres shorten (usually)3. RELAXATION PERIOD-muscle relaxes and returns to its original length

TYPES OF CONTRACTIONMUSCLE DIVERSITY

WHAT DO YOU THINK IS HAPPENING AT THE MOLECULAR LEVEL IN EACH PHASE?

LATENT PERIOD?

CONTRACTION PERIOD?

RELAXATION PERIOD?


• ISOTONIC: tension or force generated by the muscle is greater than the load and the muscle shortens

• muscle will only activate enough motor units to move the load

• ISOMETRIC: load is greater than the tension or force generated by the muscle and the muscle does not shorten

• force generated by activating all of the motor units in a muscle is still not sufficient to move the load

• tension will develop in the muscle, but the sarcomeres will not shortened because the muscle is too weak to move the load



• a muscle can contract with varying degrees of force depending on the circumstance. Force is the result of the number of muscle cells or MOTOR UNITS stimulated:

• an increase in action potentials coming from brain can prevent muscles from completely relaxing and force increases• result of Ca+2 remaining high in the muscle cell

SUMMATION: a muscle is re-stimulated while there is still some contractile activity

TETANUS: occurs when muscle fiber is stimulated so rapidly that it does not relax at all between stimuli, a smooth, sustained and forceful contraction occurs


• force generated by muscle contraction is inversely proportional to contraction velocity

• it takes longer to lift heavy objects than it does to lift lighter objects

• it takes longer to lift heavier objects

HOW CAN WE EXPLAIN THIS

RELATIONSHIP?

TYPES OF MUSCLEMUSCLE DIVERSITY

• so far have focused on striated muscle, but there are other types of muscle found in animals:

1. STRIATED MUSCLE2. CARDIAC MUSCLE: special type of striated muscle that can control its own

contraction (will describe in cardiovascular lectures)3. SMOOTH MUSCLE: refer to the involuntary muscles found surrounding the

digestive tract, respiratory tract and blood vessels

• smooth muscle lack sarcomeres and therefore a striped appearance

• smooth muscle has a different arrangement of actin and myosin


• thick and think filaments are scattered throughout the cells• as a result, smooth muscle

contracts in all directions

• no neuromuscular junctions• often use GAP JUNCTIONS

(electrical synapses) or neuron swellings called VERICOSITIES

• no T-TUBULES


• smooth mucsle also lacks TROPONIN, they key protein involved in regulating the contraction of striated muscle

• use a different protein called CALDESMON, which binds to actin thin filaments and blocks myosin binding sites when not stimulated

• when Ca+2 is released in smooth muscle cells it binds to a protein called CALMODULIN

• calmodulin then removes caldesmon from actin, exposing the myosin binding site• then events in the sliding filament model occur

MUSCLE DIVERSITY• in some animals evolution has modified muscles (and the

molecular events that regulate muscle contraction) for new functions

SOUND PRODUCING ORGANS

• for example, muscles of the rattlesnake shaker organ can contract 100 times per second (100 Hz)!

• some animals have modified muscles into sound producing organs that are specialized for high-frequency contractions

MUSCLE DIVERSITYSOUND PRODUCING ORGANS

• for example, the cicada makes its buzzing noises by vibrating a region of its exoskeleton called the TYMBAL at a rate of about 200 times per second



• 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 the formation of myosin-actin cross bridges and cycling of Ca+2

• 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


• sonic muscles have a high concentration of SARCOPLASMIC RETICULI• recall that striated muscle contracts when the sarcoplasmic reticulum releases

its Ca+2 and relaxes when calcium is removed from the cell • in sonic muscles, the high density of SARCOPLASMIC RETICULI allows calcium

cycling to occur much faster and the result is much higher contraction frequencies


1. ENHANCED ABILITY TO CYCLE CALCIUM

• sound producing organs also have another means to rapidly remove Ca+2 from muscle cells so that they can relax

• some sonic muscles have high levels of a Ca+2 binding protein called PARVALBUMIN• parvalbumin binds free Ca+2 in cells and therefore accelerates

the relaxation phase of contraction


1. ENHANCED ABILITY TO CYCLE CALCIUM


2. FAST CROSS-BRIDGE CYCLING• for a muscle to contract, the myosin head must form a cross-bridge, undergo the

powerstroke and then detach • 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 average striated muscle• the molecular basis for this has not been established


3. SHORTEN SARCOMERE LENGTH BEYOND LIMITS SEEN IN OTHER ANIMALS

• special adaptations to muscle anatomy that allow sarcomeres to shorten beyond what is typically possible• when sarcomeres shorten,

thin filaments can collide and force of contraction decreases

• however, most sound producing muscles generate high frequency, low force contractions, so the decrease in force does not effect performancetextbook Fig 5.19 pg 216

MUSCLE DIVERSITYHEATER ORGANS ARE MODIFIED MUSCLES

• in some cases, a muscle may undergo TRANS-DIFFERENTIATION, in which a muscle is diverted from its typical developmental pathway to create an organ with a new function

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

MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED

MUSCLES

• all muscles produce heat through the chemical reactions that hydrolyze ATP

• in muscles, the cycling of Ca+2 is an ATP dependent process (requires energy)

• when released Ca+2 flows down its concentration gradient (no energy), but ATP is required to transport Ca+2 back into the sarcoplasmic reticulum to prepare for another contraction

• billfish constantly cycle Ca+2 between the sarcoplasmic reticulum and the interior of the cell

• this activity produces metabolic heat, but because the heater organ contains few myofibrils and the Ca+2 is cycled so quickly, no actual contraction occurs

• to facilitate this process heater organs have high numbers of sarcoplasmic reticula and mitochondria, but very few muscle cells

MUSCLE DIVERSITYHEATER ORGANS AND ELECTRIC ORGANS ARE MODIFIED

MUSCLES

MUSCLE DIVERSITYSPECIALIZED INVERTEBRATE MUSCLES

• 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 than in vertebrates: 250-1000 contractions per second!• this frequency is much too high to be achieved by cycling Ca+2

• an alternative mechanism must be at work

MUSCLE DIVERSITYINVERTEBRATE MUSCLES

• insects 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 contraction is used by ASYNCHRONOUS FLIGHT MUSCLES

• 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 they do not rely on Ca+2 cycling

• Ca+2 concentrations instead 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 over and over• recall, binding of Ca+2 to TnC uncovers

myosin binding sitetextbook Fig 5.36 pg 241


• some invertebrates also possess muscles specialized for sustained contraction• bivalve mollusks possess muscles capable of generating long duration

contractions that expend 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 acetylcholinesterase as in vertebrates.

• instead, another neurotransmitter called SEROTONIN causes the muscle to relax• relaxation is therefore not dependent on intracellular Ca+2 levels


LECTURE SUMMARY• TWITCH is the response of striated muscle to a single action potential and occurs

in three phases: LATENT PERIOD, CONTRACTION PERIOD and RELAXATION PERIOD

• ISOMETRIC contraction occurs when the load is greater than the force generated by the muscle and ISOTONIC contraction occurs when the load is less than the force generated by the muscle

• an increase in action potentials can prevent muscles from completely relaxing and force increases causing SUMMATION or TETANUS

• smooth muscle has a different arrangement of actin and myosin and uses CALDESMON instead of troponin

• evolution has modified muscles to perform high frequency contractions that are the result of:• 1. ENHANCED ABILITY TO CYCLE CALCIUM• 2. FAST CROSS-BRIDGE CYCLING• 3. SHORTEN SARCOMERE LENGTH BEYOND LIMITS SEEN IN OTHER ANIMALS

• billfish constantly cycle Ca+2 between the sarcoplasmic reticulum and interior of the muscle cell to generate heat for vision

• insect flight muscles use a variant of TnC which has only a single Ca+2 binding site• mussel adductor muscles uses SEROTONIN to relax

Documents

BIOL 3151: Principles of Animal Physiology