Chapter 47
Effectors: Making Animals Move
Effectors: Making Animals Move
Microtubules, Microfilaments, and Cell Movement
Muscle Contraction
Muscle Strength and Performance
Skeletal Systems
Other Effectors
Microtubules, Microfilaments, and Cell Movement
Two components of the cytoskeletonmicrotubules and microfilamentsgenerate __________ movement.
Microtubules are important in changing cell shape and moving organelles.
They generate forces by polymerizing and depolymerizing the protein tubulin.
The spindle that moves chromosomes to the mitotic poles at anaphase is made of microtubules.
In the developing nervous system, microtubules help growing neurons search for the appropriate contact cells.
Microtubules, Microfilaments, and Cell Movement
Microtubules also generate the small-scale movements of cilia and flagella.
Many protists and small invertebrates use cilia for locomotion; larger multicellular animals use cilia to move fluids and particles over cell surfaces.
In humans, cilia remove wastes from the lung and sweep eggs from the ovary into the oviducts.
Flagellated cells maintain the flow of water through the bodies of sponges; flagella power the movement of sperm in most species.
Microtubules, Microfilaments, and Cell Movement
Microfilaments are proteins that generate contractile forces by changing conformation.
The dominant microfilament protein is actin. Bundles of cross-linked actin are important structural components of cells.
The microvilli of the cells lining the gut and the stereocilia of sensory hair cells are stiffened by actin microfilaments.
Microfilaments reach their highest level of organization in muscle cells.
Microtubules, Microfilaments, and Cell Movement
With myosin, actin microfilaments generate the contractile forces responsible for cell movement and shape.
The contractile ring that divides an animal cell during cytokinesis is made of actin and myosin filaments.
Endocytosis and amoeboid motion rely on interactions of actin and myosin.
Microtubules, Microfilaments, and Cell Movement
Amoeboid motion is accomplished by the cell squeezing itself into a lobe-shaped projection called a pseudopod.
By reversing its plasmasol (liquid portion of cytoplasm) with its plasmagel (thicker gel cytoplasm), it moves the cytoplasm.
It is the interacting network of contracting actin and myosin that squeezes the plasmasol into a pseudopod.
Muscle Contraction
In the animal kingdom, wherever whole tissues contract, __________ cells are responsible.
These specialized cells have high densities of actin and myosin.
Actin filaments consist of a twisted chain of actin molecules.
Myosin filaments are bundles of many myosin molecules.
The actin and myosin filaments lie parallel to each other. At contraction, the actin and myosin filaments slide past each other like a telescope.
The three types of vertebrate muscle are smooth, cardiac, and skeletal.
Muscle Contraction
Smooth muscle provides contraction for internal organs, which are under the control of the autonomic nervous system.
Smooth muscle moves food through the digestive tract, controls the flow of blood, and empties the urinary bladder.
Smooth muscle cells are the simplest muscle cells structurally; they have a single nucleus and are usually long and spindle-shaped.
Muscle Contraction
Smooth muscle cells are arranged in sheets and are in electrical connection with one another via __________ junctions.
This arrangement allows an action potential generated in the membrane to spread to all the cells in the sheet.
The resting potential of the membrane is sensitive to being stretched.
When stretched, the cells depolarize and fire action potentials, causing contraction.
Muscle Contraction
Neurotransmitters of the autonomic nervous system can also alter the membrane potential.
In the digestive tract, acetylcholine causes smooth muscle cells to depolarize, thus making them more likely to contract; norepinephrine causes them to hyperpolarize, thus making them less likely to contract.
Muscle Contraction
Cardiac muscles are branched and appear striated because of the regular arrangement of their actin and myosin filaments.
Branching creates a meshwork that resists tearing and allows the heart to withstand the high pressures of blood pumping without leaking.
Intercalated discs provide strong mechanical adhesions between adjacent cells.
Muscle Contraction
Cardiac muscle cells are also in electrical contact with one another, and depolarization begun at one point in the heart rapidly spreads through the muscle mass.
Pacemaker cells are special muscle cells that initiate the hearts rhythmic contractions.
These cells give the heart myogenic capacity (self-generated heartbeat).
Although heart activity is modified by the autonomic nervous system, the heart will beat without nervous input.
Muscle Contraction
All voluntary movements are controlled by skeletal muscle.
Skeletal muscle is also called striated muscle because of its striped appearance.
Skeletal muscle cells are called muscle fibers. They are large and have many nuclei because they are a fusion of many individual cells.
Each muscle fiber is packed with bundles of myofibrils, each made up of thin actin units surrounding thick myosin units.
Muscle Contraction
Myofibrils consist of repeating units called __________.
Each sarcomere is bounded by Z lines, which anchor the thin actin filaments.
At the center is the A band, housing all the myosin filaments.
The H zone and I band are areas where actin and myosin do not overlap and appear light.
The M band contains proteins that support the myosin filaments.
Muscle Contraction
The bundles of myosin filaments are held in a centered position within the sarcomere by the protein titin.
Titin runs the full length of the sarcomere from Z line to Z line, and each titin molecule runs through the myosin bundle.
Between the ends of the myosin bundles and the Z lines, titin molecules are very elastic, accounting for the resistance to stretch in relaxed skeletal muscle.
Muscle Contraction
When a muscle contracts, the sarcomere shortens, the H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments.
Huxley and Huxley called this mechanism the sliding filament theory of muscle contraction.
Actin and myosin slide past each other as the muscle contracts.
Muscle Contraction
Each myosin molecule consists of two long polypeptide chains coiled together, each ending in a large globular head.
A myosin filament is made of many such molecules arranged in parallel.
An actin filament consists of two chains of actin molecules __________ together.
The myosin heads have sites that bind to actin, forming bridges between the actin and myosin.
Myosin heads also have ATPase activity.
Muscle Contraction
A myosin head binds to actin and its orientation changes. This exerts a force that causes the actin to slide.
The myosin head then binds ATP and releases the actin. The myosin head returns to its original formation and can bind to actin again.
Contraction of the sarcomere involves many cycles of interaction between many myosin heads and actin.
Backsliding of actin does not occur because the many surrounding filaments create a system of interacting cycles.
Muscle Contraction
The ATP is needed to __________ the actinmyosin bonds, not to form them.
The energy is actually used to stop muscles from contracting.
This accounts for the stiffening of muscles (rigor mortis) after death. With no ATP being made, the actinmyosin bonds cant be broken.
Muscle Contraction
Muscle contractions are initiated by action potentials from motor neurons.
Each motor neuron branches and synapses with up to a hundred muscle fibers. These fibers constitute a motor unit.
Muscle fibers are excitable and are depolarized by a threshold action potential.
This opens sodium channels, permitting the muscle plasma membrane to generate action potentials just like the axon of the delivering neuron.
Muscle Contraction
Action potentials in muscle fibers also travel deep within the cell. The plasma membrane is continuous with a system of tubules that branch through the cytoplasm (sarcoplasm).
Transverse tubules are T tubules.
T tubules run close to a network of intracellular membranes called the sarcoplasmic reticulum.
The sarcoplasmic reticulum surrounds every myofibril.
At rest, there is a high concentration of Ca2+ in the sarcoplasmic reticulum and a low concentration in the sarcoplasm.
Muscle Contraction
When an action potential spreads through the T tubule system, it causes calcium channels in the sarcoplasmic reticulum to open.
Ca2+ diffuses out of the reticulum into the sarcoplasm surrounding the myofibrils.
The Ca2+ stimulates the interaction of actin and myosin by by binding to the protein troponin.
Muscle Contraction
When the muscle is at rest, two proteins, tropomyosin and troponin, block the myosin binding sites on the actin filament.
When Ca2+ is released to the sarcoplasm, it binds to troponin. Troponin and tropomyosin change shape, exposing the actinmyosin binding sites.
With the binding sites exposed, the actinmyosin bonds are made, and the filaments are pulled past each other, resulting in muscle fiber contraction.
If Ca2+ remains available, the cycle repeats and muscle contraction continues.
Muscle Contraction
In smooth muscle, Ca2+ entering the sarcoplasm combines with a protein called calmodulin.
This complex activates the enzyme myosin kinase, which phosphorylates the myosin heads.
Thus chemically activated, myosin goes through cycles of binding and releasing actin, causing muscle contraction.
Another enzyme, myosin phosphatase, dephosphorylates the myosin and helps stop the actinmyosin interactions.
Muscle Contraction
The minimum unit of contraction, a twitch, is measured in terms of the tension, or force, it generates.
The force generated by a muscle depends on how many muscle fibers are in its motor units. In muscles used for fine movement, motor units have only one or a few fibers.
If action potentials are fired rapidly, before the myofibrils return to rest, the twitches sum, and tension increases and is sustained.
At high stimulation levels, the calcium pumps in the sarcoplasmic reticulum can no longer remove Ca2+ ions between action potentials and maximum tension is generated, called tetanus.
Muscle Contraction
The length of time in tetanus depends on the supply of __________.
The actinmyosin bonds have to keep cycling to maintain muscle tension.
Faster twitching of individual fibers causes temporal summation.
An increase in the number of motor units (all the muscle fibers served by a single neuron) in the contraction results in spatial summation.
Muscle Contraction
A low level of tension maintained in many muscles is called muscle tone.
It results from a small but changing number of motor units alternating contraction and relaxation, and is constantly readjusted by the nervous system.
Muscle Strength and Performance
Skeletal muscle fibers may be of different types.
Slow-twitch fibers (red muscle) have many mitochondria and a lot of the oxygen-binding molecule myoglobin to provide steady, prolonged ATP production.
Red muscle is also well supplied with blood vessels and fuel reserves (glycogen and fat).
Long-term aerobic work such as running and swimming depend on this type of fiber.
Muscle Strength and Performance
Fast-twitch fibers (white muscle) have fewer mitochondria and very little myoglobin.
They develop maximum tension more rapidly and with greater tension, but fatigue rapidly.
The myosin of fast-twitch fibers has a high ATPase activity, but cannot replenish ATP fast enough to sustain long-time contraction.
Fast-twitch fibers are ideal for situations that require sudden, maximum strength, such as weight lifting or sprinting.
Genetics largely determines the proportion of these two fibers in skeletal muscles.
Muscle Strength and Performance
When muscle is stretched and the sarcomeres are lengthened, there is less overlap between the actin and myosin, fewer cross-bridges form, and less force can be produced.
If the sarcomeres are stretched too much, there is no overlap between the actin and myosin, and no force can be produced.
Titin molecules pull the actin and myosin back into an overlapping arrangement.
When muscle is fully contracted, the actin and mysin filaments overlap so much that the myosin bundles are pressed up against the Z lines and additional shortening is difficult.
Muscle Strength and Performance
Different types of exercise produce different physical conditioning responses.
In general, anaerobic activities (weight lifting) increase strength and aerobic activities (jogging) increase endurance.
Strength is a function of the cross-sectional area of muscles; more actin and myosin filaments can produce more tension.
Anaerobic exercise induces formation of new actin and myosin filaments, hence, bigger muscles.
Satellite cells generate new muscle fibers following muscle damage and, to some extent, in response to exercise.
Muscle Strength and Performance
Aerobic exercise enhances the oxidative capacity of muscles by increasing the number of mitochondria, enzymes, and the density of capillaries that deliver oxygen.
There is also an increase in myoglobin, which facilitates the diffusion of oxygen throughout the muscle fibers and provides a store of oxygen for use when the blood supply is insufficient.
Thus aerobic training stimulates fast-twitch fibers to increase their oxidative capacity.
Muscle Strength and Performance
Muscles have three systems for obtaining the ATP they need for contraction:
The immediate system: preformed ATP and creatine phosphate
The glycolytic system: metabolizing carbohydrates to lactate and pyruvate
The oxidative system: metabolizing carbohydrates or fats to H20 and C02
The capacity of these three systems and the rates at which they can produce ATP determine work capacity and endurance.
Muscle Strength and Performance
The immediate system uses stored ATP (very small amounts) and creatine phosphate (CP).
CP stores energy in a phosphate bond and transfers it to ADP.
The total energy available from the immediate system is only about 10 Calories but it is available immediately and enable fast-twitch fibers to generate force quickly.
The system is exhausted in only a few seconds.
Muscle Strength and Performance
The glycolytic system is able to take over within a few seconds.
The ATP generated by glycolytic enzymes in the muscle fiber cytoplasm is rapidly available to the myosin filaments.
However, glycolysis rapidly leads to the accumulation of lactic acid, which slows the process.
The glycolytic and immediate systems can provide energy for less than one minute.
Muscle Strength and Performance
Oxidative metabolism can come on line in about a minute and produce large amounts of ATP
However, it requires many reactions and takes place in the mitochondria, so the rate at which oxidative metabolism can make ATP available to the myosin filaments is slower than that of the other two systems.
Skeletal Systems
The simplest type of skeleton is the __________ skeleton of cnidarians, annelids, and other soft-bodied invertebrates.
It consists of fluid enclosed in a body cavity surrounded by muscle.
Squeezing this fluid-filled cavity by muscle action bulges the body in a particular direction.
This mechanism accounts for the extending and retracting tentacles in the sea anemone.
The earthworm uses its hydrostatic skeleton to crawl by exerting pressure on many separate, fluid-filled segments.
Skeletal Systems
Exoskeletons are hardened outer surfaces to which muscles attach internally.
Contractions of muscles cause jointed and articulated segments of the exoskeleton to move relative to each other.
The simplest example of an exoskeleton is the shell of a mollusk.
Skeletal Systems
The arthropod exoskeleton, or cuticle, covers all body surfaces, including appendages.
Except at the flexible joints, the cuticle contains waxes and stiffening materials.
The outer, waxy epicuticle protects the body from drying out, and the thicker, chitin-containing endocuticle forms most of the structure and provides protection.
The exoskeleton must be shed (molted) to allow growth of the animal to a larger size.
During molting, the animal is very vulnerable to predators and the outside environment.
Skeletal Systems
Vertebrate endoskeletons are internal scaffoldings to which muscles attach and against which they can pull.
Bones are connected by joints that allow a range of movements.
Unlike exoskeletons, endoskeletons do not provide outer protection, but they can grow and enlarge inside the body without a molt.
The human body has 206 bones that make up the axial and appendicular skeletons.
Skeletal Systems
Cartilage is connective tissue with an extracellular matrix of a rubbery mix of collagen and polysaccharide, which gives strength and resiliency.
It is found in joints and in stiff but flexible structures such as the nose and ear.
The embryonic skeleton of vertebrates is primarily cartilage, which is gradually replaced by bone during development.
Sharks and rays keep their cartilage skeletons for life.
Skeletal Systems
Bone is mostly extracellular matrix material of collagen fibers and crystals of calcium phosphate, which makes bone hard and rigid.
The living cells of bone:
Osteoblasts lay down new matrix on the bone surface and gradually become enclosed within the bone in lacunae (cavities).
Osteocytes are the enclosed osteoblasts. They are in contact through long cellular extensions that run through tiny channels.
Osteoclasts resorb bone, creating cavities and tunnels, and help the osteoblasts replace and remodel the bones.
Skeletal Systems
Bones may develop in two different ways.
Membranous bone (e.g., skull bones) forms on a scaffolding of connective tissue membrane.
Cartilage bone (e.g., limb bones) forms first as cartilage, then gradually hardens (ossifies) to become bone.
Cartilage bones can grow during ossification.
Skeletal Systems
Bone may be cancellous (with cavities) or compact (solid and hard).
Most bones have both cancellous and compact regions.
Long limb bones have compact bone surrounding a central cavity of soft __________ and cancellous ends.
Mammal compact bone is called Haversian bone; it is composed of structural units called Haversian systems, which are sets of osteocytes and blood vessels in concentric bony cylinders.
Skeletal Systems
A joint is where two bones meet. The human skeleton has several types of joints.
Movement around joints is accomplished by antagonistic muscle pairsone contracting, the other relaxing.
One muscle is the flexor (bends the joint) and the other is the extensor (straightens the joint).
Bones at a joint are held together by ligaments, flexible bands of connective tissue.
Straps of connective tissue called tendons attach the muscles to bones.
Skeletal Systems
Bones and joints work like systems of levers.
A power arm and a load arm work around a fulcrum, or pivot.
The length ratio of the two arms determines whether a particular lever can exert a lot of force over a short distance or is better at translating force into large or fast movements.
Some parts of the human body rely on the design that produces power (e.g., the jaw); others rely on the one that produces speed (e.g., lower leg).
Other Effectors
Effectors are tissues and organs that carry out responses to commands from the central nervous system.
There are many types of specialized effectors in the animal kingdom; some for defense and some for communication, capturing prey or avoiding predators.
Other Effectors
__________ are miniature harpoonlike missiles fired by certain cnidarians.
Each nematocyst consists of a tightly coiled thread inside a capsule, armed with a spinelike trigger.
When potential prey brush the trigger, the nematocyst fires.
The prey are poisoned or entangled by batteries of these barbed threads, and some large fish may be subdued in this way.
The Portuguese man-of-war has enough poison to kill a human.
Other Effectors
Some animals change color for camouflage or to communicate.
Chromatophores are pigmented cells in the skin of creatures like chameleons and flounder.
The most common type of chromatophore has fixed cell boundaries, within which pigment granules are moved by microfilaments.
When pigment is concentrated in the cell center, the animal looks pale.
When pigment is dispersed throughout the cell, the animal is darker.
Other Effectors
Another type of chromatophore is capable of amoeboid movement.
The cells can mold themselves into shapes with a minimal area, making the tissue look pale.
When they are flattened out, the tissue looks darker.
Other Effectors
The third type of chromatophore changes shape by the action of muscle fibers that radiate outward.
When the fibers are relaxed, the chromatophore is small, and the animal is pale.
When the fibers contract, the chromatophore expands, and the animal is dark.
These chromatophores can change very rapidly and are used in some species in courtship and aggressive behavior.
Other Effectors
Glands are effector organs that produce chemicals used for defense, prey capture, and communication.
Airborne chemicals used for communication are called pheromones and are used by insects in mating.
Snakes, spiders, some frogs, and other organisms have poison glands for prey capture and defense.
Some of these poisons can be highly toxic, blocking nervemuscle junctions and calcium channels in prey or predators.
Skunks have an effective __________ spray (__________) for defense, but it is not poisonous.
Other Effectors
Electric eels, catfish, and rays can generate electricity. The electric fields are used to sense the environment, to communicate, and to stun prey or predators.
Electric organs evolved from __________ and produce __________ __________ in the fashion of nerves and muscles, only with far more voltage.
An electric eel can produce 600 volts or about 100 watts, enough to light a row of light bulbs or stun a human.
Animation 47.1 Molecular Mechanism of Muscle Contraction
Animation 47.2 Smooth Muscle Action
Video 47.1 The flagellated green alga Haematococcus
Video 47.2 The flagellated euglenoid Phacus
Video 47.3 Rotifers feeding via flagella-induced vortices
Video 47.4 Cardiac muscle cell beating
Video 47.5 Contraction of skeletal muscle
Video 47.6 Chromatophore and melanophore function in fish scales
Video 47.7 Visual communication in squid