Other Effectors

Muscles are universal in animals, but many effectors are more specialized and are shared by only a few animal species. Some specialized effectors are used for defense, some for communication, and some for capturing prey or avoiding predators. In this section we mention only a few of these specialized effectors to give a sampling of their evolutionary diversity.

Lever system designed for power

Load arm: power arm = 2:1 ratio which generates much force over a small distance.

Lever system designed for speed

Load arm: power arm = 5:1 ratio which moves low weights long distances with speed.

Lever system designed for power

Load arm: power arm = 2:1 ratio which generates much force over a small distance.

Lever system designed for speed

Load arm: power arm = 5:1 ratio which moves low weights long distances with speed.

Lever Systems The Human Body

An example of a lever system designed for power is the human jaw. The power arm is long relative to the load arm.

47.18 Bones and Joints Work Like Systems of Levers A lever system can be designed for either power or speed.

An example of a lever system designed for speed is the human leg. The power arm is short relative to the load arm.

An example of a lever system designed for power is the human jaw. The power arm is long relative to the load arm.

47.18 Bones and Joints Work Like Systems of Levers A lever system can be designed for either power or speed.

An example of a lever system designed for speed is the human leg. The power arm is short relative to the load arm.

chromatophores. A change in body color is a response that some animals use to camouflage themselves in a particular environment or to communicate with other animals. Chromatophores are pigment-containing cells in the skin that can change the color and pattern of the animal. Chromatophores are under neuronal or hormonal control, or both; in most cases, they can effect a change within minutes or even seconds.

Chromatophores enable squids, sole, and flounder, all of which spend much time on the seafloor, as well as the famous chameleons (a group of African lizards; see Figure 34.19) and a few other animals, to blend in with the background on which they are resting and thus escape discovery by predators. Chromatophores with different pigments enable animals to assume different hues or to become mottled to match the background more precisely. In other mollusks, fishes, and lizards, a color change sends a signal to potential mates and territorial rivals of the same species.

There are three principal types of chromatophore cells. The most common type has fixed cell boundaries, within which pigmented granules may be moved about by microfilaments. When the pigment is concentrated in the center of each chromatophore, the animal is pale; the animal turns darker when the pigment is dispersed throughout the cell. Another type of chromatophore is capable of amoeboid motion. These cells can mold themselves into shapes with a minimal surface area, leaving the tissue relatively pale, or they can flatten out to make the tissue appear darker.

The third type of chromatophore changes shape as a result of the action of muscle fibers radiating outward from the cell (Figure 47.19a). When the muscle fibers are relaxed, the chromatophores are small and compact, and the animal is pale. To darken the animal, the muscle fibers contract and spread the chromatophores over more of the body surface. These chromatophores can change so rapidly that they are used in some species for communication during courtship and aggressive interactions. For example, the cuttlefish, a cephalopod, can signal courtship intentions to a potential mate on one side of its body while signaling aggressive threats to a rival on the other side (Figure 47.19b).

glands. Glands are effector organs that produce and release chemicals. Endocrine glands, as we saw in Chapter 42, produce hormones for internal signaling. Other glands secrete substances into the gut or onto the body surface. Some of these secretions are used defensively or to capture

47.19 Chromatophores Help Animals Camouflage Themselves or Communicate (a) Muscle fibers around chromatophores cause the chromatophores to contract. (b) Cuttlefish are cephalopod mollusks that can change color patterns so fast that these changes can be used for rapid communication.

prey. Others are pheromones, chemical signals released into the environment for communication with other individuals.

Certain snakes, frogs, salamanders, spiders, mollusks, and fishes have poison glands, which are used for capturing prey or defending against predators. Many of the poisons produced by these glands are extremely specific in their modes of action. For example, the poison dendrotoxin, which certain tribes of the Amazonian rainforest use on the tips of their arrows for hunting, comes from the skin of a frog and blocks certain potassium channels. The snake venom bungarotoxin inactivates the acetylcholine receptors at the neuromuscular junction. The puffer fish poison tetrodotoxin blocks voltage-gated sodium channels. A poison from a mollusk, conotoxin, blocks calcium channels. Not all defensive secretions are poisonous, however. A well-known example is mercaptan, the odoriferous chemical sprayed by skunks.

Chromatophore Muscle fibers compacted relaxed

(animal is pale)

Muscle fibers Chromatophore contracted spread (animal is dark)

Chromatophore
Pigment

(b) Sepia latimanus

(b) Sepia latimanus

Animals That Produce Electricity

electric organs. Various fishes can generate electricity, as we saw in Chapter 45. These species include the electric eel, the knife fish, the torpedo (a type of ray), and the electric catfish. The electric fields they generate are used for sensing the environment, for communication, and also for stunning potential predators or prey. The electric organs of these animals evolved from muscles, and they produce electric potentials in the same general way as nerves and muscles do.

Electric organs consist of very large, disc-shaped cells arranged in long rows like stacks of batteries. When these cells discharge simultaneously, the electric organ can generate far more voltage and current than can nerve or muscle tissue. Electric eels, for example, can produce up to 600 volts with an output of approximately 100 watts—enough to light a row of light bulbs or to temporarily stun a person.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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