Animal Physiology: Response Mechanisms
Motor Responses and Locomotion
Motor responses are linked to the behavior of animals. They are responses that involve some kind of movement, with or without displacement. The effector organs for these responses are the muscles.
The Muscular System
The muscular system is formed by organs called muscles, consisting of muscle tissue and elongated cells called muscle fibers. Their cytoplasm has a large number of myofilaments (actin and myosin) with contractile function.
The muscles of invertebrates and vertebrates are very similar; they are classified as:
Striated Muscle
In striated muscle, the myofilaments are grouped into units called sarcomeres and are fast-twitch. There are three types:
- Vertebrate Skeletal Muscle: Found in the skeletal system, these are fast-twitch, voluntary, and not very resistant to fatigue.
- Cardiac Muscle: Found in the heart, these are involuntary and very resistant to fatigue.
- Invertebrate Striated Muscle: The organization of myofilaments is varied.
Smooth Muscle
Smooth muscle is slow-twitch, involuntary, and resistant to fatigue. They form the muscles associated with the internal organs.
The Skeletal System
It is the static component of the musculoskeletal system, while the muscular system is the dynamic part. Most invertebrates have no skeleton, but possess hard structures that can be located internally or externally. The Porifera contain spongin, a fibrillar protein that gives them consistency. Cnidarians, such as jellyfish, have elastic, fiber-covered bodies, while polyps (e.g., corals) are covered with a limestone structure. Gastropod mollusks secrete an outer shell, while slugs have an inner plate above the respiratory cavity.
Bivalves have two hinged shells that are closed by two strong adductor muscles. Cephalopods have an internal shell, such as the limestone ‘squid pen’. Echinoderms have a chitinous exoskeleton composed of independent units that are linked to each other; their muscles are attached internally to the skeleton. Vertebrates have bones made up of bone tissue, whose intercellular substance is formed by calcium salts. The skeleton of some vertebrates, such as cartilaginous fish, is cartilaginous.
Movements
Bones join each other by joints, which sometimes allow movement. Movement of bones is facilitated by the combined action of muscles. The main types of movement are: flexion, extension, abduction, elevation, adduction, depression, supination, and pronation.
Secretory Responses and Glands
Animals can respond to a stimulus by secreting a substance. In this case, the effectors are glands and organs. They are classified depending on the nature of the substance produced and where it is released:
Types of Glands
- Exocrine Glands (Glands of External Secretion): These secrete substances out of the body or into a cavity in contact with the outside, such as salivary, sudoriferous (sweat), tear, and digestive glands.
- Endocrine Glands (Ductless Glands): These secrete hormones directly into the blood, affecting distant target organs, such as the thyroid gland.
- Mixed Glands: These possess both exocrine and endocrine functions. For example, the pancreas secretes pancreatic juice into the intestine (exocrine) and insulin and glucagon into the blood (endocrine).
Invertebrate Endocrine System
In invertebrates, hormone-secreting neurons (neurohormones) are also abundant.
Pheromones
Both vertebrates and invertebrates secrete pheromones, substances expelled to the outside that act on other animals, usually of the same species. Their chemical nature is very diverse; they can be found in sweat or urine. Pheromones are involved in functions such as signaling behavior in the field and locating individuals of the opposite sex. In invertebrates, they may be involved in processes of individual development and differentiation. For example, a pheromone secreted by the queen bee attracts other bees around her and also prevents the development of worker ovaries and the building of new queen cells.