Stimulus and Response: Nervous, Hormonal Systems
Stimulus and Response in Living Organisms
A stimulus is a change that occurs in the external or internal environment and provokes a response in the body. The relations function is carried out by coordination systems:
- Receptor organs (receive stimuli)
- Coordination centers (analyze information and elaborate a response)
- Effector organs (execute responses)
Coordination may be nervous or hormonal.
Nervous Coordination
The synapse is the functional union where the nervous impulse passes from one neuron to another via neurotransmitters. There are three types of nerves:
- Sensory nerves
- Motor nerves
- Mixed nerves
The peripheral nervous system consists of:
- Cranial nerves (12 pairs that originate from the brain)
- Spinal nerves (31 pairs that originate from the spinal cord)
- Nerve ganglia (interspersed nerve organs)
Skeletal Systems
A hydrostatic skeleton uses pressure exerted by muscles arranged in longitudinal and circular layers, allowing for body elongation. An exoskeleton consists of hard parts that cover the body. The internal skeleton of vertebrates is made up of bones; ligaments join bones together, and tendons connect muscles to bones.
Hormonal Coordination
The thyroid secretes hormones that activate metabolism and are involved in regulating calcium levels in the blood. The pancreas secretes insulin, which facilitates glucose entry into cells. The testes secrete testosterone, which controls the development of male sexual characteristics.
The Senses
| Sense | Stimulus | Organ |
|---|---|---|
| Taste | Chemical receptors | Tongue |
| Smell | Chemical substances in the air | Nose |
| Hearing | Vibrations and sounds | Eardrum |
| Sight | Light | Eye |
| Touch | Heat, cold, pain | Skin |
| Time | Intuition | – |
Reproduction and Development
Living matter is born, grows, develops, reproduces, and eventually dies. Reproduction may be asexual, involving only one parent, or sexual, involving two parents.
Asexual Reproduction
An organism undergoing asexual reproduction makes copies of itself, splitting in two, and the offspring are all identical. Types of asexual reproduction in unicellular organisms include:
- Bipartition: The nucleus divides, followed by the rest of the cell. This occurs about once a day.
- Budding: The nucleus divides and moves toward the edge of the cell to form a bud containing cytoplasm. The bud breaks off, forming a daughter cell. This happens approximately every two hours.
- Sporulation: The nucleus divides several times, producing numerous daughter cells called spores.
Regeneration is the ability of sex cells to close wounds or regrow organs and complete body parts.
Types of asexual reproduction in plants include:
- Rhizomes (underground stems growing horizontally)
- Bulbs (short, usually underground stems with modified, wrapped leaves)
- Tubers (short, thickened underground stems with buds)
- Stolons/Runners (stems growing horizontally on the ground, producing new plants at a distance from the parent)
Sexual Reproduction
Parts of an angiosperm plant include the calyx, corolla, stamens (male), and carpel (female). There are three types of pollination:
- Anemophilous (wind)
- Entomophilous (insects)
- Zoophilous (animals)
Animal breeding processes can be:
- Viviparous (the embryo develops inside the mother’s body)
- Oviparous (the mother lays an egg, and the embryo develops inside it)
- Ovoviviparous (the mother lays an egg, but it develops inside her womb until it hatches, and the already-formed embryo emerges)