Organism Response to Stimuli & Coordination

Organism Response to Stimuli

Organisms have systems that allow them to collect information from within and outside their bodies and respond to changes that occur.

Key Concepts

• Stimulus: Any factor capable of triggering a response in an organism. This can be physical or chemical.
• Response: The action an organism performs as a reaction to the stimulus received.
• Sensation: The ability to receive stimuli (e.g., sight, hearing, touch, smell, and taste).
• Perception: A function that requires the interpretation of a stimulus by the brain and the elaboration of complex responses.

Sense Organs and Receptors

Sense organs are responsible for receiving environmental stimuli. Receptors are cells within these organs that capture and transmit the stimulus to the brain through the nervous system. Sense organs receive stimuli from the outside environment.

Coordination Systems

Stimuli are transmitted to a coordinating center. Animals have two main coordination systems:

• The Nervous System: Transmits electrical signals via nerves.
• The Endocrine System: Sends chemical signals (hormones) through the bloodstream.

Both systems coordinate responses to stimuli, which are then carried out by effector organs.

The Nervous System

The nervous system is the central control and coordination system of the body. Its function is to receive information from multiple receptors, interpret it, develop responses, and transmit them to other parts of the body to initiate action.

The cells that form the nervous system are neurons.

Parts of the Nervous System

• Brain

  The control center and coordination of information; it regulates all bodily functions and is protected by the skull.

• Spinal Cord

  Protected by the vertebral column, it is the pathway through which information travels from the rest of the body to the brain and vice versa.

• Nerves

  The network that connects the central nervous system with the rest of the body.

Reflexes

Simple and quick responses are called reflexes; these actions occur automatically and involuntarily.

The Endocrine System

The endocrine system works through hormones. Hormones are substances produced in endocrine glands; they are released into the bloodstream and distributed throughout the body.

Plant Responses

Plant responses to environmental stimuli include tropisms and nasties.

Tropisms

Plant growth responses caused by an external stimulus that determines the direction of growth.

• Phototropism: Response to light.
• Geotropism: Response to gravity.
• Thigmotropism: Response to contact.

Nasties

Plant movements caused by an external stimulus, but the direction of the movement is independent of the stimulus direction.

Photoperiod and Flowering

The photoperiod is the number of hours of light a plant receives each day.

• Long-day plants: Require a light period typically longer than a critical length (e.g., 14 hours), tending to flower in spring or summer.
• Short-day plants: Require a dark period typically longer than a critical length (e.g., 9 hours of continuous darkness), often flowering in autumn.