12.1 - Coordination and Response

All living organisms have the ability to detect both internal and external environmental changes and respond to them accordingly at all times. Such ability is also called sensitivity. The changes that induce certain responses in the body are called stimuli (singular, stimulus). Stimuli can be divided into 2 types.

1. Internal stimuli

• Changes that happen inside the body
• Examples, changes in blood osmotic pressure, blood sugar level, carbon dioxide and oxygen levels.

2. External stimuli

• changes that happen outside the body
• Examples, changes in light intensity, sound, temperature, pressure and touch

Mammals detect stimuli by using specialized sensory cells called receptors. The receptors are responsible to detect any incoming stimuli. The stimuli will then be converted to nerve impulses to be sent to the brain. The brain will coordinate appropriate responses to the stimuli. A response is the way an organism reacts after a stimulus is detected. Effectors in the body carry out the responses to stimuli. Receptors and effectors work together to execute a suitable response to the stimuli.

Figure 1 - Pathways involving detecting and responding to changes in the external environment.

When stimuli are detected and eventually result in appropriate response, this is known as coordination. Coordination is important to make sure that the activities of an organism function as an integrated whole. The two different coordinating systems that are involved in the role of coordination and response are the nervous system and the endocrine system. These two systems complement each other in coordination and response.

Figure 2 - Pathways involving detecting and responding to changes in the internal environment.

Types of Receptors

Sensory receptors are specialized nerve endings which detect changes in the external and internal environments. In general, one type of receptor only detects one type of stimulus.

Sensory receptor and Stimulus

• Photoreceptor : Light
• Thermoreceptor : Changes in surrounding temperature
• Chemoreceptor : Chemical substances
• Baroreceptor : Changes in pressure
• Mechanoreceptor : Touch and pressure
• Nociceptor : Pain