Chapter 3

The changes in external and internal environment faced by an organisms

A changes in the environment that can make an organisms react and cause response in the body are called stimuli

External stimuli are the changes that occur outside the body

Internal stimuli are the changes that occur inside the body

Ex of external stimuli: light, smell, temperature, touch, sound, taste and pressure

Ex of internal stimuli: sugar level in blood, osmotic pressure in blood and body temperature

a. To enable organisms to move, feel pain and taste

b. To enable organisms to adjust to changes in the environment

c. To protect the organisms from harm

d. To ensure the survival

A response is a result of a reaction towards stimuli

In plants, response made is generally a growth movement towards the external stimuli called tropism

Nastic movement – movement of part of plant in response to external stimuli

Tactic movement – response that made up by the whole organisms to external stimuli

Stimulus

Receptor

Afferent pathway (Afferent Neurone)

Integrating

centre(brain)

Effectors

(muscle)

Efferent pathway (Efferent Neurone)

causes

Response

1. The receptor in the sensory organs detect the stimulus. Triggers a nerve impulse to produced in afferent pathway.

2. The afferent pathway carries the nerve impulse to the integrating centre which is in the brain

3. The brain interprets the information and sends out another nerve impulse

4. The efferent pathway then carries the nerve impulse from the brain to the effector

5. The effector such as muscles or glands reacts to cause a response to the stimulus

Integrating centre (brain)

Receptors in the hypothalamus

Stimulus (high glucose level in the blood)

Normal glucose level in the blood

Effector (pancreatic glands)

Response (activities to decrease high glucose

level)

A change in blood sugar level is detected

By negative feedback

Glucose level is lowered

Sugar level in our blood will increase when we take too much carbohydrate or sweet food

This situation will be detected by hypothalamus

The endocrine system and nervous system control and coordinate functions in the body maintain a constant sugar level through a negative feedback system

-ve feedback operation counteract the changes in the internal environment in order to restore it to normal

Coordination is the process involved in the detection of stimulus and the subsequent response of the organism towards the stimulus

It involves both nervous system and endocrine system

Stimulus Receptor(sensory organ)

Afferent pathway (Afferent Neurone)

Integrating centre

(brain) Effectors (muscle,

endocrine glands)

Efferent pathway (Efferent Neurone)

Response

Trigger nerve

impulse Carries nerve

impulses

Carries nerve

impulses

Send nerve impuls

es

Detected by

React s to cause