Chapter 1: Stimuli and Responses PDF

# Chapter 1: Stimuli and Responses ## 1.1 Human Nervous System - In a game of badminton, the movement of the shuttlecock serves as a stimulus that is detected by the eye. An impulse is triggered and sent to the brain. The brain interprets the impulse, estimates the shuttlecock's speed, and determi...

# Chapter 1: Stimuli and Responses ## 1.1 Human Nervous System - In a game of badminton, the movement of the shuttlecock serves as a stimulus that is detected by the eye. An impulse is triggered and sent to the brain. The brain interprets the impulse, estimates the shuttlecock's speed, and determines the direction and pattern of body movement. The brain then sends impulses to the hand and leg muscles to respond. - The human nervous system is an important control system in body coordination. Along with sight, thinking, and body movement, the human nervous system also controls and coordinates organ functions in the body and maintains a balanced internal environment through a process. ### Structure of the Human Nervous System The human nervous system consists of: - **Central nervous system:** Brain and spinal cord - **Peripheral nervous system:** - 12 pairs of cranial nerves connecting the brain to the sensory and internal organs - 31 pairs of spinal nerves connecting the spinal cord to the skeletal muscles ## Functions of the Human Nervous System The human nervous system controls and coordinates organs and parts of the body. It: - Detects stimuli - Sends information in the form of impulses - Interprets impulses - Produces appropriate responses ## Voluntary and Involuntary Actions The responses of the human body to stimuli can be divided into voluntary actions and involuntary actions. ### Voluntary actions - Are conscious actions and conducted under one's will. - Are controlled by the brain. - Examples of voluntary actions include reading, writing, speaking, eating, drinking, walking, running, and exercising. ### Involuntary actions - Occur immediately without conscious control or prior thoughts. - Can be classified into two: - Involving medulla oblongata: heartbeat, breathing, peristalsis, secretion of saliva - Involving spinal cord (reflex actions): withdrawing hand when it accidentally touches a hot object, withdrawing foot when it accidentally steps on a sharp object, sneezing when dust enters the nose ## Importance of the Network of Human Nervous System in Daily Life The network of the human nervous system controls and coordinates the organs and parts of the body to carry out processes such as breathing and body movements. A damaged nervous system normally causes a person to become temporarily, partially, or completely paralyzed. For example, if the nerves in the muscles of a leg or hand are injured, the person will face difficulty in moving his leg or hand. In the case of a more serious problem, he might have to depend on machines to carry out his physiological processes such as breathing or heart beating. Humans are blessed with a nervous system, so they should use and take good care of it. ## 1.2 Stimuli and Responses in Humans Humans face constant changes in surroundings. These changes are called stimuli. Examples of stimuli include light, sound, and chemical substances. Humans use their sensory organs to detect stimuli. Humans have five sensory organs: eyes, ears, nose, skin, and tongue. ### Eye The eye is the sensory organ of sight. - **Iris:** The coloured part of the eye that controls the size of the pupil. - **Pupil:** The opening in the centre of the iris that controls the quantity of light entering the eye. - **Eye lens:** Transparent and elastic convex lens which focuses light onto the retina. - **Retina:** Layer containing photoreceptors which detects light and produces nerve impulses. - **Yellow spot:** Part of the retina which is most sensitive to light as it has many photoreceptors. - **Blind spot:** Part of the retina which is not sensitive to light as there are no photoreceptors and an exit point for all optic nerve fibres. ### Ear The ear is the sensory organ of hearing. - **Earlobe:** Collects and directs sound waves into the ear canal. - **Ear canal:** Directs sound waves to the eardrum. - **Eardrum:** Vibrates according to the frequency of the sound waves received and transfers the vibrations to the ossicles. - **Ossicles:** Amplify sound vibrations and transfer them to the oval window. - **Oval window:** Collects and transfers sound vibrations from the ossicles to the cochlea. - **Cochlea:** Detects and converts sound vibrations into nerve impulses. - **Semicircular canals:** Detect the position of the head and help to balance the body. - **Auditory nerve:** Sends nerve impulses from the cochlea to the brain to be interpreted. ### Nose The nose is the sensory organ of smell. Smells are chemical substances present in the air. Sensory cells for smell are located at the roof of the nasal cavity. Sensory cells for smell are tiny and covered with a layer of mucous. Chemical substances in the air will dissolve in this layer of mucous and stimulate the cells to produce nerve impulses. The nerve impulses are then sent to the brain to be interpreted to determine the type of smell. ### Tongue The tongue is the sensory organ of taste. The surface of the tongue is covered by tiny nodules known as papillae. The surface of a papillae is covered by hundreds of taste buds. Taste buds contain taste receptors. Taste receptors can detect five types of basic tastes, sweet, salty, sour, bitter, and umami. When food is chewed, part or all the chemical substances in the food dissolve in the saliva. These dissolved chemical substances will diffuse into the taste buds through their pores and stimulate the taste receptors in them to produce nerve impulses. These nerve impulses are then sent to the brain to be interpreted as sweet, salty, sour, bitter, umami tastes or a combination of the basic tastes. ### Skin The skin is the largest sensory organ in the human body. The human skin is made up of a thin outer layer known as epidermis and an inner layer known as dermis. The skin has five types of receptors at different positions to detect different stimuli. When the receptor in the skin is stimulated, nerve impulses are produced and sent through the nervous system to the brain to be interpreted and to produce an appropriate response. ## 1.3 Stimuli and Responses in Plants Like humans and animals, plants can also detect stimuli and respond to them. The stimuli that can be detected by plants include light, water, gravity, and touch. ### Tropism Tropism is a directional response of plants to stimuli such as light, water, gravity, and touch coming from a certain direction. A certain part of a plant will grow towards or move away from the detected stimulus. The part of a plant which grows towards a stimulus is known as positive tropism whereas the part of a plant which grows away from a stimulus is known as negative tropism. The directional response of plants normally occurs slowly and less significantly. - **Phototropism:** The response of plants towards light. Shoots of plants show positive phototropism, which is growth towards the direction of light. - **Hydrotropism:** The response of plants towards water. Roots of plants show positive hydrotropism, which is growth towards the direction of water. - **Geotropism:** The response of plants towards gravity. Roots of plants show positive geotropism, which is downward growth towards the direction of gravity. Shoots of plants show negative geotropism, which is upward growth in the opposite direction of gravity. - **Thigmotropism:** The response towards touch. Tendrils or twining stems show positive thigmotropism when they cling onto whatever objects or other plants they come into contact with. ### Nastic movement Nastic movement is the response towards a stimulus such as touch but does not depend on the direction of the stimulus. It occurs more rapidly than tropism. For example, the Mimosa sp. responds to touch by folding its leaves inwards when touched. This nastic movement serves as a defense of the Mimosa sp. against its enemies and strong wind. ## 1.4 Importance of Responses to Stimuli in Animals ### Stereoscopic and Monocular Visions - **Stereoscopic vision:** Both eyes located in front of the head, a narrow field of vision, fields of vision overlap to a great extent, overlapping fields of vision produce vision in three dimensions. Found in humans and most predators. - **Monocular vision:** Both eyes located at the sides of the head, a wide field of vision, fields of vision do not overlap or overlap only slightly. Found in most prey. ### Stereophonic Hearing Stereophonic hearing is hearing using both ears. Stereophonic hearing allows us to determine the direction of the sound accurately. The importance of stereophonic hearing to humans is to determine the location of a source of sound. Stereophonic hearing also helps prey to determine the location of their predators and to escape from them. ### Hearing Frequencies of Animals Different animals can hear sounds of different frequencies. ## Sensory Organs Ensure the Survival of Animals on Earth Responses to stimuli ensure the survival of animals on Earth. ## Formative Practice **Question 1:** What is the function of the retina in the eyes? **Answer:** The retina receives light and sends nerve impulses to the brain to be interpreted. **Question 2:** Which part of the ear vibrates when sound waves enter the ear? **Answer:** Eardrum **Question 3:** State the five types of taste that the tongue can detect. **Answer:** Sweet, salty, sour, bitter, and umami. **Question 4:** What is the importance of a large number of touch receptors in the tip of the fingers? **Answer:** The large number of touch receptors in the tip of the fingers makes the tip of the fingers highly sensitive to touch. This is beneficial for tasks that require fine motor skills, such as picking up small objects or playing musical instruments. **Question 5:** What is one function of the skin? **Answer:** The skin is a protective barrier against the environment and helps to regulate body temperature. **Question 6:** What is the importance of negative geotropism in plants? **Answer:** Negative geotropism is the growth of the shoot in the opposite direction of gravity. This allows the shoot to reach sunlight for photosynthesis. **Question 7:** Explain how animals with stereoscopic vision can determine the location of an object. **Answer:** Animals with stereoscopic vision can determine the location of an object by using the difference in the images received by each eye. The brain then combines these images to create a three-dimensional view of the object, which allows the animal to judge its distance and depth. **Question 8:** How do animals with monocular vision benefit from their wide field of vision? **Answer:** Animals with monocular vision have a wide field of vision, which allows them to see a larger area of their surroundings. This is beneficial for prey animals that need to be alert for predators approaching from any direction. **Question 9:** How do plants benefit from hydrotropism? **Answer:** Hydrotropism is the growth of roots towards water. This allows plants to access water, which is essential for survival. **Question 10:** What is the difference between tropism and nastic movement? **Answer:** Tropism is a directional response to a stimulus, while nastic movement is a non-directional response to a stimulus. Tropism usually occurs slowly, while nastic movement is often rapid. **Question 11:** What is the importance of stereophonic hearing to humans? **Answer:** They can determine the location of a sound. This is beneficial for tasks such as driving, cycling, and walking in crowded areas.