Grade 8 Biology LESSON 1 Senses and coordination The senses The nervous system Neuron *please note that you should refer to your text book for information. The senses Specialized neurons (nerve cells) in your body called sensory receptors enable you to taste, smell, hear, see, and touch, and to detect motion and temperature. Receptors are structures that detect stimuli and the nervous system then responds accordingly. A stimulus: any change in the internal or external environment that causes an organism to react (respond). Smell and taste The senses of taste and smell are stimulated by chemicals and often function together. Specialized receptors located high in the nose respond to chemicals in the air and send the information to the brain. Taste buds are areas of specialized chemical receptors on the tongue that detect the tastes of sweet, sour, salty, and bitter. Signals from these receptors work together to create a combined effect in the brain. Try eating while holding your nose. You will find that your food loses much of its flavor. What we call “flavor” is mostly smell of the food rather than taste. This is why you may not taste food while you have a blocked nose. Sight or vision Grade 8 Biology Light first enters the eye through a transparent, layer of cells called the cornea. The cornea helps to focus the light through an opening called the pupil. The size of the pupil is regulated by muscles in the iris—the colored part of the eye. Behind the iris is the lens, which inverts (turns upside down) the image and projects it onto the retina. The image travels through the vitreous humor, which is a colorless, gelatin-like liquid between the lens and the retina. The retina contains numerous receptor cells called rods and cones. Rods are light-sensitive cells that are excited by low levels of light. Cones function in bright light and provide information about color to the brain. These receptors send nerve impulses to the brain via the neurons in the optic nerve. The brain then interprets the specific combination of signals received from the retina and forms a visual image. Hearing and balance Hearing Vibrations called sound waves cause particles in the air to vibrate. Sound waves enter the auditory, or ear canal and cause a membrane, called the eardrum, at the end of the ear canal to vibrate. These vibrations travel through three bones in the middle ear—the malleus (also called the hammer), the incus (anvil), and stapes (stirrup). As the stapes vibrates the fluid inside the cochlea to move like a wave against the hair cells. The hairs cells respond by generating nerve impulses in the auditory nerve and transmitting them to the brain. Balance The inner ear also contains organs for balance, including three semicircular canals. Semicircular canals transmit information about body position and balance to the brain. The brain then is able to determine your position and whether your body is still or in motion. Grade 8 Biology Touch (*you do not need to know the names of the receptors) Many types of sensory receptors that respond to temperature, pressure, and pain are found in the epidermis and dermis layers of the skin. Distribution of receptors is not uniform in all areas of the body. The tips of the fingers have many receptors that detect light touch. The soles of the feet have many receptors that respond to heavy pressure. Pain receptors are very simple, consisting of free nerve endings that are found in all tissues of the body except the brain. The nervous system The nervous system consists of two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The brain and the spinal cord make up the central nervous system. The peripheral nervous system consists of the sensory neurons and motor neurons that carry information to and from the CNS. The central nervous system The brain Because the brain maintains homeostasis and is involved with almost all of the body’s activities, it is sometimes called the control center of the body. The cerebrum is the largest part of the brain and is divided into two halves called hemispheres. The cerebrum carries out thought processes involved with learning, memory, language, speech, voluntary body movements, and sensory perception. The cerebellum controls balance, posture, and coordination, and is located at the back of the brain. The cerebellum controls the smooth and coordinated movement of skeletal muscles and also is involved with motor skills, such as playing the piano or riding a bike. The brain stem connects the brain to the spinal cord and is made up of two regions called the medulla oblongata and the pons. The medulla oblongata relays signals between the brain and the spinal cord. It also helps control breathing rate, heart rate, and blood pressure. Grade 8 Biology The hypothalamus regulates body temperature, thirst, appetite, and water balance. The spinal cord The spinal cord is a nerve column that extends from the brain to the lower back. It is protected by the vertebrae (the backbone). Spinal nerves extend from the spinal cord to parts of the body and connect them to the central nervous system. Reflexes are processed in the spinal cord. Neuron A neuron or neurone is a nerve cell. There are 3 main type you will study in grade 8. Neurones are a particular type of cell that carry information messages or signals to and from the brain and the rest of the body. A nerve impulse is an electrical charge traveling the length of a neuron. An impulse results from a stimulus, such as a touch or perhaps a loud bang that causes you to jump. A small gap exists between the axon of one neuron and the dendrite of another neuron. This gap is called a synapse. A motor neuron carries impulses from the CNS to the parts of the body that react (effectors) Grade 8 Biology A sensory neuron transmits impulses from the receptors to the CNS. WORKSHEET FOR LESSON 1 Activity 1 Identify 5 different sensory organs in a human and what each is able to detect. Activity 2 Draw the structure of the eye List out the structures and write down the function of each part. (you may recall the knowledge from grade 6 and also refer to the text book) Activity 3 Compare and write the differences between the following. a. rods —cones b. cochlea—semicircular canals c. retina—taste buds Activity 4 Questions from the textbook. Page number Question number 139 3,4 141 1 147 1 to 8 Activity 5 Make a poster each for the following topics (one A4 sheet size per poster only) a. One of the sense organs of your choice b. The brain c. Neurons
