Biology Past Paper PDF - Chapter 12 Coordination and Response

# Chapter 12 Coordination and response ## In this chapter you will: * learn about the human nervous system * find out how different types of neurone are involved in reflex actions * learn about the structure of the eye, as an example of a sense organ * find out about hormones and compare nervous a...

# Chapter 12 Coordination and response ## In this chapter you will: * learn about the human nervous system * find out how different types of neurone are involved in reflex actions * learn about the structure of the eye, as an example of a sense organ * find out about hormones and compare nervous and hormonal control in humans ## 12.1 The human nervous system Changes in an organism's environment are called *stimuli* (singular: *stimulus*) and are sensed by specialized cells called *receptors*. The organism responds using *effectors*. Muscles are effectors and may respond to a stimulus by contracting. Glands can also be effectors. For example, if you smell good food cooking, your salivary glands may respond by secreting saliva. Animals need fast and efficient communication systems between their receptors and effectors. This is partly because most animals move in search of food. Many animals need to be able to respond very quickly to catch their food or to avoid predators. To make sure that the right effectors respond at the right time, there needs to be some kind of communication system between receptors and effectors. If you touch something hot, pain receptors on your fingertips send an electrical impulse to your arm muscles to make them contract, pulling your hand away from the hot surface. The way in which receptors detect stimuli, and then pass information on to effectors, is called *coordination*. Most animals have two methods of sending information from receptors to effectors. The fastest is by means of *nerves*. The receptors and nerves make up the animal's nervous system. A slower method, but still a very important one, is by means of chemicals called *hormones*. Hormones are part of the endocrine system, and this is described in Topic 12.3 in this chapter. ## Neurones The human nervous system is made of special cells called *neurones*. Figure 12.2 illustrates a particular type of neurone called a motor neurone. | Part of Neurone | Description | |---|---| | Dendrite | Short, branched fibres that carry impulses towards the cell body. | | Cell Membrane | The outer layer of the cell, which controls what enters and leaves the cell. | | Cytoplasm | The jelly like substance that fills the neurone. | | Nucleus | Contains the genetic material of the cell. | | Ribosomes | Where proteins are made. | | Cell Body | The main part of the cell, containing the nucleus and other organelles. | | Mitochondria | Where energy is released from food. | | Axon | A long, thin fibre, which carries impulses away from the cell body. | | Myelin Sheath | A fatty layer that insulates the axon, speeding up the impulse transmission. | | Nucleus of cell which makes myelin sheath | The nucleus of the Schwann cell, which produces the myelin sheath. | | Nerve Ending | The tip of the axon where the impulse is transmitted to another neurone or effector. | **Myelin** Some of the nerve fibres of active animals such as mammals are wrapped in a layer of fat and protein called *myelin*. Every now and then, there are narrow gaps in this myelin sheath. We have seen that the signals that neurones transmit are in the form of electrical impulses. Myelin insulates the nerve fibres, so that they can carry these impulses much faster. For example, a myelinated nerve fibre in a cat's body can carry impulses at up to 100 metres per second. A fibre without myelin can only carry impulses at about 5 metres per second. ## The central nervous system All mammals (and many other animals) have a *central nervous system* (CNS) and a *peripheral nervous system* (PNS). The CNS is made up of the brain and spinal cord (figure 12.3). Like the rest of the nervous system, the CNS is made up of neurones. Its role is to coordinate the electrical impulses travelling through the nervous system. The *peripheral nervous system* is made up of nerves that spread out from the CNS. Each nerve contains hundreds of neurones. The peripheral nervous system also includes the receptors in our sense organs. When a receptor detects a stimulus, it sends an electrical impulse along a neurone to the brain or spinal cord. The brain or spinal cord receives the impulse, and sends an impulse on, along the appropriate nerve fibres, to the appropriate effector. ## Reflex arcs Figures 12.4 and 12.5 show how these electrical impulses travel. If your hand touches a hot plate, a sensory receptor in your finger detects this. The receptor starts off an electrical impulse, which travels to the spinal cord along the axon from the receptor cell. This cell is called a *sensory neurone*, because it is carrying an impulse from a sensory receptor (Figures 12.4 and 12.6). A nerve impulse from the motor neurone makes the muscle contract. | Part of Reflex Arc | Description | |---|---| | Pain Receptor | Detects painful stimuli, such as heat. | | Sensory Neurone | Transmits the impulse from the receptor to the spinal cord or brain. | | Cell Body of the Sensory Neurone | The main part of the sensory neurone, containing the nucleus and other organelles. | | Spinal Nerve | A bundle of nerve fibres that connects the spinal cord to the rest of the body. | | Cell Body of the Relay Neurone | The main part of the relay neurone, which transmits the impulse from the sensory neurone to the motor neurone. | | Axon of the Motor Neurone | The long, thin fibre of the motor neurone, which carries the impulse from the relay neurone to the effector. | | Cell Body of the Spinal Cord Motor Neurone | The main part of the motor neurone in the spinal cord, which transmits the impulse from the relay neurone to the effector. | | Spinal Cord | The part of the central nervous system that connects the brain to the rest of the body. | ## Synapses If you look carefully at Figure 12.4, you will see that the three neurones involved in the reflex arc do not quite connect to each other. There is a small gap between each pair. These gaps are called *synaptic gaps*. The ends of the two neurones on either side of the gap, plus the gap itself, is called a *synapse*. Figure 12.8 shows a synapse between a sensory neurone and a relay neurone in more detail. Inside the sensory neurone's axon are hundreds of tiny vacuoles, or vesicles. These each contain huge numbers of molecules of a chemical called a *neurotransmitter*. <start_of_image> Schematic of a Synapse: * **Sensory Neurone Axon ** * **Synaptic Cleft ** * **Relay Neurone ** * **Synaptic Vesicles ** * **Neurotransmitters ** * **Receptor Proteins ** When an electrical impulse arrives along the axon of the sensory neurone, it causes these vesicles to move to the cell membrane of the sensory neurone. They fuse with the membrane and empty their contents - the neurotransmitter molecules - into the synaptic gap. The neurotransmitter quickly diffuses across the tiny gap. The molecules of neurotransmitter attach to receptor proteins in the cell membrane of the relay neurone. This happens because the shape of the neurotransmitter molecules is complementary to the shape of the receptor proteins. The binding of the neurotransmitter with the receptors triggers an electrical impulse in the relay neurone. This impulse sweeps along the relay neurone, until it reaches the next synapse. Here, a similar process occurs to transmit the impulse to the motor neurone. Synapses act like one-way valves. There is only neurotransmitter on one side of the synapse, so the impulses can only go across from that side. Synapses ensure that nerve impulses travel only in one direction. ## 12.2 Sense Organs The parts of an organism's body that detect stimuli, the receptors, may be specialized cells or just the endings of sensory neurones. In animals, the receptors are often part of a *sense organ* (Figure 12.9). A sense organ is a group of receptor cells that respond to a particular stimulus. Your eye, for example, is a sense organ, and contains receptor cells in the retina. These receptor cells are sensitive to light. ## The structure of the eye Figure 12.10 shows the internal structure of the eye. The part of the eye that contains the receptor cells is the retina. This is the part which is actually sensitive to light. The rest of the eye simply helps to protect the retina, or to focus light onto it. | Part of the Eye | Description | |---|---| | Conjunctiva | A thin, transparent membrane that covers the front of the eye. | | Cornea | The transparent front part of the eye, which helps to focus light. | | Pupil | The black hole in the center of the iris. | | Iris | The colored part of the eye, which controls the amount of light that enters the eye. | | Lens | A transparent structure behind the pupil, which helps to focus light on the retina. | | Ciliary Muscle | A muscle that controls the shape of the lens, allowing the eye to focus on objects at different distances. | | Suspensory Ligament | A ring of tissue that holds the lens in place. | | Retina | The light-sensitive layer at the back of the eye, containing the receptor cells (rods and cones). | | Blind Spot | The point on the retina where the optic nerve leaves the eye, and there are no receptor cells. | | Optic Nerve | The nerve that carries impulses from the eye to the brain. | Each eye is set in a bony socket in the skull, which protects the eye. Only the very front of the eye is not surrounded by bone (Figure 12.11). The eye is filled with fluid, which helps to keep it in shape. ## The Retina The retina is at the back of the eye. This is where the receptor cells are. When light falls on a receptor cell in the retina, the cell sends an electrical impulse along the optic nerve to the brain. The brain uses the impulses from each receptor cell to build up an image. Some of these receptor cells are sensitive to light of different colours, enabling us to see coloured images. There are no receptor cells where the optic nerve leaves the retina. This part is called the *blind spot*. If light falls on this place, no impulses will be sent to the brain. ## Rods and Cones The closer together the receptor cells are, the clearer the image the brain can produce. The part of the retina where the receptor cells are packed most closely together is called the *fovea*. This is the part of the retina where light is focused when you look straight at an object. We have two kinds of receptor cells in the retina (Figure 12.13). *Rods* are sensitive to quite dim light, but they do not respond to colour. *Cones* are able to distinguish between the different colours of light, but they only function when the light is quite bright. We have three different kinds of cones, sensitive to red, green and blue light. ## The Iris In front of the lens is a circular piece of tissue called the iris. This is the coloured part of your eye. The iris contains pigments, which absorb light and stop it passing through. In the middle of the iris is a gap called the *pupil*. The size of the pupil can be adjusted. The wider the pupil is, the more light can get through to the retina. In high light intensity, the iris closes in, and makes the pupil small. This stops too much light getting in and damaging the retina. In low light intensity, the iris pulls back from the pupil, so that the pupil becomes larger. This allows more light to reach the retina. To allow it to adjust the size of the pupil, the iris contains muscles. *Circular muscles* are arranged in circles around the pupil. When they contract, they make the pupil get smaller. *Radial muscles* run outwards from the edge of the pupil. When they contract, they make the pupil dilate, or get larger (Figure 12.14). This is called the *iris reflex* or the *pupil reflex*. ## Focusing Light For the brain to see a clear image, there must be a clear image focused on the retina. Light rays must be bent so that they focus exactly onto the retina. Bending light rays is called *refraction*. Most refraction of the light entering the eye is done by the *cornea*. The *lens* makes fine adjustments. Figure 12.15 shows how the cornea and lens focus light onto the retina. The image on the retina is upside down. The brain interprets this so that you see it the right way up. ## Adjusting the Focus Not all light rays need bending by the same amount to focus them onto the retina. Light rays coming from an object in the distance are only diverging slightly. They do not need much bending (Figure 12.16). ## 12.3 Hormones So far in this chapter, we have seen how nerves can carry electrical impulses very quickly from one part of an animal's body to another. Animals also use chemicals to transmit information from one part of the body to another. The chemicals are called *hormones*. Hormones are made in special glands called *endocrine glands*. The hormones pass from the gland into the blood and are carried around the body in the blood plasma. Each hormone has particular organs that it affects, called its *target organs*. The hormone alters the activity of these target organs. Figure 12.19 shows the positions of some endocrine glands in the human body. Table 12.1 summarises their functions. | Gland | Hormone | Function | |---|---|---| | Adrenal gland | Adrenaline | prepares body for vigorous action | | Pancreas | Insulin | reduces the concentration of glucose in the blood | | Pancreas | Glucagon | increases the concentration of glucose in the blood | | Testis | Testosterone | causes the development of male secondary sexual characteristics | | Ovary | Oestrogen | causes the development of female secondary sexual characteristics, and helps in the control of the menstrual cycle | ## Adrenaline There are two adrenal glands, one above each kidney. They make a hormone called *adrenaline*. When you are frightened, excited or keyed up, your brain sends impulses along a nerve to your adrenal glands. This makes them secrete adrenaline into the blood. Adrenaline has several effects which are designed to help you to cope with danger. These effects are known as the 'fight or flight' response. For example, your heart beats faster, supplying oxygen to your brain and muscles more quickly. This allows your muscles to carry out aerobic respiration more quickly, giving them more energy for fighting or running away. Your breathing rate increases, so that more oxygen can enter the blood in the lungs. Adrenaline also causes the pupils in the eye to widen. This allows more light into the eye, which might help you to see the danger more clearly. Adrenaline causes the liver to release glucose into the blood. This extra glucose for the muscles, along with the extra oxygen provided by the increased breathing rate and heart rate, allows the muscles to increase their metabolic activity. You can read more about the control of blood glucose concentration in Chapter 13. Table 12.2 compares the nervous and endocrine systems. | System | Description | |---|---| | Nervous system | made up of neurones | | Nervous system | information transmitted in the form of electrical impulses | | Nervous system | impulses transmitted along neurones | | Nervous system | impulses travel very quickly, so action is fast | | Nervous system | effect of a nerve impulse usually only lasts for a very short time | | Endocrine system | made up of glands | | Endocrine system | information transmitted in the form of chemicals called hormones | | Endocrine system | chemicals carried in the blood plasma | | Endocrine system | chemicals travel more slowly, so action is slower | | Endocrine system | effect of a hormone may last longer | ## 12.4 Coordination in Plants Like animals, plants are able to respond to their environment, although usually with much slower responses than those of animals. In general, plants respond to stimuli by changing their rate or direction of growth. They may grow either towards or away from a stimulus. Growth towards a stimulus is said to be a positive response, and growth away from a stimulus is a negative response. These growth responses are called *tropisms*. Two important stimuli for plants are light and gravity. Growth responses to light are called *phototropism*. Growth responses to gravity are called *gravitropism*. Shoots normally grow towards light. They are positively phototropic (Figure 12.20). Roots do not usually respond to light, but in some plants, the root grows away from light. Shoots generally grow away from the pull of gravity, so they are negatively gravitropic. Roots generally grow towards the pull of gravity, so they are positively gravitropic (Figure 12.21). These responses help the plant to survive. Shoots must grow upwards, away from gravity and towards the light, so that the leaves are held out into the sunlight. The more light they have, the better they can photosynthesise. Flowers, too, need to be held up in the air, where insects, birds or the wind can pollinate them. Roots, though, need to grow downwards, into the soil in order to anchor the plant in the soil, and to absorb water and minerals from between the soil particles. ## Summary In mammals, the nervous system is made up of the central nervous system and the peripheral nervous system. The nervous system coordinates and helps to regulate body functions. Neurones transmit information in the form of electrical impulses. A reflex arc consists of a sensory neurone, relay neurone and motor neurone. An impulse produced in a receptor passes along the sensory neurone, into the relay neurone, then the motor neurone, and then to an effector. The effector takes action, bringing about a reflex action. Reflex actions are fast and automatic. A place where two neurones meet is called a synapse. The arrival of an electrical impulse in the first neurone at a synapse stimulates it to release molecules of neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap and binds with receptor proteins on the membrane of the second neurone. This stimulates an electrical impulse in the second neurone. As there is neurotransmitter on only one side at a synapse, the impulse can only cross the synapse in one direction. Sense organs are groups of receptor cells that respond to specific stimuli. In the eye, the cornea refracts (bends) light and the lens helps to focus light onto the retina, where receptor cells are found. Some of these receptor cells are sensitive to light of different colours. They produce electrical impulses that pass along the optic nerve to the brain. Cones are tightly packed in the fovea, while rods are further out on the retina. Cones respond only to bright light and give colour vision. Rods respond to dimmer light and do not respond to colour. The iris (pupil) reflex controls the diameter of the pupil, and therefore how much light passes through. The radial and circular muscles in the iris are an example of antagonistic muscles. Accommodation is the change of shape of the lens to focus light from distant or near objects. To focus on a near object, the ciliary muscle contracts and loosens the suspensory ligaments, allowing the lens to become fat. To focus on a distant object, the ciliary muscle relaxes and pulls on the suspensory ligaments, which pull on the lens and make it thinner. Hormones are chemicals that are secreted by glands and travel in the blood. They alter the activity of target organs. The adrenal glands secrete adrenaline, which prepares the body for fight or flight by increasing breathing rate, heart rate and the diameter of the pupil. Adrenaline increases metabolic activity, increasing blood glucose concentration and the supply of glucose and oxygen to body organs, by increasing heart rate and breathing rate Nervous control acts more quickly than hormonal control but lasts for a shorter time. Plants respond to stimuli by growth. Gravitropism is a growth response to gravity, and phototropism is a growth response to light. Usually, shoots are positively phototropic and negatively gravitropic. Roots are positively gravitropic and do not usually respond to light. Tropic responses are controlled by auxin, which is secreted by cells in the tip of a shoot. Auxin concentrates on the shady or lower side of a shoot, making the cells in those areas elongate faster than on the other side. This causes the shoot to bend towards light or away from gravity as it grows.