Bio 2222 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...

# 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 specialised 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 | Thin layer surrounding the cell that controls what enters and leaves the cell. | | Cytoplasm | The jelly-like substance that fills the cell and contains organelles. | | Nucleus | Contains DNA, which controls the activities of the cell. | | Ribosomes | Sites of protein synthesis. | | Cell body | Contains the nucleus and other organelles. | | Mitochondrion | Releases energy from glucose during respiration. | | Axon | Long, thin fibre that carries impulses away from the cell body. | | Myelin sheath | Fatty layer that insulates the axon, allowing impulses to travel faster. | | Nucleus of cell which makes myelin sheath | This controls the production of myelin. | | Nerve ending | Where the axon branches to form junctions with other neurones or with effectors. | **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. Neurones contain the same basic parts as any animal cell. Each has a nucleus, cytoplasm, and a cell membrane. However, their structure is specially adapted to be able to carry electrical signals very quickly. To enable them to do this, they have long, thin fibres of cytoplasm stretching out from the cell body. The longest fibre in a neurone is called an _axon_ (see Figure 12.2). Axons can be more than a metre long. The shorter fibres are called _dendrites_. ## 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). <start_of_image> Schematic diagram of a reflex arc: | | |:---| | **Sensory Neurone** - _Receptor_ --> _Relay Neurone_ --> _Motor Neurone_ - **Effector** | ## 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_. 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 specialised 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 Eye | Description | |:-----------|:--------------------------------------------------------------------------------------------------------------------------------| | Conjunctiva | Thin, transparent membrane that covers the front of the eye and helps to protect it from dust and other foreign objects. | | Cornea | Transparent, curved front part of the eye that refracts (bends) light rays. | | Pupil | Circular opening in the iris that allows light to enter the eye. | | Iris | Coloured part of the eye that controls the size of the pupil. | | Lens | Transparent, biconvex structure that focuses light onto the retina. | | Ciliary muscle| Circular muscle that controls the shape of the lens by adjusting the tension on the _suspensory ligaments_. | | Suspensory ligament | Ligaments that hold the lens in place. | | Retina | Light-sensitive layer at the back of the eye that contains _rods_ and _cones_. | | Blind spot | The point where the optic nerve leaves the retina and there are no receptor cells. Therefore, light that falls on this area cannot be seen. | | Optic Nerve | Carries impulses from the receptors in the retina 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_. The circular muscles and radial muscles are _antagonistic muscles_. They work together to control an action and have opposite effects. When one muscle contracts, the other relaxes. These responses of the iris are examples of a reflex action. Although the nerve impulses go into the brain, we do not need to think consciously about what to do. The response of the iris to light intensity (the stimulus) is fast and automatic. Like many reflex actions, this is very advantageous: it prevents damage to the retina that could be caused by very bright light falling onto it. ## 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). The shape of the lens is altered, to make it bend light rays by different amounts. The thicker the lens, the more it bends the light rays. The thinner it is, the less it bends them. This adjustment in the shape of the lens, to focus light coming from different distances, is called _accommodation_. Figure 12.18 shows how the shape of the lens is changed. It is held in position by a ring of _suspensory ligaments_. The tension on the suspensory ligaments, and thus the shape of the lens, is altered by means of the _ciliary muscle_. When this muscle contracts, the suspensory ligaments are loosened. When it relaxes, they are pulled tight. ## 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 that it secretes | Function of hormone | |:-------------|:--------------------------|:------------------------------------------------------------------------| | Adrenal gland | Adrenaline | Prepares body for vigorous action | | Pancreas | Insulin | Reduces the concentration of glucose in the blood | | | 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 | Made up of neurons; Information transmitted in the form of electrical impulses; Impulses transmitted along neurons; Impulses travel very quickly, so action is fast; Effect of a nerve impulse usually only lasts for a very short time. | | Endocrine | Made up of glands; Information transmitted in the form of chemicals called hormones; Chemicals carried in the blood plasma; Chemicals travel more slowly, so action is slower; 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.