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Answers to Mastering Concepts Questions 24.1 1. How is the nervous system’s role in maintaining homeostasis different from that of the endocrine system? The role of the nervous system in maintaining homeostasis is nearly instantaneous, whereas the role of the endocrine system is slower and longer lasting. 2. What are the roles of neurons and neuroglia? Neurons are the communication cells in the nervous system, whereas neuroglia play a support role. 3. Distinguish between the central and peripheral nervous systems. The central nervous system consists of the brain and spinal cord. The peripheral nervous system lies outside of the brain and spinal cord. 24.2 1. Describe the parts of a typical neuron. Three parts of a neuron are: - dendrites: branches that receive sensory input and bring it to the neuron’s cell body. - cell body: contains the nucleus, mitochondria, and ribosomes. The cell body carries on the normal metabolic cellular functions of the neuron. - axon: a long fiber extending from the cell body. Axons branch at their terminal ends and form junctions with other cells, such as other neurons, muscles, or glands. The role of the axon is to transmit a nerve impulse to another cell. 2. Where is the myelin sheath located? The myelin sheath is around the axons of neurons. 3. What is the usual direction in which a message moves within a neuron? Within a neuron, a usual message moves from dendrites to cell body to axon. 4. What are the functions of each of the three classes of neurons? The three classes of neurons and their functions can be summarized as: - Sensory neurons bring information to the central nervous system. - Motor neurons connect the central nervous system to muscles and glands. - Interneurons are central nervous system neurons that connect sensory neurons with motor neurons. 24.3 1.What is the difference between the resting potential, the threshold potential, and an action potential? A resting potential is when the voltage difference between the inside and outside of the membrane is not transmitting a signal. The inside of the cell is more negative than the outside. A threshold potential of around -50mV is the signal to open more Na+ channels. An action potential is a brief depolarization that propagates along a nerve fiber. 2. How does an axon generate and transmit a neural impulse? If the “trigger zone” reaches the threshold value then Na+ channels in the axon will briefly open, depolarizing the axon to its tip. This depolarization then propagates down the axon as Na+ in one local area diffuses into the next and brings it to threshold. 3. What prevents action potentials from spreading in both directions along an axon? There is a refractory period as a local area of the axon returns to resting potential. 4. How does myelin speed neural impulse transmission? The parts of an axon that are coated with a myelin sheath lack sodium channels. Areas where there are gaps in the sheath, however, do have sodium channels. Action potentials, therefore, "jump" between the gaps, which speeds the signal. 24.4 1. Describe the structure of a synapse. The neuron ends in an axon terminal that contains synaptic vesicles with neurotransmitters. The receiving cell has ion channels with receptors for the neurotransmitters. Between the two cells is a small gap called the synaptic cleft. 2. What event stimulates a neuron to release neurotransmitters? When the axon terminal depolarizes, calcium channels open, and calcium diffuses in. This triggers the release of the neurotransmitters. 3. What happens to a neurotransmitter after its release? After a neurotransmitter is released, some of it travels to receptors on the receiving cell. Some diffuses away, some is enzymatically inactivated, and some is taken back into vesicles within the sending cell. 24.5 1. Which structures make up the peripheral nervous system? The nerves that branch off the spinal cord compose the peripheral nervous system. 2. How do the sensory and motor pathways of the peripheral nervous system differ? The sensory pathways transmit action potentials to the central nervous system, and the motor pathways carry them away. 3. Describe the relationships among the motor, somatic, autonomic, sympathetic, and parasympathetic nervous systems. The motor system carries action potentials to the muscle and glands. It is divided into the somatic and autonomic systems, which carry signals to voluntary and involuntary structures, respectively. The autonomic system is further divided into the sympathetic system, which operates under stress, and the parasympathetic system for more relaxed times. 4. How do the sympathetic and parasympathetic nervous systems maintain homeostasis? These systems continually work together, in opposition to each other, to maintain balance in the body. 24.6 1. What are the functions of the spinal cord? The spinal cord transmits action potentials between the body and the brain; it also functions in reflexes. 2. What are the major structures in the hindbrain, midbrain, and forebrain, and what are their functions? Brain structures and functions: - Hindbrain: The major structures are the pons, medulla oblongata, and cerebellum. The pons connects higher brain centers with the spinal cord and connects the forebrain to the cerebellum. The medulla oblongata regulates breathing, blood pressure and heart rate, and controls reflex centers for hiccupping, sneezing, defecating, coughing, and swallowing. The cerebellum refines motor messages and coordinates muscle movements. - Midbrain: The midbrain is part of the brainstem. Nerve fibers that control voluntary motor function pass from the forebrain through the midbrain portion of the brainstem. - Forebrain: The major structures in the forebrain are the cerebrum, thalamus, and hypothalamus. The cerebrum controls the qualities of the mind: notably, personality, intelligence, and perception. The thalamus is a relay station that receives sensory information and sends it to the correct portion of the cerebrum. The hypothalamus maintains homeostasis, controlling body temperature, heartbeat, water balance, blood pressure, hunger, thirst, sexual arousal, and emotions. It also regulates secretions from the pituitary gland. 3. What are the parts and functions of the cerebral cortex? The cortex is divided into 4 lobes: frontal, parietal, temporal, and occipital. Each lobe is divided into 2 hemispheres (of the left and right cerebrum). The cerebral cortex is responsible for interpreting sensory information; control of voluntary movements; and association, interpretation, and analyzing of information. 4. How do short- and long-term memories differ? Short-term memory stores information only for moments, after which it is forgotten. Long-term memory forms more permanent connections between neurons in a pathway so that the information is available for much longer periods. 5. List some structures that protect the central nervous system. The meninges, blood-brain barrier, cerebrospinal fluid, skull, and vertebral column protect the central nervous system from damage. 6. To what extent can the nervous system heal itself? Neurons of the central nervous system cannot regenerate. However, all neurons can form new connections that compensate for the loss of other neurons. 24.7 1. Distinguish between sensation and perception. A sensation is the raw input of a receptor as it arrives at the central nervous system. Perception is the interpretation of the sensation in the CNS as all sensory input is integrated and then combined with memory. 2. What role do the senses play in maintaining homeostasis? The senses monitor internal and external stimuli, including blood pH, body temperature, levels of ions and water in interstitial fluids, and a host of other physical and chemical conditions. Information about these stimuli is transmitted to the central nervous system for processing and may trigger hormonal, chemical, or behavioral adjustments that maintain homeostasis. 3. What are the major types of sensory receptors? The major types of sensory receptors are chemo-, photo-, mechano-, thermo-, proprio-, electro-,and pain receptors. 4. What is a receptor potential? A receptor potential is a graded potential that occurs in a sensory receptor. If the receptor potential is large enough, it will generate an action potential in the sensory receptor. 5. What is sensory adaptation, and how is it beneficial? Sensory adaptation is a reduced response to a stimulus; tuning out sensations that are the equivalent of irrelevant “background noise.” 24.8 1. Which structures provide the senses of touch, temperature, and pain? Mechanoreceptors in the skin provide the sense of touch. Some free nerve endings in the skin are thermoreceptors, whereas other free nerve endings detect mechanical damage and produce the sensation of pain. 2. How does the brain participate in the general senses? The brain receives input from receptors. That input is mapped to specific body locations so that the sensation can be interpreted. 24.9 1. How does the brain detect and identify odors? Olfactory receptor cells in the nose send signals to the brain's olfactory bulb. The cerebral cortex identifies the odor based on the specific membrane-bound receptor proteins that have transmitted the impulse. 2. How does a taste bud function? A taste bud is a cluster of taste receptor cells that have concentrations of chemoreceptors. Each sends an action potential to sensory neurons that take the message to the brain for processing and integration. 3. What are pheromones? Pheromones are scent molecules made by one organism that carry information to, and cause a response in, another individual of the same species. 24.10 1. What are the parts of the vertebrate eye? The parts of the vertebrate eye include the sclera, the choroids, and the retina. The sclera includes the white of the eye and the cornea. The choroid includes blood vessels, lens, iris, and pupil. The retina is a layer of photoreceptors at the back of the eye. Most of the eye’s volume is filled with vitreous humor. Aqueous humor fills the space between the cornea, iris, and lens. 2. What are the roles of photoreceptors and pigments in vision? Rod cells and cone cells detect light. Rod cells provide black-and-white vision in dim light, and cone cells provide color vision in bright light. Both cell types contain lightsensitive pigments that absorb photons of light, and trigger receptor potentials that are passed on to other neurons that send action potentials to the brain. 3. Trace the pathway of information flow from the retina to the visual cortex of the brain. In the retina, light sensitive pigments in rods and cones absorb light energy of different wavelengths. In the presence of light, the pigment molecule changes shape and triggers a receptor potential that stimulates the retina’s neurons. These send action potentials through the visual pathway to the optic nerve. The optic nerve exits the eyeball, traveling from the retina to the brain. Optic nerves lead to neurons in the primary visual cortex of the brain. 24.11 1. What is the role of mechanoreceptors in the senses of hearing? Mechanoreceptors in the cochlea detect sound waves that vibrate the fluid of the inner ear. They transmit this information to processing centers in the brain. 2. What are the parts of the ear, and how do they transmit sound? The outer ear funnels sound waves into the auditory canal that ends in the eardrum. In response to sound waves, the eardrum and bones of the middle ear move; their movements jiggle the fluid of the cochlea. Vibration of the fluid in the cochlea causes cilia of hair cells to move relative to the overlying membrane. This movement, in turn, causes the hair cells to release a neurotransmitter that triggers action potentials in the auditory nerve. 24.12 1. What is the evidence that the presence of algal toxins is a selective force on softshell clam populations? Comparisons between two populations of clams living in two different environments with two different selective pressures, revealed that in the presence of regular algal blooms, clams exhibit resistant sodium channels. 2. How did Bricelj and her colleagues demonstrate that sodium channel structure explains toxin resistance in some clam populations? Using a laboratory set up, the researchers first demonstrated that the Bay of Fundy clams, unlike those at Lawrencetown Estuary, were not susceptible to the toxin of the algal bloom. They then investigated the DNA sequences for the sodium channels and discovered coding for one amino acid difference between the two protein channels. Finally, they grew cells in culture with DNA for expression of both sodium channel variants. The cells were then exposed to the saxitoxin and rates of sodium flow measured through the channels. The mutated channels still allowed sodium to flow when the wild type channel did not. Answers to Write It Out Questions 1. How do the nervous and endocrine systems differ in how they communicate? One difference is the speed at which the two communication systems act. The nervous system’s electrical impulses travel so rapidly that their effects are essentially instantaneous. The endocrine system acts more slowly. Neurons secrete neurotransmitters that affect adjacent cells; in contrast, endocrine glands secrete chemical messages called hormones that circulate throughout the bloodstream and take hours or longer to exert their effects. 2. Sketch two neurons, with one synapsing on the other. In your sketch, label the dendrites, axons, cell bodies, myelin sheath, and synapse. [Answers will be visual. Figures 24.2, 24.3, and 24.6 might be helpful.] 3. Describe the distribution of charges in the membrane of a resting neuron. At rest, a neuron’s membrane is polarized. The inside of the neuron carries a slightly negative electrical charge relative to the outside. This separation of charges creates an electrical potential. 4. What causes the switch in the distribution of charges when an axon propagates an action potential? Once enough sodium enters to depolarize the trigger zone’s membrane to a threshold potential, additional sodium gates open, triggering an action potential. 5. In what ways does an action potential resemble a crowd doing “the wave” in a football stadium? An action potential resembles “the wave” in a football game because it creates a series of electrochemical changes that propagate like a wave along the nerve fiber. Successive groups of ion channels open, then close, similar to how successive groups of people stand, then sit, as they do "the wave." 6. How does the myelin sheath increase the speed at which an axon conducts a neural impulse? Myelinated axons conduct impulses faster than those without a fatty sheath. Nodes between the myelin segments contain high concentrations of sodium channels. Action potentials “leap” from node to node, bypassing the myelinated portions. 7. How do neurons use neurotransmitters to communicate with other cells. Releasing the neurotransmitters to diffuse across a synaptic cleft causes ion channels to open and either excite or inhibit the receiving cell. 8. A scientist discovers a way to stop production of a protein required for recycling of synaptic vesicles. What will happen to the amount of neurotransmitter in the synaptic cleft? If synaptic vesicles could not be recycled then new batches of neurotransmitter could not be packaged and released, thus the quantity of neurotransmitter in the cleft would decrease. 9. List the main subdivisions of the human nervous system, along with their functions. Central nervous system: integrates and processes information, reflexes, regulates the body Peripheral nervous system: (a) the sensory portion gathers stimuli information and sends to the central nervous system; and (b) the motor portion carries motor signals to muscle and glands Somatic system: a subdivision of the motor PNS that carries signals to voluntary skeletal muscle Autonomic system: a subdivision of the motor PNS that carries signals to involuntary cardiac and smooth muscle and glands 10. Why can the loss of reflexes be a possible indication of damage to the central nervous system? Reflex arcs have a pathway through the spinal cord, so that is one possible place for the damage to be located. 11. How would you test the hypothesis that a nonhuman animal feels pain or thinks? Which animals would you choose to investigate this question? One way would be to measure electrical activity in regions of the brain associated with pain or problem solving. Using a range of animals with different levels of intelligence, from invertebrates to fishes to mammals, would yield interesting information on pain perception and thought (although the question of pain would certainly raise ethical questions). 12. Consider the suggestion that humans use only 10% of their brains. Given the brain’s energy demands, does this claim make sense? Do studies of brain-damaged patients support or refute this statement? Nervous tissue requires much energy to maintain resting potential; it would be disadvantageous for an organism to spend energy maintaining resting potential in parts of the brain that are unused. In addition, the plasticity of brains after trauma might suggest that many brain cells actively participate in neural pathways. Humans use much more than 10% of their brains. 13. Neuroglia outnumber neurons by about 10 to 1. In addition, neuroglia retain the ability to divide, unlike neurons. How do these two observations relate to the fact that most brain cancers begin in neuroglia? Cancers are cells that have lost the ability to regulate cell division. Cells have to divide in order to form tumors. Neurons don’t divide, so it would be less likely that they would lose the ability to regulate cell division. 14. How does the peripheral nervous system interact with the central nervous system to produce perceptions of stimuli? The peripheral nervous system is responsible for detecting stimuli and transmitting them to the central nervous system where they can be interpreted. 15. What is the role of transduction in the sensory system? How does transduction occur for each of the senses described in this chapter? Transduction is the process where the energy of external stimuli is converted to the energy of action potentials, the form of energy the nervous system is able to interpret. In touch, pressure on the mechanoreceptors generate the action potential. In temperature, free nerve endings in the skin do transduction. Pain receptors respond to mechanical damage. Chemoreceptors bind to molecules dissolved in a watery solution for transduction to occur in smell and taste. Light activates chemicals in photoreceptors, and vibrations in fluid move mechanoreceptors in hearing. 16. In what ways are the senses of smell and taste similar? In what ways are they different? The senses of smell and taste both depend on chemoreceptors that detect chemicals in the environment. Both senses require that a stimulus molecule be dissolved in a watery solution, such as saliva or the moist lining of a nasal passage. In addition, the molecule must interact with a receptor on a sensory cell’s membrane. However, the sense of taste is limited to just five stimuli, but smell has many, many stimuli. 17. List the structures of the human eye and their functions. Iris—regulates the diameter of the pupil Lens—further bends light and focuses it on the retina Pupil—the opening that lets light through to the lens Cornea—the clear outer part of the eye that first bends the light Retina—contains the photoreceptors Choroid—the vascular layer that supplies the eye with nutrients and oxygen Sclera—the outer portion of the eye providing protection and structure Optic nerve—the cranial nerve that connects the eye to the brain 18. What is the role of rods and cones in the sense of sight? Rods and cones are neurons in the retina that respond to light. Rods respond to light intensity, or brightness, and cones respond to light wavelength, which we perceive as color. Ultimately, the brain processes signals from rods and cones into the image we perceive. 19. Describe one way that each sense listed in this chapter can help the body maintain homeostasis. Answers will vary. - Touch allows the body to respond to stimuli that might cause pain. - Thermoreception allows the body to maintain the optimal temperature for enzyme function. - Pain sensing provides feedback to help the body heal and minimize injury. - Knowing the body’s position is important to resting, conserving energy, and avoiding injury. - Smell helps to avoid ingesting toxic or spoiled foods. - Taste helps reinforce consumption of nutrients that the body needs. - Sight stimulates quick hormonal responses to danger, and stimulates alternate hormonal responses when the danger has passed. - Hearing informs the body of threats before they can be seen or heard. - Equilibrium keeps the body balanced, allowing quick movement away from danger or to a food source. Answers to Pull It Together Questions 1. What are the main parts of a neuron? A neuron has a central cell body; short extensions (called dendrites) reaching in many directions from the cell body; typically only one long extension (called the axon or nerve fiber); and meeting points with other neurons called synapses that form at the end of axons. 2. Describe the functions of the central nervous and peripheral nervous systems. The central nervous system is responsible for the integration of sensory information and the direction of motor information. The peripheral nervous system receives the information and then performs the action directed by the central nervous system. 3. Add the somatic, autonomic, sympathetic, and parasympathetic nervous systems to this concept map. “Peripheral nervous system” leads with “is divided into” to “Somatic nervous system” and “Autonomic nervous system”. “Autonomic nervous system” leads with “is divided into” to “Sympathetic nervous system” and “Parasympathetic nervous system”. 4. Make a chart that lists the types of sensory receptors and the sense organs that use each type. Receptor Type Mechanoreceptors Thermoreceptors Nociceptors, Pain receptors Proprioceptors, Position receptors Chemoreceptors Photoreceptors Organ(s) Skin, ears, equilibrium Skin Everywhere except the brain Muscles and ligaments Nasal cavity, mouth and tongue Eyes