Chapter 54: the Nervous System

CHAPTER 44: THE NERVOUS SYSTEM

WHERE DOES IT ALL FIT IN?

Chapter 44 builds on the foundations of Chapter 32 and provides detailed information about animal form and function Students should be encouraged to recall the principles of eukaryotic cell structure and evolution associated with the particular features of animal cells. Multicellularity should also be reviewed. The information in chapter 44 does not stand alone and fits in with the remaining chapters on animals. Students should know that animals and other organisms are interrelated and originated from a common ancestor of all living creatures on Earth.

SYNOPSIS

Communication by neurons is extremely fast acting and provides information to specific locations. The nervous systems of vertebrates are composed of the brain, afferent nerves that send information to the brain, and efferent nerves that transmit commands from the brain. The two functional divisions are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain and the spinal cord. The PNS is composed of all of the nerve pathways outside the CNS. Within the PNS, the afferent nerves are sensory pathways and the efferent nerves are motor pathways. Motor pathways are divided into the voluntary (somatic) nervous system that innervate the skeletal muscles and the involuntary (autonomic) nervous system that innervate glands and non-skeletal muscles.

Electrical impulses are transmitted along individual neurons. The highly branched dendrites carry incoming impulses from many different sources to the cell body. Its surface integrates the information arriving from many dendrites and acts as the central processing unit. Generally a neuron has only a single potentially lengthy axon. It carries electrical impulses away from the cell body to a target cell. The end of the axon contains packets of neurotransmitters that chemically transmit the nerve impulse to the next neuron or target cell. Specialized Schwann cells wrap around the axon at specific intervals insulating the axon and forming the myelin sheath. This insulation is interrupted at locations called nodes of Ranvier. Schwann cells produce myelin in the PNS while oligodendrocytes produce myelin in the CNS. These cells are 2 of the most important kinds of neurolglia in vertebrates.

A neuron’s resting potential results from the charge differential between the inside and the outside of the neuron. This gradient results from active outward transport of sodium and inward active transport of potassium through voltage-gated membrane channels. This results in a cell that is slightly more negative on the outside than on the inside. Graded potential, caused by the opening of ligand-gated channels, can depolarize or hyperpolarize the membrane. Depolarizing graded potentials summate, resulting in the rapid inward diffusion of sodium through sodium channels [depolarization] wiping out the local electrical potential difference. This is followed by the outward diffusion of potassium through potassium channels [repolarization]. This rapid change in the membrane potential is called an action potential. The action potential of a neuron is an all-or-nothing event, although the actual electrical value differs among various types of neurons. A membrane is unable to respond to a new stimulus during the refractory period. Saltatory conduction is an extremely rapid form of transmission that jumps from one node of

76 Ranvier to the next.

An axon synapses with other neuron dendrites, with sites on muscles or with secretory cells. The gap between the axon and the target cell is called the synaptic cleft. Nerve signals cross this gap chemically. Chemicals released from the presynaptic side cause ion channels on the postsynaptic side to open initiating depolarization in the target cell. Different chemicals in different junctions allow a variety of responses not possible through direct electrical conduction. The neuromuscular junction is a typical synapse in which acetylcholine is the neurotransmitter. This chemical is rapidly degraded by acetylcholinesterase to allow for subsequent transmission. The cell body integrates signals from inhibitory and excitatory synapses. The signals either cancel or reinforce one another and affect the resulting signal sent out along the cell body’s axon. Many other neurotransmitters occur throughout the nervous system. These include glutamate, glycine, GABA, epinephrine, dopamine, norepinephrine, serotonin, neuropeptides, substance P, and even a gas, nitric oxide. Addictive drugs alter a post-synaptic neuron’s response to neurotransmitters.

The vertebrate brain is divided into three regions: the hindbrain, the midbrain, and the forebrain. The diencephalon of the forebrain integrates sensory information, while the telencephalon is devoted to associative activity. Mammalian brains are particularly large with respect to overall body mass as compared to those of fish and reptiles. In humans, there is great enlargement of the cerebrum which functions in correlation, association, and learning. The human brain is divided into left and right hemispheres connected by the corpus callosum. Each hemisphere is further divided into frontal, parietal, temporal, and occipital lobes. The outer layer of the cerebrum is the cerebral cortex, the site of most neural activity. Sensory integration is directed mostly by the thalamus. The hypothalamus controls visceral responses, like temperature, respiration, and heartbeat and secretions of the pituitary gland. It is connected to the cerebral cortex via the limbic system, the center for emotional responses.

The reticular activating system monitors all signals to the brain and sorts out important signals. The reticular system is also involved with sleep, which is studied via electroencephalograms. Higher cerebral functions are associated with the motor, sensory, and associative areas of the cerebral cortex. These regions direct sensory and motor input, higher mental activities, language, and memory. Voluntary bodily functions are under direct conscious control of the associative cortex. Involuntary homeostatic functions are not subject to conscious control. The spinal cord relays messages to and from the brain and functions in relfexes. Most neuromuscular control is regulated by feedback loops, the most simple being the muscle stretch receptors. Monosynaptic reflex arcs also provide feedback without involving the CNS.

Neurovisceral control is directed by either the parasympathetic or the sympathetic divisions of the autonomic nervous system. The neurotransmitter at the synapse between the CNS axon and the autonomic dendrite is acetylcholine in both the sympathetic and the parasympathetic systems. The second neurotransmitter in the parasympathetic system, between the autonomic axon and the target organ, is again acetylcholine, while the sympathetic system uses epinephrine or norepinephrine. The actions of the two neurotransmitters on a target organ are completely opposite. In general, the parasympathetic system stimulates activity of normal body functions while the sympathetic system prepares the body for greater, emergency activity.

77 LEARNING OUTCOMES

 Differentiate among afferent, efferent, and interneurons; CNS and PNS; somatic and autonomic motor neurons.  Describe the structure of a typical neuron and indicate the function(s) of each of its parts.  Explain what is meant by resting membrane potential and depolarization and understand how they are associated with transmission of a nerve impulse.

 Know the two kinds of gated membrane channels and how they are associated with ion movement through a cell membrane.  Understand how an action potential is transmitted along a nerve and why it spreads only in one direction.  Understand the importance of a chemical transmission of the nervous impulse across synapses.  Identify the chemical(s) and describe the events associated with transmission of a nerve impulse across a neuromuscular junction.  Know the primary divisions of the human brain and the function of each.  Understand the anatomical and associative organization of the human cerebral cortex.  Understand how the brain controls higher functions like learning and memory.  Describe the structure of the spinal cord and the peripheral nervous system.  Understand how a monosynaptic reflex arc is organized and functions.  Describe the two systems associated with the antagonistic control of the autonomic nervous system and identify the neurotransmitter(s) associated with each.

COMMON STUDENT MISCONCEPTIONS

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 44 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

 Students do not understand the evolution of endosymbionts in animal cells  Students are unsure that many of the lower animals are classified as animals  Students think that all animals evolved at about the same time  Students believe that most animals do not feel pain  Students believe that animals can sense emotions and danger  Students believe that only humans have a well-developed nervous system  Students believe that animals are purely instinctual  Students believe that most animals are vertebrates  Students do not equate humans with being animals  Students believe that all animals have identical organ system structures

INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE

78 Discuss the similarities and differences between a typical electrical system and the vertebrate nervous system. Which of the events typical with nervous conduction are lacking in electrical conduction?

Short-term memory is analogous to files stored in a computer’s RAM (random access memory), a volatile memory that requires a constant input of electricity. When the computer is shut off, these files are lost. Long-term memory is analogous to ROM (read only memory) present in special microchips or to files that have been written to or saved on a hard disk, floppy disk, zip disk, or CD; they still exist when the power is off. The few absolutely necessary bits of information (like time and date) are maintained in most computers by a tiny, internal battery on the mother board.

Discuss the affects of the following on the function of synapses: cocaine, anti-depressants known as SSRIs [selective seratonin re-uptake inhibitors], strychnine, and tetanus toxin. Explain how death occurs due to strychnine and tetanus toxin and explain the addiction of cocaine.

HIGHER LEVEL ASSESSMENT

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 44.

Application  Have students explain the effects of diseases that cause a loss of myelination.

 Have students explain the effects of blocking the movement of sodium ions on nervous system function.

 Ask students to explain the peripheral nervous system components needed to walk up the steps.

Analysis  Have students describe the effects of blocking acetylcholine on the autonomic nervous system.

 Have students assess the effects on the nervous system of too much calcium in the diet.

 Ask students to explain why complete damage to the brain still leaves the body with many intact functions.

Synthesis  Ask students explain why certain people are able to control autonomic nervous system responses.

 Have students find a medical application for a chemical that blocks sodium transport across the cell membrane.

79  Ask the students design an experiment to investigate the role of oligodendrites on central nervous system function.

Evaluation  Ask students evaluate the benefits and risks of using mood altering drugs that inhibit the uptake of certain brain neurotransmitters.

 Ask students to evaluate the accuracy science fiction books that envision a future in which human thoughts are controlled by drugs.

 Ask students to evaluate the effectiveness and safety of wasp control insecticides that block the action of acetylcholine.

VISUAL RESOURCES

Pass a message around the room using three different methods. Have students verbally pass instructions from one to another across a row to represent communication via gap junctions. Have another set of students walk a written message from one end of a row to another imitating hormonal communication. The third method requires buying or borrowing a pair of walkie talkies to simulate nervous communication. The speed at which the message passes should be obvious to your students. They may additionally find that the slowest is also the least accurate as the message may get garbled especially if it is complicated.

A copper wire wrapped with electrical tape resembles a myelinated nerve both in structure and function. A very simple mechanical associative activity is illustrated by a light that is activated only when it is dark outside (photosensitive) or when there is someone in the vicinity (heat, sound, or motion sensitive).

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Virtual Neurophysiology

Introduction

This demonstration uses a virtual electrophysiology to teach nervous system function in animal models.

Materials

 Computer with Media Player and Internet access  LCD hooked up to computer  Web browser linked to Biological Clocks Biointeractive website at http://www.hhmi.org/biointeractive/vlabs/neurophysiology/index2.html

Procedure & Inquiry

80 1. Explain to the class that they are going to observe a virtual neurophysiology demonstration using a model experimental system. 2. Load up the website and click on “Overview of equipment used in the lab”. 3. Then follow the sequence until completed with the virtual demonstration. 4. Then ask the class to the class to review what they saw and explain why the leech is used as an animal neurophysiology model.

USEFUL INTERNET RESOURCES

1. Images of animals are available from the University of California at Berkeley CalPhotos: Animal website. These images are valuable teaching resources for lecture and laboratory sessions related to animal anatomy and physiology. The site is available at http://calphotos.berkeley.edu/fauna/. 2. Faculty and students will find value in websites that simplify animal anatomy and physiology concepts. The information can be used for projects that educate children and civic groups about animals. Biology-4-kids is a model website for animal education. The website can be found at http://www.biology4kids.com/files/systems_nervous.html. 3. Case studies are an effective tool for stimulating interest in a lesson on fungi. The University of Buffalo has a case study called “It Takes A Lot of Nerve” which has students investigating the complexity of the nervous system in an entertaining way. The case study can be found at http://www.sciencecases.org/nervous_system/nervous_system.pdf. 4. Case studies are an effective tool for stimulating interest in a lesson on animals. The University of Buffalo has a case study called “Bad Fish” which has students investigating the physiology of the action potential. The case study can be found at http://www.sciencecases.org/badfish/badfish_genbio.pdf.

LABORATORY IDEAS

A. Brine Shrimp as a Nervous System Model

This activity has students design an experiment in which brine shrimp are used as a model of nervous system function.

a. Explain to students how animals and animal cells are used in medicine and research as models for human studies. b. Provide students with the following materials a. Large brine shrimp at room temperature b. Microscope c. Microscope slides d. Plastic pipettes e. Test reagents : i. 3% W/V sodium chloride solution ii. 3% W/V potassium chloride solution iii. 3% W/V calcium chloride solution

81 iv. 1-2 dilution organophosphate insecticide v. Black coffee c. Ask students to design an experiment to test the effects of the different solutions on the nervous system activity of the brine shrimp. Review the experiment before they progress with the activity. d. Have them record their findings and look up the potential effects of the treatments on the nervous system. e. Students should explain if their findings are consistent with the expected results.

LEARNING THROUGH SERVICE

Service learning is a strategy of teaching, learning and reflective assessment that merges the academic curriculum with meaningful community service. As a teaching methodology, it falls under the category of experiential education. It is a way students can carry out volunteer projects in the community for public agencies, nonprofit agencies, civic groups, charitable organizations, and governmental organizations. It encourages critical thinking and reinforces many of the concepts learned in a course.

1. Have students do a lesson do a hands-on program on the animal behavior. 2. Have students tutor high school students studying animal anatomy and physiology. 3. Have students volunteer on environmental restoration projects with a local conservation group. 4. Have students volunteer at the educational center of a zoo or marine park.