Anatomy & Functioning of Nervous System: Neurons, Glial Cells & Action Potentials, Study notes of Communication

Download Anatomy & Functioning of Nervous System: Neurons, Glial Cells & Action Potentials and more Study notes Communication in PDF only on Docsity! CHAPTER OUTLINE What part of your brain is involved in language? in memory? in emotions? Neuroscientists used to answer these questions by looking at specific types of brain damage and relating them to specific neurological problems. Now, highly sophisticated machines are peeking inside living human brains—and showing an astonishing level of detail about learning, emotions, and memory. Chief among these harmless techniques is functional magnetic resonance imaging, or fMRI. Regular MRI shows the location of soft tissue; fMRI tracks the movement of glucose through the brain. Because glucose is the basic fuel for the brain, fMRI shows which areas are active at any given moment. Results from just the past couple of years show how much fMRI can reveal about brain function: • Before surgery to correct epilepsy, fMRI can locate speech centers, which are often dam- aged by this surgery. By identifying where in the brain the patient forms words, surgeons can avoid damaging the ability to speak. • Brain images show differences between the brains of dyslexic children and normal readers. Images made after intensive lan- guage treatment show how the brain changes as the children gain language proficiency. • Men and women use their brains differ- ently, according to fMRI studies from the University of Alberta. “Sometimes males and females would perform the same tasks and show different brain activation, and sometimes they would perform dif- ferent tasks and show the same brain acti- vation,” said PhD student Emily Bell. • Scientists at the University of Wisconsin showed that brain regions associated with asthma can be activated when pa- tients hear the word “wheeze.” The study could lead to new drugs and/or a better ap- preciation of the brain’s role in asthma. 186 The Nervous System 7 ■ The Nervous System Makes Sense of Everything p. 000 ■ The Nervous System Is Categorized by Function and Structure p. 000 ■ Nerve Tissue Is Made of Neurons and Glial Cells p. 000 ■ Neurons Work through Action Potentials p. 000 ■ The Brain and Spinal Cord are Central to the Nervous System p. 000 ■ The Peripheral Nervous System Operates Beyond the Central Nervous System p. 000 FPO low res human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 186 The Nervous System Makes Sense of Everything 189188 CHAPTER 7 The Nervous System The Nervous System Makes Sense of Everything AXON Axon hillock Mitochondrion Cytoplasm DENDRITES Nucleus Rough endoplasmic reticulum Nucleus of Schwann cell Schwann cell: Node of Ranvier Axon terminal Synaptic end bulb Cytoplasm Myelin sheath Plasma membrane CELL BODY Axon collateral ift this book. Turn the page. Scan the words with your eyes and understand them with your brain. All of these con- scious movements are directed by the nervous system. Brush a bothersome hair off your face. Listen to tires crunch the pavement as a car drives past the open window. Smell the flowers outside. All of these sensations are brought to you compliments of the ner- vous system. Every conscious action that occurs in your body is governed by the nervous system. So are most of the “unconscious” or automatic actions that maintain homeostasis. When skeletal muscles contract, they do so in response to stimuli from the nervous system. We plan our movement in the brain, and the nervous system transmits that plan to the muscles. At the muscles, the nervous system stimulates contraction but stimulates only those motor units needed for that particular task. In Chapter 6 you learned about neuromuscular junc- tions. Review Figure 6.8 for a quick reminder of this structure. Although this type of nervous system activity is familiar, the nervous system has numerous other func- tions, some better understood than others. The ner- vous system is used to communicate from one end of the body to another. The nervous system receives and integrates stimuli, and formulates an appropriate re- sponse. The stimulus can be an external change, such as a shift in temperature or sound, or it can be an inter- nal change, such as a localized decrease in blood pres- sure or generally increased carbon dioxide levels in the tissues. Whatever the change, the nervous system’s job is to immediately detect it and adapt in order to main- tain homeostasis. Often that change will involve the endocrine system, which produces hormones that work in concert with the nervous system. The nervous system usually initiates immediate short-term responses, using neurons (Figure 7.1) and neuro- transmitters to produce amazingly fast results. In con- trast, the endocrine system re- lies on slower chemical inter- actions of hormones and target cells, which take longer to initiate a response than neural responses but tend to last longer. Your development from infancy to adulthood is driven by hormones, whereas your startled jump at the sound of a car’s backfire is caused by the nervous system. L Neuron Figure 7.1 The neuron is the functional unit of the nervous system. These remarkable cells are responsible for carrying sensory information into the brain, formulating a response, and sending that response out to the proper organs. Neuron A nerve cell that sends and receives electrical signals. Neurotransmitter A chemical used to transmit a nervous impulse from one cell to the next. List the functions of the nervous system. Describe the main difference between the endocrine system and the nervous system. LEARNING OBJECTIVES CONCEPT CHECK List four of the many different types of stimuli that the nervous system reacts to on a daily basis. Which works more quickly, the endocrine system or the nervous system? Why? human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 188 194 CHAPTER 7 The Nervous System Neurons Work Through Action Potentials 195 CONCEPT CHECK LEARNING OBJECTIVES Differentiate action potential from membrane potential. Describe the types of channels found in neuron membranes. List the events in an action potential. Neurons Work Through Action Potentials n action potential is a brief change in electrical conditions at a neuron’s mem- brane that occurs when a neural signal arrives; it is what happens when we say a neuron “fires.” What, at the molecular level, allows neu- rons to carry electrical impulses? How do these oddly + + + + + + + + + + + + + + + + + + + ++ ++ + + + + + ++ + + + + + + + + + + Distribution of ionsDistribution of charges Extracellular fluid Cytosol Phosphate ion Protein Potassium ion Chloride ion Sodium ion Equal numbers of + and – charges in most of ECF Equal numbers of + and – charges near plasma membrane Equal numbers of + and – charges in most of cytosol – – – – – – – – – – – – – – – – – – – – – – –– – –– – – – – – – – – – A B Membrane potential Figure 7.5 Unlike most body cells, neurons can significantly alter their membrane potential. The charge difference across the neurolemma alternates between
l70 mV and 30 mV during a typical nerve impulse. The cyclic change of charge across the A shaped cells receive, integrate, and respond to informa- tion? The answer begins with the electrical conditions surrounding the neuronal membrane, called the neu- rolemma. These electrical conditions create a mem- brane potential across the neurolemma that is ex- ploited when the nerve fires. (Figure 7.5). A resting neuron, as seen in Figure 7.6, has a membrane potential of
70 mV. The levels of positive sodium ions and negative chlo- ride ions are higher outside the Membrane potential The difference in electrical charge between two sides of a membrane. Cell body Dendrites Trigger zone Axon Axon terminal Cell body Dendrite Trigger zone Axon Axon terminal Cell body Trigger zone Axon Axon terminal Dendrites Motor neuron (multipolar)A Sensory neuron (bipolar)B Sensory neuron (unipolar) C Sensory neurons, motor neurons, and interneurons Figure 7.4 one axon, and at least one dendrite. The dendrite(s) bring information to the cell body. There can be many dendrites, with the branches providing many avenues for incoming impulses. The single axon routes the nerve impulse from the cell body to another neuron or an effector organ. The axon can have terminal branches, so each time the nerve fires, it can stimulate more than one cell. neuron than inside. Conversely, positive potassium ions are more concentrated inside the neuron than outside. Large, negatively charged proteins trapped in the neu- ron help to maintain the negative charge across the membrane. In the absence of a selectively permeable membrane, these differences would rapidly disappear as the ions each diffused down their respective concen- tration gradients. Sodium would diffuse into the cell, potassium would diffuse out, and the negative charges would balance. This diffusion does not happen, however, be- cause ions cannot simply diffuse through the lipid bilayer of the cell membrane. Instead, they must travel through channel proteins that serve as portals for ion diffusion. Channel proteins can be either passive or ac- tive. Passive channels are “leaky” and allow a constant trickle of ions. Active channel proteins allow no ion movement unless stimulated. This means the rate of ion movement across the nerve cell membrane de- pends on the physical state of the channel proteins, which can vary greatly from moment to moment. This variation in ion concentration across the cell mem- brane allows neurons to generate action potentials. neurolemma from
70mV to 30 mV and back to – 70 mV is termed the nerve impulse, or action potential. Charge differences are controlled by the movement of sodium and potassium ions entering and leaving the neuron. List three types of neuroglia and give their functions. What are the anatomical differences between sensory and motor neurons? Neuron and neuromuscular junction Figure 7.6 Resting neurons are visually no different from neurons undergoing an action potential. One way to determine the physiological state of a neuron is to measure the resting potential, and another is to look for the release of neurotransmitter. Resting neurons have a membrane potential near
70 mV, and are not actively releasing neurotransmitter. human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 194 A B Ligand-gated channel Voltage-gated channel Change in membrane potential Chemical stimulus opens the channel opens the channel Voltage-gated K+ channel closed Voltage-gated K+ channel open Voltage = –50 mVVoltage = –70 mV Acetylcholine Cation channel open Cation channel closed Na+ Ca2+ K+ K+ K+ Extracellular fluid Plasma membrane Cytosol Voltage-gated and ligand-gated channels Figure 7.7 Neurons Work Through Action Potentials 197196 CHAPTER 7 The Nervous System GATES AND CHANNELS CONTROL THE FLOW OF IONS Active channels are often called gated channels, because they allow ion transport only under specific environmen- tal conditions. Some gated channels are voltage-gated, opening and closing in response to transmembrane volt- age changes. Others are ligand-gated, or chemically reg- ulated, opening and closing when the proper chemical binds to them (Figure 7.7). Still others are mechani- cally regulated, responding to physical distortion of the membrane surface. At rest, the gated channels are closed. When open, these gates allow ions to cross the membrane in response to their concentration gradients, changing the transmembrane potential and generating a nerve impulse. The steps of an action potential are outlined in Figure 7.8. At the end of the action potential, the trans- membrane potential is
90mV. From the moment the sodium channels open until they reclose, the neuron cannot respond to another action potential. There are two phases to this inactive period. The absolute refrac- tory period lasts from 0.4 to 1.0 milliseconds. During this period, sodium and potassium channels are return- ing to their original states. The relative refractory pe- riod begins when the sodium channels are again in rest- ing condition, and continues until the transmembrane potential stabilizes at
70 mV. Thesodium potassium exchange pump (Na/K ATPase) helps stabilize the cell at the initial ion concentrations by moving three sodium ions out of the cell and two potassium ions into it. Scientists used to believe Na/K ATPase was needed for the neuron to carry another action poten- tial, but now it seems that it need not operate after every nerve impulse. An enormous number of sodium and potassium ions are on either side of the membrane, and the subtle concentration changes of one action po- tential do not block impulse transmission. It would take literally thousands of consecutive action potentials to alter the ion concentrations enough to destroy the over- all mechanism. The Na / K ATPase merely helps return the local membrane potentials quickly so a second ac- tion potential can be generated. Neuron action potential Figure 7.8 P ro ce ss D ia g ra m Resting state: All voltage-gated Na+ and K+ channels are closed. Depolarizing phase: Depolarization to threshold opens Na+ channel activation gates. Na+ inflow further depolarizes the membrane, opening more Na+ channel activation gates. Repolarizing phase: Na+ channel inactivation gates close and K+ channels open. Outflow of K+ causes repolarization. Repolarization continues: K+ outflow restores resting membrane potential. Na+ channel inactivation gates open. Return to resting state when K+ gates close. +30 0 –70 mV Time +30 0 –70 mV Time +30 0 –70 mV Time +30 0 –70 mV Time Na+ channel K+ channel Activation gate closed Inactivation gate open K+ K+ K+ Na+ Na+ Na+ Na+ K+ Extracellular fluid Plasma membrane Cytosol 4 1 2 3 www.wiley.com/ college/ireland human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 196 Oligodendrocyte in the brain Figure 7.11 physically touch one another; instead they are separated by a gap called a synapse. Neuro- transmitters released from the terminal bulb diffuse into the synapse, just as they do at the neuromuscular junction. They traverse this space, called the synaptic cleft, by simple diffu- sion. Neurotransmitters leave the presynaptic neuron and diffuse toward the postsynap- tic neuron, where they settle on receptors and initiate a reaction. Neurons Work Through Action Potentials 199 ACTION POTENTIALS WORK AT DIFFERENT SPEEDS Nerves can propagate action potentials at different speeds. Nerve impulses are sent along the axon in wave- like fashion. Impulses always begin at the swollen base of the axon, the axon hillock. These impulses travel along the membrane to the axon terminus, where they stimulate the release of neurotransmitters. Propagation speed can be influenced by the diameter of the axon (thin axons propagate faster) and by the amount of myelin on the axon (Figure 7.9). When the axon is wrapped in a myelin sheath, action poten- tials travel in a jumping pat- tern. The actual movement of sodium and potassium ions oc- curs only at the nodes, those stretches of naked axon visible between the cells that create the myelin sheath. This allows the action potential to travel much faster, jumping from one node to the next rather than moving steadily down the length of the axon. In the PNS, the neuroglial cells responsible for myelination are called Schwann cells (Figure 7.10). These cells wrap around the axon, providing a covering of phospholipids. Schwann cells also aid in regenera- tion of neural axons. If the axon is damaged, the Schwann cells remain in place, providing a tube through which the regenerating axon can grow. In this way, the axon terminus remains in association with the same muscular or glandular cells when it regenerates after being severed. Schwann cells are not present in the CNS, where myelin is provided by oligodendrocytes (Fig- ure 7.11). These are large cells with branching ap- pendages that touch and protect many axons. If an axon is damaged in the CNS, the oligodendrocyte re- treats, leaving no tube or pathway to aid in axonal re- growth. This is partially why damage to the neurons in the CNS is generally not repaired and why spinal-cord injuries are usually permanent. Although PNS neurons can recover from some damage, neurons in neither the PNS or CNS can regen- erate if the cell body is damaged. Axons will regenerate only if they are damaged beyond the axon hillock. As far as we know, new neurons do not form in adult CNS tissue with the exception of one small area of the brain called the hippocampus. Interestingly, depression seems to be linked to the inability to generate new neu- rons in this area. For the most part, however, when a CNS neuron is damaged beyond repair, it is lost. SYNAPSES SEPARATE ONE NEURON FROM ANOTHER Action potentials are carried along the neural mem- brane as a local change in voltage. Ions flow back and forth across the membrane as gated channels open and close, causing the alteration in voltage associated with the action potential. At the terminal bulb, however, the impulse must be transferred to the next neuron in line, and there is no membrane to carry it. Neurons do not 198 CHAPTER 7 The Nervous System Current flow due to opening of Na+ channels Nodes of Ranvier Na+ Na+ Cell body Trigger zone Myelinated propagation Na+ Na+ Na+ Na+ 1 msec 5 msec 10 msec Impulse conduction in a myelinated neuron Figure 7.9 Node of Ranvier Node of Ranvier Schwann cell Schwann cell Unmyelinated axons Myelin sheath Axon A B Schwann cell Figure 7.10 These cells individually wrap and protect the delicate and often extremely long axons of PNS neurons. They secrete compounds that aid in the regeneration of severed neuronal processes, as sometimes happens when we receive a deep wound. Myelin White lipids and phospholipids found wrapped around neural processes that aids in faster transmission. Terminal bulb The swollen terminal end of the axon that releases neurotransmitters into the synapse. Presynaptic neuron The neuron that lies before the synapse, whose axon leads to the synapse. Postsynaptic neuron The neuron that begins after passing the synapse, whose dendrites pick up diffusing neuro- transmitters. human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 198 204 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 205 VENTRICLES MAKE CEREBROSPINAL FLUID The brain may look like a solid mass of nervous tissue, but nothing could be further from the truth. Four rather large cavities in the brain are filled with CSF. These cavities (Figure 7.13) are literally holes in your head, but we call them ventricles. CSF is continuously produced and absorbed, creating a constant flow. If drainage back to the blood and the heart gets blocked, CSF builds up within the brain, adding a watery fluid under the skull that is rightly named hydrocephaly (“water head”). In infants whose skull bones have not yet fused, hydrocephaly forces the entire cranial cavity to expand at the fontanels. Once the skull has ossified, there are no fontanels, and hydrocephaly compresses the neurons of the cortex, effectively shutting down parts of the brain. This can be corrected by surgically implanting a shunt to drain the excess fluid. CSF formation helps maintain the blood-brain barrier, which permits only certain ions and nutrients to cross the vessels of the choroid plexus, resulting in a controlled environment for CNS neurons. Bacteria and viruses thus have difficulty entering the brain. Unfortu- nately, when bacteria do enter, they are difficult to treat, because the blood-brain barrier also keeps most antibiotics out. THE BRAIN HAS FOUR MAIN PARTS A first glance at the brain shows four major parts: the brain stem, the diencephalon and midbrain, the cere- bellum, and the cerebrum. Although the entire brain is basically involved in the integration of sensory input and motor responses, each section has slightly different roles. THE BRAIN STEM IS AN ANCIENT ROOT OF LIFE The brain stem contains vital centers that regulate heart rate, breathing, and blood pressure (Figure 7.14). It is the portion of the brain closest, anatomically and physiologically, to the spinal cord. The medulla oblongata and the pons make up the brain stem. The medulla oblongata contains the vital cen- ters of the brain stem associated with heart rate, respi- ratory function, and blood pressure. These centers, found in many animals, indicate that the medulla ob- longata evolved in ancient times. Here also are reflex centers for sneezing, coughing, hiccupping, and swal- lowing. Motor impulses generated in the higher centers of the brain travel through the medulla oblongata on their way to the PNS. You may have heard that the right side of the brain controls the left side of the body and vice versa. This is ba- sically true, because 80 per- cent of the motor information from the right side of the brain enters the medulla ob- longata and crosses to the left side before leaving the CNS. The crossing of these tracts is visible on the anterior surface of the medulla oblongata. The structures that can be seen crossing over one another are the pyramids (descending mo- tor tracts), and the technical POSTERIOR CHOROID PLEXUS OF THIRD VENTRICLE Cerebrum Cerebellum AQUEDUCT OF THE MIDBRAIN (CEREBRAL AQUEDUCT) CHOROID PLEXUS OF FOURTH VENTRICLE MEDIAN APERTURE ANTERIOR Superior cerebral vein ARACHNOID VILLUS SUBARACHNOID SPACE SUPERIOR SAGITTAL SINUS LATERAL VENTRICLE THIRD VENTRICLE Cranial meninges: Midbrain Pons FOURTH VENTRICLE Medulla oblongata Spinal cord CENTRAL CANAL SUBARACHNOID SPACE Path of: CSF Venous blood Filum terminale Sagittal section of brain and spinal cord Pia mater Arachnoid mater Dura mater Corpus callosum Sagittal plane View Each ventricle contains a choroid plexus, which forms CSF. CSF flows throughout the central nervous system, starting in the ventricles and flowing down toward the spinal cord. It flows down the central canal of the spinal cord, then up the outside of the cord and around the outside of the brain. CSF is absorbed into the bloodstream in the subarachnoid space. CSF formation and flow Figure 7.13 Medulla oblongata Portion of the brain stem immediately adjacent to the spinal cord, associ- ated with heart rate, breathing controls, and blood presure. Pons The area superior to the medulla oblongata, involved in transfer of information as well as respiratory reflexes human_ch07_186-227v2.qxd 18-01-2007 16:34 Page 204 Cerebellum A Fourth ventricle DECUSSATION OF PYRAMIDS Transverse section and anterior surface of medulla oblongata Vagus (X) nerve OLIVE Hypoglossal (XII) nerve PYRAMIDS Spinal nerve C1 Spinal cord Transverse plane Medulla View Third ventricle Pineal gland Superior colliculi Floor of fourth ventricle Thalamus Trochlear (IV) nerve Facial (VII) nerve Vestibulocochlear (VIII) nerve Glossopharyngeal (IX) nerve Vagus (X) nerve Accessory (XI) nerve Posterior view of midbrain in relation to brain stem Spinal nerve C1 Pons Cerebral peduncleInferior colliculi Tectum: B Brain stem (medulla oblongata and pons) Figure 7.14 term for crossing is decussing, therefore the entire phe- nomenon is referred to as the decussation of pyramids (Figure 7.15). The pons focuses on respiration. Most of the pons is composed of tracts that carry information up to the brain, down from the brain to the spinal cord, or laterally from the pons to the cerebellum. The only vi- tal center found in the pons is related to respiratory re- flex. The apneustic and pneumotaxic reflexes begin in the pons. The apneustic center triggers breathing even when we consciously hold the diaphragm still (despite the threats of countless children, you cannot hold your breath until you die). If you tried your hardest, you would eventually pass out, and the apneustic center would immediately restart your breathing. The pneumotaxic center works oppositely, be- cause it is charged with pre- venting overinflation of the lungs. When stretch receptors Fourth ventricle DECUSSATION OF PYRAMIDS Lateral corticospinal tract axons Anterior corticospinal tract axons PYRAMIDS Spinal nerve C1 Spinal cord in the lungs are stimulated, the pneumotaxic center sends a motor response causing you to exhale. THE CEREBELLUM FOCUSES ON MUSCLES AND MOVEMENT Posterior to the brain stem, we see something that looks like a smaller brain hanging off the back of the brain. This small, round structure is the cerebellum (Figure 7.16). It has two main functions: maintaining muscle 206 CHAPTER 7 The Nervous System Tracts Axons and/or dendrites with a common origin, destination, and function. Decussation of pyramids in the brain stem Figure 7.15 The cerebellum Figure 7.16 In this colorized scan, the cerebellum can be seen below the brain. human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 206 CEREBRUM Thalamus DIENCEPHALON: Hypothalamus Midbrain BRAIN STEM: CEREBELLUM Spinal cord POSTERIOR Sagittal section, medial view ANTERIOR Pons Medulla oblongata Pituitary gland Sagittal plane View A tone, posture, and balance; and fine-tuning conscious and unconscious movements directed by the cerebrum. Although we walk without thinking, the process re- quires exact coordination. That smooth gait, with its leg lifts and counterbalancing arm swings, is directed by the cerebellum. One job of the cerebellum is to understand where the limbs are located, using proprioception. This sensory skill allows you to lift your legs and move them forward without glancing at them, because your brain knows where your feet are at all times. The nervous pathways associated with proprioception run from the muscles and joints to the cerebellum. The cerebellum is also important in learning motor skills. Riding a bike, learning to swim, or even learning new information through repeatedly writing notes are all examples of cerebellar learning. New re- search indicates that the cerebellum may also play a The auditory reflex causes you to “jump” when you hear a car backfire. The visual reflex can also cause you to jump when you are focused on reading or studying and something flits by your peripheral vision. If you jump and rapidly turn your head to catch that fleeting vision, you’ve had a visual reflex. The thalamus and hypothalamus are also lo- cated in the diencephalon. The thalamus is a relay sta- tion for most incoming sensory information. Stimuli are sent from the thalamus to the appropriate portions of the cerebrum. The limbic system, which is responsi- ble for our emotions, communicates with the anterior portion of the thalamus. This communication forms a physical link between incoming sensory information and emotions. The hypothalamus is, as the name implies, be- low the thalamus. It secretes hormones that control the anterior pituitary gland, monitor water balance, and stimulate smooth muscle contraction. The hypothala- mus also regulates our circadian rhythm, body tempera- ture, heart rate, and blood pressure. THE CEREBRUM IS A CENTRAL PROCESSING CENTER The cerebrum is the largest portion of the brain (Fig- ure 7.18). It is here that information is processed and integrated, and appropriate responses are gener- ated. The cerebrum contacts all other parts of the brain, and is our center for higher thought processes. It is here that we learn, remember, and plan activities. Learning is the subject of many research studies, and we are only beginning to understand how the brain learns and remembers facts. (See “I wonder. . . What happens when we learn?”) CEREBRUM CEREBELLUM Sagittal section, medial view Spinal cord POSTERIOR ANTERIOR DIENCEPHALON: Thalamus BRAIN STEM: Midbrain Pons Medulla oblongata Hypothalamus B 208 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 209 The diencephalon Figure 7.17 role in sensory integration by receiving input from sen- sory neurons and directing it to inner portions of the cerebrum. Abnormal cerebellar anatomy has been de- tected in autistic children, suggesting a link between cerebellar function and autism. THE DIENCEPHALON IS A RELAY CENTER The diencephalon includes the central portion of the brain and functions mainly as a relay center for sensory information from the body and motor responses from the cerebrum (Figure 7.17). Within this portion of the brain, conscious and unconscious sensory informa- tion and motor commands are integrated. Centers for visual and auditory startle reflexes are located here. human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 208 The Brain and Spinal Cord are Central to the Nervous System 215 There are many other mental disorders that hu- mans suffer from. Table 7.4 gives some information on the most common of these ailments. THE SPINAL CORD CONNECTS TO ALMOST EVERYWHERE The spinal cord extends from the brain into the verte- bral column and is the second organ of the CNS (Fig- ure 7.20). The spinal cord is composed of white tracts surrounding gray matter, opposite the arrange- ment in the brain. This means that the exterior of the spinal cord is composed of communication tracts run- ning up and down the spinal cord, while the interior is composed of connections between spinal nerves. The spinal cord is the main route of communication be- tween the brain and the body. Sensory information en- ters the spinal cord via the dorsal root and is trans- ferred to an upward tract heading toward the brain. Motor impulses generated in the brain are passed through the downward tracts of the spinal cord to the nerves of the body. These tracts are often called pyramids. The pyramids are continuations of the tracts in the medulla oblongata that cross to carry informa- tion generated in one hemisphere over to the opposite side of the body. REFLEXES BYPASS THE BRAIN Sensory information that demands immediate atten- tion may initiate a reflex. Reflexes are extremely quick responses to sensory stimuli, running through the spinal cord from the dorsal root immediately to the ventral root and bypassing the brain entirely. Evolution honed this brilliant system to keep our vertebrate an- cestors safe from danger. Incoming sensory informa- tion is transferred to an association neuron in the in- nermost portion of the spinal cord and then directly to 214 CHAPTER 7 The Nervous System Class of disorder Common types Symptoms Treatment Anxiety disorders Phobias Extreme fear or dread Medications, cognitive and behavioral therapy Panic disorders Sudden intense feelings of terror Medications, cognitive and for no apparent reason behavioral therapy Obsessive compulsive disorder Anxiety coping strategies that include Medications, cognitive and repetitive actions or words, or ritualistic behavioral therapy behaviours Mood disorders Depression and bipolar Extreme sadness, sleeping or eating Psychotherapy and anti-depressants disorders pattern disturbances, changes in activity or energy levels, bipolar disorder includes violent mood swings Schizophrenia Schizophrenia Chemical imbalances in the brain that Prescription anti-psychotic lead to hallucinations, delusions, medications such as Haloperidol withdrawal, poor speech and reasoning (Haldol) and Loxitane patterns Dementias Alzheimers Loss of mental function, memory loss, Prescription drugs such as decline in physical abilities donepezil, anti-inflammatories, and anti-oxidant treatments. Also, increased nursing care Eating disorders Anorexia nervosa Preoccupation with food and unnatural Psychotherapy, lifestyle changes fear of becoming fat, self-starvation or over-exercising Bulimia Binging and purging cycles of huge Psychotherapy, life style change caloric intake Common mental disorders Table 7.4 Posterior (dorsal) root ganglion Spinal nerve Lateral white column Anterior (ventral) root of spinal nerve Central canal Anterior gray horn Anterior white column Cell body of motor neuron Anterior median fissure Axon of motor neuron Posterior (dorsal) root of spinal nerve Posterior gray horn Posterior median sulcus Posterior white column Axon of sensory neuron Lateral gray horn Cell body of sensory neuron Nerve impulses to effector tissues (muscles and glands) Nerve impulses for sensations Gray commissure Transverse section of thoracic spinal cordA Posterior median sulcus Posterior white column Posterior gray horn Lateral white column Gray commissure Anterior gray horn Anterior white column Anterior median fissure LM 5x Central canal View Transverse plane B Transverse section of thoracic spinal cord Spinal cord Figure 7.20 human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 214 216 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 217 Eth ics an d Issu es Attention Deficit Hyperactivity Disorder (ADHD, also called Attention Deficit Disorder, ADD) is one of the most common mental disorders among children. Characteristically, ADHD causes difficulties in concentration, taking directions, sitting still, and cooperating, all of which can lead to learning and social difficulties. In terms of brain physiology, it is not clear what causes ADHD. Unlike Parkinson’s or Alzheimer’s disease, for exam- ple, nobody has made brain scan images showing that ADHD damages the brain. Some think ADHD may even be related to sleep deprivation. Some researchers have found abnormal levels of sleep apnea (the periodic cessation of breathing during sleep: a
 without, pnea  breath) among ADHD children. This breathing problem causes re- peated awakenings at night, interfering with deep sleep. If this observation is correct, stimulants could merely be masking a condition of sleepiness that might better be treated more specifically. Whatever the cause, the diagnosis of ADHD is growing more common. Widely varying statistics show that it affects 1 to 6 percent of American youths. ADHD is also being diag- nosed among adults, with an estimated 1 percent of Ameri- cans aged 20 to 64 taking stimulants for the condition. Among adults, ADHD is less likely to cause hyperactivity than restlessness, difficulty paying attention, impulsive be- havior, and frustration with failing to reach goals. What can be done to treat ADHD? One approach is be- havioral; parents try to shape behavior by rewarding desir- able activity and imposing consequences for actions they want to discourage. The behavioral approach can be com- bined with, or replaced by, treatment with stimulant drugs, Attention Deficit Hyperactivity Disorder: Does Drug Treatment Make Sense? especially forms of amphetamine. Curiously, although am- phetamines stimulate most people, they calm people with ADHD. That unexpected effect is actually a hallmark of the disease. Still, the widespread use of prescription medication for ADHD is making some people nervous, especially those who suspect that an ADHD diagnosis is mainly a tactic to make business for psychiatrists and the pharmaceutical industry. These are reasons for concern: 1. Among 12- to 17-year-olds, abuse of prescription drugs is rising faster than abuse of illegal drugs, and ampheta- mines are addictive in some people. 2. Some college students with ADHD prescriptions say the amphetamines give them extra focus and energy during tests. 3. Stimulants have been linked to the death of 19 children and 6 adults (among an estimated 4 million people taking stimulants for ADHD) due to heart problems that may be related to the stimulants. The U.S. Food and Drug Admin- istration is considering stronger warning labels on the packages. Although some unexplained deaths are in- evitable among any group of 4 million people, the news should prompt doctors to evaluate heart health before prescribing stimulants for ADHD. 4. Shouldn’t we just “let boys be boys?” According to this logic, boys typically have more of the “ADHD personality characteristics,” like impulsivity, excess energy, and diffi- culty with planning. Should being male be considered a mental illness, especially in a society plagued by drug abuse? Like other challenges of parenting, ADHD forces par- ents to persist, improvise, and decide. Behavioral therapy can be wearing, and it may require assistance from teachers and others who are important to the child. Stimulant drugs can send a message that psychological problems can be fixed with a pill. But if the consequences of failing to treat ADHD are negative enough, parents must choose a treat- ment strategy and philosophy, and carry it through. Although scientists are improving their understanding of brain function, much remains to be understood, including the integration of different portions of the brain, and the function of various nuclei and neurotransmitters. As neuro- scientists probe deeper into the brain’s structure and func- tion, we may learn to treat or even prevent some of the se- vere mental disorders that afflict our fellow humans. a motor neuron. The motor neuron transmits an im- mediate response through the ventral root to the effec- tor organ. Reflexes generate an immediate, lifesaving mo- tor response. You pull your hand from an open flame even before you consciously recognize the heat. As you pull your hand away, the “that’s hot!” information is still traveling to your brain. There, a series of motor re- sponses begins, causing you to rub your hand, inspect it for burns, and exclaim in surprise or pain. Fortunately, before all these brain-initiated motor responses can oc- cur, the reflex has already removed your hand from danger. (See Figure 7.21.) 3 4 5 12 SENSORY NEURON (axon conducts impulses from receptor to integrating center) SENSORY RECEPTOR (responds to a stimulus by producing a generator or receptor potential) INTEGRATING CENTER (one or more regions within the CNS that relay impulses from sensory to motor neurons) MOTOR NEURON (axon conducts impulses from integrating center to effector) EFFECTOR (muscle or gland that responds to motor nerve impulses) Interneuron Reflex arc Figure 7.21 CONCEPT CHECK List the meninges in order, beginning with the one closest to the axial skeleton. Briefly define the key structures found in the diencephalon. What does the limbic system control? Where is gray matter located in the brain? in the spinal cord? Name the steps in a simple reflex arc. www.wiley.com/ college/ireland human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 216 ANTERIOR Cerebrum CRANIAL NERVES: Olfactory (I) nerve fibers Optic (II) nerve Oculomotor (III) nerve Trochlear (IV) nerve Trigeminal (V) nerve Abducens (VI) nerve Facial (VII) nerve Vestibulocochlear (VIII) nerve Glossopharyngeal (IX) nerve Vagus (X) nerve Accessory (XI) nerve Hypoglossal (XII) nerve Olfactory bulb Olfactory tract Optic tract PONS MEDULLA OBLONGATA Spinal cord Spinal nerve C1 POSTERIOR Cerebellum View Inferior aspect of brain The Peripheral Nervous System Operates Beyond the Central Nervous System he peripheral nervous system (PNS) is composed of all neural tissue other than the brain and spinal cord. The PNS in- cludes the nerves that protrude from these structures. The 12 nerves that extend from the brain are called the cranial nerves (Table 7.5). These nerves are identified by name and a Roman nu- meral number (Figure 7.22). Some are sensory 218 CHAPTER 7 The Nervous System The Peripheral Nervous System Operates Beyond the Central Nervous System 219 LEARNING OBJECTIVES T Brain with cranial nerves identified Figure 7.22 Cranial nerves Table 7.5 Olfactory nerve Olfactory tract Olfactory bulb Optic nerve Optic tract Oculomotor nerve Trochlear nerve Trigeminal nerve Abducens nerve Facial nerve Vestibulocochlear nerve Glossopharyngeal nerve Vagus nerve Accessory nerve Hypoglossal nerve Describe the difference between spinal and cranial nerves. Compare the sympathetic and parasympathetic aspects of the PNS. Compare Cranial and Spinal Nerves Olfactory (I) nerve Optic (II) nerve Oculomotor (III) nerve Trochlear (IV) nerve Trigeminal (V) nerve Abducens (VI) nerve Facial (VII) nerve Vestibulocochlear (VIII) nerve Glossopharyngeal (IX) nerve Vagus (X) nerve Accessory (XI) nerve Hypoglossal (XII) nerve human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 218 2The Nervous System Is Categorized by Function and Structure The nervous system is divided into the central and peripheral nervous systems. The CNS includes the brain and spinal cord and is the main integration center of the body. The PNS includes the autonomic, sen- sory, and somatic nerves of the body. The autonomic division is further subdivided into the sympathetic and parasympathetic divisions. A nerve is composed of a bundle of neurons, protected by layers of connec- tive tissue. Sensory information enters the CNS, which analyzes it and sends a motor response through the PNS to muscular or glandular tissue. 1. Compare the structure of a nerve to the structure of a muscle. What explains the anatomical similarities? What are the main differences? 2. Review the steps in an action potential, as well as the definition of IPSP and EPSP. Using what you know, describe a neuron that is exhibiting an IPSP. How would the ion concentrations across the membrane be different from an EPSP? Can you predict what ion conditions would cause an EPSP? 3. Why are reflexes faster than conscious thought? Why is the response slower when the brain is involved? Why do we even have reflexes? CRITICAL THINKING QUESTIONS 5The Brain and Spinal Cord Are Central to the Nervous System The spinal cord carries impulses to and from the brain. The CNS organs are nourished and protected from physical damage by CSF and meninges. The lobes and internal structures of the brain each have distinct, but overlapping, functions. The brain stem contains vital centers that regulate heart rate, breathing, and blood pressure. The cerebellum focuses on mus- 6The Peripheral Nervous System Operates Beyond the Central Nervous System The peripheral nervous system includes the nerves that protrude from the brain and spinal cord. The PNS originates with 12 cranial nerves and 31 pairs of spinal nerves. Periph- eral nerves may be sensory, motor, or mixed. The autonomic nerves are not under con- scious control. Sympathetic autonomic nerves control visceral organs in the thoracic and lumbar region of the spinal cord. Parasympathetic autonomic nerve fibers emerge from the cranial, cervical, and sacral region of the spinal cord. ■ afferent p. 000 ■ autonomic division p. 000 ■ cerebrospinal fluid (CSF) p. 000 ■ cortex p. 000 ■ efferent p. 000 ■ gyri p. 000 ■ hemispheric lateralization p. 000 ■ medulla oblongata p. 000 ■ membrane potential p. 000 ■ myelin p. 000 ■ neuroglia p. 000 ■ neuron p. 000 ■ neurotransmitter p. 000 ■ nuclei p. 000 ■ pons p. 000 ■ postsynaptic neuron p. 000 ■ presynaptic neuron p. 000 ■ proprioception p. 000 ■ somatic division p. 000 ■ special senses p. 000 ■ sulci p. 000 ■ terminal bulb p. 000 ■ tracts p. 000 224 CHAPTER 7 The Nervous System Critical Thinking Questions 225 4Neurons Work Through Action Potentials An action potential is a brief change in electrical conditions at a neuron’s mem- brane that occurs when a neuron “fires.” An action potential occurs when the charge dif- ferential across the neuron’s membrane suddenly reverses polarity, as a result of changing ion concentrations inside and out- side the neuron. Impulse speed is deter- mined by axon diameter, degree of myelina- tion, and other factors. Neurotransmitters carry signals from one neuron to the next across a tiny gap called the synapse. IPSPs and EPSPs also influence the generation of action potentials. KEY TERMS cles and movement. The diencephalon is a relay center between other parts of the brain, whereas the cerebrum is a central processing center, home of logic and skills. The reticular activating system is the brain’s alarm clock. Reflexes are two- or three-neuron circuits that bypass the brain to allow fast retreat from injury. CHAPTER SUMMARY 3Nerve Tissue Is Made of Neurons and Glial Cells The nervous system contains neurons and neuroglial cells. Neurons carry im- pulses, whereas glial cells carry out sup- porting functions. Sensory neurons detect conditions in the environment or body, mo- tor neurons carry instructions to the body, and interneurons connect the two systems. Dendrites bring signals to the cell body, and the long axons deliver signals to other neu- rons or tissue. human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 224 Self Test 227 1. The functional unit of the nervous system is a. the brain. b. the brain and spinal cord. c. the neuron. d. the neuroglia. 2. Information reaches the CNS from the a. afferent division of the PNS. b. efferent division of the PNS. c. motor neurons. d. sympathetic division. 3. True or False: The division of the autonomic nervous system that is responsible for digestion, energy storage, and relaxation is the parasympathetic division of the PNS. 4. Identify the type of neuroglion shown. a. Astrocyte b. Motor neuron c. Microglion d. Oligodendrocyte The next few questions refer to the image below SELF TEST 226 CHAPTER 7 The Nervous System +30 0 –70 mV Time +30 0 –70 mV Time +30 0 –70 mV Time +30 0 –70 mV Time Na+ channel K+ channel Activation gate closed Inactivation gate open K+ K+ K+ Na+ Na+ Na+ Na+ K+ Extracellular fluid Plasma membrane Cytosol C1 Spinal cord C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 L1 L2 L3 L4 L5 S1 S2 S3 S4 S5 7. The original membrane potential of a resting neuron is a.
70 mV. c. 0 mV. b. 90 mV. d. dependent on neuron location. 8. The first ion to enter the neuron at the beginning of an action potential is a. calcium. c. sodium. b. potassium. d. ATP. 9. The period of time immediately after an action potential, during which the neuron cannot send a second action potential is the a. relative refractory period. b. absolute refractory period. c. dead zone. d. sodium/potassium ATPase period. 10. The function of the cell shown in the diagram below is to a. myelinate PNS neurons. b. myelinate CNS neurons. c. increase action potential propagation speed. d. decrease action potential propagation speed. e. Both a and c are correct. 11. What cell provides this same function in the brain? a. Schwann cell b. Astrocyte c. Oligodendrocyte d. Microglial cell 12. True or False: An EPSP causes a slight hyperpolarization of the neuron cell membrane, making it more difficult to initiate an ac- tion potential. 13. Identify the specific layer of the meninges indicated by the letter A on this figure. a. Dura mater b. Pia mater c. Arachnoid 14. The ventricles in your brain are the site of a. sensory input. b. CSF formation. c. memory formation. d. CSF absorption. 15. Identify the portion of the brain indicated in this figure. a. Brainstem b. Cerebrum c. Cerebellum d. Diencephalon 16. The functions of this structure include a. sensory interpretation. b. proprioception. c. learning. d. heart rate control. 6. The type of membrane protein that allows ions to enter the cell only during a shift in membrane voltage is a a. mechanically regulated channel. b. ligand-gated channel. c. voltage-gated channel. d. leaky gated channel. 5. The neuron pictured here is responsible for a. sending and receiving sensory information. b. sending and receiving motor information. c. integrating information from sensory and motor neurons. d. Neuron function cannot be determined from neuron anatomy. 17. The portion of the brain that is responsible for emotions is the a. hypothalamus. b. thalamus. c. reticular formation. d. limbic system. 18. The surface of the spinal cord is white, indicating that it func- tions as a. a highway for information traveling up and down the cord. b. an integration center, where impulses are connected to one another and then passed to the brain. c. an insulation layer surrounding the functioning neurons un- derneath. d. In nervous tissue, color does not indicate function. 19. The correct sequence of structures in a reflex is a. sensory receptor S sensory neuron S spinal cord S brain S spinal cord S motor neuron S effector organ. b. sensory receptor S spinal cord S brain S motor neuron S effector organ. c. sensory receptor S motor neuron S spinal cord S sensory neuron S effector organ. d. sensory receptor S sensory neuron S spinal cord S motor neuron S effector organ. e. sensory receptor S effector organ S brain S motor neuron S spinal cord. 20. Which of the two divisions of the autonomic division of the PNS has the longer postsynaptic neurons? a. Sympathetic division b. Parasympathetic division 21. What is the function of the autonomic division of the PNS shown in this figure? a. Increased digestive activity b. Increased respiratory and heart rate c. Increased urinary output d. Decreased mental alertness human_ch07_186-227v2.qxd 18-01-2007 16:35 Page 226