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CHAPTER OUTLINE
W
hat 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.
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
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188 CHAPTER 7 The Nervous System The Nervous System Makes Sense of Everything 189
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 ner vous 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 ner vous system. The ner vous 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 ner vous system.
L
Neuron Figure 7. 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.
L EARNING O BJECTIVES
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?
192 CHAPTER 7 The Nervous System Nerve Tissue is Made of Neurons and Glial Cells 193
THE AUTONOMIC NERVOUS SYSTEM WORKS WHILE YOU SLEEP
The autonomic division of the PNS is a control system that governs your body’s re- sponses to subtle changes in homeostasis with involuntary, unconscious reactions. For ex- ample, the CNS continually generates responses to sensory input concerning blood pres- sure, blood gases, and visceral functioning. You are not aware of these inputs, nor do you control the motor responses that travel through the autonomic nervous system. The autonomic nervous system has two subdivi- sions. The first division, the sympathetic division, in- cludes those nerves that control the body when it is ac- tively moving, burning energy. The sympathetic division is sometimes called the “fight or flight” division, be- cause it is triggered when we feel threatened and must choose to remove ourselves from the danger (flight) or stay and “fight.” The parasympathetic division is respon- sible for digestion, energy storage, and relaxation (Table 7.1). These divisions are nicely separated by the con- tradictory demands of human life. Sometimes we must conser ve energy and rest; other times we must move quickly and expend energy. When competing in an ath- letic event, or running from an alligator, we must use
Outline of the nervous system, including comparison of the sympathetic and parasympathetic characteristics Table 7.
CONCEPT CHECK
List the differences between the CNS and the PNS.
What are the functions of the somatic division of the PNS?
How can you differentiate between the sympathetic and the parasympathetic divisions of the autonomic nervous system?
Somatic and special sensory receptors and neurons
Autonomic sensory receptors and neurons
Sympathetic (emergency situations)
Parasympathetic (energy storage) Sensory receptors and neurons in GI tract and enteric plexuses
Somatic motor neurons (voluntary)
Autonomic motor neurons (involuntary): sympathetic and parasympathetic divisions
Enteric motor neurons (involuntary) in enteric plexuses
Skeletal muscle
Smooth muscle, cardiac muscle and glands
Smooth muscle, glands, and endocrine cells of GI tract
SNS
ANS
ENS
Sensory part of PNS Motor part of PNS Effectors
CNS: brain and spinal cord
Autonomic
division Division
of the nervous system regulating functions such as blood vessel diameter and stomach activity.
energy. Conversely, after a feast (alligator steak, any- one?), we must digest the meal and store the energy. Almost ever y organ of your body has dual in- nervation, meaning that it is stimulated and controlled by both the sympathetic and the parasympathetic divi- sions. The two systems work antagonistically to main- tain homeostasis, with only one system stimulating the organ at any given time. Determining which system is in control is easy, based on the organ’s activity. If the or- gan is burning energy, releasing oxygen or glucose into the bloodstream, or otherwise aiding in sharp mental capacity and quick responses, the sympathetic division is working. If the organ’s function is conducive to rest and relaxation, you can bet that the parasympathetic division is in control. The functions of these two divisions are easy to remember. The sympathetic division is sympathetic to your plight. It is active when you need quick energy and rapid movement. The parasympathetic division starts with “P,” like potato. When this system is active you are relaxing—acting like a “couch potato.”
Nerve Tissue is Made of Neurons
and Glial Cells
ervous tissue, one of the four main tissue types, is composed of neurons and sup- porting cells called neuroglia (singular neuroglion ). The types and functions of
L EARNING O BJECTIVES List the functional categories of neurons. Describe the function of the supporting neuroglia.
N
the neuroglia are listed in Table 7.2. The three classes of neurons are based on func- tion: sensory neurons, motor neurons , and interneurons (Figure 7.4). Each type has a distinctive shape, allowing it to be readily identified. Despite their anatomical differences, all neurons have a cell body,
Neuroglia
Supporting and protecting cells found within the nervous system, including cells that provide nutrients, remove debris, and speed impulse transmission.
Neuroglia name, size and shape, location (CNS or PNS), and function Table 7.
www.wiley.com/college/ireland
Name Location Function Satellite cells PNS Regulate oxygen, carbon dioxide, nutrient and neurotransmitter levels around ganglia
Schwann cells PNS Surround axons in PNS, causing myelination of axons and faster impulse transmission, aid in repair after injury
Oligodendrocytes CNS Myelinate CNS neurons, provide structural support
Astrocytes CNS Maintain blood brain barrier, regulate nutrient, ion and dissolved gas concentrations, absorb and recycle neurotransmitters, form scar tissue after injury
Microglia CNS Clean up cellular debris and pathogens via phagocytosis
Ependymal cells CNS Line ventricles and central canal of cord, assist in CSF production
194 CHAPTER 7 The Nervous System Neurons Work Through Action Potentials 195
CONCEPT CHECK
L EARNING O BJECTIVES 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
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+ +
+
+
+
+ +^ +
**+ +
+
+
**+ +
+** (^) + +
**+ +
+**
**+
+
+
Distribution of charges Distribution of ions
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
- **- –
-** (^) – - (^) – - (^) **–
-** (^) **–
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A B
Membrane potential Figure 7. 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 ner ve 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
A Motor neuron (multipolar) B Sensory neuron (bipolar) C Sensory neuron (unipolar)
Sensory neurons, motor neurons, and interneurons Figure 7.
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 ner ve 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. 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.
Oligodendrocyte in the brain Figure 7.
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
Ner ves 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.
Node of Ranvier Node of Ranvier Schwann cell
Schwann cell
Unmyelinated axons
Myelin sheath
Axon
A B
Schwann cell Figure 7. 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.
Neurons Work Through Action Potentials 201
Class Name Location Effects Acetylcholine Acetylcholine Throughout CNS and PNS, neuromuscular Contracts muscle, causes glandular secretions, junctions, parasympathetic division general parasympathetic functions Biogenic amine Norepinephrine Hypothalamus, brain stem, cerebellum, Attention, consciousness, control of body spinal cord, cerebral cortex, and most temperature sympathetic division junctions Biogenic amine Epinephrine Thalamus, hypothalamus, midbrain, Uncertain spinal cord Biogenic amine Dopamine Hypothalamus, midbrain, limbic system, Regulates subconscious motor functions cerebral cortex, retina Biogenic amine Seratonin Hypothalamus, limbic system, cerebellum, Maintains emotional states, moods and body spinal cord temperature Biogenic amine Histamine Hypothalamus Sexual arousal, pain threshold, thirst and blood pressure control Amino acid Glutamate Cerebral cortex and brain stem Excitatory, aids in memory and learning Amino acid GABA Cerebral cortex Inhibitory Neuropeptide Substance P Spinal cord, hypothalamus, digestive tract Pain senses, controls digestive functions Neuropeptide Neuropeptide Y Hypothalamus Stimulates appetite and food intake Opiods Endorphins and Thalamus, hypothalamus, brain stem Pain control, behavioral effects enkephalons
200 CHAPTER 7 The Nervous System
NEUROTRANSMITTERS CARRY THE MESSAGE ACROSS THE SYNAPSE
Neurotransmitters are specific chemicals that carry an impulse across a synaptic cleft. We currently have iden- tified and studied more than 45 neurotransmitters, each with a slightly different effect on the postsynaptic neuron (Table 7.3). The most common neurotrans- mitters are acetylcholine (ACh) and norepinephrine (NE). As described in Chapter 6, ACh stimulates mus- cle contractions when picked up by receptors on the muscle cell membrane. Once released, it is broken down quite rapidly by the enzyme acetylcholinesterase. ACh is present on the muscle cell and in the synapse for approximately 20 milliseconds. Norepinephrine (NE) is responsible for the adrenaline rush we experience during tense situations. NE, unlike ACh, is mostly reabsorbed by the presynap- tic neuron instead of being broken down. Reabsorption takes longer, so NE can remain effective for 1 to 2 sec- onds at a time.
GRADED RESPONSES CREATE FINE NEURAL CONTROL
Action potentials are “all or nothing” events, meaning that once the threshold is reached, the nerve will fire completely. Because a single neuron cannot create a partial action potential, we vary the strength of nervous stimulation by changing the number of neurons that are firing. Graded responses can be obtained by hyperpo- larizing or depolarizing individual neural membranes. The hyperpolarized neuron requires a larger stimulus to reach threshold and begin an action potential. The depolarized neuron is the opposite: It requires less of a “kick” to begin an action potential, because its resting potential is closer to the action potential threshold. But once threshold is reached, a neuron generates an ac- tion potential that is indistinguishable from any other action potential. These hyperpolarized and depolarized neurons result from alterations in the resting membrane poten- tial of postsynaptic neurons. Two types of postsynaptic
Neurotransmitters Table 7.
Health, Wellness, and Disease
Psychoactive Drugs: Getting a Good Reception at the Synapse
C ompounds that affect the nervous system are called
psychoactive drugs. Psychoactive drugs, whether legal like alcohol or illegal like marijuana, share one key fea- ture: They affect the synapses where neurons communi- cate with one another. Psychoactive drugs most often act through one of two essential mechanisms: They may block or activate neural receptors in the synapses, or they may change the concentration of neurotransmitters that naturally occur in the synapses. In either case, the result is a change in the nature and/or intensity of the message that is passed across the synapse. Psychoactive drugs can subtly change these messages, prevent them entirely, or create false mes- sages where none was intended. Many psychoactive drugs affect the neurotransmitter dopamine, which is involved in movement and emotion. Normally, after a neurotransmitter is released in the synapse, it affects a change in the postsynaptic cell and is quickly taken up (removed) from the synapse. Cocaine and amphetamine both inhibit the removal, or “reuptake,” of dopamine, so dopamine remains in the synapse, con-
tinuing to activate the receptors for an abnormal period. The excess stimulation of dopamine receptors explains the “high” of cocaine and amphetamine. Neurotransmitter reuptake is also impaired by the “selective serotonin reuptake inhibitors,” the SSRI drugs, including Zoloft and Prozac. Many patients with anxiety and depression apparently lack an adequate supply of the neurotransmitter serotonin. SSRI drugs change condi- tions at the synapse by inhibiting serotonin reuptake, which has the effect of stretching the supply of serotonin and making it more likely that a serotonin signal will tran- sit the synapse. Surprisingly, the nervous system contains many re- ceptors specifically tuned to associate with compounds found in opiate drugs. The well-studied “mu opiate recep- tors” can occur on excitatory or inhibitory neurons. Exci- tatory neurons increase the activity of other neurons; in- hibitory neurons slow or stop nerve signals. Opiate drugs like heroin or morphine can activate these receptors. The body produces related compounds called “endogenous opioids” that also activate these receptors. Endogenous opioids may account for the feeling of well-being that follows physical exercise and could explain the ability to ignore pain during an emergency. The nervous system also contains a great num- ber of receptors for chemicals found in marijuana. These receptors exist in high concentration “around the hippocampus, cortex, olfactory areas, basal ganglia, cerebellum, and spinal cord,” accord- ing to Roger Pertwee, a cannabis expert at the Uni- versity of Aberdeen (United Kingdom). “This pat- tern accounts for the effects of cannabinoids on memory, emotion, cognition, and movement.” Cannabinoid receptors are also found in the male and female reproductive systems, and on neurons that cause nausea and vomiting. The prevalence of receptors for psychoactive drugs helps explain the power of these drugs, and also demonstrates how evolution finds new uses for old molecules. And it might even help treat dis- ease: The discovery of cannabinoid receptors in a part of the eye that controls fluid pressure could eventually lead to drugs to control glaucoma, a blinding disease caused excess pressure inside the eye.
Psychoactive drugs affect behavior and judgment, as they alter the functioning of cerebral synapses. Many of these compounds are ille- gally sold on the black market. Here a policeman guards confiscated drugs obtained during a successful “drugs bust.”
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 water y 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- rator y 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.
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
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 C 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 C
Pons
Inferior colliculi Cerebral peduncle
Tectum:
B
Brain stem (medulla oblongata and pons) Figure 7.
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 C 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.
The cerebellum Figure 7. In this colorized scan, the cerebellum can be seen below the brain.
210 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 211
The surface of the cerebrum has creases or sulci that separate individual raised portions called gyri. The surface of the cerebrum is composed of gray matter , whereas the interior is white. Gray matter is mainly cell bodies and nonmyelinated neural processes—in other words, naked axons and dendrites. In the gray matter, connections are made as axons meet den- drites. The cerebral cortex is entirely gray matter, folded to provide a larger surface area for these neural connections. It contains literally billions of cell bodies responsible for sen- sations, voluntary movements, and thought. The white matter in- side the cerebrum contains myelinated axons that carr y information to the spinal cord
or other areas of the brain. Myelinated axons are cov- ered in lipids, giving this tissue its characteristic white appearance and allowing for faster impulse transmis- sion. Information is passed from one area of the brain to another via tracts of white matter.
THE CEREBRAL HEMISPHERES ARE HOMES OF LOGIC AND ARTISTRY
The cerebrum has two distinct hemispheres that are quite similar anatomically. Both hemispheres are di- vided into lobes with general functions assigned to each. For example, the occipital lobe is where vision is interpreted, and the frontal lobe is involved in con- scious thought processes. The cortex of each lobe has motor areas, sensory areas, and association areas that integrate new information with stored memories. The primary motor area, in the frontal lobe, just in front of the central sulcus, formulates voluntar y motor com- mands. Each portion of the body is represented in the
I WONDER...
What Happens When We Learn?
U nderstanding learning is one of the toughest challenges
in neuroscience. Brains are sometimes compared to com- puters, but whereas it’s easy to point to where a hard drive stores certain information, that is seldom possible in the brain. The brain stores information here and there, in com- plex, threadlike networks of neurons. Our learned ability to speak, for example, is stored separately from our memory of last year’s birthday party. And both are stored separately from our ability to paddle a canoe or whistle a song. Learning is a type of memory, and memory occurs in three phases. Immediate memory prevents us from being bewildered by maintaining information in our conscious- ness so that we know, for example, where we are. Short- term memory helps us carry out tasks—keeping a conver- sation going, say, or remembering why we are writing a letter. Although much of our short-term memory is quickly erased, some of it gets adopted in long-term memory. This memory can survive for life, or it can fade, but it is what many people mean when they say “memory.” Scientists be- lieve these three types of memory may exist in different parts of the brain. Several types of change occur when the brain remembers something, but we call them all “neural plasticity,” meaning changes in the brain that alter its ability to do something. The neural plasticity associated with learning has several components. For example, during learning, spe- cific proteins are synthesized in the brain (we know this is true because when we block protein synthesis, we block learning). Synapses change in neural pathways so that impulses can travel through them faster and more easily, a change we call potentiation. When we learn to ride a bike, for example, the neural pathways that tell us to steer to avoid falling are potentiated. The next time we ride, these reactions happen faster, and take less conscious effort , until they eventually are triggered automatically whenever we ride a bike. Neural plasticity also changes the dendrites, the neural processes that bring impulses to the cell body. Recent studies teaching skills to rats and looking specifically at the rat hippocampus show that certain ion channels in the membrane at the dendrites become more numerous after just 10 minutes of training. Learning does not exist in a vacuum; the brain’s ability to learn is related to what else is going on. Lab studies show that fight-or-flight
conditions drastically reduce the ability to learn. People with post-traumatic stress disorder have difficulty learning, probably because of high levels of stress hormones. Emo- tional stress may even cause amnesia, which can destroy our memory of who we are, without harming the skill of ty- ing a shoe. Memory and learning play a critical role at both ends of life. Learning to swim, play guitar, or distinguish the pe- ripheral from the central nervous system are all things we may learn while young. In our final years, diseases like Alzheimer’s can undo the learning of a lifetime, leaving us bewildered and frustrated over simple tasks we used to ac- complish with ease. One final point in our “scratch-the-sur- face” overview of learning: The topic remains a black hole of neuroscience. Expect to learn a lot more about learning in the years to come.
Central sulcus
PRIMARY SOMATOSENSORY AREA (postcentral gyrus) SOMATOSENSORY ASSOCIATION AREA
WERNICKE’S AREA
Parietal lobe COMMON INTEGRATIVE AREA
VISUAL ASSOCIATION AREA PRIMARY VISUAL AREA Occipital lobe
POSTERIOR ANTERIOR
Temporal lobe
Lateral view of right cerebral hemisphere
AUDITORY ASSOCIATION AREA
PRIMARY AUDITORY AREA
PRIMARY GUSTATORY AREA
Lateral cerebral sulcus
Frontal lobe
FRONTAL EYE FIELD AREA
PREMOTOR AREA
PRIMARY MOTOR AREA (precentral gyrus)
BROCA'S SPEECH AREA
PREFRONTAL CORTEX
Cerebrum with lobes and general functions of lobes indicated Figure 7.
Sulci (sulcus)
Shallow grooves on the surface of the brain.
Gyri (gyrus)
Depressions separating individual sulci.
Cortex
Thin outer layer of any organ.
Association areas help us identify objects we have already seen, recognize familiar faces and voices, and remember winter holidays from a whiff of fresh-baked cookies.
212 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 213
Intra- abdominal
Pharynx
Tongue
Teeth, gums, and jaw
Lower lip
Upper lip
Face
Nose Eye
Thumb
Index
Middle RingLittle
Hand Wrist Forearm Elbow HeadNeck Trunk
Hip Leg Foot Toes Genitals
Arm Shoulder
Lips
Frontal plane through postcentral gyrus
Frontal section of primary somatosensory area in right cerebral hemisphere
Tongue
Toes
Ankle
Knee WristElbow
Hand
RingLittle Middle Index Thumb BrowNeck Face
Eyelid and eyeball
Trunk Hip
Shoulder
Lips
Frontal section of primary motor area in right cerebral hemisphere
Frontal plane through precentral gyrus
oV
ac zli at ion
aS
a (^) iv l iot n Swallowing
Jaw
tsaM taci n (^) o i
A B
primar y motor area. The more control we have over movements of a particular body part, the larger the sec- tion of the primary motor area devoted to it, as seen in the homunculus diagram (Figure 7.19). Sensory information from the skin and skeletal muscles is received in the primary somatosensory area of the cortex, just behind the primary motor area. As with the primary motor area, sensations from each body part go to a specific segment of this gyrus. The larger the segment of primary somatosensory area devoted to the body part, the more sensory receptors are found in that part. Interestingly, when any of the nerves along these sensory pathways are stimulated, the brain inter- prets the sensation as coming from the organ at the dis- tal end of the pathway, regardless of the source of the
stimulation. This causes referred pain , which also oc- curs when we interpret a painful stimulus from an inter- nal organ as pain in our skin or surface organs. This may happen because the visceral sensory pathways often join with or cross cutaneous sensory pathways in the spinal cord. When the pain stimulus reaches the brain, it is in- terpreted as coming from the skin, which is the usual site of injury. A typical example is the pain of appendici- tis. Although the appendix lies in the lower right ab- domen, appendicitis pain is usually described as right behind the umbilicus, or belly button. A few specialized motor actions are governed by areas outside the primary motor area. The forma- tion of words, for example, is organized in Broca’s area , on the left frontal lobe.
The left and right cerebral hemispheres are dis- tinct in some important ways. In most people, the right hemisphere analyzes sensory input, recognizes faces, and functions in spatial relationships. Emotional inter- pretation of conversation is a function of the right hemisphere. When you hear someone say “that’s just great,” your right hemisphere determines whether the speaker was actually impressed or speaking sarcastically. The left hemisphere usually includes the general lan- guage interpretation and speech centers, and it con- trols writing and speaking. The left hemisphere is more active during mathematical calculations, categorizing items, and making logical decisions, leading some to call it the “dominant” or “categorical” hemisphere. Special senses (see Chapter 8) are integrated in specific areas of the cerebral cortex. For example, the en- tire occipital lobe is devoted to visual interpretation. Auditory interpretation occurs in the primar y auditor y area of the temporal lobe. We even have a primary taste area in the parietal lobes that permits us to differentiate the taste of chocolate from coffee. No word yet on how that works with mocha java...
ASSOCIATION AREAS LINK THINGS TOGETHER
Association areas of the cerebral cortex integrate and coordinate information from many sources. For example, the somatosensory association area processes sensory information from the skin and muscles. The vi- sual association area associates new visual information with stored visual images. The auditory association area does the same thing with new auditory information. Although we can assign functions to each part of the brain, the various parts do not function alone. The brain is a network of incomprehensible complex-
ity. Stimuli are integrated throughout the cortex, and re- sponses are generated from many areas. The left and right sides of the brain connect through the transverse tracts of the corpus callosum, shar- ing information and generat- ing different responses (see Figure 7.18). In this way, despite hemispheric lateralization of some tasks, the entire cerebrum is aware of incoming sensory infor- mation as well as outgoing motor responses.
THE RETICULAR ACTIVATING SYSTEM IS THE BRAIN’S ALARM CLOCK
The reticular formation serves as an important connection between various parts of the brain. This series of nuclei and tracts extends throughout the brain, receiving sensory in- formation, parceling it to the higher centers, and directing motor responses to the appropriate body areas. The reticular activating system (RAS) is a portion of the reticular formation, impor- tant in maintaining alertness. Look around the next time you are trapped listening to a long-winded lecture. If your reticular activating system is doing its job, you will remain alert and attentive. But you might see some people whose RAS is not working so well. Their heads will be drooping; they might even be napping. The RAS may also be important in our ability to learn. One symptom of Attention Deficit Hyperactivity Disorder (ADHD) is the inability to filter out extrane- ous noises and focus on what is important. The RAS is responsible for this filtering, allowing you to study while the radio is on. It is possible that ADHD is partly due to poor function of the RAS. You can read more about ADHD in the Ethics and Issues box (page 216).
Motor homunculus and sensory homunculus Figure 7.
Special senses
The five senses of the body: hearing, vision, taste, smell, and touch.
Hemispheric
lateralization
The isolation of a task to either the left or right hemisphere of the cerebrum.
Nuclei
Areas of concen- trated neuronal cell bodies in the brain.
216 CHAPTER 7 The Nervous System The Brain and Spinal Cord are Central to the Nervous System 217
Ethics and Issues
A ttention 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:
- 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.
- Some college students with ADHD prescriptions say the amphetamines give them extra focus and energy during tests.
- 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.
- 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
2 SENSORY NEURON^1 (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.
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
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 C
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 ner ves 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 sensor y
218 CHAPTER 7 The Nervous System The Peripheral Nervous System Operates Beyond the Central Nervous System 219
L EARNING O BJECTIVES
T
Brain with cranial nerves identified Figure 7.
Cranial nerves Table 7.
Olfactory nerve Olfactory tract
Olfactory bulb Optic nerve Optic tract
Oculomotor nerve
Trochlear nerve Trigeminalnerve Abducens nerve
Facial nerve Vestibulocochlearnerve Glossopharyngealnerve
Vagus nerve Accessorynerve 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
Liver, gallbladder, and bile ducts
Eye Mucous membrane of nose and palate
Lacrimal gland
Spinal cord C C C C C C C C T T T T T T T T T T T T L L L L L S S S S S Coccygeal
Pelvic splanchnic nerves
Terminal ganglia
Ciliary ganglion
Ureter
Urinary bladder External genitals Uterus
Ascending colon
Small intestine
Descending colon
Transverse colon
Sigmoid colon Rectum
Pterygopalatine ganglion
Submandibular ganglion
Otic ganglion
PARASYMPATHETIC DIVISION (craniosacral)
Heart
Atrial muscle fibers SA/AV nodes
Larynx
Sublingual and submandibular glands
Trachea Bronchi
Lungs
Stomach Pancreas
Parotid gland
CN X
CN IX
CN VII
CN III
Key: Preganglionic neurons Postganglionic neurons
B
222 CHAPTER 7 The Nervous System
sympathetic nervous system includes nerves in the tho- racic and lumbar region of the spinal cord only. Sympa- thetic fibers extend from the spinal cord to a series of ganglia (group of cell bodies) called the sympathetic chain , on either side of the spinal cord. At these ganglia, neurons from the CNS synapse with a second neuron that extends to the effector organ. Thus sympathetic neurons leaving the spinal cord are shorter than those leaving the sympathetic chain. We call the neurons leav-
ing the spinal cord and synapsing in the ganglia pregan- glionic. Those that leave the ganglion and synapse with the effector organ are called postganglionic. Parasympathetic fibers are found only in the cranial, cervical, and sacral region of the cord. These neurons leave the spinal or cranial ner ve and join a ganglion near or in the effector organ. The parasympa- thetic preganglionic fibers are long, and the postgan- glionic neurons are extremely short.
CONCEPT CHECK
Define spinal nerve.
What is the sympathetic chain?
How do sympathetic neurons differ from parasympathetic neurons anatomically?
A Sympathetic and B parasympathetic nerve fibers Figure 7.24 (continued from page 221)
1
The Nervous System Makes Sense of Everything The nervous system is responsible for maintaining homeostasis by reacting al- most instantaneously to stimuli. It works in concert with the endocrine system to maintain homeostasis. The work of the system is performed by neurons, supported by neuroglial cells.
CHAPTER SUMMARY
AXON
Axon hillock
Axon terminal Synaptic end bulb
Axon collateral
Chapter Summary 223
2
The 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
5
The 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-
6
The 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
4
Neurons 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
3
Nerve 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.
