Chapter 9 Nervous System PDF - Essentials of Human Anatomy and Physiology

Because learning changes everything.® Chapter 09 Nervous System HOLE’S ESSENTIALS OF HUMAN ANATOMY & PHYSIOLOGY Fifteenth Edition Charles J. Welsh and Cynthia Prentice-Craver © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of...

Because learning changes everything.® Chapter 09 Nervous System HOLE’S ESSENTIALS OF HUMAN ANATOMY & PHYSIOLOGY Fifteenth Edition Charles J. Welsh and Cynthia Prentice-Craver © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.  9.1: Introduction to the Nervous System Major aspects of nervous system: sensory input, integration and processing (decision-making), and motor output (response) Functions of the nervous system: thinking, movement, internal processes of physiology Main cell types in the nervous system: Neurons: cells that communicate, via electrical impulses, with other neurons or other cells Neuroglia: cells that support, nourish, protect, and insulate neurons Neurotransmitters are the chemical messengers in a synapse, that convey an electrical impulse from a neuron to another cell. Nervous system controls the endocrine system, which regulates body functions and helps to maintain homeostasis via secretion of hormones © McGraw Hill, LLC 2  Figure 9.1: The Flow of Information in the Nervous System Access the text alternative for these images © McGraw Hill, LLC 3  9.2: Nervous System Organization General functions of the nervous system: sensory, integrative, and motor. Organs of the nervous system can be divided 2 groups: Central nervous system (CNS): Consists of the brain and spinal cord Responsible for integration of information and decision-making Peripheral nervous system (PNS): Made up of cranial and spinal nerves that connect the CNS to the rest of the body Contains sensory (afferent) and motor (efferent) divisions Motor functions: Somatic nervous system: controls voluntary skeletal muscles Autonomic nervous system: controls involuntary effectors (smooth and cardiac muscles and glands) © McGraw Hill, LLC 4  Figure 9.2: Nervous System Divisions Access the text alternative for these images © McGraw Hill, LLC 5  General Functions of the Nervous System Sensory function: Provided by sensory receptors, which detect internal or external changes Information travels from receptors to sensory neurons, which transport information into the CNS Integrative function: Coordination of sensory information in the CNS Processing of this information is the basis for decision-making Motor function: Nerve impulses (CNS) are conducted along motor neurons to effectors Effectors are muscles or glands that respond to decisions made in the CNS © McGraw Hill, LLC 6  9.3: Neurons Neuron (Nerve Cell) Structure A neuron contains a cell body, tubular cytoplasm-filled dendrites, and a tubular, cytoplasm-filled axon. The cell body (soma) contains mitochondria, lysosomes, a Golgi apparatus, chromatophilic substance (Nissl bodies – similar to rough ER), neurofilaments, and a large nucleus with a nucleolus. Dendrites conduct impulses toward the cell body; they are short and branching, and they provide the receptive surface for communication with other neurons. The axon conducts impulses away from the cell body; it arises from a thickening extending from the cell body, called the axon hillock. There is only 1 axon in each neuron. © McGraw Hill, LLC 7  Neuron Structure: Myelin Sheath Myelin Sheath: Larger axons are enclosed by myelin sheaths; they are called myelinated fibers. Narrow gaps in the myelin sheath are called nodes of Ranvier. Myelin sheath increases conduction speed of nerve impulses. © McGraw Hill, LLC 8  Figure 9.3: Structure of a Common Neuron Access the text alternative for these images © McGraw Hill, LLC 9  Structural Classification of Neurons There are 3 types of neurons, based on differences in size, shape, and structure: Multipolar neurons: have many dendrites and one axon arising from their cell bodies; most neurons with cell bodies in CNS (interneurons and motor neurons) are multipolar. Bipolar neurons have 2 processes extending from the cell body, a dendrite and an axon; found in some of the special senses, such as the eyes, nose, and ears. Unipolar neurons have only 1 process extending from the cell body; outside the cell body, it soon splits into 2 parts that function as 1 axon; the peripheral process has dendrites near a peripheral body part, and the central process runs into the CNS; the cell bodies are found in ganglia outside the CNS; these are sensory neurons. © McGraw Hill, LLC 10  Figure 9.4: Structural Classification of Neurons Access the text alternative for these images © McGraw Hill, LLC 11  Functional Classification of Neurons Sensory (afferent) neurons: conduct impulses from peripheral receptors to the CNS; usually unipolar, although some are bipolar. Interneurons (association or internuncial neurons): multipolar neurons lying within the CNS that form links between other neurons; the cell bodies of some interneurons aggregate in specialized masses called nuclei. Motor (efferent) neurons: multipolar neurons that conduct impulses from the CNS to peripheral effectors (muscles or glands). © McGraw Hill, LLC 12  Figure 9.5: Functional Classification of Neurons Access the text alternative for these images © McGraw Hill, LLC 13  9.4: Neuroglia Neuroglia (glial cells, “nerve glue”) are cells that support neurons. Functions: fill spaces, structurally support, protect, and insulate neurons Do not generate or conduct nerve impulses 4 types in CNS, 2 types in PNS Central nervous system neuroglia: Microglia: small cells that function as phagocytes for bacterial cells and cellular debris and produce scar tissue in sites of injury Oligodendrocytes: form the myelin sheath around axons in the brain and spinal cord Ependymal cells produce cerebrospinal fluid in CNS. © McGraw Hill, LLC 14  Types of Neuroglia Astrocytes: lie between blood vessels and neurons; functions: Structural support Regulation of nutrient and ion concentrations Formation of the blood-brain barrier, which protects brain tissue from chemical fluctuation and prevents entry of many substances Peripheral nervous system neuroglia: Schwann cells: produce the myelin sheath around PNS axons Satellite cells: provide protective coating around cell bodies of neurons in the PNS © McGraw Hill, LLC 15  Figure 9.6: Neuroglia of the Central Nervous System Access the text alternative for these images © McGraw Hill, LLC 16  Figure 9.7: Satellite Cells and Schwann Cells of the PNS Access the text alternative for these images © McGraw Hill, LLC 17  Regeneration of Neurons Damaged PNS neurons are able to regenerate their axons, because the neurilemma of their Schwann cells helps guide the growing axon to its original connection point. CNS axons are myelinated by oligodendrocytes, which lack a neurilemma, so they usually do not regenerate. © McGraw Hill, LLC 18  9.5: Charges Inside a Cell Cell membranes of neurons exhibit polarity, meaning that the charge inside the membrane is different from the charge outside the membrane. Due to unequal distribution of ions on both sides of the cell membrane, the inside of the cell is more negative than the outside. Neurons and muscle cells are excitable, since they can respond to stimuli by moving their internal charge into the positive range. Changing the charge inside the cell starts a sequence of events, which allows neurons to communicate. © McGraw Hill, LLC 19  Membrane Potential and Distribution of Ions Membrane Potential: the charge inside a cell Resting Membrane Potential: the charge in a cell when it is at rest; this is about -70 mV in neurons Charge inside a neuron results from unequal distribution of ions inside and outside of cells There is a greater concentration of sodium ions on the outside the cells than inside, and a greater concentration of potassium ions inside the cells than outside. Many large negatively charged ions and proteins are found on the inside of cells. © McGraw Hill, LLC 20  Figure 9.8: Resting Membrane Potential Access the text alternative for these images © McGraw Hill, LLC 21  Stimulation and the Action Potential 1 A neuron remains at rest until stimulated. A stimulus can change resting potential in either direction. An excitatory stimulus opens chemically-gated Na+ channels; Na+ ions flow into cell due to concentration gradient, causing inside of neuron to become less negative. Threshold stimulus: a stimulus strong enough to cause so many Na+ ions to enter neuron, that potential changes from -70 to -55 mV (the threshold potential). Upon reaching threshold potential, voltage-gated Na+ channels open, changing charge to about +30 mV; this begins an action potential. Change from negative to positive charge inside neuron is called depolarization, since now, inside and outside are both positive. © McGraw Hill, LLC 22  Stimulation and the Action Potential 2 Reaching an action potential is all-or-none response: Action potential either occurs or does not. An action potential occurs when the charge reaches -55 mV. Action potentials of a neuron are all of the same strength. When an action potential is reached, cell responds by returning to resting potential (-70 mV) by process of repolarization. Repolarization returns the polarized state and is accomplished by outward flow of potassium ions through potassium channels. At end of repolarization, a slight overshoot called hyperpolarization occurs, in which potential dips below -70 mV. Finally, the Na+ /K+pump moves Na+ ions back out of cell, and Na+ back into cell. © McGraw Hill, LLC 23  Figure 9.9: Ion Channels and the Action Potential Access the text alternative for these images © McGraw Hill, LLC 24  Figure 9.10: A Recording of an Action Potential Access the text alternative for these images © McGraw Hill, LLC 25  9.6: Impulse Conduction An action potential at the trigger zone causes an electrical current to flow to adjacent regions of the axon’s membrane. This spreads by a local current flowing down the fiber that stimulates the next region and continues down the axon to the axon terminal. This process is called impulse conduction. Refractory period: period during and after an action potential, during which a threshold stimulus will not cause another action potential: Limits frequency of action potentials Ensures the impulse is only transmitted in one direction – down the axon © McGraw Hill, LLC 26  Figure 9.11: Illustration of Impulse Conduction Access the text alternative for these images © McGraw Hill, LLC 27  The Process of Impulse Conduction TABLE 9.1 Impulse Conduction 1. Neuron membrane maintains resting potential. 2. Threshold stimulus is received. 3. Sodium channels in the trigger zone of the axon open. 4. Sodium ions diffuse inward, depolarizing the axon membrane. 5. Potassium channels in the axon membrane open. 6. Potassium ions diffuse outward, repolarizing the axon membrane. 7. The resulting action potential causes a local electric current that stimulates the adjacent portions of the axon membrane. 8. A series of action potentials occurs along the axon. © McGraw Hill, LLC 28  Types of Impulse Conduction Continuous conduction: Occurs in unmyelinated axons Conduct impulses sequentially over the entire length of their membrane Saltatory conduction: Occurs in myelinated axons The myelin sheath insulates axons from ion movement across the cell membrane Impulses “jump” from one Node of Ranvier to the next, since sodium and potassium channels occur only at the nodes Speed of impulse conduction is proportional to axon diameter: Thick, myelinated motor axons conduct at 120 m/s Thin, unmyelinated sensory axons conduct at 0.5 m/s © McGraw Hill, LLC 29  Figure 9.12: Saltatory Conduction in a Myelinated Axon Access the text alternative for these images © McGraw Hill, LLC 30  9.7: The Synapse A synapse is a junction between 2 communicating neurons. The small gap between the neurons is called the synaptic cleft; the impulse must be conveyed across the cleft. The neuron sending the impulse is the presynaptic neuron. The neuron receiving the impulse is the postsynaptic neuron. Neural communication across the cleft is called synaptic transmission. Communication is accomplished by a chemical called a neurotransmitter, which is stored in synaptic vesicles and released from an expansion at the distal end of the presynaptic neuron, called the synaptic knob. Neurotransmitters are released in response to a nerve impulse reaching the synaptic knob; they diffuse across the cleft and bind to receptors on the membrane of the postsynaptic neuron. © McGraw Hill, LLC 31  Figure 9.13: Synapses Separate Neurons and Allow for Communication Access the text alternative for these images © McGraw Hill, LLC 32  Figure 9.14: Synaptic Transmission Access the text alternative for these images © McGraw Hill, LLC 33  Figure 9.15: Electron Micrograph of a Synaptic Knob Don W. Fawcett/Science Source Access the text alternative for these images © McGraw Hill, LLC 34  9.8: Synaptic Transmission Excitatory and Inhibitory Actions: Excitatory Neurotransmitters: Increase entry of Na+ ions into postsynaptic neuron Bring membrane closer to threshold, making action potential more likely Inhibitory Neurotransmitters: Increase flow of Cl- ions into neuron or flow of K+ ions out of the neuron Makes charge inside the neuron more negative, making action potential less likely The postsynaptic neuron may have many presynaptic neurons influencing it, so it sums the excitatory & inhibitory inputs from all of these neurons to derive its response. © McGraw Hill, LLC 35  Figure 9.16: Excitatory versus Inhibitory Stimulus Access the text alternative for these images © McGraw Hill, LLC 36  Neurotransmitters More than 100 neurotransmitters are produced in synaptic knobs and stored in synaptic vesicles. Neurotransmitters include acetylcholine, monoamines, amino acids, neuropeptides. The action of the neurotransmitter depends on type of receptors in a specific synapse. Some neurons produce one type of neurotransmitter, while others produce two or three. © McGraw Hill, LLC 37  Some Neurotransmitters & Their Actions 1 TABLE 9.2 Some Neurotransmitters and Representative Actions Neurotransmitter Location Major Actions Acetylcholine CNS Controls skeletal muscle actions. PNS Stimulates skeletal muscle contraction at neuromuscular junctions; may excite or inhibit autonomic nervous system actions, depending on receptors. Monoamines Norepinephrine CNS Creates a sense of feeling good; low levels may lead to depression. PNS May excite or inhibit autonomic nervous system actions, depending on receptors. Dopamine CNS Creates a sense of feeling good; deficiency in some brain areas is associated with Parkinson disease. PNS Limited actions in autonomic nervous system; may excite or inhibit, depending on receptors. Serotonin CNS Primarily inhibitory; leads to sleepiness; action is blocked by LSD, enhanced by selective serotonin reuptake inhibitor drugs (SSRIs). Histamine CNS Release in hypothalamus promotes alertness. © McGraw Hill, LLC 38  Some Neurotransmitters & Their Actions 2 TABLE 9.2 Some Neurotransmitters and Representative Actions Neurotransmitter Location Major Actions Amino acids GABA CNS Generally inhibitory. Glutamic acid CNS Generally excitatory. Neuropeptides Substance P PNS Excitatory; pain perception. Endorphins, CNS Generally inhibitory; reduce pain by inhibiting substance P release. enkephalins Gases Nitric oxide PNS Vasodilation. CNS May play a role in memory. © McGraw Hill, LLC 39  Events Leading to Release of a Neurotransmitter TABLE 9.3 Events Leading to the Release of a Neurotransmitter 1. Action potential passes along an axon and over the surface of its synaptic knob. 2. Synaptic knob membrane becomes more permeable to calcium ions, and they diffuse inward. 3. In the presence of calcium ions, synaptic vesicles fuse to synaptic knob membrane. 4. Synaptic vesicles release their neurotransmitter into synaptic cleft. © McGraw Hill, LLC 40  Neurotransmitter Recycling After acting on postsynaptic cell, neurotransmitter effects must be stopped. Destruction or removal of the neurotransmitter prevents continuous stimulation of the postsynaptic neuron. Enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose the neurotransmitters after their release; example: acetylcholinesterase breaks down acetylcholine. Some neurotransmitters travel back into the presynaptic neuron for reuse; this is called reuptake. © McGraw Hill, LLC 41  9.9: Impulse Processing How impulses are processed is dependent upon how neurons are organized in the brain and spinal cord. Neuronal Pools: Neurons within the CNS are organized into neuronal pools with varying numbers of cells that work together. Each pool receives input from afferent neurons and processes the information according to the special characteristics of the pool. A neuron in a pool might receive both excitatory and inhibitory input. If net input is excitatory, but does not sum to the threshold potential, there will be no nerve impulse. If net excitatory input sums to the threshold potential, a nerve impulse occurs. © McGraw Hill, LLC 42  Facilitation, Convergence, and Divergence Facilitation: An increase in the release of neurotransmitter in response to one impulse, which occurs in an excitatory presynaptic neuron upon repeated stimulation; this response increases the likelihood that the postsynaptic neuron will reach threshold Convergence: The transmission of nerve impulses to a single neuron within a pool from two or more fibers; this makes it possible for the neuron to summate impulses from different sources Divergence: The transmission of nerve impulses from a neuron in a pool to several output fibers; this serves to amplify an impulse © McGraw Hill, LLC 43  Figure 9.17: Impulse Processing: Convergence and Divergence Access the text alternative for these images © McGraw Hill, LLC 44  9.10: Types of Nerves A nerve is a bundle of nerve fibers (axons) in the PNS. Types of nerves: Sensory (afferent) nerves conduct impulses to the CNS; their axons are called sensory or afferent fibers. Motor (efferent) nerves carry impulses from the CNS to effectors; their axons are called motor or efferent fibers. Mixed nerves carry both sensory and motor fibers; most nerves are of this type. Connective tissue coverings: Epineurium: outer covering of a nerve. Perineurium: covering around fascicles (bundles) of nerve fibers. Endoneurium: covering around individual nerve fibers (axons). © McGraw Hill, LLC 45  Figure 9.18: Connective Tissue Coverings and Nerve Structure Access the text alternative for these images © McGraw Hill, LLC 46  9.11: Neural Pathways The routes nerve impulses travel are called neural pathways, the simplest of which is a reflex arc. Reflex arcs provide the basis for involuntary actions called reflexes. Components of a reflex arc: A sensory receptor that detects changes A sensory neuron, that carries the information from a receptor toward the CNS An interneuron in the CNS (reflex center) A motor neuron, that carries a command to effectors An effector (muscle or gland that carries out the reflex) that responds to the initial change © McGraw Hill, LLC 47  Figure 9.19: A Reflex Arc Access the text alternative for these images © McGraw Hill, LLC 48  Parts of a Reflex Arc TABLE 9.4 Parts of a Reflex Arc Part Description Function Receptor Receptor end of a dendrite or Senses specific type of internal or a specialized receptor cell in a external change. sensory organ. Sensory Dendrite, cell body, and axon Carries information from receptor neuron of a sensory neuron. into brain or spinal cord. Interneuron Dendrite, cell body, and axon Carries information from sensory of a neuron within the brain neuron to motor neuron. or spinal cord. Motor Dendrite, cell body, and axon Carries instructions from brain or neuron of a motor neuron. spinal cord out to effector. Effector Muscle or gland. Responds to stimulation (or inhibition) by motor neuron and produces reflex or behavioral action. © McGraw Hill, LLC 49  Reflex Behavior 1 Reflexes: automatic responses to changes (stimuli) inside or outside of the body, that help maintain homeostasis. Reflexes control heart rate, blood pressure, etc. and carry out automatic responses such as vomiting, sneezing, swallowing, etc. The patellar (knee-jerk) reflex is an example of a simple reflex; it contains only 2 neurons, sensory & motor, and lacks an interneuron. Striking the patellar ligament stretches the quadriceps femoris muscle and tendon, stimulating stretch receptors. Sensory neurons transmit nerve impulses to the spinal cord, where they synapse with motor neurons, and issue a motor command. Motor neurons transmit the impulses to the quadriceps femoris muscle group, which contracts in response; this extends the knee. This reflex helps maintain upright posture. © McGraw Hill, LLC 50  Figure 9.20: The Patellar Reflex Access the text alternative for these images © McGraw Hill, LLC 51  Reflex Behavior 2 Withdrawal reflex: Occurs in response to touching something painful, such as stepping on a tack (or laying a hand on a hot stove) Involves sensory neurons, interneurons, and motor neurons: Sensory receptors send pain messages along sensory neurons to the spinal cord Sensory neurons send impulses to interneurons, where information is coordinated Interneurons issue motor commands to motor neurons Motor signals are sent to flexor muscles to contract At the same time, the antagonistic extensor muscles are inhibited, and message is sent to the brain for awareness Serves a protective function, as it limits tissue damage © McGraw Hill, LLC 52  Figure 9.21: A Withdrawal Reflex Access the text alternative for these images © McGraw Hill, LLC 53  9.12: Meninges The brain and spinal cord are surrounded by 3 membranes called meninges that lie between the skull bones & vertebrae and the soft CNS tissues. The meninges consist of the dura mater, arachnoid mater, and pia mater. Dura mater: Outermost layer of meninges Made up of tough, dense connective tissue, and is very thick Contains many blood vessels Forms the internal periosteum of the skull bones In some areas, the dura mater forms partitions between lobes of the brain, and in others, it forms dural sinuses. The sheath around the spinal cord is separated from the vertebrae by an epidural space. © McGraw Hill, LLC 54  Meninges Arachnoid mater: The middle layer of meninges Thin, weblike layer that lacks blood vessels Between the arachnoid and pia mater is the subarachnoid space, which contains cerebrospinal fluid (CSF) Pia mater: The innermost layer of the meninges Thin layer, which contains many blood vessels and nerves Attached to the surface of the brain and spinal cord and follows their contours © McGraw Hill, LLC 55  Figure 9.22: The Meninges of the Brain Access the text alternative for these images © McGraw Hill, LLC 56  Figure 9.23: The Meninges of the Spinal Cord Access the text alternative for these images © McGraw Hill, LLC 57  9.13: Spinal Cord Spinal cord: Begins at the base of the brain at the foramen magnum Extends as a thin cord to the level of the intervertebral disc between the 1st and 2nd lumbar vertebrae Cervical enlargement: A thickened area near top of spinal cord Provides nerves to upper limbs Lumbar enlargement: A thickened region near the bottom of the spinal cord Gives rise to nerves that serve the lower limbs. Cauda equina (horse’s tail): Structure formed where spinal cord tapers to a point inferiorly Consists of spinal nerves in the lumbar & sacral areas © McGraw Hill, LLC 58  Figure 9.24: Lateral View of Spinal Cord Access the text alternative for these images © McGraw Hill, LLC 59  Structure of the Spinal Cord Spinal cord consists of 31 segments, each of which connects to a pair of spinal nerves. Two deep grooves (anterior median fissure and posterior median sulcus) divide the cord into right and left halves. White matter, made up of bundles of myelinated nerve fibers (nerve tracts), surrounds a butterfly-shaped core of gray matter housing interneurons and neuron cell bodies. Cell bodies of sensory neurons that enter the spinal cord are found in the posterior root ganglia outside the spinal cord. The upper and lower wings of gray matter form the posterior and anterior horns; between them is the lateral horn. The gray matter divides the white matter into three regions: anterior, lateral and posterior funiculi (columns), each consisting of longitudinal bundles of axons called tracts. A central canal in the middle of the gray matter contains cerebrospinal fluid. © McGraw Hill, LLC 60  Figure 9.25: A Cross Section of the Spinal Cord (b): Ed Reschke/Photolibrary/Getty Images Access the text alternative for these images © McGraw Hill, LLC 61  Functions of the Spinal Cord Major functions: transmit impulses to and from the brain, and to house spinal reflexes Ascending tracts carry sensory information to the brain; descending tracts carry motor information from brain to muscles or glands The names that identify nerve tracts identify the origin and termination of the fibers in the tract: Spinothalamic tracts: carry sensory information from the spinal cord to the thalamus Corticospinal tracts (pyramidal tracts): carry motor impulses from the cerebral cortex to the spinal cord; pass through pyramid-shaped areas in the medulla oblongata Extrapyramidal tracts: descending tracts involved with balance and posture Spinal reflexes: controlled by reflex arcs that pass through the spinal cord © McGraw Hill, LLC 62  Figure 9.26: Ascending Tracts Access the text alternative for these images © McGraw Hill, LLC 63  Figure 9.27: Descending Tracts Access the text alternative for these images © McGraw Hill, LLC 64  9.14: The Brain The brain is the largest, most complex portion of the nervous system, containing 100 billion multipolar neurons, and many neuroglia to support the neurons. Structure is reversed from that of spinal cord; gray matter outside and white matter inside. The 4 main parts of the brain: Cerebrum: largest portion; associated with higher mental functions, and sensory & motor functions Diencephalon: processes sensory input and controls many homeostatic processes Cerebellum: coordinates muscular activity Brainstem: coordinates and regulates visceral activities and connects different parts of the nervous system © McGraw Hill, LLC 65  Figure 9.28a: Midsagittal Section of the Brain (Diagram) Access the text alternative for these images © McGraw Hill, LLC 66  Figure 9.28b: Midsagittal Section of the Brain (Photo) (b): Martin Rotker/Science Source Access the text alternative for these images © McGraw Hill, LLC 67  Structure of the Cerebrum 1 The Cerebrum is the largest portion of the mature brain. Consists of 2 cerebral hemispheres, which are mirror images Corpus callosum: flat bundle of nerve fibers that connects the hemispheres. The surface of the brain is marked by these features: Gyri (singular is gyrus): ridges Sulci (singular is sulcus): grooves Fissures (longitudinal and transverse): deep grooves © McGraw Hill, LLC 68  Structure of the Cerebrum 2 Four lobes of the cerebrum are named according to the bones they underlie: frontal, parietal, temporal, and occipital lobes. The fifth lobe is the insula; it lies deep in the lateral sulcus. A thin layer of gray matter, the cerebral cortex, lies on the outside of the cerebrum, and contains 75% of the neuron cell bodies in the nervous system. Beneath the cortex lies a mass of white matter made up of myelinated nerve fibers connecting the cell bodies of the cerebral cortex with the rest of the nervous system. © McGraw Hill, LLC 69  Figure 9.29a: Lateral View of the Human Brain (Diagram) Access the text alternative for these images © McGraw Hill, LLC 70  Figure 9.29b: View of the Human Brain and Part of Spinal Cord (Photo) (b): Rebecca Gray/Wise Anatomy Collection/McGraw Hill © McGraw Hill, LLC 71  Functions of the Cerebrum The cerebrum provides higher brain functions: Interpretation of sensory input Initiating voluntary muscular movements Stores information for memory Integrates information for reasoning Intelligence Personality © McGraw Hill, LLC 72  Functional Areas of the Cerebral Cortex 1 The functional areas of the brain overlap, but the cortex can generally be divided into sensory, association, and motor areas. The sensory areas are located in several areas of the cerebrum; they interpret sensory input, producing feelings or sensations: Cutaneous senses: anterior parietal lobe Visual area: posterior occipital lobe Auditory area: posterior temporal lobe Taste area: base of central sulcus and insula Smell area: deep in temporal lobe Sensory fibers from the PNS cross over in the spinal cord or the brainstem; this results in sensory impulses from the right side of the body being interpreted by centers in the left cerebral hemisphere. © McGraw Hill, LLC 73  Functional Areas of the Cerebral Cortex 2 Association areas of the brain analyze and interpret sensory impulses, and function in reasoning, judgment, emotions, verbalizing ideas, and storing memory: Association areas of the frontal lobe control a number of higher intellectual processes (planning, problem solving). Association areas of the parietal lobe function in understanding speech and choosing the proper words. Association areas in occipital lobe help analyze visual patterns and combine visual images with other sensory information. Association areas next to sensory areas are important for analyzing the sensory input. A general interpretive area is found at the junction of the parietal, temporal, and occipital lobes, and plays a primary role in complex thought processing and integration. Not all association areas are bilateral; Wernicke’s area of the temporal lobe is usually on the left side only; it helps with understanding of written and spoken language. © McGraw Hill, LLC 74  Functional Areas of the Cerebral Cortex 3 The primary motor areas lie in the posterior frontal lobes, anterior to the central sulcus. This region includes the pyramidal cells that are also called upper motor neurons; they synapse with lower motor neurons that exit the spinal cord and reach the skeletal muscles. There is also crossover in the brainstem in motor systems, so that the right cerebral hemisphere controls muscles on the left side of the body. Broca’s motor speech area is in the frontal lobe, usually on the left side; controls muscle movements for speech. Frontal eye field in the frontal lobe controls voluntary eye movements. © McGraw Hill, LLC 75  Figure 9.29a: Sensory, Association, & Motor Areas of the Brain Access the text alternative for these images © McGraw Hill, LLC 76  Hemisphere Dominance Both cerebral hemispheres receive and analyze sensory input and send motor impulses to the opposite side of the body, but one side is the dominant hemisphere in most people. For most people, the left hemisphere is dominant for the language- related activities of speech, writing, and reading, as well as complex intellectual functions. In some individuals, the right hemisphere as dominant, and others show equal dominance in both hemispheres. The nondominant hemisphere specializes in nonverbal functions, such as body orientation in space, and controls emotions and intuitive thinking. The nerve fibers of the corpus callosum connect the two hemispheres; this allows the dominant hemisphere to control motor cortex of nondominant side and allows sensory impulses from the nondominant side to transfer to the dominant side. © McGraw Hill, LLC 77  The Basal Nuclei The basal nuclei are masses of gray matter (nuclei) located deep within the cerebral hemispheres. Basal nuclei are also called basal ganglia. Consist of the caudate nucleus, the putamen, and the globus pallidus Produce the inhibitory neurotransmitter, dopamine Relay motor impulses from the cerebrum and help control motor activities by interacting with the motor cortex, thalamus, and cerebellum Help facilitate voluntary movement Altered activity of these nuclei neurons produces the signs of Parkinson disease and Huntington disease. © McGraw Hill, LLC 78  Figure 9.30: Frontal Section of Left Cerebral Hemisphere, Showing Basal Nuclei Access the text alternative for these images © McGraw Hill, LLC 79  Ventricles and Cerebrospinal Fluid Ventricles: a series of connected cavities within the cerebral hemispheres and brainstem. Continuous with central canal of spinal cord and subarachnoid space; all of these cavities are filled with cerebrospinal fluid (CSF). Flow of CSF proceeds through the ventricles and channels in this order: 2 Lateral ventricles Interventricular foramina Third ventricle Cerebral aqueduct Fourth ventricle, which is continuous with the central canal of the spinal cord and the subarachnoid space of the meninges © McGraw Hill, LLC 80  Figure 9.31: Ventricles of the Brain Access the text alternative for these images © McGraw Hill, LLC 81  Choroid Plexuses Choroid plexuses are masses containing specialized capillaries from the pia mater Found in all 4 ventricles Secrete cerebrospinal fluid (CSF) into the ventricles; most CSF arises in the lateral ventricles CSF circulates through ventricles and connecting passageways into the subarachnoid space, where it is reabsorbed back into the blood. CSF completely surrounds brain and spinal cord. CSF has nutritive as well as protective (cushioning) functions. © McGraw Hill, LLC 82  Figure 9.32: The Choroid Plexuses and the Pathway of CSF Access the text alternative for these images © McGraw Hill, LLC 83  The Diencephalon The diencephalon lies between the cerebral hemispheres and above the midbrain: Surrounds the third ventricle Consists of mainly gray matter Main parts are the thalamus and hypothalamus Other portions of the diencephalon: Optic tracts and optic chiasma Infundibulum (attachment of the pituitary gland to the hypothalamus) Posterior pituitary Mammillary bodies Pineal gland © McGraw Hill, LLC 84  Functions of the Thalamus Functions of the thalamus include: Sorting and directing sensory information arriving from other parts of the nervous system to the cerebral cortex Channeling all sensory impulses, except those for the sense of smell Producing general awareness of the sensation, such as pain, touch and temperature © McGraw Hill, LLC 85  Functions of the Hypothalamus The hypothalamus maintains homeostasis by regulating a wide variety of visceral activities, and by linking the endocrine system with the nervous system: Regulates heart rate and arterial blood pressure Regulates body temperature, water and electrolyte balance, hunger and body weight Controls movements and secretions of the digestive tract Helps to regulate sleep and wakefulness Stimulates the posterior pituitary gland to secrete stored hormones Produces hormones that cause the anterior pituitary gland to secrete its hormones © McGraw Hill, LLC 86  The Limbic System The limbic system, in the area of the diencephalon, controls emotional experience and expression. Consists of several structures, including parts of the cerebral cortex, deep masses of gray matter (thalamus, hypothalamus, basal nuclei) Modifies behavior by producing feelings of fear, anger, pleasure, sorrow By generating pleasant or unpleasant feelings about experiences, the limbic system guides behavior that may enhance the chance of survival © McGraw Hill, LLC 87  The Brainstem The brainstem consists of: Midbrain Pons Medulla oblongata Lies at the base of the cerebrum Connects the cerebrum, diencephalon, and cerebellum to the spinal cord © McGraw Hill, LLC 88  Figure 9.33: Ventral & Dorsal Views of the Brainstem, Showing the Thalamus & Fourth Ventricle Access the text alternative for these images © McGraw Hill, LLC 89  The Brainstem: Midbrain and Pons Midbrain: Located between the diencephalon and pons Contains bundles of myelinated nerve fibers that convey impulses to and from higher centers of the brain Contains masses of gray matter that serve as centers for auditory and visual reflexes Contains main motor pathways between cerebrum and lower portions of the nervous system Pons: Lies between the midbrain and medulla oblongata Transmits impulses to & from medulla oblongata and cerebrum Also conducts impulses from cerebrum to cerebellum Contains centers that help regulate the rate and depth of breathing © McGraw Hill, LLC 90  The Brainstem: Medulla Oblongata The medulla oblongata: Transmits all ascending and descending impulses between the brain and spinal cord Extends from pons to foramen magnum Most of the corticospinal tracts cross over in the pyramids of the medulla oblongata The medulla oblongata houses nuclei that control visceral functions: Cardiac center: alters heart rate Vasomotor center: controls vasoconstriction & vasodilation of blood vessels; helps control blood pressure Respiratory center: Controls rate and depth of breathing Also contains nuclei that control reflexes such as coughing, sneezing, swallowing, vomiting © McGraw Hill, LLC 91  The Brainstem: Reticular Formation Reticular formation or reticular activating system: Network of nerve fibers connecting small masses of gray matter scattered throughout the brainstem Neurons in reticular formation connect parts of hypothalamus, basal nuclei, cerebellum, and cerebrum with the major ascending and descending tracts. Decreased activity in the reticular formation results in sleep; increased activity results in wakefulness. Injury to the reticular formation results in a comatose state. The reticular formation filters incoming sensory impulses. © McGraw Hill, LLC 92  Cerebellum The cerebellum: Located beneath the occipital lobes of the cerebrum, posterior to the brainstem Consists of 2 hemispheres connected by the vermis A thin layer of gray matter called the cerebellar cortex lies outside a core of white matter called the arbor vitae. The cerebellum communicates with other parts of the central nervous system through 3 pairs of tracts, the cerebellar peduncles. Functions of cerebellum: Integrates sensory information about the position of body parts Coordinates skeletal muscle activity Maintains posture Ensures that movement occurs in the desired manner © McGraw Hill, LLC 93  Figure 9.34: A Midsagittal Section Through the Cerebellum Access the text alternative for these images © McGraw Hill, LLC 94  9.15: Peripheral Nervous System Peripheral nervous system (PNS): Consists of nerves that connect the CNS to body parts Consists of cranial nerves, arising from the brain, and spinal nerves, arising from the spinal cord Contains sensory and motor divisions The motor part of the PNS is made up of 2 portions: Somatic nervous system, which connects the CNS to skeletal muscles and the skin and oversees conscious activities. Autonomic nervous system, which connects the CNS to viscera, and controls subconscious activities. © McGraw Hill, LLC 95  Subdivisions of the Nervous System TABLE 9.5 Subdivisions of the Nervous System 1. Central nervous system (CNS) a. Brain b. Spinal cord 2. Peripheral nervous system (PNS) a. Cranial nerves arising from the brain and brainstem 1. Sensory fibers connecting peripheral sensory receptors to the CNS 2. Somatic fibers connecting to skin and skeletal muscles 3. Autonomic fibers connecting to viscera b. Spinal nerves arising from the spinal cord 1. Sensory fibers connecting peripheral sensory receptors to the CNS 2. Somatic fibers connecting to skin and skeletal muscles 3. Autonomic fibers connecting to viscera © McGraw Hill, LLC 96  Cranial Nerves Twelve pairs of cranial nerves arise from the underside of the brain. Most are mixed nerves, containing sensory & motor nerve fibers, but some are only sensory, and others are primarily motor. The first pair arises from the cerebrum, and the second pair from the thalamus, and the rest arise from the brainstem. The 12 pairs are designated by number and name; the numbers are in order, from superior to inferior. © McGraw Hill, LLC 97  Figure 9.35: The Location of the Cranial Nerves Access the text alternative for these images © McGraw Hill, LLC 98  Functions of the Cranial Nerves 1 TABLE 9.6 Functions of the Cranial Nerves Nerve Type Function 1 Olfactory Sensory Sensory fibers conduct impulses associated with the sense of smell. 2 Optic Sensory Sensory fibers conduct impulses associated with the sense of vision. 3 Oculomotor Primarily Motor fibers conduct impulses to muscles that raise eyelids, move eyes, adjust motor the amount of light entering the eyes, and focus lenses. Some sensory fibers conduct impulses associated with the condition of muscles. 4 Trochlear Primarily Motor fibers conduct impulses to muscles that move the eyes. motor Some sensory fibers conduct impulses associated with the condition of muscles. 5 Trigeminal Mixed Ophthalmic division Sensory fibers conduct impulses from the surface of the eyes, tear glands, scalp, forehead, and upper eyelids. Maxillary division Sensory fibers conduct impulses from the upper teeth, upper gum, upper lip, lining of the palate, and skin of the face. Mandibular division Sensory fibers conduct impulses from the skin of the jaw, lower teeth, lower gum, and lower lip. Motor fibers conduct impulses to muscles of mastication and to muscles in the floor of the mouth. 6 Abducens Primarily Motor fibers conduct impulses to muscles that move the eyes. motor Some sensory fibers conduct impulses associated with the condition of muscles. © McGraw Hill, LLC 99  Functions of the Cranial Nerves 2 TABLE 9.6 Functions of the Cranial Nerves Nerve Type Function 7 Facial Mixed Sensory fibers conduct impulses associated with taste receptors of the anterior tongue. Motor fibers conduct impulses to muscles of facial expression, tear glands, and salivary glands. 8 Vestibulocochlear Sensory Vestibular branch Sensory fibers conduct impulses associated with the sense of equilibrium. Cochlear branch Sensory fibers conduct impulses associated with the sense of hearing. 9 Glossopharyngeal Mixed Sensory fibers conduct impulses from the pharynx, tonsils, posterior tongue, and carotid arteries. Motor fibers conduct impulses to muscles of the pharynx used in swallowing and to salivary glands. 10 Vagus Mixed Somatic motor fibers conduct impulses to muscles associated with speech and swallowing; autonomic motor fibers conduct impulses to the heart, smooth muscle, and glands in the thorax and abdomen. Sensory fibers conduct impulses from the pharynx, larynx, esophagus, and viscera of the thorax and abdomen. 11 Accessory Primarily motor Cranial branch Motor fibers conduct impulses to muscles of the soft palate, pharynx, and larynx. Spinal branch Motor fibers conduct impulses to muscles of the neck and back. 12 Hypoglossal Primarily Motor fibers conduct impulses to muscles that move the tongue. motor Note: The cranial nerves described as primarily motor do have some sensory fibers associated with specialized receptors (proprioceptors) that give information about length and force of contraction of skeletal muscles. Because this information is part of motor control, these nerves are still considered motor nerves. © McGraw Hill, LLC 100  Spinal Nerves 31 pairs of spinal nerves arise from spinal cord. All except the first pair are mixed nerves. Grouped according to the level from which they arise Numbered in sequence: 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and 1 pair of coccygeal nerves Each arises from two roots: a sensory posterior root, and a motor anterior root. Each posterior root contains a posterior root ganglion, which houses the cell bodies of sensory neurons entering the spinal cord. The anterior and posterior roots unite to form a spinal nerve, which extends out of the vertebral canal through the intervertebral foramen. © McGraw Hill, LLC 101  Figure 9.36: Location and Branching of the Spinal Nerves Access the text alternative for these images © McGraw Hill, LLC 102  Spinal Nerve Plexuses The main branches of spinal nerves, except in the thoracic region, form networks called plexuses: Cervical Plexuses (C1 to C4): lie on either side of the neck; supply muscles and skin of the neck; include the phrenic nerves, which control the diaphragm. Brachial Plexuses (C5 to T1): arise from lower cervical and upper thoracic nerves; supply muscles and skin of arms, forearms, and hands; lead into the upper limbs; include the musculocutaneous, ulnar, median, radial, and axillary nerves. Lumbosacral Plexuses (L1 to S4): arise from the lower spinal cord; supply muscles and skin of the lower abdomen, external genitalia, buttocks, and legs; include the obturator, femoral, and sciatic nerves. Anterior branches of the thoracic spinal nerves do not form plexuses but become the intercostal nerves Plexuses serve to sort and recombine spinal nerve axons, so that axons derived from different spinal nerves extend to the same part of the body in the same peripheral nerve. © McGraw Hill, LLC 103  9.16: Autonomic Nervous System Autonomic nervous system (ANS): Portion of the PNS that functions constantly and independently, without conscious effort Controls visceral motor functions of smooth muscle, cardiac muscle, and glands Helps maintain homeostasis, responds to emotional stress, and prepares the body for strenuous activity Controls heart rate, blood pressure, breathing rate, body temperature © McGraw Hill, LLC 104  General Characteristics of the ANS Autonomic activities are regulated by reflexes that have sensory receptors in the viscera and skin. Impulses are conducted to the brain or spinal cord; then motor impulses travel through cranial and spinal nerves, then through ganglia, and finally to effectors (muscles or glands). Two divisions of the ANS, which exert opposing effects on target organs in many cases: Sympathetic division: active in conditions of stress or emergency (fight or flight) Parasympathetic division: active under normal, restful conditions (rest and digest) © McGraw Hill, LLC 105  Autonomic Neurons ANS neurons are all motor neurons. In the ANS, motor pathways consist of 2 neurons: A preganglionic neuron, that leaves the CNS, and synapses with one or more neurons, which have cell bodies in an autonomic ganglion in the PNS A postganglionic neuron, whose fiber (axon) leaves an autonomic ganglion, and innervates a visceral effector © McGraw Hill, LLC 106  Figure 9.37: Autonomic and Somatic Motor Pathways Access the text alternative for these images © McGraw Hill, LLC 107  Sympathetic Division Short preganglionic fibers in the sympathetic division arise from neurons in the gray matter in the thoracic and lumbar regions of the spinal cord (T1 to L2). Axons exit the spinal cord via ventral roots of spinal nerves. The axons then leave the spinal nerves, and proceed into the sympathetic (paravertebral) ganglia, a chain of sympathetic ganglia close to the vertebral column on each side. There they synapse with postganglionic neurons, whose long axons return to spinal nerves and then proceed to a visceral effector. Sometimes, preganglionic fibers pass right through the paravertebral ganglia, and synapse in collateral ganglia, closer to the target organs. © McGraw Hill, LLC 108  Figure 9.38: The Sympathetic Division of the ANS Access the text alternative for these images © McGraw Hill, LLC 109  Parasympathetic Division Long preganglionic fibers in the parasympathetic division arise from the brainstem and sacral region of the spinal cord. The preganglionic fibers extend outward in cranial or sacral nerves, and synapse in terminal ganglia close to or in visceral effector organs. Short postganglionic fibers continue into the effector organs (muscles or glands). © McGraw Hill, LLC 110  Figure 9.39: The Parasympathetic Division of the ANS Access the text alternative for these images © McGraw Hill, LLC 111  Autonomic Neurotransmitters Preganglionic fibers of sympathetic and parasympathetic divisions all release acetylcholine; they are called cholinergic fibers. Parasympathetic postganglionic fibers are cholinergic fibers and release acetylcholine. Most sympathetic postganglionic fibers release norepinephrine (noradrenalin); they are called adrenergic fibers. The effects of the 2 divisions of the ANS are often different, due to the effects of the different postganglionic neurotransmitters. Most organs receive innervation from both divisions, usually with opposing effects. Some effectors are innervated by only one division; example: blood vessels are only under sympathetic control. © McGraw Hill, LLC 112  Figure 9.40: Neurotransmitters of the Preganglionic and Postganglionic Fibers of the Autonomic Nervous System Access the text alternative for these images © McGraw Hill, LLC 113  Effects of Neurotransmitters on Effectors TABLE 9.7 Effects of Neurotransmitter Substances on Visceral Effectors or Actions Visceral Effector or Action Response to Adrenergic Response to Cholinergic Stimulation (Sympathetic) Stimulation (Parasympathetic) Pupil of the eye Dilation Constriction Heart rate Increases Decreases Bronchioles of lungs Dilation Constriction Muscle of intestinal wall Slows peristaltic action Speeds peristaltic action Intestinal glands Secretion decreases Secretion increases Blood distribution More blood to skeletal More blood to digestive organs; less muscles; less blood to blood to skeletal muscles digestive organs Blood glucose concentration Increases Decreases Salivary glands Secretion decreases Secretion increases Tear glands No action Secretion Muscle of gallbladder wall Relaxation Contraction Muscle of urinary bladder wall Relaxation Contraction © McGraw Hill, LLC 114  Control of Autonomic Activity The autonomic nervous system is mainly controlled by control centers in the brain and spinal cord. The limbic system and cerebral cortex alter the reactions of the autonomic nervous system through emotional influence. © McGraw Hill, LLC 115  Because learning changes everything. ® www.mheducation.com © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.