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Chapter 13: Central Nervous System COVERINGS OF THE BRAIN AND SPINAL CORD Two protective coverings (Figure 13-2) Outer covering is bone; cranial bones encase the brain and vertebrae encase the spinal cord (Figure 13-1) Inner covering is the meninges; the meninges of the cord continue inside the spinal cavity beyond the end of the spinal cord COVERINGS OF THE BRAIN AND SPINAL CORD: MENINGES Meninges have three membranous layers (Figure 13-3) Dura mater: strong, white, fibrous tissue; outer layer of meninges and inner periosteum of the cranial bones; has three important extensions • Falx cerebri Projects downward into the longitudinal fissure between the two cerebral hemispheres Dural sinuses: function as veins, collecting blood from brain tissues for return to the heart Superior sagittal sinus—one of several dural sinuses • Falx cerebelli: separates the two hemispheres of the cerebellum • Tentorium cerebelli: separates the cerebellum from the cerebrum COVERINGS OF THE BRAIN AND SPINAL CORD: MENINGES (cont.) Meninges have three membranous layers (cont.) Arachnoid mater: delicate, cobweblike layer between the dura mater and pia mater Pia mater: innermost, transparent layer; adheres to the outer surface of the brain and spinal cord; contains blood vessels; beyond the spinal cord, forms a slender filament called filum terminale Several spaces exist between and around the meninges • Epidural space: located between the dura mater and inside the bony covering of the spinal cord; contains a supporting cushion of fat and other connective tissues (virtually absent around brain because dura is continuous with periosteum of bone) • Subdural space: located between the dura mater and arachnoid mater; contains lubricating serous fluid • Subarachnoid space: located between the arachnoid and pia mater; contains a significant amount of cerebrospinal fluid (CSF) CEREBROSPINAL FLUID Functions Provides a supportive, protective cushion Reservoir of circulating fluid monitored by the brain to detect changes in the internal environment Fluid spaces CSF: found within the subarachnoid space around the brain and spinal cord and within the cavities and canals of the brain and spinal cord Ventricles: four fluid-filled spaces within the brain (Figure 13-4) • First and second ventricles (lateral): one located in each hemisphere of the cerebrum • Third ventricle: thin, vertical pocket of fluid below and medial to the lateral ventricles • Fourth ventricle: tiny, diamond-shaped space where the cerebellum attaches to the back of the brainstem CEREBROSPINAL FLUID (cont.) Formation and circulation of CSF (Figure 13-5) Occurs by separation of fluid from blood in the choroid plexuses • Fluid from the lateral ventricles seeps through the interventricular foramen (of Monro) into the third ventricle • From the third ventricle fluid goes through the cerebral aqueduct (of Sylvius) into the fourth ventricle • From the fourth ventricle fluid goes to two different areas Some fluid flows directly into the central canal of the spinal cord Some fluid leaves the fourth ventricle through openings in its roof and goes into the cisterna magna, a space that is continuous with the subarachnoid space • Fluid circulates in the subarachnoid space and then is absorbed into venous blood through the arachnoid villi SPINAL CORD Structure of the spinal cord (Figure 13-6) Lies within the spinal cavity and extends from the foramen magnum to the lower border of the first lumbar vertebra Oval-shaped cylinder that tapers slightly from above downward Two bulges, one in the cervical region and one in the lumbar region Anterior median fissure and posterior median sulcus are two deep grooves; anterior fissure is deeper and wider SPINAL CORD (cont.) Nerve roots • Fibers of dorsal nerve root Carry sensory information into the spinal canal Dorsal root ganglion: cell bodies of unipolar, sensory neurons make up a small region of gray matter in the dorsal nerve root • Fibers of ventral nerve root Carry motor information out of the spinal cord Cell bodies of multipolar motor neurons are in the gray matter of the spinal cord SPINAL CORD (cont.) Interneurons are located in the spinal cord’s gray matter core Spinal nerve: a single mixed nerve on each side of the spinal cord where the dorsal and ventral nerve roots join together Cauda equina: bundle of nerve roots extending (along with the filum terminale) from the conus medullaris (inferior end of spinal cord) (Figure 13-7) SPINAL CORD (cont.) Gray matter • Columns of gray matter extend the length of the cord • Consists predominantly of cell bodies of interneurons and motor neurons • In transverse section, looks like an H, with the limbs called the anterior, posterior, and lateral horns of gray matter; crossbar of H is the gray commissure SPINAL CORD (cont.) White matter • Surrounds the gray matter and is subdivided in each half on the cord into three funiculi: anterior, posterior, and lateral white columns • Each funiculus consists of a large bundle of axons divided into tracts • Names of spinal tracts indicate the location of the tract, the structure in which the axons originate, and the structure in which they terminate SPINAL CORD (cont.) Functions of the spinal cord Provides conduction routes to and from the brain • Ascending tracts conduct impulses up the cord to the brain • Descending tracts conduct impulses down the cord from the brain • Bundles of axons comprise all tracts SPINAL CORD (cont.) Provides conduction routes to and from the brain (cont.) • Tracts are both structural and functional organizations of nerve fibers Structural: all axons of any one tract originate in the same structure and terminate in the same structure Functional: all axons that comprise one tract serve one general function SPINAL CORD (cont.) Provides conduction routes to and from the brain (cont.) • Important ascending (sensory) tracts (Figure 13-8) Lateral spinothalamic tracts: crude touch, pain, and temperature Anterior spinothalamic tracts: crude touch, pressure Fasciculi gracilis and cuneatus: discriminating touch and conscious kinesthesia Spinocerebellar tracts: subconscious kinesthesia Spinotectal: touch SPINAL CORD (cont.) • Important descending (motor) tracts (Figure 13-8) Lateral corticospinal tracts: voluntary movements on opposite side of the body Anterior corticospinal tracts: voluntary movements on same side of body Reticulospinal tracts: maintain posture during movement Rubrospinal tracts: transmit impulses that coordinate body movements and maintenance of posture Tectospinal tracts: head and neck movements during visual reflexes Vestibulospinal tracts: coordination of posture and balance • Spinal cord: reflex center for all spinal reflexes, which are located in the gray matter of the cord BRAIN Structures of the brainstem (Figures 13-9 and 13-10) Medulla oblongata • Lowest part of the brainstem • Part of the brain that attaches to spinal cord; located just above the foramen magnum • A few centimeters in length and separated from the pons above by a horizontal groove • Composed of white matter and a network of gray and white matter called the reticular formation network • Pyramids: two bulges of white matter located on the ventral side of the medulla; formed by fibers of the pyramidal tracts • Olive: oval projection located lateral to the pyramids • Nuclei: clusters of neuron cell bodies located in the reticular formation BRAIN (cont.) Structures of the brainstem (cont.) Pons • Located above the medulla and below the midbrain • Composed of white matter and reticular formation Midbrain BRAIN (cont.) • Located above the pons and below the cerebrum; forms the midsection of the brain • Composed of white tracts and reticular formation • Extending divergently through the midbrain are cerebral peduncles, which conduct impulses between the midbrain and cerebrum • Corpora quadrigemina: landmark in midbrain Composed of two inferior colliculi and two superior colliculi Forms the posterior, upper part of the midbrain that lies just above the cerebellum Inferior colliculus contains auditory centers Superior colliculus contains visual centers Red nucleus and substantia nigra: clusters of cell bodies of neurons involved in muscular control BRAIN (cont.) Functions of the brainstem Performs sensory, motor, and reflex functions Spinothalamic tracts, fasciculi cuneatus and gracilis, spinoreticular tracts, corticospinal and reticulospinal tracts pass through brainstem Nuclei in medulla contain reflex centers • Of primary importance: cardiac, vasomotor, and respiratory centers • Nonvital reflexes: vomiting, coughing, sneezing, etc. Pons contains reflexes mediated by fifth, sixth, seventh, and eighth cranial nerves and pneumotaxic centers that help regulate respiration Midbrain contains centers for certain cranial nerve reflexes (visual and auditory) BRAIN (cont.) Structure of the cerebellum (Figure 13-11) Second largest part of the brain; contains more neurons than the rest of the nervous system Located just below the posterior portion of the cerebrum; transverse fissure separates these two parts of the brain Gray matter makes up the cortex and white matter predominates in the interior Arbor vitae: internal white matter of the cerebellum; distinctive pattern similar to the veins of a leaf Cerebellum has numerous sulci and delicate, parallel gyri (folia) Consists of the cerebellar hemispheres and the vermis BRAIN (cont.) Structure of the cerebellum (cont.) Internal white matter: composed of short and long tracts • Shorter tracts conduct impulses within the cerebellum • Longer tracts conduct impulses to and from the cerebellum; fibers enter or leave by way of three pairs of peduncles Inferior cerebellar peduncles: composed chiefly of tracts into the cerebellum from the medulla and cord Middle cerebellar peduncles: composed almost entirely of tracts into the cerebellum from the pons Superior cerebellar peduncles: composed principally of tracts from dentate nuclei in the cerebellum through the red nucleus of the midbrain to the thalamus BRAIN (cont.) Structure of the cerebellum (cont.) Dentate nuclei • Important pair of cerebellar nuclei, one of which is located in each hemisphere • Nuclei connected with thalamus and motor areas of the cerebral cortex by tracts • Through the tracts, cerebellar impulses influence the motor cortex and the motor cortex influences the cerebellum Functions of the Cerebellum Provides involuntary coordination of body movements Helps control posture Controls balance and allows body movements to be smooth and coordinated BRAIN: DIENCEPHALON Diencephalon (Figure 13-13) Located between the cerebrum and the midbrain Consists of several structures located around the third ventricle: thalamus, hypothalamus, optic chiasma, pineal gland, and several others Thalamus • Dumbbell-shaped mass of gray matter composed of many nuclei • Each lateral mass forms one lateral wall of the third ventricle • Intermediate mass extends through the third ventricle and joins the two lateral masses • Geniculate bodies: two of the most important groups of nuclei comprising the thalamus; play role in processing auditory and visual input • Serves as a major relay station for sensory impulses on their way to the cerebral cortex BRAIN: DIENCEPHALON (cont.) Thalamus (cont.) • Plays two parts in mechanism responsible for sensations Impulses produce conscious recognition of the crude, less-critical sensations of pain, temperature, and touch Neurons relay all kinds of sensory impulses, except possibly olfactory, to the cerebrum • Plays part in the mechanism responsible for emotions by associating sensory impulses with feeling of pleasantness and unpleasantness • Plays part in arousal mechanism • Plays part in mechanisms that produce complex reflex movements BRAIN: DIENCEPHALON (cont.) Hypothalamus • Consists of several structures that lie beneath the thalamus • Forms floor of the third ventricle and lower part of lateral walls • Prominent structures found in the hypothalamus Supraoptic nuclei Paraventricular nuclei Mamillary bodies: posterior part of hypothalamus, involved with olfactory sense • Infundibulum: the stalk leading to the posterior lobe of the pituitary gland • Small but functionally important area of the brain; performs many functions of greatest importance for survival and enjoyment • Links mind and body • Links nervous system to endocrine system BRAIN: DIENCEPHALON (cont.) Hypothalamus (cont.) • Summary of hypothalamic functions Regulator and coordinator of autonomic activities Major relay station between the cerebral cortex and lower autonomic centers; crucial part of the route by which emotions express themselves in changed bodily functions Synthesizes hormones secreted by posterior pituitary and plays an essential role in maintaining water balance Some neurons function as endocrine glands Plays crucial role in arousal mechanism Crucial part of mechanism regulating appetite Crucial part of mechanism maintaining normal body temperature BRAIN: DIENCEPHALON (cont.) Pineal gland • Located just above the corpora quadrigemina of the midbrain • Involved in regulating the body’s biological clock (Figure 13-14) • Produces melatonin as a “timekeeping” hormone Melatonin is made from the neurotransmitter serotonin Levels increase when sunlight is absent and decrease when sunlight is present, thus regulating the circadian (daily) biologic clock (Figure 13-15) Melatonin is the “sleep hormone” BRAIN: CEREBRUM Structure of the cerebrum Cerebral cortex • Largest and uppermost division of the brain; consists of right and left cerebral hemispheres, each divided into five lobes (Figure 13-16) Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula (island of Reil) • Cerebral cortex: outer surface composed of six layers of gray matter • Gyri: convolutions; some are named: precentral gyrus, postcentral gyrus, cingulate gyrus, and hippocampal gyrus • Sulci: shallow grooves BRAIN: CEREBRUM (cont.) Cerebral cortex (cont.) • Fissures: deeper grooves that divide each cerebral hemisphere into lobes; four are prominent Longitudinal fissure: deepest fissure; divides cerebrum into two hemispheres Central sulcus (fissure of Rolando): groove between frontal and parietal lobes Lateral fissure (fissure of Sylvius): groove between temporal lobe below and parietal lobes above; island of Reil lies deep in lateral fissure Parietooccipital fissure: groove that separates occipital lobe from parietal lobes BRAIN: CEREBRUM (cont.) Cerebral tracts and basal nuclei • Basal nuclei Structure: islands of gray matter located deep inside the white matter of each hemisphere (Figure 13-18); include the following: • Caudate nucleus • Lentiform nucleus: consists of putamen and pallidum • Amygdaloid nucleus Function: regulation of voluntary (conscious) motor control related to posture, walking, and other repetitive movements; possible roles in thinking and learning BRAIN: CEREBRUM (cont.) Cerebral tracts and basal nuclei (cont.) • Three types of cerebral tracts make up cerebrum’s white matter (Figure 13-17) Projection tracts: extensions of the sensory spinothalamic tracts and motor corticospinal tracts Association tracts: most numerous cerebral tracts; extend from one convolution to another in the same hemisphere Commissural tracts: extend from one convolution to a corresponding convolution in the other hemisphere; comprise the corpus callosum and anterior and posterior commissure BRAIN: CEREBRAL CORTEX Functions of the cerebral cortex Certain areas of the cerebral cortex engage in predominantly one particular function (Figures 13-19 and 13-20) • Postcentral gyrus: mainly general somatic sensory area; receives impulses from receptors activated by heat, cold, and touch stimuli • Precentral gyrus: chiefly somatic motor area; impulses from neurons in this area descend over motor tracts and stimulate skeletal muscles • Transverse gyrus: primary auditory area • Occipital lobe: primary visual area BRAIN: CEREBRAL CORTEX (cont.) Sensory functions of the cortex • Somatic senses: sensations of touch, pressure, temperature, proprioception, and similar perceptions that require complex sensory organs • Cortex contains a somatic sensory map of the body • Information sent to primary sensory areas is relayed to sensory association areas and other parts of the brain • The sensory information is compared and evaluated, and the cortex integrates separate bits of information into whole perceptions BRAIN: CEREBRAL CORTEX (cont.) Motor functions of the cortex • For normal movements to occur, many parts of the nervous system must function • Precentral gyrus: primary somatic motor area; controls individual muscles • Secondary motor area: in the gyrus immediately anterior to the precentral gyrus; activates groups of muscles simultaneously Integrative functions of the cortex CONSCIOUSNESS State of awareness of one’s self, one’s environment, and other human beings (Figure 13-21) Depends on excitation of cortical neurons by impulses conducted to them by the reticular activating system Two current concepts about the reticular activating system Functions as arousal system for the cerebral cortex Functioning is crucial for maintaining consciousness LANGUAGE Ability to speak and write words and understand spoken and written words Speech centers: areas in the frontal, parietal, and temporal lobes Left cerebral hemisphere contains speech centers in approximately 90% of the population; contained in either the right hemisphere or both in the remaining 10% Aphasias: lesions in speech centers EMOTIONS Subjective experiencing and objective expressing of emotions involve functioning of the limbic system (Figure 13-22) Limbic system: also known as the emotional brain Most structures of limbic system lie on the medial surface of the cerebrum (cingulate gyrus and hippocampus) Have primary connections with other parts of the brain, such as the thalamus, fornix, septal nuclei, amygdaloid nucleus, and hypothalamus MEMORY A major mental activity Cortex is capable of storing and retrieving both short- and long-term memory Temporal, parietal, and occipital lobes are among the areas responsible for shortand long-term memory Structural changes in the neural pathways of the cerebral cortex store long-term memories Limbic system plays a key role in memory SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEM For the cerebral cortex to perform its sensory functions, impulses must first be conducted to the sensory areas by sensory pathways (Figure 13-23) Three main pools of sensory neurons Primary sensory neurons conduct impulses from the periphery to the central nervous system Secondary sensory neurons • Conduct impulses from the cord or brainstem to the thalamus • Dendrites and cell bodies are located in the gray matter of the cord and brainstem • Axons ascend in tracts up the cord and through the brainstem, terminating in the thalamus, where they synapse with dendrites or cell bodies of tertiary sensory neurons SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEM (cont.) Three main pools of sensory neurons (cont.) Tertiary sensory neurons • Conduct impulses from thalamus to the postcentral gyrus of the parietal lobe • Bundle of axons of tertiary sensory neurons form the thalamocortical tracts • Extend through the internal capsule to the cerebral cortex Sensory pathways to the cerebral cortex are crossed SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEM (cont.) Two sensory pathways conduct impulses that produce sensations of touch and pressure Medial lemniscal system • Consists of tracts that make up the fasciculi cuneatus and gracilis, and the medial lemniscus • Axons of secondary sensory neurons make up medial lemniscus • Functions: transmit impulses that produce discriminating touch and pressure sensations and kinesthesia Spinothalamic pathway: functions are crude touch and pressure sensation SOMATIC SENSORY PATHWAYS IN THE CENTRAL NERVOUS SYSTEM (cont.) For the cerebral cortex to perform its motor functions, impulses are conducted from its motor areas to skeletal muscles by somatic motor pathways Consist of motor neurons that conduct impulses from the central nervous system to skeletal muscles; some motor pathways are extremely complex and others are very simple SOMATIC MOTOR PATHWAYS IN THE CENTRAL NERVOUS SYSTEM Two methods used to classify somatic motor pathways: pyramidal and extrapyramidal tracts (Figure 13-24) Pyramidal tracts: also known as corticospinal tracts • Approximately three quarters of the fibers decussate in the medulla and extend down the cord in the crossed corticospinal tract located on the opposite side of the spinal cord in the lateral white column • Approximately one quarter of the fibers do not decussate but extend down the same side of the spinal cord as the cerebral area from which they came SOMATIC MOTOR PATHWAYS IN THE CENTRAL NERVOUS SYSTEM Extrapyramidal tracts: much more complex than pyramidal tracts • Consist of all motor tracts from the brain to the spinal cord anterior horn motor neurons except the corticospinal tracts • Within the brain, consist of numerous relays of motor neurons between motor areas of the cortex, basal nuclei, thalamus, cerebellum, and brainstem • Within the spinal cord, some important tracts are the reticulospinal tracts • Conduction by extrapyramidal tracts plays a crucial part in producing large, automatic movements • Conduction by extrapyramidal tracts plays an important part in emotional expressions • Motor program: set of coordinated commands that control the programmed motor activity mediated by extrapyramidal pathways (Figure 13-25)