CHAPTER 12: THE CENTRAL NERVOUS SYSTEM MODULE 12.1 OVERVIEW OF THE CENTRAL NERVOUS SYSTEM

Transcription

1 CHAPTER 12: THE CENTRAL NERVOUS SYSTEM MODULE 12.1 OVERVIEW OF THE CENTRAL NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM Central nervous system (CNS) includes brain and spinal cord; involved in movement, interpreting sensory information, maintaining homeostasis, and functions relating to mind OVERVIEW OF CNS FUNCTIONS Functions of nervous system can be broken down into three categories: Motor functions include stimulation of a muscle cell contraction or a gland secretion; function of peripheral nervous system (PNS) Sensory functions detection of sensations within and outside body; also is a function of PNS OVERVIEW OF CNS FUNCTIONS Functions of nervous system (continued): Integrative functions include decision-making processes; exclusive function of CNS; includes a wide variety of functions: o Interpretation of sensory information o Planning and monitoring movement o Maintenance of homeostasis o Higher mental functions such as language and learning BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Brain soft, whitish-gray organ, anatomically continuous with spinal cord; resides in cranial cavity and directly or indirectly controls most of body s functions Weighs between 1250 and 1450 grams; made of mostly nervous tissue; contains epithelial and connective tissues as well Internal cavities called ventricles; filled with cerebrospinal fluid Receives about 20% of total blood flow during periods of rest; reflects its requirements for huge amounts of oxygen, glucose, and nutrients BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Brain consists of four divisions, each distinct in type of input it receives and where it sends its output: Cerebrum Diencephalon Cerebellum Brainstem 1

2 BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Cerebrum enlarged superior portion of brain; divided into left and right cerebral hemispheres Each cerebral hemisphere is further divided into five lobes containing groups of neurons that perform specific tasks Responsible for higher mental function such as learning, memory, personality, cognition (thinking), language, and conscience Performs major roles in sensation and movement as well BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Diencephalon deep underneath cerebral hemispheres; central core of brain Consists of four distinct structural and functional parts Responsible for processing, integrating, and relaying information to different parts of brain, homeostatic functions, regulation of movement, and biological rhythms BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Cerebellum posterior and inferior portion of brain Divided into left and right hemispheres Heavily involved in planning and coordination of movement, especially complex activities such as playing a sport or an instrument BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Brainstem connects brain to spinal cord Involved in basic involuntary homeostatic functions Control of certain reflexes Monitoring movement Integrating and relaying information to other parts of nervous system BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Spinal cord long tubular organ enclosed within protective vertebral cavity; blends with inferior portion of brainstem; ends between first and second lumbar vertebrae cm (17 18 inches) in length and only ranges from cm ( inches) in diameter Central canal an internal cavity within spinal cord that is continuous with brain s ventricles; filled with cerebrospinal fluid BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD White matter found in both brain and spinal cord; consists of myelinated axons Each lobe of cerebrum contains bundles of white matter called tracts; receives input from and sends output to clusters of cell bodies and dendrites in cerebral gray matter called nuclei (Figure 12.2a) Spinal cord contains white matter tracts that shuttle information processed by nuclei in spinal gray matter (Figure 12.2b) 2

3 BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Gray matter found in both brain and spinal cord; consists of neuron cell bodies, dendrites, and unmyelinated axons Outer few millimeters of cerebrum is gray matter; deeper portions of brain are mostly white matter with some gray matter scattered throughout Spinal cord is mostly gray matter that processes information (in cord center); surrounded by tracts of white matter (outside); relays information to and from brain BASIC STRUCTURE OF THE BRAIN AND SPINAL CORD Communication between gray and white matter connects different regions of brain and spinal cord with one another; myelinated axons enable near instantaneous communication between locations Make note organization of gray and white matter in brain and spinal cord is reversed; spinal white matter is superficial while it is deep in brain OVERVIEW OF CNS DEVELOPMENT Brain and spinal cord develop from a tube with an enlarged end in embryo and fetus (Figure 12.3) Neural tube hollow tube from which nervous tissue develops; completely developed by fourth week of gestation Caudal end of neural tube forms spinal cord; cranial end forms three saclike structures (primary brain vesicles) which include forebrain, midbrain, and hindbrain Cavity within hollow neural tube becomes ventricles in brain and central canal in spinal cord OVERVIEW OF CNS DEVELOPMENT Primary brain vesicles give rise to five secondary brain vesicles by fifth week of development Secondary brain vesicles create four divisions of mature brain: Forebrain expands into two secondary brain vesicles (telencephalon and diencephalon); two lobes of telencephalon become cerebral hemispheres; diencephalon retains its name in mature brain OVERVIEW OF CNS DEVELOPMENT Secondary brain vesicles create four divisions of mature brain (continued): Midbrain expands into secondary brain vesicle called mesencephalon; develops into mature midbrain Hindbrain develops into two secondary brain vesicles (metencephalon and myelencephalon), both of which develop into remainder of brainstem; metencephalon also matures to become cerebellum 3

4 MODULE 12.2 THE BRAIN THE CEREBRUM Cerebrum structure responsible for higher mental functions (Figures , Table 12.1) Gross anatomical features of cerebrum include: o Sulci shallow grooves on surface of cerebrum; gyri elevated ridges found between sulci; together increase surface area of brain; maximizing limited space within confines of skull; example of Structure-Function Core Principle THE CEREBRUM Gross anatomical features (continued): o Fissures deep grooves found on surface of cerebrum o Longitudinal fissure long deep groove that separates left and right cerebral hemispheres o A cavity is found deep within each cerebral hemisphere; right hemisphere surrounds right lateral ventricle; left hemisphere surrounds left lateral ventricle THE CEREBRUM Five lobes are found in each hemisphere of cerebrum (Figure 12.4): Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula THE CEREBRUM Five lobes of cerebrum (continued): Frontal lobes most anterior lobes o Posterior border called central sulcus; sits just behind precentral gyrus o Neurons in these lobes are responsible for planning and executing movement and complex mental functions such as behavior, conscience, and personality THE CEREBRUM Five lobes of cerebrum (continued): Parietal lobes just posterior to frontal lobes o Contains postcentral gyrus posterior to central sulcus o Neurons in these lobes are responsible for processing and integrating sensory information and function in attention 4

5 THE CEREBRUM Five lobes of cerebrum (continued): Temporal lobes form lateral surfaces of each cerebral hemisphere o Separated from parietal and frontal lobes by lateral fissure o Neurons in these lobes are involved in hearing, language, memory, and emotions THE CEREBRUM Five lobes of cerebrum (continued): Occipital lobes make up posterior aspect of each cerebral hemisphere o Separated from parietal lobe by parieto-occipital sulcus o Neurons in these lobes process all information related to vision THE CEREBRUM Five lobes of cerebrum (continued): Insulas deep underneath lateral fissures; neurons in these lobes are currently thought to be involved in functions related to taste and viscera (internal organs) THE CEREBRUM-GRAY MATTER Gray Matter: Cerebral Cortex functionally most complex part of cortex; covers underlying cerebral hemispheres Most of cerebral cortex is neocortex (most recently evolved region of brain); has a huge surface area Composed of 6 layers (of neurons and neuroglia) of variable widths (Figure 12.5) All neurons in cortex are interneurons THE CEREBRUM-GRAY MATTER Gray Matter: Cerebral Cortex (continued): Functions of neocortex revolve around conscious processes such as planning movement, interpreting incoming sensory information, and complex higher functions Neocortex is divided into three areas: primary motor cortex, primary sensory cortices, and association areas (next slide) THE CEREBRUM-GRAY MATTER Gray Matter: Cerebral Cortex (continued): Neocortex is divided into three areas: primary motor cortex, primary sensory cortices, and association areas (continued): o Primary motor cortex plans and executes movement o Primary sensory cortices first regions to receive and process sensory input o Association areas integrate different types of information: Unimodal areas integrate one specific type of information Multimodal areas integrate information from multiple different sources and carry out many higher mental functions 5

6 THE CEREBRUM-GRAY MATTER Motor areas most are located in frontal lobe; contain upper motor neurons which are interneurons that connect to other neurons (not skeletal muscle) Primary motor cortex; involved in conscious planning of movement; located in precentral gyrus of frontal lobe Upper motor neurons of each cerebral hemisphere control motor activity of opposite side of body via PNS neurons called lower motor neurons; execute order to move THE CEREBRUM-GRAY MATTER Movement requires input from many motor association areas such as large premotor cortex located anterior to primary motor cortex Motor association areas are unimodal areas involved in planning, guidance, coordination, and execution of movement Frontal eye fields paired motor association areas; one on each side of brain anterior to premotor cortex; involved in back and forth eye movements as in reading THE CEREBRUM-GRAY MATTER Sensory Cortices Two main somatosensory areas in cerebral cortex; deal with somatic senses; information about temperature, touch, vibration, pressure, stretch, and joint position Primary somatosensory area (S1) in postcentral gyrus of parietal lobe Somatosensory association cortex (S2) posterior to S1 THE CEREBRUM-GRAY MATTER Sensory Cortices (continued): Special senses touch, vision, hearing, smell, and taste each have a primary and a unimodal association area as does sense of equilibrium (balance); found in all lobes of cortex except frontal lobe Primary visual cortex at posterior end of occipital lobe; first area to receive visual input; transferred to visual association area which processes color, object movement, and depth THE CEREBRUM-GRAY MATTER Sensory Cortices (continued): Special senses (continued): Primary auditory cortex in superior temporal lobe; first to receive auditory information; input is transferred to nearby auditory association cortex and other multimodal association areas for further processing THE CEREBRUM-GRAY MATTER Sensory Cortices (continued): Special senses (continued): Gustatory cortex taste information processing; scattered throughout both insula and parietal lobes Vestibular areas deal with equilibrium and positional sensations; located in parietal and temporal lobes 6

7 THE CEREBRUM-GRAY MATTER Sensory Cortices (continued): Special senses (continued): Olfactory cortex processes sense of smell; in evolutionarily older regions of brain; consists of several areas in limbic and medial temporal lobes THE CEREBRUM-GRAY MATTER Multimodal association areas regions of cortex that allow us to perform complex mental functions: Language processed in two areas of cortex: Broca s area in anterolateral frontal lobe; premotor area responsible for ability to produce speech sounds Wernicke s area (integrative speech area) in temporal and parietal lobes; responsible for ability to understand language THE CEREBRUM-GRAY MATTER Multimodal association areas (continued): Prefrontal cortex occupies most of frontal lobe; communicates with diencephalon, other regions of cerebral gray matter, and association areas located in other lobes; many functions including modulating behavior, personality, learning, memory, and an individual s personality state THE CEREBRUM-GRAY MATTER Multimodal association areas (continued): Parietal and temporal association areas occupy most of their respective lobes; perform multiple functions including integration of sensory information, language, maintaining attention, recognition, and spatial awareness THE CEREBRUM-GRAY MATTER Basal nuclei, found deep within each cerebral hemisphere; cluster of neuron cell bodies, involved in movement; separated from diencephalon by a region of white matter called internal capsule; includes (Figure 12.6): Caudate nuclei Putamen Globus pallidus THE CEREBRUM-GRAY MATTER Basal nuclei (continued): Caudate nuclei C-shaped rings of gray matter; lateral to lateral ventricle of each hemisphere with anteriorly oriented tail Putamen posterior and inferior to caudate nucleus; connected to caudate nucleus by small bridges of gray matter; combination of putamen and caudate are sometimes called corpus striatum Globus pallidus sits medial to putamen; contains more myelinated fibers than other regions 7

8 THE CEREBRUM-WHITE MATTER Cerebral white matter can be classified as one of three types (Figure 12.7): Commissural fibers connect right and left hemispheres; corpus callosum, largest of four groups in this category, lies in middle of brain at base of longitudinal fissure Projection fibers connect cerebral cortex of one hemisphere with other areas of same hemisphere, other parts of brain, and spinal cord; corona radiata are fibers that spread out in a radiating pattern; condense around diencephalon to form two V-shaped bands called internal capsules Association fibers restricted to a single hemisphere; connect gray matter of cortical gyri with one another THE CEREBRUM-LIMBIC SYSTEM Possible pathway for information transferred by conduction of an action potential from one region of brain to another (Figure 12.8): 1. Action potential originates in gray matter 2. Action potential is sent to another area of gray matter by projection fibers 3. Second (new) action potential is generated by gray matter; spreads to neighboring gray matter by association fibers 4. Lastly, a third action potential is generated; can be sent to other cerebral hemisphere by commissural fibers THE CEREBRUM-LIMBIC SYSTEM Limbic system important functional brain system, includes limbic lobe (region of medial cerebrum), hippocampus, amygdala, and pathways; connect each of these regions of gray matter with rest of brain (Figure 12.9) Found only within mammalian brains Involved in memory, learning, emotion, and behavior THE CEREBRUM-LIMBIC SYSTEM Limbic system (continued): Limbic lobe and associated structures form a ring on medial side of cerebral hemisphere; contain two main gyri: cingulate gyrus and parahippocampal gyrus Hippocampus in temporal lobe; connected to a prominent C-shaped ring of white matter (fornix) which is its main output tract; involved in memory and learning Amygdala anterior to hippocampus; involved in behavior and expression of emotion, especially fear 8

9 THE DIENCEPHALON Diencephalon at physical center of brain; composed of four components, each with its own nuclei that receive specific input and send output to other brain regions (Figure 12.10): Thalamus Hypothalamus Epithalamus Subthalamus THE DIENCEPHALON Thalamus main entry route of sensory data into cerebral cortex (Figure 12.10a, b) Consists of two egg-shaped regions of gray matter; make up about 80% of diencephalon Third ventricle is found between these two regions Thalamic nuclei receive afferent fibers from many other regions of nervous system excluding information about the sense of smell THE DIENCEPHALON Thalamus (continued): Regulates cortical activity by controlling which input should continue to cerebral cortex Each half of thalamus has three main groups of nuclei separated by thin layers of white matter Specific nuclei function as relay stations that receive input, integrate information, then send information to specific motor or sensory areas in cerebral cortex THE DIENCEPHALON Thalamus (continued): Association nuclei o Receive input from variety of sources o Process information related to emotions, memory, and integration of sensory information o Processed information is sent on to appropriate association areas of cerebral cortex THE DIENCEPHALON Thalamus (continued): Nonspecific nuclei o Receive information from basal nuclei, cerebellum, and motor cortex o Send information to a wide range of locations including cerebral cortex and other brain regions o Involved in controlling arousal, consciousness, and level of responsiveness and excitability of cerebral cortex 9

10 THE DIENCEPHALON Hypothalamus collection of nuclei anterior and inferior to larger thalamus Neurons perform several vital functions critical to survival; include regulation of autonomic nervous system, sleep/wake cycle, thirst and hunger, and body temperature THE DIENCEPHALON Hypothalamus (continued): Inferior hypothalamus anatomically and functionally linked to pituitary gland by an extension called infundibulum; hypothalamic tissue makes up posterior portion of this endocrine gland Hypothalamus secretes a number of different releasing and inhibiting hormones; affect function of pituitary gland; in turn, pituitary gland secretes hormones that affect activities of other endocrine glands throughout body THE DIENCEPHALON Hypothalamus (continued): Antidiuretic hormone and oxytocin, hypothalamic hormones that do not affect pituitary gland, have their effect on water balance and stimulation of uterine contraction during childbirth, respectively Input to hypothalamus arrives from many sources including cortex and basal nuclei THE DIENCEPHALON Hypothalamus (continued): Mammillary bodies connect hypothalamus with limbic system; receive input from hippocampus; involved in memory regulation and behavior Input from outside nervous system; endocrine system (among others) provides information from receptors that detect changes in body temperature and receptors that detect changes in osmotic concentration of blood THE DIENCEPHALON Epithalamus superior to thalamus; most of its posterosuperior bulk is an endocrine gland called pineal gland; secretes melatonin; hormone involved in sleep/wake cycle Subthalamus inferior to thalamus; functionally connected with basal nuclei; together, they control movement CEREBELLUM Cerebellum makes up posterior and inferior portion of brain; functionally connected with cerebral cortex, basal nuclei, brainstem, and spinal cord; interactions between these regions together coordinate movement (Figure 12.11) Anatomically, divided into two cerebellar hemispheres connected by structure called vermis (Figure 12.11a) Ridges called folia cover exterior cerebellar surface; separated by shallow sulci; increases surface area of region 10

11 CEREBELLUM Cerebellum (continued): Divided into three lobes: anterior, posterior, and flocculonodular lobes (Figure 12.11b) Cerebellar cortex outer layer of gray matter Cortex is extremely folded and branching white matter is called arbor vitae because it resembles tree branches CEREBELLUM Cerebellum (continued): Inner white matter contains clusters of gray matter (deep cerebellar nuclei) scattered throughout White matter converges into three large tracts called cerebellar peduncles; only connection between cerebellum and brainstem THE BRAINSTEM Brainstem one of oldest components of brain; vital to our immediate survival as its nuclei control many basic homeostatic functions such as heart rate and breathing rhythms (Figures ) Controls many reflexes (programmed, automatic responses to stimuli); functions in movement, sensation, and maintaining alertness THE BRAINSTEM Brainstem (continued): Located inferior to diencephalon, anterior to cerebellum, and superior to spinal cord, on midsagittal section of brain (Figure 12.12) Extends to level of foramen magnum; along with fourth ventricle (deep within brainstem), continuous with spinal cord and its central canal THE BRAINSTEM Brainstem (continued): Three subdivisions; superior midbrain, middle pons, and inferior medulla oblongata, where following structures reside: Fibers of cerebellum and related nuclei travel through portions of brainstem where they either synapse with nuclei found there or progress on to other destinations in cerebral cortex or spinal cord THE BRAINSTEM Brainstem (continued): Nuclei of reticular formation group of connected nuclei scattered throughout brainstem, many functions including regulation of respiration, blood pressure, sleep/wake cycle, pain perception, and consciousness Tracts of white matter between spinal cord and brain nearly all pathways to and from brain and spinal cord travel through brainstem Cranial nerve nuclei many cranial nerves originate in brainstem; their nuclei have many sensory, motor, and autonomic responsibilities 11

12 THE BRAINSTEM Midbrain Inferior to diencephalon; surrounds cerebral aqueduct (connects third and fourth ventricles) (Figure 12.14b) Also known as mesencephalon; shortest and most superior brainstem region THE BRAINSTEM Midbrain (continued): Includes following structures: o Superior and inferior colliculi, protrude from posterior surface of brainstem; two paired projections that form roof of midbrain (tectum); involved in visual and auditory functions respectively; project to thalamus o Descending tracts white matter tracts that originate in cerebrum and form anteriormost portion of midbrain; called crus cerebri THE BRAINSTEM Midbrain (continued): Includes following structures (continued): o Substantia nigra posterior to crus cerebri, is a darkly pigmented region whose neurons work with basal nuclei to control movement o Red nucleus posterior to substantia nigra; communicates with cerebellum, spinal cord, and other regions involved in regulating movement; role in humans not yet understood THE BRAINSTEM Midbrain (continued): Includes following structures (continued): o Many cranial nerve nuclei are found in midbrain o Midbrain tegmentum region of midbrain between cerebral aqueduct and substantia nigra; contains numerous nuclei, many of which are components of reticular formation; both ascending and descending white matter tracts are found in this region as well THE BRAINSTEM Pons inferior to midbrain; has a prominent anterior surface that contains descending motor tracts from crus cerebri, some of which pass through pons en route to spinal cord Other tracts enter cerebellum by way of middle cerebellar peduncle Reticular formation and cranial nerve nuclei are located posterior to these tracts THE BRAINSTEM Pons (continued): Pontine tegmentum surrounded by middle cerebellar peduncles Pontine nuclei have many roles including: regulation of movement, breathing, reflexes, and complex functions associated with sleep and arousal 12

13 THE BRAINSTEM Medulla oblongata most inferior structure of brainstem; continuous with spinal cord at foramen magnum (Figure 12.14c) Pyramids on anterior surface of medulla, contain upper motor neuron fibers of corticospinal tract (also called the pyramidal tract) as they travel from cerebral cortex to spinal cord THE BRAINSTEM Medulla oblongata (continued): Right and left corticospinal fibers decussate (crossover) within pyramids; motor fibers originating from right side of cerebral cortex descend through left side of spinal cord and vice versa Posterior columns paired tracts of white matter found on medulla s posterior surface; carry sensory information from spinal cord to nucleus gracilis and nucleus cuneatus THE BRAINSTEM Medulla oblongata (continued): Posterior columns also decussate within medulla so sensory information from right side of spinal cord is processed by left side of cerebral cortex and vice versa Olive lateral to each pyramid; protuberance that contains inferior olivary nucleus; receives sensory fibers from spinal cord and directs information to cerebellum Several cranial nerve and reticular formation nuclei are found in medulla LOCKED-IN SYNDROME Caused by damage to motor tracts of pons; cerebral cortex is unable to communicate with spinal cord; no purposeful movement is possible, but sensory pathways to brain remain intact (as do other cortical functions) Patients are therefore literally locked in their bodies; yet fully aware of everything going on around them; some can communicate using eye movements controlled by midbrain Small number of patients recover some motor functions but most do not; overall prognosis is poor; most succumb to infections or other paralytic complications THE BRAINSTEM Reticular formation collection of over 100 nuclei found in central core of three brainstem subdivisions making this one of most complex regions of brain (Figure 12.15) Input is received from multiple sources including: cerebral cortex, limbic system, and sensory stimuli Output is sent throughout entire brain and spinal cord 13

14 THE BRAINSTEM Reticular formation (continued): Central nuclei (center of reticular formation) function in sleep, pain transmission, and mood Nuclei surrounding central nuclei serve motor functions for both skeletal muscles and autonomic nervous system Other nuclei are instrumental in homeostasis of breathing and blood pressure Lateral nuclei play a role in sensation and in alertness and activity levels of cerebral cortex MODULE 12.3 PROTECTION OF THE BRAIN BRAIN PROTECTION Three features within protective shell of skull provide additional shelter for delicate brain tissue: Cranial meninges three layers of membranes that surround brain Cerebrospinal fluid (CSF) fluid that bathes brain and fills cavities Blood-brain barrier prevents many substances from entering brain and its cells from blood BRAIN PROTECTION Cranial meninges composed of three protective membrane layers of mostly dense irregular collagenous tissue Structural arrangement from superficial to deep: epidural space, dura mater, subdural space, arachnoid mater, subarachnoid space, and pia mater (Figure 12.18) BRAIN PROTECTION Cranial meninges (continued): Epidural space between inner surface of cranial bones and outer surface of dura mater; only a potential space as dura is normally tightly bound to bone only allowing for passage of blood vessels Dura mater (dura) outermost meninx; thickest and most durable of three meningeal layers; double-layered membrane composed mostly of collagen fibers with few elastic fibers BRAIN PROTECTION Cranial meninges (continued): Dura mater (dura) (continued): o Two layers of dura (Figure 12.18a): Periosteal dura outer layer; attached to inner surface of bones of cranial cavity; functions as periosteum with extensive blood supply in epidural space Meningeal dura inner layer; avascular and lies superficial to arachnoid mater 14

15 Mostly fused, creating a single inelastic membrane except in regions where cavities called dural sinuses are found BRAIN PROTECTION Cranial meninges (continued): Dura mater (dura) (continued): o Dural sinuses venous channels; drain CSF and deoxygenated blood from brain s extensive network of veins; also found where meningeal dura folds over itself and courses between structures in brain BRAIN PROTECTION Cranial meninges (continued): Dura mater (dura) (continued): o Dural folds include falx cerebri, tentorium cerebelli, and falx cerebelli (Figure 12.14b) Falx cerebri within longitudinal fissure; forms partition between left and right cerebral hemispheres; superior sagittal sinus (large dural sinus) found superior to falx cerebri Tentorium cerebelli partition between cerebellum and occipital lobe of cerebrum Falx cerebelli partition between left and right hemispheres of cerebellum BRAIN PROTECTION Cranial meninges (continued): Subdural space serous fluid-filled space; found deep to dura mater and superficial to arachnoid mater; houses veins that drain blood from brain Arachnoid mater second meningeal layer deep to subdural space; thin weblike membrane composed of dense irregular collagenous tissue with some degree of elasticity (Figure 12.18c) BRAIN PROTECTION Cranial meninges (continued): Arachnoid mater (continued): o Arachnoid trabeculae composed of collagen fiber bundles and fibroblasts; anchor arachnoid to deep pia mater o Arachnoid granulations (villi) small bundles of arachnoid; project superficially through meningeal dura into the dural sinuses; allow for return of CSF to bloodstream BRAIN PROTECTION Cranial meninges (continued): Subarachnoid space found deep to arachnoid mater and superficial to pia mater; contains major blood vessels of brain; filled with CSF Pia mater deepest meningeal layer; only meninx in physical contact with brain 15

16 tissue BRAIN PROTECTION Cranial meninges (continued): Pia mater (continued): o Follows contour of brain, covering delicate tissue of every sulcus and fissure o Permeable to substances in brain extracellular fluid and CSF; allows for substances to move between these two fluid compartments; helps to balance concentration of different solutes found in each fluid THE VENTRICLES AND CEREBROSPINAL FLUID Four ventricles within brain are linked cavities that are continuous with central canal of spinal cord (Figures 12.19, 12.20) Lined with ependymal cells Filled with cerebrospinal fluid THE VENTRICLES AND CEREBROSPINAL FLUID Right and left lateral ventricles (first and second ventricles); within their respective cerebral hemisphere (Figure 12.19): Resemble ram s horns when observed in anterior view; horseshoe-shaped appearance in lateral view Three regions: anterior horn, inferior horn, and posterior horn THE VENTRICLES AND CEREBROSPINAL FLUID Third ventricle narrow cavity found between two lobes of diencephalon; connected to lateral ventricles by interventricular foramen Fourth ventricle between pons and cerebellum; connected to third ventricle by cerebral aqueduct (small passageway through midbrain) Continuous with central canal of spinal cord Contains several posterior openings that allow CSF in ventricles to flow into subarachnoid space (Figure 12.19a) THE VENTRICLES AND CEREBROSPINAL FLUID Cerebrospinal fluid (CSF) clear, colorless liquid similar in composition to blood plasma; protects brain in following ways: Cushions brain and maintains a constant temperature within cranial cavity Removes wastes and increases buoyancy of brain; keeps brain from collapsing under its own weight THE VENTRICLES AND CEREBROSPINAL FLUID Choroid plexuses where majority of CSF is manufactured; found in each of four ventricles where blood vessels come into direct contact with ependymal cells (also produce some CSF themselves) Fenestrated capillaries have gaps between endothelial cells; allow fluids and electrolytes to exit from blood plasma to enter extracellular fluid (ECF) 16

17 THE VENTRICLES AND CEREBROSPINAL FLUID Choroid plexuses (continued): About 150 ml (2/3 cup) of CSF circulates through brain and spinal cord ml of CSF is produced daily so old CSF must be removed as choroid plexuses make new CSF Process of CSF production and removal occurs constantly; CSF is completely replaced every 5 6 hours THE VENTRICLES AND CEREBROSPINAL FLUID Pathway for formation, circulation, and reabsorption of CSF (Figure 12.20): Fluid and electrolytes leak out of capillaries of choroid plexuses into ECF of ventricles Taken up into ependymal cells; then secreted into ventricles as CSF Circulated through and around brain and spinal cord in subarachnoid space; assisted by movement of ependymal cell cilia Some CSF is reabsorbed into venous blood in dural sinuses via arachnoid granulations THE BLOOD-BRAIN BARRIER Blood-brain barrier protective safeguard that separates CSF and brain ECF from chemicals and disease-causing organisms sometimes found in blood plasma (Figure 12.21) Consists mainly of simple squamous epithelial cells (endothelial cells) of blood capillaries, their basal laminae, and astrocytes THE BLOOD-BRAIN BARRIER Unique features of endothelial cells found in barrier: Neighboring endothelial cells are bound together by many tight junctions; prevent fluids and molecules from passing between them; influenced by activity of astrocytes on developing brain Limited capacity for movement of molecules and substances into and out of cell by endocytosis and exocytosis THE BLOOD-BRAIN BARRIER Substances that easily pass through plasma membranes are able to pass through blood-brain barrier; include water, oxygen, carbon dioxide, and nonpolar, lipid-based molecules Protein channels or carriers allow for passage of other essential molecules across blood-brain barrier; include glucose, amino acids, and ions Most large, polar molecules are effectively prevented from crossing blood-brain barrier in any significant amount; while barrier is protective, it can hinder access of medications into brain 17

18 CONCEPT BOOST: WHERE EXACTLY IS BLOOD-BRAIN BARRIER? No single structure is labeled blood-brain barrier on any figure because blood-brain barrier isn t around brain; it s within brain Not located in one distinct place but found throughout entire brain To understand this, we must first understand body s tiniest blood vessels; capillaries are vessels that deliver oxygen and nutrients to body s cells and remove any wastes produced by cells CONCEPT BOOST: WHERE EXACTLY IS BLOOD-BRAIN BARRIER? Capillaries found in most organs and tissues are fairly leaky; allow a wide variety of substances to move from blood to extracellular fluid (and vice versa) Capillaries of brain are specialized to allow only selected substances to enter its extracellular fluid; effectively act as a barrier ; prevents other substances from doing so (Figure 12.21) Blood-brain barrier, therefore, is actually a property of capillaries found throughout brain rather than a distinct physical barrier INFECTIOUS MENINGITIS Potentially life-threatening infection of meninges in subarachnoid space; inflammation occurs, causing classic signs: headache, lethargy, stiff neck, fever Diagnosis examination of CSF for infectious agents and white blood cells (cells of immune system); bacteria and viruses are most common causative agents: Viral generally mild; resolves in 1 2 weeks Bacterial can rapidly progress to brain involvement and death; aggressive antibiotic treatment necessary; some most common forms are preventable with vaccines MODULE 12.4 THE SPINAL CORD THE SPINAL CORD Spinal cord composed primarily of nervous tissue; responsible for both relaying and processing information; less anatomically complex than brain but still vitally important to normal nervous system function; two primary roles: Serves as a relay station and as an intermediate point between body and brain; only means by which brain can interact with body below head and neck Processing station for some less complex activities such as spinal reflexes; do not require higher level processing PROTECTION OF THE SPINAL CORD Brain s meninges pass through foramen magnum to provide a continuous protective covering of spinal cord and distal nerves at base (Figure 12.22) Three spinal meninges include dura mater, arachnoid, and pia mater; structurally similar to brain meninges except that spinal cord dura has only one layer and pia mater has some structural enhancements (Figure 12.22a) 18

19 PROTECTION OF THE SPINAL CORD Three spinal meninges include dura mater, arachnoid, and pia mater; structurally similar to brain meninges except that spinal cord dura has only one layer and pia mater has some structural enhancements (Figure 12.22a) (continued): Spinal dura mater does not have periosteal layer found in dura mater of brain; consists of only a meningeal layer Spinal pia mater has added function of anchoring spinal cord to surrounding vertebral cavity; thin pia extensions called denticulate ligaments pass through arachnoid and adhere to dura mater PROTECTION OF THE SPINAL CORD Actual or potential spaces between spinal cord meninges are same as those found between cranial meninges with following features (Figure 12.22b): Epidural space actual space due to absence of a periosteal dura; found between meningeal dura and walls of vertebral foramina; space is filled with veins and adipose tissue; cushions and protects spinal cord PROTECTION OF THE SPINAL CORD Actual or potential spaces between spinal cord meninges are same as those found between cranial meninges with following features (Figure 12.22b): Subdural space only a potential space much like epidural space surrounding brain; dura and arachnoid are normally adhered to one another Subarachnoid space found between arachnoid and pia mater; filled with CSF; base of spinal cord contains a large volume of CSF; useful site for withdrawing samples for clinical laboratory testing EPIDURAL ANESTHESIA AND LUMBAR PUNCTURES Epidural (spinal) anesthesia local anesthetic medication is injected into epidural space through an inserted needle Causes numbing (inability to transmit motor or sensory impulses) of nerves extending off spinal cord below level of injection Commonly given during childbirth and other surgical procedures EPIDURAL ANESTHESIA AND LUMBAR PUNCTURES Lumbar puncture (spinal tap) needle inserted into subarachnoid space between fourth and fifth lumbar vertebrae; avoids possibility of injuring spinal cord CSF is withdrawn for analysis; used to assess conditions like meningitis, encephalitisand multiple sclerosis 19

20 EXTERNAL SPINAL CORD ANATOMY Spinal cord extends proximally from foramen magnum to region between first and second lumbar vertebrae; following structural features can be seen on spinal cord (Figure 12.23): Narrow posterior median sulcus can be seen on entire length of posterior side of cord Wider anterior median sulcus can be seen on entire length of anterior side of cord Conus medullaris is cone-shaped distal end of cord EXTERNAL SPINAL CORD ANATOMY Filum terminale found between first and second lumbar vertebrae; composed of spinal pia mater; thin layer of pia continues through vertebral cavity to form an anchor that is attached to first coccygeal vertebra Spinal cord has two enlarged regions (cervical and lumbar enlargements); nerve roots fuse together to form spinal nerves (serve upper and lower extremities respectively) in these enlargements (Figure 12.23a) EXTERNAL SPINAL CORD ANATOMY Spinal nerves are components of PNS; carry sensory and motor impulses to and from spinal cord Projections visible on each side of spinal cord between vertebrae are called posterior and anterior nerve roots Roots of spinal nerves extend inferiorly from conus medullaris and fill remainder of vertebral cavity; this bundle of spinal nerve roots is called cauda equina due to its horsetail-like appearance INTERNAL SPINAL CORD ANATOMY Butterfly-shaped spinal gray matter is surrounded by tracts of white matter; following features are seen on cross section of spinal cord (Figures 12.24, 12.25): Central canal filled with CSF; seen in center of spinal cord; surrounded by two thin strips of gray matter (gray commissure); connects each butterfly wing INTERNAL SPINAL CORD ANATOMY Anterior to posterior orientation of spinal cord can be determined by using shape and size of gray matter wings Anterior wings are broader while thinner posterior wings extend almost to outer surface of spinal cord INTERNAL SPINAL CORD ANATOMY Spinal gray matter makes up three distinct regions found within spinal cord; houses neurons with specific functions and includes (Figure 12.24): Anterior horn (ventral horn) makes up anterior wing of gray matter and gives rise to anterior motor nerve roots; neuron cell bodies found in this region are involved in somatic motor functions (skeletal muscle contraction) 20

21 INTERNAL SPINAL CORD ANATOMY Spinal gray matter (continued): Posterior horn (or dorsal horn) makes up posterior wing of gray matter and gives rise to posterior sensory nerve roots; neuron cell bodies found in this region are involved in processing incoming somatic and visceral sensory information Lateral horn, found only in spinal cord between first thoracic vertebra and lumbar region; contains cell bodies of neurons involved in control of viscera via autonomic nervous system INTERNAL SPINAL CORD ANATOMY Spinal White Matter: Ascending and Descending Tracts Contains axons of neurons that travel to and from brain; allows spinal cord to fulfill one of its primary functions as a relay station Organized into general regions called funiculi; three (posterior funiculus, lateral funiculus, and anterior funiculus) lie on each side of spinal cord (Figure 12.25) INTERNAL SPINAL CORD ANATOMY Spinal White Matter (continued): Funiculi features o White matter in each funiculus is organized into tracts or columns; bilaterally symmetrical (left and right side of spinal cord have identical tracts serving their respective side of body) o Ascending and descending tracts bring information to and from a specific part of brain o Sensory pathways travel in posterior and lateral funiculi while motor pathways travel in anterior and lateral funiculi INTERNAL SPINAL CORD ANATOMY Spinal White Matter (continued): Ascending tracts carry various kinds of sensory information (Figure 12.25a): o Posterior columns found in posterior funiculus; made up of two tracts, medial fasciculus gracilis and lateral fasciculus cuneatus; carry somatosensory information, such as proprioception and touch; nucleus gracilis transmits stimuli from lower limbs and lower trunk while nucleus cuneatus transmits stimuli from upper limbs and upper trunk INTERNAL SPINAL CORD ANATOMY Spinal White Matter (continued): Ascending tracts (continued): o Spinocerebellar tracts found in lateral funiculi; carry information about joint position and muscle stretch from entire body to cerebellum o Anterolateral system includes spinothalamic tracts that travel in anterior and lateral funiculi; transmit pain and temperature stimuli from entire body to brain 21

22 INTERNAL SPINAL CORD ANATOMY Spinal White Matter (continued): Descending tracts transmit motor information from specific regions in brain down spinal cord to specific regions in body (Figure 12.25b) o Corticospinal tracts largest of descending tracts; help control skeletal muscles below head and neck o Originate from motor areas of cerebral cortex; descend as part of internal capsule then decussate within brainstem o Travel through lateral funiculi of spinal cord; fibers deliver motor information to appropriate locations in anterior horn MODULE 12.5 SENSATION PART I: ROLE OF THE CNS IN SENSATION SENSORY STIMULI Sensory stimuli those effects that cause senses to respond; multiple sensory stimuli from different regions of brain can be pulled together into a single mental picture Each of these disparate stimuli reaches brain in following two-part process: o Stimulus is detected by neurons in PNS and sent as sensory input to CNS o In CNS, sensory input is sent to cerebral cortex for interpretation SENSORY STIMULI Sensory stimuli (continued): When CNS has received all different sensory inputs, it integrates them into a single perception (a conscious awareness of sensation) Sensations can be grouped into two basic types: o Special senses detected by special sense organs and include vision, hearing, equilibrium, smell, and taste o General senses detected by sensory neurons in skin, muscles, or walls of organs; can be further subdivided into general somatic senses that involve skin, muscles, and joints and general visceral senses that involve internal organs GENERAL SOMATIC SENSES General somatic senses pertain to touch, stretch, joint position, pain, and temperature (Figures ) Two types of touch stimuli are delivered to appropriate part of cerebral cortex by different pathways: Tactile senses (fine or discrimination touch) include vibration, two-point discrimination, and light touch Nondiscriminative touch (crude touch) lacks fine spatial resolution of tactile senses 22

23 GENERAL SOMATIC SENSES Most of general somatic senses are considered mechanical senses; neurons that detect them are responsive to mechanical deformation Two major ascending tracts in spinal cord carry somatic sensory information to brain: posterior columns/medial lemniscal system and anterolateral system GENERAL SOMATIC SENSES Basic pathway consists of following: First-order neuron detects initial stimulus in PNS; axon of this neuron then synapses on a second-order neuron Second-order neuron interneuron located in posterior horn of spinal cord or in brainstem; relays stimulus to a third-order neuron Third-order neuron an interneuron found in thalamus; delivers impulse to cerebral cortex GENERAL SOMATIC SENSES Posterior columns/medial lemniscal system includes axons of neurons that transmit tactile information about discriminative touch and axons that convey information regarding proprioception (Figure 12.26) Ascend through posterior columns; medial fasciculus gracilis (carries impulses from lower limbs) and fasciculus cuneatus (carries impulses from upper limbs, trunk, and neck) GENERAL SOMATIC SENSES Posterior columns/medial lemniscal system (continued): Fasciculus gracilis and cuneatus axons synapse with second-order neurons when they enter medulla, nucleus gracilis, and nucleus cuneatus, respectively Axons of second-order neurons decussate and form tracts called medial lemniscus Fibers of medial lemniscus ascend through pons and midbrain until they reach third-order neurons in thalamus; axons of these third-order neurons proceed to cerebral cortex GENERAL SOMATIC SENSES Anterolateral system fibers transmit pain, temperature, and nondiscriminative touch stimuli in anterolateral spinal cord (Figure 12.27) First-order neurons synapse on second-order neurons in posterior horn; then decussate Spinothalamic tract largest member of anterolateral system; transmits signals through spinal cord to sensory relay nuclei of thalamus; third-order neurons in thalamus then transmit impulses to cerebral cortex 23

24 GENERAL SOMATIC SENSES Role of Cerebral Cortex in Sensation, S1 and Somatotopy: Thalamus relays most incoming information to primary somatosensory cortex (S1) in postcentral gyrus Each part of body is represented by a specific region of S1, a type of organization called somatotopy (Figure 12.28) GENERAL SOMATIC SENSES Role of Cerebral Cortex (continued): Mapping of primary somatosensory cortex (S1) illustrates that different parts of body are unequally represented (Figure 12.28a) More S1 space is dedicated to hands and face; represents importance of manual dexterity, facial expression, and speech to human existence Unequal representation of body parts in S1 is exemplified by sensory homunculus (Figure 12.28b) GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Touch Stimuli: Thalamic nuclei relay touch information from spinothalamic tracts and posterior columns primarily to S1 for conscious perception Once sensory information reaches S1, it is processed, perceived, and passed along to cortical association areas GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Touch Stimuli (continued): Somatosensory association cortex (S2) plays a major role in processing sensory input and sending it to limbic system Limbic system is involved in tactile learning and memory S1 also sends sensory input to parietal and temporal association areas which integrate and relay information to motor areas of frontal lobe GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Pain Stimuli: perception of pain stimuli is called nociception Thalamus relays pain stimuli to several brain regions including S1 and S2 where sensory discrimination (localization, intensity, and quality) is perceived and analyzed Also sent to basal nuclei, regions of limbic system, hypothalamus, and prefrontal cortex, where emotional and behavioral aspects of pain are processed 24

25 GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Pain Stimuli (continued): Cerebral cortex appears to have a significant influence on perception and modulation of pain; evident by a phenomenon called placebo effect where a dummy treatment with no active pain-killing ingredients produces pain relief GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Pain Stimuli (continued): A descending pathway originating mostly in S1, amygdala, and a region of midbrain called periaqueductal gray matter provides an explanation for placebo effect Neurons in the periaqueductal gray matter release neurotransmitters called endorphins GENERAL SOMATIC SENSES Role of the Cerebral Cortex in Sensation Processing of Pain Stimuli (continued): Endorphins decrease sensitivity to pain stimuli of posterior horn neurons o Pain input is still present and its intensity is unaffected o CNS neurons perceive pain as being less intense (or absent) o Example of Cell-Cell Communication Core Principle PHANTOM LIMB PAIN Phantom limb occurs after amputation of limb, digit, or even breast; patients perceive body part is still present and functional in absence of sensory input; small percentage develop phantom pain (burning, tingling, or severe pain) in missing part Difficult to treat due to complex way CNS processes pain; supports idea that S1 has map of body that exists independently of PNS Over time, map generally rearranges itself so body is represented accurately; phantom sensations decrease INTRODUCTION TO SPECIAL SENSES Special senses include vision, hearing (audition), taste (gustation), smell (olfaction), and balance (vestibular sensation) Each involves neurons that detect a stimulus and send it to CNS for processing and integration Thalamus gateway for entry of special sensory stimuli into cerebral cortex; interprets majority of this information; olfaction is exception 25