Download The Central Nervous System

Chapter 12
The Central Nervous System
Regions and Organization
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Adult brain regions
1.Cerebral hemispheres
2.Diencephalon
3.Brain stem (midbrain, pons, and medulla)
4.Cerebellum
Regions and Organization of the CNS
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Spinal cord
– Central cavity surrounded by gray matter
– External white matter composed of myelinated fiber tracts
Regions and Organization of the CNS
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Brain
– Similar pattern
– Additional areas of gray matter in brain
– Cerebral hemispheres and cerebellum
• Outer gray matter called cortex
– Cortex disappears in brain stem
• Scattered gray matter nuclei amid white matter
Ventricles of the Brain
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Filled with cerebrospinal fluid (CSF)
Lined by ependymal cells
Connected to one another and to central canal of spinal cord
– Lateral ventricles  third ventricle via interventricular foramen
– Third ventricle  fourth ventricle via cerebral aqueduct
Ventricles of the Brain
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Paired, C-shaped lateral ventricles in cerebral hemispheres
– Separated anteriorly by septum pellucidum
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Third ventricle in diencephalon
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Fourth ventricle in hindbrain
– Three openings: paired lateral apertures in side walls; median aperture in
roof
• Connect ventricles to subarachnoid space
Cerebral Hemispheres
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Surface markings
– Ridges (gyri), shallow grooves (sulci), and deep grooves (fissures)
– Longitudinal fissure
• Separates two hemispheres
– Transverse cerebral fissure
• Separates cerebrum and cerebellum
Cerebral Hemispheres
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Five lobes
– Frontal
– Parietal
– Temporal
– Occipital
– Insula
Cerebral Hemispheres
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Central sulcus
– Separates precentral gyrus of frontal lobe and postcentral gyrus of
parietal lobe
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Parieto-occipital sulcus
– Separates occipital and parietal lobes
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Lateral sulcus outlines temporal lobes
Cerebral Hemispheres
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Three basic regions
– Cerebral cortex of gray matter superficially
– White matter internally
– Basal nuclei deep within white matter
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Cerebral Cortex
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Thin (2–4 mm) superficial layer of gray matter
40% mass of brain
Site of conscious mind: awareness, sensory perception, voluntary motor
initiation, communication, memory storage, understanding
4 General Considerations of Cerebral Cortex
1.Three types of functional areas
– Motor areas—control voluntary movement
– Sensory areas—conscious awareness of sensation
– Association areas—integrate diverse information
2.Each hemisphere concerned with contralateral side of body
3. Lateralization of cortical function in hemispheres
4. Conscious behavior involves entire cortex in some way
Motor Areas of Cerebral Cortex
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In frontal lobe; control voluntary movement
Primary (somatic) motor cortex in precentral gyrus
Premotor cortex anterior to precentral gyrus
Broca's area anterior to inferior premotor area
Frontal eye field within and anterior to premotor cortex; superior to Broca's
area
Primary Motor Cortex
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Large pyramidal cells of precentral gyri
Long axons  pyramidal (corticospinal) tracts of spinal cord
Allows conscious control of precise, skilled, skeletal muscle movements
Motor homunculi - upside-down caricatures represent contralateral motor
innervation of body regions
Premotor Cortex
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Helps plan movements; staging area for skilled motor activities
Controls learned, repetitious, or patterned motor skills
Coordinates simultaneous or sequential actions
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Controls voluntary actions that depend on sensory feedback
Broca's Area
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Present in one hemisphere (usually the left)
Motor speech area that directs muscles of speech production
Active in planning speech and voluntary motor activities
Frontal Eye Field
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Controls voluntary eye movements
Sensory Areas of Cerebral Cortex
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Conscious awareness of sensation
Occur in parietal, insular, temporal, and occipital lobes
Primary somatosensory cortex
• Vestibular cortex
Somatosensory association
cortex
• Olfactory cortex
Visual areas
• Gustatory cortex
Auditory areas
• Visceral sensory area
Primary Somatosensory Cortex
• In postcentral gyri of parietal lobe
• Receives general sensory information from skin, and proprioceptors of
skeletal muscle, joints, and tendons
• Capable of spatial discrimination: identification of body region being
stimulated
• Somatosensory homunculus upside-down caricatures represent
contralateral sensory input from body regions
Somatosensory Association Cortex
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Posterior to primary somatosensory cortex
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Determines size, texture, and relationship of parts of objects being felt
Integrates sensory input from primary somatosensory cortex for understanding
of object
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Visual Areas
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Primary visual (striate) cortex
– Extreme posterior tip of occipital lobe
– Most buried in calcarine sulcus of occipital lobe
– Receives visual information from retinas
Visual Areas
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Visual association area
– Surrounds primary visual cortex
– Uses past visual experiences to interpret visual stimuli (e.g., color, form,
and movement)
• E.g., ability to recognize faces
– Complex processing involves entire posterior half of cerebral hemispheres
Auditory Areas
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Primary auditory cortex
– Superior margin of temporal lobes
– Interprets information from inner ear as pitch, loudness, and location
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Auditory association area
– Located posterior to primary auditory cortex
– Stores memories of sounds and permits perception of sound stimulus
Vestibular Cortex
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Posterior part of insula and adjacent parietal cortex
Responsible for conscious awareness of balance (position of head in space)
OIfactory Cortex
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Primary olfactory (smell) cortex
– Medial aspect of temporal lobes (in piriform lobes)
– Part of primitive rhinencephalon, along with olfactory bulbs and tracts
– Remainder of rhinencephalon in humans part of limbic system
– Region of conscious awareness of odors
Gustatory Cortex
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In insula just deep to temporal lobe
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Involved in perception of taste
Visceral Sensory Area
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Posterior to gustatory cortex
Conscious perception of visceral sensations, e.g., upset stomach or full
bladder
Multimodal Association Areas
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Receive inputs from multiple sensory areas
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Sensations, thoughts, emotions become conscious – makes us who we are
Send outputs to multiple areas, including premotor cortex
Allows meaning to information received, store in memory, tying to previous
experience, and deciding on actions
Three broad parts:
– Anterior association area (prefrontal cortex)
– Posterior association area
– Limbic association area
Anterior Association Area (Prefrontal Cortex)
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Most complicated cortical region
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Development depends on feedback from social environment
Involved with intellect, cognition, recall, and personality
Contains working memory needed for abstract ideas, judgment, reasoning,
persistence, and planning
Posterior Association Area
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Large region in temporal, parietal, and occipital lobes
Plays role in recognizing patterns and faces and localizing us in space
Involved in understanding written and spoken language (Wernicke's area)
Limbic Association Area
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Part of limbic system
Involves cingulate gyrus, parahippocampal gyrus, and hippocampus
Provides emotional impact that makes scene important and helps establish
memories
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Lateralization of Cortical Function
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Hemispheres almost identical
Lateralization - division of labor between hemispheres
Cerebral dominance - hemisphere dominant for language (left hemisphere 90% people)
Lateralization of Cortical Function
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Left hemisphere
– Controls language, math, and logic
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Right hemisphere
– Visual-spatial skills, intuition, emotion, and artistic and musical skills
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Hemispheres communicate almost instantaneously via fiber tracts and
functional integration
Cerebral White Matter
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Myelinated fibers and tracts
Communication between cerebral areas, and between cortex and lower CNS
– Association fibers— horizontal; connect different parts of same
hemisphere
– Commissural fibers— horizontal; connect gray matter of two hemispheres
– Projection fibers— vertical; connect hemispheres with lower brain or
spinal cord
Diencephalon
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Three paired structures
– Thalamus
– Hypothalamus
– Epithalamus
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Encloses third ventricle
Thalamus
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80% of diencephalon
Superolateral walls of third ventricle
Bilateral nuclei connected by interthalamic adhesion (intermediate mass)
– Contains several nuclei, named for location
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– Nuclei project and receive fibers from cerebral cortex
Thalamic Function
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Gateway to cerebral cortex
Sorts, edits, and relays ascending input
– Impulses from hypothalamus for regulation of emotion and visceral function
– Impulses from cerebellum and basal nuclei to help direct motor cortices
– Impulses for memory or sensory integration
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Mediates sensation, motor activities, cortical arousal, learning, and memory
Hypothalamus
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Forms inferolateral walls of third ventricle
Contains many nuclei
– Example: mammillary bodies
• Paired anterior nuclei
• Olfactory relay stations
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Infundibulum—stalk that connects to pituitary gland
Hypothalamic Function
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Controls autonomic nervous system (e.g., blood pressure, rate and force of
heartbeat, digestive tract motility, pupil size)
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Physical responses to emotions (limbic system)
– Perception of pleasure, fear, and rage, and in biological rhythms and drives
Hypothalamic Function
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Regulates body temperature – sweating/shivering
Regulates hunger and satiety in response to nutrient blood levels or hormones
Regulates water balance and thirst
Regulates sleep-wake cycles
– Suprachiasmatic nucleus (biological clock)
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Controls endocrine system
– Controls secretions of anterior pituitary gland
– Produces posterior pituitary hormones
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Epithalamus
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Most dorsal portion of diencephalon; forms roof of third ventricle
Pineal gland (body)—extends from posterior border and secretes melatonin
– Melatonin—helps regulate sleep-wake cycle
Brain Stem
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Three regions
– Midbrain
– Pons
– Medulla oblongata
Brain Stem
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Similar structure to spinal cord but contains nuclei embedded in white matter
Controls automatic behaviors necessary for survival
Contains fiber tracts connecting higher and lower neural centers
Nuclei associated with 10 of the 12 pairs of cranial nerves
Midbrain
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Between diencephalon and pons
Cerebral peduncles ventrally
– Contain pyramidal motor tracts
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Cerebral aqueduct
– Channel connecting third and fourth ventricles
Midbrain Nuclei
• Periaqueductal gray matter
– Pain suppression; links amygdaloid body and ANS; controls cranial nerves
III (oculomotor) and IV (trochlear)
• Corpora quadrigemina— dorsal protrusions
– Superior colliculi—visual reflex centers
– Inferior colliculi—auditory relay centers
• Substantia nigra—functionally linked to basal nuclei
• Red nucleus—relay nuclei for some descending motor pathways; part of
reticular formation
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Pons
• Fourth ventricle seperates pons and cerebellum
• Fibers of pons
– Connect higher brain centers and spinal cord
– Relay impulses between motor cortex and cerebellum
• Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)
• Some nuclei of reticular formation
• Nuclei help maintain normal rhythm of breathing
Medulla Oblongata (Medulla)
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Joins spinal cord at foramen magnum
Forms part of ventral wall of fourth ventricle
Contains choroid plexus of fourth ventricle
Pyramids—two ventral longitudinal ridges formed by pyramidal tracts
Decussation of the pyramids—crossover of corticospinal tracts
• Inferior olivary nuclei—relay sensory information from muscles and joints to
cerebellum
• Cranial nerves VIII, IX, X, and XII are associated with medulla
• Vestibular nuclei (pons and medulla)—mediate responses that maintain
equilibrium
• Several nuclei (e.g., nucleus cuneatus and nucleus gracilis) relay sensory
information
Medulla Oblongata: Functions
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Autonomic reflex center
– Functions overlap with hypothalamus
• Hypothalamus relays instructions via medulla
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Cardiovascular center
– Cardiac center adjusts force and rate of heart contraction
– Vasomotor center adjusts blood vessel diameter for blood pressure
regulation
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Respiratory centers
– Generate respiratory rhythm
– Control rate and depth of breathing (with pontine centers)
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Additional centers regulate
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– Vomiting, Hiccuping, Swallowing, Coughing, and Sneezing
Cerebellum
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11% of brain mass
Dorsal to pons and medulla
Input from cortex, brain stem and sensory receptors
Allows smooth, coordinated movements
Cerebellar hemispheres connected by vermis
Folia—transversely oriented gyri
Each hemisphere has three lobes
– Anterior, posterior, and flocculonodular
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Arbor vitae—treelike pattern of cerebellar white matter
Cerebellar Processing of Motor Activity
• Cerebellum receives impulses from cerebral cortex of intent to initiate
voluntary muscle contraction
• Signals from proprioceptors and visual and equilibrium pathways continuously
"inform" cerebellum of body's position and momentum
• Cerebellar cortex calculates the best way to smoothly coordinate muscle
contraction
• "Blueprint" of coordinated movement sent to cerebral motor cortex and brain
stem nuclei
Cognitive Function of Cerebellum
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Role in thinking, language, and emotion
May compare actual with expected output and adjust accordingly
Functional Brain Systems
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Networks of neurons that work together but span wide areas of brain
– Limbic system
– Reticular formation
Limbic System
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Structures on medial aspects of cerebral hemispheres and diencephalon
Includes parts of diencephalon and some cerebral structures that encircle
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brain stem
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Emotional or affective brain
– Amygdaloid body—recognizes angry or fearful facial expressions,
assesses danger, and elicits fear response
– Cingulate gyrus—role in expressing emotions via gestures, and resolves
mental conflict
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Puts emotional responses to odors
– Example: skunks smell bad
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Most output relayed via hypothalamus
Limbic System: Emotion and Cognition
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Limbic system interacts with prefrontal lobes
– React emotionally to things we consciously understand to be happening
– Consciously aware of emotional richness in our lives
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Hippocampus and amygdaloid body—play a role in memory
Reticular Formation
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Three broad columns run length of brain stem
Has far-flung axonal connections with hypothalamus, thalamus, cerebral
cortex, cerebellum, and spinal cord  can govern brain arousal
Reticular Formation: RAS and Motor Function
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Reticular activating system (RAS)
– Sends impulses to cerebral cortex to keep it conscious and alert
– Filters out repetitive, familiar, or weak stimuli (~99% of all stimuli!)
– Inhibited by sleep centers, alcohol, drugs
– Severe injury results in permanent unconsciousness (coma)
Protection of the Brain
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Bone (skull)
Membranes (meninges)
Watery cushion (cerebrospinal fluid)
Blood brain barrier
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Meninges
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Cover and protect CNS
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid (CSF)
Form partitions in skull
Meninges
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Three layers
– Dura mater
– Arachnoid mater
– Pia mater
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Meningitis
– Inflammation of meninges
Dura Mater
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Strongest meninx
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Dural septa limit excessive movement of brain
Two layers of fibrous connective tissue (around brain) separate to form dural
venous sinuses
– Falx cerebri—in longitudinal fissure; attached to crista galli
– Falx cerebelli—along vermis of cerebellum
– Tentorium cerebelli—horizontal dural fold over cerebellum and in
transverse fissure
Arachnoid Mater
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Middle layer with weblike extensions
Separated from dura mater by subdural space
Subarachnoid space contains CSF and largest blood vessels of brain
Arachnoid villi protrude into superior sagittal sinus and permit CSF
reabsorption
Pia Mater
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Delicate vascularized connective tissue that clings tightly to brain
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Cerebrospinal Fluid (CSF)
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Composition
– Watery solution formed from blood plasma
• Less protein and different ion concentrations than plasma
– Constant volume
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Functions
– Gives buoyancy to CNS structures
• Reduces weight by 97%
– Protects CNS from blows and other trauma
– Nourishes brain and carries chemical signals
Choroid Plexuses
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Hang from roof of each ventricle; produce CSF at constant rate; keep in
motion
– Clusters of capillaries enclosed by pia mater and layer of ependymal cells
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Ependymal cells use ion pumps to control composition of CSF and help
cleanse CSF by removing wastes
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Normal volume ~ 150 ml; replaced every 8 hours
Blood Brain Barrier
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Helps maintain stable environment for brain
Separates neurons from some bloodborne substances
Composition
– Continuous endothelium of capillary walls
– Thick basal lamina around capillaries
– Feet of astrocytes
• Provide signal to endothelium for formation of tight junctions
Blood Brain Barrier: Functions
• Selective barrier
– Allows nutrients to move by facilitated diffusion
– Metabolic wastes, proteins, toxins, most drugs, small nonessential amino
acids, K+ denied
– Allows any fat-soluble substances to pass, including alcohol, nicotine, and
anesthetics
• Absent in some areas, e.g., vomiting center and hypothalamus, where
necessary to monitor chemical composition of blood
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Homeostatic Imbalances of the Brain
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Traumatic brain injuries
– Concussion—temporary alteration in function
– Contusion—permanent damage
– Subdural or subarachnoid hemorrhage—may force brain stem through
foramen magnum, resulting in death
– Cerebral edema—swelling of brain associated with traumatic head injury
Homeostatic Imbalances of the Brain
• Cerebrovascular accidents (CVAs or strokes)
– Ischemia
• Tissue deprived of blood supply; brain tissue dies, e.g., blockage of
cerebral artery by blood clot
– Hemiplegia (paralysis on one side), or sensory and speech deficits
– Transient ischemic attacks (TIAs)—temporary episodes of reversible
cerebral ischemia
– Tissue plasminogen activator (TPA) is only approved treatment for stroke
Homeostatic Imbalances of the Brain
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Degenerative brain disorders
– Alzheimer's disease (AD): a progressive degenerative disease of brain
that results in dementia
• Memory loss, short attention span, disorientation, eventual language
loss, irritable, moody, confused, hallucinations
• Plaques of beta-amyloid peptide form in brain
– Toxic effects may involve prion proteins
• Neurofibrillary tangles inside neurons kill them
• Brain shrinks
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Parkinson's disease
– Degeneration of dopamine-releasing neurons of substantia nigra
– Basal nuclei deprived of dopamine become overactive  tremors at rest
– Cause unknown
• Mitochondrial abnormalities or protein degradation pathways?
– Treatment with L-dopa; deep brain stimulation; gene therapy; research into
stem cell transplants promising
• Huntington's disease
– Fatal hereditary disorder
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– Caused by accumulation of protein huntingtin
• Leads to degeneration of basal nuclei and cerebral cortex
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Initial symptoms wild, jerky "flapping" movements
Later marked mental deterioration
Treated with drugs that block dopamine effects
Stem cell implant research promising
Spinal Cord: Gross Anatomy and Protection
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Location
– Begins at the foramen magnum
– Ends at L1 or L2 vertebra
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Functions
– Provides two-way communication to and from brain
– Contains spinal reflex centers
Spinal Cord: Gross Anatomy and Protection
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Bone, meninges, and CSF
Epidural space
– Cushion of fat and network of veins in space between vertebrae and spinal
dura mater
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CSF in subarachnoid space
Dural and arachnoid membranes extend to sacrum, beyond end of cord at L1
or L2
– Site of lumbar puncture or tap
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Terminates in conus medullaris
Filum terminale extends to coccyx
– Fibrous extension of conus covered with pia mater
– Anchors spinal cord
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Denticulate ligaments
– Extensions of pia mater that secure cord to dura mater
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Spinal nerves (Part of PNS)
– 31 pairs
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Cervical and lumbosacral enlargements
– Nerves serving upper and lower limbs emerge here
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Cauda equina
– Collection of nerve roots at inferior end of vertebral canal
Cross-sectional Anatomy
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Two lengthwise grooves partially divide cord into right and left halves
– Ventral (anterior) median fissure
– Dorsal (posterior) median sulcus
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Gray commissure—connects masses of gray matter; encloses central canal
Gray Matter
•Dorsal horns - interneurons that receive somatic and visceral sensory input
•Ventral horns - some interneurons; somatic motor neurons; axons exit cord via
ventral roots
•Lateral horns (only in thoracic and superior lumbar regions) - sympathetic
neurons
•Dorsal roots – sensory input to cord
•Dorsal root (spinal) ganglia—cell bodies of sensory neurons
White Matter
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Myelinated and nonmyelinated nerve fibers allow communication between
parts of spinal cord, and spinal cord and brain
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Run in three directions
– Ascending – up to higher centers (sensory inputs)
– Descending – from brain to cord or lower cord levels (motor outputs)
– Transverse – from one side to other (commissural fibers)
Ascending Pathways
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Three main pathways:
– Two transmit somatosensory information to sensory cortex via thalamus
•Dorsal column–medial lemniscal pathways
•Spinothalamic pathways
•Provide discriminatory touch and conscious proprioception
– Spinocerebellar tracts terminate in the cerebellum
Dorsal Column–Medial Lemniscal Pathways
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Transmit input to somatosensory cortex for discriminative touch and vibrations
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Composed of paired fasciculus cuneatus and fasciculus gracilis in spinal cord
and medial lemniscus in brain (medulla to thalamus)
Spinothalamic Pathways
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Lateral and ventral spinothalamic tracts
Transmit pain, temperature, coarse touch, and pressure impulses within lateral
spinothalamic tract
Spinocerebellar Tracts
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Ventral and dorsal tracts
Convey information about muscle or tendon stretch to cerebellum
– Used to coordinate muscle activity
Descending Pathways and Tracts
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Deliver efferent impulses from brain to spinal cord
Two groups
– Direct pathways—pyramidal tracts
– Indirect pathways—all others
Descending Pathways and Tracts
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Motor pathways involve two neurons:
– Upper motor neurons
•Pyramidal cells in primary motor cortex
– Lower motor neurons
•Ventral horn motor neurons
•Innervate skeletal muscles
The Direct (Pyramidal) Pathways
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Impulses from pyramidal neurons in precentral gyri pass through pyramidal
(corticospinal)l tracts
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Descend without synapsing
Axons synapse with interneurons or ventral horn motor neurons
Direct pathway regulates fast and fine (skilled) movements
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Indirect (Multineuronal) System
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Complex and multisynaptic
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These pathways regulate
Includes brain stem motor nuclei, and all motor pathways except pyramidal
pathways
– Axial muscles maintaining balance and posture
– Muscles controlling coarse limb movements
– Head, neck, and eye movements that follow objects in visual field
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Reticulospinal and vestibulospinal tracts—maintain balance
Rubrospinal tracts—control flexor muscles
Superior colliculi and tectospinal tracts mediate head movements in
response to visual stimuli
Poliomyelitis
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Destruction of ventral horn motor neurons by poliovirus
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Destruction of ventral horn motor neurons and fibers of pyramidal tract
Muscles atrophy
Death may occur from paralysis of respiratory muscles or cardiac arrest
Survivors often develop postpolio syndrome many years later from neuron
loss
Amyotrophic Lateral Sclerosis (ALS) (Lou Gehrig's Disease)
– Symptoms—loss of ability to speak, swallow, and breathe
– Death typically occurs within five years
– Caused by environmental factors and genetic mutations involving RNA
processing
•Involves glutamate excitotoxicity
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Drug riluzole interferes with glutamate signaling – only treatment
Epilepsy
•Victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable
jerking; 1% of population
Assessing CNS Dysfunction
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Reflex tests
Imaging techniques
– CT, MRI, PET, radiotracer dyes for Alzheimer's, ultrasound, cerebral
angiography
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