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Chapter 12

Central Nervous System (CNS)

 

Central Nervous System (CNS)

•      CNS – composed of the brain and spinal cord

•      Cephalization

•    Elaboration of the anterior portion of the CNS

•    Increase in number of neurons in the head

•    Highest level has been reached in the human brain

The Brain

•      Composed of wrinkled, pinkish gray tissue

•      Surface anatomy includes cerebral hemispheres, cerebellum, and brain stem

Embryonic Development

•      During the first 26 days of development:

•    Ectoderm thickens along dorsal midline to form the neural plate

•    The neural plate invaginates, forming a groove flanked by neural folds

•    The neural groove fuses dorsally and forms the neural tube

Primary Brain Vesicles

•      The anterior end of the neural tube expands and constricts to form the three primary brain vesicles

•    Prosencephalon – the forebrain

•    Mesencephalon – the midbrain

•    Rhombencephalon – hindbrain

Secondary Brain Vesicles

•      In week 5 of embryonic development, secondary brain vesicles form:

•    Telencephalon and diencephalon arise from the forebrain

•    Mesencephalon remains undivided

•    Metencephalon and myelencephalon arise from the hindbrain

Adult Brain Structures

•      Fates of the secondary brain vesicles:

•    Telencephalon – cerebrum:  cortex, white matter, and basal nuclei

•    Diencephalon – thalamus, hypothalamus, and epithalamus

•    Mesencephalon – brain stem: midbrain

•    Metencephalon – brain stem: pons

•    Myelencephalon – brain stem: medulla oblongata

Adult Neural Canal Regions

•      Adult structures derived from the neural canal

•    Telencephalon – lateral ventricles

•    Diencephalon – third ventricle

•    Mesencephalon – cerebral aqueduct

•    Metencephalon and myelencephalon – fourth ventricle

Basic Pattern of the Central Nervous System

•      Spinal Cord

•    Central cavity surrounded by a gray matter core

•    External to which is white matter composed of myelinated fiber tracts

•      Brain

•    Similar to spinal cord but with additional areas of gray matter

•    Cerebellum has gray matter in nuclei

•    Cerebrum has nuclei and additional gray matter in the cortex

Ventricles of the Brain

•      Arise from expansion of the lumen of the neural tube

•      The ventricles are:

•    The paired C-shaped lateral ventricles

•    The third ventricle found in the diencephalon

•    The fourth ventricle found in the hindbrain dorsal to the pons

Cerebral Hemispheres

•      Form the superior part of the brain and make up 83% of its mass

•      Contain ridges (gyri) and shallow grooves (sulci)

•      Contain deep grooves called fissures

•      Are separated by the longitudinal fissure

•      Have three basic regions: cortex, white matter, and basal nuclei

Major Lobes, Gyri, and Sulci of
the Cerebral Hemisphere

•      Deep sulci divide the hemispheres into five lobes:

•    Frontal, parietal, temporal, occipital, and insula

•      Central sulcus – separates the frontal and parietal lobes

•      Parietal-occipital sulcus – separates the parieto and occipital lobes

•      Lateral sulcus – separates the parietal and temporal lobes

•      The precentral and postcentral gyri border the central sulcus

Cerebral Cortex

•      The cortex – superficial gray matter; accounts for roughly 40% of the mass of the brain

•      It enables sensation, communication, memory, understanding, and voluntary movements

•      Each hemisphere acts contralaterally (controls the opposite side of the body)

•      Hemispheres are not equal in function

•      No functional area acts alone; conscious behavior involves the entire cortex

Functional Areas of the Cerebral Cortex

•      Three types of functional areas are:

•    Motor areas – control voluntary movement

•    Sensory areas – conscious awareness of sensation

•    Association areas – integrate diverse information

Cerebral Cortex: Motor Areas

•      Primary (somatic) motor cortex

•      Premotor cortex

•      Broca’s area

•      Frontal eye field

Primary Motor Cortex

•      Located in the precentral gyrus

•      Composed of pyramidal cells whose axons make up the corticospinal tracts

•      Allows conscious control of precise, skilled, voluntary movements

•      Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function

Premotor Cortex

•      Located anterior to the precentral gyrus

•      Controls learned, repetitious, or patterned motor skills

•      Coordinates simultaneous or sequential actions 

•      Involved in the planning of movements

Broca’s Area and Frontal Eye Field

•      Broca’s area

•    Located anterior to the inferior region of the premotor area

•    Present in one hemisphere (usually the left)

•    A motor speech area that directs muscles of the tongue

•    Is active as one prepares to speak

•      Frontal eye field

•    Located anterior to the premotor cortex and superior to Broca’s area

•    Controls voluntary eye movement

Sensory Areas

•      Primary somatosensory cortex

•      Somatosensory association cortex

•      Visual areas

•      Auditory areas

•      Olfactory cortex

•      Gustatory cortex

•      Vestibular cortex

Primary Somatosensory Cortex

•      Located in the postcentral gyrus, this area:

•    Receives information from the skin and skeletal  muscles

•    Exhibits spatial discrimination

•      Somatosensory homunculus – caricature of relative amounts of cortical tissue devoted to each sensory function

Somatosensory Association Area

•      Located posterior to the primary somatosensory cortex

•      Integrates sensory information

•      Forms comprehensive understanding of the stimulus

•      Determines size, texture, and relationship of parts

Visual Area

•      Primary visual cortex

•    Located on the extreme posterior tip of the occipital lobe

•    Receives visual information from the retinas

•      Visual association area

•    Surround the primary visual cortex

•    Interprets visual stimuli (e.g., color, form, and movement)

Auditory Areas

•      Primary auditory cortex

•    Located at the superior margin of the temporal lobe

•    Receives information related to pitch, rhythm, and loudness

•      Auditory association area

•    Located posterior to the primary auditory cortex

•    Stores memories of sounds and permits perception of sounds

Association Areas

•      Prefrontal cortex

•      Language areas

•      General (common) interpretation area

•      Visceral association area

Prefrontal Cortex

•      Location – anterior portions of the frontal lobe

•      Involved with intellect, cognition, recall, and personality

•      Necessary for judgment, reasoning, persistence, and conscience

•      Closely linked to the limbic system (emotional part of the brain)

Language Areas

•      Located in a large area surrounding the left (or language-dominant) lateral sulcus

•      Major parts and functions:

•    Wernicke’s area –  involved in sounding out unfamiliar words

•    Broca’s area – speech preparation and production

•    Lateral prefrontal cortex – language comprehension and word analysis

•    Lateral and ventral temporal lobe – coordinate auditory and visual aspects of language

General (Common) Interpretation Area

•      Ill-defined region including parts of the temporal, parietal, and occipital lobes

•      Found in one hemisphere, usually the left

•      Integrates incoming signals into a single thought

•      Involved in processing spatial relationships

Visceral Association Area

•      Located in the cortex of the insula

•      Involved in conscious perception of visceral sensations

Lateralization of Cortical Function

•      Lateralization – each hemisphere has abilities not shared with its partner

•      Cerebral dominance – designates the hemisphere dominant for language

•      Left hemisphere – controls language, math, and logic

•      Right hemisphere – controls visual-spatial skills, emotion, and artistic skills

Cerebral White Matter

•      Consists of deep myelinated fibers and their tracts

•      It is responsible for communication between:

•    The cerebral cortex and lower CNS center, and areas of the cerebrum

•      Types include:

•    Commissures – connect corresponding gray areas of the two hemispheres

•    Association fibers – connect different parts of the same hemisphere

•    Projection fibers – enter the hemispheres from lower brain or cord centers

Basal Nuclei

•      Masses of gray matter found deep within the cortical white matter

•      The corpus striatum is composed of three parts

•    Caudate nucleus

•    Lentiform nucleus – composed of the putamen and the globus pallidus

•    Fibers of internal capsule running between and through caudate and lentiform nuclei

Functions of Basal Nuclei

•      Though somewhat elusive, the following are thought to be functions of basal nuclei:

•    Influence muscular activity

•    Regulate attention and cognition

•    Regulate intensity of slow or stereotyped movements

•    Inhibit antagonistic and unnecessary movement

Diencephalon

•      Central core of the forebrain

•      Consists of three paired structures –
thalamus, hypothalamus, and epithalamus

•      Encloses the third ventricle

Thalamus

•      Paired, egg-shaped masses that form the superolateral walls of the third ventricle

•      Connected at the midline by the intermediate mass

•      Contains four groups of nuclei – anterior, ventral, dorsal, and posterior

•      Nuclei project and receive fibers from the cerebral cortex

Thalamic Function

•      Afferent impulses from all senses converge and synapse in the thalamus

•      Impulses of similar function are “sorted out,” edited, and relayed as a group

•      All inputs ascending to the cerebral cortex pass through the thalamus

•      Plays a key role in mediating sensation, motor activities, cortical arousal, learning, and memory

Hypothalamus

•      Located below the thalamus, it caps the brainstem and forms the inferolateral walls of the third ventricle

•      Mammillary bodies:

•    Small, paired nuclei bulging anteriorly from the hypothalamus

•    Relay station for olfactory pathways

•      Infundibulum – stalk of the hypothalamus; connects to the pituitary gland

•    Main visceral control center of the body

Hypothalamic Function

•      Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, and many other visceral activities

•      Is involved with perception of pleasure, fear, and rage

•      Controls mechanisms needed to maintain normal body temperature

•      Regulates feelings of hunger and satiety

•      Regulates sleep and the sleep cycle

Endocrine Functions of the Hypothalamus

•      Releasing hormones control secretion of hormones by the anterior pituitary

•      The supraoptic and paraventricular nuclei produce ADH and oxytocin

Epithalamus

•      Most dorsal portion of the diencephalon; forms roof of the third ventricle

•      Pineal gland – extends from the posterior border and secretes melatonin

•    Melatonin – a hormone involved with sleep regulation, sleep-wake cycles, and mood

•      Choroid plexus – a structure that secretes cerebral spinal fluid (CSF)

Brain Stem

•      Consists of three regions – midbrain, pons, and medulla oblongata

•      Similar to spinal cord but contains embedded nuclei

•      Controls automatic behaviors necessary for survival

•      Provides the pathway for tracts between higher and lower brain centers

•      Associated with 10 of the 12 pairs of cranial nerves

Midbrain

•      Located between the diencephalon and the pons

•      Midbrain structures include:

•    Cerebral peduncles – two bulging structures that contain descending pyramidal motor tracts

•    Cerebral aqueduct – hollow tube that connects the third and fourth ventricles

•    Various nuclei

Midbrain Nuclei

•      Nuclei that control cranial nerves III (oculomotor) and IV (trochlear)

•      Corpora quadrigemina – four domelike protrusions of the dorsal midbrain

•      Superior colliculi – visual reflex centers

•      Inferior colliculi – auditory relay centers

•      Substantia nigra – functionally linked to basal nuclei

•      Red nucleus – largest nucleus of the reticular formation; red nuclei are relay nuclei for some descending motor pathways

Pons

•      Bulging brainstem region between the midbrain and the medulla oblongata

•      Forms part of the anterior wall of the fourth ventricle

•      Fibers of the pons:

•    Connect higher brain centers and the spinal cord

•    Relay impulses between the motor cortex and the cerebellum

•      Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)

•      Contains nuclei of the reticular formation

Medulla Oblongata

•      Most inferior part of the brain stem

•      Along with the pons, forms the ventral wall of the fourth ventricle

•      Contains a choroid plexus on the ventral wall of the fourth ventricle

•      Pyramids – two longitudinal ridges formed by corticospinal tracts

•      Decussation of the pyramids – crossover points of the corticospinal tracts

Medulla Nuclei

•      Inferior olivary nuclei – gray matter that relays sensory information

•      Cranial nerves X, XI, and XII are associated with the medulla

•      Vestibular nuclear complex – synapses that mediate and maintain equilibrium

•      Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis

•      Cardiovascular control center – adjusts force and rate of heart contraction

•      Respiratory centers – control rate and depth of breathing

The Cerebellum

•      Located dorsal to the pons and medulla

•      Protrudes under the occipital lobes of the cerebrum

•      Makes up 11% of the brain’s mass

•      Provides precise timing and appropriate patterns of skeletal muscle contraction

•      Cerebellar activity occurs subconsciously

Anatomy of the Cerebellum

•      Two bilaterally symmetrical hemispheres connected medially by the vermis

•      Folia – transversely oriented gyri

•      Each hemisphere has three lobes – anterior, posterior, and flocculonodular

•      Neural arrangement – gray matter cortex, internal white matter, scattered nuclei

•      Arbor vitae – distinctive treelike pattern of the cerebellar white matter

Cerebellar Peduncles

•      Three paired fiber tracts that connect the cerebellum to the brain stem

•      All fibers in the cerebellum are ipsilateral

•      Superior peduncles connect the cerebellum to the midbrain

•      Middle peduncles connect the pons to the cerebellum

•      Inferior peduncles connect the medulla to the cerebellum

Cerebellar Processing

•      Cerebellum receives impulses of the intent to initiate voluntary muscle contraction

•      Proprioceptors and visual signals “inform” the cerebellum of the body’s condition

•      Cerebellar cortex calculates the best way to perform a movement

•      A “blueprint” of coordinated movement is sent to the cerebral motor cortex

Cerebellar Cognitive Function

•      Plays a role in language and problem solving

•      Recognizes and predicts sequences of events

Functional Brain System

•      Networks of neurons working together and spanning wide areas of the brain

•      The two systems are:

•    Limbic system

•    Reticular formation

Limbic System

•      Structures located on the medial aspects of cerebral hemispheres and diencephalon

•      Includes the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus

•      Parts especially important in emotions:

•    Amygdala – deals with anger, danger, and fear responses

•    Cingulate gyrus – plays a role in expressing emotions via gestures, and resolves mental conflict

•      Puts emotional responses to odors – e.g., skunks smell bad

Limbic System: Emotion and Cognition

•      The limbic system interacts with the prefrontal lobes, therefore:

•    One can react emotionally to conscious understandings

•    One is consciously aware of emotion in one’s life

•      Hyppocampal structures – convert new information into long-term memories

Reticular Formation

•      Composed of three broad columns along the length of the brain stem

•    Raphe nuclei

•    Medial (large cell) group

•    Lateral (small cell) group

•      Has far-flung axonal connections with hypothalamus, thalamus, cerebellum, and spinal cord

Reticular Formation: RAS and Motor Function

•      RAS – reticular activating system

•    Sends impulses to the cerebral cortex to keep it conscious and alert

•    Filters out repetitive and weak stimuli

•      Motor function

•    Helps control coarse motor movements

•    Autonomic centers regulate visceral motor
functions – e.g., vasomotor, cardiac, and respiratory centers

Protection of the Brain

•      The brain is protected by bone, meninges, and cerebrospinal fluid

•      Harmful substances are shielded from the brain by the blood-brain barrier

Meninges

•      Three connective tissue membranes that lie external to the CNS – dura mater, arachnoid mater, and pia mater

•      Functions of the meninges include:

•    Cover and protect the CNS

•    Protect blood vessels and enclose venous sinuses

•    Contain cerebrospinal fluid (CSF)

•    Form partitions within the skull

Meninges

Dura Mater

•      Leathery, strong meninx composed of two fibrous connective tissue layers

•      The two layers separate in certain areas and form dural sinuses

•      Three dural septa extend inward and limit excessive movement of the brain

•    Falx cerebri – fold that dips into the longitudinal fissure

•    Falx cerebelli – runs along the vermis of the cerebellum

•    Tentorium cerebelli – horizontal dural fold extends into the transverse fissure

Arachnoid Mater

•      The middle meninx, which forms a loose brain covering

•      It is separated from the dura mater by the subdural space

•      Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels

•      Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood

Pia Mater

•      Deep meninx composed of delicate connective tissue that clings tightly to the brain

Cerebrospinal Fluid (CSF)

•      Watery solution similar in composition to blood plasma

•      Contains less protein and different ion concentrations than plasma

•      Forms a liquid cushion that gives buoyancy to the CNS organs

•      Prevents the brain from crushing under its own weight

•      Protects the CNS from blows and other trauma

•      Nourishes the brain and carries chemical signals throughout it

Choroid Plexuses

•      Clusters of capillaries that form tissue fluid filters, which hang from the roof of each ventricle

•      Have ion pumps that allow them to alter ion concentrations of the CSF

•      Help cleanse CSF by removing wastes

Blood-Brain Barrier

•      Protective mechanism that helps maintain a stable environment for the brain

•      Bloodborne substances are separated from neurons by:

•    Continuous endothelium of capillary walls

•    Relatively thick basal lamina

•    Bulbous feet of astrocytes

Blood-Brain Barrier: Functions

•      Selective barrier that allows nutrients to pass freely

•      Is ineffective against substances that can diffuse through plasma membranes

•      Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood

•      Stress increases the ability of chemicals to pass through the blood-brain barrier

Cerebrovascular Accidents (Strokes)

•      Caused when blood circulation to the brain is blocked and brain tissue dies

•      Most commonly caused by blockage of a cerebral artery

•      Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis

•      Transient ischemic attacks (TIAs) – temporary episodes of reversible cerebral ischemia

Spinal Cord

•      CNS tissue is enclosed within the vertebral column from the foramen magnum to L1

•      Provides two-way communication to and from the brain

•      Protected by bone, meninges, and CSF

•      Epidural space – space between the vertebrae and the dural sheath (dura mater) filled with fat and a network of veins

•      Conus medullaris – terminal portion of the spinal cord

•      Filum terminale – fibrous extension of the pia mater; anchors the spinal cord to the coccyx

•      Denticulate ligaments – delicate shelves of pia mater; attach the spinal cord to the vertebrae

•      Spinal nerves – 31 pairs attach to the cord by paired roots

•      Cervical and lumbar enlargements – sites where nerves serving the upper and lower limbs emerge

•      Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

Embryonic Development of the Spinal Cord

•      Develops from caudal portion of neural tube

•      By week 6, there are two clusters of neuroblasts:

•    Alar plate –
will become interneurons

•    Basal plate –
will become motor neurons

•      Neural crest cells form the dorsal root ganglia

Cross-Sectional Anatomy of the Spinal Cord

•      Anterior median fissure – separates anterior funiculi

•      Posterior median sulcus – divides posterior funiculi

Gray Matter and Spinal Roots

•      Gray matter consists of soma, unmyelinated processes, and neuroglia

•      Gray commissure – connects masses of gray matter; encloses central canal

•      Posterior (dorsal) horns – interneurons

•      Anterior (ventral) horns – interneurons and somatic motor neurons

•      Lateral horns – contain sympathetic nerve fibers

Gray Matter: Organization

•      Dorsal half – sensory roots and ganglia

•      Ventral half – motor roots

•      Dorsal and ventral roots fuse laterally to form spinal nerves

•      Four zones are evident within the gray matter – somatic sensory (SS), visceral sensory (VS), visceral motor (VM), and somatic motor (SM)

White Matter in the Spinal Cord

•      Fibers run in three directions – ascending, descending, and transversely

•      Divided into three funiculi (columns) – posterior, lateral, and anterior

•      Each funiculus contains several fiber tracks

•    Fiber tract names reveal their origin and destination

•    Fiber tracts are composed of axons with similar functions

White Matter: Pathway Generalizations

•      Pathways decussate

•      Most consist of two or three neurons

•      Most exhibit somatotopy (precise spatial relationships)

•      Pathways are paired (one on each side of the spinal cord or brain)

Spinal Cord Trauma: Paralysis

•      Paralysis – loss of motor function

•      Flaccid paralysis – severe damage to the ventral root or anterior horn cells

•    Lower motor neurons are damaged and impulses do not reach muscles

•    There is no voluntary or involuntary control of muscles

•      Spastic paralysis – only upper motor neurons of the primary motor cortex are damaged

•    Spinal neurons remain intact and muscles are stimulated irregularly

•    There is no voluntary control of muscles

Spinal Cord Trauma: Transection

•      Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut

•      Paraplegia – transection between T1 and L1

•      Quadriplegia – transection in the cervical region

Poliomyelitis

•      Destruction of the anterior horn motor neurons by the poliovirus

•      Early symptoms – fever, headache, muscle pain and weakness, and loss of somatic reflexes

•      Vaccines – Salk and Sabin polio vaccines are available and can prevent infection

Amyotrophic Lateral Sclerosis (ALS)

•      Lou Gehrig’s disease – neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract

•      Symptoms – loss of the ability to speak, swallow, and breathe

•      Death occurs within five years

•      Linked to malfunctioning genes for glutamate transporter and/or superoxide dismutase

Developmental Aspects of the CNS

•      CNS is established during the first month of development

•      Gender-specific difference appear in response to testosterone (or lack thereof)

•      Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the fetus’ developing CNS

•      Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage

Developmental Aspects

•      The hypothalamus is one of the last area of the CNS to develop

•      Visual cortex develops slowly over the first 11 weeks

•      Growth and maturation of the nervous system occurs throughout childhood and reflects progressive myelination

•      Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s

•      Excessive use of alcohol causes signs of senility unrelated to the aging process