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