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12

The Central Nervous System

Part A

Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint^® Lecture Slides prepared by Vince Austin, University of Kentucky

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Central Nervous System (CNS)

CNS – composed of the brain and spinal cord
Cephalization

Elaboration of the cranial portion of the CNS
Increase in number of neurons in the head
Highest level is reached in the human brain

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The Brain

Composed of wrinkled, pinkish gray tissue
Surface anatomy includes cerebral hemispheres, cerebellum and brain stem

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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

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Embryonic Development

Surface ectoderm

(a) 19 days

(b) 20 days

(c) 22 days

(d) 26 days

Neural folds

Neural crest

Surface ectoderm

Neural groove

Neural tube

Anterior (rostral) end

Figure 12.1

Level of section

Neural plate

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Day 13

Day 16

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21-23 days

19-21 days

17-19 days

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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

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Neural Tube and Primary Brain Vesicles

Figure 12.2a, b

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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

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Secondary Brain Vesicles

Figure 12.2c

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Adult Brain Structures

Fates of the secondary brain vesicles:

Telencephalon – cerebrum
Diencephalon – thalamus, hypothalamus, and epithalamus
Mesencephalon – brain stem: midbrain
Metencephalon – brain stem: pons and cerebellum
Myelencephalon – brain stem: medulla oblongata

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Adult Neural Canal Regions

Figure 12.2c, d

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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

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Adult Neural Canal Regions

Figure 12.2c, e

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Space Restriction and Brain Development

Figure 12.3

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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

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Ventricles of the Brain

Figure 12.5

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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

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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

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Major Lobes, Gyri, and Sulci of the Cerebral Hemisphere

Parieto-occipital sulcus – separates the parietal and occipital lobes
Lateral sulcus – separates the parietal and temporal lobes
The precentral and postcentral gyri border the central sulcus

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Cerebral Cortex

The cortex – superficial gray matter; accounts for 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

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Functional Areas of the Cerebral Cortex

The three types of functional areas are:

Motor areas – control voluntary movement
Sensory areas – conscious awareness of sensation
Association areas – integrate diverse information

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Functional Areas of the Cerebral Cortex

Figure 12.8a

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Functional Areas of the Cerebral Cortex

Figure 12.8b

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Cerebral Cortex: Motor Areas

Primary (somatic) motor cortex
Premotor cortex
Broca’s area
Frontal eye field

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Primary Motor Cortex

Located in the precentral gyrus
Allows conscious control of precise, skilled, voluntary movements
Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function

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Primary Motor Cortex

Figure 12.9.1

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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
Think “MUSCLE MEMORY”

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Broca’s Area

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

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Frontal Eye Field

Frontal eye field

Located anterior to the premotor cortex and superior to Broca’s area
Controls voluntary eye movement

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Sensory Areas

Primary somatosensory cortex
Somatosensory association cortex
Visual and auditory areas
Olfactory, gustatory, and vestibular cortices

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Sensory Areas

Figure 12.8a

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Primary Somatosensory Cortex

Located in the postcentral gyrus, this area:

Receives information from the skin and skeletal muscles

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

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Primary Somatosensory Cortex

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Complete Homunculus

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Somatosensory Association Cortex

Located posterior to the primary somatosensory cortex
Integrates sensory information
Forms comprehensive understanding of the stimulus
Determines size, texture, and relationship of parts

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Visual Areas

Primary visual cortex

Seen on the extreme posterior tip of the occipital lobe
Receives visual information from the retinas

Visual association area

Surrounds the primary visual cortex
Interprets visual stimuli (e.g., color, form, and movement)

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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
Wernicke’s area

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Association Areas

Prefrontal cortex
Language areas
General (common) interpretation area
Visceral association area

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Association Areas

Figure 12.8a

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Prefrontal Cortex

Located in the anterior portion 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)

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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

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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

6^th sense?

Involved in processing spatial relationships

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Visceral Association Area

Located in the cortex of the insular lobe
Involved in conscious perception of visceral sensations and addictive behaviors

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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

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Diencephalon

Central core of the forebrain
Consists of three paired structures – thalamus, hypothalamus, and epithalamus
Encloses the third ventricle

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Diencephalon

Figure 12.12

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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, learning, and memory

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Hypothalamus

Located below the thalamus, it caps the brainstem and forms the inferolateral walls of the third ventricle
Infundibulum – stalk of the hypothalamus; connects to the pituitary gland

Main visceral control center of the body

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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

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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)

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Epithalamus

Figure 12.12

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Brain Stem

Consists of three regions – midbrain, pons, and medulla oblongata
Similar to spinal cord
Controls automatic behaviors necessary for survival
Associated with 10 of the 12 pairs of cranial nerves

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Brain Stem

Figure 12.15c

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Midbrain

Located between the diencephalon and the pons
Origin of cranial nerves III (oculomotor) and IV (trochlear)

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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

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Pons

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

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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
Origin of cranial nerves X (vagus), XI (accessory) and XII (hypoglossal)

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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 (coordination)
Cerebellar activity occurs subconsciously

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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

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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

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Meninges

Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater
Functions of the meninges

Cover and protect the CNS
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid (CSF)
Form partitions within the skull

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Meninges

Figure 12.23a

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Dura Mater

Leathery, strong meninx composed of two fibrous connective tissue layers
The two layers separate in certain areas and form dural sinuses

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Dura Mater

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

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Dura Mater

Figure 12.24

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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

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Arachnoid Mater

Figure 12.23a

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Pia Mater

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

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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

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Choroid Plexuses

Clusters of capillaries that form tissue fluid filters, which hang from the roof of each ventricle
Help cleanse CSF by removing wastes

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Choroid Plexuses

Figure 12.25a

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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

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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

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12

The Central Nervous System

Part D

Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint^® Lecture Slides prepared by Vince Austin, University of Kentucky

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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

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Spinal Cord

Figure 12.28a

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Spinal Cord

Conus medullaris – terminal portion of the spinal cord
Filum terminale – fibrous extension of the pia mater; anchors the spinal cord to the coccyx

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Spinal Cord

Spinal nerves – 31 pairs attach to the cord by paired roots
Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

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Cross-Sectional Anatomy of the Spinal Cord

Figure 12.30a

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Gray Matter: Organization

Dorsal half – sensory roots
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)

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Gray Matter: Organization

Figure 12.31

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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

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Spinal Cord Trauma: Paralysis

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

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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

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Poliomyelitis

Destruction of the anterior horn motor neurons by the polio virus
Early symptoms – fever, headache, muscle pain and weakness, and loss of somatic reflexes
Vaccines are available and can prevent infection

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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