Chapter 15 A, Sp 10

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Human Anatomy, Second Edition

McKinley & O'Loughlin

Chapter 15A Lecture Outline: Brain and Cranial Nerves

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Brain and Cranial Nerves An adult brain weighs 1.35 - 1.4 kg

(about 3 pounds) and has a volume of about 1200 cc

Brain size is not directly correlated with intelligence

It is the number of active synapses!

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The Brain’s 4 Major Regions The cerebrum, diencephalon,

brainstem, and cerebellum. Left and right cerebral hemispheres.

Each has five functional areas called lobes. Outer surface of an adult brain exhibits

folds and shallow depressions Inferior portion has 12 pairs of cranial

nerves.

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Brain Development and Tissue Organization

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Embryonic Development of the Brain

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Organization of Brain Tissue Gray matter has cell bodies,

dendrites, axon terminals, and unmyelinated axons.

White matter is composed of myelinated axons.

The gray cortex covers the surface of most of the adult brain. About 2-4 mm thick.

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Organization of Neural Tissue Areas in the Brain White matter lies deep to the

gray matter of the cortex. Within the white matter are

clusters of neuron cell bodies,cerebral nuclei.

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Support and Protection of the Brain The brain is protected and isolated

by several structures: bony cranium meninges cerebrospinal fluid (CSF) blood-brain barrier

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3 Cranial Meninges Connective tissue layers separate the

soft tissue of the brain from the cranial bones.

Enclose and protect blood vessels that supply the brain.

Contain and circulate CSF. Help form some veins that drain blood

from the brain.

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Pia Mater (“Gentle Mother”) Thin layer of delicate areolar tissue

that follows every contour of the brain surface.

Not visible except with a microscope.

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Arachnoid (Spider Web Like) Partially a delicate web of collagen

and elastic fibers, termed the arachnoid trabeculae.

Between the subdural space and the subarachnoid space.

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Dura Mater (“Tough Mother”) Strongest meninx. Two dense, irregular C.T. layers.

periosteal layer is the periosteum of the cranial bones

meningeal layer is deep to the periosteal layer

Fused, except at the dural venous sinuses.

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4 Cranial Dural Septa The meningeal layer of the dura mater

extends as flat partitions (septa) deep into the cranial cavity as the cranial dural septa.

These separate parts of the brain and give stabilization and support to the entire brain. Falx cerebri Falx cerebelli Tentorium cerebelli Diaphragma sellae

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Hematomas Epidural hematoma

Usually from a severe blow at the pterion: junction of temporal, sphenoid, frontal, and parietal bones

Fracture results in arterial hemorrhage. Typically, lose consciousness, awake and seem fairly

normal, few hours later brain damaged and neurologic injury and death if not treated

Subdural hematoma Usually from broken veins from fast or violent

rotation of the head Blood accumulates slowly, but still a serious medical

emergency

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Concussion Most common brain injury Loss of consciousness after a blow or sudden stop No obvious physical defect but many symptoms may

occur Cumulative effect

Contusion Visible bruising of the brain May be from torn pia mater Loss of consciousness briefly May have respiration abnormalities and decreased

blood pressure

Brain Injuries

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4 Brain Ventricles Cavities derived from the lumen

(opening) of the embryonic neural tube.

Continuous with one another as well as with the central canal of the spinal cord.

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Cerebrospinal Fluid A clear, colorless liquid that

circulates in the ventricles and subarachnoid space.

Performs several important functions. buoyancy protection environmental stability

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CSF Formation Formed from blood plasma by

the choroid plexus, capillaries covered by ependymal cells, in each ventricle.

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CSF Circulation 500 ml produced per day 100-160 ml total volume

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Hydrocephalus (“Water on the Brain”) Excessive CSF Prior to suture closure, head enlarges

and neurologic damage may occur After suture closure, brain compressed

and results in permanent brain damage Treatment of severe cases: shunt

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Blood-Brain Barrier There are differential rates of

passage for different substances in order to protect the brain’s neurons.

Prevents exposure of neurons in the brain to drugs, toxins, waste products in the blood, and variations in levels of normal substances (ions, hormones) that could adversely affect brain function.

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Examples Quickly pass through the barrier;

Lipid-soluble substances and water-soluble substances with a carrier by active transport

Glucose, gases such as O2 and CO2, water, and most lipid soluble substances (ethanol and anaesthetics)

Caffeine, nicotine, and heroin Slowly pass through the barrier;

Na+, K+, Cl-, creatinine, and urea Do NOT pass through the barrier:

Proteins, most antibiotics, and chemotherapeutic agents

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The Barrier The brain capillaries are the

least permeable in the body. Tight junctions A continuous basement

membrane that totally surrounds the capillaries.

Astrocytes act as “gatekeepers.”

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Barrier Missing or Lacking in Places Circumventricular organs (organs

around the 3rd and 4th ventricles) lack the BBB: hypothalamus, pituitary, pineal gland, and other nearby glands

Missing where there is a need to monitor blood chemical changes to coordinate endocrine and nervous systems. Missing in the choroid plexus where CSF made.

HIV virus may enter where the BBB is lacking.

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Damage to the BBB Trauma can damage the BBB During prolonged emotional stress, the

tight junctions are opened, so the BBB fails and toxic substances in the blood can enter the brain tissue. Implicated in the neurological symptoms associated with Gulf War syndrome.

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Tricking the BBB High glucose solutions can be used

to shrink the cells to make gaps for antibiotics to enter.

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Cerebrum Location of conscious thought

processes and origin of all complex intellectual functions

Two large hemispheres Gray cerebral cortex with inner white

matter and islands of gray cerebral nuclei

Folded ridges are gyri separated by shallow depressions called sulci or deeper grooves called fissures

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Cerebral Hemispheres Joined by white matter tracts in

some places. Hard to assign specific function to

a specific region. Usually contralateral. Anatomically almost mirror images

but some functional differences, hemispheric lateralization.

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Lobes of the Cerebrum Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula (deep to the lateral

sulcus)

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Mapping Some specific structural areas have distinct

motor and sensory functions Some higher mental functions are dispersed

over large areas Three categories of functional areas

Motor (voluntary movement) Sensory (conscious awareness of

sensation) Association (primarily integrate and

store information)

Functional Areas of the Cerebrum

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Primary motor cortex in precentral gyrus Voluntary skeletal muscle activity Motor homunculus (surface area proportional to the

number of motor units for the area) Motor speech area (Broca’s area) - left side

Regulates patterns of breathing and controls the muscle movements for speech

Frontal eye field Control and regulate eye movements for reading and

coordinating binocular vision Anterior to premotor cortex, a motor association area

Motor Areas in Frontal Lobes

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Sensory Areas Primary somatosensory cortex

General somatic sensory information (conscious)

Postcentral gyrus in parietal lobe Sensory homunculus (surface area proportional

to the number of receptors for the area) Primary visual cortex in occipital lobe Primary auditory and olfactory

cortices in temporal lobe Gustatory cortex in insula

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Association Areas Connected to primary areas - process and

interpret incoming information or coordinate a motor response. They integrate new sensory inputs with memories of past experiences.

Premotor cortex mainly coordinates learned, skilled motor activities

Somatosensory association area integrates and interprets sensations to determine the texture, temperature, pressure, and shape of objects

Auditory association area interprets the characteristics of sound and store memories of sounds heard in the past.

Visual association area analyzes color, movement, and form to ID what we see

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Wernicke’s area is involved in recognizing, understanding, and comprehending spoken or written language. Must work with Broca’s area.

Gnostic area (common integrative area or general interpretation area) integrates all sensory association area information to give a comprehensive understanding of a current activity

Multi-Association Areas

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Process incoming information from several association areas and ultimately direct either extremely complex motor activity or complicated analytical functions in response.

Examples: speech, cognition, understanding spatial relationships, and general interpretation

Higher-Order Processing Centers

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Prefrontal Cortex Most complex area

Intelligence, complex learning abilities, personality

Also: abstract ideas, judgment, reasoning, persistence, planning, concern for others, and conscience

Prefrontal lobotomy used from 1930-1950s to treat severe mental illness Epilepsy and abnormal personality changes Use drugs now

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The Case of Phineas Gage Railroad accident in 1848 drove a 3 1/2

ft. tamping rod through his frontal lobe Survived, but changed his personality

Formerly, capable, well-balanced and shrewd, etc.

After, irreverent, fitful, profane, inconsiderate, and could not make decisions

Frontal lobes important in personality and basic elements of decision making

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The Insula Not well understood The insula and anterior cingulate cortex are crucial

centers of emotional cognition. They are also necessary for attending to feelings that arise from your body and for experiencing pain.

The more viscerally aware you are, the more emotionally attuned you are.

Your ability to read and interpret sensations arising from within your own body, interoception, is essential to fundamental features of being human: sentiment, sentience, and emotional awareness.

(from Scientific American Mind, Aug./Sept. 2007)

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Central White Matter Association tracts

Connect nearby gyri within the same hemisphere

Commissural tracts Connect the two hemispheres Corpus callosum, anterior and posterior

commissures Projection Tracts

Connect the cerebral cortex to the inferior brain regions and the spinal cord

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Cerebral or Basal Nuclei Corpus striatum (largest part)

Caudate nucleus stimulates muscles to produce the pattern and rhythm of arm and leg movements associated with walking

Lentiform nucleus Putamen controls muscular movement

at the subconscious level Globus pallidus controls and adjusts

muscle tone

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