The Brain and Cranial Nerves

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The average male adult brain weighs about 3.5 lbs.
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Composed of 3 divisions:
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Cerebrum = Cerebral cortex and Basal nuclei
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Cerebellum – posterior and inferior to cerebrum
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Brainstem: Diencephalon (Thalamus, Hypothalamus and
Epithalamus), Midbrain (superior and inferior colliculi),
Pons and Medulla oblongata

Absorbs CSF back into venous sinuses and blood

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Cavities within the cerebrum and brain stem where cerebrospinal fluid
(CSF) is manufactured and circulated.
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Two lateral ventricles over thalamus and roll out into temporal lobes, medial third and fourth ventricles joined by cerebral aqueduct.
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Lateral ventricles look like a Rocky Mountain sheep Rams horns and communicate to third ventricle via interventricular foreamen.
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CSF is formed by the ependymal cells of choroid plexus in all four ventricles; is an ultrafiltrate of blood- no cells present; high in Na and K.
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CSF circulates through ventricular system as well as central canal of spinal cord and subarachnoid space around brain and spinal cord.

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CSF formation and circulation into and from blood vascular system .
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CSF provides protection and nourishment to the brain and spinal cord.
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Has higher levels of Na and K than blood.

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Cerebrum composes the vast majority of the brain mass (80%). It is divided into 2 halves or hemispheres, a right and a left hemisphere.
Longitudinal fissure- midsagittal groove dividing the two hemispheres into a right and left side.
Consists of an outer layer of gray matter over white matter (like crust on white bread).
Sulci and gyri = valleys and ridges on surface of cerebral hemispheres.
Five lobes: Frontal, parietal, occipital, temporal and insula.

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Blood flow to the brain is provided by the internal carotid arteries and the vertebral arteries.
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The blood brain barrier (BBB) is thought to be due to specialized endothelial cells that are influenced by the glial astrocytes.
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In the choroid plexus there is also a CSF-BBB formed by the ependymal cells.
The BBB is absent in some places of the 3 rd and 4 th ventricles at patches called circumventricular organs where some substances may pass into the brain tissue.

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The cortex is the outer layer of the cerebrum is composed of gray matter and is where our
“consciousness” is located.
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It allows us to be who we are, perceive sensations, comprehend and understand, as well as learn and, communicate with the ability to act and react at a conscious level.

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 sulcus/sulci – the valleys or grooves found between the gyri. Some of these also have specific names.
central sulcus – separates the frontal and parietal lobes..
lateral sulcus – separates the temporal lobe from the parietal/ frontal lobes.
parieto-occipital sulcus – separates the parietal and occipital lobes.

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gyrus/gyri – the elevated ridges of tissue in the cortex
1. precentral gyrus- motor cortex area 4
2. post central gyrus- sensory cortex areas 1-3
- homunculus (little man)- portrays motor and sensory areas of body overlaying precentral and postcentral gyri of the cortex

Three important areas of cortex
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Motor areas control voluntary movement.
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Sensory areas provide consciousness and awareness of sensations.
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Association areas act mainly to integrate diverse information into decisive and meaningful actions. This is done via interneurons or association neurons.

Cortical motor, sensory and association areas

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1° (primary) Motor Cortex – located in the precentral
gyrus. Also known as Broadmann’s area # 4.
-Contains large Pyramidal neurons that allow us to perform precise and skilled movements with our skeletal muscles.
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-Motor innervation is contralateral i.e. the left side of the brain controls the right side of the body and vice versa.
Broca’s Area – located superior to lateral sulcus and anterior to precentral gyrus. Associated with motor speech and is usually located in the left hemisphere
Premotor cortex anterior to precentral gyrus (Broadmann 6). thought to be site of memory bank for fine motor skils.

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1° (primary) Somatosensory Cortex – located in the postcentral gyrus of the parietal lobe. Areas 1-3
Visual Sensory Area – Located on the extreme posterior tip of the occipital lobe. Receives input from retina; is largest cortical sensory area.
Auditory Area – medial aspects of the temporal lobes.
Gustatory Area –parietal lobe deep to the temporal lobe at the tip of the tongue of humunculus.

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Longitudinal and transverse fissures
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Lateral, Central and Parieto-occipital sulci
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Precentral (motor #4) and Postcentral (sensory #1-3) gyri
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Lobes: Frontal, Parietal, Temporal, Occipital and
Insula
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Basal nuclei: Caudate nucleus, Putamen, Globus pallidus
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Corpus callosum, septum pellucidum, fornix, and internal capsule

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White matter is myelinated neuronal tissue in brain and spinal cord and contains neuronal tracts.
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Gray matter is unmyelinated and contains mostly neuronal nuclear groups or nuclei and unmyelinated tracts.

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is composed of myelinated processes. Within the white matter are “roads” of ordered groups of neuron processes called tracts. There are three major types of tracts in the cerebral cortex:
Commissural fibers – connect the gray matter between the two hemispheres. e.g. corpus callosum
Association fibers – connect adjacent gyri in same hemisphere. e.g. visual and auditory association areas.
Projection fibers – these connect to regions outside of the cerebrum e.g. internal capsule; corona radiata

Communication fibers

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Masses of cerebral gray matter buried deep in each hemisphere in the white matter and lateral to the thalamus.
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Caudate nucleus, globus pallidus and putamen
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Communicate with cortex in controlling movement
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Complete function is ??, but may regulate start, stop and intensity of voluntary body movements from cortical levels.

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Concerned with sense of smell in association with long term memory and emotional responses (anger, fear, rage, sex and hunger).
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Includes cingulate gyrus, amygdala, hippocampus, septum, fornix, thalamus, hypothalamus and olfactory bulbs.

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Thalamus is sensory relay nucleus from spinal cord and brainstem regions to the cerebral cortex. Located at top of brainstem, each half is joined by intermediate mass.
Hypothalamus nuclear group just inferior to thalamus consisting of ~ 12 nuclei. Third ventricle is roof.
-Main visceral control center for many systems: regulates
ANS, GI, cardiovascular centers, sleep/wake, emotions, behavior, hunger/thirst, body temp, memory, sex drive and endocrine system.
Epithalamus pineal gland and habenula produce melatonin for sleep/wake.

Diencephalon, Midbrain, Pons and Medulla

Thalamus

Epithalamus
-Pineal gland
-Habenular nuc.

Cerebral and Diencephalon
Functions

(see Figure 15.7; p. 422)
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Extends from posterior border of diencephalon to the beginning of spinal cord at foreamen magnum.
Consists of Midbrain, Pons, and Medulla oblongata
-controls automatic behavior of systems necessary for life
(e.g. cardiovascular, respiratory, emetic, etc.)
-passageway for all tracts to cerebrum and spinal cord
-contains nuclei of many cranial nerves to face and neck
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Has same pattern of gray/white matter as spinal cord (i.e. white matter surrounds gray matter)

(Lateral view)

Brain Stem
(Diencephalon, Midbrain and Hindbrain)

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Lies between a plane that extends from behind pineal gland down to posterior end of mamillary bodies and caudally to the rostral part of the pons.
Central cavity is cerebral aqueduct with tectum (roof) dorsally and cerebral peduncles ventrally.
Periaqueductal gray is involved in fight or flight (Sym)
Contains copra quadrigemina (sup & inf colliculi) which are relay nuclei for vision and hearing respectively.
Other nuclei include substantia nigra and red nucleus
Contains nuclei for occulomotor (CN-III) and trochlear (CN-
IV).

(X-section)

(lateral view)

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Composed of 100’s of small gray nuclei that extend from medulla, pons and midbrian and project up to the diencephalon, cerebellum and cerebrum.
It receives diverse input and sends a continuous stream of information to cerebral cortex keeping it alert to all activity during sleep and consciousness. It is often referred to as the reticular activating system
(RAS).
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The motor portion sends neurons to the spinal cord via reticulospinal tracts.
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Functions include cardiovascular control, pain control, breathing, swallowing, and habituation to environment

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Bulging region on inferior surface of brain stem.
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Lies between the midbrain and medulla oblongata.
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Forms a bridge between left and right halves of cerebellum.
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Fourth ventricle separates pons from cerebellum.
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Cranial nerves V (trigeminal), VI (abducens), and VII
(facial) arise from pontine nuclei.
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Middle cerebellar peduncles carry fibers from Pons to cerebellum.

(lateral view)

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Extends from Pons to spinal cord
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Pyramids (pyramidal tracts) decussate (crossover) at caudal border and are motor corticospinal tracts.
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Cranial nerves VIII (vestibulocochlear), IX
(glossopharyngeal), X (Vagus), XI (accessory) and XII
(hypoglossal) arise from medullary nuclei near fourth ventricle.
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Visceral centers for cardiac, vasomotor and respiratory centers are in medulla.

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Cauliflower-like looking mass at posterior part of brain and dorsal to the pons and medulla.
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Coordinates and smoothes body movements, and maintains posture and equilibrium.
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Two lateral hemispheres are connected by vermis
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Hemispheres subdivided into 3 lobes:
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1. anterior, 2. posterior and 3. flocculonodular
Anterior and posterior coordinate movements
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Flocculonodular adjusts posture and equilibrium

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Has three regions: Outer cortical gray matter, Inner white matter (arbor vitae) and Deep cerebellar nuclei.
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Cerebellar cortical nuclei function to smooth body movements.
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White matter contains tracts that carry information to and from cerebral cortex.
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Deeper nuclei relay instructions to other areas of brain from cerebellar cortex to refine body movements.

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Cerebellar cortex receives 3 forms of information :
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1). Equilibrium, 2). Position of muscles in limbs, neck and trunk and 3). Info from cerebral cortex.
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Cerebellar peduncles are fibers tracts that connect cerebellum with different parts of brain.
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Unlike cerebral motor outflow which is contralateral, cerebellar outflow is ipsilateral (same side).

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Twelve pairs of cranial nerves arise from nuclei in brain and migrate out of cranium to innervate peripheral organs.
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First 2 pairs arise from forebrain, the remainder come from brainstem nuclei
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They are numbered in Roman numerals I thru XII.
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All cranial nerves except for X innervate head and neck.
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Cranial nerve X (Vagus) innervates head, neck, thorax and abdomen.

I Olfactory nerve (S)
II Optic nerve (S)
III Occulomotor (M)
IV Trochlear (M)
V Trigeminal (B)
VI Abducens (M)
VII Facial (B)
VIII Vestibulochochlear (S)
IX Glossopharyngeal (B)
X Vagus (B)
XI Accessory (M)
XII Hypoglossal (M)

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Strictly sensory for smell.
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Nerve fibers are in cribriform plate of ethmoid bone

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Strictly sensory for vision
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Enters cranium via optic foramen anterior to pituitary gland in sella turcica.

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Primarily motor nerve to extrinsic eye muscles
(inferior oblique, sup., inf. and medial rectus muscles)

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Primarily motor to superior oblique eye muscles

IV Trochlear nerve to sup. oblique

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Mixed nerve with 3 divisions to face; largest cranial nerve.

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Primarily motor to lateral rectus muscle of eye

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Mixed nerve with 5 branches to neck and face.

VII Facial nerve distribution – five branches to face & neck

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Purely sensory for hearing and equilibrium

VIII Vestibulocochlear to inner ear

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Mixed nerve to pharynx, post. tongue, parotid glands

IX Glossopharyngeal to tongue and salivary glands

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Mixed nerve extends to thoracic and abdominal cavities; parasympathetic to heart, lungs and abdomen

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Motor nerves to layrnx, pharynx and neck muscles for swallowing

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Motor to muscles of tongue

XII Hypoglossal to tongue muscles