The Brain and Cranial Nerves

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The Brain and Cranial Nerves
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The Brain

• Weighs about 3 pounds
• Composed of about 100 billion neurons (and many
  more neuroglia), each of which forms about 1000
  synapses with other neurons
• A metabolically active organ needs a constant
  blood supply (with oxygen and glucose)

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The Brain (continued)

• Protected by the blood-brain barrier
• Many harmful substances cannot pass from the
  blood into the brain
• Cells in the walls of the blood vessels that
  supply the brain are sealed together to prevent
  these materials from passing into the brain

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The Brain (continued)

• Consists of 4 main parts
• Brain stem
• Cerebellum
• Cerebrum
• Diencephalon

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Fig.14.01a
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Fig.14.01b
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The Brain (continued)

• Brain stemcontinuous with the spinal cord
  consists of the medulla oblongata, pons, and
  midbrain
• Cerebellumlocated behind the brain stem and
  beneath the cerebrum
• Cerebrumthe largest and most superiorly located
  part of the brain
• Diencephalonlocated above the brain stem
  consists of the thalamus, hypothalamus, and
  pineal gland

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Cranial Meninges

• Continuous with the spinal meninges and have the
  same names
• Dura materouter the falx cerebri and the falx
  cerebelli are extensions of the dura mater that
  separate the two halves of the cerebrum and the
  cerebellum
• Arachnoid matermiddle
• Pia materinner

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Fig. 13.02
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Cerebrospinal Fluid (CSF)

• A clear, colorless liquid that protects the brain
  and spinal cord against chemical and physical
  injuries
• About 3-5 ounces in an adult
• Carries oxygen, glucose, and other substances
  from the blood to the cells of the CNS
• Circulates continuously through cavities of the
  brain and spinal cord and in the subarachnoid
  space

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Ventricles

• Four cavities in the brain filled with
  cerebrospinal fluid
• Lateral ventricles (2)one in each hemisphere of
  the cerebrum
• Third ventriclebetween the right and left halves
  of the thalamus
• Fourth ventriclebetween the brain stem and the
  cerebellum

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Fig. 14.03
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Ventricles (continued)

• Cerebrospinal fluid is produced by ependymal
  cells (a type of neuroglia) and a choroid plexus
  in each ventricle
• A choroid plexus is a network of capillaries in
  the wall of a ventricle
• The blood vessels form the CSF from blood plasma

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Fig. 14.04b
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Circulation of CSF

• Cerebrospinal fluid constantly circulates from
  one ventricle to another
• Each lateral ventricle is connected to the third
  ventricle by an interventricular foramen
• The cerebral aqueduct connects the third
  ventricle to the fourth ventricle
• The CSF passes into the subarachnoid space and
  the central canal of the spinal cord from the
  fourth ventricle
• It is reabsorbed into the blood through arachnoid
  villi , fingerlike extensions of the arachnoid
  that project into blood-filled spaces around the
  brain called sinuses

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Fig. 14.04a
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Medulla Oblongata

• A continuation of the spinal cord
• Forms the inferior part of the brain stem
• The white matter of the medulla consists of
  tracts (both ascending sensory tracts and
  descending motor tracts) that go to and from the
  brain
• Within the medulla, most of the large motor
  tracts on the left side cross over to the right,
  and most on the right side cross over to the left
  at the decussation of pyramids

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Medulla Oblongata (continued)

• The gray matter of the medulla contains several
  nuclei which control vital body functions
• Cardiovascular centerregulates the rate and
  force of the heartbeat and the diameter of blood
  vessels
• Medullary rhythmicity areaadjusts the basic
  rhythm of breathing
• Nuclei for cranial nerves VIII-XII
• Other nuclei control reflexes for swallowing,
  vomiting, coughing, sneezing, and hiccupping

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Fig. 14.06
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Pons

• Located above the medulla in the brain stem
• Bridge that connects parts of the brain with
  one another
• Like the medulla, contains tracts and nuclei
• The pneumotaxic area and the apneustic area help
  the medulla control breathing
• Contains nuclei for cranial nerves V-VIII

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Midbrain

• Located above the pons in the brain stem
• Also contains tracts and nuclei
• The cerebral aqueduct passes through it to
  connect the third ventricle to the fourth
  ventricle
• On the posterior side of the midbrain are a pair
  of superior colliculi and a pair of inferior
  colliculi
• The superior colliculi are visual reflex centers
  (ex. adjusts the pupil size, adjusts the lens
  shape for near and far vision)
• The inferior colliculi are auditory reflex
  centers (ex. startle reflex caused by a loud
  noise)

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Midbrain (continued)

• Contains nuclei that control subconscious muscle
  activities these neurons release dopamine in
  Parkinson disease, these neurons are lost
• Contains nuclei for cranial nerves III and IV

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Fig. 14.07a
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Cerebellum

• Located beneath the cerebrum and behind the brain
  stem
• Has a right and left hemisphere
• The cortex is the superficial layer which
  contains gray matter white matter is located
  deeper
• Three paired cerebellar peduncles (inferior,
  superior, and middle) attach the cerebellum to
  the brain stem

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Cerebellum (continued)

• Controls skilled muscular activities,
  subconscious skeletal muscle movements,
  equilibrium, and balance

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Fig. 14.08c
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Thalamus

• Consists primarily of gray matter organized into
  a right and left half
• A bridge of gray matter, the intermediate mass,
  joins the two halves
• The major relay station for most sensory impulses
  to the cerebrum
• Provides crude perception of touch, pressure,
  pain, and temperature

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Hypothalamus

• Located inferior to the thalamus
• The major link between the nervous and endocrine
  systems produces some hormones
• Connects to the pituitary gland, the master
  endocrine gland, by the stalk-like infundibulum
• One of the main regulators of homeostasis

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Hypothalamus (continued)

• Controls the ANS (regulates heart rate, movement
  of food through the GI tract, contraction of the
  urinary bladder)
• Helps regulate emotional and behavioral patterns
  (ex. rage, aggression, pain, pleasure, sexual
  arousal)
• Regulates eating and drinking (ex. thirst center
  located here)
• Controls body temperature
• Regulates circadian rhythms and states of
  consciousness

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Pineal Gland

• An endocrine gland that looks like a tiny pine
  cone
• Secretes the hormone melatonin, which promotes
  sleepiness and contributes to setting the bodys
  biological clock

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Cerebrum

• The center for intelligence
• Allows us to read, write, speak, calculate,
  imagine, plan
• Consists of a right and left hemisphere,
  separated by the longitudinal fissure
• Has an outer layer of gray matter, the cerebral
  cortex, which is only about a tenth of an inch
  thick but contains billions of neurons
• White matter is deep to the cortex

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Cerebrum (continued)

• The cortex has folds called gyri and grooves
  called sulci
• The hemispheres are connected internally by a
  band of white matter called the corpus callosum
• Each hemisphere is subdivided into four lobes a
  frontal lobe, a parietal lobe, a temporal lobe,
  and an occipital lobe

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Cerebrum (continued)

• The central sulcus separates the frontal and
  parietal lobes the lateral cerebral sulcus
  separates the frontal and temporal lobes and the
  parieto-occipital sulcus separates the parietal
  and occipital lobes
• The emotional brain, or limbic system is an
  internal part of the cerebrum which functions in
  the emotional aspects of behavior related to
  survival

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Cerebral Cortex Areas and Functions

• Specific types of nerve impulses are processed in
  certain regions of the cerebral cortex
• Sensory impulses are received and interpreted
  mainly in the posterior half of both hemispheres
  (behind the central sulcus)
• Motor impulses flow mainly from the anterior part
  of each hemisphere
• Association areas are found in all lobes of the
  cerebral cortex

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Fig. 14.11
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Sensory Areas

• Primary somatosensory arealocated in the
  parietal lobes, directly behind the central
  sulcus in the postcentral gyrus receives sensory
  impulses from all over the body (touch,
  proprioception/position, pain, itch, tickle,
  thermal sensations)
• Primary visual arealocated at the rear of the
  occipital lobes receives impulses for vision
• Primary auditory arealocated in the temporal
  lobes, just below the lateral cerebral sulcus
  receives impulses for hearing
• Primary gustatory arealocated in the parietal
  lobes, just above the lateral cerebral sulcus
  receives impulses for taste

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Motor Areas

• Primary motor arealocated in the frontal lobes,
  directly in front of the central sulcus in the
  precentral gyrus controls voluntary contractions
  of specific muscles
• Brocas speech areain 97 of the population,
  this area is located near the rear of the left
  frontal lobe controls the production of speech

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Main Association Areas

• Somatosensory association arealocated just
  behind the primary somatosensory area in the
  parietal lobes integrates and interprets
  sensations
• Visual association arealocated just in front of
  the primary visual area in the occipital lobes
  relates and recognizes what is seen
• Auditory association arealocated just below and
  behind the primary auditory area in the temporal
  lobes distinguishes various sounds from one
  another
• Wernickes language arealocated in the left
  parietal and temporal lobes interprets the
  meaning of speech and recognizes spoken words
• Premotor arealocated in front of the primary
  motor area in the frontal lobes controls learned
  skilled movements

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Fig. 14.15
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Hemispheric Lateralization

• The brain is generally symmetrical on its right
  and left sides
• The left hemisphere controls the right side of
  the body the right hemisphere controls the left
  side of the body
• The two hemispheres share performance of many
  functions, but each specializes in certain unique
  functions
• There is also considerable variation in the two
  hemispheres from one individual to another

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Hemispheric Lateralization (continued)

• The left hemisphere is generally more important
  for reasoning, numerical and scientific skills,
  spoken and written language, and use of sign
  language
• The right hemisphere is generally more
  specialized for musical and artistic awareness,
  space and pattern perception, recognition of
  faces and expressions, and distinguishing between
  odors

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Cranial Nerves

• Part of the PNS
• 12 pair of nerves which arise from the brain
• Cranial nerve I (olfactory nerve)
• Cranial nerve II (optic nerve)
• Cranial nerve III (oculomotor nerve)
• Cranial nerve IV (trochlear nerve)
• Cranial nerve V (trigeminal nerve)
• Cranial nerve VI (abducens nerve)
• Cranial nerve VII (facial nerve)
• Cranial nerve VIII (vestibulocochlear nerve)
• Cranial nerve IX (glossopharyngeal nerve)
• Cranial nerve X (vagus nerve)
• Cranial nerve XI (accessory nerve)
• Cranial nerve XII (hypoglossal nerve)
• Oh, oh, oh, to touch and feel very green
  vegetablesah!

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Fig. 14.05
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Cranial Nerve I (Olfactory Nerve)

• A sensory nerve, since it contains only sensory
  axons
• These axons conduct impulses for olfaction, or
  the sense of smell, from the nasal cavity to the
  brain

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Fig. 14.T03a
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Cranial Nerve II (Optic Nerve)

• A sensory nerve
• Axons conduct impulses for vision from the retina
  of the eye to the brain
• About ½ inch from the eyeball, the two optic
  nerves merge to form the optic chiasm within the
  chiasm, axons from the medial half of each eye
  cross over to the opposite side

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Fig. 14.T03b
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Cranial Nerve III (Oculomotor Nerve)

• A mixed nerve, since it contains axons of both
  sensory and motor neurons
• Nucleus is located in the midbrain
• Control movements of the eyeball and upper eyelid

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Fig. 14.T03c
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Cranial Nerve IV (Trochlear Nerve)

• A mixed nerve
• The smallest of the cranial nerves
• Nucleus is located in the midbrain
• Also controls movement of the eyeball

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Fig. 14.T03d
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Cranial Nerve V (Trigeminal Nerve)

• A mixed nerve
• The largest of the cranial nerves has 3 branches
• Nucleus is in the pons
• Control chewing movements and involved in facial
  sensations of touch, pain, and temperature

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Fig. 14.T03e
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Cranial Nerve VI (Abducens Nerve)

• A mixed nerve
• Nucleus is in the pons
• Also controls movement of the eyeball

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Fig. 14.T03f
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Cranial Nerve VII (Facial Nerve)

• A mixed nerve
• Nucleus is in the pons
• Controls muscles of facial expression and
  involved with the sense of taste

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Fig. 14.T03g
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Cranial Nerve VIII (Vestibulocochlear Nerve)

• A mixed nerve
• Has 2 branches, the vestibular and the cochlear
• Nucleus for the vestibular branch is in the pons
  nucleus for the cochlear branch is in the medulla
  oblongata
• Conveys impulses for hearing and equilibrium

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Fig. 14.T03h
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Cranial Nerve IX (Glossopharyngeal Nerve)

• A mixed nerve
• Nucleus is in the medulla oblongata
• Involved in swallowing and tasting

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Fig. 14.T03i
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Cranial Nerve X (Vagus Nerve)

• A mixed nerve
• Nucleus is in the medulla oblongata
• Has a wide distribution, from the head and neck
  to the thorax and abdomen
• Involved in swallowing, tasting, and voice
  production
• Functions in the parasympathetic division of the
  ANS to slow heart rate

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Fig. 14.T03j
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Cranial Nerve XI (Accessory Nerve)

• A mixed nerve
• Nucleus is in the medulla oblongata
• Controls swallowing movements and movement of the
  shoulders and head

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Fig. 14.T03k
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Cranial Nerve XII (Hypoglossal Nerve)

• A mixed nerve
• Nucleus is in the medulla oblongata
• Controls the movement of the tongue during speech
  and swallowing

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Fig. 14.T03L
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Brain Waves

• Brain waves are electrical signals generated by
  neurons in the brain
• Electroencephalogram (EEG)a record of the
  electrical activity of the brain
• Detected by electrodes, sensors placed on the
  forehead and scalp
• Used to diagnose certain disorders (ex.
  epilepsy), provide information about
  sleep/wakefulness, and confirm brain death

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Aging and the Nervous System

• During the first years of life, the brain grows
  rapidly, primarily due to an increase in the size
  of neurons and an increase in the number and size
  of neuroglia
• From early adulthood on, brain mass declines
• The number of neurons does not decrease much, but
  the number of synapses does
• In addition, the speed of impulse conduction
  slows, so the response time increases