Chapter 14 Part 1

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Chapter 14Part 1

• The Brain and Cranial Nerves
• Lecture Outline

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INTRODUCTION

• The brain is the center for registering
  sensations, correlating them with one another and
  with stored information, making decisions, and
  taking action.
• It is also the center for intellect, emotions,
  behavior, and memory.
• It also directs our behavior towards others.
• In this chapter we will consider the principal
  parts of the brain, how the brain is protected
  and nourished, and how it is related to the
  spinal cord and to the 12 pairs of cranial nerves.

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Chapter 14The Brain and Cranial Nerves

• Largest organ in the body at almost 3 lb.
• Brain functions in sensations, memory, emotions,
  decision making, behavior

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OVERVIEW OF BRAIN ORGANIZATION AND BLOOD SUPPLY

• The major parts of the brain are the brain stem,
  diencephalon, cerebrum, and cerebellum (Figure
  14.1).
• The CNS develops from an ectodermal neural tube
• Three primary vesicles prosencephalon,
  mesencephalon, and rhombencephalon develop from
  the neural tube. (Figure 14.29)
• The embryologic development of the CNS is
  summarized in table 14.1

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Principal Parts of the Brain

• Cerebrum
• Diencephalon
• thalamus hypothalamus
• Cerebellum
• Brainstem
• medulla, pons midbrain

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Blood Supply to Brain

• Arterial blood supply is branches from circle of
  Willis on base of brain
• Vessels on surface of brain----penetrate tissue
• Uses 20 of our bodies oxygen glucose needs
• blood flow to an area increases with activity in
  that area
• deprivation of O2 for 4 min does permanent injury
• at that time, lysosome release enzymes
• Blood-brain barrier (BBB)
• protects cells from some toxins and pathogens
• proteins antibiotics can not pass but alcohol
  anesthetics do
• tight junctions seal together epithelial cells,
  continuous basement membrane, astrocyte processes
  covering capillaries

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Blood Flow and the Blood-Brain Barrier

• An interruption of blood flow for 1 or 2 minutes
  impairs neuronal function.
• A total deprivation of oxygen for 4 minutes
  causes permanent injury.
• Because carbohydrate storage in the brain is
  limited, the supply of glucose to the brain must
  be continuous.
• Glucose deficiency may produce mental confusion,
  dizziness, convulsions, and unconsciousness.

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BBB

• A blood-brain barrier (BBB) protects brain cells
  from harmful substances and pathogens by serving
  as a selective barrier to prevent passage of many
  substances from the blood to the brain.
• An injury to the brain due to trauma,
  inflammation, or toxins causes a breakdown of the
  BBB, permitting the passage of normally
  restricted substances into brain tissue.
• The BBB may also prevent entry of drugs that
  could be used as therapy for brain cancer or
  other CNS disorders, so research is exploring
  ways to transport drugs past the BBB.

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Protective Covering of the Brain

• The brain is protected by the cranial bones
  (Figure 7.4) and the cranial meninges (Figure
  14.2).
• The cranial meninges are continuous with the
  spinal meninges and are named dura mater,
  arachnoid, and pia mater.
• Three extensions of the dura mater separate parts
  of the brain the falx cerebri, falx cerebelli,
  and the tentorium cerebelli.

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Protective Coverings of the Brain

• Bone, meninges fluid
• Meninges same as around the spinal cord
• dura mater
• arachnoid mater
• pia mater
• Dura mater extensions
• falx cerebri
• tentorium cerebelli
• falx cerebelli

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CEREBROSPINAL FLUID

• Cerebrospinal fluid (CSF) is a clear, colorless
  liquid that protects the brain and spinal cord
  against chemical and physical injuries.

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Cerebrospinal Fluid (CSF)

• 80-150 ml (3-5oz)
• Clear liquid containing glucose, proteins, ions
• Functions
• mechanical protection
• floats brain softens impact with bony walls
• chemical protection
• optimal ionic concentrations for action
  potentials
• circulation
• nutrients and waste products to and from
  bloodstream

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Ventricles

• There are four CSF filled cavities within the
  brain called ventricles (Figure 14.3).
• A lateral ventricle is located in each hemisphere
  of the cerebrum. The lateral ventricles are
  separated by the septum pellucidum.
• The third ventricle is a narrow cavity along the
  midline superior to the hypothalamus and between
  the right and left halves of the thalamus.
• The fourth ventricle is between the brain stem
  and the cerebellum.

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Origin of CSF

• Choroid plexus capillaries covered by
  ependymal cells
• 2 lateral ventricles, one within each cerebral
  hemisphere
• roof of 3rd ventricle
• fourth ventricle

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Drainage of CSF from Ventricles

• One median aperture two lateral apertures allow
  CSF to exit from the interior of the brain

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Flow of Cerebrospinal Fluid
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Reabsorption of CSF

• Reabsorbed through arachnoid villi
• grapelike clusters of arachnoid penetrate dural
  venous sinus
• 20 ml/hour reabsorption rate same as production
  rate
• Reabsorption of CSF
• Reabsorbed through arachnoid villi
• grapelike clusters of arachnoid penetrate dural
  venous sinus
• 20 ml/hour reabsorption rate same as production
  rate

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Hydrocephalus

• Blockage of drainage of CSF (tumor, inflammation,
  developmental malformation, meningitis,
  hemorrhage or injury)
• Continued production cause an increase in
  pressure --- hydrocephalus
• In newborn or fetus, the fontanels allow this
  internal pressure to cause expansion of the skull
  and damage to the brain tissue
• Neurosurgeon implants a drain shunting the CSF to
  the veins of the neck or the abdomen

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THE BRAIN STEM
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Medulla Oblongata

• Continuation of spinal cord
• Ascending sensory tracts
• Descending motor tracts
• Nuclei of 5 cranial nerves
• Cardiovascular center
• force rate of heart beat
• diameter of blood vessels
• Respiratory center
• medullary rhythmicity area sets basic rhythm of
  breathing
• Information in out of cerebellum
• Reflex centers for coughing, sneezing, swallowing
  etc.

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Ventral Surface of Medulla Oblongata

• Ventral surface bulge
• pyramids
• large motor tract
• decussation of most fibers
• left cortex controls right muscles
• Olive olivary nucleus
• neurons send input to cerebellum
• proprioceptive signals
• gives precision to movements

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Dorsal Surface of Medulla Oblongata

• Nucleus gracilis nucleus cuneatus sensory
  neurons
• relay information to thalamus on opposite side of
  brain
• 5 cranial nerves arise from medulla -- 8 thru 12

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XII Hypoglossal Nerve

• Controls muscles of tongue during speech and
  swallowing
• Injury deviates tongue to injured side when
  protruded
• Mixed, primarily motor

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XI Spinal Accessory Nerve

• Cranial portion
• arises medulla
• skeletal mm of throat soft palate
• Spinal portion
• arises cervical spinal cord
• sternocleidomastoid and trapezius mm.

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X Vagus Nerve

• Receives sensations from viscera
• Controls cardiac muscle and smooth muscle of the
  viscera
• Controls secretion of digestive fluids

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IX Glossopharyngeal Nerve

• Stylopharyngeus m. (lifts throat during
  swallowing)
• Secretions of parotid gland
• Somatic sensations taste on posterior 1/3 of
  tongue

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VIII Vestibulocochlear Nerve

• Cochlear branch begins in medulla
• receptors in cochlea
• hearing
• if damaged deafness or tinnitus (ringing) is
  produced
• Vestibular branch begins in pons
• receptors in vestibular apparatus
• sense of balance
• vertigo (feeling of rotation)
• ataxia (lack of coordination)

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Injury to the Medulla

• Hard blow to the back of the head may be fatal
• Cranial nerve malfunctions on same side as
  injuryloss of sensation or paralysis of throat
  or tongue irregularities in breathing and heart
  rhythm

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Pons

• The pons is located superior to the medulla. It
  connects the spinal cord with the brain and links
  parts of the brain with one another by way of
  tracts (Figures 14.1, 14.5).
• relays nerve impulses related to voluntary
  skeletal movements from the cerebral cortex to
  the cerebellum.
• contains the pneumotaxic and apneustic areas,
  which help control respiration along with the
  respiratory center in the medulla (Figure 23.24).
• contains nuclei for cranial nerves V trigeminal,
  VI abducens, VII facial, and VIII
  vestibulocochlear (vestibular branch
  only).(Figure 14.5).

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Pons

• One inch long
• White fiber tracts ascend and descend
• Pneumotaxic apneustic areas help control
  breathing
• Middle cerebellar peduncles carry sensory info to
  the cerebellum
• Cranial nerves 5 through 7

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VII Facial Nerve

• Motor portion
• facial muscles
• salivary nasal and oral mucous glands tears
• Sensory portion
• taste buds on anterior 2/3s of tongue

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VI Abducens Nerve

• Lateral rectus eye muscle

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V Trigeminal Nerve

• Motor portion
• muscles of mastication
• Sensory portion
• touch, pain, temperature receptors of the face
• ophthalmic branch
• maxillary branch
• mandibular branch

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Midbrain

• One inch in length
• Extends from pons to diencephalon
• Cerebral aqueduct connects 3rd ventricle above to
  4th ventricle below

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Midbrain in Section

• Cerebral peduncles---clusters of motor sensory
  fibers
• Substantia nigra---helps controls subconscious
  muscle activity
• Red nucleus-- rich blood supply iron-containing
  pigment
• cortex cerebellum coordinate muscular movements
  by sending information here from the cortex and
  cerebellum

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Dorsal Surface of Midbrain

• Corpora quadrigemina superior inferior
  colliculi
• coordinate eye movements with visual stimuli
• coordinate head movements with auditory stimuli

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IV Trochlear Nerve

• Superior oblique eye muscle

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III Oculomotor Nerve

• Levator palpebrae raises eyelid (ptosis)
• 4 extrinsic eye muscles
• 2 intrinsic eye muscles
• accomodation for near vision (changing shape of
  lens during reading)
• constriction of pupil

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Reticular Formation

• Scattered nuclei in medulla, pons midbrain
• Reticular activating system
• alerts cerebral cortex to sensory signals (sound
  of alarm, flash light, smoke or intruder) to
  awaken from sleep
• maintains consciousness helps keep you awake
  with stimuli from ears, eyes, skin and muscles
• Motor function is involvement with maintaining
  muscle tone

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Cerebellum

• 2 cerebellar hemispheres and vermis (central
  area)
• Function
• correct voluntary muscle contraction and posture
  based on sensory data from body about actual
  movements
• sense of equilibrium

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Cerebellum

• Transverse fissure between cerebellum cerebrum
• Cerebellar cortex (folia) central nuclei are
  grey matter
• Arbor vitae tree of life white matter

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Cerebellar Peduncles

• Superior, middle inferior peduncles attach to
  brainstem
• inferior carries sensory information from spinal
  cord
• middle carries sensory fibers from cerebral
  cortex basal ganglia
• superior carries motor fibers that extend to
  motor control areas

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THE DIENCEPHALON
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Diencephalon Surrounds 3rd Ventricle

• Surrounds 3rd ventricle
• Superior part of walls is thalamus
• Inferior part of walls floor is hypothalamus

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Thalamus

• The thalamus is located superior to the midbrain
  and contains nuclei that serve as relay stations
  for all sensory impulses, except smell, to the
  cerebral cortex (Figure 14.9).
• seven major groups of thalamic nuclei on each
  side (Figure 14.9 c and d).
• They are the Anterior nucleus, medial nuclei,
  lateral group, ventral group, intralaminar
  nuclei, midline nucleus, and the reticular
  nucleus.
• It also registers conscious recognition of pain
  and temperature and some awareness of light touch
  and pressure.
• It plays an essential role in awareness and the
  acquisition of knowledge (cognition.)

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Thalamus

• 1 inch long mass of gray mater in each half of
  brain (connected across the 3rd ventricle by
  intermediate mass)
• Relay station for sensory information on way to
  cortex
• Crude perception of some sensations

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Thalamic Nuclei

• Nuclei have different roles
• relays auditory and visual impulses, taste and
  somatic sensations
• receives impulses from cerebellum or basal
  ganglia
• anterior nucleus concerned with emotions, memory
  and acquisition of knowledge (cognition)

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Hypothalamus

• The hypothalamus
• inferior to the thalamus, has four major regions
  (mammillary, tuberal, supraoptic, and preoptic)
• controls many body activities, and is one of the
  major regulators of homeostasis (Figure 14.10).
• The hypothalamus has a great number of functions.
• It controls the ANS.
• It produces hormones.
• It functions in regulation of emotional and
  behavioral patterns.
• It regulates eating and drinking through the
  feeding center, satiety center, and thirst
  center.
• It aids in controlling body temperature.
• It regulates circadian rhythms and states of
  consciousness.

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Hypothalamus

• Dozen or so nuclei in 4 major regions
• mammillary bodies are relay station for olfactory
  reflexesinfundibulum suspends the pituitary
  gland
• Major regulator of homeostasis
• receives somatic and visceral input, taste, smell
  hearing information monitors osmotic pressure,
  temperature of blood

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Epithalamus

• The epithalamus lies superior and posterior to
  the thalamus and contains the pineal gland and
  the habenular nuclei (Figure 14.7).
• The pineal gland secretes melatonin to influence
  diurnal cycles in conjunction with the
  hypothalamus.
• The habenular nuclei (Figure 14.7a) are involved
  in olfaction, especially emotional responses to
  odors.

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Epithalamus

• Pineal gland
• endocrine gland the size of small pea
• secretes melatonin during darkness
• promotes sleepiness sets biological clock
• Habenular nuclei
• emotional responses to odors

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Subthalamus

• The subthalamus lies immediately inferior to the
  thalamus and includes tracts and the paired
  subthalamic nuclei, which connect to motor areas
  of the cerebrum.
• The subthalamic nuclei and red nucleus and
  substantia nigra of the midbrain work together
  with the basal ganglia, cerebellum, and cerebrum
  in control of body movements.
• Table 14.2 summarizes the functions of the parts
  of the diencephalon.

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Circumventricular Organs

• Parts of the diencephalon, called
  circumventricular organs (CVOs), can monitor
  chemical changes in the blood because they lack a
  blood-brain barrier.
• CVOs include
• part of the hypothalamus,
• the pineal gland,
• the pituitary gland, and a few other nearby
  structures.
• They function to coordinate homeostatic
  activities of the endocrine and nervous systems.
• They are also thought to be the site of entry
  into the brain of HIV.

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THE CEREBRUM

• The cerebrum is the largest part of the brain .
• The surface layer, the cerebral cortex, is 2-4 mm
  thick and is composed of gray matter. The cortex
  contains billions of neurons.
• The cortex contains gyri (convolutions), deep
  grooves called fissures, and shallower sulci.
  (Figure 14.11a)
• Beneath the cortex lies the cerebral white
  matter, tracts that connect parts of the brain
  with itself and other parts of the nervous
  system.
• The cerebrum is nearly separated into right and
  left halves, called hemispheres, by the
  longitudinal fissure.
• Internally it remains connected by the corpus
  callosum, a bundle of transverse white fibers.
  Figure 14.12)