Transcript brain

Chapter 14
Part 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 14
The 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
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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 injury;
loss 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
2/3’s 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 reflexes;
infundibulum 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)
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