Transcript Chapter_14_Brainx

Anatomy
and
Physiology
Biology 2401
Chapter-14
Brain and Cranial Nerves
Functions
• Recognizing and integrating sensations
– Among the incoming sensations
– With stored information (memory)
• Making decisions
• Initiating actions
– Behavior
– Emotions
• Memory
• Center of intellect
Blood Supply
• Arterial blood rich in
oxygen and nutrients
arrives at a ring of blood
vessels on the inferior
surface of the braincerebral arterial circle
(Circle of Willis).
• Vessels branch out from
the circle and those vessels
that run along the surface
have branches that
penetrate into the tissue.
Oxygen and Glucose Needs
• 20% of the oxygen and glucose needs of the body
– As activity of an area increases, so does blood supply
– After 1-2 minutes without O2, neuronal function is impaired
– After 4 minutes without O2, lysosomes release enzymes and
cells die
– Cells cannot be replaced
• Very little carbohydrate stored in the brain. Must have
continuous supply of glucose, therefore maintenance
of constant blood levels of glucose is an important
aspect of homeostasis.
– Glucose deficiency results in confusion, dizziness,
convulsion, and possibly unconsciousness, coma and death.
Blood Brain Barrier (BBB)
• Selective barrier prevents many substances in the blood
from entering the brain tissues.
• Protects brain cells from pathogens and other harmful
substances
• Brain injuries can cause breakdown of the barrier
• BBB prevents therapeutic substances such as antibiotics
from reaching brain cells
Meninges
• Protective coverings
of the brain
• These cranial
meninges are
continuous with the
meninges of the
spinal cord
• Dura mater,
arachnoid, and pia
mater
Dura mater
• Superficial, tightly bound to internal periosteum except at three areas where it
forms folds that extend into the fissures of the brain:
– Falx cerebri : in the longitudinal fissure between the two cerebral hemispheres
– Tentorium cerebelli: between cerebellum and cerebrum
– Falx cerebelli: between two cerebellar hemispheres
• Dural venous sinuses: spaces within the dura where large veins are located, for
example, the superior sagittal sinus. Located in the bases of all three of the
folds.
Other Meninges and Spaces
– Subdural space: deep to
dura mater
• Arachnoid: intermediate
meninges. Site of
cerebrospinal fluid
reabsorption
– Subarchnoid space: filled
with cerebrospinal fluid
• Pia mater: delicate fibrous
connective tissue layer
closely applied to the
brain tissue
Cerebrospinal Fluid (CSF)
• Clear, colorless liquid. 80-150
mL.
• Produced and reabsorbed at
the rate of 20 mL/h
• Contains glucose, proteins,
and ions
• Mechanical protection: brain
floats within CSF cushioning
brain; brain is surrounded by
CSF in the subarachnoid space
• Chemical protection: correct
ion concentrations for action
potentials
• Circulation: provides nutrients
and removes nutrients
Ventricles of the Brain
• Four CSF filled cavities connected by channels
• Two lateral ventricles, one within each hemisphere of the
cerebrum. Between these two ventricles is a barrier, the septum
pellucidum.
– Interventricular foramen between lateral ventricles and third ventricle
• Third ventricle: along the midline superior to the hypothalamus
and between the right and left halves of the thalamus.
– Cerebral aqueduct between the third and fourth ventricles
• Fourth ventricle: between brain stem and cerebellum.
• Fourth ventricle continuous with central canal of spinal cord
– Median and lateral apertures open into subarachnoid space
Ventricles
• Production: choroid plexi in
all four ventricles. Capillaries
covered by ependymal cells
• CSF is manufactured from
plasma and is similar in
composition except CSF does
not have large plasma
proteins.
• CSF flows through ventricles
and out medial and lateral
apertures to subarachnoid
space
• CSF reabsorbed through
arachnoid villi: grape-like
clusters of arachnoid matter
that penetrates into the dural
sinuses. CSF returned to
plasma.
CSF Production,
Flow, Reabsorption
Flow of Cerebrospinal Fluid
Hydrocephalus
• “Water on the brain”
• Reabsorption of CSF is blocked, usually because CSF
blocked from leaving ventricles
• Production of CSF continues, putting pressure on brain
tissue
• In fetus, fontanels allow internal pressure to cause
expansion of the skull as well as damaging brain tissue.
• In adult, can blockage of CSF flow can occur because of
tumor, inflammation, hemorrhage, injury
• Neurosurgeons can implant drains that shunt CSF to
veins of neck or abdomen. Can be done in utero.
Brain Injuries
• Causes of damage
–
–
–
–
Displacement or distortion of tissue at impact
Increased intracranial pressure
Infections
Free radical damage after ischemia (lack of blood flow)
• Three types of injury
– Concussion: temporary loss of consciousness
accompanied by headache, drowsiness, confusion, lack
of concentration, unequal pupil size
– Contusion: bruising of the brain.
– Laceration: tearing of brain with accompanying
hematoma causing increased intracranial pressure
Major Parts of the Brain
Brain stem
Cerebellum
Diencephalon
Cerebrum
BRAINSTEM: Medulla
• Continuation of spinal cord
• Contains both ascending and
descending fiber tracts.
– Ascending: sensory
– Descending: motor
• Cardiovascular center controls
– Force and rate of heart beat
– Diameter of blood vessels
• Respiratory center: medullary
rhythmicity area sets basic
rhythm of breathing
• Cranial nerves VIII to XII
• Reflex centers for coughing,
sneezing, swallowing.
oblongata
Medulla Oblongata: Ventral Surface
• Pyramids
– Motor tract
– Decussation of most motor
fibers- left cerebral cortex
controls muscles on right
side of body and vice versa.
• The bridge
• Ascending and descending
myelinated fiber tracts. Relays
impulses from the cerebral
cortex to the cerebellum that
is related to voluntary skeletal
movements
• Pneumotaxic and apneustic
areas help control breathing
pattern along with medulla
oblongata
• Nuclei for cranial nerves V
through VIII
BRAINSTEM: Pons
BRAINSTEM: Midbrain
• From pons to diencephalon
• Contains cerebral aqueduct
• Substantia nigra: subconscious
muscle activities (muscle tone,
coordinating movements)
• Red nuclei: coordinate
muscular movements
• Corpora quadrigemina: reflexes
of movements of eyes, head
and neck, and movements of
head and trunk in response to
auditory stimuli.
• Serves as the origin for 2 cranial
nerves: III and IV
BRAINSTEM: Reticular Formation
• Scattered nuclei in medulla oblongata, pons, and midbrain
• Sensory: contains the reticular activating system (RAS)
– Awakens the cerebral cortex from sleep by allowing sensory signals to reach the cortex.
– Maintains consciousness because of sensory input such as sound, touch, movement
(proprioception), vision
• Motor: helps maintain muscle tone
DIENCEPHALON
• Surround the
third ventricle
• Superior aspect is
thalamus; inferior
is hypothalamus
• Other parts:
epithalamus,
subthalamus
Thalamus
• Left and right sides
connected by
intermediate mass
• Relay station for
sensory information
ascending to cortex
• Crude perception of
some sensations
• Emotions, memory,
cognition (acquisition
of knowledge)
Hypothalamus
• One of the major regulators of
homeostasis
– Controls the ANS
– Produces releasing and inhibiting
factors that affect the pituitary
– Produces hormones (ADH and
Oxytocin)
– Regulation of emotional and
behavioral patterns
– Feeding, satiety, and thirst centers
– Body temperature regulation
– Circadian rhythms
– States of consciousness
– Monitors osmotic pressure
Hypothalamus, cont.
• More than a dozen nuclei
within these regions
• Mammillary bodies relay
olfactory reflexes
• Infundibulum: suspends
the pituitary, contains
fiber tracts, and a subset
of the circulatory system.
Epithalamus
• Pineal gland: secretes melatonin during darkness,
promotes sleepiness and sets biological clock in
conjunction with the hypothalamus
CEREBELLUM
• Functions:
– Compares intended
movements/posture signaled
by the cerebrum with actual
movements/posture
– Sense of equilibrium
• Anatomy
– Two hemispheres with an
intermediate vermis. Each
hemisphere has an anterior
and a posterior lobe.
– External folia, outer gray cortex
and inner central nuclei
– Inner white matter- arbor vitae
CEREBRUM
• Largest part of the brain
• Right and left hemispheres
are connected internally by
the corpus callosum,
externally separated by
longitudinal fissure
• Gray matter: outer cortex
inner basal ganglia
• Cortex contains convolutions
(gyri) with shallow grooves
in between (sulci). Deeper
grooves are fissures.
• Basal ganglia are buried
within the white matter
(fiber tracts)
Lobes of the Cerebrum
• Four lobes in each hemisphere that are visible externally and
named after the bones with which they are associated: frontal,
parietal, temporal, occipital
• Sulci and fissures divide the lobes from each other:
• One lobe deep to the lateral surface of each hemisphere: insula.
White Matter: Fiber Tracts
• Association fibers: between gyri in the same hemisphere
• Commissural fibers: connect gyri in one cerebral hemisphere to
the corresponding gyri in the opposite hemisphere
• Projection fibers: fibers that leave or enter the cerebrum, form
ascending (sensory) and descending (motor) tracts. Information
from or to other parts of the brain and spinal cord
Basal Ganglia
• Paired masses of gray matter in each cerebral
hemisphere
• Control large automatic movements of skeletal muscles
– Connect to red nucleus and substantia nigra of the subthalamus
• Receives input and sends output from cerebral cortex,
thalamus and hypothalamus
Damage to Basal Ganglia
• Damage to basal ganglia: tremor, rigidity, involuntary
muscle movements.
• Parkinson’s disease: neurons from the substantia
nigra that communicate with the putamen and
caudate nucleus degenerate
• Besides motor control, basal ganglia help initiate and
terminate some cognitive processes; therefore
damage can cause obsessive compulsive disorder,
schizophrenia, and chronic anxiety. Thought to
involve dysfunction between the basal ganglia and
the limbic system
Limbic System
• Found in cerebral hemispheres and diencephalon
• Functions in emotional aspects of behavior and
memory
• Associated with pleasure and pain
• Strong emotions increase efficiency of memory
Overview of Functional Areas
of the Cerebral Cortex
• Sensory areas: receive and interpret sensory impulses.
• Motor areas: govern muscular movement. Beginning of
descending fibers
• Association areas: integrative functions such as memory,
emotions, reasoning, will, judgment, personality, and
intelligence
Sensory Areas
• Sensory areas: receive and interpret sensory impulses.
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–
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–
Primary somatosensory area: postcentral gyrus (1, 2, 3)
Primary visual area (17)
Primary auditory area (41, 42)
Primary gustatory area (43)
Motor Areas
• Govern muscular movement. Beginning of descending fibers
• Primary motor area: precentral gyrus (4). Controls voluntary
contractions of skeletal muscles on opposite side
• Motor speech area: Broca’s area (44). Production of speechcontrols tongue and airway
Association Areas
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•
•
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Integrative functions such as memory, emotions, reasoning, will, judgment, personality,
and intelligence
Somatosensory area: integrate and interpret (5, 7)
Visual association area: recognize and evaluate (18, 19)
Auditory association (Wernicke’s) area: words become speech (22)
Gnostic area: integrate all senses and respond (5, 7, 39, 40)
Premotor area: learned skilled movements such as playing piano (6)
Frontal eye field: scanning eye movements such as reading (8)
Aphasia
• Inability to use or comprehend speech
• Nonfluent aphasia: inability to properly form
words
– Damage to motor speech (Broca’s) area
– Person knows what they want to say, but cannot speak
• Fluent aphasia: faulty understanding of spoken or
written words
– Damage to either the auditory association (Wernicke’s)
area or to the common integrative area
– Word deafness: inability to understand spoken words
– Word blindness: inability to understand written words
Hemispheric Lateralization
•
•
•
•
•
•
Left hemisphere is more important for right-handed control as well as those things
depicted
Right hemisphere is more important for left-handed control as well as those things
depicted
Functional specialization more pronounced in men
Females generally have larger connections between two sides
If speech areas on the left are damaged, aphasia results
If speech areas on the right are damaged, speech has little emotional inflection
Electroencephalogram (EEG)
• Brain waves: summation
of millions of nerve action
potentials in cerebral
cortex
– Alpha: awake and resting
with eyes closes
– Beta: intense mental
activity
– Theta: normal in children,
but found in adults only
during emotional stress,
frustration, brain disorders
– Delta: deep sleep, infancy,
brain disorders
Use of EEGs
• Diagnosis of epilepsy, other seizure disorders, infectious
diseases, tumors, trauma, hematoma, metabolic
abnormalities, degenerative diseases, confusion, periods
of unconsciousness
• Information regarding sleep and wakefulness
• Determination of brain death
Cranial Nerves
Cranial Nerves
Cranial Nerve I: Olfactory
• Sense of smell
• Sensory; passes through the cribiform plate of the ethmoid
• Extends from olfactory mucosa of nasal cavity to olfactory bulb
Cranial Nerve II: Optic
• Vision
• Sensory; fibers
pass through the
optic foramen
• Optic nerve is
composed of
fibers from the
retina
• Fibers of optic
tract project to
the visual cortex
in the occipital
lobe
Cranial Nerve VIII: Vestibulocochlear
Two branches
• Cochlear branch goes to the cochlea of the inner ear and carries
impulses concerning hearing. Damage can cause tinnitus.
• Vestibular branch (associated with pons) carries information from
the vestibule of the inner ear. Carries information on balance.
Damage can cause vertigo and ataxia (lack of coordination).
Table 14-9 Cranial Nerve Branches and Functions
Table 14-9 Cranial Nerve Branches and Functions