The Brain and Cranial Nerves
• Brain functions in sensations, memory, emotions, decision making, behavior
Principal Parts of the Brain
• Cerebrum
• Diencephalon
– thalamus & hypothalamus
• Cerebellum
• Brainstem
– medulla, pons & midbrain
Protective Coverings of the Brain
• Bone, meninges & fluid
• Meninges same as around the spinal cord
– dura mater
– arachnoid mater
– pia mater
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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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Cerebrospinal Fluid (CSF)
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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
Origin of CSF
• Choroid plexus = capillaries covered by ependymal cells
– 2 lateral ventricles, one within each cerebral hemisphere
– roof of 3rd ventricle
– fourth ventricle
Drainage of CSF from Ventricles
• One median aperture & two lateral apertures allow CSF to exit from the interior of the
brain
Flow of Cerebrospinal Fluid
Reabsorption of CSF
• Reabsorbed through arachnoid villi
– grapelike clusters of arachnoid
penetrate dural venous sinus
• 20 ml/hour reabsorption
rate = same as production rate
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
• Cranial nerves 8-12
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 thru 7
Midbrain
• One inch in length
• Extends from pons to diencephalon
• Cerebral aqueduct connects 3rd ventricle
above to 4th ventricle below
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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
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
Transverse fissure between cerebellum & cerebrum
Cerebellar cortex (folia) & central nuclei are grey matter
Arbor vitae = tree of life = white matter
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
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)
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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Functions of Hypothalamus
Controls and integrates activities of the ANS which regulates smooth, cardiac muscle
and glands
Synthesizes regulatory hormones that control the anterior pituitary
Contains cell bodies of axons that end in posterior pituitary where they secrete
hormones
Regulates rage, aggression, pain, pleasure & arousal
Feeding, thirst & satiety centers
Controls body temperature
Regulates daily patterns of sleep
Epithalamus
• Pineal gland
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– endocrine gland the size of small pea
– secretes melatonin during darkness
– promotes sleepiness & sets biological clock
Habenular nuclei
– emotional responses to odors
Subthalamus & CVO
Subthalamus
– small area just inferior to thalamus
– work with basal ganglia, cerebrum & cerebellum to control body movements
Circumventricular organs
– in walls of 3rd & 4th ventricles
– monitor changes in blood chemistry because lack blood brain barrier (parts of
hypothalamus, pineal & pituitary gland)
– sites of entry of HIV virus into brain (dementia)
Cerebrum
(Cerebral Hemispheres)
Cerebral cortex is gray matter
overlying white matter
– 2-4 mm thick containing billions
of cells
– grew so quickly formed folds
(gyri) and grooves (sulci or fissures)
Longitudinal fissure separates left
& right cerebral hemispheres
Corpus callosum is band of white matter connecting left and right cerebral
hemispheres
Each hemisphere is subdivided into 4 lobes
Cerebral White Matter
• Association fibers between gyri in same hemisphere
• Commissural fibers from one hemisphere to other
• Projection fibers form descending & ascending tracts
Basal Ganglia
• Input & output with cerebral cortex, thalamus & hypothalamus
• Control large automatic movements of skeletal muscles
Limbic System
• Emotional brain--intense pleasure & intense pain
• Strong emotions increase efficiency of memory
Brain Injuries
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Causes of damage
– displacement or distortion of tissue at impact
– increased intracranial pressure
– infections
– free radical damage after ischemia
Concussion---temporary loss of consciousness
– headache, drowsiness, confusion, lack of concentration
Contusion--bruising of brain (less than 5 min unconsciousness but blood in CSF)
Laceration--tearing of brain (fracture or bullet)
– increased intracranial pressure from hematoma
I -- Olfactory Nerve
Extends from olfactory mucosa of nasal cavity to olfactory bulb
Sense of smell
II -- Optic Nerve
Connects to retina supplying vision
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
IV = Trochlear Nerve
• Superior oblique eye muscle
V = Trigeminal Nerve
• Motor portion
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– muscles of mastication
Sensory portion
– touch, pain, & temperature receptors of the face
• ophthalmic branch
• maxillary branch
• mandibular branch
VI = Abducens Nerve
Lateral rectus eye muscle
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
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)
IX = Glossopharyngeal Nerve
Stylopharyngeus m. (lifts throat during swallowing)
Secretions of parotid gland
Somatic sensations & taste on posterior 1/3 of tongue
X = Vagus Nerve
Receives sensations from viscera
Controls cardiac muscle and smooth muscle of the viscera
Controls secretion of digestive fluids
XI = Spinal Accessory Nerve
• Cranial portion
– arises medulla
– skeletal mm of throat & soft palate
• Spinal portion
– arises cervical spinal cord
– sternocleidomastoid and trapezius mm.
XII = Hypoglossal Nerve
• Controls muscles of tongue during speech and swallowing
• Injury deviates tongue to injured side when protruded
• Mixed, primarily motor