THE NERVOUS SYSTEM: NEURAL TISSUE

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C H A P T E R T H I R T E E N

• THE NERVOUS SYSTEM NEURAL TISSUE

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Two organ systems coordinate and direct
activities of body

• Nervous system
• Swift, brief responses to stimuli
• Endocrine system
• Adjusts metabolic operations
• Directs long-term changes

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Nervous system includes all neural tissue in body

• Central Nervous System
• Brain and spinal cord
• Peripheral Nervous System
• All neural tissue outside CNS

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Functional divisions of nervous system

• Afferent
• Sensory information from receptors to CNS
• Efferent
• Motor commands to muscles and glands
• Somatic division
• Voluntary control over skeletal muscle
• Autonomic division
• Involuntary regulation of smooth and cardiac
  muscle, glands

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Cells in Nervous Tissue

• Neurons

• Neuroglia

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Neuroglia (Glia)

• about half the volume of cells in the CNS
• smaller than neurons
• 5 to 50 times more numerous
• do NOT generate electrical impulses
• divide by mitosis
• Four types in the CNS
• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells

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Astrocytes

• Largest of glial cells
• Most numerous
• Star shaped with many processes
• projecting from the cell body
• Help form and maintain blood-brain barrier
• Provide structural support for neurons
• Maintain the appropriate chemical
• environment for generation of nerve
  impulses/action potentials
• Regulate nutrient concentrations for neuron
  survival
• Regulate ion concentrations - generation of
  action potentials by neurons
• Take up excess neurotransmitters
• Assist in neuronal migration during brain
  development
• Perform repairs to stabilize tissue

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Oligodendrocytes

• Most common glial cell type
• Each forms myelin sheath around the axons of
  neurons in CNS
• Analogous to Schwann cells of PNS
• Form a supportive network around CNS neurons

• fewer processes than astrocytes
• round or oval cell body

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Microglia

• few processes
• derived from mesodermal cells
• that also give rise to monocytes
• and macrophages

• Small cells found near blood vessels
• Phagocytic role - clear away dead cells
• protect CNS from disease through phagocytosis of
  microbes
• migrate to areas of injury where they clear away
  debris of
• injured cells - may also kill healthy cells

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Representative Neuron
http//www.horton.ednet.ns.ca/staff/selig/Activiti
es/nervous/na1.htm
-neurofilaments or neurofibrils give cell shape
and support - bundles of intermediate
filaments -microtubules move material inside
cell -lipofuscin pigment clumps (harmless aging)
- yellowish brown
1. cell body or soma -single nucleus with
prominent nucleolus -Nissl bodies -rough ER
free ribosomes for protein synthesis -proteins
then replace neuronal cellular components for
growth and repair of damaged axons in the PNS
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Neurons
2. Cell processes dendrites (little trees) -
the receiving or input portion of the
neuron -short, tapering and highly
branched -surfaces specialized for contact with
other neurons -cytoplasm contains Nissl bodies
mitochondria
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• 3. Cell processes axons
• Conduct impulses away from cell body-propagates
  nerve impulses to another neuron
• Long, thin cylindrical process of cell
• contains mitochondria, microtubules
  neurofibrils - NO ER/NO protein synth.
• joins the soma at a cone-shaped elevation axon
  hillock
• first part of the axon initial segment
• most impulses arise at the junction of the axon
  hillock and initial segment trigger zone
• cytoplasm axoplasm
• plasma membrane axolemma
• Side branches collaterals arise from the axon
• axon and collaterals end in fine processes called
  axon terminals
• Swollen tips called synaptic end bulbs contain
  vesicles filled with neurotransmitters

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Excitability
http//bcs.whfreeman.com/thelifewire/content/chp44
/4401s.swf

• Ability of cell membrane to conduct electricity
• Skeletal muscle fibers
• Most neurons
• Changes in membrane potential induces an action
  potential (AP)
• the PM of neurons exhibits a membrane potential
  electrical voltage difference across the membrane
• in excitable cells like neurons this potential
  resting potential
• due to differences in sodium and potassium ion
  concentration in and out of the cell
• potassium is higher inside cell, sodium is higher
  outside
• inside of the cell has a higher concentration of
  negative phosphate ions and proteins potential
  of -40 to -90 mV

• the cell is said to be polarized
• membrane has sodium/potassium pumps
• to maintain specific concentrations of these ions
  in
• out of the neurons and therefore maintain the
• resting membrane potential
• -move three Na out and 2 K in
• -inside of the neuron is slightly
• negative

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Action Potential

• Resting membrane potential is -70mV
• threshold usually -55 MV
• Depolarization is the change from -70mV to 30 mV
• Repolarization is the reversal from 30 mV back
  to -70 mV)

http//www.blackwellpublishing.com/matthews/channe
l.html

• action potential nerve impulse
• takes place in two stages depolarizing phase
  (more positive) and repolarizing phase (more
  negative - back toward resting potential)
• followed by a hyperpolarizing phase or refractory
  period in which no new AP
• can be generated

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Local Anesthetics

• Prevent opening of voltage-gated Na channels
• Nerve impulses cannot pass the anesthetized
  region
• Novocaine and lidocaine blocks nerve impulses
  along nerves that detect pain

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Synapse

• Synapse
• Site of intercellular communication between 2
  neurons or between a neuron and an effector (e.g.
  muscle)
• Originates in the soma
• Travels along axons
• Permit communication between neurons and other
  cells
• Initiating neuron presynaptic neuron
• Receiving neuron postsynaptic neuron
• two types chemical electrical

• NT will cause either and excitatory or inhibitory
  response
• If the NT depolarizes the postsynaptic neuron
  excitatory

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Neurotransmitters

• More than 100 identified
• Some bind receptors and cause channels to open
• Others bind receptors and result in a second
  messenger system
• Results in either excitation or inhibition of the
  target

botulism causes paralysis through blockage of AcH
release from motor neurons
1. small molecules e.g. Acetylcholine (ACh) -All
neuromuscular junctions use ACh -ACh also
released at chemical synapses in the PNS and by
some CNS neurons -Can be excitatory at some
synapses and inhibitory at others

• 2. Amino acids glutamate aspartate GABA
• Stimulate most excitatory neurons in the CNS
  (about ½ the neurons in the brain)
• Binding of glutamate to receptors opens calcium
  channels excitatory response
• GABA (gamma amino-butyric acid) is an inhibitory
  neurotransmitter for 1/3 of all brain synapses

Valium is a GABA agonist - enhancing its
inhibitory effect
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Neurotransmitters

• 3. Biogenic amines modified amino acids
• catecholamines norepinephrine (NE), epinephrine,
  dopamine (tyrosine)
• serotonin - concentrated in neurons found in the
  brain region raphe nucleus
• derived from trytophan
• sensory perception, temperature regulation, mood
  control, appetite, sleep induction
• feeling of well being
• NE - role in arousal, awakening, deep sleep,
  regulating mood
• epinephrine (adrenaline) - flight or fight
  response
• dopamine - emotional responses and pleasure,
  decreases skeletal muscle tone

• Parkinsons - muscle stiffness due to
  degeneration of dopanergic nerves
• patients given L-Dopa (dopamine precursor)

• amphetamines promote dopamine and NE release

• isoproterenol binds to epinephrine receptors
• - used in asthma to mimic the effects of
  epinephrine
• schizophrenia - excess of dopamine
• Zyprexa blocks dopamine and serotonin receptors
• -antagonizes the effects of serotonin and
  dopamine

• cocaine blocks transporters for dopamine
  reuptake
• Prozac, Paxil blocks transporters for serotonin
  reuptake

Other types Nitric oxide - formed on demand in
the neuron then release (brief lifespan) -role
in memory and learning -produces vasodilation -
Viagara enhances the effect of NO
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Neuropeptides

• widespread in both CNS and PNS
• excitatory and inhibitory
• act as hormones elsewhere in the body
• -Substance P -- enhances our perception of pain
• -opoid peptides endorphins - release during
  stress, exercise
• enkephalins - analgesics
• (200x stronger than morphine)
• -pain-relieving effect by blocking the
  release of
• substance P
• dynorphins - regulates pain and emotions

acupuncture may produce loss of pain sensation
because of release of opioid-like substances such
as endorphins or dynorphins
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Divisions of the nervous system

• Sensory pathway
• Ascending
• Information from sensory receptors to CNS
• Motor pathway
• Descending
• Information from CNS to skeletal muscle or glands
• Direct pathways cause precise, voluntary
  movements
• Indirect pathways result in involuntary
  movement (from brain stem)

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Functional Classification of Neurons

• Sensory (afferent) neurons
• transport sensory information from skin, muscles,
  joints, sense organs viscera to CNS
• Motor (efferent) neurons
• send motor nerve impulses to muscles glands
• Interneurons (association) neurons
• connect sensory to motor neurons
• 90 of neurons in the body

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Sensory Neurons

• Afferent division of PNS
• Deliver sensory information from sensory
  receptors to CNS
• free nerve endings bare dendrites associated
  with pain, itching, tickling, heat and some touch
  sensations
• Exteroceptors located near or at body surface,
  provide information about external environment
• Proprioceptors located in inner ear, joints,
  tendons and muscles, provide information about
  body position, muscle length and tension,
• position of joints
• Interoceptors located in blood vessels, visceral
  organs and NS
• -provide information about
  internal environment
• -most impulses are not perceived those
  that are,
• are interpreted as pain or pressure

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Sensory Neurons

• Sensory receptors cont
• mechanoreceptors detect pressure, provide
  sensations of touch, pressure,
• vibration, proprioception, blood vessel stretch,
  hearing and equilibrium
• thermoreceptors detect changes in temperature
• nociceptors respond to stimuli resulting from
  damage (pain)
• photoreceptors light
• osmoreceptors detect changes in OP in body
  fluids
• chemoreceptors detect chemicals in mouth
  (taste), nose (smell)
• and body fluids

-analgesia relief from pain -drugs aspirin,
ibuprofen block formation of prostaglandins
that stimulate the nociceptors -novocaine
block nerve impulses along pain
nerves -morphine, opium derivatives (codeine)
pain is felt but not perceived in brain
(blocks morphine and opiate receptors in pain
centers)
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Motor Neurons

• Efferent pathways
• Stimulate peripheral structures
• Somatic motor neurons
• Innervate skeletal muscle
• Visceral motor neurons
• Innervate all other peripheral effectors
• Preganglionic and postganglionic neurons

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Somatic nervous system (SNS) 1. sensory -
neurons that convey sensory information from
somatic receptors in the head, body wall, senses
- to the CNS 2. control of motor output -
neurons that conduct voluntary impulses to
skeletal muscles -contributions from the basal
ganglia, cerebellum, brain stem and SC 3. one
neuron pathway somatic motor neurons synapse
directly with the effector 4. neurotransmitter
usually acetylcholine 5. effectors skeletal
muscles 6. responses - contraction
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• Autonomic nervous system (ANS)
• sensory - neurons that convey info from autonomic
  sensory receptors in the visceral organs - to the
  CNS
• 2. control of motor output - neurons that
  conduct impulses from the CNS to
• smooth and cardiac muscle glands
• 3. two neuron pathway preganglionic neurons
  extend from CNS and synapse with postganglionic
  neurons in an autonomic ganglion, postganglionic
  neurons then synapse with the effector
• 4. neurotransmitter preganglionic ACh
• -postganglionic ACh or norepinephrine
• 5. effectors smooth cardiac muscle, glands,
• 6. responses contraction or relaxation of SM
• -increased or decrease heart contraction
• -increased or decreased gland secretions

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- motor output branch has two divisions 1.
sympathetic 2. parasympathetic -most organs are
innervated by both divisions which
have opposing functions e.g. sympathetic
increases heart rate parasympathetic
decreases rate
-
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An Introduction to the Organization of the Brain
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Figure 15.1 Major Divisions of the Brain
Major Regions of the Brain
http//www.wisc-online.com/objects/framz.asp?objID
OTA502
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The Cranial Nerves
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I - Olfactory II - Optic III - Oculomotor IV-Troch
lear V - Trigeminal VI - Abducens VII -
Facial VIII - Acoustic IX - Glossopharyngeal X -
Vagus XI - Accessory XII - Hypoglossal

• -cranial nerves 12 pairs
• -considered part of the peripheral nervous system
  (PNS)
• -olfactory optic contain only sensory axons
  sensory nerves
• -some are motor nerves e.g. oculomotor,
  trochlear etc.
• remaining are mixed nerves both motor and
  sensory axons
• some say my mother bought my brother some bitter
  beer, my, my

http//www.wisc-online.com/objects/index.asp?objID
AP11504
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The Olfactory Nerve (I)

• Carries sensory information
• Sense of smell from nasal mucosa to brain
• Branches enter skull through cribiform plate
• Synapses within olfactory bulbs

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• The optic nerve (II) (sensory)
• Carries visual information
• enters skull through optic canal
• of the sphenoid
• -right and left join at the
• optic chiasma (site of cross-over)
• -continue to brain as optic tracts

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• The abducens nerve (VI) Motor
• Innervates lateral rectus muscle of eye
• exits through S.O.F

• The oculomotor nerve (III) Motor
• Primary source of innervation for extra-ocular
  muscles
• also carries postganglionic fibers that innervate
  the ciliary muscles (lens shape)
• exits through superior orbital fissure
• The trochlear nerve (IV) Motor
• Smallest cranial nerve
• Innervates superior oblique eye muscle
• also provides proprioception info
• exits through S.O.F

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The Trigeminal Nerve (V) Mixed

• Largest cranial nerve
• Mixed nerve
• sensory touch, pain thermal
• Ophthalmic branch
• sensory upper eyelid, eyeball
• lacrimal glands, side of nose, forehead
• and scalp
• Maxillary branch
• sensory nose, palate, part
• of pharynx, upper teeth, upper
• lip and lower eyelid
• Mandibular branch
• sensory tongue, cheek,
• lower teeth, skin over mandible
• and side of head anterior to ear
• -motor muscles of chewing

-inferior alveolar nerve (branch of mandibular)
-often anesthetized in dental procedures lower
jaw -numbs to mental nerve (branch of the
IAN) -superior alveolar nerve (branch of the
maxillary) -numbs the upper jaw
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The Facial Nerve (VII) Mixed

• Mixed nerve
• Controls muscles of scalp and face
• Pressure sensations from face
• Taste sensations from tongue

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Facial Nerve VII

• efferent branches supply muscles of facial
  expression
• also carries preganglionic parasympathetic fibers
  to the lacrimal, sub-mandibular and sub-maxillary
  glands
• afferent branches serves a tiny patch of skin
  behind the ear
• also provides taste information and sensation to
  the body of the tongue

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Facial Nerve VII

• Greater Petrosal
• branches off before exiting skull
• motor fibers Pre/para fibers to pterygopalatine
  ganglion
• postganglionic fibers leave the ganglion to join
  with branches of the maxillary division or V -gt
  lacrimal gl, nasal cavity and minor salivary
  glands of the palate
• also taste sensation
• Chorda Tympani
• parasympathetic, motor for SMn and SL salivary
  glands
• sensory for taste at the body of the tongue
• crosses the tympanic membrane before exiting the
  skull
• travels with the lingual n. to the floor of the
  mouth
• Posterior Auricular, Diagastic and Stylohyoid
• branches after VII exits the stylomastoid f.
• all are motor epicranial m., diagastic and
  stylohyoid muscles
• facial expression
• temporal (anterior to ear), zygomatic (inferior
  orbicularis oculi ZMj, ZMn), buccal (upper lip,
  nose, buccinator, risorius and orbicularis oris),
  mandibular ( lower lip and mentalis) and cervical
  (plastysma)

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The Vestibulocochlear Nerve (VIII) Sensory

• Vestibular nerve
• Monitors sense of balance, position and movement
• Cochlear nerve
• Monitors hearing

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The Glossopharyngeal Nerve (IX) Mixed

• Innervates the tongue, pharyngeal muscles,
  stylopharyngeus m.
• Controls swallowing
• the efferent portion also sends pre/para fibers
  to the parotid gland (salivation)
• also receives sensory info from taste receptors
  and general sensation from the tongue

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The Vagus Nerve (X) Mixed

• Vital to autonomic control of visceral function
• large efferent portion to the soft palate,
  pharynx and larynx
• many other parasympathetic fibers to the organs
  of the gut, respiratory and CV systems
• small afferent portion receives sensory
  information from around the ear and for taste
  info from the epiglottis
• passes through the jugular foramen

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• The accessory nerve (XI)
• Internal branch
• Innervates swallowing muscles
• External branch
• Controls muscles associated with pectoral girdle

• The hypoglossal nerve (XII)
• Voluntary motor control over tongue movements

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Trigeminal Nerve and Branches

• bulge in the dorsal root of V trigeminal
  ganglion
• also called the semilunar ganglion
• ganglion collection of neuronal cell bodies
• comprised of a motor root and a sensory root
• supplies the muscles of mastication
• exits via the forament ovale in the sphenoid
• travels with mandibular division of V (V3)
• sensory root divides into three portions
• 1. Opthalmic
• 2. Maxillary
• 3. Mandibular

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Opthalmic Division

• smallest division V1
• sensory information from conjuctiva, cornea,
  eyeball, orbit, forehead, ethmoid and frontal
  sinuses
• also part of dura mater
• carries its sensory info by way of the superior
  orbital fissure along with III, IV VI
• formed from the union of
• the frontal, lacrimal and
• nasociliary branches

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• Frontal branch supraorbital supratrochlear
• SO forehead, anterior scalp
• ST bridge of nose, upper eyelid, medial forehead
• runs along roof of orbit
• Lacrimal branch
• lateral eyelid, conjuctive lacrimal gland
• also provides post/para fibers to the lacrimal
  gland tear production
• Nasociliary branch infratrochlear ciliary
  nerves anterior ethmoid n.
• runs superior to II within the orbit
• IT medial eyelid skin, side of nose
• Ciliary eyeball
• AE nasal cavity
• paranasal sinuses

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Maxillary Division

• V2
• sensory information from maxilla skin,
  maxillary sinuses, nasal cavity, palate
  nasopharynx part of dura mater (meningeal
  branches)
• forms in the pterygopalatine fossa
• enters skull through the foramen rotundum
• prior to branching pterygopalatine ganglion
• parasympathetic relay station for branches that
• arise from the facial nerve
• branches
• zygomatic
• infraorbital
• anterior superior alveolar
• middle superior alveolar
• posterior superior alveolar
• greater lesser palatine
• nasopalatine

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• Zygomatic
• zygomaticotemporal zygomaticofacial
• enters pterygopalatine fossa through the
  infraorbital fissure and joins to contribute to
  the maxillary nerve
• zygo.facial skin of cheek
• through the frontal process of the zygomatic bone
  and enters the orbit thru the lateral wall
• zygo.temporal skin of the temporal bone
• through the temporal process of the zygomatic
  bone and travels along the lateral wall of the
  orbit
• Infraorbital IO
• formed from cutaneous branches
• from the upper lip, medial portion of cheek
• lower eyelid and side of nose
• runs into the infraorbital foramen of
• the maxilla
• travels along the infraorbital canal
• with the infraorbital blood vessels
• joins with the anterior superior
• alveolar nerve

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• Anterior Superior Alveolar (ASA)
• sensation pain from the maxillary central
  incisors, lateral incisors, canines and their
  tissues facial gingiva
• originates as dental branches supplying the pulp
  interdental branches of the associated
  periodonteum dental plexus of the maxilla
• joins with the IO within the IO canal
• Middle Superior Alveolar (MSA)
• sensation pain from the maxillary premolars and
  first molar
• periodonteum and their buccal gingiva
• originates from dental, interdental and
  interradicular branches (dental plexus) pulp
  and periodonteum
• forms this plexus with the ASA and PSA
• joins the IO
• MSA is not present in all patients
• can be replaced by the ASA or PSA

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• Posterior Superior Alveolar (PSA)
• sensation from 2nd and 3rd molar, root of 1st ,
  their periodonteum buccal gingiva, maxillary
  sinuses
• joins the IO in the pterygopalatine fossa
• branches
• a. dental branches interdental interradicular
• (dental plexus)
• b. internal branches exit from the posterior
  superior alveolar foramina (with the posterior
  superior alveolar arteries)
• c. external receive information from buccal
  gingiva and associated molars

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• Nasapalatine
• originates in the mucosa of the anterior hard
  palate posterior to the maxillary teeth
• enters the skull through the incisive foramen
• travels along the nasal septum
• receives info from the anterior hard palate,
  lingula gingiva of anterior maxillary teeth
• and nasal septum
• Greater palatine (GP)
• lies between the periosteum and bone of the
  anterior hard palate
• sensory for the posterior hard palate posterior
  lingual gingiva
• enters through the greater palatine foramen near
  the 2nd or 3rd molar
• travels through the pterygopalatine canal with
  the GP blood vessels
• Lesser Palatine (LP)
• sensory for the palatine tonsils and soft palate
• enters through the LP foramen (near the junction
  of the palatine and the pterygoid processes of
  the sphenoid)
• joins with the GP in the canal
• joined by the nasal branches which
• receive info from the nasal cavity

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Mandibular Division

• V3
• forms from the union of smaller anterior and
  larger posterior trunks in the infratemporal
  fossa
• passes through the foramen ovale
• joins with the Opthal. and Maxillary at the
  trigeminal ganglion
• before dividing into the trunks sends off small
  meningeal branches to the dura mater small
  muscular branches to the medial pterygoid m.
• anterior trunk buccal muscular branches
• posterior trunk auriculotemporal, lingual and
  inferior alveolar branches

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• Buccal nerve sensory
• sensory
• skin of cheek, buccal mucosa and buccal gingiva
  of posteior mandibular teeth
• found on the surface of the buccinator and
  travels deep to the masseter
• Muscular branches
• motor
• from the motor/ventral root of V
• deep temporal branch motor to temporalis
• masseteric branch motor to masseter
• lateral pterygoid branch

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• Auriculotemporal
• travels with the superficial temporal artery and
  vein
• sensory for external ear and scalp
• also carries parasympathetic fibers from IX and
  communicates with II

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• Lingual
• sensory from the body of tongue, floor of mouth,
  and lingual gingiva
• passes from the medial to the lateral side of the
  duct of submandibular gland
• base of tongue runs between the medial
  pterygoid and mandible (slightly medial to the
  inferior alveolar nerve)
• near the root of the last mandibular molar is can
  be visualized clinically (can be damaged upon
  extraction of 3rd molar)
• communicates with the submandibular ganglion
  located superior to the SMn gland (part of
  Parasympathetic NS)
• Inferior Alveolar (IAN)
• sensory from mandibular teeth
• merger of the mental and the incisive
• travels through the mandibular canal and is
  joined by the mandibular dental plexus
• exits through the mandibular foramen
• joined by a mylohyoid branch
• can be found paired on one or both sides
• with two mandibular canals and foramen

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Mental branch of the IAN - union of several
external branches - sensory from the chin, lower
lip and labial mucosa - enters the skull at the
mental foramen - then merges with the lingual
branch of the IAN Lingual branch of the IAN -
made up of dental branches from anterior
mandibular teeth, interdental branches form the
surrounding periodonteum dental
plexus -sensory information for the anterior
mandibular teeth Mylohyoid branch of the IAN -
joins with the mental and lingual after it
emerges from the mandibular foramen - pierces
the sphenomandibular ligament - runs in the
mylohyoid groove then onto the mylohyoid
muscle - motor commands to this muscle
anterior belly of the diagastric (posterior
belly is innervated by the facial nerve)
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External Anatomy of Spinal Cord
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Inferior End of Spinal Cord

• Conus medullaris
• cone-shaped end of spinal cord
• Filum terminale
• thread-like extension of pia mater
• stabilizes spinal cord in canal
• Caudae equinae (horses tail)
• dorsal ventral roots of lowest spinal nerves
• Spinal segment
• area of cord from which each pair of spinal
  nerves arises

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Spinal Nerve

• after passing through intervertebral foramina the
  spinal nerve branches ramus/rami
• Dorsal ramus
• Sensory/motor innervation to skin and muscles of
  back

• Ventral ramus
• -Ventrolateral body surface, body wall
  structures, muscles of the upper and lower limbs
• -Pairs of spinal nerves monitor dermatomes

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Nerve Plexuses

• Four major plexuses
• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus

• Joining of ventral rami of spinal nerves to form
  nerve networks or plexuses
• Found in neck, arm, low back sacral regions
• No plexus in thoracic region
• intercostal nn. innervate intercostal spaces
• T7 to T12 supply abdominal wall as well

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Cervical Plexus

• Cervical plexus
• C1-C4 ventral rami
• Some fibers from C5
• Innervates muscles of the neck and diaphram
• Phrenic nerve

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The Cervical and Brachial Plexus