Download I. Neurons are the anatomical elements of neural systems

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The Nerves of the Eye and Orbit
Optic nerve (II)
Oculomotor nerve (III)
Trochlear (IV)
Trigeminal (V)
Abducens (VI)
Facial (VII)
I.
Neurons are the anatomical elements of neural systems
a. neuron= a single nerve cell, usually generates an action potential, but
several nerve cells in the retina DO NOT, they produce graded potentials
b. typically bipolar—receive input from one end and provide outputs at the
other end
c. dendrites= small branches of neuron that receive inputs
d. axon= a single long process that provide the output
e. cell body contains metabolism apparatus and protein synthesis
f. action potential= brief change in the electrical polarity of the cell’s
membrane that can be propagated from one end of the cell to the other
without the amplitude being diminished
II.
Neural circuits consist of neurons linked mostly by unidirectional
chemical synapses
a. synapse= site of close approximation between two neurons, or between
neuron and effector (cell to cell connections acting as communication
sites)
b. neurotransmitter= chemical released by a pre-synaptic swelling that
interacts with chemical receptors on the dendrites of the second neuron
c. presynaptic cell= cell that releases the neurotransmitter
d. postsynaptic cell= cell on which the neurotransmitter acts
e. the effect is to excite or inhibit the electrical activity in the target cell
f. chemical synapses are unidirectional= info flows from the presynaptic to
the postsynaptic cell, but not vice versa
g. “recurrent connections” (common in retina) act as a feedback mechanism
to presynaptic cell
h. Gap Junctions – create bidirectional ion flow
III.
The direction of neural information flow distinguishes between sensory and
motor nerves
a. optic nerve fibers, as well as pain touch, cold and hot receptors of the eye
and orbit conduct signals from the eye to the brain, but not vice versa
(sensory nerves)
b. information from the brain to the eye occurs by other sets of fibers (motor
nerves)
IV.
Motor outputs are divided anatomically and functionally into somatic and
autonomic systems
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a. autonomic nervous system= output of this system is to smooth muscles
and glands
b. somatic (voluntary) nervous system= cell bodies of this system are
contained within the brain and spinal cord and connect to striated muscle
V.
The autonomic nervous system is subdivided into the sympathetic and
parasympathetic systems
a. sympathetic NS (fibers enters eye by passing through the sympathetic
root of ciliary ganglion, no synapsis present)= uses norepinepherine as a
neurotransmitter (FIGHT or FLIGHT)
b. parasympathetic NS= uses acetylcholine as a neurotransmitter, cell
bodies of nerve close to target organ (ciliary & Meckal’s ganglia)
c. the two systems of neurons often innervate the same targets, but usually
with antagonistic effects
VI.
In the optic nerve, the location of axons from retinal ganglion cells
corresponds to their location on the retina
a. optic nerve= CRANIAL NERVE II, eye’s main sensory nerve
b. contains about 1 million nerve fibers, all of which are axons of the
ganglion cells that form the innermost cellular layer of the retina
c. bundle of axons exits through the optic foramen as the optic nerve
d. optic nerves from each eye join at the optic chiasm
e. ganglion cells in the superior retina have axons in the superior half of the
nerve
f. axons in the inferior part of the nerve are form ganglion cells in the
inferior retina
g. Figure 5.3 , temporal retinal fiber begin segregated into superior and
inferior bundles but coalesce at the chiasm, the foveal bundle of ganglion
cell nerves are temporal as the exit the globe but centered at the chiasm
VII.
Axons from the two optic nerves are redistributed in the optic chiasm
a. after the two optic nerves pass through their respective optic canals and
exit the sphenoid bone, they merge as the optic chiasm, where optic
nerves combine, from which the bilateral optic tracts arise
b. axons from the nasal hemiretinas cross to the opposite side of the brain
(the contralateral projection)
c. temporal retinal axons remain on the side from which they originated
(ipsilateral side)
d. not all of the crossing axons take the most direct and shortest route from
one side to the other
i. some cross the midline and turn anteriorly and then loop back
ii. other nasal axons remain on the ipsilateral side and then change
direction to cross the midline at the back of the chiasm
iii. between these extremes, are axons that run directly through the
center of the chiasm
iv. Figure 5.5
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e. the fovea is represented bilaterally in both optic tracts
VIII.
Spatial ordering of axons changes in the optic tracts
a. retinotopic= the pattern of projection of axons such that neighboring
areas on the retina remain neighbors in the target nuclei
b. such an arrangement suggests that a small bundle of axons in a particular
part of the tract will be from corresponding parts of the two retinas and
therefore correspond to the same region of the visual field
IX.
In the lateral geniculate nuclei, which are primary targets in the optic tracts,
inputs from the two eyes are separated into different layers
a. lateral geniculate nuclei= Main output of retina, large six-layered nuclei
in the midbrain to which most retinal ganglion cell project, main relay to
cortex
b. neurons here send axons to the primary visual cortex via the optic
radiations
c. layers 1,4, and 6 receive inputs from the contralateral eye
d. layers 2,3, and 5 receive inputs from the ipsilateral eye
e. left LGN receives axons from the left temporal retina and the right nasal
retina (right half of the visual world)
f. right LGN receives axons from the right temporal retina and the left nasal
retina (left half of the visual world)
X.
Axons terminating in the lateral geniculate nuclei are spatially ordered
a. ganglion cells that are neighbors in the retina will have their terminals as
neighbors in the LGN
b. destruction of a particular part of the LGN will always produce a scotoma
(an area of relative or complete blindness) in the same part of the visual
field
c. the fovea and surrounding areas are over-represented spatially in the LGN
(the large # of foveally derived axons is evenly spread across the LGN,
and because
XI.
Some axons leave the optic tracts for other destinations
a. some retinal ganglion cells do not contribute to the primary visual
pathway and are not directly involved in visual perception
b. some projections appear to have signals that convey info to secondary
targets and are used for other purposes (i.e. reflex eye movements)
c. Superior colliculus, olivary nuclei, accessory optic nuclei, supraoptic
nucleas.
XII.
Lesions of the optic nerves and tracts produce defects in the visual fields
a. Figure 5.12
b. any lesion between the chiasm and the eye will affect the visual field of
only one eye
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c. lesions of the chiasm or the optic tracts will affect both monocular fields
d. if one optic tract were completely severed, the scotomas would be
hemianopias, in which half of each monocular field is blind
XIII.
Lesions in the secondary visual pathways can be observed only as motor
deficits
a. it is possible to interrupt the primary visual pathway, so as to produce
complete while certain visual behaviors, such as the pupillary light
reflexes, remain intact and normal
b. brain lesions that involve one or more of the subsidiary pathways without
affecting the primary visual pathway will not produce visual field defects
The Trigeminal Nerve: Signals for Touch and Pain
A. Two of the three trigeminal divisions carry signals from the eye and surrounding
tissues.
a. Trigeminal Nerve (Cranial Nerve V) differs from other cranial nerves
because it collects the cell bodies of its neurons in a large ganglion, the
gasserian (semilunar) ganglion, outside the brainstem
b. Connected to the brainstem via the sensory and motor roots
i. Ophthalmic division (V1): Has three major branches
1. Nasociliary nerve: Only one of the three to innervate the
eye
2. Frontal nerve: Has two dependent branches-the
supratrochlear and supraorbital nerves both of which go to
the skin of the brow and forehead
3. Lacrimal nerve-has only a single branch which is the route
used by parasympathetic fibers going to the lacrimal gland
via the lacrimal nerve
ii. Maxillary division (V2): Extend to the teeth and gums in the upper
jaw, the lips and facial skin, and the mucosa of the maxillary sinus
1. Zygomatic nerve: the major branch that enters the orbit
and runs beneath the orbit floor to innervate the skin of the
lower eyelid and face
2. Zygomatio-facial and zygomatico-temporal nerves are
small branches that exit the orbit through small foramina in
the zygomatic bone to the skin of the cheek
iii. Mandibular division (V3): Has numerous branches that innervates
the teeth, gums, and skin of the lower jaw
B. All somatosensory information from the eye is conveyed by the nasociliary nerve
to the ophthalmic division of the trigeminal
a. Two long ciliary nerves exit the posterior sclera on either side of the optic
nerve, and they run medially to join the nasociliary nerve.
b. Several short ciliary nerves exit the sclera all around the optic nerve. They
are mixed nerves containing sensory, sympathetic, and parasympathetic
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C.
D.
E.
F.
G.
H.
I.
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fibers and they all run from the eye to the ciliary ganglion that lies on the
lateral side of the optic nerve
Sensory nerve fibers from the cornea, conjunctiva, limbus, and anterior sclera join
to form the long ciliary nerves.
a. In the general classification of sensory receptors they are nociceptors,
activated by noxious or potentially noxious stimuli including mechanical
pressure, which is perceived as pain, and substances that are released by
damaged cells or inflammatory reactions.
b. The majority of nerve endings are in the cornea where they end in the
epithelium
c. Nerve fibers from the cornea, iris, limbus, and ciliary body join on either
side of the eye to run within the choroids as two long ciliary nerves. They
exit the sclera on either side of the optic nerve, running back and medially
to join the nasociliary nerve.
d. Majority of fibers in the long ciliary nerves are from the cornea
Stimulation of corneal or conjunctival nerve endings elicits sensations of touch or
pain, a blink reflex, and reflex lacrimation
Other sensory fibers from the eye are conveyed by the short ciliary nerves and the
sensory root of the ciliary ganglion
a. The sensory fibers in the short ciliary nerves run directly through the
ganglion without synapsing and they exit as a single fiber bundle called
the sensory root
Most other branches of the ophthalmic nerve carry somatosensory fibers from the
skin of the eyelids and face
a. The merging of the anterior ethmoidal and infratrochlear nerves form the
nasociliary nerve
b. The anterior ethmoidal nerve innervates the nasal mucosa and skin at the
tip of the nose
c. The posterior ethmoidal nerve innervates the nasal mucosa and joins the
nasociliary just after it enters the orbit
d. Supratrochlear and supraorbital nerves, two branches of the frontal nerve
that innervates the upper eyelid and forehead
A few branches of the maxillary nerve pass through the orbit from the facial skin
and maxillary sinus
a. Zygomatico-temporal and zygomatico-facial join the zygomatic nerve
from the skin overlying the cheek bone
b. Pterygopalatine ganglion (sphenopalatine, or Meckel’s) carry
parasympathetic fibers for the lacrimal gland
Lesions in the branching hierarchy of the ophthalmic nerve produce anesthesia
that helps identify the lesion site
a. Retrobulbar block: an injection of anesthetic behind the eye
Viral infection of the trigeminal system can produce severe corneal damage
Extraocular Motor Nerves
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A. The three cranial nerves that innervate the six extraocular muscles contain axons
from clusters of cells in the brainstem that arise from the nuclei of motor neurons.
Their cell bodies have extensive dendritic processes
a. Oculomotor (III) to the superior rectus, medial rectus, inferior rectus, and
inferior oblique; also the levator muscle
b. Trochlear (IV) to the superior oblique
c. Abducens (VI) to the lateral rectus
B. Most of the cells in the oculomotor nuclei send their axons into the ipsilateral
oculomotor nerve and to the muscles on the ipsilateral side. EXCEPTION: the
superior recti---the axons cross the midline between the two oculomotor nuclei to
pass through the contralateral nucleus and enter the contralateral oculomotor
nerve.
(ie: RIGHT superior rectus innervated by axons originating in the LEFT
oculomotor nucleas and vice versa)
C. Severing ONE oculomotor nerve will still affect only the muscles on the
ipsilateral eye
D. Axons destined for different muscles run together in the oculomotor nerve until it
exits the cavernous sinus just behind the orbit
a. The superior branch of the oculomotor nerve innervates the levator and
superior rectus
b. The inferior branch innervates the medial and inferior recti and inferior
oblique
E. The oculomotor nerve contains parasympathetic fibers bound for the ciliary
ganglion
a. Motor root (of the ciliary ganglion): a small bundle of axons leaves the
inferior oculomotor branch going to the inferior oblique and runs upward
to the ciliary ganglion
b. Edinger-Westphal nucleus: Lies in the brainstem near the ipsilateral
oculomotor nucleus. Nerve fibers from these nuclei join the oculomotor
nerve before they exit the brain stem
F. Cells in the trochlear nerve (CN IV) nucleus
a. innervate the contralateral superior oblique (only extraocular muscle with
contralateral innervation along with superior rectus muscle
b. the smallest and longest nerves going to the extraocular muscles making
them vulnerable to trauma
G. Abducens nerve cells (CN VI)
a. innervate a single muscle ipsilateral lateral rectus
b. lesions along the abducens nerve are confined to lateral rectus mainly
c. most lateral of the extraocular nerves and is subject to damage associated
with trauma to the side of the head
H. Cavernous sinus- a large venous sinus alongside the sphenoid bone
a. All of the oculomotor nerves pass through the cavernous sinus on their
way to the orbit
b. Carotid arteries pass through the cavernous sinuses
c. The optic nerve does NOT pass through the cavernous sinuses
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I. The extraocular motor nerves probably contain sensory axons from muscle
spindles and tendon organs as well as motor axons
Innervation of the Muscles of the Eyelids
A. Three sets of muscles are associated with the eyelids
B. The orbicularis oculi (striated muscle) is innervated by the facial nerve (VII)
a. Contraction causes narrowing of the palpebral fissure to close eye
b. Supplied by 2 branches of the facial nerve: upper and lower zygomatic
branches of the facial nerve
c. Lesions that irritate the facial nerve and produce unusual levels of activity
in the nerve fibers going to the orbicularis muscle can produce
blepharospasm
d. Ectropian: may result do to failed transmission to orbicularis and muscle
paralysis leading to this eversion of the lower lids
C. Levator is innervated by the oculomotor (III) nerve
D. The superior and inferior tarsal muscles are made of smooth muscle and are
innervated by the sympathetic system
E. Ptosis may result from either oculomotor or sympathetic lesions
a. Normally the upper lid is held in position by the tonus in the levator and
superior tarsal muscles.
b. Lesion to CN III (oculomotor nerve) can affect innervation of levator or a
sympathetic lesion affecting the superior tarsal muscle can result in ptosis
(drooping upper lid)
I. Autonomic Innervation of the Smooth Muscle within the Eye
A. Sympathetic nerve pathway to the eye
i. Most sympathetic fibers originate in superior cervical ganglion
located in the upper part of the neck below the skull
ii. Fibers run upward as the internal carotid nerves along with internal
carotid arteries and begin branching at the carotid plexus
(sympathetic plexus)
iii. One of the bungles of sympathetic fibers becomes the sympathetic
root of the ciliary ganglion which runs along the ophthalmic
artery and enters the orbit at the optic foramen
iv. The fibers run along the optic nerve and pass through the ciliary
ganglion, but do not make synaptic connections with the ganglion
v. While passing through, the bundle branches and enters the short
ciliary nerves
vi. The short ciliary nerves pass through the sclera to the choroid and
branch to their target structures
B. Dilator muscle of the iris has sympathetic innervation
i. Drugs mimicking sympathetic neurotransmitters (a common
sympathetic neurotransmitter that is mimicked is norepinephrine)
produce mydriasis (pupil dilation) NOT miosis (pupil constriction)
ii. Dilation is the result of the contraction of the dilator muscle
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II.
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C. Sympathetic innervation of arterioles in the uveal tract produce
vasoconstriction
i. Most of the arterioles in the uveal tract (iris, choroid, and ciliary
body) appear to receive innervation from sympathetic innervation
ii. Allows choroidal blood flow to remain constant while arterial
pressure increases
iii. Under normal circumstances (when arterial and venous pressures
on either side of the vascular bed are constant) the blood flow will
be INVERSELY proportional to the resistance in the vascular bed
1. Ex: increase resistance = decreased flow (in choroid
activation of sympathetic system reduces blood flow)
D. Horner’s Syndrome
i. results from an isolated lesion in the sympathetic pathway that
could be far from the eye
ii. Symptoms include: ptosis, miosis, and anhidrosis (lack of
sweating) to the face and neck of the affected side
iii. Other possible signs: decreased IOP, vasodilation of conjunctival
blood vessels, and enophthamos (sinking of the eye into the orbit)
E. Parasympathetic nerve pathway to the eye
i. Preganglionic fibers originating in the Edinger-Westphal nucleus
travel along the oculomotor nerve and motor root of the ciliary
ganglion before reaching the ciliary ganglion, while postganglionic
fibers originate in cell bodies of the ciliary ganglion and enter the
eye by the short ciliary nerves
1. These fibers innervate the sphincter and ciliary muscles
ii. Rami oculares are postganglionic fibers the orginate in the
pterygopalatine ganglion and enter the orbit through the superior
orbital fissure
1. They partially innervate the choroidal vasculature
F. Pupil constriction is controlled by the parasympathetic system
i. Retinal ganglion cells monitor the amount of light entering the eye
and transmit information to the olivary pretectal nucleus (there are
2 olivary pretectal nuclei, 1 for each eye)
ii. Olivary pretectal nucleus sends signals to both Edinger-Westphal
nuclei
iii. Edinger-Westphal nuclei innervate the sphincter muscle
1. Pupil constriction in the eye receiving light stimulus is
direct reflex, constriction in other eye is consensual reflex
G. Argyll Robertson pupil is characterized by partially constricted pupils
(or pupil) without direct or consensual reflexes, but miosis with
accommodation is present
i. Lesion site is most likely between the olivary pretectal nuclei and
Edinger-Westphal nuclei
ii. Present in neurosyphili
Innervation of the Lacrimal Gland
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III.
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A. Postganglionic parasympathetic fibers to the lacrimal gland originate
from the pterygopalatine ganglion
i. Pathway
1. pterygopalatine ganglion
2. zygomatic nerve
3. zygomatico-temporal nerve
4. communicating branch
5. lacrimal nerve
ii. Axons terminate around secretory cells of lacrimal gland
B. Sympathetic inputs are also present in the lacrimal gland, but pathway is
unclear
C. Preganglionic parasympathetic fibers to the pterygopalatine ganglion
originate in a portion of the facial nerve nucleus
i. Provides connection between innervation of the cornea and
conjunctiva and lacrimal gland
D. Lacrimal gland stimulation is associated with pain, emotional distress,
and very intense light stimuli
E. Lacrimal gland is also influenced by hormones
F. Afferent or efferent pathway lesions prevent reflex weeping, but efferent
pathway lesion also prevents tonic lacrimal gland stimulation thereby
affecting basal tear production
i. Efferent lesions could contribute to dry eye syndrome
Issues in Neural development
A. Specialized growth cones guide the extension of axons and dendrites
i. Growth cones – specialized regions of developing neural processes
that contain filipodia the crawl to a region where it will receive
synaptic inputs or to the target cells to which it will be presynaptic
B. Pathfinding by growth cones depends on recognition of local directions
signs
i. Neuroglial cells – supporting cells found throughout the nervous
system that act as adhesives to guide growth cones on particular
paths
C. Target recognition and acquisition may require specific markers
produced by the target cells
i. Tropic factors guide developing neurons to their final position; the
final position is dependent upon the timing that the tropic factors
are being produced
D. Many early neurons are eliminated as mature patterns of connectivity are
established
E. Adult connectivity patterns are not always complete at birth, and
postnatal development is subject to modification
i. Critical periods – times in which the nervous system can be
extensively modified
1. Pathfinding is completed early in life
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2. Synapse establishment and maintenance persists for
considerably longer, and could persists, on some level,
throughout life
F. Ocular albinism is associated with a pathfinding error in the
development of optic nerve axons
G. Anomalous innervation of the extraocular muscles may be the result of
pathfinding or target recognition errors
H. Some forms of amblyopia may be related to problems with postnatal
establishment and maintenance of synaptic connections
i. Amblyopia – reduced visual acuity that is not accounted for by
refractive error or pathology
I. Innervation of the extraocular muscles begins early in gestation sensory
innervation much later
IV.
Postnatal Neuron Growth and Regeneration
A. Most postnatal neuron growth is interstitial growth
i. Interstial growth – neuron growth that occurs when growth cones
disappear and lengthening is no longer needed, neurons get larger
and a new cell membrane is formed
B. Neurons do not undergo mitosis postnatally
i. We are born with all neurons that our body will ever form
C. Spinal neurons in peripheral nerves can regenerate after being damaged
D. Central nervous system neurons do not regenerate following major
damage
E. Corneal nerve endings will regenerate following local damage
i. Damaged corneal nerve fibers initially degenerate, but eventually
regenerate to innervated damaged area; also, fibers from
undamaged regions provide innervation to damaged area
ii. Nerve endings in cornea can regenerate because only a small,
uncomplicated area of the pathway is damaged
F.
Neuronal degeneration can affect other, undamaged neurons
i. Transneuronal atrophy – degeneration of nerve tissue that arises
because the nerve from which the tissue received signals is no
longer functional
G. Parasympathetic nerve pathway to the eye originate in the ciliary or the
pterygopalatine ganglion (Fig. 5.26)
i. Pre-ganglionic fibers originating in the Edinger-Westphal
nucleus travel along the oculomotor nerve and motor root
of the ciliary ganglion before reaching the ciliary ganglion.
ii. Post-ganglionic fibers originate in the cell bodies of the ciliary
ganglion and enter the eye by the short ciliary nerves.
1. These fibers innervate the sphincter and ciliary muscle
iii. Rami oculares are post-ganglionic parasympathetic fibers also
that originate in the pterygopalatine ganglion.
1. From the facial nerve synapse, the fibers pass through
the superior orbital fissure, form a plexus around
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the carotid artery, bypass the ciliary ganglion, and
innervate the choroid.
H. Pupil constriction is controlled by the parasympathetic innervation of the
sphincter (Fig 5.28)
i. Retinal ganglion cells monitor the amount of light entering the
eye then leave the optic tract and run to the pretectal
nucleus cells (there are 2 PNCs one for each eye).
ii. Olivary pretectal nucleus sends signals to both EdingerWestphal nuclei that accounts for the direct and
consensual reflex. (the path for OPN to the contralateral
Edinger-Westphal nucleus passes through the posterior
commissure.
iii. Edinger-Westphal nuclei innervate the sphincter muscle and
cause pupillary light reflexes.
I. Argyll Robertson pupil
i. Characterized by constricted pupils with no direct or consensual
reflex.
ii. Caused by lesions between the olivary pretectal nuclei and
Edinger-Westphal nuclei.
iii. Retention of miosis occurs despite loss of pupil reflex
iii. Found many times in syphilis patients (Whore's pupilaccommodation but no reflex)
Innervation of Lacrimal Gland (Fig 5.27 and 29)
A. Postganglionic parasympathetic fibers in the lacrimal gland originate
from the pterygopalatine ganglion (same area as Rami oculares)
i. Pathway: Leave pterygopalatine ganglion and join the
maxillary nerve briefly before branching off to the
zygomatico-temporal branch. The fibers then leave via
the communicating branch to join the lacrimal gland.
ii. Parasympathetic axons terminate around the secretory cells of
the glands.
iii. Sympathetic inputs are unclear
B. Preganglionic parasympathetic fibers to the pterygopalatine ganglion
originate in a portion of the facial nerve nucleus
i. Provides connection between innervation of the cornea and
conjunctiva and lacrimal gland
C. Other inputs on the parasympathetic pathway can lead to lacrimal
gland stimulation and tear production.
i. Inputs can be pain, emotional distress, and very intense light.
D. Lacrimal gland can also be stimulated by local and nonlocal hormones.
E. Lesions in the afferent and efferent pathway can prevent reflex weeping
i. Efferent lesions are more serious because they eliminate all tonic
innervation of lacrimal gland
ii. A possible causative agent for dry eye
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I. Neural Development
A. Once neurons differentiate, they grow processes that become axons and dentrites. Growth originates at the
cell body.
i.
Growth cones- specialized region that contain filipodia that allow developing processes to crawl to
specific regions.
ii.
Dentrites find the region where it will receive synaptic input and axons find the presynaptic region of
target cells.
B. Pathfinding of growth cones has several factors.
Laminin on the surface of neuroglial cells act as stepping stones that guide the growth cones along
particular paths.
b)
The first neurons to reach their destinations act as “pioneer” cells that make it easier for later neurons to
reach their destinations.
a)
a)
b)
c)
C. Growing neurons must recognize when they have reached their targets.
Tropic factors are specific markers produced by target cells.
Axons grow towards these markers.
Tropic factors are produced at different times. This is how one neuron can innervate several cells.
D. Many more neurons are produced than are needed.
a) Fine tuning the pattern of connectivity requires the elimination of neurons with poor connections to target cells.
b)
This establishes mature patterns of neuron connective
E. Adult connectivity patterns are not always complete at birth. Critical periods are times when neurons are
susceptible to modification.
a)
Critical period for pathfinding happens early and ends when the target is acquired.
b)
Critical period for synapse establishment and maintenance is much longer. Some capacity for synapse
modification may persist throughout life.
F. Developmental abnormalities that involve errors in pathfinding, target recongnition, and synapse
establishment, can interfere with normal neuron interactions.
a) Ocular albinism is associated with errors in pathfinding in the development of the optic nerve axons. This
misrouting occurs at the chiasm.
b) Recognition errors can cause inaccurate innervation of the extraocular muscles that result in improper function of
those muscles.
c) Amblyopia that is not accounted for by refractive error or pathology is associated with postnatal synaptic
establishment and maintenance anomalies.
II. Post natal Neuron Growth and Regeneration
A. Once target is reached by developing neurons, the growth cones disappear and subsequent increase in size is
by interstitial growth.
b.
Peripheral nerves retain a capacity for interstitial growth for many years.
c.
Growth in central neurons does not last as long. Brain reaches adult size earlier than the rest of the body
B. Neurons do not undergo mitosis postnatally. We are born with all the neurons that are body will ever have.
This is why there is an overproduction of them in the embryo.
C. Because neurons are unable to reproduce, ability to regenerate depends on where the damage is.
a) Spinal neurons in peripheral nerves can regenerate after damage. Axons can grow to reach the target structure
again, but the innervation is never as good as the original.
b) Central neurons do not regenerate at all after damage. Intricate
pathways and need for precision may prevent it.
c) Corneal nerve ending will regenerate if damage is confined. Fibers from undamaged regions provide innervation
to damaged area. Regeneration is possible because only a small, uncomplicated pathway area is damaged.
Neuronal degeneration after damage can affect other neurons causing transneuronal atrophy. Neurons that used to
receive signals from the damaged nerve, will no longer function and will degenerate too.
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