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Cranial nerve
From Wikipedia, the free encyclopedia
Jump to: navigation, search Nerve: Cranial nerves
Inferior view of the brain and brain stem showing cranial nerves. An unlabelled version is here
Latin
nervus cranialis (plural: nervi craniales)
Code
TA A14.2.00.038
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which
emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial
nerves. Only the first and the second pair emerge from the cerebrum; the remaining ten pairs emerge
from the brainstem.
The cranial nerves are part of the peripheral nervous system (PNS) with the exception of cranial nerve II
or 'optic nerve', along with the retina, which is not a true peripheral nerve but a tract of the
diencephalon.[1] Cranial nerve ganglia originate in the central nervous system (CNS). The remaining
eleven axons extend beyond the brain and are therefore considered part of the PNS.[2]
Cranial nerves in non-human vertebrates[edit source | editbeta]
Human cranial nerves are nerves similar to those found in many other vertebrates. Cranial nerves XI and
XII evolved in other species to amniotes (non-amphibian tetrapods), thus totaling twelve pairs. In some
primitive cartilaginous fishes, such as the spiny dogfish or mud shark (Squalus acanthias), there is a
terminal nerve numbered zero, since it exits the brain before the traditionally designated first cranial
nerve.
Cranial nerve zero
The terminal nerve appears just ahead of the other cranial nerves bilaterally as a microscopic plexus of
unmyelinated peripheral nerve fascicles in the subarachnoid space that covers gyrus rectus. This plexus
appears near the cribriform plate and travels posteriorly toward the olfactory trigone, medial olfactory
gyrus, and lamina terminalis.[2]
The nerve is often overlooked in autopsies because it is unusually thin for a cranial nerve, and is often
torn out upon exposing the brain.[3] Careful dissection is necessary to visualize the nerve. Its purpose
and mechanism of function is still open to debate; consequently, nerve zero is often not mentioned in
anatomy textbooks.[1]
Function[edit source | editbeta]
Although very close to[4] (and often confused for a branch of) the olfactory nerve, nerve zero is not
connected to the olfactory bulb, where smells are analyzed. This fact suggests that the nerve is either
vestigial or may be related to the sensing of pheromones. This hypothesis is further supported by the
fact that nerve zero projects to the medial and lateral septal nuclei, and the preoptic areas, all of which
are involved in regulating sexual behavior in mammals.[1]
Development[edit source | editbeta]
The zebrafish has been used as a developmental model in recent research.[5]
The connections between cranial nerve zero and the olfactory system has been extensively studied in
human embryos. It was found to enter the brain at stages 17 and 18 from olfactory origins.
Olfactory nerve
From Wikipedia, the free encyclopedia
Jump to: navigation, search Nerve: Olfactory nerve
The Olfactory Nerve
Inferior view of the human brain, with the cranial nerves labelled.
Latin
nervus olfactorius
MeSH Olfactory+Nerve
Cranial nerves
CN I – Olfactory
CN II – Optic
CN III – Oculomotor
CN IV – Trochlear
CN V – Trigeminal
CN VI – Abducens
CN VII – Facial
CN VIII – Vestibulocochlear
CN IX – Glossopharyngeal
CN X – Vagus
CN XI – Accessory
CN XII – Hypoglossal
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Optic nerve
The olfactory nerve, or cranial nerve I, is the first of twelve pairs of cranial nerves. It is instrumental in
the sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is capable of
regeneration. The olfactory nerve is sensory in nature and originates on the olfactory mucosa in the
anterosuperior nasal cavity.[1] From the olfactory mucosa, the nerve travels down the olfactory tract
until it reaches the olfactory bulb, where the fascicles of the olfactory nerve pass through foramina on
the cribriform plate, which resides on the roof of the nasal cavity. These fascicles are not visible on a
cadaver brain because they are severed upon removal
The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central
nervous system, as it is derived from an outpouching of the diencephalon during embryonic
development. As a consequence, the fibers are covered with myelin produced by oligodendrocytes,
rather than Schwann cells, which are found in the peripheral nervous system, and are encased within
the meninges. Peripheral neuropathies like Guillain-Barré syndrome do not affect the optic nerve.
The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than
the epineurium, perineurium, and endoneurium found in peripheral nerves. Fiber tracks of the
mammalian central nervous system (as opposed to the peripheral nervous system) are incapable of
regeneration, and, hence, optic nerve damage produces irreversible blindness. The fibres from the
retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay
inputs into the primary visual cortex.
Oculomotor nerve
From Wikipedia, the free encyclopedia
Jump to: navigation, search Nerve: Oculomotor nerve
Nerves of the orbit. Seen from above.
Inferior view of the human brain, with the cranial nerves labelled.
Latin
nervus oculomotorius
Gray's subject #198 884
Innervates
Superior rectus, Inferior rectus, Medial rectus, Inferior oblique, Levator palpebrae,
sphincter pupillae (parasympathetics), ciliaris muscle (parasympathetics)
From
oculomotor nucleus, Edinger-Westphal nucleus
To
superior branch, inferior branch
MeSH Oculomotor+Nerve
Cranial nerves
CN I – Olfactory
CN II – Optic
CN III – Oculomotor
CN IV – Trochlear
CN V – Trigeminal
CN VI – Abducens
CN VII – Facial
CN VIII – Vestibulocochlear
CN IX – Glossopharyngeal
CN X – Vagus
CN XI – Accessory
CN XII – Hypoglossal
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital
fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an
open eyelid by innervating the levator palpebrae superioris muscle. The oculomotor nerve is derived
from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of
eye movement.