Cranial nerves are the nerves that emerge directly from the brain (including the
brainstem), in contrast to spinal nerves (which emerge from segments of the spinal
cord). Cranial nerves relay information between the brain and parts of the body,
primarily to and from regions of the head and neck.|
The cranial nerves are considered components of the peripheral nervous system (PNS),
although on a structural level the olfactory, optic and terminal nerves are more
accurately considered part of the central nervous system (CNS).
The 12 pairs of cranial nerves are special nerves associated with the brain. The
fibers in cranial nerves are of diverse functional types. Some cranial nerves are
composed of only one type, others of several. In their attachment to the brain,
the first two cranial nerves are associated with the forebrain, nerves III and
IV with the midbrain, and nerves V to XII with the hindbrain.
Cranial nerves differ significantly from spinal nerves, especially in their development
and their relation to the special senses and because some cranial nerves supply
pharyngeal arch structures. They are attached to the brain at irregular rather than
regular intervals; they are not formed of dorsal and ventral roots; some have more
than one ganglion, whereas others have none; and the optic nerve is actually a tract
of the CNS rather than a peripheral nerve.
(a) Ophthalmic nerve
(b) Maxillary nerve
(c) Mandibular nerve
|Afferent and Efferent|
||Afferent and Efferent|
||Afferent and Efferent|
||Afferent and Efferent|
The Functional Components
Cranial nerves act basically as either a one-way or a two-way road, meaning that
some of them transmit information only to the brain (afferent), others only transmit
instructions out (efferent), while the remainder are built to receive and transmit
information. Following this, the cranial nerves that are exclusively or largely afferent
are (I, II, and VIII), the others that are largely efferent are (III, IV, VI, XI, and XII),
and the ones that contain both afferent and efferent fibers are (V, VII, IX, and X).
The efferent fibers of the cranial nerves arise within the brain from groups of nerve
cells termed motor nuclei. The afferent fibers arise outside the brain from groups of
nerve cells, generally in a sensory ganglion along the course of the nerve. The central
processes of these nerve cells then enter the brain, where they end in groups of nerve
cells termed sensory nuclei.
The four functional types of fibers found in spinal nerves are present also in some of
the cranial nerves: somatic afferent, visceral afferent, visceral efferent, and somatic
efferent. These four types are termed "general." In certain cranial nerves, however,
components that are "special" to the cranial nerves are present. The special afferent
fibers comprise visual, auditory, equilibratory, olfactory, taste, and visceral reflex
fibers. (The first three are usually classified as somatic, and the last three as visceral.)
The special efferent fibers (which are classified as visceral) are those to skeletal
muscles either known or thought to be derived from the pharyngeal arches (muscles of
mastication, facial muscles, muscles of pharynx and larynx, sternomastoid, and trapezius).
The cranial nerves may be grouped as follows:
|Olfactory, optic, and vestibulocochlear nerves (I, II, and VIII)
pertain to organs of special sense (special afferent).
|Oculomotor, trochlear, abducent, and hypoglossal nerves (III, IV,
VI, and XII) supply skeletal muscle of specific regions of the head
(eyeballs in the case of III, IV, and VI; tongue in the case of XII).
Nerve III also contains parasympathetic fibers to the smooth muscle
of the sphincter pupillae and the ciliary muscle (general visceral
|Trigeminal nerve (V) contains motor fibers to the muscles of
mastication (special visceral efferent) and sensory fibers from
various parts of the head, e.g., face, nasal cavity, tongue, and
teeth (general somatic afferent).
|Facial, glossopharyngeal, vagus, and accessory nerves (VII, IX,
X and XI) contain several components:
||Motor fibers to the muscles of facial expression
(VII) and the muscles of the pharynx and larynx (IX and X)
(special visceral efferent). Many of the fibers to the pharynx
and larynx are derived from nerve XI (internal branch) and
travel by way of nerve X (hence XI is "accessory" to the vagus).
||Parasympathetic secretory fibers to the lacrimal
and salivary glands (nervus intermedius of VII), the salivary
glands (IX), and certain glands associated with the respiratory
and digestive systems (X) (general visceral efferent). Nerve X
also supplies most of the smooth muscle of the respiratory and
digestive systems, as well as cardiac muscle.
||Taste fibers (nervus intermedius of VII; also IX
and X) (special visceral afferent).
||Fibers from the mucous membrane of the tongue and
pharynx (hence the name glossopharyngeal) and of much of the
respiratory and digestive systems (general visceral afferent)
are contained in nerves IX and X.
||The spinal part of nerve XI supplies the
sternomastoid and trapezius, two muscles of disputed development.
The brainstem is the lower posterior extension of the brain, located in front of
the cerebellum. Its superior or top portion, is the center for visual and auditory
reflexes such as blinking and adjusting the ear to sound volume. The middle
section, or the pons, bridges the cerebellum hemispheres and higher brain
centers with the spinal cord. The brain stem provides the main motor and sensory
innervation to the face and neck via the cranial nerves.|
The brainstem connects the cerebrum and cerebellum to the spinal cord. It's
an extremely important part of the brain as the nerve connections of the motor
and sensory systems from the main part of the brain to the rest of the body pass
through the brain stem. This includes the corticospinal tract for motor function,
the posterior column-medial lemniscus pathway for fine touch and vibration sensation,
and the spinothalamic tract for pain, temperature, itch and crude touch sensations.
The brain stem contains collections of cells that comprise major integrating
centers for motor and sensory functions, form the nuclei of most cranial nerves
(all of the cranial nerves except the first are attached to the brain stem),
form centers concerned with the regulation of a variety of visceral, endocrinological,
behavioral, and other activities, are functionally associated with most of the
special senses, control muscular activity in the head and part of the neck, supply
pharyngeal arch structures, and are connected with the cerebellum.
The brainstem consists of three structures: The midbrain,
medulla oblongata and pons.
It's basically a relay station, passing messages back and forth between
various parts of the body and the cerebral cortex. The brain stem also plays an
important role in the regulation of cardiac and respiratory function. Many
simple or primitive functions that are essential for survival are located here.
The midbrain is an important center for
ocular motion while the pons is involved with coordinating eye and facial
movements, facial sensation, hearing and balance. The
medulla oblongata controls breathing, blood pressure, heart rhythms
and swallowing. Messages from the cortex to the spinal cord and nerves that
branch from the spinal cord are sent through the pons
and the brainstem. Destruction of these regions of the brain will cause "brain
death." Without these key functions, a person can't survive.
The reticular activating system is found in the midbrain, pons, medulla and part
of the thalamus. It controls levels of wakefulness, maintaining consciousness,
enables people to pay attention to their environments, and is involved in sleep
patterns and regulating the sleep cycle.
Originating in the brainstem are 10 of the 12 cranial nerves that control
hearing, eye movement, facial sensations, taste, swallowing and movements of the
face, neck, shoulder and tongue muscles. The cranial nerves for smell and vision
originate in the cerebrum. Four pairs of cranial nerves (cranial nerves 5
through 8) originate from the pons.
The symptoms of multiple sclerosis (MS) lesions of the brainstem include
diplopia or double vision, vertigo, urinary problems (hesitancy or
incontinence), hemiplegia, facial pain and paralysis, and even hearing problems.
Many of these are associated with damage to cranial nerve transmissions.