Cranial nerves are the nerves that emerge directly from the brain, in contrast to spinal nerves. 10 of the cranial nerves originate in the brainstem. Cranial nerves relay information between the brain and parts of the body to and from regions of the head and neck. Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head emerging in the space above the first cervical vertebra; the cranial nerves, emerge from the central nervous system above this level. Each cranial nerve is present on both sides. Depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I–XII, sometimes including cranial nerve zero; the numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back. The terminal nerves, olfactory nerves and optic nerves emerge from the cerebrum or forebrain, the remaining ten pairs arise from the brainstem, the lower part of the brain; the cranial nerves are considered components of the peripheral nervous system, although on a structural level the olfactory and trigeminal nerves are more considered part of the central nervous system.
Most humans are considered to have twelve pairs of cranial nerves, with the terminal nerve more canonized. They are: the olfactory nerve, the optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, hypoglossal nerve. Cranial nerves are named according to their structure or function. For example, the olfactory nerve supplies smell, the facial nerve supplies motor innervation to the face; because Latin was the lingua franca of the study of anatomy when the nerves were first documented and discussed, many nerves maintain Latin or Greek names, including the trochlear nerve, named according to its structure, as it supplies a muscle that attaches to a pulley. The trigeminal nerve is named in accordance with its three components, the vagus nerve is named for its wandering course. Cranial nerves are numbered based on their rostral-caudal position. If the brain is removed from the skull the nerves are visible in their numeric order, with the exception of the last, CN XII, which appears to emerge rostrally to CN XI.
Cranial nerves have paths outside the skull. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by. All cranial nerves are paired, which means that they occur on both the right and left sides of the body; the muscle, skin, or additional function supplied by a nerve on the same side of the body as the side it originates from, is referred to an ipsilateral function. If the function is on the opposite side to the origin of the nerve, this is known as a contralateral function. Intracranial course of cranial nerves is important regarding diagnosis of various intracranial lesions like brain tumors and intracranial arterial aneurysms. Dysfunction of one or more cranial nerves indicates stimulation by some lesion. For example an acoustic schwanoma may cause disturbance in hearing but with further growth of tumor it may involve other cranial nerves and the patient may present with pain resembling trigeminal neuralgia when the tumor involves trigeminal nerve or diplopia due to abducent nerve involvement facial palsy with facial nerve compression.
These findings along with cerebellar signs will suggest the diagnosis of a cerebellopontine angle lesion. A patient presenting with ptosis may have a posterior communicating artery aneurysm compressing the oculomotor nerve during its intracranial course. Facial pain in the distribution of any one or all divisions of trigeminal nerve suggests stimulation of trigeminal nerve roots by a near by vessel; the cell bodies of many of the neurons of most of the cranial nerves are contained in one or more nuclei in the brainstem. These nuclei are important relative to cranial nerve dysfunction because damage to these nuclei such as from a stroke or trauma can mimic damage to one or more branches of a cranial nerve. In terms of specific cranial nerve nuclei, the midbrain of the brainstem has the nuclei of the oculomotor nerve and trochlear nerve; the fibers of these cranial nerves exit the brainstem from these nuclei. Some of the cranial nerves have sensory or parasympathetic ganglia of neurons, which are located outside the brain.
The sensory ganglia are directly correspondent to dorsal root ganglia of spinal nerves and are known as cranial sensory ganglia. Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X. There are parasympathetic ganglia, which are part of the autonomic nervous system for cranial nerves III, VII, IX and X; the trigeminal ganglia of the trigeminal nerve occupies a space in the dura mater called Trigeminal cave. This ganglion contains the cell bodies of the sensory fibers of the three branches of the trig
Anatomical terms of location
Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. All vertebrates have the same basic body plan – they are bilaterally symmetrical in early embryonic stages and bilaterally symmetrical in adulthood; that is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other organisms as well. While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories. Standardized anatomical and zoological terms of location have been developed based on Latin and Greek words, to enable all biological and medical scientists to delineate and communicate information about animal bodies and their component organs though the meaning of some of the terms is context-sensitive.
The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used for location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way. For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position, thus what is on "top" of a human is the head, whereas the "top" of a dog may be its back, the "top" of a flounder could refer to either its left or its right side. For invertebrates, standard application of locational terminology becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not bilaterally symmetrical. In these species, terminology depends on their type of symmetry.
Because animals can change orientation with respect to their environment, because appendages like limbs and tentacles can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position. All descriptions are with respect to the organism in its standard anatomical position when the organism in question has appendages in another position; this helps avoid confusion in terminology. In humans, this refers to the body in a standing position with arms at the side and palms facing forward. While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing. Many anatomical terms can be combined, either to indicate a position in two axes or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position, both anterior and lateral to the body axis. In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.
There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. The modifier term is truncated and an "o" or an "i" is added in prefixing it to the qualified term. For example, a view of an animal from an aspect at once dorsal and lateral might be called a "dorsolateral" view. Again, in describing the morphology of an organ or habitus of an animal such as many of the Platyhelminthes, one might speak of it as "dorsiventrally" flattened as opposed to bilaterally flattened animals such as ocean sunfish. Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral"; such terms sometimes used to be hyphenated. There is however little basis for any strict rule to interfere with choice of convenience in such usage. Three basic reference planes are used to describe location; the sagittal plane is a plane parallel to the sagittal suture. All other sagittal planes are parallel to it, it is known as a "longitudinal plane".
The plane is perpendicular to the ground. The median plane or midsagittal plane is in the midline of the body, divides the body into left and right portions; this passes through the head, spinal cord, and, in many animals, the tail. The term "median plane" can refer to the midsagittal plane of other structures, such as a digit; the frontal plane or coronal plane divides the body into ventral portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical. A longitudinal plane is any plane perpendicular to the transverse plane; the coronal plane and the sagittal plane are examples of longitudinal planes. A transverse plane known as a cross-section, divides the body into cranial and caudal portions. In human anatomy: A transverse plane is an X-Z plane, parallel to the ground, which s
The endocranium in comparative anatomy is a part of the skull base in vertebrates and it represents the basal, inner part of the cranium. The term is applied to the outer layer of the dura mater in human anatomy. Structurally, the endocranium consists of a boxlike shape, open at the top; the posterior margin exhibit the foramen magnum, an opening for the spinal cord. The floor of the endocranium has several paired openings for the cranial nerves, the anterior margin holds a spongy construction, allowing for the external nasal nerves to pass through. All bones of the structure derive from the cranial neural crest during fetal development. In humans and other mammals, the endocranium forms during fetal development as a cartilaginous neurocranium, that ossifies from several centers. Several of these bones merge, in the adult primates, the endocranium is composed of only five bony elements: The Ethmoid bone, lying behind the nose; the Sphenoid bone, underlying the forward portion of the brain Paired petrous part of the temporal bones, containing the inner ear structures The Occipital bone, surrounding the foramen magnum The endocranium in mammals is much reduced in relative size and number of bones compared to the condition in the ancestral land vertebrates, though the occipital bone occur as several bony elements in several mammal groups.
The occipital bone is found as several bony elements in birds and reptiles, while the skull of modern amphibians is reduced with a simplified endocranium. The skull of early labyrinthodonts were rather complex, contained in addition to the bones mentioned above several small cartilaginous components that are fused to temporal and occipital bones in mammals: Paired prootic and opisthotic bones above each fenestra ovalis, fused to the petrous part of the temporal bones in mammals. Paired exoccipital bones medially and a single basioccipital bone below the foramen magnum, part of the occipital bone in mammals. While the endocranium is an integral part of the skull in mammals and reptiles, its connection to the roofing parts of the skull is more loose in the lower vertebrates. In Agnathans and Chondrichthyes, the skull lacks the skull roof dermal elements, their whole cranium being composed of the endocranium, properly called a chondrocranium. In most Osteichthyes, the skull is only loosely joined, the endocranial elements do not form a unit with the skull roof.
An endocast or endocranial cast is a cast made of the mold formed by the impression the brain makes on the inside of the neurocranium, providing a replica of the brain with most of the details of its outer surface. Endocasts can form when sediments fill the empty skull, after which the skull is destroyed and the cast fossilized. Scientists are utilizing computerized tomography scanning technology to create digital endocasts without damaging valuable specimens; this gives a 3D representation of the brain. Brain size and complexity can be determined. Endocasts were used for looking at the brains of Homo sapiens to find hemispheric specialization. Dermatocranium Splanchnocranium
The dorsum sellae is part of the sphenoid bone in the skull. In the sphenoid bone, the anterior boundary of the sella turcica is completed by two small eminences, one on either side, called the middle clinoid processes, while the posterior boundary is formed by a square-shaped plate of bone, the dorsum sellae, ending at its superior angles in two tubercles, the posterior clinoid processes, the size and form of which vary in different individuals; this article incorporates text in the public domain from page 147 of the 20th edition of Gray's Anatomy "Anatomy diagram: 34257.000-2". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01. Anatomy photo:22:os-0912 at the SUNY Downstate Medical Center
The superior surface of the body of the sphenoid bone is bounded behind by a ridge, which forms the anterior border of a narrow, transverse groove, the chiasmatic groove and behind which lies the optic chiasma of cranial nerve 2. The groove ends on either side in the optic foramen, which transmits the optic nerve and ophthalmic artery into the orbital cavity; this article incorporates text in the public domain from page 147 of the 20th edition of Gray's Anatomy figures/chapter_42/42-18. HTM: Basic Human Anatomy at Dartmouth Medical School Anatomy image: skel/internal2 at Human Anatomy Lecture, Pennsylvania State University
The foramen rotundum is a circular hole in the sphenoid bone that connects the middle cranial fossa and the pterygopalatine fossa. The foramen rotundum is one of the several circular apertures located in the base of the skull, in the anterior and medial part of the sphenoid bone; the mean area of the foramina rotunda is not considerable, which may suggest that they play a minor role in the dynamics of blood circulation in the venous system of the head. The foramen rotundum evolves in shape throughout the fetal period, from birth to adolescence, it achieves a perfect ring-shaped formation in the fetus after the 4th fetal month. It is oval-shaped in the fetal period, round-shaped after birth. After birth, the rotundum is about 2.5 mm and in 15- to 17-year olds about 3 mm in length. The average diameter of the foramen rotundum in adults is 3.55 mm. The maxillary branch of the trigeminal nerve passes through and exits the skull via the pterygopalatine fossa and the foramen rotundum. Foramen is the Latin term designating a hole-like opening.
It derives from the Latin forare meaning to perforate. Here, the opening is round as indicated by the Latin rotundum meaning round. Foramina of skull This article incorporates text in the public domain from page 150 of the 20th edition of Gray's Anatomy Anatomy photo:22:os-0905 at the SUNY Downstate Medical Center cranialnerves at The Anatomy Lesson by Wesley Norman Superior view of the base of the skull at winona.edu
The foramen magnum is a large oval opening in the occipital bone of the skull in humans and various other animals. It is circular openings in the base of the skull; the spinal cord, an extension of the medulla, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments, it transmits the spinal component of the accessory nerve into the skull. The opisthion is the midpoint on the posterior margin of the foramen magnum and is a cephalometric landmark. Another landmark is the basion located at the midpoint on the anterior margin of the foramen magnum; the foramen magnum is a important feature in bipedal mammals. One of the attributes of a bipedal animal’s foramen magnum is a forward shift of the anterior border. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion.
The forward shift of the foramen magnum is apparent in bipedal hominins, including modern humans, Australopithecus africanus, Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was bipedal, may be the earliest known bipedal ape; the discovery of this feature has given scientists another form of identifying bipedal mammals. The alar ligament, attached on each side to the tubercle of occipital condyle on each side of Foramen magnum divides it into anterior smaller compartment and posterior larger compartment. Thus, in humans, the neck muscles do not need to be as robust. Comparisons of the position of the foramen magnum in early hominid species are useful to determine how comfortable a particular species was when walking on two limbs rather than four. Posterior cranial fossa This article incorporates text in the public domain from page 129 of the 20th edition of Gray's Anatomy "Anatomy diagram: 34257.000-1".
Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01. Diagram 1 Diagram 2 3D animation showing position of basion on YouTube