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| Anatomical terminology |
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This article describes anatomical terminology that is used to describe the central and peripheral nervous systems - including the brain, brainstem, spinal cord, and nerves.
Neuroanatomy, like other aspects of anatomy, uses specific terminology to describe anatomical structures. This terminology helps ensure that a structure is described accurately, with minimal ambiguity. Terms also help ensure that structures are described consistently, depending on their structure or function. Terms are often derived from Latin and Greek, and like other areas of anatomy are generally standardised based on internationally accepted lexicons such as Terminologia Anatomica.
To help with consistency, humans and other species are assumed when described to be in standard anatomical position , with the body standing erect and facing observer, arms at sides, palms forward.
Anatomical terms of location depend on the location and species that is being described.
To understand the terms used for anatomical localisation, consider an animal with a straight CNS, such as a fish or lizard. In such animals the terms "rostral", "caudal", "ventral" and "dorsal" mean respectively towards the rostrum, towards the tail, towards the belly and towards the back. For a full discussion of those terms, see anatomical terms of location.
For many purposes of anatomical description, positions and directions are relative to the standard anatomical planes and axes. Such reference to the anatomical planes and axes is called the stereotactic approach.
Standard terms used throughout anatomy include anterior / posterior for the front and back of a structure, superior / inferior for above and below, medial / lateral for structures close to and away from the midline respectively, and proximal / distal for structures close to and far away from a set point.
Some terms are used more commonly in neuroanatomy, particularly:
Standard anatomical planes and anatomical axes are used to describe structures in animals. In humans and many other primates the axis of the central nervous system is not straight, but bent to allow for forward vision when the body is vertical. This means that differences in terminology are needed to reflect the differences between the brains of primates and the brains of nearly all other vertebrates. For example, to describe the human brain, "rostral" still means "towards the beak or snout (Latin rostrum)", or at any rate, the interior of the cranial cavity just behind the face. "Caudal" means "towards the tail (Latin cauda"), but not "towards the back of the cranial cavity", which is "posterior" (behind, in ordinary motion). The rostro-caudal axis of the human central nervous system (magenta in the diagram) makes a near 90° bend at the level of the midbrain and continues through the brain-stem and spinal cord. In human anatomy, the occipital lobes and the back of the head are posterior but not caudal to the frontal lobes and the face.
"Superior" and "inferior" are adjectives from human anatomy, respectively meaning towards to top of the head or the soles of the feet when standing. The brain is superior to the spinal cord in people, but in quadrupeds the brain is anterior (forward in motion) to the spinal cord.
"Dorsal" means "in the direction away from the ridge of the human back or its equivalent in other animals. In human neuroanatomy the word is somewhat distorted, becoming synonymous with "superior" in the forebrain, i.e. in the direction of the roof of the cranial cavity"cranial cavity and thence to the body. "Ventral" in the central nervous system also refers to the rostro-caudal axis, which changes within the head.
These three axes of the human brain match the three planes within which they lie, even though the terms for the planes have not been changed from the terms for the bodily planes. The most commonly used reference planes are:
Specific terms are used for peripheral nerves that originate from, or arrive at, a specific point.
An afferent nerve fiber is a fibre originating at the present point. For example, a striatal afferent is an afferent originating at the striatum.
An efferent nerve fiber is one that arrives at the present point. For example, a cortical efferent is a fibre coming from elsewhere, and arriving to the cortex. That is the opposite of the direction in which the nerve fibre conducts signals.
Specific terms are also used to describe the route of a nerve or nerve fibre:
A chiasm (from Greek Chi ) is used to describe different types of crossings of or within peripheral nerve fibres between the cerebral hemispheres. The major example in the human brain is the Optic chiasm.
A decussation (from Latin decussis 'ten', written as a capital X) refers to nerve fibers that cross the sagittal plane from one side of the central nervous system to the other, and connect different brain regions. There are two kinds:
The first type is known also for invertebrates, whereas the second type only occurs in vertebrates. The second type is thought to be due to an axial twist, such that each hemisphere of the forebrain represents predominantly the contralateral side of the body.
A commissure is a bilateral connection of axons connecting the left and right side of the same brain region. For example, nerve fibre tracts that cross between the two cerebral hemispheres, are the anterior commissure, posterior commissure, corpus callosum, hippocampal commissure, and habenular commissure. The spinal cord contains a commissure as well: the anterior white commissure.
A ganglion can also have the form of crossing nerves, but a ganglion always contains synapses between neurons as well as their cell bodies. The other kinds of nerve crossings never contain synapses of cell bodies of neurons.
The difference between a chiasm and a decussation is that the first refers to peripheral nerves whereas the latter refers to crossings inside central nervous system. A commissure connects the same brain region of each side whereas a decussation connects different brain regions.
Specific terms are used to represent the gross anatomy of the brain:
A gyrus is an outward folding of the brain, for example the precentral gyrus. A sulcus is an inward fold, or valley in the brain's surface - for example the central sulcus. Additional terms used to describe these may include:
A fissure is used to describe:
Specific acronyms are used to represent imaging. Some common acronyms include MRI (Magnetic Resonance Imaging) and CT (Computed tomography).
In neuroanatomy, the optic chiasm, or optic chiasma, is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes, it is located within the brain.
Standard anatomical terms of location are used to describe unambiguously the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provides a definition of what is at the front ("anterior"), behind ("posterior") and so on. As part of defining and describing terms, the body is described through the use of anatomical planes and anatomical axes.
Articles related to anatomy include:
The brainstem is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch, and sometimes the diencephalon is included in the brainstem.
The third ventricle is one of the four connected cerebral ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ventricles, and is filled with cerebrospinal fluid (CSF).
The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum.
The spinothalamic tract is a nerve tract in the anterolateral system in the spinal cord. This tract is an ascending sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.
The cerebral aqueduct is a small, narrow tube connecting the third and fourth ventricles of the brain. The cerebral aqueduct is a midline structure that passes through the midbrain. It extends rostrocaudally through the entirety of the more posterior part of the midbrain. It is surrounded by the periaqueductal gray, a layer of gray matter.
The medial longitudinal fasciculus (MLF) is a prominent bundle of nerve fibres which pass within the ventral/anterior portion of periaqueductal gray of the mesencephalon (midbrain). It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.
The spinocerebellar tracts are nerve tracts originating in the spinal cord and terminating in the same side (ipsilateral) of the cerebellum. The two main tracts are the dorsal spinocerebellar tract, and the ventral spinocerebellar tract. Both of these tracts are located in the peripheral region of the lateral funiculi. Other tracts are the rostral spinocerebellar tract, and the cuneocerebellar tract.
The subarachnoid cisterns are spaces formed by openings in the subarachnoid space, an anatomic space in the meninges of the brain. The space is situated between the two meninges, the arachnoid mater and the pia mater. These cisterns are filled with cerebrospinal fluid (CSF).
The dorsal longitudinal fasciculus (DLF) is a nerve fiber tract extending from the hypothalamus rostrally to the spinal cord caudally. It containins both descending and ascending fibers. Its fibers form a distinct bundle in the midbrain.
In the human brain, the superior cerebellar peduncle is one of the three paired cerebellar peduncles of bundled fibers that connect the cerebellum to the brainstem. The superior cerebellar peduncle connects to the midbrain. It consists mainly of efferent fibers, the cerebellothalamic tract that runs from a cerebellar hemisphere to the contralateral thalamus, and the cerebellorubral tract that runs from a cerebellar hemisphere to the red nucleus. It also contains afferent tracts, most prominent of which is the ventral spinocerebellar tract. Other afferent tracts are the ventral trigeminal tract, tectocerebellar fibers, and noradrenergic fibers from the locus coeruleus. The superior peduncle emerges from the upper and medial parts of the white matter of each cerebellar hemisphere and is placed under cover of the upper part of the cerebellum.
A commissure is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology.
Decussation is used in biological contexts to describe a crossing. In Latin anatomical terms, the form decussatio is used, e.g. decussatio pyramidum.
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal cord is hollow and contains a structure called the central canal, which contains cerebrospinal fluid. The spinal cord is also covered by meninges and enclosed by the neural arches. Together, the brain and spinal cord make up the central nervous system.
A nerve tract is a bundle of nerve fibers (axons) connecting nuclei of the central nervous system. In the peripheral nervous system, this is known as a nerve fascicle, and has associated connective tissue. The main nerve tracts in the central nervous system are of three types: association fibers, commissural fibers, and projection fibers. A nerve tract may also be referred to as a commissure, decussation, or neural pathway. A commissure connects the two cerebral hemispheres at the same levels, while a decussation connects at different levels.
An anatomical plane is a hypothetical plane used to transect the body, in order to describe the location of structures or the direction of movements. In human and non-human anatomy, three principal planes are used:
In anatomy a chiasm is the spot where two structures cross, forming an X-shape. Examples of chiasms are: