Cerebral hemisphere
The vertebrate cerebrum is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres; each of these hemispheres has an outer layer of grey matter, the cerebral cortex, supported by an inner layer of white matter. In eutherian mammals, the hemispheres are linked by the corpus callosum, a large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix join the hemispheres and these are present in other vertebrates; these commissures transfer information between the two hemispheres to coordinate localized functions. There are three known poles of the cerebral hemispheres: the occipital pole, the frontal pole, the temporal pole; the central sulcus is a prominent fissure which separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex. Macroscopically the hemispheres are mirror images of each other, with only subtle differences, such as the Yakovlevian torque seen in the human brain, a slight warping of the right side, bringing it just forward of the left side.
On a microscopic level, the cytoarchitecture of the cerebral cortex, shows the functions of cells, quantities of neurotransmitter levels and receptor subtypes to be markedly asymmetrical between the hemispheres. However, while some of these hemispheric distribution differences are consistent across human beings, or across some species, many observable distribution differences vary from individual to individual within a given species; each cerebral hemisphere has an outer layer of cerebral cortex, of grey matter and in the interior of the cerebral hemispheres is an inner layer or core of white matter known as the centrum semiovale. The interior portion of the hemispheres of the cerebrum includes the lateral ventricles, the basal nuclei, the white matter. There are three poles of the cerebrum, the occipital pole, the frontal pole, the temporal pole. If the upper part of either hemisphere be removed, at a level about 1.25 cm above the corpus callosum, the central white matter will be exposed as an oval-shaped area, the centrum ovale minus, surrounded by a narrow convoluted margin of gray substance, studded with numerous minute red dots, produced by the escape of blood from divided bloodvessels.
If the remaining portions of the hemispheres be drawn apart a broad band of white substance, the corpus callosum, will be observed, connecting them at the bottom of the longitudinal fissure. Each labium is part of the cingulate gyrus described. If the hemispheres be sliced off to a level with the upper surface of the corpus callosum, the white substance of that structure will be seen connecting the two hemispheres; the large expanse of medullary matter now exposed, surrounded by the convoluted margin of gray substance, is called the centrum ovale majus. The blood supply to the centrum ovale is from the superficial middle cerebral artery; the cortical branches of this artery descend to provide blood to the centrum ovale. The cerebral hemispheres are derived from the telencephalon, they arise five weeks after conception as bilateral invaginations of the walls. The hemispheres grow round in a C-shape and back again, pulling all structures internal to the hemispheres with them; the intraventricular foramina allows communication with the lateral ventricles.
The choroid plexus is formed from vascular mesenchyme. Broad generalizations are made in popular psychology about certain functions being lateralized, that is, located in the right or left side of the brain; these claims are inaccurate, as most brain functions are distributed across both hemispheres. Most scientific evidence for asymmetry relates to low-level perceptual functions rather than the higher-level functions popularly discussed. In addition to this lateralization of some functions, the low-level representations tend to represent the contralateral side of the body; the best example of an established lateralization is that of Broca's and Wernicke's Areas where both are found on the left hemisphere. These areas correspond to handedness however, meaning the localization of these areas is found on the hemisphere opposite to the dominant hand. Function lateralization such as semantics, intonation, prosody, etc. has since been called into question and been found to have a neuronal basis in both hemispheres.
Perceptual information is processed in both hemispheres, but is laterally partitioned: information from each side of the body is sent to the opposite hemisphere. Motor control signals sent out to the body come from the hemisphere on the opposite side. Thus, hand preference is related to hemisphere lateralization. In some aspects, the hemispheres are asymmetrical. There are higher levels of the neurotransmitter norepinephrine on the right and higher levels of dopamine on the left. There is more white matter on the more grey matter on the left. Linear reasoning functions of language such as grammar and word production are late
Cerebellum
The cerebellum is a major feature of the hindbrain of all vertebrates. Although smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or larger. In humans, the cerebellum plays an important role in motor control, it may be involved in some cognitive functions such as attention and language as well as in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium and motor learning in humans. Anatomically, the human cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres, its cortical surface is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex.
These parallel grooves conceal the fact that the cerebellar cortex is a continuous thin layer of tissue folded in the style of an accordion. Within this thin layer are several types of neurons with a regular arrangement, the most important being Purkinje cells and granule cells; this complex neural organization gives rise to a massive signal-processing capability, but all of the output from the cerebellar cortex passes through a set of small deep nuclei lying in the white matter interior of the cerebellum. In addition to its direct role in motor control, the cerebellum is necessary for several types of motor learning, most notably learning to adjust to changes in sensorimotor relationships. Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum; these models derive from those formulated by David Marr and James Albus, based on the observation that each cerebellar Purkinje cell receives two different types of input: one comprises thousands of weak inputs from the parallel fibers of the granule cells.
The basic concept of the Marr–Albus theory is that the climbing fiber serves as a "teaching signal", which induces a long-lasting change in the strength of parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial. At the level of gross anatomy, the cerebellum consists of a folded layer of cortex, with white matter underneath and a fluid-filled ventricle at the base. Four deep cerebellar nuclei are embedded in the white matter; each part of the cortex consists of the same small set of neuronal elements, laid out in a stereotyped geometry. At an intermediate level, the cerebellum and its auxiliary structures can be separated into several hundred or thousand independently functioning modules called "microzones" or "microcompartments"; the cerebellum is located in the posterior cranial fossa. The fourth ventricle and medulla are in front of the cerebellum, it is separated from the overlying cerebrum by a layer of leathery dura mater, the tentorium cerebelli.
Anatomists classify the cerebellum as part of the metencephalon, which includes the pons. Like the cerebral cortex, the cerebellum is divided into two hemispheres. A set of large folds is, by convention, used to divide the overall structure into 10 smaller "lobules"; because of its large number of tiny granule cells, the cerebellum contains more neurons than the total from the rest of the brain, but takes up only 10% of the total brain volume. The number of neurons in the cerebellum is related to the number of neurons in the neocortex. There are about 3.6 times as many neurons in the cerebellum as in the neocortex, a ratio, conserved across many different mammalian species. The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made up of a tightly folded layer of gray matter: the cerebellar cortex; each ridge or gyrus in this layer is called a folium. It is estimated that, if the human cerebellar cortex were unfolded, it would give rise to a layer of neural tissue about 1 meter long and averaging 5 centimeters wide—a total surface area of about 500 square cm, packed within a volume of dimensions 6 cm × 5 cm × 10 cm.
Underneath the gray matter of the cortex lies white matter, made up of myelinated nerve fibers running to and from the cortex. Embedded within the white matter—which is sometimes called the arbor vitae because of its branched, tree-like appearance in cross-section—are four deep cerebellar nuclei, composed of gray matter. Connecting the cerebellum to different parts of the nervous system are three paired cerebellar peduncles; these are the superior cerebellar peduncle, the middle cerebellar peduncle and the inferior cerebellar peduncle, named by their position relative to the vermis. The superior cerebellar peduncle is an output to the cerebral cortex, carrying efferent fibers via thalamic nuclei to upper motor neurons in the cerebral cortex; the fibers arise from the deep cerebellar nuclei. The middle cerebellar peduncle is connected to the pons and receives all of its input from the pons from the pontine nuclei; the input to the pons is from the cerebral cortex and is relayed from the pontine nuclei via transverse pontine fibers to the cerebellum
Motor neuron
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, whose axon projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs muscles and glands. There are two types of motor neuron -- lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and directly onto lower motor neurons; the axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, gamma motor neurons. A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch; as a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch.
The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, tension rises smoothly reaching a plateau. Motor neurons begin to develop early in embryonic development, motor function continues to develop well into childhood. In the neural tube cells are specified to either ventral-dorsal axis; the axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis. This homeodomain is known as the motor neural progenitor domain. Transcription factors here include Pax6, OLIG2, Nkx-6.1, Nkx-6.2, which are regulated by sonic hedgehog. The OLIG2 gene being the most important due to its role in promoting Ngn2 expression, a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development.
Further specification of motor neurons occurs when retinoic acid, fibroblast growth factor, TGFb, are integrated into the various Hox transcription factors. There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character. In the spinal column, Hox 4-11 sort motor neurons to one of the five motor columns. Upper motor neurons originate in the motor cortex located in the precentral gyrus; the cells that make up the primary motor cortex are Betz cells. The axons of these cells descend from the cortex to form the corticospinal tract. Corticomotorneurons are neurons in the primary cortex which project directly to motor neurons in the ventral horn of the spinal cord. Axons of corticomotorneurons terminate on the spinal motor neurons of multiple muscles as well as on spinal interneurons, they are unique to primates and it has been suggested that their function is the adaptive control of the distal extremities including the independent control of individual fingers.
Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas. Nerve tracts are bundles of axons as white matter. In the spinal cord these descending tracts carry impulses from different regions; these tracts serve as the place of origin for lower motor neurons. There are seven major descending motor tracts to be found in the spinal cord: Lateral corticospinal tract Rubrospinal tract Lateral reticulospinal tract Vestibulospinal tract Medial reticulospinal tract Tectospinal tract Anterior corticospinal tract Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets; the target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories lower motor neurons, which can be further divided in sub-categories. According to their targets, motor neurons are classified into three broad categories: Somatic motor neurons Special visceral motor neurons General visceral motor neurons Somatic motor neurons originate in the central nervous system, project their axons to skeletal muscles, which are involved in locomotion.
The three types of these neurons are the alpha efferent neurons, beta efferent neurons, gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system to the periphery. Alpha motor neurons innervate extrafusal muscle fibers, which are the main force-generating component of a muscle, their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average; the motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: Main Article: Motor Unit Slow motor units stimulate small muscle fibers, which contract slowly and provide small amounts of energy but are resistant to fatigue, so they are used to sustain muscular contraction, such as keeping the body upright, they gain their energy via oxidative hence require oxygen. They are called red fibers. Fast fatiguing motor units stimulate larger muscle groups, which apply large amounts of force but fatigue quickly.
They are used for tasks that require large brief bursts on energy, such as jumping or
Anatomical terminology
Anatomical terminology is a form of scientific terminology used by anatomists and health professionals such as doctors. Anatomical terminology uses many unique terms and prefixes deriving from Ancient Greek and Latin; these terms can be confusing to those unfamiliar with them, but can be more precise, reducing ambiguity and errors. Since these anatomical terms are not used in everyday conversation, their meanings are less to change, less to be misinterpreted. To illustrate how inexact day-to-day language can be: a scar "above the wrist" could be located on the forearm two or three inches away from the hand or at the base of the hand. By using precise anatomical terminology such ambiguity is eliminated. An international standard for anatomical terminology, Terminologia Anatomica has been created. Anatomical terminology has quite regular morphology, the same prefixes and suffixes are used to add meanings to different roots; the root of a term refers to an organ or tissue. For example, the Latin names of structures such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for "two-headed", brachii as in the brachial region of the arm.
The first word describes what is being spoken about, the second describes it, the third points to location. When describing the position of anatomical structures, structures may be described according to the anatomical landmark they are near; these landmarks may include structures, such as the umbilicus or sternum, or anatomical lines, such as the midclavicular line from the centre of the clavicle. The cephalon or cephalic region refers to the head; this area is further differentiated into the cranium, frons, auris, nasus and mentum. The neck area is called cervical region. Examples of structures named according to this include the frontalis muscle, submental lymph nodes, buccal membrane and orbicularis oculi muscle. Sometimes, unique terminology is used to reduce confusion in different parts of the body. For example, different terms are used when it comes to the skull in compliance with its embryonic origin and its tilted position compared to in other animals. Here, Rostral refers to proximity to the front of the nose, is used when describing the skull.
Different terminology is used in the arms, in part to reduce ambiguity as to what the "front", "back", "inner" and "outer" surfaces are. For this reason, the terms below are used: Radial referring to the radius bone, seen laterally in the standard anatomical position. Ulnar referring to the ulna bone, medially positioned when in the standard anatomical position. Other terms are used to describe the movement and actions of the hands and feet, other structures such as the eye. International morphological terminology is used by the colleges of medicine and dentistry and other areas of the health sciences, it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research and medical care. The international morphological terminology refers to morphological sciences as a biological sciences' branch. In this field, the form and structure are examined as well as the changes or developments in the organism, it is functional.
It covers the gross anatomy and the microscopic of living beings. It involves the anatomy of the adult, it includes comparative anatomy between different species. The vocabulary is extensive and complex, requires a systematic presentation. Within the international field, a group of experts reviews and discusses the morphological terms of the structures of the human body, forming today's Terminology Committee from the International Federation of Associations of Anatomists, it deals with the anatomical and embryologic terminology. In the Latin American field, there are meetings called Iberian Latin American Symposium Terminology, where a group of experts of the Pan American Association of Anatomy that speak Spanish and Portuguese and studies the international morphological terminology; the current international standard for human anatomical terminology is based on the Terminologia Anatomica. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998.
It supersedes Nomina Anatomica. Terminologia Anatomica contains terminology for about 7500 human gross anatomical structures. For microanatomy, known as histology, a similar standard exists in Terminologia Histologica, for embryology, the study of development, a standard exists in Terminologia Embryologica; these standards specify accepted names that can be used to refer to histological and embryological structures in journal articles and other areas. As of September 2016, two sections of the Terminologia Anatomica, including central nervous system and peripheral nervous system, were merged to form the Terminologia Neuroanatomica; the Terminologia Anatomica has been perceived with a considerable criticism regarding its content including coverage and spelling mistakes and errors. Anatomical terminology is chosen to highlight the relative location of body structures. For instance, an anatomist might describe one band of tissue as "inferior to" another or a physician might describe a tumor as "superficial to" a deeper body structure.
Anatomical terms used to describe location
Brain
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
Astrocyte
Astrocytes known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. The proportion of astrocytes in the brain is not well defined. Depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to 40% of all glia, they perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, a role in the repair and scarring process of the brain and spinal cord following traumatic injuries. Research since the mid-1990s has shown that astrocytes propagate intercellular Ca2+ waves over long distances in response to stimulation, similar to neurons, release transmitters in a Ca2+-dependent manner. Data suggest that astrocytes signal to neurons through Ca2+-dependent release of glutamate; such discoveries have made astrocytes an important area of research within the field of neuroscience.
Astrocytes are a sub-type of glial cells in the central nervous system. They are known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. Astrocytes are classically identified using histological analysis. Several forms of astrocytes exist in the central nervous system including fibrous and radial; the fibrous glia are located within white matter, have few organelles, exhibit long unbranched cellular processes. This type has "vascular feet" that physically connect the cells to the outside of capillary walls when they are in proximity to them; the protoplasmic glia are the most prevalent and are found in grey matter tissue, possess a larger quantity of organelles, exhibit short and branched tertiary processes. The radial glial cells are disposed in planes perpendicular to the axes of ventricles. One of their processes abuts the pia mater, while the other is buried in gray matter. Radial glia are present during development, playing a role in neuron migration.
Müller cells of the retina and Bergmann glia cells of the cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane. Astrocytes are macroglial cells in the central nervous system. Astrocytes are derived from heterogeneous populations of progenitor cells in the neuroepithelium of the developing central nervous system. There is remarkable similarity between the well known genetic mechanisms that specify the lineage of diverse neuron subtypes and that of macroglial cells. Just as with neuronal cell specification, canonical signaling factors like Sonic hedgehog, Fibroblast growth factor, WNTs and bone morphogenetic proteins, provide positional information to developing macroglial cells through morphogen gradients along the dorsal–ventral, anterior–posterior and medial–lateral axes; the resultant patterning along the neuraxis leads to segmentation of the neuroepithelium into progenitor domains for distinct neuron types in the developing spinal cord.
On the basis of several studies it is now believed that this model applies to macroglial cell specification. Studies carried out by Hochstim and colleagues have demonstrated that three distinct populations of astrocytes arise from the p1, p2 and p3 domains; these subtypes of astrocytes can be identified on the basis of their expression of different transcription factors and cell surface markers. The three populations of astrocyte subtypes which have been identified are 1) dorsally located VA1 astrocytes, derived from p1 domain, express PAX6 and reelin 2) ventrally located VA3 astrocytes, derived from p3, express NKX6.1 and SLIT1 and 3) and intermediate white-matter located VA2 astrocyte, derived from the p2 domain, which express PAX6, NKX6.1, reelin and SLIT1. After astrocyte specification has occurred in the developing CNS, it is believed that astrocyte precursors migrate to their final positions within the nervous system before the process of terminal differentiation occurs. In medical science, the neuronal network was considered the only important function of astrocytes, they were looked upon as gap fillers.
More the function of astrocytes has been reconsidered, they are now thought to play a number of active roles in the brain, including the secretion or absorption of neural transmitters and maintenance of the blood–brain barrier. Following on this idea the concept of a tripartite synapse has been proposed, referring to the tight relationship occurring at synapses among a presynaptic element, a postsynaptic element and a glial element. Structural: They are involved in the physical structuring of the brain. Astrocytes get their name because they are "star-shaped", they are the most abundant glial cells in the brain that are associated with neuronal synapses. They regulate the transmission of electrical impulses within the brain. Glycogen fuel reserve buffer: Astrocytes contain glycogen and are capable of gluconeogenesis; the astrocytes next to neurons in hippocampus store and release glucose. Thus, astrocytes can fuel neurons with glucose during periods of high rate of glucose consumption and glucose shortage.
A recent research on rats suggests there may be a connection between this activity and physical exercise. Metabolic support: They provide neurons with nutrients such as lactate. Glucose sensing: associated w
Spinal cord
The spinal cord is a long, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord; the brain and spinal cord together make up the central nervous system. In humans, the spinal cord begins at the occipital bone where it passes through the foramen magnum, meets and enters the spinal canal at the beginning of the cervical vertebrae; the spinal cord extends down to between the second lumbar vertebrae where it ends. The enclosing bony vertebral column protects the shorter spinal cord, it is around 45 cm in men and around 43 cm long in women. The spinal cord has a varying width, ranging from 13 mm thick in the cervical and lumbar regions to 6.4 mm thick in the thoracic area. The spinal cord functions in the transmission of nerve signals from the motor cortex to the body, from the afferent fibers of the sensory neurons to the sensory cortex, it is a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.
It is the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking; the spinal cord is the main pathway for information connecting the brain and peripheral nervous system. Much shorter than its protecting spinal column, the human spinal cord originates in the brainstem, passes through the foramen magnum, continues through to the conus medullaris near the second lumbar vertebra before terminating in a fibrous extension known as the filum terminale, it is about 45 cm long in men and around 43 cm in women, ovoid-shaped, is enlarged in the cervical and lumbar regions. The cervical enlargement, stretching from the C5 to T1 vertebrae, is where sensory input comes from and motor output goes to the arms and trunk; the lumbar enlargement, located between L1 and S3, handles sensory input and motor output coming from and going to the legs. The spinal cord is continuous with the caudal portion of the medulla, running from the base of the skull to the body of the first lumbar vertebra.
It does not run the full length of the vertebral column in adults. It is made of 31 segments from which branch one pair of sensory nerve roots and one pair of motor nerve roots; the nerve roots merge into bilaterally symmetrical pairs of spinal nerves. The peripheral nervous system is made up of these spinal roots and ganglia; the dorsal roots are afferent fascicles, receiving sensory information from the skin and visceral organs to be relayed to the brain. The roots terminate in dorsal root ganglia, which are composed of the cell bodies of the corresponding neurons. Ventral roots consist of efferent fibers that arise from motor neurons whose cell bodies are found in the ventral gray horns of the spinal cord; the spinal cord are protected by three layers of tissue or membranes called meninges, that surround the canal. The dura mater is the outermost layer, it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space; the epidural space is filled with adipose tissue, it contains a network of blood vessels.
The arachnoid mater, the middle protective layer, is named for its spiderweb-like appearance. The space between the arachnoid and the underlying pia mater is called the subarachnoid space; the subarachnoid space contains cerebrospinal fluid, which can be sampled with a lumbar puncture, or "spinal tap" procedure. The delicate pia mater, the innermost protective layer, is associated with the surface of the spinal cord; the cord is stabilized within the dura mater by the connecting denticulate ligaments, which extend from the enveloping pia mater laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral vertebra. In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing sensory and motor axons. Internal to this peripheral region is the grey matter, which contains the nerve cell bodies arranged in the three grey columns that give the region its butterfly-shape; this central region surrounds the central canal, an extension of the fourth ventricle and contains cerebrospinal fluid.
The spinal cord is elliptical in cross section, being compressed dorsolaterally. Two prominent grooves, or sulci, run along its length; the posterior median sulcus is the groove in the dorsal side, the anterior median fissure is the groove in the ventral side. The human spinal cord is divided into segments. Six to eight motor nerve rootlets branch out of right and left ventro lateral sulci in a orderly manner. Nerve rootlets combine to form nerve roots. Sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots; the ventral and dorsal roots combine to form one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside the intervertebral foramen; these rootlets form the demarcation between the peripheral nervous systems. The grey column, in the center of the cord, is shaped like a butterfly and consists of cell bodies of interneurons, motor neurons, neuroglia cells and unmyelinated axons; the anterior and posterior grey column present as projections of the grey matter and are known as the horns of the spinal cord.
Together, the gr
