Microglia are a type of neuroglia located throughout the brain and spinal cord. Microglia account for 10–15% of all cells found within the brain; as the resident macrophage cells, they act as the first and main form of active immune defense in the central nervous system. Microglia are distributed in large non-overlapping regions throughout the CNS. Microglia are key cells in overall brain maintenance—they are scavenging the CNS for plaques, damaged or unnecessary neurons and synapses, infectious agents. Since these processes must be efficient to prevent fatal damage, microglia are sensitive to small pathological changes in the CNS; this sensitivity is achieved in part by the presence of unique potassium channels that respond to small changes in extracellular potassium. The brain and spinal cord, which make up the CNS, are not accessed directly by pathogenic factors in the body's circulation due to a series of endothelial cells known as the blood–brain barrier, or BBB; the BBB prevents most infections from reaching the vulnerable nervous tissue.
In the case where infectious agents are directly introduced to the brain or cross the blood–brain barrier, microglial cells must react to decrease inflammation and destroy the infectious agents before they damage the sensitive neural tissue. Due to the lack of antibodies from the rest of the body, microglia must be able to recognize foreign bodies, swallow them, act as antigen-presenting cells activating T-cells. Microglial cells are plastic, undergo a variety of structural changes based on location and system needs; this level of plasticity is required to fulfill the vast variety of functions that microglia perform. The ability to transform distinguishes microglia from macrophages, which must be replaced on a regular basis, provides them the ability to defend the CNS on short notice without causing immunological disturbance. Microglia adopt a specific form, or phenotype, in response to the local conditions and chemical signals they have detected; the microglial sensome is a new biological concept that appears to be playing a large role in neurodevelopment and neurodegeneration.
The sensome refers to the unique grouping of protein transcripts used for sensing ligands and microbes. In other words, the sensome represents the genes required for the proteins used to sense molecules within the body; the sensome can be analyzed with a variety of methods including qPCR, RNA-seq, microarray analysis, direct RNA sequencing. Genes included in the sensome code for receptors and transmembrane proteins on the plasma membrane that are more expressed in microglia compared to neurons, it does not include secreted proteins or transmembrane proteins specific to membrane bound organelles, such as the nucleus and endoplasmic reticulum. The plurality of identified sensome genes code for pattern recognition receptors, there are a large variety of included genes. Microglial share a similar sensome to other macrophages, however they contain 22 unique genes, 16 of which are used for interaction with endogenous ligands; these differences create a unique microglial biomarker that includes over 40 genes including P2ry12 and HEXB.
DAP12 appears to play an important role in sensome protein interaction, acting as a signalling adaptor and a regulatory protein. The regulation of genes within the sensome must be able to change in order to respond to potential harm. Microglia can take on the role of neurotoxicity in order to face these dangers. For these reasons, it is suspected. Sensome genes that are upregulated with aging are involved in sensing infectious microbial ligands while those that are downregulated are involved in sensing endogenous ligands; this analysis suggests a glial-specific regulation favoring neuroprotection in natural neurodegeneration. This is in contrast to the shift towards neurotoxicity seen in neurodegenerative diseases; the sensome can play a role in neurodevelopment. Early-life brain infection results in microglia that are hypersensitive to immune stimuli; when exposed to infection, there is an upregulation of sensome genes involved in neuroinflammation and a downregulation of genes that are involved with neuroplasticity.
The sensome’s ability to alter neurodevelopment may however be able to combat disease. The deletion of CX3CL1, a expressed sensome gene, in rodent models of Rett syndrome resulted in improved health and longer lifespan; the downregulation of Cx3cr1 in humans without Rett syndrome is associated with symptoms similar to schizophrenia. This suggests that the sensome not only plays a role in various developmental disorders, but requires tight regulation in order to maintain a disease-free state; this form of microglial cell is found at specific locations throughout the entire brain and spinal cord in the absence of foreign material or dying cells. This "resting" form of microglia is composed of a small cellular body. Unlike the amoeboid forms of microglia, the cell body of the ramified form remains in place while its branches are moving and surveying the surrounding area; the branches are sensitive to small changes in physiological condition and require specific culture conditions to observe in vitro.
Unlike activated or ameboid microglia, ramified microglia do not phagocytose cells and secrete fewer immunomolecules. Microglia in this state are able to search for and identify immune threats while maintaining homeostasis in the CNS. Although this is considered the res
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane, controlling the flow of ions across secretory and epithelial cells, regulating cell volume. Ion channels are present in the membranes of all excitable cells. Ion channels are one of the two classes of ionophoric proteins, the other being ion transporters; the study of ion channels involves biophysics, electrophysiology, pharmacology, while using techniques including voltage clamp, patch clamp, immunohistochemistry, X-ray crystallography, RT-PCR. Their classification as molecules is referred to as channelomics. There are two distinctive features of ion channels that differentiate them from other types of ion transporter proteins: The rate of ion transport through the channel is high. Ions pass through channels down their electrochemical gradient, a function of ion concentration and membrane potential, "downhill", without the input of metabolic energy.
Ion channels are located within the membrane of all excitable cells, of many intracellular organelles. They are described as narrow, water-filled tunnels that allow only ions of a certain size and/or charge to pass through; this characteristic is called selective permeability. The archetypal channel pore is just one or two atoms wide at its narrowest point and is selective for specific species of ion, such as sodium or potassium. However, some channels may be permeable to the passage of more than one type of ion sharing a common charge: positive or negative. Ions move through the segments of the channel pore in single file nearly as as the ions move through free solution. In many ion channels, passage through the pore is governed by a "gate", which may be opened or closed in response to chemical or electrical signals, temperature, or mechanical force. Ion channels are integral membrane proteins formed as assemblies of several individual proteins; such "multi-subunit" assemblies involve a circular arrangement of identical or homologous proteins packed around a water-filled pore through the plane of the membrane or lipid bilayer.
For most voltage-gated ion channels, the pore-forming subunit are called the α subunit, while the auxiliary subunits are denoted β, γ, so on. Because channels underlie the nerve impulse and because "transmitter-activated" channels mediate conduction across the synapses, channels are prominent components of the nervous system. Indeed, numerous toxins that organisms have evolved for shutting down the nervous systems of predators and prey work by modulating ion channel conductance and/or kinetics. In addition, ion channels are key components in a wide variety of biological processes that involve rapid changes in cells, such as cardiac and smooth muscle contraction, epithelial transport of nutrients and ions, T-cell activation and pancreatic beta-cell insulin release. In the search for new drugs, ion channels are a frequent target. There are over 300 types of ion channels just in the cells of the inner ear. Ion channels may be classified by the nature of their gating, the species of ions passing through those gates, the number of gates and localization of proteins.
Further heterogeneity of ion channels arises when channels with different constitutive subunits give rise to a specific kind of current. Absence or mutation of one or more of the contributing types of channel subunits can result in loss of function and underlie neurologic diseases. Ion channels may be classified by i.e. what opens and closes the channels. For example, voltage-gated ion channels open or close depending on the voltage gradient across the plasma membrane, while ligand-gated ion channels open or close depending on binding of ligands to the channel. Voltage-gated ion channels close in response to membrane potential. Voltage-gated sodium channels: This family contains at least 9 members and is responsible for action potential creation and propagation; the pore-forming α subunits are large and consist of four homologous repeat domains each comprising six transmembrane segments for a total of 24 transmembrane segments. The members of this family coassemble with auxiliary β subunits, each spanning the membrane once.
Both α and β subunits are extensively glycosylated. Voltage-gated calcium channels: This family contains 10 members, though these members are known to coassemble with α2δ, β, γ subunits; these channels play an important role in both linking muscle excitation with contraction as well as neuronal excitation with transmitter release. The α subunits have an overall structural resemblance to those of the sodium channels and are large. Cation channels of sperm: This small family of channels referred to as Catsper channels, is related to the two-pore channels and distantly related to TRP channels. Voltage-gated potassium channels: This family contains 40 members, which are further divided into 12 subfamilies; these channels are known for their role in repolarizing the cell membrane following action potentials. The α subunits have six transmembrane segments, homologous to a single domain of the sodium channels. Correspondingly, they assemble; some transient receptor potential channels: This group of channels referred to as
Spindle neurons called von Economo neurons, are a specific class of neurons that are characterized by a large spindle-shaped soma tapering into a single apical axon in one direction, with only a single dendrite facing opposite. Other neurons tend to have many dendrites, the polar-shaped morphology of spindle neurons is unique. Spindle neurons are found in two restricted regions in the brains of hominids: the anterior cingulate cortex and the fronto-insular cortex, but they have been discovered in the dorsolateral prefrontal cortex of humans. Spindle cells are found in the brains of the humpback whales, fin whales, killer whales, sperm whales, bottlenose dolphin, Risso's dolphin, beluga whales and Asian elephants, to a lesser extent in macaque monkeys and raccoons; the appearance of spindle neurons in distantly related clades suggests that they represent convergent evolution an adaptation to larger brains. Austrian psychiatrist and neurologist Constantin von Economo discovered spindle neurons and described them in 1929, why they are sometimes called von Economo neurons.
Spindle neurons are large cells that may allow rapid communication across the large brains of great apes and cetaceans. Although rare in comparison to other neurons, spindle neurons are abundant, comparatively large, in humans; the discovery of spindle neurons in diverse whale species has led to the suggestion that they are "a possible obligatory neuronal adaptation in large brains, permitting fast information processing and transfer along specific projections and that evolved in relation to emerging social behaviors."p. 254 The apparent presence of these specialized neurons only in intelligent mammals may be an example of convergent evolution. Their restriction among the primates to great apes leads to the hypothesis that they developed no earlier than 15–20 million years ago, prior to the divergence of orangutans from the African great apes. Primitive forms of spindle neurons have been discovered in macaque monkey brains and raccoons. In 1999, neuroscientist Prof. John Allman and colleagues at the California Institute of Technology first published a report on spindle neurons found in the anterior cingulate cortex of hominids, but not in any other species.
Neuronal volumes of ACC spindle neurons were larger in humans and bonobos than the spindle neurons of the common chimpanzee and orangutan. Allman and his colleagues have delved beyond the level of brain infrastructure to investigate how spindle neurons function at the superstructural level, focusing on their role as «air traffic controllers' for emotions... at the heart of the human social emotion circuitry, including a moral sense». Allman's team proposes that spindle neurons help channel neural signals from deep within the cortex to distant parts of the brain. Allman's team found signals from the ACC are received in Brodmann's area 10, in the frontal polar cortex, where regulation of cognitive dissonance is thought to occur. According to Allman, this neural relay appears to convey motivation to act, concerns the recognition of error. Self-control – and avoidance of error – is thus facilitated by the executive gatekeeping function of the ACC, as it regulates the interference patterns of neural signals between these two brain regions.
In humans, intense emotion activates the anterior cingulate cortex, as it relays neural signals transmitted from the amygdala to the frontal cortex by functioning as a sort of lens to focus the complex texture of neural signal interference patterns. The ACC is active during demanding tasks requiring judgment and discrimination, when errors are detected by an individual. During difficult tasks, or when experiencing intense love, anger, or lust, activation of the ACC increases. In brain imaging studies, the ACC has been found to be active when mothers hear infants cry, underscoring its role in affording a heightened degree of social sensitivity; the ACC is a ancient cortical region, is involved with many autonomic functions, including motor and digestive functions, while playing a role in the regulation of blood pressure and heart rate. Significant olfactory and gustatory capabilities of the ACC and fronto-insular cortex appear to have been usurped, during recent evolution, to serve enhanced roles related to higher cognition – ranging from planning and self-awareness to role playing and deception.
The diminished olfactory function of humans, compared to other primates, may be related to the fact that spindle cells located at crucial neural network hubs have only two dendrites rather than many, resulting in reduced neurological integration. At a Society for Neuroscience meeting in 2003, Allman reported on spindle cells his team found in another brain region, the fronto-insular cortex, a region which appears to have undergone significant evolutionary adaptations in mankind – as as 100,000 years ago; this fronto-insular cortex is connected to the insula, a region, the size of a thumb in each hemisphere of the human brain. The insula and fronto-insular cortex are part of the insular cortex, wherein the elaborate circuitry associated with spatial awareness are found, where self-awareness and the complexities of emotion are thought to be generated and experienced. Moreover, this region of the right hemisphere is crucial to navigation and perception of three-dimensional rotatio
The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. In contrast to commissural fibers, association fibers connect regions within the same hemisphere of the brain, projection fibers connect each region to other parts of the brain or to the spinal cord; the commissural fibers make up tracts that include the corpus callosum, the anterior commissure, the posterior commissure. The corpus callosum is the largest commissural tract in the human brain, it consists of about 200-300 million axons. The corpus callosum is essential to the communication between the two hemispheres. A recent study of individuals with agenesis of the corpus callosum suggests that the corpus callosum plays a vital role in problem solving strategies, verbal processing speed, executive performance; the absence of a developed corpus callosum is shown to have a significant relationship with impaired verbal processing speed and problem solving. Another study of individuals with multiple sclerosis provides evidence that structural and microstructural abnormalities of the corpus callosum are related to cognitive dysfunction.
Verbal and visual memory, information processing speed, executive tasks were shown to be impaired when compared to healthy individuals. Physical disabilities in multiple sclerosis patients seem to be related to abnormalities of the corpus callosum, but not to the same extent of other cognitive functions. Using diffusion tensor imaging, researchers have been able to produce a visualization of this network of fibers, which shows the corpus callosum has an anteroposterior topographical organization, uniform with the cerebral cortex; the anterior commissure is a tract that connects the two temporal lobes of the cerebral hemispheres across the midline, placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, over 10 times larger than the anterior commissure, other routes of communication pass through the hippocampal commissure or, via subcortical connections; the anterior commissure is a significant pathway that can be distinguished in the brains of all mammals.
Using diffusion tensor imaging, researchers were able to approximate the location of the anterior commissure where it crosses the midline of the brain. This tract can be observed to be in the shape of a bicycle as it branches through various areas of the brain. Through diffusion tensor imaging results, the anterior commissure was categorized into two fiber systems: 1) the olfactory fibers and 2) the non-olfactory fibers; the posterior commissure is a rounded nerve tract crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in the bilateral pupillary light reflex. Evidence suggests the posterior commissure is a tract that plays a role in language processing between the right and left hemispheres of the brain, it connects the pretectal nuclei. A case study described in The Irish Medical Journal discussed the role the posterior commissure plays in the connection between the right occipital cortex and the language centers in the left hemisphere; this study explains how visual information from the left side of the visual field is received by the right visual cortex and transferred to the word form system in the left hemisphere though the posterior commissure and the splenium.
Disruption of the posterior commissure can cause alexia without agraphia. It is evident from this case study of alexia without agraphia that the posterior commissure plays a vital role in transferring information from the right occipital cortex to the language centers of the left hemisphere; the lyra or hippocampal commissure. Aging Age-related decline in the commissural fiber tracts that make up the corpus callosum indicate the corpus callosum is involved in memory and executive function; the posterior fibers of the corpus callosum are associated with episodic memory. Perceptual processing decline is related to diminished integrity of occipital fibers of the corpus callosum. Evidence suggests that the genu of the corpus callosum does not contribute to any one cognitive domain in the elderly; as fiber tract connectivity in the corpus callosum declines due to aging, compensatory mechanisms are found in other areas of the corpus callosum and frontal lobe. These compensatory mechanisms, increasing connectivity in other parts of the brain, may explain why elderly individuals still display executive function as a decline of connectivity is seen in regions of the corpus callosum.
Older adults compared to younger adults show poorer performance in balance tests. A decline in white matter integrity of the corpus callosum in older individuals may explain declines in the ability to balance. Changes in the white matter integrity of the corpus callosum may be related to cognitive and motor function decline as well. Decreased white matter integrity effects proper transmission and processing of sensorimotor information. White matter degeneration of the genu of the corpus callosum is associated with gait, balance impairment, the quality of postural control; the corpus callosum allows for communication between the two hemispheres and is found only in placental mammals, while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles and fish. The anterior commissure serves as the primary mode of interhemispheric communication in marsupials, which carries all the commissural fibers arising from the neocortex, whereas in pl
Nervous tissue called neural tissue or nerve tissue, is the main tissue component of the nervous system. The nervous system regulates and controls bodily functions and activity and consists of two parts: the central nervous system comprising the brain and spinal cord, the peripheral nervous system comprising the branching peripheral nerves, it is composed of neurons, or nerve cells, which receive and transmit impulses, neuroglia known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons. Nervous tissue is made up of different types of nerve cells. An axon is the long stem-like part of the cell. Bundles of axons make up the nerves in the PNS and tracts in the CNS. Functions of the nervous system are sensory input, control of muscles and glands and mental activity. Nervous tissue is composed of neurons called nerve cells, neuroglial cells. Four types of neuroglia found in the CNS are astrocytes, microglial cells, ependymal cells and oligodendrocytes.
Two types of neuroglia found in the PNS are Schwann cells. In the central nervous system, the tissue types found are white matter; the tissue is categorized by its neuroglial components. Neurons are cells with specialized features that allow them to receive and facilitate nerve impulses, or action potentials, across their membrane to the next neuron, they possess a large cell body, with cell projections called an axon. Dendrites are thin, branching projections that receive electrochemical signaling to create a change in voltage in the cell. Axons are long projections that carry the action potential away from the cell body toward the next neuron; the bulb-like end of the axon, called the axon terminal, is separated from the dendrite of the following neuron by a small gap called a synaptic cleft. When the action potential travels to the axon terminal, neurotransmitters are released across the synapse and bind to the post-synaptic receptors, continuing the nerve impulse. Neurons are classified both structurally.
Functional classification: Sensory neurons: Relay sensory information in the form of an action potential from the PNS to the CNS Motor neurons: Relay an action potential out of the CNS to the proper effector Interneurons: Cells that form connections between neurons and whose processes are limited to a single local area in the brain or spinal cordStructural classification: Multipolar neurons: Have 3 or more processes coming off the soma. They include interneurons and motor neurons. Bipolar neurons: Sensory neurons that have two processes coming off the soma, one dendrite and one axon Pseudounipolar neurons: Sensory neurons that have one process that splits into two branches, forming the axon and dendrite Unipolar brush cells: Are excitatory glutamatergic interneurons that have a single short dendrite terminating in a brush-like tuft of dendrioles; these are found in the granular layer of the cerebellum. Neuroglia encompasses the non-neural cells in nervous tissue that provide various crucial supportive functions for neurons.
They are smaller than neurons, vary in structure according to their function. Neuroglial cells are classified as follows: Microglial cells: Microglia are macrophage cells that make up the primary immune system for the CNS, they are the smallest neuroglial cell. Astrocytes: Star-shaped macroglial cells with many processes found in the CNS, they are the most abundant cell type in the brain, are intrinsic to a healthy CNS. Oligodendrocytes: CNS cells with few processes, they form myelin sheaths on the axons of a neuron, which are lipid-based insulation that increases the speed at which the action potential, can travel down the axon. NG2 glia: CNS cells that are distinct from astrocytes and microglia, serve as the developmental precursors of oligodendrocytes Schwann cells: The PNS equivalent of oligodendrocytes, they help maintain axons and form myelin sheaths in the PNS. Satellite glial cell: Line the surface of neuron cell bodies in ganglia Enteric glia: Found in the enteric nervous system, within the gastrointestinal tract.
In the central nervous system: Grey matter is composed of cell bodies, unmyelinated axons, protoplasmic astrocytes, satellite oligodendrocytes and few myelinated axons. White matter is composed of myelinated axons, fibrous astrocytes, myelinating oligodendrocytes, microglia. In the Peripheral Nervous System: Ganglion tissue is composed of cell bodies and satellite glial cells. Nerves are composed of myelinated and unmyelinated axons, Schwann cells surrounded by connective tissue; the three layers of connective tissue surrounding each nerve are: Endoneurium. Each nerve axon, or fiber is surrounded by the endoneurium, called the endoneurial tube, channel or sheath; this is a thin, protective layer of connective tissue. Perineurium; each nerve fascicle containing one or more axons, is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. This plays a important role in the protection and support of the nerve fibers and serves to prevent the passage of large molecules from the epineurium into a fascicle.
Epineurium. The epineurium is the outermost layer of dense connective tissue enclosing the nerve; the function of nervous tissue is to form the communication network
A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system; each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as Schwann cells that coat the axons in myelin. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium; the axons are bundled together into groups called fascicles, each fascicle is wrapped in a layer of connective tissue called the perineurium. The entire nerve is wrapped in a layer of connective tissue called the epineurium. In the central nervous system, the analogous structures are known as tracts; each nerve is covered on the outside by a dense sheath of the epineurium. Beneath this is a layer of flat cells, the perineurium, which forms a complete sleeve around a bundle of axons.
Perineurial septae subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium; this forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, meshwork of collagen fibres. Nerves are bundled and travel along with blood vessels, since the neurons of a nerve have high energy requirements. Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid; this acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood-nerve barrier similar to the blood-brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation, the amount of endoneurial fluid may increase at the site of irritation; this increase in fluid can be visualized using magnetic resonance neurography, thus MR neurography can identify nerve irritation and/or injury.
Nerves are categorized into three groups based on the direction that signals are conducted: Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin. Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands. Mixed nerves contain both afferent and efferent axons, thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. Nerves can be categorized into two groups based on where they connect to the central nervous system: Spinal nerves innervate much of the body, connect through the vertebral column to the spinal cord and thus to the central nervous system, they are given letter-number designations according to the vertebra through which they connect to the spinal column. Cranial nerves innervate parts of the head, connect directly to the brain, they are assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included.
In addition, cranial nerves have descriptive names. Specific terms are used to describe their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to "innervate" that section of the body or organ. Other terms relate to whether the nerve affects the same side or opposite side of the body, to the part of the brain that supplies it. Nerve growth ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling". If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells; this is referred to as neuroregeneration. The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud.
When one of the growth processes finds the regeneration tube, it begins to grow towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect. A nerve conveys information in the form of electrochemical impulses carried by the individual neurons that make up the nerve; these impulses are fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, the message is converted from electrical to chemical and back to electrical. Nerves can be categorized into two groups based on function: An afferent nerve fiber conducts sensory information from a sensory neuron to the central nervous system, where the information is processed. Bundles of fibres or axons, in the peripheral nervous system are called nerves, bundles of afferent fibers are known as sensory nerves.
An efferent nerve fiber conducts signals from a motor neuron in the central nervous system to muscles. Bundles of these fibres are known as efferent nerves; the nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. In vertebrates it consists of two main par
An organ system is a group of organs that work together as a biological system to perform one or more functions. Each organ system does a particular job in the body, is made up of certain tissues; these specific systems are studied in anatomy. They are present in many types of animals. Respiratory system: the organs used for breathing, the pharynx, bronchi and diaphragm. Digestive system: digestion and processing food with salivary glands, stomach, gallbladder, intestines and anus. Cardiovascular system: and channeling blood to and from the body and lungs with heart and blood vessels. Urinary system: kidneys, ureters and urethra involved in fluid balance, electrolyte balance and excretion of urine. Integumentary system: skin, hair and nails. Skeletally system: structural support and protection with bones, cartilage and tendons. Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal gland, thyroid and adrenal glands.
Lymphatic system: the transfer of lymph between tissues and the blood stream. Includes the functions of immune responses and the development of antibodies. Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living; the job of the circulatory system is to move blood, oxygen, carbon dioxide, hormones, around the body. It consists of the heart, blood vessels and veins. Immune system: protects the organism from foreign bodies. Nervous system: collecting and processing information with brain, spinal cord, peripheral nervous system and sense organs. Sensory systems: visual system, auditory system, olfactory system, gustatory system, somatosensory system, vestibular system. Muscular system: allows for manipulation of the environment, provides locomotion, maintains posture, produces heat. Includes skeletal muscles, smooth muscles and cardiac muscle. Reproductive system: the sex organs, such as ovaries, fallopian tubes, vagina, mammary glands, testes, vas deferens, seminal vesicles and prostate