Somatosensory system
The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and pathways that responds to changes at the surface or inside the body; the axons of sensory neurons connect with, or respond to, various receptor cells. These sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional name to the responding sensory neuron, such as a thermoreceptor which carries information about temperature changes. Other types include mechanoreceptors and nociceptors which send signals along a sensory nerve to the spinal cord where they may be processed by other sensory neurons and relayed to the brain for further processing. Sensory receptors are found all over the body including the skin, epithelial tissues, muscles and joints, internal organs, the cardiovascular system. Somatic senses are sometimes referred to as somesthetic senses, with the understanding that somesthesis includes the sense of touch and haptic perception.
The mapping of the body surfaces in the brain is called somatotopy. In the cortex, it is referred to as the cortical homunculus; this brain-surface map is not immutable, however. Dramatic shifts can occur in response to injury; the four mechanoreceptors in the skin each respond to different stimuli for long periods. Merkel cell nerve endings are found in hair follicles. Due to having a small receptive field, they are used in areas like fingertips the most. Tactile corpuscles react to moderate light touch, they are located in the dermal papillae. They respond unlike Merkel nerve endings, they are responsible for the ability to feel gentle stimuli. Lamellar corpuscles distinguish rough and soft substances, they react in quick action potentials to vibrations around 250 Hz. They have large receptor fields. Pacinian reacts only to sudden stimuli so pressures like clothes that are always compressing their shape are ignored. Bulbous corpuscles react and respond to sustained skin stretch, they are responsible for the feeling of object slippage and play a major role in the kinesthetic sense and control of finger position and movement.
Merkel and bulbous cells - slow-response - are myelinated. All of these receptors are activated upon pressures that squish their shape causing an action potential. All afferent touch/vibration info ascends the spinal cord via the posterior column-medial lemniscus pathway via gracilis or cuneatus. Cuneatus sends signals to the cochlear nucleus indirectly via spinal grey matter, this info is used in determining if a perceived sound is just villi noise/irritation. All fibers cross in the medulla; the postcentral gyrus includes the primary somatosensory cortex collectively referred to as S1. BA3 receives the densest projections from the thalamus. BA3a is involved with the sense of relative position of neighboring body parts and amount of effort being used during movement. BA3b is responsible for distributing somato info, it projects texture info to BA1 and shape + size info to BA2. Region S2 divides into parietal ventral area. Area S2 is involved with specific touch perception and is thus integrally linked with the amygdala and hippocampus to encode and reinforce memories.
Parietal ventral area is the somatosensory relay to the premotor cortex and somatosensory memory hub, BA5. BA5 is association area. BA1 processes texture info. Area S2 processes light touch, visceral sensation, tactile attention. S1 processes the remaining info. BA7 integrates visual and proprioceptive info to locate objects in space; the insular cortex plays a role in the sense of bodily-ownership, bodily self-awareness, perception. Insula plays a role in conveying info about sensual touch, temperature and local oxygen status. Insula is a connected relay and thus is involved in numerous functions; the somatosensory system is spread through all major parts of the vertebrate body. It consists both of sensory receptors and afferent neurons in the periphery, to deeper neurons within the central nervous system. A somatosensory pathway will have three long neurons: primary and tertiary; the first neuron always has its cell body in the dorsal root ganglion of the spinal nerve. The second neuron has its cell body either in the brainstem.
This neuron's ascending axons will cross to the opposite side either in the spinal cord or in the brainstem. In the case of touch and certain types of pain, the third neuron has its cell body in the VPN of the thalamus and ends in the postcentral gyrus of the parietal lobe. Photoreceptors, similar to those found in the retina of the eye, detect damaging ultraviolet radiation (
Proprioception
Proprioception, is the sense of self-movement and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by mechanically-sensitive proprioceptor neurons distributed throughout an animal's body. Most vertebrates possess three basic types of proprioceptors: muscle spindles, which are embedded in skeletal muscle fibers, Golgi tendon organs, which lie at the interface of muscles and tendons, joint receptors, which are low-threshold mechanoreceptors embedded in joint capsules. Many invertebrates, such as insects possess three basic proprioceptor types with analogous functional properties: chordotonal neurons, campaniform sensilla, hair plates; the central nervous system integrates information from proprioception and other sensory systems, such as vision and the vestibular system, to create an overall representation of body position and acceleration. The sense of proprioception is ubiquitous across mobile animals, is essential for the motor coordination of the body.
More proprioception has been described in flowering land plants. Proprioception is from Latin proprius, meaning "one's own", "individual", capio, capere, to take or grasp, thus to grasp one's own position in space, including the position of the limbs in relation to each other and the body as a whole. The word kinesthesia or kinæsthesia refers to movement sense, but has been used inconsistently to refer either to proprioception alone or to the brain's integration of proprioceptive and vestibular inputs. Kinesthesia is a modern medical term composed of elements from Greek; the position-movement sensation was described in 1557 by Julius Caesar Scaliger as a "sense of locomotion". Much in 1826, Charles Bell expounded the idea of a "muscle sense", credited as one of the first descriptions of physiologic feedback mechanisms. Bell's idea was that commands are carried from the brain to the muscles, that reports on the muscle's condition would be sent in the reverse direction. In 1847 the London neurologist Robert Todd highlighted important differences in the anterolateral and posterior columns of the spinal cord, suggested that the latter were involved in the coordination of movement and balance.
At around the same time, Moritz Heinrich Romberg, a Berlin neurologist, was describing unsteadiness made worse by eye closure or darkness, now known as the eponymous Romberg's sign, once synonymous with tabes dorsalis, that became recognised as common to all proprioceptive disorders of the legs. In 1880, Henry Charlton Bastian suggested "kinaesthesia" instead of "muscle sense" on the basis that some of the afferent information comes from other structures, including tendons and skin. In 1889, Alfred Goldscheider suggested a classification of kinaesthesia into three types: muscle and articular sensitivity. In 1906, Charles Scott Sherrington published a landmark work that introduced the terms "proprioception", "interoception", "exteroception"; the "exteroceptors" are the organs that provide information originating outside the body, such as the eyes, ears and skin. The interoceptors provide information about the internal organs, the "proprioceptors" provide information about movement derived from muscular and articular sources.
Using Sherrington's system and anatomists search for specialised nerve endings that transmit mechanical data on joint capsule and muscle tension, which play a large role in proprioception. Primary endings of muscle spindles "respond to the size of a muscle length change and its speed" and "contribute both to the sense of limb position and movement". Secondary endings of muscle spindles detect changes in muscle length, thus supply information regarding only the sense of position. Muscle spindles are stretch receptors, it has been accepted that cutaneous receptors contribute directly to proprioception by providing "accurate perceptual information about joint position and movement", this knowledge is combined with information from the muscle spindles. A major component of proprioception is joint position sense, determined by measuring the accuracy of joint–angle replication. Clinical aspects of joint position sense are measured in joint position matching tests that measure a subject's ability to detect an externally imposed passive movement, or the ability to reposition a joint to a predetermined position.
These involve an individual's ability to perceive the position of a joint without the aid of vision. It is assumed that the ability of one of these aspects will be related to another; this suggests that while these components may well be related in a cognitive manner, they may in fact be physiologically separate. More recent work into the mechanism of ankle sprains suggests that the role of reflexes may be more limited due to their long latencies, as ankle sprain events occur in 100 ms or less. In accordance, a model has been proposed to include a'feedforward' component of proprioception, whereby the subject will have central information about the body's position before attaining it. Kinesthesia is a key component in muscle memory and hand-eye coordination, training can improve this sense; the ability to swing a golf club or to catch a ball requires a finely tuned sense of the position o
Inferior colliculus
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The inferior colliculus has three subdivisions: the central nucleus, a dorsal cortex by which it is surrounded, an external cortex, located laterally, its bimodal neurons are implicated in auditory-somatosensory interaction, receiving projections from somatosensory nuclei. This multisensory integration may underlie a filtering of self-effected sounds from vocalization, chewing, or respiration activities; the inferior colliculi together with the superior colliculi form the eminences of the corpora quadrigemina, part of the tectal region of the midbrain. The inferior colliculus lies caudal to its counterpart – the superior colliculus – above the trochlear nerve, at the base of the projection of the medial geniculate nucleus and the lateral geniculate nucleus; the inferior colliculi of the midbrain are located just below the visual processing centers known as the superior colliculi.
The inferior colliculus is the first place where vertically orienting data from the fusiform cells in the dorsal cochlear nucleus can synapse with horizontally orienting data. Sound location data thus becomes integrated by the inferior colliculus. IC left sides of the midbrain, it is divided into the Central Nucleus of IC, dorsal cortex and lateral cortex. The input connections to the inferior colliculus are composed of many brainstem nuclei. All nuclei except the contralateral ventral nucleus of the lateral lemniscus send projections to the central nucleus bilaterally, it has been shown that great majority of auditory fibers ascending in the lateral lemniscus terminate in the CNIC. In addition, the IC receives inputs from the auditory cortex, the medial division of the medial geniculate body, the posterior limitans, suprapeduncular nucleus and subparafascicular intralaminar nuclei of the thalamus, the substantia nigra pars compacta lateralis, the dorsolateral periaqueductal gray, the nucleus of the brachium of the inferior colliculus and deep layers of the superior colliculus.
The inferior brachium carries auditory afferent fibers from the inferior colliculus of the mesencephalon to the medial geniculate nucleus. The inferior colliculus receives input from both the ipsilateral and contralateral cochlear nucleus and the corresponding ears. There is some lateralization, the dorsal projections only project to the contralateral inferior colliculus; this inferior colliculus contralateral to the ear it is receiving the most information from projects to its ipsilateral medial geniculate nucleus. The medial geniculate body is the output connection from inferior colliculus and the last subcortical way station; the MGB is composed of ventral and medial divisions, which are similar in humans and other mammals. The ventral division receives auditory signals from the central nucleus of the IC; the majority of the ascending fibers from the lateral lemniscus project to IC, which means major ascending auditory pathways converge here. IC switchboard as well, it is involved in the integration and routing of multi-modal sensory perception the startle response and vestibulo-ocular reflex.
It is responsive to specific amplitude modulation frequencies and this might be responsible for detection of pitch. In addition, spatial localization by binaural hearing is a related function of IC as well; the inferior colliculus has a high metabolism in the brain. The Conrad Simon Memorial Research Initiative measured the blood flow of the IC and put a number at 1.80 cc/g/min in the cat brain. For reference, the runner up in the included measurements was the somatosensory cortex at 1.53. This indicates that the inferior colliculus is metabolically more active than many other parts of the brain; the hippocampus considered to use up a disproportionate amount of energy, was not measured or compared. Skottun et al. measured the interaural time difference sensitivity of single neurons in the inferior colliculus, used these to predict behavioural performance. The predicted just noticeable difference was comparable to that achieved by humans in behavioral tests; this suggested that by the level of the inferior colliculus, integration of information over multiple neurons is unnecessary.
Axiomatically determined functional models of spectro-temporal receptive fields in inferior colliculus have been determined by Lindeberg and Friberg in terms of derivatives of Gaussian functions over the log-spectral domain and either Gaussian kernels over time in the case of non-causal time or first-order integrators coupled in cascade in the case of time-causal operations, optionally in combination with local glissando transformations to account for variations in frequencies over time. The shapes of the receptive field functions in these models can be determined by necessity from structural properties of the environment combined with requirements about the internal structure of the auditory system to enable theoretically well-founded processing of sound signals at different temporal and log-spectral scales. Thereby, the receptive fields in inferior colliculus can be seen as well adapted to handling natural sound transformations. Auditory system List of regions in the human brain Stained brain slice images which include the "inferior colliculus" at the BrainMaps project N
Lateral lemniscus
The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and the contralateral inferior colliculus of the midbrain. Three distinct inhibitory, cellular groups are located interspersed within these fibers, are thus named the nuclei of the lateral lemniscus; the brainstem nuclei include: the superior olive the intermediate nucleus of the lateral lemniscus the ventral nucleus of the lateral lemniscus the dorsal nucleus of the lateral lemniscus Fibers leaving these brainstem nuclei ascending to the inferior colliculus rejoin the lateral lemniscus. In that sense, this is not a'lemniscus' in the true sense of the word, as there is third order information coming out of some of these brainstem nuclei; the lateral lemniscus is located where the cochlear nuclei and the pontine reticular formation crossover. The PRF descends the reticulospinal tract where it innervates spinal interneurons, it is the main auditory tract in the brainstem that connects the superior olivary complex with the inferior colliculus.
The dorsal cochlear nucleus has input from the LL and output to the contralateral LL via the ipsilateral and contralateral Dorsal Acoustic Stria. There are three small nuclei on each of the lateral lemnisci: the ventral and the intermediate; the two lemnisci communicate via the commissural fibers of Probst. The function of the complex of Nuclei of the lateral lemniscus is not known, it is involved in the acoustic startle reflex. The cells of the DNLL respond best to bilateral inputs, have onset and complexity tuned sustained responses; the nucleus is GABAergic, projects bilaterally to the inferior colliculus, contralaterally to the DNLL, with different populations of cells projecting to each IC. In rat, the DNLL has a prominent columnar organization. Nearly all neurons are stained for GABA in the central part of the nucleus, the remaining GABA negative cells are interspersed with the positive, stain for glycine. Two populations of GABA+ cells are visible: larger stained cells that project to the contralateral IC, smaller, darker stained cells that project ipsilaterally.
GABAergic axon terminals form dense groups surrounded by GABA-lemniscal fibers throughout the nucleus, synapse on both somata and in the neuropil. Glycinergic axon terminals, on the other hand, are more finely localized, with the majority of recipient neurons located laterally in the nucleus. INLL has little spontaneous activity and broad tuning curves; the temporal responses are different from cells of the VNLL. This structure is hypertrophied in the rat, forming a prominent bulge on the surface of the brainstem. GAD, GABA, Glycine staining reveals several distinct regions that are not evident in standard cytoarchitectural preparations. A modest number of GABA-stained neurons are arranged in small groups in the center of the nucleus, whereas glycine-stained neurons are more common and dispersed, with regional concentrations in the dorsolateral and ventrolateral portions of the nucleus. Most GABA+ cells are gly+ as well. Sound in the contralateral ear leads to the strongest responses in the VNLL, which deals with some temporary processing.
The VNLL may be essential to the IC’s decoding of amplitude modulated sounds. VNLL cells have little spontaneous activity and moderately complex tuning curves. In rat, the VNLL is composed of the ventral and dorsal regions; the columnar region contains many glycine-positive neurons, whereas the dorsal region contains clusters of GABA+ neurons intermingled with gly+ cells, with some cells containing both. The table below shows that each of the nuclei have a complicated arrangement of ipsilateral and contralateral afferent inputs and outputs
Periosteum
The periosteum is a membrane that covers the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones; the periosteum consists of dense irregular connective tissue. It is divided into an outer "fibrous layer" and inner "cambium layer"; the fibrous layer contains fibroblasts, while the cambium layer contains progenitor cells that develop into osteoblasts. These osteoblasts are responsible for increasing the width of a long bone and the overall size of the other bone types. After a bone fracture the progenitor cells develop into osteoblasts and chondroblasts, which are essential to the healing process; as opposed to osseous tissue, the periosteum has nociceptive nerve endings, making it sensitive to manipulation. It provides nourishment by providing the blood supply to the body from the marrow; the periosteum is attached to the bone by strong collagenous fibers called Sharpey's fibres, which extend to the outer circumferential and interstitial lamellae.
It provides an attachment for muscles and tendons. The periosteum that covers the outer surface of the bones of the skull is known as the "pericranium", except when in reference to the layers of the scalp; the word periosteum is derived from the Greek Peri-, meaning "surrounding", -osteon, meaning "bone". The Peri refers to the fact that the Periosteum is the outermost layer of long bones, surrounding other inner layers. Periostitis Endochondral ossification Intramembranous ossification Brighton, Carl T.. "Early histologic and ultrastructural changes in microvessels of periosteal callus". Journal of Orthopaedic Trauma. 11: 244–253. PMID 9258821. Periosteum - InnerBody
Axon
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart
Human
Humans are the only extant members of the subtribe Hominina. Together with chimpanzees and orangutans, they are part of the family Hominidae. A terrestrial animal, humans are characterized by their erect bipedal locomotion. Early hominins—particularly the australopithecines, whose brains and anatomy are in many ways more similar to ancestral non-human apes—are less referred to as "human" than hominins of the genus Homo. Several of these hominins used fire, occupied much of Eurasia, gave rise to anatomically modern Homo sapiens in Africa about 315,000 years ago. Humans began to exhibit evidence of behavioral modernity around 50,000 years ago, in several waves of migration, they ventured out of Africa and populated most of the world; the spread of the large and increasing population of humans has profoundly affected much of the biosphere and millions of species worldwide. Advantages that explain this evolutionary success include a larger brain with a well-developed neocortex, prefrontal cortex and temporal lobes, which enable advanced abstract reasoning, problem solving and culture through social learning.
Humans use tools better than any other animal. Humans uniquely use such systems of symbolic communication as language and art to express themselves and exchange ideas, organize themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an wide variety of values, social norms, rituals, which together undergird human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, mythology, religion and numerous other fields of knowledge. Though most of human existence has been sustained by hunting and gathering in band societies many human societies transitioned to sedentary agriculture some 10,000 years ago, domesticating plants and animals, thus enabling the growth of civilization; these human societies subsequently expanded, establishing various forms of government and culture around the world, unifying people within regions to form states and empires.
The rapid advancement of scientific and medical understanding in the 19th and 20th centuries permitted the development of fuel-driven technologies and increased lifespans, causing the human population to rise exponentially. The global human population was estimated to be near 7.7 billion in 2015. In common usage, the word "human" refers to the only extant species of the genus Homo—anatomically and behaviorally modern Homo sapiens. In scientific terms, the meanings of "hominid" and "hominin" have changed during the recent decades with advances in the discovery and study of the fossil ancestors of modern humans; the clear boundary between humans and apes has blurred, resulting in now acknowledging the hominids as encompassing multiple species, Homo and close relatives since the split from chimpanzees as the only hominins. There is a distinction between anatomically modern humans and Archaic Homo sapiens, the earliest fossil members of the species; the English adjective human is a Middle English loanword from Old French humain from Latin hūmānus, the adjective form of homō "man."
The word's use as a noun dates to the 16th century. The native English term man can refer to the species as well as to human males, or individuals of either sex; the species binomial "Homo sapiens" was coined by Carl Linnaeus in his 18th-century work Systema Naturae. The generic name "Homo" is a learned 18th-century derivation from Latin homō "man," "earthly being"; the species-name "sapiens" means "wise" or "sapient". Note that the Latin word homo refers to humans of either gender, that "sapiens" is the singular form; the genus Homo evolved and diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids branch of the primates. Modern humans, defined as the species Homo sapiens or to the single extant subspecies Homo sapiens sapiens, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000–60,000 years ago, Australia around 40,000 years ago, the Americas around 15,000 years ago, remote islands such as Hawaii, Easter Island and New Zealand between the years 300 and 1280.
The closest living relatives of humans are gorillas. With the sequencing of the human and chimpanzee genomes, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%. By using the technique called a molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated; the gibbons and orangutans were the first groups to split from the line leading to the h
