Nociception
Encyclopedia
Nociception is defined as "the neural processes of encoding and processing noxious stimuli." It is the afferent
Afferent
Afferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve fiber...

 activity produced in the peripheral and central nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 by stimuli that have the potential to damage tissue. This activity is initiated by nociceptor
Nociceptor
A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.-History:...

s, (also called pain receptors), that can detect mechanical, thermal or chemical changes above a set threshold. Once stimulated, a nociceptor transmits a signal along the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

, to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. Nociception triggers a variety of autonomic responses and may also result in the experience of pain
Pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...

 in sentient beings.

Detection of noxious stimuli

Mechanical, thermal, and chemical stimuli are detected by nerve endings called nociceptors, which are found in the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 and on internal surfaces such as the periosteum
Periosteum
Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

 or joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...

 surfaces. The concentration of nociceptors varies throughout the body, mostly found in the skin and less so in deep internal surfaces.
All nociceptors are free nerve ending
Free nerve ending
A free nerve ending is an unspecialized, afferent nerve ending, meaning it brings information from the body's periphery toward the brain. They function as cutaneous receptors and are essentially used by vertebrates to detect pain.-Structure:...

s that have their cell bodies outside the spinal column in the dorsal root ganglia
Dorsal root ganglion
In anatomy and neuroscience, a dorsal root ganglion is a nodule on a dorsal root that contains cell bodies of neurons in afferent spinal nerves.-Unique unipolar structure:...

 and are named according to their appearance at their sensory ends.

Nociceptors have a certain threshold; that is, they require a minimum level of stimuli before they trigger a signal. Once this threshold is reached a signal is passed along the axon of the nerve into the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

.

In some conditions, excitation of pain fibers becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia
Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...

.

Spinothalamic tract

Before reaching the brain, the spinothalamic tract splits into the lateral, "neospinothalamic" tract and the medial, "paleospinothalamic" tract.

Neospinothalamic tract


Fast pain travels via type Aδ fiber
A delta fiber
A delta fibers, or Aδ fibers, are a type of sensory fiber.They are associated with cold and pressure, and as nociceptors stimulation of them is interpreted as fast/first pain information....

s to terminate on the dorsal horn of the spinal cord where they synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

 with the dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

s of the neospinothalamic tract. The axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s of these neurons travel up the spine to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 and cross the midline through the anterior white commissure
Anterior white commissure
The anterior white commissure, also known as the alba anterior medullae spinalis, is a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray commissure...

, passing upwards in the contralateral anterolateral columns. These fibres terminate on the ventrobasal complex
Ventrobasal complex
-Function:The ventrobasal complex receives sensory input from the medial lemniscus , from spinothalamic tracts, and from the trigeminal nerve. It subsequently relays the inputs to the primary sensory area of the cerebral cortex. Synchronously, it distinguishes sensory inputs from deep sensory...

 of the thalamus and synapse with the dendrites of the somatosensory cortex
Postcentral gyrus
The lateral postcentral gyrus is a prominent structure in the parietal lobe of the human brain and an important landmark. It is the location of primary somatosensory cortex, the main sensory receptive area for the sense of touch...

. Fast pain is felt within a tenth of a second of application of the pain stimulus and is a sharp, acute, prickling pain felt in response to mechanical and thermal stimulation. It can be localised easily if Aδ fibres are stimulated together with tactile receptors.

Paleospinothalamic tract


Slow pain is transmitted via slower type C fiber
Group C nerve fiber
-Location:C fibers are found in the peripheral nerves of the somatic sensory system. They are afferent fibers, conveying input signals from the periphery to the central nervous system.-Structure:...

s to laminae II and III of the dorsal horns, together known as the substantia gelatinosa
Substantia gelatinosa of Rolando
The apex of the posterior horn of the gray matter of the spinal cord is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando , which contains both neuroglia cells, and small nerve cells...

. Impulses are then transmitted to nerve fibers that terminate in lamina V, also in the dorsal horn, synapsing with neurons that join fibers from the fast pathway, crossing to the opposite side via the anterior white commissure, and traveling upwards through the anterolateral pathway. These neurons terminate throughout the brain stem
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

, with one tenth of fibres stopping in the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 and the rest stopping in the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

, pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

 and periaqueductal grey of the midbrain tectum
Midbrain tectum
The tectum is a region of the brain, specifically the dorsal part of the mesencephalon . This is contrasted with the tegmentum, which refers to the region ventral to the ventricular system...

. Slow pain is stimulated by chemical stimulation, is poorly localized and is described as an aching, throbbing or burning pain.

Regulation

The body possesses an endogenous analgesia system, which can be supplemented with analgesic drugs
Analgesic
An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....

 to regulate nociception and pain. There is both an analgesia system in the central nervous system and peripheral receptors that decreases the grade in which nociception reaches the higher brain areas. The degree of pain can be modified by the periaqueductal gray
Periaqueductal gray
Periaqueductal gray is the gray matter located around the cerebral aqueduct within the tegmentum of the midbrain. It plays a role in the descending modulation of pain and in defensive behaviour...

 before it reaches the thalamus and consciousness. According to gate control theory of pain, this area can also reduce pain when non-painful stimuli are received in conjunction with nociception.

Central

The central analgesia system is mediated by 3 major components: the periaquaductal grey matter, the nucleus raphe magnus
Nucleus raphe magnus
The nucleus raphes magnus, located directly rostral to the nucleus raphes obscurus, is afferently stimulated from axons in the spinal cord and cerebellum....

 and the nociception inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit nociception-transmitting neurons also located in the spinal dorsal horn.

Peripheral

The peripheral regulation consists of several different types of opioid receptor
Opioid receptor
Opioid receptors are a group of G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin receptors...

s that are activated in response to the binding of the body's endorphin
Endorphin
Endorphins are endogenous opioid peptides that function as neurotransmitters. They are produced by the pituitary gland and the hypothalamus in vertebrates during exercise, excitement, pain, consumption of spicy food, love and orgasm, and they resemble the opiates in their abilities to produce...

s. These receptors, which exist in a variety of areas in the body, inhibit firing of neurons that would otherwise be stimulated to do so by nociceptors.

Factors

The gate control theory of pain, proposed by Patrick David Wall
Patrick David Wall
Patrick David "Pat" Wall was a leading British neuroscientist described as 'the world's leading expert on pain' and best known for the Gate control theory of pain.-Early life and academia:...

 and Ronald Melzack
Ronald Melzack
Ronald Melzack, is a Canadian psychologist.After studying for his Ph.D. in 1954 with D. O. Hebb at McGill University in Montreal, he began to work with patients who suffered from "phantom limb" pain — people who feel pain in an arm or leg that has been removed...

, postulates that nociception (pain) is "gated" by non-nociception stimuli such as vibration. Thus, rubbing a bumped knee seems to relieve pain by preventing its transmission to the brain. Pain is also "gated" by signals that descend from the brain to the spinal cord to suppress (and in other cases enhance) incoming nociception (pain) information.

Nociception response

When nociceptors are stimulated they transmit signals through sensory neurons in the spinal cord. These neurons release the excitatory neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 glutamate at their synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

s.

If the signals are sent to the reticular formation
Reticular formation
The reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...

 and thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

, the sensation of pain enters consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

 in a dull poorly localized manner. From the thalamus, the signal can travel to the somatosensory cortex in the cerebrum
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...

, when the pain is experienced as localized and having more specific qualities.

Nociception can also cause generalized autonomic responses
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 before or without reaching consciousness to cause pallor
Pallor
Pallor is a reduced amount of oxyhaemoglobin in skin or mucous membrane, a pale color which can be caused by illness, emotional shock or stress, stimulant use, lack of exposure to sunlight, anaemia or genetics....

, diaphoresis
Diaphoresis
Diaphoresis is excessive sweating commonly associated with shock and other medical emergency conditions.Diaphoretic is the state of perspiring profusely, or something that has the power to cause increased perspiration....

, tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

, hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...

, lightheadedness
Lightheadedness
Light-headedness is a common and often unpleasant sensation of dizziness and/or feeling that one may be about to faint, which may be transient, recurrent, or occasionally chronic. In some cases, the individual may feel as though his or her head is weightless. The individual may also feel as...

, nausea
Nausea
Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

 and fainting.

Nociception in non-mammalian animals

Nociception has been documented in non-mammalian animals, including fishes and a wide range of invertebrates
Pain in invertebrates
Pain in invertebrates is a problematic issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of...

, including leeches, nematode worms, sea slugs, and fruit flies. As in mammals, nociceptive neurons in these species are typically characterized by responding preferentially to high temperature (40 degrees C or more), low pH, capsaicin, and tissue damage.

History of term

The term nociception was coined by Charles Scott Sherrington
Charles Scott Sherrington
Sir Charles Scott Sherrington, OM, GBE, PRS was an English neurophysiologist, histologist, bacteriologist, and a pathologist, Nobel laureate and president of the Royal Society in the early 1920s...

to make clear the difference between the physiological nature of nervous activity signalling tissue damage and the psychological response of pain to this physiological event.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK