Receptor cell

As a receptor cell or receptor (from the Latin recipere , 'to receive', 'to receive'), sensor or sensor cell , also sensory cell , a specialized cell is called in physiology , which receives certain chemical or physical stimuli from the environment of a body or its interior and in transferred (transduced) a neuronally comparable form. It only exists in multicellular organisms with nerve tissue and serves as a sensory cell for the perception of external or internal changes ( exteroception or interoception ). Sensory cells can be scattered all over the body surface or inside the body or special sense organs be summarized.

features

Receptor cells are sensitive to changes in different forms of energy and allow an organism to become sensitive to their changes. In this sense, they not only fulfill a function in a living being comparable to a sensor in technology and convert stimuli - such as light that falls on the retina in the eye - into electrical signals that can be passed on to the brain. Rather, these signals and their changes are filtered in a living being, compared in terms of time and space, integrated, interpreted and can thus be experienced with a special quality. The susceptibility to certain stimulus qualities in receptor cells often depends on specific molecular structures, which are also referred to as receptors in biochemistry .

As a sensory cell, a receptor cell is the first link in the signal chain of our senses . It is mostly designed for special types of stimuli, depending on the form and strength of the mode of action. It converts such adequate stimuli particularly sensitively above a certain stimulus threshold into a receptor potential (generator potential ) as a signal, depending on stimulus strength and change. In a (assigned 1) afferent neuron this generator potentials are mapped and dissolve when the respective here threshold potential exceed, action potentials , which as neural signals to the central nervous system (CNS) forwarded be.

A distinction must be made between the reception of stimuli by sensory cells and the sensory perception of sensations as an experienced event. For example, photoreceptors in the retina of the eye convert light stimuli into signals that are passed on to neurons, compared with one another and sent to different regions of the brain via the optic nerves . You have based on the transmitted signals in the midbrain are differences in brightness of patterns and directions of movement differed; Projected to areas of the visual cortex in the cerebrum , they are interpreted for processing a visual impression. Signals transmitted to the core areas of the midbrain are used to answer moving patterns with (reflex) eye movements or sudden differences in brightness with a pupil constriction. Retinal signals transmitted to nuclei of the hypothalamus are important for the synchronization of the day-night rhythm . The external stimulus or stimulus can be the same in all of these cases, changes in the input of electromagnetic energy ( photons ).

A sensory cell does not perceive objects, but rather stimuli. Stimuli are effects which, depending on the type and amount of energy, can also affect other cells, but in sensory cells they change a cell's own receptive structure - for example a photopigment such as rhodopsin , iodopsin or melanopsin - in a special way. The change in receptors suffered is initially not compensated for by cell processes, but is converted into a signal that can be communicated to nerve cells by means of a sensitive cell-internal sensitive process - which is usually considerably increased during the transduction process : the electrical signal of a changed membrane potential .

May sensor cells so stimuli of different energy form record reflect differences in the energy input and this, into a same form of energy, as a signal representing ( transduction ). The (analog) receptor potential generated in this way is picked up by assigned nerve cells as a generator potential and then transformed into the (digital) action potential ( transformation ). As an action potential series, nerve cells transmit their excitation via the axon , and convert it at the ends of chemical synapses into a secondary signal, in transmitter quanta ( transmission ), with which an excitation can be transmitted to other cells . If these are nerve cells as part of a sensory or sensitive system - possibly enabling conscious perception - then we speak of sensory cells in the narrow sense.

classification

Receptor cells can be classified according to various criteria.

According to the adequate stimulus

receptors are first differentiated according to the corresponding form of energy that stimulates them:

• Chemoreceptors are addressed by chemical stimuli. This includes
  • Sensory olfactory cells of the olfactory mucous membrane , in humans several hundreddifferent types of the total of approx. 10 million olfactory cells , depending on the type of their specific olfactory receptor protein , also for olfactory perception
  • Additional olfactory receptor cells of the Jacobson organ , in humans in the nasal mucous membrane stunted after birth, with special olfactory receptor protein types, especially for pheromone perception or accessory olfactory perception
  • Taste sensory cells ofthe oral mucosa , in humans mainly in the taste papillae on the tongue, five / six different taste receptor proteins , especially for gustatory perception , including the sensory impression of a so-called taste
  • central chemoreceptors in regions of the brain in humans Mark brain for CO ₂ - partial pressure and pH of the cerebrospinal fluid (reflex circulatory and respiratory regulation), and in the area postrema for various toxic metabolites (reflexive vomiting)
  • peripheral chemoreceptors in large blood vessels, so in the carotid body for the partial pressure of oxygen (pO ₂ ) and carbon dioxide (pCO ₂ ), and the acidity ( pH ) of the blood
  • intestinal chemoreceptors in the lining of the esophagus and upper gastrointestinal tract
  • Osmoreceptors as receptors for the osmotic concentration of extracellular fluid are usually assigned to the chemoreceptors. They occur, for example, in the hypothalamus and in the kidney .
• Photoreceptors are addressed by light stimuli.
• Magnetoreceptors are addressed by magnetic fields.
• Electroreceptors are addressed by electrical fields.
• Thermoreceptors are addressed by thermal stimuli.
• Mechanoreceptors are addressed by mechanical stimuli.
  • Baroreceptors are mechanoreceptors that are addressed by pressure differences (in arterial blood vessels).
• Nociceptors are sensory free nerve endings thatare stimulatedby chemical, thermal or mechanical effects of noxious substances in the event of tissue damage or injuryand thengenerateelectrical signals (action potentials).

According to membrane potential signals

Sensory cells can respond differently to a constant stimulus over time:

• tonic sensory cells - with constant stimulus intensity, show a constant receptor potential or a constant signal frequency; they map the stimulus proportionally (the logarithm of the stimulus strength) according to its intensity ( P-sensors ) and only adapt slowly
• phasic sensory cells - show a continuously reduced receptor potential or decreasing signal frequency with constant stimulus intensity; they adapt quickly and reproduce the stimulus differentially according to the speed of the change in intensity ( D sensors ); if the intensity remains unchanged, the pulse frequency finally falls to zero
• phasic-tonic sensory cells - if the stimulus intensity remains the same, they show a response roughly proportional to the intensity ; if the stimulus intensity changes, they also show a response that depends on the speed of the change in intensity ( PD sensors ); Most sensory cells can be assigned to this type

Sensory cells of cranial animals generally generate a receptor potential that depicts a stimulus as a depolarization according to intensity and duration; however, their photoreceptors are hyperpolarized by exposure to light .

According to the form of neural assignment

The neuroectoderm produces , among other things, nerve cells that become sensory cells - for example, those of the sense of smell . These are called “primary” sensory cells because they are both a sensor and the first neuron to transmit signals; in contrast to those that are only sensors and are connected to the first afferent neuron via a synapse as so-called “secondary” sensory cells - for example those of the sense of taste .

1. primary sensory cells are neurons with an axon. In addition to the olfactory cells already mentioned, this also includes the nociceptors as free nerve endings . Various mechanoreceptors in the body wall and intestines are also primary sensory cells. For example, the specialized receptors with nerve fibers that are excited by mechanical stimuli such as stretching or pressure, such as the touch receptors of the skin (the tactile sense of surface sensitivity ), but also the proprioceptors in muscles , tendons , ligaments , joint capsules and joints (the sense of strength and position sense the depth sensitivity ).
2. secondary sensory cells do not generate action potentials themselves, but have a synapse with the first afferent neuron . In addition to the taste cells mentioned above, this also includes, for example, the type I glomus cells of the glomus organs . Also the sensory cells of the sense of hearing and the sense of balance in the sensory epithelium of the inner ear - the hair cells of Corti organ in the cochlea and hair cells in the ampullary crests of the three semicircular canal organs and the two maculae of macular organs - transmit their signals via synapses (dendritic) extensions the assigned first afferent neurons of the 8th cranial nerve ( vestibulocochlear nerve ).

Anatomists and physiologists do not always use the same definitions for primary sensory cells . For example, from an anatomical point of view, the photoreceptors in the retina are primary sensory cells, since they are also nerve cells (and come from the neuroepithelium ). Sensory-physiologically, sensory cells are primary when the sensor is at the same time a nerve cell with an axon and thus represents the first neuron in an afferent system such as the visual pathway ; if there is a synapse between the sensor and the afferent nerve fiber, the sensory cells are called secondary .

Electrophysiologically, in addition to the transduction of the stimulus, the transformation of the neuronal signal (into action potentials) is often required for the term primary sensory cell . In this sense, photoreceptors can occasionally also be addressed as secondary sensory cells, since they do not yet generate an action potential themselves (only the third afferent neuron, the retinal ganglion cell ).

According to the position in the body

receptor cells can be differentiated into - based on an underlying scheme of the body

• somatic sensors on the outer surface of the body or its abdominal wall; As somatosensitive, these are primarily changed by (external) stimuli from the body's surroundings.
  • this includes, for example, skin sensitivity
• visceral sensors on the internal surfaces of the body or in its intestines; as viscerosensitive, these are primarily changed by (internal) stimuli in the body as the environment
  • this includes, for example, mucosal sensitivity

Beyond this rough scheme, finer distinctions are also possible with assignments according to the position in tissue layers or in regions or in individual organs.

According to a functional system

are sensory cells a afferent system of different sensory modality associated, so

• sensory dem
  • olfactory system of the olfactory sense via the olfactory tract
  • visual system of the sense of sight via the visual pathway
  • auditory system of the auditory sense via the auditory pathway
  • vestibular system of the sense of equilibrium via the equilibrium nerves
  • gustatory system of the sense of taste via the taste fibers

• sensitive to that
  • protopathic system for pain, temperature, and gross tactile sensations
  • epicritical system for subtle sensations of the sense of touch
  • proprioceptive system for the body's own sensations of muscles, tendons and joints as a sense of posture, movements and applied force

According to the perception concept

An often-based approach puts perceived sensations a concept of space for the body reference based on which it allows, for example in the perception of external objects to part from internal or an external world or inner world belong. A distinction can thus be made between

• Exteroception as the perception of changes in external processes
• Interoception as the perception of changes in internal processes

A distinction is to be made between this, however, with what this difference is made, i.e. what in this regard borders and delimits on the outside and embraces and includes on the inside. For this body in space and as a space, a more or less sharply outlined idea can be formed, a body scheme . However, one's own body is usually not perceived as an object or object, but rather felt as a state, perceived as one's own, as experienced in relation to the body that one is. In this context, a distinction is often made between

• Proprioception as the perception of orientation, position, position and movement of the body in space together with the feeling for heaviness, tension, force and speed
• Visceroception as the perception of states and activities of internal organs

The Viszerozeption is also called Enterozeption referred to and equal to the perception in some concepts interoception set. Other concepts of perception include both enteroception and proprioception under interoception.

literature

• Christopher D. Moyes, Patricia M. Schulte: Animal Physiology , Pearson Studies, 2010. ISBN 978-3-86894-124-1 .
• Roger Eckert, Raimund Apfelbach: Tierphysiologie , 4th edition, Thieme, Stuttgart 2002, ISBN 978-3-13-664004-3 .

Individual evidence

1. ^ Stefan Silbernagl , Agamemnon Despopoulos : Taschenatlas Physiologie , 8th edition, Thieme Verlag, 2012, ISBN 978-3-13-567708-8 , p. 330.