Topographic map (Neuroanatomy)
Encyclopedia
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system
. Topographic maps can be found in all sensory system
s and in many motor system
s.
refers to the part of the central nervous system
that allows an organism to see. It interprets information from visible light to build a representation of the world. The ganglion cells of the retina
project in an orderly fashion to the lateral geniculate nucleus
of the thalamus
and from there to the primary visual cortex
(V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex. This projection pattern has been termed retinotopy
.
The retinotopic layout of the visual topographic map refers to the fact that afferent connections from the retina that hold their particular order through sub-cortical structures, to V1, and throughout other cortical visual regions. The primary visual cortex (V1, Brodmann's area
17) is the first cortical area to receive visual input. The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1.
, chemical labels are distributed in a graded fashion across the retina and tectum. This allows each retinal ganglion cell to recognize its proper termination site. Experiments with the compound eyes in Xenopus demonstrate that not only the ganglion cells, but also their axons, carry these specificities. Axons must be able to communicate with each other to ensure that ones with the same positional tags innervate the same area of the superior colliculus
.
Second-order representations, also known as a field discontinuity map, are maps that are organized such that it appears that a discontinuity has been introduced in either the visual field or the retina. The maps in V2 and other extrastriate cortex
are second-order representations.
is the sensory system for hearing in which the brain interprets information from the frequency of sound waves, yielding the perception of tones. Sound waves enter the ear
through the auditory canal. Theses waves arrive at the eardrum
where the properties of the waves are transduced into vibrations. The vibrations travel through the bones of the inner ear
to the cochlea
. In the cochlea, the vibrations are transduced into electrical information through the firing of hair cells in the organ of Corti. The organ of Corti
projects in an orderly fashion to structures in the brainstem (namely, the cochlear nuclei
and the inferior colliculus
), and from there to the medial geniculate nucleus of the thalamus
and the primary auditory cortex; adjacent sites on the organ of Corti
, which are themselves selective for the sound
frequency, are represented by adjacent neurons in the aforementioned CNS structures. This projection pattern has been termed tonotopy
.
The tonotopic layout of sound information begins in the cochlea
where the basilar membrane
vibrates at different positions along its length depending upon the frequency of the sound. Higher frequency sounds are at the base of the cochlea, if it were unrolled, and low frequency sounds are at the apex. This arrangement is also found in the auditory cortex in the temporal lobe
. In areas that are tonotopically organized, the frequency varies systematically from low to high along the surface of the cortex, but is relatively constant across cortical depth. The general image of topographic organization in animal
s is multiple tonotopic maps distributed over the surface of the cortex.
comprises a diverse range of receptors and processing centers to produce the perception of touch, temperature, proprioception
, and nociception
. Receptors are located throughout the body including the skin
, epithelia, internal organs, skeletal muscle
s, bone
s, and joint
s. The cutaneous receptors of the skin project in an orderly fashion to the spinal cord
, and from there, via different afferent pathways (dorsal column-medial lemniscus tract and spinothalamic tract
), to the thalamus
and the primary somatosensory cortex
. Again, adjacent areas on the skin are represented by adjacent neurons in all aforementioned structures. This projection pattern has been termed somatotopy.
One common diagram of the somatotopic map is the cortical homunculus
. This illustration is a fairly accurate representation of how much cortical area represents each body part or region. It also maps what part of the cortex represents each region of the body.
s of the motor cortex are efferent neurons that exit the brain instead of bringing information to the brain through afferent
connections. The motor system
is responsible for initiating voluntary or planned movements (reflex
es are mediated at the spinal cord
level, so movements that associated with a reflex are not initiated by the motor cortex). The activation from the motor cortex travels through Betz cell
s down the corticospinal tract
through upper motor neurons, terminating at the anterior horn of the grey matter where lower motor neurons transmit the signal to peripheral motor neurons and, finally, the voluntary muscle
s.
. One instance would be (as already stated) in some reflex
es. Reflexes usually are mediated at the level of the spinal cord through reflex arc
s. In humans, mono-, oligo-, and poly-synaptic reflex arcs, propriospinal interneuron
systems, and internuncial gray matter neurons collectively participate continuously to produce spinal cord reflex that activates muscle.
The higher motor centers of octopus
es (large brained invertebrate
s) are notable for organizing (unlike vertebrate
s) highly skilled movements without the use of somatotopic maps of their bodies.
is the sensory system used for olfaction
, or the sense of smell. It detects volatile, airborne substances.
Most sensory systems spatially segregate afferent input from primary sensory neurons to construct a topographic map that defines the location of a sensory stimulus within the environment, as well as the quality of the stimulus itself. Unlike the other sensory systems, the olfactory system does not code any spatial information. Relieved of the requirement to map the position of an olfactory stimulus in space, the olfactory system employs spatial segregation of sensory input to encode the quality of an odorant
.
The topographic map revealed in the olfactory system differs in quality form the orderly representation inherent in the retinotopic, tonotopic, or somatotopic sensory maps. The olfactory system does not maintain an orderly representation of receptor cells in the periphery. Olfactory neurons differ from one another by the nature of the receptor that they possess, not by their location on a receptor sheet.
Individual olfactory sensory neurons express only one of the thousand receptor gene
s, such that neurons are functionally distinct. Cell
s expressing a given receptor in the olfactory epithelium are randomly dispersed within 1 of 4 broad zones. Sensory neurons extend a single unbranched axon to the olfactory bulb
such that the projections from neurons expressing a specific receptor converge on 2 out of the 1800 glomeruli
. The pattern of convergence is absolute and invariant in all individuals in a species. The bulb therefore provides a spatial map that identifies which of the numerous receptors have been activated within the sensory epithelium such that the quality of an olfactory stimulus would be encoded by specific combinations of glomeruli activated by a given odorant.
Since the positions of individual glomeruli are topographically defined, the bulb provides a spatial map that identifies which of the receptors have been activated in the sensory epithelium. Therefore, the quality of the stimulus would be encoded by the specific combination of glomeruli activated by a given odorant.
This topographic map encoding odor quality in the bulb has been shown to develop without apparent contribution from activity-dependent processes. Mice lacking the olfactory cyclic nucleotide-gated ion channel fail to exhibit odor-evoked electrophysiological responses in the sensory epithelium, but the pattern of convergence of like axons in the bulb is unaltered in these mutant mice, arguing strongly that olfactory experience is not necessary for the establishment or refinement of the topographic map.
These findings, however, do not exclude a role for activity-dependent processes in the maintenance or potential plasticity of the map after it is established. For example, neurons that do not express functional odorant receptors have been observed to have a significantly shorter half-life. Activity-dependent processes may therefore be essential for the survival of the olfactory neurons postnatally, and in that manner, may alter the input to individual glomeruli, altering the sensitivity to individual odors.
is the sensory system for taste
. Like olfaction, taste requires a process of chemoreception
. The receptors for taste are taste buds on the tongue
. The tongue contains taste receptors, as well as mechanoreceptors. Afferents from taste receptors and mechanoreceptors of the tongue access different ascending systems in the brainstem. However, it is uncertain how these two sources of information are processed in cortex. The primary gustatory cortex (G) is located near the somatotopic region for the tongue (S1), in the insular cortex
deep in the lateral fissure with the secondary taste areas in the opercula
.
The peripheral taste system likely maintains a specific relationship between taste bud cells selectively responsive to one taste quality and the ganglion cells signaling that particular quality. This explains the response specificity of some individual taste nerve, particularly because sweet, amino acid, and bitter receptors are expressed in distinct populations of taste cells. Although anatomical evidence for such an exclusive relationship is lacking at the level of single receptor and ganglion cells, the relationship between single buds and their innervating ganglion cells is tractable neuroanatomically. In taste, attempts to identify a spatial representation of taste receptors or taste qualities have revealed only an indistinct functional topography in the brain. Nevertheless, taste ganglion cells must distribute peripheral fibers to particular receptor cell types and disseminate impulses centrally in a structurally organized manner.
. Single-cell recording
, Transcranial magnetic stimulation
(TMS), electrical stimulation of the cortex, and Functional magnetic resonance imaging
(fMRI) are some of the techniques used to study maps in the brain. Many of the existing topographic maps have been further studied or refined using fMRI. For example, Hubel
and Wiesel
originally studied the retinotopic maps in the primary visual cortex using single-cell recording. Recently, however, imaging of the retinotopic map in the cortex and in sub-cortical areas, such as the lateral geniculate nucleus, have been improved using the fMRI technique.
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
. Topographic maps can be found in all sensory system
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
s and in many motor system
Motor system
The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system....
s.
Visual system
The visual systemVisual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
refers to the part of the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
that allows an organism to see. It interprets information from visible light to build a representation of the world. The ganglion cells of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
project in an orderly fashion to the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
of 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 from there to the primary visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
(V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex. This projection pattern has been termed retinotopy
Retinotopy
Retinotopy describes the spatial organization of the neuronal responses to visual stimuli. In many locations within the brain, adjacent neurons have receptive fields that include slightly different, but overlapping portions of the visual field. The position of the center of these receptive fields...
.
The retinotopic layout of the visual topographic map refers to the fact that afferent connections from the retina that hold their particular order through sub-cortical structures, to V1, and throughout other cortical visual regions. The primary visual cortex (V1, Brodmann's area
Brodmann area
A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...
17) is the first cortical area to receive visual input. The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1.
Development
According to the Chemoaffinity hypothesisChemoaffinity hypothesis
The Chemoaffinity hypothesis states that neurons make connections with their targets based on interactions with specific molecular markers and, therefore, that the initial wiring diagram of an organism is determined by its genotype...
, chemical labels are distributed in a graded fashion across the retina and tectum. This allows each retinal ganglion cell to recognize its proper termination site. Experiments with the compound eyes in Xenopus demonstrate that not only the ganglion cells, but also their axons, carry these specificities. Axons must be able to communicate with each other to ensure that ones with the same positional tags innervate the same area of the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
.
Categories of retinotopic maps
First-order representations are those in which adjacent points of the same hemifield always map to adjacent columns in the contralateral cortex. An example of this would be the map in primary visual cortex (V1).Second-order representations, also known as a field discontinuity map, are maps that are organized such that it appears that a discontinuity has been introduced in either the visual field or the retina. The maps in V2 and other extrastriate cortex
Extrastriate cortex
The extrastriate cortex is the region of the occipital cortex of the mammalian brain located next to the primary visual cortex, which is also named striate cortex because of its appeareance in the microscope. The extrastriate cortex encompasses multiple functional areas, including V3, V4, V5/MT...
are second-order representations.
Auditory system
The auditory systemAuditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
is the sensory system for hearing in which the brain interprets information from the frequency of sound waves, yielding the perception of tones. Sound waves enter the ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
through the auditory canal. Theses waves arrive at the eardrum
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
where the properties of the waves are transduced into vibrations. The vibrations travel through the bones of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
to the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
. In the cochlea, the vibrations are transduced into electrical information through the firing of hair cells in the organ of Corti. The organ of Corti
Organ of Corti
The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...
projects in an orderly fashion to structures in the brainstem (namely, the cochlear nuclei
Cochlear nuclei
The cochlear nuclei are two heterogeneous collections of neurons in the mammalian brainstem that receive input from the cochlear nerve, which carry sound information from the cochleae...
and the inferior colliculus
Inferior colliculus
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex...
), and from there to the medial geniculate nucleus of 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 primary auditory cortex; adjacent sites on the organ of Corti
Organ of Corti
The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...
, which are themselves selective for the sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
frequency, are represented by adjacent neurons in the aforementioned CNS structures. This projection pattern has been termed tonotopy
Tonotopy
In physiology, tonotopy is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain...
.
The tonotopic layout of sound information begins in the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
where the basilar membrane
Basilar membrane
The basilar membrane within the cochlea of the inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani .-Function:...
vibrates at different positions along its length depending upon the frequency of the sound. Higher frequency sounds are at the base of the cochlea, if it were unrolled, and low frequency sounds are at the apex. This arrangement is also found in the auditory cortex in the temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. In areas that are tonotopically organized, the frequency varies systematically from low to high along the surface of the cortex, but is relatively constant across cortical depth. The general image of topographic organization in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s is multiple tonotopic maps distributed over the surface of the cortex.
Somatosensory system
The somatosensory systemSomatosensory system
The somatosensory system is a diverse sensory system composed of the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception , and nociception . The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal...
comprises a diverse range of receptors and processing centers to produce the perception of touch, temperature, proprioception
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
, and nociception
Nociception
Nociception is defined as "the neural processes of encoding and processing noxious stimuli." It is the afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue...
. Receptors are located throughout the body including 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...
, epithelia, internal organs, skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
s, bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s, and 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:...
s. The cutaneous receptors of the skin project in an orderly fashion to 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...
, and from there, via different afferent pathways (dorsal column-medial lemniscus tract and spinothalamic tract
Spinothalamic tract
The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch...
), to 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 primary 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...
. Again, adjacent areas on the skin are represented by adjacent neurons in all aforementioned structures. This projection pattern has been termed somatotopy.
One common diagram of the somatotopic map is the cortical homunculus
Cortical homunculus
A cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex , i.e., the portion of the human brain directly responsible for the movement and exchange of sense and motor information of the rest of the body.There...
. This illustration is a fairly accurate representation of how much cortical area represents each body part or region. It also maps what part of the cortex represents each region of the body.
Motor System
Unlike the topographic maps of the senses, the neuronNeuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s of the motor cortex are efferent neurons that exit the brain instead of bringing information to the brain through 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...
connections. The motor system
Motor system
The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system....
is responsible for initiating voluntary or planned movements (reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es are mediated at 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...
level, so movements that associated with a reflex are not initiated by the motor cortex). The activation from the motor cortex travels through Betz cell
Betz cell
Betz cells are pyramidal cell neurons located within the fifth layer of the grey matter in the primary motor cortex, M1. They are named after Vladimir Alekseyevich Betz, who described them in his work published in 1874. These neurons are the largest in the central nervous system, sometimes reaching...
s down the corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
through upper motor neurons, terminating at the anterior horn of the grey matter where lower motor neurons transmit the signal to peripheral motor neurons and, finally, the voluntary muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s.
Movements not requiring use of topographic maps
There are several instances of movements that do not require the participation of topographic maps, or even the participation of the brainBrain
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,...
. One instance would be (as already stated) in some reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es. Reflexes usually are mediated at the level of the spinal cord through reflex arc
Reflex arc
A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord...
s. In humans, mono-, oligo-, and poly-synaptic reflex arcs, propriospinal interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
systems, and internuncial gray matter neurons collectively participate continuously to produce spinal cord reflex that activates muscle.
The higher motor centers of octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
es (large brained invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s) are notable for organizing (unlike vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s) highly skilled movements without the use of somatotopic maps of their bodies.
Olfactory System
The olfactory systemOlfactory system
The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...
is the sensory system used for olfaction
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
, or the sense of smell. It detects volatile, airborne substances.
Most sensory systems spatially segregate afferent input from primary sensory neurons to construct a topographic map that defines the location of a sensory stimulus within the environment, as well as the quality of the stimulus itself. Unlike the other sensory systems, the olfactory system does not code any spatial information. Relieved of the requirement to map the position of an olfactory stimulus in space, the olfactory system employs spatial segregation of sensory input to encode the quality of an odorant
Aroma compound
An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor...
.
The topographic map revealed in the olfactory system differs in quality form the orderly representation inherent in the retinotopic, tonotopic, or somatotopic sensory maps. The olfactory system does not maintain an orderly representation of receptor cells in the periphery. Olfactory neurons differ from one another by the nature of the receptor that they possess, not by their location on a receptor sheet.
Individual olfactory sensory neurons express only one of the thousand receptor gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, such that neurons are functionally distinct. Cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s expressing a given receptor in the olfactory epithelium are randomly dispersed within 1 of 4 broad zones. Sensory neurons extend a single unbranched axon to the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
such that the projections from neurons expressing a specific receptor converge on 2 out of the 1800 glomeruli
Glomerulus (olfaction)
The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells...
. The pattern of convergence is absolute and invariant in all individuals in a species. The bulb therefore provides a spatial map that identifies which of the numerous receptors have been activated within the sensory epithelium such that the quality of an olfactory stimulus would be encoded by specific combinations of glomeruli activated by a given odorant.
Since the positions of individual glomeruli are topographically defined, the bulb provides a spatial map that identifies which of the receptors have been activated in the sensory epithelium. Therefore, the quality of the stimulus would be encoded by the specific combination of glomeruli activated by a given odorant.
This topographic map encoding odor quality in the bulb has been shown to develop without apparent contribution from activity-dependent processes. Mice lacking the olfactory cyclic nucleotide-gated ion channel fail to exhibit odor-evoked electrophysiological responses in the sensory epithelium, but the pattern of convergence of like axons in the bulb is unaltered in these mutant mice, arguing strongly that olfactory experience is not necessary for the establishment or refinement of the topographic map.
These findings, however, do not exclude a role for activity-dependent processes in the maintenance or potential plasticity of the map after it is established. For example, neurons that do not express functional odorant receptors have been observed to have a significantly shorter half-life. Activity-dependent processes may therefore be essential for the survival of the olfactory neurons postnatally, and in that manner, may alter the input to individual glomeruli, altering the sensitivity to individual odors.
Gustatory System
The gustatory systemGustatory system
The gustatory system is the sensory system for the sense of taste.- Importance :The gustatory system allows humans to distinguish between safe and harmful food. Bitter and sour foods we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation...
is the sensory system for taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....
. Like olfaction, taste requires a process of chemoreception
Chemosensor
A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. In more general terms, a chemosensor detects certain chemical stimuli in the environment.- Classes :...
. The receptors for taste are taste buds on the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...
. The tongue contains taste receptors, as well as mechanoreceptors. Afferents from taste receptors and mechanoreceptors of the tongue access different ascending systems in the brainstem. However, it is uncertain how these two sources of information are processed in cortex. The primary gustatory cortex (G) is located near the somatotopic region for the tongue (S1), in the insular cortex
Insular cortex
In each hemisphere of the mammalian brain the insular cortex is a portion of the cerebral cortex folded deep within the lateral sulcus between the temporal lobe and the frontal lobe. The cortical area overlying it towards the lateral surface of the brain is the operculum...
deep in the lateral fissure with the secondary taste areas in the opercula
Operculum (brain)
Operculum, derived from Latin, meaning "little lid", refers to the cerebral cortex on the outside surface of the brain bordering the lateral sulcus, and the roof and floor of the lateral sulcus. Neuroscience divides the operculum into orbital, frontal, parietal and temporal regions, after the...
.
The peripheral taste system likely maintains a specific relationship between taste bud cells selectively responsive to one taste quality and the ganglion cells signaling that particular quality. This explains the response specificity of some individual taste nerve, particularly because sweet, amino acid, and bitter receptors are expressed in distinct populations of taste cells. Although anatomical evidence for such an exclusive relationship is lacking at the level of single receptor and ganglion cells, the relationship between single buds and their innervating ganglion cells is tractable neuroanatomically. In taste, attempts to identify a spatial representation of taste receptors or taste qualities have revealed only an indistinct functional topography in the brain. Nevertheless, taste ganglion cells must distribute peripheral fibers to particular receptor cell types and disseminate impulses centrally in a structurally organized manner.
Benefits
For neurons, it is metabolically costly to make long connections, and the trade-off of connection distance is time. Thus, it is good design to group neurons together that are to be highly interconnected. Multiple topographic maps is a feature that is advantageous because it allows maps of different sizes that would accommodate varying levels of acuity and details in signals. A more detailed map has more neurons that would take up more area than a more global map, which would require fewer connections.Techniques
A variety of techniques have been used to establish the topographic maps in the brain. The existence of topographical maps was shown early by electrical stimulation of the cortex, tracing patterns of epileptic seizures, stimulation sequences, and impairments due to lesions. Details in the maps came later through microelectrode stimulation and recording techniques became commonly used in demonstrating somatotopic maps and later in the auditory and visual systems, both cortically and in subcortical structures such as the colliculi and geniculate nuclei of the thalamusThalamus
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...
. Single-cell recording
Single-unit recording
In neurophysiology and neurology, single-unit recording is the use of an electrode to record the electrophysiological activity from a single neuron.-History:...
, Transcranial magnetic stimulation
Transcranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
(TMS), electrical stimulation of the cortex, and Functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(fMRI) are some of the techniques used to study maps in the brain. Many of the existing topographic maps have been further studied or refined using fMRI. For example, Hubel
David H. Hubel
David Hunter Hubel is the John Franklin Enders Professor of Neurobiology, Emeritus, at Harvard Medical School. He was co-recipient with Torsten Wiesel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was...
and Wiesel
Torsten Wiesel
Torsten Nils Wiesel was a Swedish co-recipient with David H. Hubel of the 1981 Nobel Prize in Physiology or Medicine, for their discoveries concerning information processing in the visual system; the prize was shared with Roger W...
originally studied the retinotopic maps in the primary visual cortex using single-cell recording. Recently, however, imaging of the retinotopic map in the cortex and in sub-cortical areas, such as the lateral geniculate nucleus, have been improved using the fMRI technique.