Neural pathway

A neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable a signal to be sent from one region of the nervous system to another. Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus.[1] Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.

In the hippocampus there are neural pathways involved in its circuitry including the perforant pathway, that provides a connectional route from the entorhinal cortex[2] to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1),[3] and the subiculum.

Descending motor pathways of the pyramidal tracts travel from the cerebral cortex to the brainstem or lower spinal cord.[4][5] Ascending sensory tracts in the dorsal column–medial lemniscus pathway (DCML) carry information from the periphery to the cortex of the brain.

Neural pathway diagram
A neural pathway connects one part of the nervous system to another using bundles of axons called tracts. The optic tract that extends from the optic nerve is an example of a neural pathway because it connects the eye to the brain; additional pathways within the brain connect to the visual cortex.

Naming

Architecture of the 3 layers in the cerebella cortex
Neural pathways of cerebellar cortex
Ciliary ganglion pathways
Pathways in the ciliary ganglion. Green=Parasympathetic; Red=Sympathetic; Blue=Sensory

The first named pathways are evident to the naked eye even in a poorly preserved brain, and were named by the great anatomists of the Renaissance using cadaver material. Examples of these include the great commissures of the brain such as the corpus callosum (Latin, "hard body"; not to be confused with the Latin word "colossus" – the "huge" statue), anterior commissure, and posterior commissure. Further examples include the pyramidal tract, crus cerebri (Latin, "leg of the brain"), and cerebellar peduncles (Latin, "little foot of the cerebellum"). Note that these names describe the appearance of a structure but give one no information on its function or location.

Later, as neuroanatomical knowledge became more sophisticated, the trend was toward naming pathways by their origin and termination. For example, the nigrostriatal pathway runs from the substantia nigra (Latin, "black substance") to the corpus striatum (Latin, "striped body"). This naming can extend to include any number of structures in a pathway, such that the cerebellorubrothalamocortical pathway originates in the cerebellum, synapses in the red nucleus ("ruber" in Latin), on to the thalamus, and finally terminating in the cerebral cortex.

Sometimes, these two naming conventions coexist. For example, the name "pyramidal tract" has been mainly supplanted by lateral corticospinal tract in most texts. Note that the "old" name was primarily descriptive, evoking the pyramids of antiquity, from the appearance of this neural pathway in the medulla oblongata. The "new" name is based primarily on its origin (in the primary motor cortex, Brodmann area 4) and termination (onto the alpha motor neurons of the spinal cord).

In the cerebellum one of the two major pathways is that of the mossy fibers. Mossy fibers project directly to the deep nuclei, but also give rise to the following pathway: mossy fibers → granule cells → parallel fibers → Purkinje cells → deep nuclei. The other main pathway is from the climbing fibers and these project to Purkinje cells and also send collaterals directly to the deep nuclei.[6]

Functional aspects

Posterior Parietal Lobe
Diagram showing cortical pathways

In general, neurons receive information either at their dendrites or cell bodies. The axon of a nerve cell is, in general, responsible for transmitting information over a relatively long distance. Therefore, most neural pathways are made up of axons. If the axons have myelin sheaths, then the pathway appears bright white because myelin is primarily lipid. If most or all of the axons lack myelin sheaths (i.e., are unmyelinated), then the pathway will appear a darker beige color, which is generally called grey.

Some neurons are responsible for conveying information over long distances. For example, motor neurons, which travel from the spinal cord to the muscle, can have axons up to a meter in length in humans. The longest axon in the human body belongs to the Sciatic Nerve and runs from the great toe to the base of the spinal cord. These are archetypal examples of neural pathways.

Basal ganglia pathways and dopamine

Neural pathways in the basal ganglia in the cortico-basal ganglia-thalamo-cortical loop, are seen as controlling different aspects of behaviour. This regulation is enabled by the dopamine pathways. It has been proposed that the dopamine system of pathways is the overall organiser of the neural pathways that are seen to be parallels of the dopamine pathways.[7] Dopamine is provided both tonically and phasically in response to the needs of the neural pathways.[7]

Major neural pathways

See also

References

  1. ^ Moore, Keith; Dalley, Arthur (2005). Clinically Oriented Anatomy (5th ed.). LWW. p. 47. ISBN 0-7817-3639-0. A bundle of nerve fibers (axons) connecting neighboring or distant nuclei of the CNS is a tract.
  2. ^ Witter, Menno P.; Naber, Pieterke A.; Van Haeften, Theo; Machielsen, Willem C.M.; Rombouts, Serge A.R.B.; Barkhof, Frederik; Scheltens, Philip; Lopes Da Silva, Fernando H. (2000). "Cortico-hippocampal communication by way of parallel parahippocampal-subicular pathways". Hippocampus. 10 (4): 398–410. doi:10.1002/1098-1063(2000)10:4<398::AID-HIPO6>3.0.CO;2-K. PMID 10985279.
  3. ^ Vago, David R.; Kesner, Raymond P. (2008). "Disruption of the direct perforant path input to the CA1 subregion of the dorsal hippocampus interferes with spatial working memory and novelty detection". Behavioural Brain Research. 189 (2): 273–83. doi:10.1016/j.bbr.2008.01.002. PMC 2421012. PMID 18313770.
  4. ^ Purves, Dale (2011). Neuroscience (5. ed.). Sunderland, Mass.: Sinauer. pp. 375–378. ISBN 9780878936953.
  5. ^ Purves, Dale; Augustine, George J.; Fitzpatrick, David; Katz, Lawrence C.; LaMantia, Anthony-Samuel; McNamara, James O.; Williams, S. Mark (1 January 2001). "Damage to Descending Motor Pathways: The Upper Motor Neuron Syndrome".
  6. ^ Llinas RR, Walton KD, Lang EJ (2004). "Ch. 7 Cerebellum". In Shepherd GM. The Synaptic Organization of the Brain. New York: Oxford University Press. ISBN 0-19-515955-1.
  7. ^ a b Hong, Simon (2013). "Dopamine system: manager of neural pathways". Frontiers in Human Neuroscience. 7. doi:10.3389/fnhum.2013.00854.
Cough reflex

The cough reflex has both sensory (afferent) mainly via the vagus nerve and motor (efferent) components. Pulmonary irritant receptors (cough receptors) in the epithelium of the respiratory tract are sensitive to both mechanical and chemical stimuli. The bronchi and trachea are so sensitive to light touch that slight amounts of foreign matter or other causes of irritation initiate the cough reflex. The larynx and carina are especially sensitive. Terminal bronchioles and even the alveoli are sensitive to chemical stimuli such as sulfur dioxide gas or chlorine gas. Rapidly moving air usually carries with it any foreign matter that is present in the bronchi or trachea. Stimulation of the cough receptors by dust or other foreign particles produces a cough, which is necessary to remove the foreign material from the respiratory tract before it reaches the lungs.

Direct pathway

The direct pathway, sometimes known as the direct pathway of movement, is a neural pathway within the central nervous system (CNS) through the basal ganglia which facilitates the initiation and execution of voluntary movement. It works in conjunction with the indirect pathway. Both of these pathways are part of the cortico-basal ganglia-thalamo-cortical loop.

Doppler Shift Compensation

When an echolocating bat approaches a target, its outgoing sounds return as echoes, which are Doppler shifted upward in frequency. In certain species of bats, which produce constant frequency (CF) echolocation calls, the bats compensate for the Doppler shift by lowering their call frequency as they approach a target. This keeps the returning echo in the same frequency range of the normal echolocation call. This dynamic frequency modulation is called the Doppler Shift Compensation (DSC), and was discovered by Hans Schnitzler in 1968.CF bats employ the DSC mechanism to maintain the echo frequency within a narrow frequency range. This narrow frequency range is referred to as the acoustic fovea. By modulating the frequency of the outgoing calls, the bats can ensure that the returning echoes stay nearly constant within this range of optimal sensitivity. Ultimately, by keeping the echoes in this optimal range, the bats can quickly ascertain certain properties (such as distance and velocity) about the target.

This behavior appears to have evolved independently in several species of the Rhinolophidae and Mormoopidae families. The common features shared by bats with DSC are that they produce CF sounds, and that they have a specialized cochlea that is adapted to receiving a narrow range of frequencies with high resolution. DSC allows these bats to utilize these features to optimize the echolocation behavior.

Intermittent photic stimulation

In medicine, Intermittent Photic Stimulation, or IPS, is a form of visual stimulation used in conjunction with electroencephalography to investigate anomalous brain activity triggered by specific visual stimuli, such as flashing lights or patterns.

IPS and EEGs are often used to diagnose conditions such as photosensitive epilepsy. The field is relatively new and the details of use of IPS have not been widely standardized. IPS is often used in conjunction with other controllable generators of visual stimuli, such as low-level visual stimulation LLVS.

Photic stimulation may also be used to elicit myoclonus, especially cortical reflex myoclonus when present in photo-sensitive forms.

IPS may be used to stimulate the visual system for patients with amblyopia. This system uses a visual stimulus that is usually red in color with a frequency of about 4 Hz to stimulate the neural pathway between the retina and the visual cortex. The objective is to improve the visual acuity of an amblyopic (lazy) eye.

James Papez

James Wenceslas Papez (;

1883–1958) was an American neuroanatomist. Papez received his MD from the University of Minnesota College of Medicine and Surgery. He is most famous for his 1937 description of the Papez circuit which is a neural pathway in the brain thought to be involved in the cortical control of emotion. He was a neurologist at Cornell University and curator of the Wilder Brain Collection, when he published a journal article in which he outlined a "new" circuit to account for emotion. He hypothesized that the hippocampus, the cingulate gyrus (Broca's callosal lobe), the hypothalamus, the anterior thalamic nuclei, and the interconnections among these structures constituted a harmonious mechanism which elaborate the functions of emotions. Papez never mentioned Broca's limbic lobe but others noted that his circuit was very similar to Broca's great limbic lobe.

Lazarus sign

The Lazarus sign or Lazarus reflex is a reflex movement in brain-dead or brainstem failure patients, which causes them to briefly raise their arms and drop them crossed on their chests (in a position similar to some Egyptian mummies). The phenomenon is named after the Biblical figure Lazarus of Bethany, whom Jesus raised from the dead in the Gospel of John.

Medial forebrain bundle

The medial forebrain bundle (MFB), is a neural pathway containing fibers from the basal olfactory regions, the periamygdaloid region and the septal nuclei, as well as fibers from brainstem regions, including the ventral tegmental area.

Menace reflex

The menace response is one of three forms of blink reflex. It is the reflex blinking that occurs in response to the rapid approach of an object. The reflex comprises blinking of the eyelids, in order to protect the eyes from potential damage, but may also include turning of the head, neck, or even the trunk away from the optical stimulus that triggers the reflex.Stimulating the menace reflex is used as a diagnostic procedure in veterinary medicine, in order to determine whether an animal's visual system, in particular the cortical nerve, has suffered from nerve damage. Cortical damage, particularly cerebral lesions, can cause loss of the menace reflex while leaving the other blink reflexes, such as the dazzle reflex, unaffected. The presence or absence of the menace reflex, in combination with other reflexes, indicates a locus of damage. For example, an animal with polioencephalomalacia will lack the menace reflex, but will still have the pupillary light reflex. Polioencephalomacia damages the visual cortex, impairing the menace reflex, but leaves the optic nerve, oculomotor nucleus, and oculomotor nerve intact, leaving the pupillary light reflex unaffected. Contrastingly, an animal with ocular hypovitaminosis-A will suffer from degeneration of the optic nerve, and such an animal presents with a lack of both reflexes.Testing the menace reflex has to be done with care. Waving an object close to an animal's eyes or face does not necessarily demonstrate a functioning menace reflex, in part because the animal can sense such objects and react to them via senses other than sight. Clinical testing of the menace reflex usually involves precautions such as waving an object from behind a sheet of glass, so as to shield the animal from any drafts caused by the motion of the object through the air, which it might otherwise sense. Such reactions to non-visual stimuli are a widespread cause of false positives and false negatives when pet owners test their own animals for the presence of the menace reflex.The neural pathway of the menace reflex comprises the optic (II) and facial (VII) nerves. It is mediated by tectobulbar fibres in the rostral colliculi of the midbrain passing from the optic tract to accessory nuclei, and thence to the spinal cord and lower motor neurones that innervate the head, neck, and body muscles affected by the reflex. The facial nerve is mediated through a corticotectopontocerebellar pathway.

Nerve conduction velocity

Nerve conduction velocity is an important aspect of nerve conduction studies. It is the speed at which an electrochemical impulse propagates down a neural pathway. Conduction velocities are affected by a wide array of factors, including age, sex, and various medical conditions. Studies allow for better diagnoses of various neuropathies, especially demyelinating conditions as these conditions result in reduced or non-existent conduction velocities.

Neurocomputational speech processing

Neurocomputational speech processing is computer-simulation of speech production and speech perception by referring to the natural neuronal processes of speech production and speech perception, as they occur in the human nervous system (central nervous system and peripheral nervous system). This topic is based on neuroscience and computational neuroscience.

Pierre Mollaret

Pierre Mollaret (10 July 1898 – 3 December 1987) was a neurologist who made significant scientific contributions to the study of infectious diseases and neurology. He was born in Auxerre, France and died in Paris. A rare disease characterized by recurrent episodes of aseptic meningitis was discovered by Mollaret, and subsequently named after him - called Mollaret's meningitis, this disease is typically caused by herpes simplex virus infection of the brain.He is also credited with the characterization of a neural pathway known as the Guillain-Mollaret triangle or Myoclonic triangle, and the discovery of the causative agent of cat-scratch disease.

Pupillary light reflex

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

Reflex arc

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. However, the brain will receive the sensory input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). However, autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.During a somatic reflex, nerve signals travel along the following pathway:

Somatic receptors in the skin, muscles and tendons

Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem

An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse

Efferent nerve fibers carry motor nerve signals from the anterior horn to the muscles

Effector muscle innervated by the efferent nerve fiber carries out the response.A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b) carry the response generated by the spinal cord to effector organ(s) during a reflex action.

Stereognosis

Stereognosis (also known as haptic perception or tactile gnosis) is the ability to perceive and recognize the form of an object in the absence of visual and auditory information, by using tactile information to provide cues from texture, size, spatial properties, and temperature, etc. In humans, this sense, along with tactile spatial acuity, vibration perception, texture discrimination and proprioception, is mediated by the dorsal column-medial lemniscus pathway of the central nervous system. Stereognosis tests determine whether or not the parietal lobe of the brain is intact. Typically, these tests involved having the patient identify common objects (e.g. keys, comb, safety pins) placed in their hand without any visual cues.

Stereognosis is a higher cerebral associative cortical function.Astereognosis is the failure to identify or recognize objects by palpation in the absence of visual or auditory information, even though tactile, proprioceptive, and thermal sensations may be unaffected. It may be caused by disease of the sensory cortex or posterior columns. People suffering from Alzheimer's disease show a reduction in stereognosis. Astereognosis can be caused by damage to the posterior association areas of the parietal, temporal, or occipital lobes, or the postcentral gyrus of either hemisphere. For other types of dementia, stereognosis does not appear to decline.

Mechanism of neural pathway

The dorsal column medial lemniscus pathway is responsible for relaying sensory information regarding proprioception, vibration, and fine touch. First order neurons carry sensory information from proprioceptive and tactile receptors to the medulla oblongata where they synapse in either the medullary gracilus or cunate nuclei. Information carried from regions above spinal level T6 synapse at the cunate nuclei, while information carried from T6 and below synapse at the gracilus nuclei. At this point, second order neurons decussate and relay information through the contralateral medial lemniscus to the thalamus. At the thalamus, second order neurons synapse with third order neurons, which continue through the internal capsule to the primary sensory cortex of the post central gyrus where the tract terminates. Stereognosis determines whether or not this tract is properly functioning.An individual who cannot properly identify an object using stereognosis, could suffer from lesions in nerve roots, peripheral nerves, the spinal cord, thalamus, or primary sensory cortex. Because the dorsal column medial lemniscus pathway travels through and relays information to these areas, a lack of proper sensation indicates a problem with this neural sensory tract. Administered tests and recognition of pattern sensory loss can identify lesions in particular nerves or areas.

Substantia gelatinosa of Rolando

The apex of the posterior grey column, one of the three grey columns of the spinal cord, is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando (or SGR) (or gelatinous substance of posterior horn of spinal cord), which contains both neuroglia cells, and small nerve cells. The gelatinous appearance is due to a very low concentration of myelinated fibers. It extends the entire length of the spinal cord and into the medulla oblongata where it becomes the spinal nucleus of the trigeminal nerve.

It is named after Luigi Rolando.

It corresponds to Rexed lamina II.

Upper motor neuron lesion

An upper motor neuron lesion (also known as pyramidal insufficiency) occurs in the neural pathway above the anterior horn cell of the spinal cord or motor nuclei of the cranial nerves. Conversely, a lower motor neuron lesion affects nerve fibers traveling from the anterior horn of the spinal cord or the cranial motor nuclei to the relevant muscle(s).Upper motor neuron lesions occur in the brain or the spinal cord as the result of stroke, multiple sclerosis, traumatic brain injury and cerebral palsy.

Whitten effect

The Whitten effect occurs when male pheromones stimulate synchronous estrus in a female population.Social signals, or social stimuli, have an effect on reproduction in all mammals. For certain female mice, the pheromones contained in the urine of male mice can act as a social stimulus, and induce synchronous estrus.Estrus is a stage of the female reproductive cycle, and if a female is in estrus, it means that she is both fertile and sexually receptive. Synchronous estrus occurs when multiple females are in estrus at the same time.

When the pheromones contained in the urine of male mice stimulate synchronous estrus in a population of female mice, it is known as the Whitten effect. This is a phenomenon observed by Wesley K. Whitten (1956, 1966, 1968), whereby male mouse pheromone-laden urine synchronizes the estrus cycle "among unisexually grouped females," and is an example of male-to-female pheromonal effects in mice, similar to the Bruce effect.The Whitten effect occurs when a group of female mice are exposed to the urine produced by a male mouse. The male’s urine contains certain volatile, or airborne, pheromones that affect the hormonal processes of the females that control their reproductive status. A sexually mature and viable male must produce the urine, as the pheromones that produce the Whitten effect are dependent on male sex hormones such as testosterone.The female mice do not require direct contact with the male’s urine to produce the Whitten effect, as the pheromone contained in the urine is airborne and therefore is taken up by the females through their olfactory system. The reproductive cycle of female mice in isolation is approximately 4 to 5 days, and the reproductive cycles of grouped females are often longer and more irregular. However, when grouped female mice are exposed to the pheromones contained in a male’s urine, the Whitten effect occurs, and the majority of the female mice will enter a new estrus cycle by the third day of exposure. However, there is little evidence for a similarly functioninging vomeronasal, or olfactory, system (thought to be the sensory organ that initiates the Bruce, Vandenbergh, and Whitten effects) in humans. These differences, in putative stimulus and neural pathway (as well as species observed), stringently distinguishes the Whitten and McClintock effect, as the latter does not posit a role for male pheromones.

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