Pathophysiology of Sensory Neurons

The obligatory function of the sensorial nervous system of the body is operating sensory information. Sensory neurons, neuronal pathways and various compartments of the brain play their role in sensory perception.

Pathophysiology of Sensory NeuronsAbstract

Sensory information is carried out by receptive fields and processed in cranial and spinal nervous systems, Pathophysiology of Sensory or peripheral neurons lead to acute as well as chronic diseases. They may be due to genetic factors, chemotherapeutic side effects or toxic-metabolic abnormalities. The genetic factor is less common because normal functions of mutant proteins have been identified. SNDs most commonly occur due to neuron degeneration, loss of fibres or unmyelinated of axonal parts. Acquired SNDs relate with systemic immune-mediated disorders, vitamins intoxication or inadequacy, neurons toxicity and cancer. Chronic idiopathic sensory ataxic neuropathy (ISAN), carcinomatous neuropathy, Sjogren syndrome-associated neuropathy, acute autonomic and sensory neuropathy (AASN) are different types of sensory ataxic neuropathy. Pandysautonomia is due to cholinergic dysautonomia in which destruction of myelinated and unmyelinated nerves and loss of autonomic function occurs and the consequence is acute autonomic and chronic neuropathy. Guillian-Barre neuropathy is autonomic malfunction. Treatment through drugs comforts neuropathic anxiety but can leave side effects.

Introduction

Various senses are transformed from the physical scenario to the brain field where information is processed and create perception (Krantz and John, 2013). Gautama and Aristotle identify universally accepted, five normal senses i.e. touch, taste, smell, sight and hearing. Nociception, equilibrioception, kinesthesia and thermoception are other senses found in humans as well as mammals. Some alternated senses like magnetoception and electroreception are discovered in nonhuman animals (Hofle et al; 2010).

The sensory neurons dorsal ganglionic bodies are present in the dorsal ganglionic region of the spinal cord (Purves et al; 2008). Afferent neurons carry out sensory transduction. They have ability to convert a particular type of stimulus, using receptors, and form action/graded potential (Parson and Richard, 2018).

Three types of issues are solved by organisms by using information. Homeostasis, synchronizing various functions with conspecifics and detection and responding of resources and danger. Behaviour is also built up due to the transformation and processing of information. Like identification and notification of conspecifics of threats, cooperative actions or to deceive (Bowdan and Wyse, 1996).

Physiology of sensory neurons

Receptor organ and receptor cells together form the receptive field in the body. For example, the receptive field of an eye, it is every rod and cone is about what part of world and colour can be detected by them respectively (Kolb and Whishaw, 2003). Visual, auditory and somatosensory systems have exhibited their receptive fields.

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Different types of sensory receptors are present on sensory neurons. These respond to various types of stimuli either external or internal. External receptors include smell, taste, vision, auditory, temperature and mechanoreceptors. On the other hand internal receptors include blood and nociceptors. Cranial nerves are responsible for transferring information from head to the central nervous system which is conveyed by sensory neurons. Data in the lower part of the head is carried out through the 31st spinal nerves pair towards the brain from spinal cord sensory neurons (Kalat and James, 2013).

When a specific receptor responds to a physical stimulus the sensation is initialized. These types of receptors which respond to specific stimuli are classified into four different groups i.e. chemoreceptors, photoreceptors, mechanoreceptors and thermoreceptors. They encounter with specific environmental stimuli and transform the signals into an electrical action potential which is passed to distinct brain regions via afferent neurons and it is analyzed as well as interpreted there.

Pathology of sensory neurons

            Sensory neuropathy was reported by Denny-Brown in 1948 and mostly referred to describe the disorders characterized by the primary deterioration i.e. demyelination of sensory neurons of the peripheral region (Denny-Brown, 1948). Sensory neuropathies are frequently discussed under the chronic diseases, chemotherapy reactions or harmful metabolic irregularities and they are conspicuously produced by genetic mutations, but these mutated proteins have been identified as performing normal functions especially exposures that implicate mitochondrial dynamics and axonal transport mechanisms. Phenotypical associated diseases, generated due to monogenic mutation include hereditary sensory and autonomic neuropathies which influence sympathetic as well as sensory neurons. Disturbance in axonal transport is also a genetic aspect of hereditary neuropathies i.e. genetic sensory and autonomic neuropathies. Sympathetic and sensory neurons criticality depends on specific nerves growth factors signalling which is sometimes disrupted by certain confluence on gene mutation (Tourtellotte, 2016).

Acquired Sensory neurons diseases (SNDs) show chronic or subacute time period and relate with systemic immune-mediated diseases, intoxication or inadequacy of vitamins, neurotoxicants and uncontrolled cell division. Often they are referred to as idiopathic but can be considered as a hereditary transmitted abnormality and show specific clinic picture. Destruction of dorsal root ganglionic regions sensory neurons is a source of declination of short and long axons of peripheral region and central sensory projections of posterior columns regions and consequence of this procedure of pathological importance are length independent pattern of sensory nerves degeneration. It explains discrete clinical and neurophysiological disorders (Sghirlanzoni et al; 2005).

Various forms of Neuropathies      

The lesions of sensory ataxic neuropathy include different types of neuropathies i.e. chronic idiopathic sensory ataxic neuropathy, cancer-causing disorders, Sjogren syndrome-associated neuropathy and acute autonomic and sensory neuropathies are large-diameter sensory neurons and column of the spinal cord and large myelinated fibres in peripheral nerves trunks (Sobue, 1996). SANs are specified by disappearing of proprioceptive sensation and conservation of muscle strength. Studies reveal that idiopathic neuropathies are due to toxic, infectious or autoimmune diseases. Reactivity against gangliosides containing disialosyl groups, usually GDlb, has been described in isolated cases of acute and chronic idiopathic ataxic neuropathies (iSAN) and different experimental findings (in vivo animal models and in vitro preparations) suggest that antidisialosyl or antiSDlb antibodies might have a key part in the pathological pathway of ataxic neuropathies (Illa et al; 2001).

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Severe sympathetic and parasympathetic impairment with partial or complete conservation of somatic motor and sensory purpose result in acute pandysautonomia and acute sensory neuropathy. Some cases have only shown cholinergic dysautonomia, while others have presented desolation of the autonomic role along with other abnormalities of nervous system. Coalin et al; 1980, reported a patient suffering from acute autonomic and sensory neuropathy, manifesting severe sensory disability and disautonomia with marked loss of myelinated and unmyelinated fibres. Several other similar cases have appeared in literature. It is not clear whether this disorder is a new syndrome that is different from acute pandysautonomia or merely a subtype of it. The causes of above two syndromes are unknown. Acute pandysautonomia which includes acute autonomic and sensory neuropathy becomes more common. An account of such cases has been reported globally which includes many areas of Japan (Okada and Hokkaodo, 1998).

Detection of small fibre neuropathies and large fibre neuropathies

Malfunction of small fibres can be observed in isolation or linked with lengthy fibres disorders. Autonomic failure intensifies the diagnosis in Guillain-Barre syndrome. Recently, chronic idiopathic distal small-fibre neuropathy is often detected and more highlighted abnormalities, diabetic and amyloidotic polyneuropathies are very prevalent. In real autonomic failure, autonomic failure of peripheral nerves occurs which creates a difference with multiple-system atrophy. When small fibres which carry out functions of malfunctioning signs as well as symptoms and as a result instrumental recordings are the only choice for accurate treatment, this severe condition is complex regional syndrome. Large myelinated fibres are predominantly evaluated by standard neurophysiological techniques. Somatic neuropathy includes small-fibre neuropathy while thin myelinated and non-myelinated fibres work as thermoreceptors, nociceptors as well as performs autonomic function. Some clinical conditions require full autonomic evaluation for diagnosis of autonomic dysfunction in orthostatic intolerance syndrome (Santiago et al; 2000).

One of the Pathophysiology of Sensory Neurons related disease is Phantom limb syndrome in which amputees feel pain in their amputated limbs. To relief the pain which Phantom limb syndrome patient senses, V. S. Ramachandran developed a mirror box. It is a simple device with a mirror box which generates an illusion to show two limbs, instead of one by sensory system perception. In this way Phantom limb syndrome is controlled by reducing pain. This method acclimatizes amputated limb and quietens the syndrome (Blakeslee et al; 1998).

Current Status of Sensory Neurons Pathophysiology

Pathophysiology of Sensory Neurons related diseases are being cured by the use of drugs available in the market at present. For example, Gabapentin is used for the treatment of neuropathic pain when an interaction is developed between among voltage-dependent calcium channels located on non-receptive neurons (Lee et al; 2005). Along with that, many other drugs used for the treatment of other ailments can cause side effects which can disturb nervous system i.e. aminoglycoside antibiotic is an ototoxic drug and influence cochlea but it also causes damage to stereocilia. Due to hair cells damage, their K+ pumping is stopped. As a result, energy is not produced by endochoclear potential and auditory signals are not transduced, thus leading to deafness (Priuska and Schacht, 1997).

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Conclusion

Sensory nervous system transduces the information from the external environment to the nervous system and brain where interpretation occurs, creating perception. Except for major five senses, nociception, equilibrioception, kinesthesia and thermoception are present in human and magnetoception and electroception has also been shown in some non-human animals. Animals’ sensory perception takes place via receptive field. Sensory information is processed in cranial as well as spinal nerves of brain and spinal cord respectively. Initiation of sensation process occurs through commonly characterized distinct categories of receptors i.e. chemoreceptors, photoreceptors, mechanoreceptors and thermoreceptors.

Summary  

Pathophysiology of sensory neurons describes the diseases in which axonal parts are demyelinated. However, Peripheral neuropathies lead to chronic diseases. This may occur due to the reaction of chemotherapy or poisonous metabolic disorders.

Genetic factors are also involved to some extent. Gene mutation interferes with nerve growth factors signalling upon which sympathetic and sensory neurons criticality depends. Acquired Sensory neurons diseases (SNDs) show chronic or subacute time period. SND’s relate with systemic immune-mediated diseases, intoxication or inadequacy of vitamins, neurotoxicants and uncontrolled cell division. It explains discrete clinical and neurophysiological disorders. The lesions of sensory ataxic neuropathy include different types of pathophysiology of sensory neurons.

Studies reveal that idiopathic neuropathies are due to toxic, infectious or autoimmune diseases. Reactivity against gangliosides containing disialosyl groups, usually GDlb, has been described in isolated cases of acute and chronic idiopathic ataxic neuropathies and results in antidisialosyl or antiSDlb antibodies might have a key part in the pathology of some ataxic neuropathies. Acute pandysautonomia which includes acute autonomic and sensory neuropathy becomes more common.

An account of such cases has been reported globally which includes many areas of Japan. Chronic idiopathic distal small-fibre neuropathy is often detected as well as highlighted abnormalities, diabetic and amyloidotic polyneuropathies, small-fibres dysfunction is highly prevalent. In real autonomic failure, autonomic failure of the peripheral region takes place which creates difference with multiple-system atrophy. Large myelinated fibres are predominantly evaluated by standard neurophysiological techniques. Somatic neuropathy includes small-fibre neuropathy while thin myelinated and non-myelinated fibres work as thermoreceptors, nociceptors as well as performs autonomic function. Phantom limb syndrome in which amputees feel pain in their amputated limbs. To relief the pain which Phantom limb syndrome patient senses, V. S. Ramachandran developed a mirror box. Some drugs used for the cure of other health problems can cause side effects on voltage-dependent calcium channels.

Zohaib Saleem

Zohaib Saleem

Basically I am a veterinarian completed my DVM from the University of Agriculture Faisalabad. Currently, I am pursuing my M.Phil in Physiology. My research interest is oncosuppressor genes and bacteriophage therapy.

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