Neurogenesis in the olfactory epithelium continues throughout the entire life of mammals, and it is the axons of these newly formed olfactory receptor neurons that grow into the target tissue after the first cranial nerve is injured, not the regenerating axons of mature cells. These axons are able to enter and grow within the CNS of adult animals, unlike regenerating axons in injured dorsal roots, the majority of which are prevented from penetrating very far into the spinal cord. One reason why the olfactory axons are so successful in entering the CNS may be due, at least partially, to the fact that they are ensheathed by a type of glial cell (the ensheathing cell) that expresses phenotypic features of both astrocyte and Schwann cells. The presence of both L1/Ng-CAM and N-CAM in the plasma membranes of both ensheathing cells and immature olfactory receptor neurons would enable the olfactory axons to use the glial cell surfaces as a substratum on which to grow. It is probably also true that ensheathing cells synthesize and secrete laminin, thus providing an additional adhesive substrate for the olfactory axons, as well as glia-derived nexin and nerve growth factor, both of which are neurite-promoting agents.

Glial influences on axonal growth in the primary olfactory system.

The general features of the olfactory system are remarkably consistent across vertebrates. A phylogenetic analysis of central olfactory projections indicates that at least three distinct olfactory subsystems may be broadly present in vertebrates and that a fourth, the accessory olfactory or vomeronasal system, arose in tetrapods. The origin and function of the vomeronasal system have been the subject of much controversy, but some conclusions can be drawn. The vomeronasal system did not arise as an adaptation to terrestrial life, as indicated by the presence of a vomeronasal system in modern aquatic amphibians and the increasing paleontological evidence that the last common ancestor of amphibians and amniotes was aquatic. The vomeronasal system is involved in both foraging and reproductive behaviors in reptiles and has been shown to be involved in some pheromonally mediated behaviors in mammals. However, among mammals, some pheromonal responses are not mediated by the vomeronasal system, and the possible involvement of the vomeronasal system in other type of behaviors has not yet been investigated. Thus, the relative functions of the olfactory and vomeronasal systems of tetrapods remain unclear. Other hypotheses that features of the olfactory system are specialized for aquatic chemoreception or for pheromone detection are similarly insupportable. For example, the suggestion that members of the olfactory receptor family can be separated into two groups that function for transduction of air-borne or water-borne odorants is contradicted by the presence of both groups in aquatic amphibians and by a phylogenetic analysis of the sequences for these genes. Interestingly, the putative odorant receptors from the vomeronasal epithelium share little sequence similarity with those from the olfactory epithelium, indicating that these receptors may have been independently co-opted from the larger family of seven transmembrane domain receptors for use in odor transduction. A phylogenetic analysis of the distribution of olfactory receptor cell types indicates that microvillar olfactory receptor cells are widespread among vertebrates and are not restricted to aquatic animals or to the vomeronasal epithelium of tetrapods. Previous suggestions that all microvillar receptor cells are specialized for the detection of pheromones are not tenable. Attempts to recognize features of the olfactory system that are common to all vertebrates and might be specialized for the detection of pheromones vs. more general odorants, or for the detection of water-borne vs. air-borne odorants, are not supported by current evidence.

Evolution of Vertebrate Olfactory Systems

Molecular Neurobiology of the Olfactory System

The characteristics of the electro-olfactogram (EOG): its loss and recovery following olfactory nerve section in rainbow trout (Salmo gairdneri).

The initial bearings of pigeons are commonly not only homeward oriented, but also polarized toward a preferred compass direction (PCD). The PCD is loft-specific; at ten out of eleven loft sites investigated it is in the western semicircle. It can become shifted in the course of general homing experience. The PCD appeared stable against several experimental treatments (long-term clock shifts, olfactory deprivation, exposure to different winds at the home site), but could be reduced or reversed, respectively, by certain screenings of the aviary in which the birds were raised. It is concluded that “release-site biases” are not necessarily caused by distortions of the physical environment.

Preferred compass directions in initial orientation of homing pigeons

Homing pigeons were subjected to uni- or bilateral resection of the olfactory nerves. A control of the operated region carried out five months later, showed that the olfactory nerves were continuous from the olfactory mucosa to the bulb. Cross sections of the regenerated nerve, observed under the electron microscope, showed morphological alterations with respect to the normal. Total measurement of the cross surface of both olfactory nerves showed that the regenerated nerve is always smaller than the intact nerve; this is also apparent in the total number of the fibres counted. Electrophysiological tests, performed on the regenerated nerve showed that the conduction along the resected nerve is restored.

Olfactory nerve reconstitution in the homing pigeon after resection: ultrastructural and electrophysiological data.

Experimental resection of the olfactory nerve in the homing pigeon induces a total degeneration of the nerve and olfactory epithelium. The orthograde degenerative process starts before the retrograde one. Ten days after resection, new neurons begin to differentiate from the basal cells. The axon forms earlier than the distal dendritic process, and the speed of growth increases slowly. The regenerated axons only reach the bulb in the 5th month. Two months after resection the olfactory epithelium is similar to that of the intact control side. The ultrastructural features of the mucosa and olfactory axons are similar to those of normal ones.

Degenerative and regenerative processes in the olfactory system of homing pigeons.

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

A. Lanfranchi

Author Tools

Chat about Author

Papers

Olfactory nerve reconstitution in the homing pigeon after resection: ultrastructural and electrophysiological data.

Degenerative and regenerative processes in the olfactory system of homing pigeons.