Olfactory system
The olfactory system is the sensory system used for the sense of smell. Olfaction is one of the special senses directly associated with specific organs. Most mammals and reptiles have a main olfactory system and an accessory olfactory system. The main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli.
The senses of smell and taste are often referred to together as the chemosensory system, because they both give the brain information about the chemical composition of objects through a process called transduction.
Structure
Peripheral
The peripheral olfactory system consists mainly of the nostrils, ethmoid bone, nasal cavity, and the olfactory epithelium. The primary components of the layers of epithelial tissue are the mucous membranes, olfactory glands, olfactory neurons, and nerve fibers of the olfactory nerves.Odor molecules can enter the peripheral pathway and reach the nasal cavity either through the nostrils when inhaling or through the throat when the tongue pushes air to the back of the nasal cavity while chewing or swallowing. Inside the nasal cavity, mucus lining the walls of the cavity dissolves odor molecules. Mucus also covers the olfactory epithelium, which contains mucous membranes that produce and store mucus, and olfactory glands that secrete metabolic enzymes found in the mucus.
Transduction
in the epithelium detect odor molecules dissolved in the mucus and transmit information about the odor to the brain in a process called sensory transduction. Olfactory neurons have cilia containing olfactory receptors that bind to odor molecules, causing an electrical response that spreads through the sensory neuron to the olfactory nerve fibers at the back of the nasal cavity.Olfactory nerves and fibers transmit information about odors from the peripheral olfactory system to the central olfactory system of the brain, which is separated from the epithelium by the cribriform plate of the ethmoid bone. Olfactory nerve fibers, which originate in the epithelium, pass through the cribriform plate, connecting the epithelium to the brain's limbic system at the olfactory bulbs.
Central
The main olfactory bulb transmits pulses to both mitral and tufted cells, which help determine odor concentration based on the time certain neuron clusters fire. These cells also note differences between highly similar odors and use that data to aid in later recognition. The cells are different with mitral having low firing-rates and being easily inhibited by neighboring cells, while tufted have high rates of firing and are more difficult to inhibit. How the bulbar neural circuit transforms odor inputs to the bulb into the bulbar responses that are sent to the olfactory cortex can be partly understood by a mathematical model.The uncus houses the olfactory cortex which includes the piriform cortex, amygdala, olfactory tubercle, and parahippocampal gyrus.
The olfactory tubercle connects to numerous areas of the amygdala, thalamus, hypothalamus, hippocampus, brain stem, retina, auditory cortex, and olfactory system. In total it has 27 inputs and 20 outputs. An oversimplification of its role is to state that it:
- checks to ensure odor signals arose from actual odors rather than villi irritation,
- regulates motor behavior brought on by odors,
- integrates auditory and olfactory sensory info to complete the aforementioned tasks, and
- plays a role in transmitting positive signals to reward sensors.
The bed nuclei of the stria terminalis act as the information pathway between the amygdala and hypothalamus, as well as the hypothalamus and pituitary gland. BNST abnormalities often lead to sexual confusion and immaturity. The BNST also connect to the septal area, rewarding sexual behavior.
Mitral pulses to the hypothalamus promote/discourage feeding, whereas accessory olfactory bulb pulses regulate reproductive and odor-related-reflex processes.
The hippocampus receives almost all of its olfactory information via the amygdala. The hippocampus forms new memories and reinforces existing ones.
Similarly, the parahippocampus encodes, recognizes and contextualizes scenes. The parahippocampal gyrus houses the topographical map for olfaction.
The orbitofrontal cortex is heavily correlated with the cingulate gyrus and septal area to act out positive/negative reinforcement. The OFC is the expectation of reward/punishment in response to stimuli. The OFC represents the emotion and reward in decision making.
The anterior olfactory nucleus distributes reciprocal signals between the olfactory bulb and piriform cortex. The anterior olfactory nucleus is the memory hub for smell.
When different odor objects or components are mixed, humans and other mammals sniffing the mixture are often unable to identify the components in the mixture even though they can recognize each individual component presented alone. This is largely because each odor sensory neuron can be excited by multiple odor components. It has been proposed that, in an olfactory environment typically composed of multiple odor components, feedback from the olfactory cortex to the olfactory bulb suppresses the pre-existing odor background via olfactory adaptation, so that the newly arrived foreground odor can be singled out from the mixture for recognition. Image:Olfactory system.svg|thumb|280px|1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus 6: Olfactory receptor cells
Clinical significance
Loss of smell is known as anosmia. Anosmia can occur on both sides or a single side.Olfactory problems can be divided into different types based on their malfunction. The olfactory dysfunction can be total, incomplete, distorted, or can be characterized by spontaneous sensations like phantosmia. An inability to recognize odors despite a normally functioning olfactory system is termed olfactory agnosia. Hyperosmia is a rare condition typified by an abnormally heightened sense of smell. Like vision and hearing, the olfactory problems can be bilateral or unilateral meaning if a person has anosmia on the right side of the nose but not the left, it is a unilateral right anosmia. On the other hand, if it is on both sides of the nose it is called bilateral anosmia or total anosmia.
Destruction to olfactory bulb, tract, and primary cortex results in anosmia on the same side as the destruction. Also, irritative lesion of the uncus results in olfactory hallucinations.
Damage to the olfactory system can occur by traumatic brain injury, cancer, infection, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. These conditions can cause anosmia. In contrast, recent finding suggested the molecular aspects of olfactory dysfunction can be recognized as a hallmark of amyloidogenesis-related diseases and there may even be a causal link through the disruption of multivalent metal ion transport and storage. Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy.
Doctors must exclude other diseases that inhibit or eliminate 'the sense of smell' such as chronic colds or sinusitis before making the diagnosis that there is permanent damage to the olfactory system.
Prevalence of olfactory dysfunction in the general US population was assessed by questionnaire and examination in a national health survey in 2012–2014. Among over a thousand persons aged 40 years and older, 12.0% reported a problem with smell in the past 12 months and 12.4% had olfactory dysfunction on examination. Prevalence rose from 4.2% at age 40–49 to 39.4% at 80 years and older and was higher in men than women, in blacks and Mexican Americans than in whites and in less than more educated. Of concern for safety, 20% of persons aged 70 and older were unable to identify smoke and 31%, natural gas.
Causes of olfactory dysfunction
The olfactory system is a vital sense, and its dysfunction may lead to a reduced quality of life, an inability to determine hazardous odors, decreased pleasure in eating, and poor mental health.The common causes of olfactory dysfunction include advanced age, viral infections, exposure to toxic chemicals, head trauma, and neurodegenerative diseases.