What Is the Olfactory Bulb?

Olfactory bulb: The olfactory bulb is a part of the telencephalon cortex. It is located below and at the front of the olfactory sulcus on the frontal orbital surface of the cerebral hemisphere. The olfactory nerve consisting of the central processes of the olfactory mucosa bipolar cells enters the frontal lobe through the holes of the sieve plate, and most of the fibers finally reach the front end of the olfactory bulb. Therefore, the olfactory bulb can be regarded as the terminating nucleus of the olfactory nerve and the primary center of the olfactory sense. A cluster of cells and fibers at the base of the skull. After the olfactory nerve fibers enter it, they are connected with other fibers to form an olfactory bundle, and then enter the brain forward.

Olfactory bulb: The olfactory bulb is a part of the telencephalon cortex. It is located below and at the front of the olfactory sulcus on the frontal orbital surface of the cerebral hemisphere. The olfactory nerve consisting of the central processes of the olfactory mucosa bipolar cells enters the frontal lobe through the holes of the sieve plate, and most of the fibers finally reach the front end of the olfactory bulb. Therefore, the olfactory bulb can be regarded as the terminating nucleus of the olfactory nerve and the primary center of the olfactory sense. A cluster of cells and fibers at the base of the skull. After the olfactory nerve fibers enter it, they are connected with other fibers to form an olfactory bundle, and then enter the brain forward.

Chinese name

Olfactory bulb

Foreign name

olfactorius bulbus, olfactory bulb

Brain area

Frontal lobe

Features

Participate in olfactory function

Anatomy of the olfactory bulb :

1. Olfactory bulb:

The olfactory bulb is an oval gray matter on the front end of each hemisphere, belonging to the olfactory brain. The olfactory bundle is connected backward, and the olfactory filament (olfactory nerve fiber) is connected to it in front. It has a layered structure, consisting of an olfactory nerve fiber layer, a gray matter layer, and an olfactory bundle fiber layer. Accepts afferent fibers of the olfactory nerve; emitted fibers project to the anterior olfactory nucleus, olfactory nodules, a portion of the amygdala, the piriform cortex, the septal nucleus, and the hypothalamus. It not only transmits olfactory impulses to the central relay station, but also collects olfactory impulses to regulate the introduction of olfactory information.

2. Telencephalon:

The telencephalon develops from the forebrain and includes the left and right hemispheres. The surface of each hemisphere is covered with a layer of cortex, called the cerebral cortex; the deep side of the cortex is the medulla, which consists of fibers that enter and leave the cortex and connect the cortex; the gray matter is buried in the medulla and is the basal nucleus; The internal ventricle is the lateral ventricle.

The earliest differentiation of the forebrain is related to the sense of smell. In fish and amphibians, there is only the olfactory leaf related to smell. The neocortex has only begun to sprout since the reptiles started. In the later evolution, the higher the animal, the more developed the neocortex. In humans, the neocortex accounts for 96% of the total cortex, and the rest belong to the ancient olfactory cortex. Under the influence of labor and language, the human cerebral cortex has acquired new properties, both in structure and function, and has produced human-specific thinking and consciousness activities.

The human hemisphere is highly developed. It covers the midbrain, and the back is covered with the midbrain and cerebellum. There is a longitudinal fissure between the left and right hemispheres. The hemisphere and cerebellum are separated by a split brain. The base of the longitudinal fissure has a corpus callosum connecting the two hemispheres. On the median sagittal section of the brain, the corpus callosum connects the end plate forward and downward, which is the foremost part of the early embryonic brain. From here, the wing plate of the neural tube expands outward into the cerebral hemisphere. Although the endplate constitutes the anterior wall of the third ventricle, in fact, the structure before the interventricular foramen and the optic cross line should belong to the telencephalon, which is the middle part connecting the left and right cerebral hemispheres.

Layering of olfactory bulb:

The olfactory bulb is a flat oval-shaped gray matter block, which is the primary center of smell. The olfactory bulb is located between the frontal lobe of the cerebral hemisphere and the ethmoidal plate of the ethmoid bone, and travels backwards through the olfactory tract. The olfactory nerve, composed of the central processes of the olfactory mucosa bipolar cells, finally this ball. The olfactory bulb can be divided into 6 layers from the surface inward: the olfactory nerve fiber layer, synaptic globule layer, outer granular layer, cap cell layer, inner granular layer, olfactory bundle fiber and so on. There are cap cells, brush cells, granule cells and short axis cells in the olfactory bulb. Each neuron and its protrusion form a complex synapse, which is connected with the nerve fibers that travel to and from the olfactory tract.

Olfactory and olfactory-related diseases:

Olfactory disorders:

Among the many causes of olfactory disorders, in addition to nasal and paranasal sinus diseases, secondary upper respiratory tract infections, head trauma and idiopathic causes account for more than 85% of the total. The high incidence of olfactory loss and hyposmia due to nasal and paranasal sinus disorders underscores the need for ENT examinations. Intracranial causes (apart from head trauma and intracranial surgery) rarely cause loss of smell.

Olfactory disorder is the main manifestation of olfactory nerve disease. Olfactory disorders include hyposmia, loss of smell, olfactory hallucinations, olfactory allergy, and abnormal smell. Olfactory decline and olfactory loss are mainly seen when the olfactory conduction pathway is blocked; olfactory hallucinations and olfactory allergies are mainly caused by olfactory central lesions. There are many reasons for olfactory disorders. In addition to those with olfactory nervous system disease, there are many other diseases that can also produce olfactory disorders, such as certain upper respiratory tract diseases that cause loss of smell, certain brain trauma and mass lesions Vascular diseases can also be caused. Some psychiatric patients (hysteresis, etc.) can have various olfactory disorders, and some are caused by congenital deficiency. Clinically important is the olfactory disorder caused by intracranial space-occupying lesions. Early detection is important for early diagnosis.

(A) olfactory sulcus meningiomas:

Olfactory loss can be the first symptom of olfactory meningiomas. If undetected, it often increases and causes epilepsy, vision loss, and dementia. Headaches often occur. Brain MRI can confirm the diagnosis.

(B) head trauma:

Olfactory loss may be caused by damage to the olfactory nerve from the ethmoidal plate into the skull, olfactory bulb damage, or cerebral cortex damage. Shear forces, fractures of the anterior cranial cavity, and direct bruises are the most common mechanisms. The incidence of olfactory loss and decline after head trauma is 10% to 20%, and is proportional to the severity of the injury. MRI changes are present in 88% of patients, mainly below the olfactory bulb and frontal lobe. More than 40% of cases can be recovered, but those who have not recovered more than 1 year after the injury cannot be recovered.

(C) aging:

Changes in olfactory sensation due to aging include decreased sensitivity, strength, discrimination, and discrimination. Age-related olfactory defects can alter food choices and impair nutritional status.

(4) Other reasons:

In Parkinson's disease, the extent and age of olfactory damage, the duration of the disease, the severity of the disease, impaired cognitive ability, and dopaminergic or cholinergic treatment are not relevant. Olfactory dysfunction occurs in 40% of patients with multiple sclerosis and is related to the number of demyelinating plaques on the MRI forehead and below the temporal lobe. Olfactory allergy is uncommon and can occur in depression and exposure to toxic gases. Olfactory inversion can occur during temporal lobe epilepsy or tumors, olfactory bulb damage, depression, or sinusitis. About 75% of patients with olfactory insufficiency are accompanied by hyposmia or loss of smell.

Olfactory hallucinations can occur during Alzheimer's disease, depression, schizophrenia, alcohol withdrawal, and sulciform epilepsy. During epilepsy, hallucinations are characteristically unpleasant or unpleasant, and rarely occur as isolated epileptic events. Smoking, drugs, radiotherapy, hemodialysis, and environmental and industrial toxins can cause olfactory disorders.