Krause end bulbs, also known as bulboid corpuscles, are a type of specialized sensory receptor found in various tissues throughout the body. They are part of the somatosensory system, translating physical stimuli into electrical signals that the nervous system can interpret. Like many nerve endings, Krause end bulbs are classified as encapsulated receptors, meaning the nerve fiber is enclosed within a protective layer of connective tissue. The specific type of sensation they detect has historically been debated, with modern studies suggesting a different primary role than early research.
Microscopic Structure of the End Bulbs
The physical anatomy of the Krause end bulb is centered around a terminal axon that is encased by a thin capsule. This enveloping layer is composed of a collagenous connective tissue sheath, often described as a perineurial capsule, which provides the structure with its distinct shape. The capsule itself frequently exhibits a multilayered organization, with concentric rows of flattened cells providing a compact enclosure.
Within this connective tissue sheath lies the inner core, which contains the terminal branches of the sensory nerve fiber. The myelinated axon loses its insulating layer shortly before entering the capsule, where it then forms a complex, nonmyelinated ending. This nerve ending is often described as coiled or branched, forming a dense mass within the soft core of the structure.
Krause end bulbs can appear as minute cylindrical, oval, or globular bodies, depending on their location. The presence of a multilayered structure gives them a superficial resemblance to other, larger encapsulated receptors, such as the Pacinian corpuscles. Some are classified as “complex” due to their tightly coiled axons, while “simple” forms contain a more linear axonal ending.
Distribution Across the Body
Krause end bulbs are not distributed uniformly across the skin but are concentrated in specific, sensitive areas of the body, particularly those featuring mucocutaneous tissues. They are notably prevalent in mucous membranes, the moist linings found at the body’s orifices. Examples include the mucosa of the lips and the tongue, where their dense concentration contributes to the high sensitivity of these areas.
A high density is also found in the conjunctiva, the transparent membrane covering the front of the eye and the inner surface of the eyelids. They are also found in transitional zones where skin meets mucous membrane, such as the nasal passage.
These nerve endings are further located in the connective tissue surrounding other structures, including the epineurium, the outermost sheath of peripheral nerve trunks. Certain joints, such as those in the fingers, also contain these structures, where they are sometimes referred to as articular end-bulbs. Their presence in the genitalia of both males and females, including the penis and clitoris, is so significant that they are sometimes termed genital corpuscles.
Sensory Function and Scientific Debate
Historically, the Krause end bulb was widely classified as a thermoreceptor specializing in the detection of cold temperatures. This traditional view suggested that these receptors began to fire when the skin surface dropped to approximately 35°C, with their maximum signaling rate occurring at temperatures between 20°C and 30°C. This designation placed them in a category distinct from mechanoreceptors, which sense physical deformation.
The modern scientific understanding of their function challenges this rigid historical designation. Many contemporary histological and physiological studies reclassify Krause end bulbs as low-threshold mechanoreceptors. This interpretation suggests they are primarily sensitive to light touch, pressure, or stretching of the surrounding tissue, rather than temperature alone.
This shift in understanding is supported by their structural similarity to certain mechanoreceptors, with their multilayered capsules suggesting a mechanism for sensing mechanical distortion. Recent, highly specific research has provided evidence for a mechanoreceptive role, particularly in the genital regions. Studies have identified these corpuscles as vibrotactile sensors that are acutely sensitive to vibration and mechanical stimulation.
In the genitalia, Krause corpuscles are thought to be crucial for normal sexual behavior in mammals, serving as highly specialized sensors for tactile feedback. The ongoing debate highlights the difficulty in assigning a single, simple function to encapsulated nerve endings, suggesting that their physiological role may vary depending on their specific anatomical location.