When the terminals of sensory nerves do not show any particular specialisation of structure they are called free nerve endings. Such endings are widely distributed in the body. They are found in connective tissue. They are also seen in relation to the epithelial lining of the skin, cornea, alimentary canal, and respiratory system.
Free nerve endings are particularly numerous in relation to hair follicles. They respond mainly to deformation of hair i.e., they are fast adapting mechanoreceptors. The abundance of free nerve endings in relation to hair follicles is to be correlated with the fact that hair increase the sensitivity of skin to touch. Free nerve endings may also be thermoreceptors and nociceptors.
Some free nerve endings present in relation to hair follicles are described as lanceolate endings. These terminals are seen running along the hair root, below the opening of the sebaceous duct. The terminations of the nerve fibres are flattened with sharp edges that make direct contact with epithelial cells of the hair root.
These are small oval or cylindrical structures seen in relation to dermal papillae in the hand and foot, and in some other situations. These corpuscles are believed to be responsible for touch. They are slow adapting mechano-receptors.
Pacinian corpuscles are circular or oval structures. These are much larger than tactile corpuscles. They may be up to 2 mm in length, and up to 0.5 mm across. They are found in the subcutaneous tissue of the palm and sole, in the digits, and in various other situations. Lamellated corpuscles are believed to be fast adapting mechanoreceptors specially sensitive to vibration. They also respond to pressure.
Each corpuscle has a capsule, an intermediate zone, and a central core. The capsule is arranged in about thirty concentric layers (like the layers of an onion). The intermediate zone is cellular. The core consists of an outer layer of cells from which cytoplasmic lamellae project inwards and interdigitate with each other. In the centre of the core there is generally a single nerve fibre. The terminal part of the fibre is expanded into a bulb. Pacinian corpuscles are supplied by thick myelinated nerve fibres (Type A).
These are spherical structures about 50 μm in diameter. They consist of a capsule within which a nerve fibre terminates in a club-shaped manner. Their significance is controversial. Some authorities regard them to be degenerating or regenerating terminals of nerve fibres rather than as specialised endings.
These are small disc-like structures seen in relation to specialised epithelial cells (Merkel cells) present in the stratum spinosum of the epidermis. The discs are expanded ends of nerve fibres. Merkel cells bear spine-like protrusions that interdigitate with surrounding epidermal cells. Tactile menisci are slow adapting mechanoreceptors sensitive to pressure. They are supplied by large myelinated nerve fibres. Apart from surface epithelium of the skin, Merkel cell receptors may be found in relation to the sheaths of hair follicles.
These are spindle-shaped structures present in the dermis of hairy skin. Some are also found in non-hairy skin. Similar receptors are also present in relation to joints, in the gums, and in the glans penis.
Within a fibrocellular sheath there are collagen fibres amongst which there are numerous unmyelinated endings of myelinated nerve fibres. Ruffini endings are slow adapting cutaneous mechanoreceptors responsive to stresses in dermal collagen. They resemble the Golgi tendon organs described below.
We can summarise the functions of cutaneous receptors as follows.
a. Merkel discs and Ruffini endings are slowly adapting mechanoreceptors.
b. Pacinian corpuscles and some types of free nerve endings act as rapidly adapting mechanoreceptors.
c. Other free nerve endings act as nociceptors and thermoreceptors.
d. Pacinian corpuscles and Ruffini endings lie deep to skin in the dermis or in tissue deep to skin. Their receptive fields are large and sensations mediated through them are not accurately localised. Pacinian corpuscles are useful mainly for appreciation of vibration. Ruffini endings respond to stretching of the dermis.
e. In contrast to Pacinian corpuscles and Ruffini endings, Merkel cell receptors and Meissner’s corpuscles have small receptor fields (specially over the fingers) and allow good tactile localisation.
f. Apart from their sensory functions afferent nerve fibres may play a role in inflammation and repair of tissue, probably by releasing peptides (in particular substance P) at their endings. However, these views are not fully established at present.
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