What is facet joints, mechanoreceptors and nociceptors?

*Please note that the following content is solely a brief introduction to facet joint type I-IV nerve endings for the purpose of promoting the clarification of chiropractic care.

Facet joints are joints found in the spine that guides the normal movement of the spine. Mechanoreceptors are receptors found within the tissue of the capsule of the facet joint which tell the brain continuously the position and movement of the spine for the purpose of providing more stability to the spine, as well as aid in the normal movement of the spine by influencing the activity of the associated muscles which serve to move the spine. Nociceptors are pain receptors found within the tissue of the capsule of the facet joint and when activated cause the sensation of spinal pain; neck pain, back pain and low-back pain.

The central nervous system is divided into a cephalic (upper) brain and caudal spinal cord (below). The spinal cord with its associated spinal nerves are found within a cavity that is made of the joining of successive vertebral bodies and their vertebral arches (bones of the spine) with intervertebral discs in between the vertebrae (serve as cushions between the vertebral bodies); and are collectively known together with the spinal cord as the spine. Facet joints are diarthrotic joints of the spine and are formed by the articulation of the superior and inferior articular process of adjacent vertebrae. The facet joint is composed of a fluid-filled space bordered by hyaline cartilage over the subchondral bone and is enclosed by a synovial membrane and a thin fibrous capsular ligament known as the facet joint capsule or facet joint capsular ligament. Invaginations of the facet joint capsule form synovial folds or menisci in the synovium deep to the facet joint capsule which fill voids, absorb shock, distribute pressure and maintain stability during facet joint articulation (during movement of the spine). The outer layer of the facet joint capsule is densely packed with parallel bundles of collagen and the inner layer is composed of elastic fibers. The facet joint capsular tissue is well innervated by the medial branch of the dorsal rami (part of spinal nerves). Each medial branch segmentally innervates two adjacent facet joints, thus providing dual innervation. The extracellular matrix of the hyaline cartilage covering the facet joint surfaces is mainly made of collagen and aggrecan macromolecules. Collagen is the mostly abundant macromolecule of the two and is a highly organized structure to provide tensile and shear strength during facet joint loading. Aggrecan is a large proteoglycan which is loaded with numerous charged glycosaminoglycan molecules to provide compressive strength during facet joint loading. The facet joints restraint vertebral motion and guide and limit intersegmental motion. During axial loading of the spine between two adjacent vertebrae, the load transmission is shared by the intervertebral disc anteriorly (in front) and by the two segmental facet joints posteriorly (behind). The amount of axial loading on the facet joints is dependent on posture, segmental alignment and the presence of degenerative changes. Normally, between 5% and 25% of segmental loading is placed on the facet joints, and increase with extension movement of the spine. In the setting of degenerative changes; the intervertebral disc height reduces and the segmental loading on the facet joints can increase to about 50%. In addition, the facet joints limit axial rotation and stabilize the motion spinal segment to prevent translation and dislocation during flexion and extension movements of the spine. During flexion movement of the spine; the forward shear forces acting on the facet joint capsular tissue increases, particularly in the lower lumbar region of the spine due to the higher weight and longer leverage of the lumbar vertebral bodies. In lower levels of the spine; the orientation of the facet joint increase sagittaly to adapt to the forward shearing forces on the facet joints.

Nerve endings in joints can be classified into four types of nerve endings, namely type I-IV. Ruffini end-organs are encapsulated type I nerve endings; Pacinian corpuscles are encapsulated type II nerve endings; and Golgi endings are encapsulated type III nerve endings. Joint type I-III nerve endings are supplied by large myelinated Aβ-fibre afferent. Free nerve endings are non-capsulated type IV nerve endings and are supplied by myelinated Aδ-fibres or unmyelinated C-fibres. There are two types of free nerve endings; type IVa which have a vasomotor function and is associated with blood vessels, and type IVb which lies between the collagen and elastic fibres of joint ligamentous and capsular tissues. Both type IV nerve endings signal pain and are classified as nociceptors. Histology and anatomical studies have identified the existence of type I-IV nerve endings, substance P (pain mediator) and calcitonin gene-related peptide (pain mediator) in the facet joint capsular tissue and synovium. Inflammatory cytokines (these expressions initiate inflammation and produce pain as part of the normal physiological mechanism of the human body in response to injury) have also been identified in the facet joint’s synovium and cartilage. Cartilage erosion channels along with substance P, innervation of the subchondral bone of the facet joint has also been linked to synovial joint pain. Each of these receptors have relatively large receptive fields and when stimulated can possibly be sufficient to cover an individual facet joint.

Ruffini end-organ nerve endings are slowly adapting, low-threshold, static and dynamic mechanoreceptors. They have been found around the collagen fibers of the superficial layers of the facet capsular tissue and are stimulated by the displacement of collagen. Type I joint mechanoreceptors signal a change in intra-articular pressure and static joint position, as well as the direction, amplitude and velocity of joint movement. They are therefore extremely sensitive to a change in capsular stretch. Pacinian corpuscle nerve endings are rapid adapting, low threshold and dynamic mechanoreceptors. They have been found in the deeper layers of the facet capsular tissue. Type II joint mechanoreceptors signal dynamic joint position and a change in stress applied to the joint capsular tissue during the joint movement; from the start to end and during deceleration and acceleration. Also, Pacinian corpuscle is stimulated mainly by compression and tensile loading on the joint capsular tissue and is also sensitive to a change in capsular stretch. Golgi ending nerve endings are very slowly adapting, high threshold and dynamic mechanoreceptors. These encapsulated nerve endings have been found at the junction between the inner and more superficial layers of the facet joint capsular ligament. Type III joint mechanoreceptors signal extreme range of joint movement and are stimulated by tension in the joint capsular tissue. They are also sensitive to a change in capsular stretch. Joint mechanoreceptors play an important role in maintaining articular (joint) congruity; assist in distributing the load on a joint; influence the activity of the muscles crossing the joint (spinal segmentally innervated muscles) via arthrokinetic reflex activity; and relay information to the central nervous system via the posterior-medial lemniscal pathways reporting conscious proprioception and via the spinocerebellar pathways reporting non-conscious proprioception (proprioception is the ability of the human body to be continuously aware where his/ her body parts are around him/ her in space).

Joint pain receptors or known as nociceptors are non-adapting and high threshold receptors and have been found in both the superficial and inner capsular tissue and in the synovial tissues of the facet joints. The free nerve endings modulate pain on the basis of the character of the insult; inflammatory, metabolic, vascular or mechanical stimuli. Facet joint nociceptors are likely to be stimulated when the facet joint capsule is stretched; during facet joint over loading; in the presence of inflammation; and during degenerative changes occurring in the joint. Thus, irritation of the mechanical or chemical joint nociceptors in the facet joint capsular tissue and synovium may produce facet joint pain. Articular nociceptors send their nociceptive information to the central nervous system by way of the anterolateral system (spinothalamic tracts) for pain perception (the sensation of pain).