4 Joint Senses and Proprioception

Elisabet Hagert and Susanne Rein

4 Joint Senses and Proprioception

“The experience of hands is tactile …

They live in the land of feeling where touch is everything And where the mystery of touch is the bridge between nerve and soul.”

—Harry Martinsson, Human Hands ¹

4.1 Introduction

Martinsson’s poem about the human hand ¹ and its intimate role in bridging the outer world with our innermost perceptions is an eloquent description of the role of proprioception and joint sense in allowing us conscious and unconscious interaction with the world around us. Although tactility pertains to the sensory role of the skin, this sensory function originates from fine nerve endings and mechanoreceptors located primarily in the pulp of the fingertips. These mechanoreceptors are also present in the joints of the human hand, indicating that joints, in addition to their mechanical function, have a sensory function. In this chapter, we briefly describe the known sensory innervation of joints in the human hand, the different joint senses, and their respective function in hand proprioception.

4.2 Innervation of Joints in the Human Hand

Sensory nerve endings, so-called mechanoreceptors, are the primary focus of innervation studies in joints. They are able to detect mechanical stimuli, such as changes in joint position and velocity, transform them into neural excitations, and signal this information from the joint via afferent nerves and dorsal root ganglia to the spinal cord. Sensory nerve endings in ligaments are classified according to Freeman and Wyke ² into four types, based on their typical shape (▶Fig. 4.1) and neurophysiological traits (▶Table 4.1). Sensory nerve endings are found mostly close to ligament insertions into bone as well as in the epiligamentous region of ligaments, ³ ^– ⁶ where they can act as monitors of tension and force applied to the ligament. ⁷

**Fig. 4.1** Immunohistochemical staining of sensory nerve endings using low-affinity nerve growth factor receptor p75 (p75) (magnification × 400). (a) A Ruffini ending characterized by p75 immunoreactive (IR) dendritic nerve endings (*arrowhead*), a clearly visible central axon without IR (*arrow*), and a thin, at times partial, encapsulation of the corpuscle. (b) In contrast, the Pacini corpuscle has an onion-layered p75 IR capsule (*arrowhead*) and central axon (*arrow*). (c) The Golgi-like ending is larger, with an afferent nerve fascicle (*arrow*) coursing to the center of the corpuscle. Typically smaller corpuscles within the Golgi-like ending are seen (*arrowhead*). (d) Finally, free nerve endings (*arrow*) are p75 IR and often located close to vessels (*star*).

**Table 4.1** Classification of mechanoreceptors in ligaments based on Freeman and Wyke, ² modified by Hagert, ⁸ outlining the morphology and function of the various sensory nerve endings found in the human hand
Type	Eponym/Name (descriptive)	Characteristics	Neurophysiological trait	Role in joint function	IR patterns in mechanoreceptors
I	Ruffini (dendritic)	Coil-shaped. Partial encapsulation. Arborizing nerve branches with bulbous terminals. 50–100 µm.	Slowly adapting Low-threshold	Static joint position Changes in velocity/amplitude	Central axon – PGP9.5, S100 Terminal nerve branches – PGP9.5, trkB Incomplete capsule – p75
II	Pacini (lamellated)	Rounded, ovular corpuscle. Thick lamellar capsule. 20–50 µm.	Rapidly adapting Low-threshold	Joint acceleration/deceleration	Central axon – PGP9.5, S100 Thick capsule – p75
III	Golgi-like (grouped dendritic)	Large, spherical. Partial encapsulation. Groups of arborizing and terminal nerve endings. >150 µm.	Rapidly adapting High-threshold	Extreme ranges of joint motion	Terminal nerve branches – PGP9.5, S100, trkB Incomplete capsule – p75
IV	Free nerve endings	Varicose appearance, often close to arterioles. Groups or single fibers	A∂ fibers – fast C fibers – slow	Noxious, nociceptive, inflammatory	Axon – PGP9.5, trkB
V	Unclassifiable	Variable size, appearance, and degree of encapsulation.	Unknown	Unknown	Incomplete capsule – p75 Variable IR pattern
Abbreviations: Protein Gene Product 9.5 (PGP9.5); S-100 protein (S100); tyrosine kinase receptor B (trkB); low-affinity neurotrophic receptor p75 (p75); immunoreactions (IR). Source: Reprinted from Hagert. ⁸

4.2.1 Wrist

The innervation of wrist ligaments has been intensively studied. ³ ^, ⁵ ^, ⁶ ^, ⁹ ^– ¹² Its pattern was found to vary distinctly, with a pronounced innervation in the dorsal ligaments and in the entire scapholunate interosseous ligament (SLIL), ¹⁰ ^, ¹¹ an intermediate innervation in the volar triquetral ligaments, and only limited to occasional innervation of the volar radial ligaments. ³ ^, ⁹ Based on this, the dorsal ligaments and the SLIL are considered sensory important ligaments in wrist proprioception, with the Ruffini ending (▶Fig. 4.1a) being the predominant mechanoreceptor type. ⁵

The triangular fibrocartilage complex also represents a richly innervated region with a homogenous distribution of Ruffini endings, but also free nerve endings (▶Fig. 4.1d) in the ulnar and dorsal areas, Pacini corpuscles (▶Fig. 4.1b) in the radial and dorsal areas, and Golgi-like endings (Fig. 4.1c) in the ulnar and ventral areas. ¹³

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