3 Sense and Proprioception
Movements with precision and dexterity are essential for achieving the multitude of functions demanded of the human hand; although it is the cerebral motor cortex that sends commands to the hand to produce well-coordinated movements, without adequate sensory input this cannot be achieved. Great advances have been made in our understanding of the mechanics of sensory input in the past two decades using information gathered through three techniques: behavioral methods (psychophysics), electrophysiological methods (microneurography, nerve conduction studies, and evoked potential recordings), and functional magnetic resonance imaging (fMRI). In this chapter, we review the structural and functional anatomy of sensory receptors of the hand in general and thereafter focus on proprioception.
3.1 Sensations
Detection of a stimulus and recognition that an event has occurred constitute sensation, whereas interpretation and appreciation of that event constitute perception. 1 The major categories of stimuli are visual, auditory, gustatory (taste), olfactory (smell), mechanical, thermal, and nociceptive (pain). From the various stimuli reaching the brain, an image of the body and of the external world is constructed and updated continuously, a process essential for generating prompt and appropriate responses to changes within and outside the body. To deal with different varieties of stimuli, it is necessary to have a variety of sensors (receptors) and separate populations of neurons to transmit them to the brain. It is crucial not only to sense and discriminate between different types of stimuli but also to acquire information regarding the intensity, topography, and frequency of the stimuli. These needs have led to the evolution of receptors with unique characteristics, a variety of neurotransmitters, and axons with differing conduction velocities.
3.1.1 The Somatosensory Unit
Each somatosensory unit consists of the dorsal root ganglion (DRG) cell, its axon, and all the receptors it innervates. The DRGs are pseudounipolar cells (▶Fig. 3.1), with the peripheral portion of the axon terminating in receptors (either as bare nerve ending or in an encapsulated structure) and the proximal portion in the dorsal horn or more proximally in the central nervous system. ▶Table 3.1 shows the various sensory units and the receptors.
Somatosensory Receptors
General Principles
Receptors are akin to transducers and generate electrical discharges in response to specific stimuli. Appropriate stimulus induces a change in the transmembrane potential (receptor potential) by changing the permeability of the ion channels of the cell membrane; this may be triggered by mechanical deformation of the membrane causing opening of ion channels, by change in temperature altering permeability of the membrane, or by release of a chemical which may open the channels. The usual change in the membrane is opening of the sodium channels, causing influx of positively charged sodium ions. This in turn leads to migration of sodium ions from the surrounding tissues, eventually leading to change in the membrane potential of the adjacent node of Ranvier. When the receptor potential is large enough to cause depolarization of the nerve fiber terminating in the receptor, an action potential is triggered, which propagates along the axon toward the DRG (▶Fig. 3.2).
Determination of stimulus strength is crucial for meaningful sensory perception. As the stimulus strength increases, the amplitude of the receptor potential increases in proportion initially, along with an increase in the frequency of the action potential; however, with an intense stimulation, after the initial increase, there is progressively less and less additional increase in frequency of action potential. This general characteristic of receptors bestows an important attribute of being able to respond to an extreme range of stimuli, from very weak to very intense. 2
Spatial resolution is essential for perception and recognition of complex stimuli. Two techniques are used to modulate spatial resolution: divergence and convergence. When a single receptor sends input to more than one ganglion cell, divergence occurs; on the other hand, when a single ganglion cell receives input from several receptors, convergence occurs, leading to poor spatial resolution but allowing detection of relatively weak stimuli.
Information regarding intensity of stimuli is transmitted by change in the rate and temporal codes. From the firing rate of individual neurons and the number of neurons responding, the intensity of the stimulus is derived.
Surround inhibition of receptor field serves to increase contrast. Tactile receptors show a central area of high sensitivity with surround or lateral inhibition, so that each receptor responds to stimulus occurring in one specific area.
Sensory pathways and their destinations adhere to strict topographic representation at each step to facilitate accurate generation of body schema.
Anatomy of Somatosensory Receptors (▶Fig. 3.3)
Nociceptors
Free nerve endings (FNEs) sense pain and are ubiquitous in the skin. They often penetrate the epidermis and end in stratum granulosum; there are FNEs that surround the hair follicles as well. They are also present in connective tissue, bones, and joints. They are unencapsulated and exhibit little structural specialization. FNEs may be terminals of C fibers or A delta fibers; the C fiber terminals lack Schwann cell investment and are intimately associated with the epithelium, unlike the A delta terminals.
A variety of stimuli may be perceived as pain, including chemicals related to tissue damage, pH alterations, and heat above 45°C. Noxious stimuli can activate C and/or A delta FNEs. The A delta terminals respond to intense mechanical stimuli and the C terminals respond to noxious mechanical, noxious heat, and chemical stimuli. The first pain is rapidly perceived and is discriminative (location and source). The second pain is of longer duration and is characterized as severe and often described as of agonizing quality. The first pain results from stimuli carried through the A delta and the second pain through the C fibers.
Certain C terminals contain two neuroactive peptides: calcitonin gene-related peptides (CGRPs) and substance P. Tissue damage leads to release of prostaglandin, bradykinin, and serotonin, which can initiate action potential in the nociceptor; branches of the stimulated axon can release substance P and CGRP through local axon reflex, which can lead to histamine release from mast cells and capillary dilation, leading to inflammatory response.
Some FNEs can serve as multimodal receptors; an example is the nerve ending containing certain ion channels. Thus nerve terminals with a subfamily of transient receptor potential vanilloid, a cation channel that opens when the temperature increases, generally serve as thermal receptors; high concentration of prostaglandin or bradykinin can lower the threshold at which these channels open, thus transforming the terminal into nociceptors. 3
Thermoceptors
Some FNEs in the epidermis and dermis serve as thermoceptors; these slowly adapting receptors may respond to cold, warmth, or temperature-sensitive nociception. The cold and warm receptors occur separately as distinct spots. Cold receptors respond to decrease in temperature over a range of 5 to 43°C and are more numerous than warm receptors; the latter respond to increasing temperature of up to 45°C. 4 When temperature goes over 45°C or falls below 13°C, the sensation of pain occurs. Warm receptors are innervated by unmyelinated C fibers and cold receptors by thinly myelinated A delta fibers.
Mechanoreceptors
Mechanoreceptors respond to physical deformation of skin or subcutaneous structures, from touch, pressure, stretch, or vibration. A delta FNEs in the skin, muscles, tendons, ligaments, and joint capsules as well as A beta terminals around hair follicles (peritrichial nerve endings), along with more specialized receptors such as Meissner’s and pacinian corpuscles, constitute mechanoreceptors. Based on microneurographic studies, four categories of mechanoreceptors can be identified (▶Table 3.2): fast-adapting (phasic) FA I and FA II and slow-adapting (tonic) SA I and SA II. The FA I and FA II receptors respond promptly to mechanical stimuli such as skin indentation but adapt quickly; the slow-adapting receptors show regular and sustained response. FA I afferents connect to Meissner’s corpuscles and FA II to pacinian corpuscles. 5 The SA I afferents connect to Merkel’s cell neurite complexes and SA II to Ruffini’s endings. The FA I and SA I units are quite dense at the fingertips.
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