Receptors

Sensation begins most distally with the transduction of mechanical stimulation by mechanoreceptors in the dermis, epidermis, muscles, and joints. To stimulate the axon associated with a mechanoreceptor, the stimulus must first pass through intervening tissues. This process is referred to as stimulus accession. This is followed by stimulus transduction, in which the stimulus energy is transformed into electrical energy by depolarization of the axon terminal in proportion to the amount of mechanical energy applied. The amplitude of the mechanical stimulus determines the frequency at which the action potentials are initiated. [8 This temporal summation of afferent impulses is preserved throughout the afferent course and is decoded centrally as subjective stimulus sensation magnitude. Increasing the intensity of a mechanical stimulus not only increases the firing frequency of the discharging mechanoreceptor but also recruits more sensory units. The intrinsic properties of the mechanoreceptors, however, determine how sustained the impulse activity is when stimulated. [9 Rapid adaptation of a mechanoreceptor indicates that the unit is firing only as long as the stimulus is moving. Slow adaptation indicates that the unit also fires when the stimulus is held constant. y Four subtypes of mechanoreceptors that have large fiber afferent nerves associated with them have been identified: (1) Meissner's end organ (RA) with rapid adaptation, a distinct receptor field border, and small receptive field size; (2) paciniform end organ (PC) with rapid adaptation, obscure field borders, and large field size; (3) Merkel's end organ (SA I) with slow adaptation, distinct field borders, and a small receptive field; and (4) Ruffini's end organ (SA II) with slow adaptation, obscure field borders, and large field size. y

Any given mechanoreceptor may respond to various types of mechanical energy; however, most are particularly sensitive to one form of stimulation. This selectivity tends to be greatest at or near threshold levels of stimulation, that is, activation of the smallest number of sensory units necessary for stimulus perception. y

It has been demonstrated, from intraneural microstimulation of a single myelinated fiber associated with a mechanoreceptor, that elementary sensations can be perceived and sensory quality, magnitude, and localization can be resolved at a cognitive level. [5 Microstimulation of a single RA unit produces a sensation of intermittent tapping or fluttering. These receptors are related to texture discrimination

TABLE 19-3 -- CLINICO-ANATOMICAL CORRELATION OF LESIONS INVOLVING PROPRIOCEPTION AND VIBRATION SENSATIONS

Anatomic Site of Damage

Sensory Findings

Other Neurological and Medical Findings

Sensory receptors

Local diminution of all modalities

Edema or inflammation of skin

Primary neuron Peripheral nerve

Distal to proximal gradient loss of all modalities

Distal weakness and atrophy of lower extremity museles, areflexia

Sensory root ganglion

Vibration and propnoceptive loss greater than pain and temperature in distal extremities; arms and legs equally affected; limb ataxia.

Areflexia; may have xerophthalmia, xerostomia, arthritis, rash, or pulmonary mass lesion

Dorsal columns and other spinal tracts

Propnoceptive and vibratory loss generally in bilateral lower extremities

Pupillary abnormalities, areflexia, ataxia in lower extremities

Secondary neurons

Nucleus cuneatus and gracilis

Selective loss of vibration and proprioception, in lower extremities for gracilis and in upper extremities for cuneatus

Cranial nerve deficit in lower brain stem

Medial lemniseus

Loss of proprioception, vibration, and disenminative touch on the eontralateral half of the body usually in an incomplete distribution

Cranial nerve palsy; the particular nerve involved is important in localizing at what level the medial lemniscus is lesioned as it courses through the length of the brain stem.

Tertiary neurons

Ventroposterolateral and ventroposteromedial nucleii

Loss or decrease of all modalities on entire contralateral half of body, frequently accompanied by spontaneous pain

May be associated thalamic aphasia, memory deficit, or rubral tremor

Internal capsule

Loss of all modalities or eombinations of seleetive loss of pain and temperature or vibration, propnoception, and disenminative modalities. Distlibution is near complete or incomplete hemibody contralateral to the lesion; associated hemiparesthesia may be noted

Sensory cortex

Mild and often transient loss of pain, temperature, and vibration; more severe and sustained loss of propnoception and diseriminative touch. There is vanability in distnbution and severity from morment to moment. Distal extremities are affected more severely than proximal extremities, and extremities are affected more than the trunk. Unilateral fate and arm or leg alone may be affected.

Neglect, aphasia, extincton on double simultaneous stimulation

in glabrous skin such as the fingertips. y Microstimulation of a PC receptor produces a sensation of vibration or tickle. Microstimulation of an SA I receptor produces a sensation of pressure,^ and these receptors appear to be critical in resolving the spatial structures of objects or surfaces. y , U Microstimulation of SA II receptors evokes no sensation when stimulated in isolation; however, they are excited by skin stretching and joint movement. y The Ia afferents that represent the primary endings of muscle spindles also produce no sensation when stimulated in isolation. y

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