A7 Loss of Sensory Function

Topic

The loss of sensory function in mammals compromises the nervous system's ability to acquire information regarding the mechanical state of the body itself—proprioception—and the surrounding environment—exteroception. Each sensory modality functions physically as a specialized transducer that converts a distinct form of physical energy—such as mechanical deformation, sound pressure, magnetic fields, or vibration—into electrical action potentials that the nervous system can interpret. Degradation of any link in this transduction chain reduces the quality of sensory feedback available for motor control.

Proprioception relies on specialized mechanoreceptors—muscle spindles and Golgi tendon organs—that act as viscoelastic transducers of mechanical deformation in muscles and tendons: the rate at which these receptors fire action potentials is proportional to the sum of two components—one related to the extent of tissue deformation at a given moment, and the other to the rate at which that deformation changes over time. The onset of neuropathy reduces both the receptors' sensitivity to sustained deformation and their sensitivity to the rate of deformation, thereby raising the minimum threshold of joint movement required for the animal to detect that the joint is moving.

The sensory modality that transduces sound pressure reaches its most sophisticated form in the echolocation used by bats and cetaceans, which functions like active sonar: the animal emits a sound signal and compares its intensity with that of the returning signal reflected off a target. This comparison also factors in the target's sound-reflecting properties and the amount of signal energy lost during transit due to absorption by the medium and geometric wave spreading. When hearing deteriorates, the minimum intensity threshold required for the animal to detect an echo rises, and the resolution with which it can distinguish between closely spaced targets—in terms of both angle and distance—diminishes.

The sensory modality that transduces the magnetic field operates via pairs of internal magnetic structures—magnetosomes—that detect magnetic field differences between different points on the animal's body; the sensitivity of this detection drops as internal thermal noise increases—the same type of random molecular agitation that competes with any weak signal the organism attempts to detect.

When proprioception specifically is lost, the motor control relying on the feedback mentioned earlier suffers in a characteristic way: postural oscillations increase while the animal is at rest; an ataxic gait emerges—worsening markedly when the animal closes its eyes and can no longer compensate for the loss of proprioception with visual information; and the time it takes for the animal to react to an unexpected mechanical disturbance increases.

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