29.1 Sensory systems

Introduction

The act of smelling something, anything, is remarkably like the act of thinking. Immediately at the moment of perception, you can feel the mind going to work, sending the odor around from place to place, setting off complex repertories through the brain, polling one center after another for signs of recognition, for old memories and old connection.

Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction . We are going to focus on the five special senses of humans.

There are three types of stimuli that are detected by the human sensory systems. The first is chemical stimulus , where molecules stimulate a sensory neuron. Chemical stimuli are detected by the olfactory system, when molecules in the air bind to sensory cells in the nasal epithelia, and in the gustation (taste) system, when molecules in your food stimulate your taste buds. The second is electromagnetic radiation ; light interacts with molecules in the sensory cells (rods and cones) of your retina, and those sensory cells send a signal to your brain. The third is mechanical stimulation , where the sensory cells are activated by movement or touch. Mechanical stimuli are detected by the cells in the inner ear that help you detect balance and body position, and by other cells in your inner ear detect sound (the sense of hearing). Additionally, mechanical stimuli are involved in other somatosensory systems, such as pressure, pain, or vibration, in proprioception (position of legs, arms and other body parts), and in kinesthesia (detection of motion of those same body parts.

Sensory perception

Reception

The first step in sensation is reception , which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can propagate for many kilometers). For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances.