Cutaneous Senses and Vision
September 24/26, 2012
Our bodies are capable of sensing a wide spectrum of stimuli. We are consciously aware of some of the information our bodies perceive, but much of the information that is sensed is beyond our consciousness. Receptors responsible for perception of stimuli are found in many places: skin, eyes, ears, mouth, blood vessels, lungs, brain—frankly, every cell in the body has sensory receptors. These receptors are broadly classified as photoreceptors (sensitive to light), chemoreceptors (sensitive to chemicals), thermoreceptors (sensitive to warm or cold), mechanoreceptors (sensitive to stretch/pressure), and nociceptors (sensitive to damage).
Sensory receptors are found on neurons that bring information INTO the central nervous system; thus these neurons are called afferent neurons. Efferent neurons are neurons that carry information away from the central nervous system; these neurons are also called motor neurons or effector neurons since they are responsible for controlling responses of muscles, glands, and organs.
The cutaneous senses provide us with information about our immediate environment, allowing our integumentary system (skin) to detect differences in temperature, texture, weight, and shape of objects that we touch as well as the air. The receptors involved with these sensations are: cold receptors, warm receptors, touch receptors, pressure receptors, and nociceptors. Since it is difficult to distinguish between touch and pressure, we will consider the two together.
Cutaneous receptors are usually found in groups, and not alone. This is sometimes called a punctate arrangement of receptors. One small area of the skin can distinguish touch, while a neighboring area is sensitive to warmth. Nociceptors are unique in that they are spread in a more ubiquitous pattern.
Each receptor type is most sensitive to a particular energy type/stimulus that is called its adequate stimulus. Again, nociceptors are an exception to this rule. Any stimulus that causes damage leads to the release of chemical transmitters that activate pain receptors. Although each receptor, excluding pain, has an adequate stimulus, it is possible for a strong stimulus that is not an adequate stimulus to elicit sensation. For example, if a warmth receptor is stimulated by a strong mechanical stimulus (such as a hammer blow), the sensation perceived may be pressure, pain, and warmth.
Temperature perception is based on the activation of cold receptors, warm receptors, and nociceptors. The sensation of hot is caused by simultaneous activation of warm receptors and nociceptors while the sensation of cold is caused by combined activation of cold receptors and nociceptors.
A sensory unit is an afferent neuron with all of its receptors. In general, each afferent neuron is connected to many receptors. An exception to this generality is the fovea of the retina where there is a one receptor: one neuron relationship. The area (for instance, area of the skin) containing all of the receptors of a sensory unit is called the receptive field. The degree to which we can localize sensations to a specific area of the body, or distinguish two separate simultaneous stimuli (two point discrimination) depends on the size and degree of overlap of adjacent sensory fields.
The intensity of a sensation can be increased in two ways: increase the number of receptors activated, and/or increase the frequency of receptors that are activated. When a stimulus remains constant over a period time, almost all types of sensory receptors adapt so that they become less sensitive with time. Some sensory receptors adapt slowly while others adapt rapidly. Cutaneous Senses
Experiment 1. Arrangement of Cutaneous Receptors.
HYPOTHESIZE whether sensory receptors for different types of adequate stimuli are uniformly distributed on your skin. 1. Mark...