GENERAL CUTANEOUS SENSATIONS
This lab involves the examination of cutaneous receptors. There are four exercises. For this lab, report your observations in a formal report, according to the formatting prescribed in Lab Report Format.doc which is found on Blackboard under Assignments/Before You Begin.
Metal calipers or compass with pencil
2 or more blunt probes (ball point pens or forks with blunt tines) 3 pennies
1 liter ice water in large container
1 liter hot water in large container (make sure it is bearable to the touch) 1 liter room-temperature water in large container
Felt-tip markers, blue, red, and black (not permanent) Stop watch or digital watch
A sensation is defined as a state of awareness, of the internal or external environment. For a sensation to occur, four criteria must be met. First, there must be a stimulus. This is a change in the environment, to which we will become aware. Next, there must be a receptor. A receptor is a cell, or an organ, which is sensitive to the stimulus. There also must be an afferent (sensory) nerve pathway, to carry signals to the central nervous system. Finally, there must be sensory cortex, where the signals will be analyzed and interpreted consciously.
The skin is responsive to a number of stimuli. These include light touch, deep pressure, vibration, and changes in temperature. Any stimulus must be of threshold magnitude to be detected. All sensory systems have a limit to their sensitivity; any stimulus below a minimum magnitude cannot elicit a response.
The receptors for general cutaneous sensations are small, modified nerve endings. They are scattered throughout the skin and underlying fascia. These receptors are adapted to be most sensitive to certain stimuli:
Type of receptorStimulusTypical Sensation
Mechanoreceptorsmechanical agitationtouch, pressure, vibration
Thermoreceptorschanges in temperatureheat, cold
Nociceptorsdamage to tissuespain
Proprioceptorsmuscle lengthbody position.
Receptors must be capable of transduction, the conversion of stimulus energy into graded generator potentials that eventually reach and propagate as action potentials along nerve fibers. Typically, the stimulus causes sodium channels to open across cell membranes or subcellular membranes, depolarizing the receptor, to some degree. The magnitude of the generator potential is usually proportional to the magnitude of the stimulus:
Weak stimulusfew ion channels openedweak depolarization
Strong stimulusmore channels openedstrong depolarization.
The generator potential results in the release of neurotransmitter from the receptor, where it synapses with an afferent neuron. The neurotransmitter then elicits action potentials in the afferent neuron. The stream of action potentials eventually arrives at the sensory cortex, where it is interpreted mentally as some sort of sensation.
Stimulus magnitude (also called intensity) is encoded by action potential firing rate. This is the only means of encoding intensity, because action potentials do not vary in magnitude. A low rate of firing is interpreted as a weak sensation; a fast rate is interpreted as a more intense sensation.
EXERCISE 1: STRUCTURE OF TOUCH RECEPTORS
Meissner’s corpuscles are receptors for light touch. These are located in the upper dermis, usually within dermal papillae. This location places them as close to the body surface as possible, thus enabling them to respond to light agitation of the skin. Within the corpuscle, the sensory nerve ending is coiled, parallel to the plane of the skin surface. Compression of the skin (by touching) deforms the receptor, causing depolarization.
Pacinian corpuscles are receptors for deep pressure. These are typically located in the lower dermis or underlying fascia. Their deep...