9. Investigating human perception – physical and physiological tests
9.4. Sometosensory receptors
Thesomatosensory systemincludes several modalities of sensation from the body – touch, pressure, pain, tem-perature and proprioception. These modalities are transmitted to the central nervous system through three different pathways. The first modality is calleddiscriminative touch.This means sensation of touch, pressure, and vibration by which one can describe the shape and texture of an object without seeing it. The second types of modalities are pain and temperature(and also itch and tickle). The third modality is the somatosensory proprioception -muscle stretch, tendon tension, etc.
Tactile stimuliare mechanical forces that get in contact with the skin. Touch is caused by a force that produces little distortion of the skin. Pressure means greater distortion of the skin and also the underlying tissue. Tactile stimuli varying in time produce sensations such as flutter or vibration.
The cell bodies of the somatosensory afferent neurons are located in the posterior root ganglia of the spinal cord.
They are pseudounipolar cells with a single process that divides to form one peripheral and one central axon. The peripheral axon ends in the skin (muscle, etc.), and the central axon travels to the central nervous system. Most of the somatosensory afferent terminals are enveloped in a connective tissue capsule or end on hair follicles. The complex of the capsule/hair follicle and the afferent endings form a "somatosensory receptor". The sensitivity of the receptors to specific stimuli is determined by the non-neural tissue surrounding the afferent terminal (Table 9.1). Other somatosensory axons branch and terminate in skin as free nerve endings.
Table 9.1. Skin senses and receptors
Sense in the basal layer of the
epidermis
Investigating human perception – physical and physiological tests
flutter and move-ment
in a hairless skin, near the surface of the skin fast
Egg shaped. Stack of flattened epi-thelial cells with afferent terminal of collagenous fibers with nerve end-ing branches intertwined with them. epithelial cells with a single nerve ending in its center.
Vater-Pacini:
movement in hair follicles
fast Nerve endings spiral around or run parallel to the hair shaft
Thereceptive fieldof a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. In the somatosensory system, receptive fields are regions of the skin. The fingers, which require the ability to detect fine detail, have many mechanoreceptors with small receptive fields, while the back and legs, for example, have fewer receptors with large receptive fields.
Thermoceptionorthermoreceptionis the sense by which an organism perceives the rate of heat flux. The organs of Ruffini are the heat receptors while the bulbs of Krause are the cold receptors. (Both are considered to be mechanoreceptors too, because they respond also to mechanical stimuli, e.g. vibration.) Other thermoreceptors are free nerve endings.
Cold receptors respond to lower temperatures (10°C -40°C), warm receptors are activated by heat (30°C -45°C).
Warm receptors are the terminals of unmyelinated C-fibers (low conduction velocity), while those responding to cold belong to thinly myelinated Aδ fibers (faster conduction velocity). Some cold receptors also respond to sudden high temperatures (typically above 45°C), and some warm receptors respond to sudden cold temperatures (below 10°C). These are known as paradoxical cold and heat responses, respectively. Both might be followed by a sensation of pain. The neutral zone (between 32 to 35 °C) is defined as the range of temperatures in which heat and warm receptors are equally active and small adjustments of skin blood flow are sufficient to maintain core temperature.
Long lasting cold stimuli block thermoreceptors – and other nerve fibers too – cooling is a well known anesthetic procedure. Localization of the heat (or cold-) source is better if it is coupled with tactile senses.
Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Normally, pain motivates the individual to withdraw from damaging situations and to avoid similar events in the future, and also to protect damaged tissues during healing.
Nociceptionis initiated by nociceptors (pain receptors), that can detect mechanical, thermal or chemical changes above a given threshold. All nociceptors are free nerve endings. Nociceptors are found in the skin and on internal surfaces such as the periosteum or joint surfaces. A characteristic feature of nociceptors is their tendency to be sensitized by prolonged stimulation, leading to a condition called hyperalgesia.
Skin nociceptors may be divided into two categories. The first type includes the high threshold mechanical nociceptors responding only to painful mechanical stimulation, and the thermal nociceptors, which respond to ex-treme thermal stimuli (specific nociceptors). The second types are the polymodal nociceptors, which respond to all (mechanical, thermal, chemical) high intensity stimuli. Chemicals which can activate nociceptors include serotonine, histamine and bradikinin. Other chemical mediators, like prostaglandins act as sensitizers, altering pain sensitivity by increasing the responsiveness of peripheral nociceptors. Aspirin-like pain-killing drugs act by inhib-iting prostaglandin synthesis (Figure. 9.6).
Investigating human perception – physical and physiological tests
Figure 9.6. Sensitivity zones of the thermoreceptors
Primary sensory neurons involved in pain sensation release predominantly substance P and glutamate in the dorsal horn of the spinal cord. Nociceptive information is transmitted to the brain via the spinothalamic tracts. Noxious signals are sent to the reticular formation and thalamus and enter consciousness. From the thalamus, the signal travels to the somatosensory cortex, where the pain is experienced and localized. Nociception can also cause gen-eralized autonomic responses before or without reaching consciousness to cause tachycardia, hypertension, pallor, cold sweating, nausea and fainting. Finally, the ascending information can activate descending analgesic pathways, from the midbrain periaqueductal grey area, which exerts an inhibitory control over the dorsal horn. Opioid receptors are present in many regions of the nervous system that are involved in pain control. The opioid drugs produce an-algesia by inhibiting neurotransmitter release from the primary afferent terminals in the spinal cord, by inhibiting the projection neurons transmitting pain information or by activating the descending analgesic pathway in the midbrain.
Itchis „a sensation that causes the desire or reflex to scratch”. Itch has many similarities to pain, but while both are unpleasant sensory experiences, their behavioral response patterns are different. Unmyelinated nerve fibers for itch and pain both originate in the skin, however, pain creates a withdrawal reflex while itch leads to a scratch reflex.
9.4.1. Examination of tactile receptors and thermorecept-ors of the skin.
Aim of the practice:study the functioning of tactile and thermoreceptors of the skin Materials needed:beakers, thermometer, compass, ruler
Thermoreceptor adaptation test
Prepare three beakers. First container marked „HOT” should be filled with 40 C° water (hot tap water, warmed up if needed). Second container marked „ROOM TEMPERATURE” should be filled with room-temperature water.
Third container marked „COLD” should be filled with 10 C° water (cold tap water with ice cubes). Check the temperatures with a thermometer.
Steps of the experiment:
• Subject should place one finger in the COLD container and an other finger in the HOT container!
• Leave them in the water for 15 seconds! How do the fingers feel? Does it change over the course of 15 seconds?
Does it feel warmer or colder than it did originally? Note any changes!
• Place both fingers in the ROOM TEMPERATURE container! How does the fingers feel? Warmer or colder?
Investigating human perception – physical and physiological tests
The experiment demonstrates the adaptation of themoreceptors, and relativity of thermal sensation. Activity of cold receptors decreases in the cold water because of the long lasting cold stimuli. The same happens to the warm receptors in the warm water. Because perception of temperature depends on the relative activity of warm and cold receptors, the finger kept in hot water senses ROOM TEMPERATURE water colder then the finger kept in cold water.
Note every changes in temperature sensation during the whole experiment! Explain your results! Compare the adaptation time of cold vs. warm receptors. Find an explanation of this difference!
Two point touch discrimination test
The spatial resolution to touch can be evaluated by measuring two-point discrimination thresholds. The subject has to report whether his/her skin is touched with one or two pointy objects (e.g. a pair of compass with two spikes).
If the two stimulated points belong to the same receptive field, the sensation is felt as one stimulus. If the points belong to separate receptive fields, the subject feels two stimuli. The distance between the points, at which the subject loses the ability to feel two points is thediameter of the receptive field. The two-point discrimination threshold is less than 5 mm at the finger tips and is about 40 mm at the thigh.
Steps of the experiment:
• The subject should close his/her eyes. Start with setting the compass spikes 2-3 cm apart! Touch to subject’s skin with both spikes simultaneously. If he/she can feel two points, decrease slightly the distance and repeat! If he/she can feel one point, open the legs of the compass slightly and repeat!
• Find the shortest distance of compass spikes at which the subject still senses two touches. (Use a ruler to measure the distance!)
• Repeat this measurement in different regions of the body (index finger, palm, lower arm, back etc. ).