III: To define the mechanoreceptive and thermoreceptive sensations.

Transcription

1 Somatic Sensations

2 I: To explain the proper way of sensory testing. II: To describe the somatic sensations. III: To define the mechanoreceptive and thermoreceptive sensations.

3 SOMATIC SENSATIONS Somatic sensations are sensations which are conducted by the somatic nerves and come to conscious perception. Testing of all sensations should be: (a) Bilateral, symmetric and systematic. (b) By using the same strength of stimulation. (c) Including all dermatomes. (d) Testing are performed while the eyes are closed to eliminate the effect of vision on recognizing the sensation. Somatic sensations are classified into three categories: [I] MECHANORECEPT1VE SENSATIONS [II] THERMORECEPTIVE SENSATIONS [III] PAIN SENSATION

4 MECHANORECEPTIVE SENSATIONS These sensations produced by mechanical stimuli which stimulate mechanoreceptors. MECHANORECEPTORS Mechanoreceptors are receptors which are especially sensitive to mechanical stimuli and they are classified into: 1. CUTANEOUS MECHANORECEPTORS They are of three types (Fig. 3-1): i. Naked nerve endings; The free nerve endings and the basket endings around hair follicles. ii. Expanded nerve endings; Merkel discs and Ruffini corpuscles. iii. Encapsulated nerve endings; Pacinian and Meissner corpuscles.

5 Figure 3-1: Cutaneous mechanoreceptors

6 [II] DEEP MECHANORSCEPTORS These receptors are of two types: i. Muscle spindles: which are stretch receptors found in the fleshy part of skeletal muscles? ii. Golgi tendon organs which are tension receptors found in the tendons of the skeletal muscles.

7 b. Itch; is a sensation of skin irritation which leads to the desire for scratching of the skin (the scratch reflex). Tickle and itch signals are transmitted by the slowly conducting type IV nerve fibers.scratching relieves itch by removing the irritating stimulus and by presynaptic lateral inhibition of the central terminals of the primary itching conducting fibers. MECHANORECEPTIVE SENSATIONS 1. TOUCH OR TACTILE SENSATION It is the cutaneous sensation produced by light mechanical stimuli. There are two types of touch sensation: (A) CRUDE TOUGH This sensation without accurate identification of the locality or the number of stimuli and it is of two types: a. Tickle; is a sensation produced by mild tactile stimulation of certain areas of skin, usually leading to reflex involuntary laughter.

8 (B) FINE TOUCH It is a sensation with accurate identification of the locality and the number of stimuli and it is classified into: a. TACTILE LOCALIZATION (Topgnosis) It is the ability to identify the point where the stimulus is applied. The acuity of this sense is inversely proportionate to the distance of error in localizing the stimulus. b. TACTILE DISCRIMINATION It is the ability to identify two tactile stimuli applied simultaneously as two separate points of contact. It is tested by the compass test or using discriminator. Acuity of this sensation is inversely proportionate to the two-point threshold which is the minimal distance at which the two stimuli are felt as two separate points. This sensation is conducted by the gracile and cuneate pathway.

9 Figure 3-2: Two-point threshold and the discriminator test

10 Three conditions which favor the development of high degree of tactile discrimination in a skin area. i. High density of touch receptors. ii. Minimal convergence in the sensory pathway. iii. Large area of sensory cortical representation in the sensory cortex. These conditions are all found in the skin of the lips and fingertips.

11 2. PRESSURE SENSATION There are two types of pressure receptors: the rapidly adapting Pacinian corpuscles, and the slowly adapting Ruffini endings. The acuity of pressure sensation is tested by applying different weights on a supported hand of a blind-folded subject, then, the subject is asked to identify which weight is heavier and which is lighter. There are two types of pressure sensation: A. CRUDE PRESSURE SENSATION This is pressure sensation with low ability to discriminate different weights. This sensation is conducted by the anterolateral pathway. B. FINE PRESSURE SENSATION This is pressure sensation with high ability to discriminate different weights. This sensation is conducted by the gracile and cuneate pathway.

12 STEREOGNOSIS This is special type of sensation which is defined as the ability to identify objects by handling them without seeing them (a key, a coin). This ability depends on intact touch and pressure sensations as well as intact cortical sensory somatic association area of the parietal lobe. This sensation is conducted by the gracile and cuneate pathway.

13 3. MUSCLE TENSION SENSATION This is the sensation produced by traction on muscle tendons. The receptors are the Golgi tendon organs. It enables the person to discriminate different weights by lifting them. It is tested by applying different weights on an unsupported hand of a blind-folded subject, then, the subject is asked to identify the lighter and the heavier weight. 4. PROPRIOCEPTIVE SENSATIONS Proprioception is the sensation of the position of different parts of the body relative to each other and the position of the body in space. Proprioception is divided into two types:

14 (A)STATIC PROPRIOCEPTION (SENSE OF POSITION) The receptors are the deep mechanoreceptors (muscle spindles and Golgi tendon organs). It is tested by putting a limb in an unusual position and asking the blind-folded subject to put the other limb in a similar position. (B) DYNAMIC PROPRIOCEPTION (KINESTHETIC SENSATION) The receptors are the Pacinian corpuscles and Golgi tendon organs in ligaments and synovial membranes of joints. This sense is tested by moving a joint and the blind-folded subject is asked to tell when the movement begins and when it stops or when the rate of movement changes.

15 THERMORECEPTIVE SENSATIONS There are two types of specialized thermoreceptors, one is sensitive to warmth and the other is sensitive to cold which are found in the base of the epidermal layer of the skin. There are warmth sensitive spots on the skin where there are warmth receptors only and cold sensitive spots where there are cold receptors only. The number of cold spots on the skin is (4-10) times as many as those of the warmth spots.

16 Thermoreceptors are also found in the abdominal viscera, the spinal cord and around great veins. These receptors are concerned mainly with informing the hypothalamic thermostat of any increase in the body core temperature. Cutaneous thermoreceptors monitor the temperature of the skin, not that of the body. Thermoreceptors adapt at temperatures between C, giving a feeling of thermeneutrality, but no adaptation occurs outside this range. At temperatures below 15 o C or above 45 C, thermosensitive pain receptors are activated giving rise to pain sensation (cold pain and heat pain).

17 THERMORECEPTORS There are two types of thermoreceptors: 1. WARMTH RECEPTORS Warmth receptors are specialized free nerve endings which are stimulated at temperatures between o C with maximum frequency of discharge at about 40 C (Fig. 4-3). At temperatures lower than 40 o C the frequency of discharge decreases and the discharged impulses take the separated impulses pattern (ungrouped pattern). At temperatures higher than 40 C, the frequency decreases and the discharged impulses take a grouped pattern, "volley pattern" (Fig. 4-5). Warmth sensation is conducted by the thin, type IV fibers.

18 2. COLD RECEPTORS Cold receptors are the Krause end-bulbs which are specialized, encapsulated nerve endings (Fig. 4-4). They are stimulated at temperatures between C with maximum frequency of discharge at about 25 C (Fig. 7-3). At temperatures lower than 25 C, the frequency of discharge decreases and the discharged impulses take the grouped pattern (volley pattern). At temperatures higher than 25 C, the frequency of discharge decreases and the discharged impulses take the ungrouped pattern. Cold sensation is transmitted by type III nerve fibers. Figure 3-4: Krause end bulb.

19 Figure 3-5: The change in frequency and pattern of discharge from thermoreceptors with the change of temperature

20 Figure 3-6: The frequency of discharge from thermoreceptors and thermal pain receptors at different temperatures.

21 PARADOXICAL COLD SENSATION Rapid warming of the skin to temperatures between o C gives a transient false sense of cold (paradoxical cold sensation). This is because at this temperature range, cold receptors are transiently stimulated. Paradoxical cold, like any cold sensation, produces reflex vasoconstriction and rise in arterial blood pressure (the cold pressor effect). NON SPECIFIC THERMORECEPTORS Some pressure receptors (Ruffini endings) are stimulated also by cold. This explains why a colder of two otherwise identical weights placed on the hand is felt heavier than the warmer weight (Weber's illusion).