SENSATIONS.pptx physiology FOR first BHMS

SENSATIONS

1. SENSATIONS DR POTNURU SRINIVASA SUDHAKAR

2. 1. Sensations are feelings aroused by change of environment . If the organism has to fight for its existence , it must be kept informed about the various events in and around it. The different ways by which the organism may be aware of its surrounding are called sensations.

3. 2. Sensory units: The area supplied by one sensory fibre is called a sensory unit.

4. 3. Dermatome : The area of skin supplied by single spinal root fibre forms the dermatome. It is supplied by sensory neurons that arise from a spinal root ganglion.

5. Dermatome maps are commonly used in clinical neurology . These maps are valuable for the localization of varied sensory phenomena in patients with neurological disorders.

6. 4 . Anaesthetic zone : It is done by cutting a posterior root and noting the corresponding anaesthetic zone on the skin. It is not a perfect interpretation as adjoining nerves overlap.

7. 5. Residual sensation : It is identified by cutting posterior roots above and below, and noting the residual sensory area. Vasodilatation : The stimulation of the peripheral end of the cut posterior root causes cutaneous vasodilatation of the corresponding area.

8. SENSORY MECHANISM For each sensation the following mechanism is involved : 1. An exposure to appropriate stimulus. 2. Sensitization of a specific nerve ending which is selectively sensitive to that stimulus. 3. The sensory pathway which carries the impulse to the central nervous system. 4. The nerve centre where the impulse is finally interpreted as a particular sensation. Psychical centre where the 'meaning' of the sensation is analyzed and understood .

9. PROPERTIES OF SENSATIONS Sensations differ from one another in various respects M odality, Q uality , I ntensity , adaptation , D uration , and Localisation or Projection .

10. 1.Modality A .The ability to distinguish the characteristic of a sensation from all other sensations is known as its modality . Sensations aroused by stimulation of sense organs , viz. the eye or ear , bear no resemblance to each other; to most people it is inconceivable that the stimulation of the eyes could evoke the sensation of taste or hearing . B. The modality of a sensation aroused by the stimulation of a given sense organ is fixed-Muller's law or law of specific nerve energies.

11. 2 . Quality Quality means the nature of sensation. Sensation of the same modality may vary in quality. Thus , we speak of a warm or cold breeze and of a red or green light. Quality depends on the nature of receptor stimulated (peripheral analyzer or surface detector) and the interpretation by the central nervous system. For instance , some individuals cannot distinguish between red and green( colour blindness) and there is also tone deafness.

12. 3 . Intensity It denotes the degree of sensation. It depends on the frequency of afferent impulses passing up the sensory nerve. Stronger the stimulus, higher will be frequency and more intense will be the sensation. Two sensations of the same quality may differ in intensity. A warm the object delivers little energy and a hot object delivers much energy to the receptors.

13. 4 . Adaptation It will be recalled that the structures of muscles and nerves adapt to a constant stimulus. The frequency of impulses gradually decreases due to adaptation. It has been stated that the speed with which adaptation sets in differs in various receptors. For instance, a slight bending of a hair causes a distinct sensation, but, if the hair is kept in this new position the sensation quickly disappears . This is true for touch in general. The adaptation to pain sensation is exceedingly slow or absent .

14. 5 . Extent It indicates the area from which the sensation arises. It depends upon the number of receptors simultaneously stimulated .

15. 6 . Duration It is self-evident that a sensation occupies a certain length of time. However , a sensation is not necessarily coexistent with the stimulus. The duration of a sensation may be shorter than that of stimulation because of adaptation. On the other hand, sensations may outlast the period of stimulation and thereby give rise to after-sensation.

16. 7 . Localisation or Projection It is the ability to locate the exact spot from which the sensation arises. Sensations are concerned more intimately with the brain than with any other part of the body. Yet when a sensation is experienced we are never conscious of our brain.

17. Sensations are invariably projected or referred either to some part of our own body or to some part of the environment.

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