stimuli (1).pptx

stimuli

1. STIMULUS & RESPONSe TYPES OF STIMULI DR.FARIDULLAH KHAN DEPARTMENT OF PHYSIOLOGY RMU

2. Stimulus In general, a  stimulus  is something that provokes or causes an action or response. A stimulus is an external force which when applied to an excitable cell is capable of producing a change in the internal environment of the cell.

3. Definition A stimulus is a detectable change in the internal or external environment of an organism. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be made, is called sensitivity. 

4. Sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body , in chemo receptors  and mechanoreceptors. When a stimulus is detected by a sensory receptor, it can elicit a reflex via stimulus transduction.

5. INTERNAL & EXTERNAL STIMULUS An INTERNAL STIMULUS is often the first component of a homeostatic control system. EXTERNAL STIMULI are capable of producing systemic responses throughout the body, as in the fight-or-flight response.

6. INTERNAL STIMULUS EXAMPLE FEVER Chemicals released by white blood cells raise the body temperature by 2-3 degrees causing the temperature to elevate. This helps to kill bacteria and inhibit viruses. Internal Stimuli – Bacterial Infection Response - Fever

7. EXTERNAL STIMULUS Example “Fight or Flight Response ” A person has just been confronted with a grizzly bear on a narrow mountain trail. External Stimuli – Grizzly Bear Response – Fight or Flight Response Heart rate increases, blood flow is shunted to the major muscle groups, digestive activity slows, blood pressure rises.

8. CLASSIFICATION ON the basis of Strength of stimulus: In order for a stimulus to be detected with high probability, its level of strength must exceed the  absolute threshold ; if a signal does reach threshold, the information is transmitted to the CNS, where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.

9. Threshold stimulus The minimum strength required for  stimuli  to initiate the response for example in a muscle i -e contraction is called  Threshold stimulus  in muscle contraction. The  stimuli  other than nerve  threshold stimulus are  mechanical  stimuli  like pressure, electrical  stimuli  like shock, chemical  stimuli .

10. Sub-threshold   /Sub- liminal stimulus Sub-threshold  : refers to a  stimulus  that is too small/weak in magnitude to produce an action potential in excitable cells. In general, a  sub-threshold stimulus  leads to the depolarization of the membrane, but the magnitude of the depolarization is not large enough to reach the threshold voltage

11. Supra-threshold Supra-threshold  : refers to a  stimulus  that is large enough in magnitude to produce an action potential in excitable cells. ... Therefore,  supra-threshold stimuli  elicit action potentials. A stimulus having its strength more than a threshold stimulus. It will produce the same response as after application of the threshold stimulus

12. Subthreshold, Threshold and Suprathreshold Stimuli

13. In this graph to the right, we are shown a neuron and it shows how different intensities of stimuli affect the voltage changes. On the bottom of the graph we see stimuli: starting with a weak stimulus and slowly going to a stronger one. On the top we see our mv and when we give a really weak stimulus we have this small little response labeled sub threshold potential.

14. That’s not an action potential, it’s called a “local response” or “local depolarization” or “ sub threshold potential. ” It depolarizes a little bit but it’s not an action potential because an AP is a complete reversal of the polarity of the cell and this is not a reversal yet. If we keep giving it a stronger and stronger stimulus, strong enough to pass the  threshold potential,  then it generates an action potential. This stimulus is called the threshold stimulus.  The threshold stimulus is the weakest stimulus that could generate an action potential in a neuron.

15. RESPONSE Qualitative or quantitative change in the internal enviornment of an excitable cell secondary to application of a stimulus Depending upon the strength of stimulus it can be: Local response/ Graded pot Propagated Action potential

16. nnnnkk ’’’’[[[[[‘[‘[‘’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’]’]’’’’’’’’’’’’]’]’]’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’

17. Stimulus & Modalities A stimulus is a change detectable by the body. Stimuli exist in a variety of energy forms, or modalities, such as heat, light, sound, pressure, and chemical changes. Sometimes we perceive sensory signals when they reach a level of consciousness, but other times they are processed completely at the subconscious level. All the information regarding all these senses is send to the CNS via AFFERENT NEURONS .

18. Different types/modalities of Stimuli Nervous stimuli (Sympathetic & Parasympathetic) Mechanical Stimuli Chemical Stimuli Electrical Stimuli Thermal Stimuli Hormonal Stimuli

19. Sensory transduction Because the only way afferent neurons can transmit information to the CNS about stimulus via action potential propagation, these forms of energy must be converted into electrical signals. The conversion of stimulus energy into a graded potential is called Sensory transduction and is done by sensory receptors.

20. SENSORY RECEPTORS  Receptors are sensory afferent nerve endings that terminate in periphery as either part of a neuron or in the form of specialized capsulated structures. They act as biological transducers and convert various forms of energy acting on them into action potentials.

21. The following are the properties of the Sensory Receptors: Receptor Potential. Specificity of stimulus & the Adequate stimulus. Effect of strength of stimulus. Adaptation (also Desensitization). Muller’s doctrine of specific nerve energies Law of projection. Threshold. Sensory unit Receptive field.

22. SPECIFICITY OF STIMULUS & ADEQUATE STIMULUS If all stimuli are converted to action potentials in sensory neurons and all action potentials are identical, how can the central nervous system tell the difference between heat and pressure, or between a pinprick to the toe and one to the hand?

23. All stimuli once received by the receptor are converted into action potentials and all of them are carried by the afferent neurons. This means that the CNS must distinguish four properties of a stimulus to be able to specify a stimulus :(1) its nature, or modality and(2) its location(3) Intensity(4) Duration

24. ADEQUATE STIMULUS Each sensory receptor has an adequate stimulus, a particular form of energy to which it is most responsive. For example, thermo receptors are more sensitive to temperature changes than to pressure, and mechanoreceptors respond preferentially to stimuli that deform the cell membrane, receptors in the eye are sensitive to light, receptors in the ear to sound waves, and warmth receptors in the skin to heat energy. Because of this differential sensitivity of receptors, we cannot “see” with our ears or “hear” with our eyes.

25. ADEQUATE STIMULUS Some receptors can respond weakly to stimuli other than their adequate stimulus, but even when activated by a different stimulus, a receptor still gives rise to the sensation usually detected by that receptor type. They respond to most other forms of energy if the intensity is high enough. Photoreceptors of the eye respond most readily to light, for instance, but a blow to the eye may cause us to “see stars”, an example of mechanical energy of sufficient force to stimulate the photoreceptors. Sensory receptors can be incredibly sensitive to its preferred stimulus.

26. Modality/ Nature of the stimulus The 1:1 association of a receptor with a sensation is called labeled line coding. Stimulation of a cold receptor is always perceived as cold, whether the actual stimulus was cold or an artificial depolarization of the receptor. The blow to the eye that causes us to see a flash of light is another example of labeled line coding. A blow to the eye is seen as “white light” as the photoreceptors were stimulated.

27. The afferent neuron with its peripheral receptor that first detects the stimulus is known as a first-order sensory neuron . It synapses on a second-order sensory neuron , either in the spinal cord or the medulla, depending on which sensory pathway is involve d.

28. This neuron then synapses on a third-order sensory neuron in the thalamus, and so on. With each step, the input is processed further.

29. Effect of Strength & Duration of Stimulus:   For individual sensory neurons, intensity discrimination begins at the receptor. If a stimulus is below threshold, the primary sensory neuron does not respond. Once stimulus intensity exceeds threshold, the primary sensory neuron begins to produce action potentials. As stimulus intensity increases, the receptor potential amplitude (strength) increases in proportion, and the frequency of action potentials in the primary sensory neuron increases, up to a maximum rate. Similarly, the duration of a stimulus is coded by the duration of action potentials in the sensory neuron. In general, a longer stimulus generates a longer series of action potentials in the primary sensory neuron

30. ADAPTATION also called Desensitization. It is the decrease in response of receptors on being continuously stimulated.  When a stimulus persists continuously, some receptors adapt, or cease to respond. Thus, the receptor “adapts” to the stimulus by no longer responding to it to the same degree. Receptors fall into one of two classes, depending on how they adapt to continuous stimulation: Tonic receptors Phasic receptors

31. Tonic Receptors are slowly adapting receptors that respond rapidly when first activated, then slow down and maintain their response. Pressure sensitive baroreceptors , irritant receptors, and some tactile receptors and proprioceptors fall into this category. In general, the stimuli that activate tonic receptors are parameters that must be monitored continuously by the body. It is important that these receptors do not adapt to a stimulus and continue to generate action potentials to relay this information to the CNS.

32. Phasic receptors rapidly adapting receptors that respond when they first are receive a stimulus but stop responding if the strength of the stimulus remains constant. Many of the tactile receptors in the skin belong to this class. Some phasic receptors, most notably the Pacinian corpuscle, respond with a slight depolarization called the off response when the stimulus is removed. They are important in situations where it is important to signal a change in stimulus intensity rather than the status quo information.

33. A good example is of the touch receptors. When we wear a ring in the morning, because these receptors adapt rapidly, you are not continually conscious of wearing your watch, rings, and clothing. the tactile receptors are stimulated. However, because the stimulation is continuous, after a little while the receptors show adaptation. Now we are not continuously aware of the ring while we go about doing different things, otherwise, we would not be able to focus. When we take off the ring, then the receptor is switched office. it is stimulated again to make us aware of the fact that the ring is being taken off.

34. MULLER’S DOCTRINE OF SPECIFIC NERVE ENERGIES Muller’s doctrine states that the action potential produced by all receptors is the same in all nerve fibers provided the diameter of the nerve fiber is the same. The nature of perception of a stimulus by the CNS is defined by the pathway over which the sensory information is carried. Hence, the origin of the sensation is not important.

35. LAW OF PROJECTIONE Stimulation of a cold receptor over the knee by cold or electrical stimulation of the fiber originating from this receptor before and after its entry into the brain or spinal cord will all produce a sensation of cold over the knee. Stimulation of nerve fiber anywhere along its course produces the specific sensation in the area of the body from where it originated.

36. THRESHOLD All receptors need a minimum strength of stimulus to start showing activity; this strength is called the threshold.

37. SENSORY UNIT A sensory unit is just like a motor unit. The sensory unit is a single primary afferent nerve including all its peripheral branches.

38. RECEPTIVE FIELD. THE AREA OF THE BODY WHOSE SENSORY NERVE SUPPLY COMES FROM a single SENSORY UNIT IS CALLED A RECEPTIVE FIELD.