Baraa Ayed. Salsabeel Fleifel. Faisal Muhammad

Transcription

1 23 Baraa Ayed Salsabeel Fleifel Faisal Muhammad

2 Review for the previous lecture: - Last time, we talked about receptors and the objectives to define receptors. We define receptors as transducers that convert any type of energy to electrical energy. Also, we talked about receptor potential which is due to the change in the permeability of ligand gated channels (chemical gated channels), this potential could be depolarizing potential (Excitatory potential) because the receptor is coupled with N+ channels or hyperpolarizing potential (inhibitory potential) because the receptor is coupled with K+ channels. - Also, we classify the sensory receptors according to modality (" type of sensation ") into many types: *Mechanoreceptors: detect deformation, Touch and Pressure *Thermoreceptors: detect change in temperature *Nociceptors: detect tissue damage (pain receptors) *Electromagnetic (Photoreceptors): detect light (Rods and Cones) *Chemoreceptors: taste, smell, CO2, O2, chemical, etc. - Also, we classify the sensory receptors according to Location into: *Exteroceptors *Interceptors *Proprioceptors - We talked about types of receptors : *Free nerve ending *Expanded tip receptors *Tactile hair 1 P a g e

3 *Pacinian corpuscle *Meissner s corpuscle *Krause s corpuscle *Ruffini's organ end *Golgi tendon *Muscle spindle Then we talked about the connection between the receptors and afferent neurons: Type of sensory receptor Type of Neuron fibers Time Free nerve ending A nerve 5-30 m/sec C unmyelinated m/sec Tactile hair A nerve m/sec Pacinian corpuscle "onion" A nerve m/sec Meissner's corpuscle A nerve m/sec Ruffini's end organ A nerve m/sec Markel's discs A nerve m/sec Note: The fast fibers are Aβ and they are found in non hairy skin (glabrous skin), the receptors in glabrous skin are Meissner s corpuscle. These fibers are found also in hairy skin but the receptors in the hairy skin are Merkel s disk. Merkel s disk receptors are found in hairy and non hairy skin but Meissner s corpuscle are found only in non hairy skin 2 P a g e

4 - How the potential of sensory receptor happens? There are two types of sensory potential: 1) Generator potential: when the receptor area is the terminal of the afferent neuron. 2) Receptor potential: when the receptor area is separated from the afferent neuron. * Both of them have the same mechanism of sensory potential either receptor or generator potential. -Why the receptor area is considered as non excitable area? In order to differentiate the different intensities otherwise if it is excitable it needs to reach a certain value to produce action potential. - How the receptors are specific to certain sensation? Because the sensation is qualitative not quantitative, that means that a certain receptor will respond to a certain stimulus either it weak or strong. For example, touch receptors are sensitive to touch and the threshold of touch for the touch receptors is very low. But the threshold of touch in other receptors (Not touch 3 P a g e

5 receptors) is very high. So, it can be sensitive by other types of receptors, but it needs very high level of stimulus. Regardless to how sensory neurons are stimulated only one sensory modality can be perceived. For example, pain receptors can be stimulated by low temperature or high temperature but you still feel pain (the same mechanism of sensation). We listed the types of receptors in the skin which are called somatic receptors and we started to explain the coding for: 1) Type of sensation The modality ": *Each of the principle types sensation; touch, pain, sight, sound, is called a Modality of sensation *Each receptor is responsive to one type of stimulus energy. Specificity is a key property of a receptor, it underlines the most important coding mechanism the labeled line principle. *How the sensation is perceived is determined by the characteristics of the receptor and the central connections of the axon connected to the receptor. 2(Receptor Excitation: *mechanical deformation which stretches the membrane and opens ion channels Application of chemicals which also opens ion channels *change in temperature which alters the permeability of the membrane through changing the metabolic rate *electromagnetic radiation that changes the membrane characteristics 4 P a g e

6 3) Strength "intensity" of stimulus: -By the frequency rate of action potential (the discharge rate): so the higher the frequency rate, the stronger the stimulus is because when the stimulus is strong, it able to stimulate the sensory neuron in the relative refractory period and the action potential become shorter and shorter because this increase the rate of frequency. - By the number of neuron fibers that stimulated: weak stimulus is able to stimulate few neurons, but strong stimulus is able to stimulate more neurons. So, the cerebral cortex determines the strength of the stimulus by the number of neurons that transmit the impulses to the cerebral cortex. So high number of neurons stimulated, the stronger the stimulus is and few number of neurons stimulated, the weaker the stimulus is. Note: The different intensities can be transmitted either by using increasing numbers of parallel fibers (spatial summation) or by sending more action potentials along a single fiber (temporal summation). An example of spatial summation. 5 P a g e

7 4) The location: - By the labeled line principle: the sensation of each part in the body is represented by specific area in the cerebral cortex and this representation is contralateral for example, the left side of the body is represented in the right side of the cerebral cortex and the right side of the body is represented in the left side of the cerebral cortex and vice versa and the representation is upside down. - Some parts of the body are represented in large area in the cerebral cortex and some parts are represented in small area in the cerebral cortex, for example the lips have the greatest area in the cerebral cortex then the thumb and the face so: *Lips > thumb > face. According the large area of them in the cerebral cortex *the lower limb has the smallest represented area in the cerebral cortex But what determine the size of the area in the cerebral cortex? That depends on the number of the sensory receptors in the certain area in the body. So, we predict that the lips have the highest number of sensory receptors "high number of nerve endings" and the lower limb and the skin have few sensory receptors. *This is summary for coding of sensory information 6 P a g e

8 (Adaptation) - We classify the sensory receptors in term of adapting into two kinds: 1) Rapidly adapting receptors and we called them "phasic receptors" 2) Slowly adapting receptors and we called them "tonic receptors" So, as we see in the picture there are 4 types of receptors. The broken line which represents Pacinian corpuscle is stimulated and this lead to the increase in the rate of the potential, it reaches 250 impulses per second and then in less than second, this rate become equal to zero. So, Pacinian corpuscle represents very fast adapting "Rapidly adapting" And when the potential become zero, that means Complete adaption in very short time. The second Lines which represent Hair receptor is also represent Rapid adaption but the other represent slow adaption. So, the rate of adaption varies according to the type Of the receptor. Therefore, receptor responds when the Change is taking place. *Rapid adapting receptors include: Receptors responding to pressure, touch, and smell *Slow adapting receptors include: Merkel s discs, Ruffini s corpuscles, and interoceptors that respond to chemical levels in the blood and proprioceptors Note: pain receptors and propripoceprtors do not exhibit adaptation. 7 P a g e

9 *pain receptors are considered as non-adapting receptors because pain is due to damage of tissue. So when we feel pain in certain area in our body, we try to cure this damage. Suppose that pain receptors are adapting so, we cannot feel pain and the tissue damage increase more and more without feeling it and that lead to gangrene. So, the tissue will die and doctors cut this area. Medical situation: - In diabetic patients, some of them have complications and they might lose their sensation of pain. This case is called "Neuropathy": a condition in which a neuron cannot sense the pain. If those patients injure or burn, they will not feel pain. Another example, if the patient wears narrow shoes, he will not feel that نقص ischemia and therefore, there is decreased of blood flow and that lead to color. Then the tissue will die and become blue in."التروية Now, we will discuss rapid and slow adapting receptors in more details: 1) Slow adapting receptors "tonic (static) receptors": - They continue to transmit impulses to the brain as long as the stimulus is there. - They keep the brain apprised to the status of the body - They will adapt to extinction as long as the stimulus is present and that may take hours or days. This point means that if the stimulus is presented in the same strength for long time, these receptors may adapt but that take very long hours and that why they are slowly adapting. - These receptors include: muscle spindle, Golgi tendon apparatus, Ruffini s endings, Markel s discs, Macula, chemo- and baroreceptors. 8 P a g e

10 2) Rapid adapting receptors "phasic receptors": - It responds only when the change is taking place. This means that these receptors cannot be used to transmit continuous signal because they are stimulated only when the stimulus strength changes. These receptors have basal rate of firing. For example, if the stimulus is represented, they transmit more and more electrical impulses. But, if the stimulus is decreased, the number of impulses is decreased rapidly. The importance of basal rate is the control of positive and negative "increasing and decreasing of the rate of impulses". -Fast adaption is important to know two things: 1) strength of stimulus 2) the rate of change *Rate and Strength of the response is related to the Rate and Intensity of the stimulus * Because fast adaption tells us the rate of change, so we can multiply this rate by time (predicting the future position or condition of the body) *very important for balance and movement *types of rapidly adapting receptors: Pacinian corpuscle, semicircular canals In the inner ear ***REMEMBER: *Signal intensity is critical for interpretation of the signal by the brain (i.e., pain). 9 P a g e

11 *Gradations in signal intensity can be achieved by: 1) Increasing the number of fibers stimulated, spatial summation. 2) Increasing the rate of firing in a limited number of fibers, temporal summation. - What is the main difference between phasic and tonic receptor? * As we see in the picture (a), when the stimulus is applied, and when it is decreased the rate of action potential still constant without changing. This till us that there is no adapting, it needs long time to adapt. (Tonic receptors) * As we see in picture (b), there is a stimulus applied. It started with very high rate of impulses(burst of activity), then it started to reduce firing rate although the stimulus still there and this till us that they are fast adapting. (Phasic receptors) SO, the main difference is: -the rate of potentials is constant in Tonic receptors - the rate of potentials is changed (start with increasing and then decreasing until they reach zero which mean complete adaption) in Phasic receptors. 10 P a g e

12 (Receptive field) Receptive field: it is the area from which we collect information for that receptor. And there is an inversely relationship between the size of receptive field and the density of receptors in this area. So when the receptive field is smaller, the density of receptors is higher. For example, the fingertip has small cutaneous receptive field but high density of receptors. Also, when the receptive field is larger, the lower density of receptors in this area (back and legs have few sensory endings). Note: The smaller the receptive field, the sharper the information (the better the localization), and the larger the receptive field, the loss of localization. If the sensation is coming to the brain from one area or from two areas, the stimulus stimulates one or two receptors. If it stimulate one receptor-even if they are two hits, I will perceive them as one point. But, if the stimulus hits two receptors (two receptive fields) I will perceive them as two points. - Two point discrimination: Minimum distance at which 2 points of touch can be perceived as separate. This distance depends on the density of receptors in certain area for example; fingertip has high density of cutaneous receptors so, the minimum distance between two touch points in fingertip is 2mm in order to discriminate them as two points. But, in other parts of the body which have large receptive fields for example in the back of the body this distance is larger, it almost about more than 5 cm. 11 P a g e

13 Note: If distance between 2 points is less than minimum distance, only 1 point will be felt. (Neural processing) Neural processing means the way in which sensation processed. The area in the center of the receptive field is the most stimulated area so, a weak stimulus will only stimulate the area in the center (which is called Discharge zone) so, the center area reaches threshold, but the peripheral area is not stimulated so the receptor potential in the peripheral area is close to threshold but it does not reach threshold when the stimulus is weak (this area is called facilitated zone). But, if the stimulus is strong, it will stimulate more area on the periphery. (Neuronal Pools) - Groups of neurons with special characteristics of organization. 12 P a g e

14 * Types of neuronal circuits: 1. Divergence: the neuron divides at the axon. There will be loss of the localization because the stimulus spreads too many neurons. (Output from one neuron onto many) *This type is bad for localization. 2. Convergence: (output from many neurons onto one), this type includes summation of EPSPs (temporal or spatial) and this summation will reach the threshold *There are two types of convergence: A. Convergence from a single source. (Temporal summation): the summation of EPSPs reaches the threshold (End with action potential). This type is good for Localization B. Convergence from multiple sources. (Spatial summation): Also, the summation of EPSPs reaches the threshold (End with action potential).this type is bad for localization 13 P a g e

15 - (Lateral inhibition): Lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition prevents the spreading of action potential from the excited neuron (in the center) to the peripheral division in the lateral direction. This creates contrast in stimulation that allows increased sensory perception which mean sharpening of sensation. Note: when there is no lateral inhibition, there is diffusion of sensation but when there is lateral inhibition, there is no diffusion of sensations (sharpening of sensation). Lateral inhibition and the resultant sharpening of sensation occur within the central nervous system. (CNS) 14 P a g e

16 *Example for sharpening of sensation by lateral inhibition: When a blunt object touches the skin, a number of receptive fields are stimulated. The receptive fields in the center areas where the touch is strongest will be stimulated more than those in the neighboring fields where the touch is lighter. Stimulation will gradually diminish from the point of greatest contact, without a clear, sharp boundary. What we perceive, however, is not the fuzzy sensation that might be predicted. Instead, only a single touch with well-defined borders is felt. This sharpening of sensation is due to a process called lateral inhibition. 15 P a g e

17 * In the picture above we notice that there is sharpening of stimulation in(c) because of lateral inhibition. - Other types of sensations for prolonging time of signal: 1-Reverberating Circuits 2-Synaptic after discharge 3-Parallel circuits 1) Reverberating circuits - An incoming signal travels along a chain made up of neurons; each neuron along the path is linked with the previous cell by collateral synapses. So, there is one stimulus that will result in action potential that moves in a circle and that lead to prolongation the time of signal. 2) Synaptic after discharge Remember that EPSP is a local, summated, graded depolarization, it takes time more than action potential, but it decay faster. since the time of EPSP (15-20 msec) is longer than the time of Action Potential ( msec) then more Number of Action Potential per one EPSP (if the EPSP was above the threshold for 10 msec it will generate action potential for 10 msec) EX: when you close your eyes after looking to the sun for a long time, you are going to see a light (shadow of the sun) even your eyes are closed!, WHY?? 16 P a g e

18 The sun has stimulated your electromagnetic receptors, made a synapse and many discharges, this discharges won't stop until depletion of synaptic vesicles that house neurotransmitters in the synapse. 3-Parallel circuits -here we have too many outputs, with different number of synapse in each output, but it will not prolongate as long as Reverberating Circuits. ( each action potential <output> will reach the cell body at different time, depending on number of neurons <synapse> on its way, so we will have at first output 1, then output 2 and so on, this will Prolong the time of action potential ) Stabilization of neuronal discharge We have seen previously that Reverberating Circuits will never stop theoretically, actually, our systems will stabilize neuronal discharge by many ways: Synaptic fatigue: It is caused by a temporary depletion of synaptic vesicles that house neurotransmitters in the synapse, generally produced by persistent high frequency neuronal stimulation. 17 P a g e

19 Neuronal inhibitory circuits: *Gross inhibition Basal ganglia inhibits muscle tone *Feed back inhibition-cortico-fugal fibers from cerebral cortex descending fibers to control the intensity and sharpness *Downregulation and upregulation- Long term stabilization through modification of the receptor availability (internalization or externalization) (We mean here neurotransmitters receptors which are membrane receptors, not sensory receptors) " ال ت صح ب ع ل و الم ست وي ات ب د ن و الن يات " إن ما ال ع مال بالن يات 18 P a g e