راما ندى أسامة الخضر. Faisal Muhammad

Transcription

1 22 راما ندى أسامة الخضر Faisal Muhammad

2 Revision Last time we started talking about sensory receptors, we defined them and talked about the mechanism of their reaction. Now we will talk about sensory receptors, our objectives are to: define these receptors talk about their mechanism of reaction and the coding mechanism how the system calls for intensity and how it calls for modality Talk about adaptation and the neural processing. Neural processing: transmit of the impulses from the receptors to the cerebral cortex. You don t feel the sensation consciously unless these kinds of sensation reach the cortex, if they don t reach they are inhibited down and we don t feel them. Usually 90% of the sensation aren t felt (they are inhibited down or adapted before they reach cerebral cortex). We classified receptors according to the modality (type of sensation) to: 1. Mechanoreceptors: they detect mechanical deformation of organs. 2. Thermoreceptors: they sense the change in temperature. 3. Nociceptors: these receptors are for pain (pains are usually caused by a tissue damage). 4. Electromagnetic (photoreceptors): they detect light. 5. Chemoreceptors: they detect changes in chemical concentration. Examples: changes in oxygen concentration, changes in partial pressure of carbon dioxide and so on. So these are the classifications ;) Another classification is according to location: 1. Exteroceptors: are those that are located in the surface of the body (we can find them in the external skin), they sense changes in temperature, pressure, touch and pain. 2. Interceptors (visceral receptors): they are found inside the visceral organs (internal organs), like: abdomen, heart, lung, urinary track and kidney. 3. Sometimes we consider the receptors that sense changes in position as exteroceptors, other times we consider them as a special type of receptors called proprioceptors. 1 P a g e

3 So, proprioceptors sense the change in position by changing the angulation of the joint. These receptors are usually found in the muscles, tendons or around the joints. Note: Whatever the type of receptors is, the mechanism of action is the same. Tactile receptors (receptors that are found in skin): 1. Free nerve ending: they are connected to two types of neurons which are Aδ (5-30 m/sec) And C (0.5-2 m/sec). They are found in the skin and they might be sensitive to pressure, temperature, pain or touch. Note: c fibers are very slow in conducting impulses. Before we continue you should know that there are two types of pain: a) Slow pain: it s connected to C fibers and it does accumulate, like: toothache. b) Fast pain: it s connected to Aδ fibers. Example: in case of injury by a knife the first part of pain called fast pain. Now let s return to tactile receptors. 2. Merckel's disc (expanded tip tactile receptors): The name refers to Merckel who defined them. The expanded tip of neuron form a dome called Iggo dome (the name of the person who described it) which projects upward towards the epithelium. Iggo dome is innervated by a single large myelinated nerve fiber (Aβ). They are found in hairy and nonhairy (glabrous) skin and they are sensitive to touch since we can find them in skin. 2 P a g e

4 Note: Glabrous skin: nonhairy skin as in lips and nail buds. 3. Tactile hair receptors: they are sensitive to hair movement and we can find them around hair follicles, they are called hair plexus or hair shaft. 4. Pacinian corpuscle: According to the name of the person who described it. It has a central nerve fiber extending through its core, surrounding this central nerve fiber are multiple concentric capsule layers. They are found in dermis (beneath the skin (epidermis)) so they are responsible for pressure and they are found even around the joints. They are sensitive to pressure, vibration and changes in the mechanical state of the tissue, therefore sometimes we consider them as proprioceptors. These receptors adapt very fast that s why they are responsible for vibration. Like the vibration of tuning fork, it switches rapidly between on and off, these kinds of sensation need type of receptors that adapt rapidly. 3 P a g e

5 5. Meissner's corpuscle: They are found in the dermis and around the joints (therefore they are considered as proprioceptors). There are corpuscles around them so they adapt within parts of a second after they are stimulated which means that they are particularly sensitive to the movement of objects over the surface of the skin as well as to low frequency vibration. They are connected to Aβ (fast) nerve fibers (30-70 m/sec), they are found in nonhairy skin (glabrous skin), like: finger tips and lips, and they are sensitive to touch and vibration. 6. Golgi tendon and muscle spindle: (Golgi: according to the name of the person who defined it) They are found in muscles (Golgi tendon in the tendon of the muscles and muscle spindle inside muscle fibers), they are classified as proprioceptors. 7. Krause's corpuscle: They are found in the dermis and around the joints (similar to Meissner's corpuscle). 8. Ruffini's ending: They are found in the dermis and sensitive to pressure (adapting slowly). What do we mean by saying rapidly and slowly adaptation? Slow adaptation receptors continue to transmit impulses to the brain as long as the stimulus is present. For instance, an impulse from the muscle spindle and Golgi tendon allows the nervous system to know the status of muscle 4 P a g e

6 contraction and the load on the muscle tendon at each instance.rapid adaptation receptors cannot be used to transmit continuous signals because they are stimulated only when the stimulus strength changes. Thus, in the case of Pacinian corpuscle, a sudden pressure applied to the tissue excites these receptors for a few milliseconds, and then its excitation is over even though the pressure continues. Later, however, it transmits a signal again when the pressure is released for an example. Note: Usually the locations of the receptors contribute to the type of sensation to which they respond. Examples: Receptors which are found in the dermis are sensitive to pressure as in Pacinian corpuscle. Receptors which are found in the epidermis are sensitive to touch, pain and temperature as in Merkel's disc. Note: The rapidly adapting receptors are stimulated only when the stimulus strength changes. Ex: paroreceptors sense the change in pressure (not pressure). Mechanism: Receptors in general are two types either separated from the neuron or a terminal part for an efferent neuron. If it's separated from neuron, the communication between the neuron and the receptor occurs by transmission of chemical messengers (neurotransmitters), but if it is a terminal part for a neuron, the communication occurs directly. 5 P a g e

7 What does happen when there is a stimulus? The mechanism is the same in both types, when there is a stimulus, it changes the permeability of the ionic channels which are called a stimulus gated channels. They open or close according to the presence or absence of stimulus, suppose that the channels here are sodium channels so, when there is a stimulus, they will open and the membrane will be depolarized, this will change membrane potential in this area, this change in membrane potential is called receptor potential. Definition: Receptor potential: the change in membrane potential in the area of receptor. If the receptor is a terminal part for a neuron, we call it generator potential but if the receptor is separated from the neuron, we call it receptor potential. So the mechanism is the same, stimulus changes the permeability of the receptor gated channels and causes membrane depolarization. If the receptor potential reaches the threshold, it will cause action potential in the afferent neuron, but what does happen if it goes above the threshold? 6 P a g e It depends on how much the amplitude. When receptor potential rises above the threshold level, it increases the intensity of the amplitude that makes the action potential frequency greater as shown in diagram below-.

8 To illustrate the meaning of the amplitude suppose that resting membrane potential is -70 mv and when the stimulus occurs the membrane potential becomes -30 mv then the amplitude is 40 mv. The effect of stimulus strength on RP amplitude Note that the amplitude increases rapidly at first but then progressively less at high stimulus strength. Note that the receptor area is non-excitable tissue, which means that the membrane potential will go to less negative until it reaches theoretically +61 mv if it was sodium channels. ****Excitable means that if the stimulus reaches the threshold it will cause action potential. 7 P a g e

9 Why the receptor area is non-excitable? If it was excitable once it reaches the threshold there will be an action potential and we won t have an amplitude for the action potential (none or all principle), so it's non-excitable to discriminate the differences in intensity of different stimuli. Example for illustration: If there are two actions potential one is weak and the other is stronger but both reach threshold, how does the receptor area discriminate the differences in intensity between the two stimuli? **Before we start, suppose that the resting membrane potential is -70 mv, the threshold is -40 mv and the duration of action potential is 10 milliseconds, so let's begin the party In the first condition when the stimulus is weak, it changes the membrane potential from -70 mv to -40 mv; the amplitude equals to 30 mv, it reaches the threshold but it didn't exceed it, in this case, we have one action potential and the duration of it is 10 milliseconds so the frequency is 100 Hz. 8 P a g e

10 Definition: Frequency: is how many action potentials a neuron can generate per unit of time (rate of action potential). If the stimulus is stronger and the changes membrane potential from -70 mv to -20 mv, the amplitude here equals to 50 mv, it's stronger than the threshold so, it's going to stimulate the neuron during relative refractory period. In this case, the period of the action potential is 5 milliseconds and the frequency is: The cerebral cortex when receives 200 mv action potential per second, it will be considered as a high intensity action potential. What is the maximum frequency of neuron? 1000 Hz when the action potential is generated at the end of the absolute refractory period and can't be exceeded since the duration of this period is 1 millisecond. So, the importance of the non-excitable tissue in the receptor area is to discriminate different strengths of stimuli. Remember!!!!! In the excitable tissue when the stimulus reaches threshold it will cause action potential regardless of the intensity of stimulus. *This mechanism is the same for receptor potential and generator potential. Summary: - The receptor potential is a local 9 P a g e *The first node of Ranvier is spike generating region (action potential generation region) and the axon called conducting region.

11 potential, can be summated by either spatial or temporal summation (discussed in handout 3), doesn t follow non-or all principle and it's not propagated. ending. Each receptor is sensitive to one type of modality. For instance, temperature receptors are sensitive to temperature and the pain receptors are sensitive to pain although both are free nerve If you touch your skin, you will stimulate the receptors of touch and these receptors are attached to a specific pathway to a specific area in the cerebral cortex, then the cerebral cortex determines its type according to the path and the receptors that it came from, the Labeled line principle, as we call it. Definition: Labeled line principle: the specificity of nerve fibers in transmitting only one modality of sensation. The question here is by what the type of sensation is determined (modality)? 1* The specificity of the receptors. 2* The labeled line principle (pathway). We said that the receptor is sensitive to one modality of sensation, this modality is called adequate stimulus for this receptor. Note: نوعي وليس كمي quantitative. Adequate is qualitative not Examples: The adequate stimulus for pain receptors is pain. The adequate stimulus for touch receptors is touch. The adequate stimulus for thermoreceptors is temperature. 10 P a g e

12 That means that these receptors have very low threshold for this type of sensation and they can be stimulated by another type of sensation (must be strong!). For instance, if pain fibers are stimulated, the person perceives pain regardless of the type of stimulus excites the fiber. As an example, it can be stimulated be electricity, overheating of the fibers, crushing of the fiber or stimulation of the pain nerve endings by a tissue damage. This is a lateral view of the cerebral cortex. Imagine that our body is represented on it, if the sensation comes from a specific region in the body, it will be transmitted to a specific area in the cerebral cortex that is responsible for this area. This is how you can feel by your leg or your hand, so the labeled line extends from the receptor that locates in a specific organ to the area where is responsible for it in the cerebral cortex. Note that: The imaginary representation on the cerebral cortex is for our somatic organs (our body) not for visceral organs. The representation is upside down (inverted), which means that the head is represented in the most lateral portion of somatosensory area, and the lower part of the body is represented medially. 11 P a g e

13 The representation is contra lateral, that means that each lateral side of the cortex receives sensory information almost exclusively from the opposite side from the body. The representation is not proportional to the size of the organ, the size of these areas is directly proportional to the number of specialized sensory receptors in each respective peripheral area of the body. For instance, a great number of specialized nerve endings are found in the lips and thumb, whereas only a few present in the skin of the body trunk. You have to study slides 12 P a g e in addition to this sheet