sensory nerve fiber classification

Transcription

1 Before you start this sheet, you should notice that the order of subjects is not the same as the lecture. and you should refer to the slides and handout for extra information and pictures. Today's lecture is going to be about: sensory fiber classification. sensory pathways, and associated disease. tactile hallucinations. sensory nerve fiber classification sensory nerve fibers were first classified into general classification until the muscle spindle and the golgi tendon were found, the sensory (alternative) classification appeared. 1- General classification: Type A fibers which is subdivided into: A- α, A - β, A - γ and A - δ. Type A is large and medium sized myelinated nerve fiber with a conduction velocity from m / sec. Type C fibers: small unmyelinated nerve fibers. 2-Alternative classification: this was established when the muscle spindle and the golgi tendon were found. where type A α was subdivided into two groups: Group Ia fibers (Sensory fibers from the annulospiral endings of Muscle Spindle). Group Ib fibers (Sensory fibers from the Golgi Tendon Organs). other groups included are group II, III, IV. the doctor said that we don't have to memorize all of them, we should only memorize the important ones that we'll come across during this course, understand them and be able to identify the different nerve fibers. sensory inputs pathway almost all sensory information from somatic segments of the body enter the spinal cord through the dorsal roots of spinal nerve and then to the brain. in this lecture we talked about one of the pathways which is the dorsal column-medial lemniscal system, and as the name implies it carries signals in the dorsal columns of the cord. Modalities carried by the dorsal column : This pathway is responsible for discriminative touch, and whenever we say discriminative touch we mean by it the following : 1- precise accurate touch. 2- conscious proprioception(the most important) : when I close my eyes and bend my elbow, my brain knows that my elbow is bent. 1

2 3- pressure. 4- vibration (first sensation to be lost in diabetic patients). Transmission in the pathway First : sensory inputs of the lower limb will enter the spinal cord through the dorsal root and reside in the midline, medially. and ascend in the spinal cord without any synapses. these ascending fibers are called fasciculus gracilus (fasciculus: a group of nerve fibers, gracilus: cylindrical). fibers coming from the upper part of the body -upper arm and upper chest-are going to lie laterally in the fasciculus cuneates (widge shape) which will accompany the gracilus. These two fasciculi will ascend ipsilaterally until they reach the lower medulla where they will first synapse (first synapse) giving a second order neuron,then decussate (cross to other site). As the nerve fibers cross they will be called medial lemniscus. when decussating happen, fibers will invert. meaning that the fibers coming from the lower part of the body which are residing in the spinal cord medially are going to go laterally in the thalamus at the ventroposterior lateral nucleus. in this pathway through the brain stem the medial lemniscus will be joined by additional fibers from the sensory nuclei of the trigeminal nerve which carry sensation from face (these sensations subserve the same modality of sensation the dorsal column subserve from the body),and reside medially in the thalamus. second: in the thalamus these fibers will make a second synapse and a third neuron will ascend to cortex. here it's called the thalamocortical radiation. ventroposterior lateral nucleus of thalamus is responsible for fibers coming from the whole body. ventroposterior medial nucleus of thalamus is responsible for sensation of the face. (it's called radiation because it spreads to cortex and will be more representative in the next area it's going to). then finally it will cross the internal capsule between the thalamus and putamen and end at the sensory cortex. note: you should always remember that the thalamus function as a secretary, it doesn't allow any information to go directly to the cortex (which can order any part) without checking it, that's why it's called the clearing house. what are we suppose to know from each pathway as physician. the sensory modality carried by the pathway because if we lost the sensation we can tell which pathway is affected. know the decussation because it help us to identify site of lesion (right or left). here in the dorsal column it happen in the lower medulla. 2

3 know spatial orientation: in dorsal column the lower part is medial in the spinal cord then goes laterally in the thalamus, then back medially in the cortex. it's very important in spinal cord because it helps us to determine the origin of tumors, wither from the cord material itself or metastasized from other sites ( both cases we have different sign and symptoms and prognosis). you have to be careful about the things that were mentioned above in the central nervous system. the fibers that are carried from the body fast or slow conducting? the dorsal column is composed of large,myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110m/sec.(guyton) what is the information that reach within a fraction of second to the brain? the answer is equilibrium. why? because if a person bent down and the brain didn't get the position of his body within fraction of seconds, the person will fall and get injured or fractured. as we said equilibrium and proprioception are the fastest sensation so here the velocity makes a difference, because any delay could lead to injury. pain is related to the slow conducting fibers, fiber C and A- δ. pain need to be tonic, continuously alarming the brain about painful situation (tonic receptor--> no adaptation). so wither it's transmitted within 0.5 m/sec or 100m/sec it doesn't make a big difference. cross section in the spinal cord. 1- sacral- lumbar-thoracic-cervical from medial to lateral. in the dorsal column(posterior aspect of the spinal cord). 2- it's very important in diagnosing disorders. we face a lot of patients who suffer from cancers, which could be primary (ex: breast cancer in female, prostate cancer in male) or secondary. secondary's in vertebra press on the posterior column and this will lead to loss of proprioception and vibration in upper part of the body. cervical is the first to be affected because it's lateral and the injury will be on the same side(ipsilateral) because the tract hasn't decussate yet. 3

4 tumor arising from the material of spinal cord pressing on dorsal column will lead to loss of proprioception and vibration in the lower limb, also ipsilateral. tumors from inside are more destructive and have more bad prognosis than the secondary or outside tumors. spatial orientation is very important for diagnosing of disorder, any lesion in the CNS above the level of lower medulla, the effect will be contralateral. Drawing of a thalamus from monkey(chimpanzee). the doctor here explained what am going to write through a picture, but it's not in the slides but i wrote it anyway. monkeys are very similar to humans beings and they have been very well studied because they are the closest to us. as you can see stimulation (like touch and proprioception) from the lower part will give electrical activity to the lateral part of thalamus. in the picture the ventroposterior lateral was in red color while the ventroposterior medial was in blue. the more you go to the face area, electrical activity can be recorded from neurons in the medial aspect of thalamus and this is a proof that there is turning in the medial lemniscus pathway. By the time the thalamocortical radiation arrive to the somatosensory cortex the upper part will be presented laterally and the lower part medially. important notes to notice when you see how your brain present the body in the cortex. the actual intense sensation that the brain care about is the face area specifically the mouth area(lips) and upper limb usually the hand majorly the thumb. we conclude from this that the mouth has important functions other than taste, like talking. for example if you lost a little sensation in tongue or cheek area you will be stuttering and you will have a problem with one or more of the letters. for hand we can see that the thumb resemble 50 % of the capability of hand while the finger 20 % and the other each 10%. why is that? because if you lost the index you can replace it by the middle finger or ring finger but whenever the thumb is injured we can't replace it. so keep in mind, in sensory cortex, the libs, face, hand and thumb are majorly affected in injury. 4

5 Major diseases affect the dorsal column There are diseases that affect the dorsal column with highly specific manner, leading to sensory ataxia(a: without; taxia: order), so the patient will sway right and left as he move or stand ( sign of disequilibrium ). 1- Vitamin B12 deficiency: it's one of the major health problem in Jordan along with hypertension, diabetes, lung cancer and others. it specifically hits the dorsal column leading to a disease known as subacute combined degeneration of the spinal cord. subacute reflects that the vitamin B12 deficiency is prolonged and it needs weeks to month to develop this disease so whenever the patient starts to sway left and right this means that there is degeneration in the dorsal column and it should be treated abruptly or otherwise the patient will lose equilibrium combined reflects that it involve more than one pathway : The dorsal column (sensory). The lateral corticospinal tract (motor). it will hit both of these pathway bilaterally. The patient will have spastic paresis (the lower limbs become very spastic because of the damage to the corticospinal tract) and loss of equilibrium, proprioception and vibration in the areas affected below of the lesion. Vitamin B12 deficiency are more common seen in vegetarians, fruitarians, alcoholics, people that don t care about their diets, GI surgery with removal of large part of the stomach and intestine and crohn's disease. 2- STD (syphilis) : syphilis has been present from the ancient time up till now, and it continue now because it comes as a co-morbidity with AIDS. it starts with the entry of a bacteria (Treponema pallidum), which usually enters from the sides after contact with an infected partner. and it develop in three stages: 5

6 primary syphilis. approximately 2 weeks after infection, an ulcer will usually appear on the side of entry like mouth or genitalia. this ulcer is called Chancre. and wither you treat it or not, it will disappear after two weeks. It's very much similar to the herpes simplex virus that comes with the flu. so not everyone with an ulcer on his lips have syphilis. the question now is how can we differentiate? we can do this by one important question, is the ulcer painful or not? if the answer is yes and it's painful, then you don't have to worry about it. But if the answer is NO and it's painless, then you should worry because it is an indicator for something bad, such as primary syphilis, cancer, or autoimmune disorder, and you should do a syphilis test. note: you should be careful when you deal with this ulcer because it's very infective and Treponema pallidum is transmitted through body fluids! Secondary syphilis. After two months or more the patient will come suffering from a flu-like disease, lymphadenopathy and rash that involves even the palms and the soles, this is almost diagnostic and pathognomonic for Secondary syphilis and these patient are highly infective. note: the rash that results from certain drugs or foods is rarely or highly unlikely to involve the palms and soles. Primary and secondary syphilitic patient are easily treated with Penicillin! A single shot of penicillin enough to treat the disease! Tertiary syphilis. There will be involvement of the dorsal root and the dorsal column so the patients will suffer from the 3 P's : Pain. Paresthesia. Polyurea. why pain? even though we didn't mention pain transmission in the dorsal column? because all sensations will enter the dorsal root then pain will go on different pathway. so involvement of pain pathway will lead to shoot pain. 6

7 Typically patient with tertiary syphilis comes with high-strike (the leg is lifted more than normal), because he doesn t have proprioception. There is eye involvement which is called argyll robertson pupil (Prostitute's Pupil ) where the eye can accommodate (when I look at far object-->dilation, look at near object-->constrict) but it doesn t react to the light. A scandal. it's about an experiment that was made between in the U.S, where scientists kept 400 African-American without treatment from syphilis, just to observe and study the path and stages of syphilis, keeping in mind that the penicillin was available at that time, which is highly unacceptable and it made a huge scandal. As a result now there is a medical and ethical committee, that doesn't allow any research except if there is objectives that will help and improve the care of patient. Tactile hallucination is a feeling of touch without the presence of an external stimuli, which occurs in the case of : withdrawal of certain drugs. in psychiatric disturbances. Delirium tremens : which is sudden withdrawal of alcohol after a heavy chronic consumption. a disturbance in the brain chemistry (disturbance in the sensory cortex ). ( hallucination is always related to change in brain chemistry.) It has two types: pleasant In the form of hugging, kissing & sexual sensations. Typically the patient is male, young, years old. He is laughing all the time, spending a lot of time in his room, neglecting his personal hygiene, and doesn t want to socialize or eat. The patient doesn't want to heal and doesn't seek help because he is happy, but his family will seek help from the doctors. because if he has a work he will leave it, if he is a student he will quit school..etc. Usually they heal perfectly, if they were treated, and within 6 months they will come well-groomed, have returned back to work or school and talking all sense. in the majority the stimulus present all day, and they are very hyperactive and rarely sleep. 7

8 unpleasant Comes in the forms of pressure, strangulation, hitting. The patient will seek medical help. They are treated perfectly. so as we said hallucination results from disturbances in the brain chemistry so they are wellcontrolled & treatable in compare to the other neurological diseases which are hopeless, even though relapses may present, but they usually recover. sorry for any mistakes. Done by: Aseel Yaseen. 8