Reverberating Circuit Example

 Neuronal pathway: group of neurons along with the nervous system structure that function to transmit certain information from its initiation (start point) to its destination (end point). Some of them are long pathways like sensory pathway (ex: from the foot to the cortex, almost 1.5 miter), and some of them are small pathways for short distance. The purpose of having more than one neuron is for processing of the information whenever this information goes through new neuron, new synapse and new dendrite. Every pathway contains more than one neuron which is important for processing (usually we have first order, second order, and third order neurons). not all the pathways are only series of axons, there is also what we call:  Divergence “one signal is transmitted to more than one area “: we start with the first neuron which splits into two branches (called branching axon or collateral axons) or more than two branches (destinations).  Convergence: when there is more than one information are end into one axon which transmits them to the second and third destination.  Neuronal circuits or pools: Groups of neurons with special characteristics of organization which work together. they have a specific arrangement and it’s more complex than the simple divergence and/or the convergence. Ex: Reverberating circuit (positive feedback): certain neuron will diverge into two branches, the first continues to the destination(output) and the second return back to reinforce the same circuit (re-excite the input of the same circuit).So if this neuron was firing 10 action potentials per second, the output will be also 10 per second, but these 10 action potentials will reoccur again and again (you will continue firing 10 action potentials and it won’t stop alone unless you stop the circuit). This is important when you need a continuous signal from a certain stimulus such as when you want a certain rhythm of heart rate or breathing, you just send an order (stimulus) which will continue signaling by this circuit; so, you do not need to send the same order every minute or second. we also need an outside control to increase or decrease the rhythm. Some parts of the cortex contain reverberating circuits that are concern with Short term memory; this means that you will remember an information just for a moment then when you go away you forget it. this is due to the activation of the facilitated reverberating circuit for short time then inhibit it and activate another circuit  Parallel after discharge: the most complex and the most important one in the cortex, important for high functional processing and timing. Here there is collaterals of more than one pathway that end on the same output; it’s primarily a divergence but what happen exactly is that after the divergence all the three pathways for example will be converted to one neuron. in addition to that each pathway has a different number of neurons and synapses alongside the pathway. consequently; the output neuron will receive three action potentials; the first from the first pathway then after some delay due to the number of synapses it will receive the second then the third. This arrangement gives a complex processing, because you start with one information then diverge it into three parts through three pathways with different timing and processing. and so, you end up with very complex processing and a complex output. What is the importance of these pathways? As you know every pathway has more than one axon, also they differ in their arrangement which is important for the type of sensation you may sense. I n other wards, in the upcoming lectures we will talk about somatosensory pathways. and as we have two types of touch; one of them will be transmitted through PCML (posterior column-medial lemniscal) pathway and the other through ALS(anterolateral-spinothalamic) pathway, but why do we have two pathways although they are both touch? because they differ in their types; one of them is discriminative (fine touch) and the other is non-discriminative (crude touch). as every type has different destination and gives different information, it needs a different pathway even though they start at the same point (same receptor).  if we stimulate two receptors and they stimulate two different neurons (first neuron) but at the end in the synapse they converge into one neuron (second neuron) and this neuron continues to the cortex in the brain. here if you stimulate one of the first neurons or both, the brain won’t differentiate where is the original stimulus because it will receive the signal from one neuron which is the last one. (non-discriminative)  if the pathway does not have any convergence, this will help in the discrimination of the two signals(stimulus). (discriminative) To sum up: Although we have the same receptor, area, and distance the difference in the pathway will give different types of touch; discriminative and nondiscriminative. the arrangement of the pathway also can help to get better resolution of the sensation. this is achieved by lateral inhibition which is a type of divergence where the most stimulated neuron will inhibit the other less stimulated neurons. If you are pressing on three neurons in your hand for example , the most pressed one will inhibit the others resulting in better resolution . this applies also for other types of information such as in vision. ✓ In all this course, you must be familiar with MRI pictures and detect the side of the lesion and the corresponding defects. in the exam the lesion will be marked by an arrow to make it obvious and easier to detect, now your mission is to know what this part of the brain is just by being familiar with the outlines of each part. when you know the part of the brain that has the lesion you can determine the corresponding defects through the functions you now of this part. To sum up, if you have an MRI picture: 1- By the outline you can detect the targeted part of the brain. is it midbrain, medulla, cortex … etc. 2- As you know the function of each part you can detect the defects. 3- The lesion may be presented with hypodensity or hyperdensity, or with change of shape without any change of density. ❖ Localization of the problem To determine the original site of lesion, stroke or the problem by examining some defects or loss of functions in the patient. It could be easy if the defect was localized in a confined area with specific function such as nucleus X with Y function, whereas if the defect was in a long pathway such as in sensory system which extends from spinal cord to the cortex, here the loss of sensation will differ between the sites of lesion along the pathway; is it in the spinal cord, or the brainstem or the cortex? (different location will give different symptoms), and this also applies to the motor pathways that extend from the cortex to the target organ. Motor order to your hand will be transmitted from the cortex to the upper part of the spinal cord then through brachial plexus to the hand). Motor order to your face will be transmitted from the cortex to the facial nucleus then to the facial nerve and to facial muscles. Question (refer to slides) …you will face at least 2 Q like this in the exam The answer is E. Illustration: because the patient is conscious and can understand the cortex is healthy. the third and fourth nerves are functioning well and because they are located in the midbrain, anything above the midbrain is healthy, whereas anything below is defected. this syndrome called pontine locked in syndrome (the patient can only move his eyes). (through this course you will be given a table of syndromes to memorize) The reason of the symptoms could be:  Inflammation and demylination: associated with relapse improvement (e.g.: multiple sclerosis)  Stroke or sudden injury: very acute; the patient is healthy and suddenly becomes symptomatic due to vascular deficit (stroke or hematoma or leakage) or injury (stab wound).  Tumor: when the patient has gradual loss of function (gradual loss of sensation for example starts with his hand then extends to the whole arm); this indicates a growing tumor. ✓ Sensations Sensation is a type of input which sends informations to the CNS, they are like the input devices of the computer. we have different types of sensation such as vision, hearing, body position, touch, pressure, and therma. Every type of sensation has a specific receptor; so, if we want to detect a mechanical change and convert it to an action potential we need a mechanical receptor. The function of the receptor is to receive (detect) the signal and convert it (processing) to an action potential. so, we have different receptors that detect different types of energy and convert them to an electrical energy. Usually sensation is subdivided depending on the embryological origin into:  Special: it’s only found in the head area such as smelling, hearing, vision, taste, and balance.  General: it’s found in other parts in the body with or without the head such as touch, thermal, pain, vibration, pressure, types of touch, tension of the muscles, muscle and joint movements, and length of the muscles. scientifically called “somato sensation “ Proprioception or propriosensation Sensation in the muscles and joints which gives informations about muscle and joint position , muscle tension and muscle length . it’s a type of general sensation. Every type of sensation need a special type of receptor so that our body has a huge number of receptors with different types and located in different layers (we don’t need to memorize all of them :P just to understand the basic idea ) .