Omar Sami. Muhammad Abid. Muhammad khatatbeh

Transcription

1 10 Omar Sami Muhammad Abid Muhammad khatatbeh

2 Let s shock the world In this lecture we are going to cover topics said in previous lectures and then start with the nerve cells (neurons) and the synapses and integration of responses. Rhythmicity of some excitable tissues: Well, at first what does the word Rhythm mean? Rhythm: repetitive self-induced discharges which occur normally in the heart, smooth muscles and in many neurons. Simply, it means that some structures undergo action potentials in different rates, for example heart muscle undergo 75 action potentials per minute, while others undergo 60, 40 action potentials per minute and so on You may wonder why there is a difference in potentials between tissues. This is due to the difference in properties, as some tissues may have high leakage for sodium ions while other may have a lower leakage in sodium ion which in turn will result in different rhythm. Let s take a little bit closer look at the cardiac conduction: Firstly, the conducting system provides the heart its automatic rhythmic beat, the electrical signals are transported from the SA (sinoatrial) node to the AV (artrioventricular) node via the conductive tissue of the heart, which allows the action potential to be spread towards muscle cells. Secondly, the cardiac muscle potential has a phenomenon called plateau, so what is plateau and why does it prolong the period of depolarization? Well, in some instances, the excited membrane does not repolarize immediately after depolarization; instead the potential remains in a plateau for many milliseconds, and only after that does the repolarization begins. ** this type of action potential occurs in heart muscle fibers, where the plateau lasts for seconds and causes contraction of heart muscle to last for this same long period But why does the plateau occur? Well, the cause of plateau is a combination of several factors: 1 P a g e

3 A- In heart muscle, there is two types of channels that participates into depolarization process, (1) the usual voltage activated sodium channels, called fast channels, and (2) voltage activated calcium channels, which are slow to open so they are called slow channels. So what actually happens due to these channels? Opening of fast channels causes the spike portion of the action potential, but don t forget that calcium channels are slow channels, so calcium ions will prolong the action potential because of their slow opening which will cause the plateau. B- The second factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower to open than usual, often not opening much until the end of the plateau. This factor delays the return of the membrane potential toward its normal negative value of -80 to -90 millivolts. The plateau ends when the calcium and sodium channels close and permeability to potassium ions increases. Now let s start with the nerve cells (neurons) and the synapses and integration of responses. Firstly, let us have a closer look at the component of the neural cell, and then approach more to the propagation of the action potential along the neural cell. 2 P a g e

4 ** Around the neural cells there is another types of cells, called supportive cells. Studying supportive cells is a huge field in neurology, which can t be covered in a sheet. 3 P a g e So, how are action potentials generated at neural cells? And how are they getting transported?

5 Synapses and integration of responses: After neurotransmitters are synthesized in the cell body, they are transported to the terminals and stored there, now let s assume that a stimulus occurred. When the impulse from the presynaptic membrane reaches the synaptic knob (end bulb), this will cause activation of voltage dependent Ca++ channels, which in turn will result in Ca++ diffusion into the synaptic knob, the increase in Ca++ concentration inside the axon terminal will trigger the release of the neurotransmitters into the synaptic cleft by exocytosis *** In some cases after the neurotransmitters are attached to their receptors, hyperpolarization may occur instead of De-Polarization, but what determines either ways? 1- De-Polarization: depolarization will occur if the receptors that the neurotransmitter binds to at the postsynaptic membrane are linked to Na+ channels. However, this depolarization is fairly little which can t reach the threshold, and won t cause any action potential. **The developed postsynaptic potential is called EPSPs (Excitatory Post Synaptic Potentials) 2- Hyperpolarization: hyperpolarization will occur if the receptors of the postsynaptic membrane that the neurotransmitter binds to are linked to K+ channels. ** The developed postsynaptic potential is called IPSPs (Inhibitory Post Synaptic Potentials) ** The part of the neuron where the action potential is generated is called the axon hillock Question: let s assume that the receptors at the postsynaptic membrane are linked to chloride channels what would happen when the neurotransmitters are attached to their receptors? 4 P a g e

6 ***** Answer: the effect of this process will be inhibitory on neural activity. This inhibition is achieved by holding the membrane at its resting potential and preventing depolarization.(prevents depolarization in general). There are a lot of terminals ( of one neuron ) that are synapsing with the cell body ( of another neuron ), so whenever the neurotransmitters are released at the synaptic cleft it will result in many de-polarizations and many hyper-polarizations, so if the sum of all of these polarizations reached the threshold, action potential will be generated at the second neuron. This process of finding the sum of these polarizations is called Summation. However, there are two types of summations: 1- Spatial summation : *So let s assume that there is three presynaptic neurons synapsing with a fourth neuron, post synaptic. So what will happen? Actually, each neuron will transport its stimulus to the fourth neuron, either it was depolarization or repolarization, if the sum of these polarizations reach the threshold, an action potential will be generated in the fourth neuron in the axon hillock. *** Spatial stands for space. 2- Temporal summation: Temporal summation occurs when a single presynaptic neuron fires many times in succession, causing the postsynaptic membrane to reach its threshold and fire. *** Temporal stands for Time. Note that the summation isn t only between excitatory potentials only; it can be between any kind of potentials. (Excitatory with excitatory, excitatory with inhibitory or inhibitory with inhibitory) 5 P a g e

7 The sum of everything is taken by axon hillock to result in action potential or nothing. However, in our nervous system there is more inhibition than excitation. Remember: -most chemical-gated channels are found on the cell body of the neuron or on the dendrites, while most voltage-gated channels are located in the rest of the neuron, that is why the action potential is generated In the axon hillock where voltage-gated channels concentrate. -Action potential travels in one direction (unidirectional) Let s assume this hypothetical situation, which doesn t happen in our bodies. What if we had stimulated a neuron at the middle, what will happen? Well, the action potential will move in both directions, right and left, although it is moving in two ways still we are considering it as Unidirectional; because the point is that the potential always moves from the excited region to the polarized region despite the hypothetical situations. What if we had stimulated a neuron at two opposite sides (may occur in some body movements), what will happen? Well, in this case the two action potentials will propagate towards the middle, and they will not by-pass each other, instead both potentials will die at the middle. We know that the action potential is generated from the body towards the terminals, but can an action potential be generated from the terminals towards the body of the axon? The answer is yes, and this situation occurs in sensory neurons, which carry sensation from terminals to the body, as they have receptors on their terminals. However, these receptors may respond to temperature, touch or any other stimulus, by stimulation an action potential will be generated and transported to the body of the axon, but not to the same axon body. However, the action potential is generated at one end and travels within the first axon towards the cell body of the second axon. Conclusion: Sensory neurons are generating action potentials from terminal of one neuron, towards the cell body of the second neuron, not the same cell body. 6 P a g e

8 There are two types of nerve fibers (neurons), Myelinated & Unmyelinated nerve fibers so what is the difference and how action potential is conducted in both of them? Myelinated Nerve Fiber: This nerve fiber is surrounded by myelin sheath which results in having a special way in conducting action potential along the neuron. In myelinated nerve fiber the action potential is conducted through a mechanism called Saltatory (jumping) conduction, the action potential will propagate using Ranvier nodes to skip larger distances, which will result in depolarization in each node, so it is considered a fast method for propagation & also it conserves energy for the axon because only the nodes Depolarize. Unmyelinated Nerve Fiber: In this nerve fiber no myelin sheath is present so the action potential can be propagated along the axon easily though more slowly than the saltatory conduction. Let s assume that there is two Unmeylinated nerve fibers with two different diameters, in which one of them the velocity will be higher? Answer: the nerve fiber with the larger diameter will result in faster propagation due to low resistance through the action potentials way So the internal current is faster in bigger diameter than in small one 7 P a g e

9 So far all of us know that in order to measure the potential across a plasma membrane, two electrodes are placed, one inside and the other outside, but is there any other method? A- Monophasic action potential: Is by placing one electrode inside the cell and other electrode outside the cell. B- Biphasic action potential: is by placing the two electrodes outside the cell membrane. ** Two waves are obtained in the recording of biphasic action potential, the first always represents Depolarization and the second is in the reverse direction of the first and always represents Repolarization. 8 P a g e