Nervous System Physiology Essentials 3 PDF 2024-2025

Nervous System Physiology Essentials 3 CHAPTER 5 Physiology Synapse The connection between a neuron and another functional total of 2 cells is called a synapse. In the CNS, both cells are neurons; In the peripheral nervous system, the second cell can be a ne...

Nervous System Physiology Essentials 3 CHAPTER 5 Physiology Synapse The connection between a neuron and another functional total of 2 cells is called a synapse. In the CNS, both cells are neurons; In the peripheral nervous system, the second cell can be a neuron, muscle, or gland cell. The synapse between the neuron and muscle cell is called a neuromuscular junction. The cell before the synapse is the presynaptic cell, The cell behind the synapse is the postsynaptic cell.  Synapse types Synapses are connections that allow information to be transmitted between neurons and are divided into various types according to their functions or structural features. The main types of synapses are: Electrical synapse, Chemical Synapse Synapses According to Their Structural Features Axodendritic Synapse, Axosomatic Synapse, Axoaxonic Synapse Synapses According to Their Functional Features Excitatory Synapses, Inhibitory synapses  Electrical Synapse Electrical synapses establish a direct connection between neurons, allowing ions to flow freely between two cells Ions are provided by proteins called gap junctions. Stimulus is transmitted very quickly and directly with gap junctions. Gap junctions provide almost close contact between cells. Gap junctions create a distance of 2-4 nm between two cells. This distance is almost 6 times shorter than a chemical synapse. Syncytium is formed between the cells. (cells are stimulated simultaneously in the form of a Mexican wave.) In electrical communication, nerve conduction is based on an ion balance No chemical substance is needed to initiate the conduction. These gap junctions are present in smooth muscle and heart muscle. Gap junctions  Chemical synapse Chemical synapses are more common A physical space between nerve endings is the synaptic cleft. The synapstic cleft is 20-40 nanometers, Chemical transmission begins at the axon end of the presynaptic neuron. When an AP arrives, neurotransmitters stored in synaptic vesicles are released by calcium ions entering the cell. These neurotransmitters pass through the synaptic cleft and bind to receptors on the postsynaptic neuron. The receptors detect these chemicals and produce a response, thus re-establishing the electrical signal. The entire postsynaptic structure is called the motor end plate.  Chemical transmission 1. The process begins with the AP wave, which travels along the membrane of the presynaptic cell until it reaches the synapse. The terminal membrane depolarizes. 2. Presynaptic neuron depolarization causes the voltage-gated Ca+2 channels in the presynaptic to open, Ca+2 flow into the presynaptic begins, and intracellular Ca+2 concentration increases rapidly. 3. Within the presynaptic nerve terminal, vesicles containing neurotransmitters are localized near the synaptic membrane. 4. The V-SNAREs in the synaptic vesicle and the T-SNAREs in the presynaptic terminal membrane combine, a complex is formed, and fusion develops. 5. After fusion, the clear vesicle content Ach is released into the synaptic cleft by exocytosis. The number of vesicles discharged into the synaptic cleft is directly dependent on the amount of Ca+2. 6. AcH binds to the postsynaptic nicotinic Ach receptor (nAcHR) located on the postsynaptic cell membrane. This receptor is a ligand-gated ion channel and binds 2 Ach. It is also a nonselective cation channel, meaning it is permeable to Na+ and K+. 7. With Ach binding, while some Na+ enters from the nAcHR receptors, K+ exits, and a miniature motor end plate potential is formed in this region. The motor end plate potential that forms is a graded potential. Therefore, only the potential change occurs in the region where it occurs and in neighboring regions, it is not transmitted. Excitatory postsynaptic potentials (EPP) occur. 8. With the accumulation of EPP, the membrane resting potential reaches the threshold value and also causes the voltage-gated Na+ channels in the subneural parts to open, and more voltage-gated Na+ channels in neighboring regions open. As a result, a large progressive true AP occurs. Neurotransmitters Neurotransmitters are chemicals that nerve cells use to transmit signals to each other. They are stored in synaptic vesicles and released into the synaptic cleft via exocytosis when an action potential reaches the presynaptic neuron. They bind to receptors on the postsynaptic cell and trigger a response from the cell. Glutamate: The most common excitatory neurotransmitter in the CNS. It plays an important role in functions such as learning and memory by transmitting excitatory signals. GABA (Gamma-aminobutyric acid): The main inhibitory neurotransmitter in the CNS. It has a calming effect by preventing overstimulation of nerve cells and plays a role in anxiety control. Acetylcholine: It is the neurotransmitter that initiates muscle contractions and is also involved in learning and memory in the CNS. It is found in both the CNS and the PNS. Serotonine, Dopamine, Glycine…  Comparison of Electrical and Chemical Synapses Electrical Synapse Chemical Synapse Narrow synaptic gap Wide synaptic gap Mediator: Ion current Mediator: Neurotransmitter No synaptic delay – but Synaptic delay present resistance can alter regions Transmission is bidirectional Transmission is unidirectional Transmission is fast Transmission is slow  V-SNARE and T-SNARE V-SNARE and T-SNARE are SNARE proteins that play an important role in the process of exocytosis. V-SNARE (Vesicle-SNARE): These proteins are found on the surface of the vesicle. The letter "V" means "vesicule". Carrier structures such as synaptic vesicles need V-SNAREs to reach their targets. T-SNARE (Target-SNARE): These proteins are located on the target cell or organelle membrane. The letter "T" means "target". T-SNAREs start the merging process by pairing with V-SNAREs in the vesicles. Merging Process: V-SNARE and T-SNARE proteins lock together and bring the vesicle and target membrane together=SNARE COMPLEX Thanks to this merging, the neurotransmitters in the vesicle are discharged into the target cell. SNARE proteins Proteins such as synaptotagmin, syntaxin, SNAP-25 and synaptobrevin take part in this process and support the function of SNAREs. 1. Synaptobrevin (V-SNARE) 2. Syntaxin (T-SNARE) ❖ Syntaxin and SNAP-25 binds together. ❖ Together they interact with synaptobrevin. 3. SNAP-25 (T-SNARE) ❖ This trio (synaptobrevin, syntaxin and SNAP-25) forms a 4. Synaptotagmin (Calcium "SNARE complex" Sensor, V-SNARE) ❖ Synaptotagmin binds to Ca2+, enhancing the assembly of the SNARE complex. Conditions or diseases that affect the neuromuscular junction Lambert-Eaton Syndrome, Botulism, Tetanus, Myasthenia Gravis Curare Effect  Botulinum toxin Botulinum toxin is a powerful neurotoxin produced by the bacterium Clostridium botulinum. This toxin stops muscle movements by acting on the nervous system and can be effective even in very small doses. Botulinum toxin causes paralysis in the muscles by blocking the release of neurotransmitters from synaptic vesicles. It specifically stops the release of acetylcholine, which blocks muscle contraction. Botulinum toxin A-E ___ destroys SNAP-25 Mechanism of Action of Botulinum Toxin: Botulinum toxin C _____ destroys syntaxin Botulinum toxin binds to SNARE proteins, especially SNAP-25, synaptobrevin Botulinum toxin D_____ destroys synaptobrevin and syntaxin, Blocking the function of these proteins. This prevents synaptic vesicles from releasing neurotransmitters from nerve endings to muscle cells. Clinical Use Aesthetic Use: It is widely used in the treatment of facial wrinkles to reduce muscle contractions. - Botox Medical Treatments: Botulinum toxin is also used in the treatment of muscle spasms, migraine, excessive sweating, and some neuromuscular diseases.  Common ways that people can be infected with botulinum toxin: Botulism Botulinum toxin is a toxin produced by the bacterium Clostridium botulinum and can be found naturally in soil, water, and some food products. It thrives in oxygen-free environments and produces toxins. Common ways that people can be infected with botulinum toxin: Homemade canned foods: Bacterial spores can produce toxins, especially in poorly sterilized, homemade canned foods. Low-acid foods (meat, canned vegetables) are risky. Commercial canned foods: Although rare, toxins can sometimes develop in poorly sealed commercial foods. Honey: Botulinum spores can naturally be found in honey. Therefore, it is recommended that babies do not consume honey. Tetanus Tetanus toxin selectively destroys synaptobrevin in inhibitory interneurons. It is produced by the anaerobic bacterium Clostridium tetani found in soil. It causes blockade of interneurons that block the release of inhibitory neurotransmitters. Resulting in overactivity of motor neurons, leading to severe muscle contraction and spastic paralysis. Lambert-Eaton Syndrome L-E syndrome is an autoimmune disease A rare autoimmune disease in which autoantibodies destroy a specific subtype of voltage-gated Ca2+ channels, reducing calcium-dependent Acethylcolin release in the nerves innervating skeletal muscle, causing muscle weakness. Myasthenia Grevis Autoantibody formation against NAchR leads to ACh accumulation in the synaptic cleft, resulting in delayed neurotransmission.  Curare effect Curare is a natural compound obtained from tropical plants with muscle relaxant properties. Traditionally, it was used by natives in South America to paralyze animals by applying it to the tip of arrows used in hunting. Curare acts at the neuromuscular junction and prevents muscle contraction by blocking acetylcholine receptors. This leads to muscle relaxation and paralysis. It competes with Ach and blocks the nicotinic acetylcholine receptor (nAChR). Ach accumulates in the synaptic cleft, therefore the number of AchE receptors also increases, the result is paralysis of the respiratory muscles in particular (asphyxy)  EPSP and IPSP Postsynaptic Potentials Transient electrical changes in the postsynaptic neuron caused by chemical signals transmitted from the presynaptic neuron to the postsynaptic neuron across a synapse. These changes can be excitatory (EPSP) or inhibitory (IPSP) and increase or decrease the probability of the postsynaptic cell producing an action potential. For the postsynaptic cell to be stimulated; the stimuli reaching the postsynaptic neuron must reach the threshold potential. Therefore, the stimulation-inhibition of the postsynaptic neuron depends on the sum of the EPSP and IPSP ending in the postsynaptic. If the total result is high in EPSP, the postsynaptic neuron is stimulated. If the total result is high in IPSP, the postsynaptic neuron is inhibited. EPSP (Excitatory Post-Synaptic Potential): Signals in which excitatory neurotransmitters positively change the membrane potential of the postsynaptic neuron, making it less negative. This makes the cell more likely to produce an action potential. For example, excitatory neurotransmitters such as glutamate produce EPSP. If Na channels open—depolarization occurs—excitatory ----EPSP --- excitatory postsynaptic potential IPSP (Inhibitory Post-Synaptic Potential): This is when the membrane potential of the postsynaptic neuron becomes more negative under the influence of inhibitory neurotransmitters. This makes the cell less likely to produce an action potential. Inhibitory neurotransmitters such as GABA produce IPSP and hyperpolarize the cell, suppressing neural impulses. If K and Cl channels open --- hyperpolarization occurs---- inhibitory ---IPSP---inhibitory postsynaptic potential  Convergence and Divergence, Facilitation of neurons Convergence: The sending of signals from many presynaptic neurons to a single postsynaptic neuron. This allows information from different sources to be collected in one cell. Divergence: The sending of signals from a presynaptic neuron to more than one postsynaptic neuron. Thus, a stimulus spreads to many points; it is important in reflexes and movements. Facilitation: The temporary increase in the excitability of a neuron with repeated stimulation. Facilitation plays an important role in neural learning and adaptation processes.  Summation Sumation refers to the sum of signals coming from a postsynaptic cell to stimulate and produce an AP. Summation occurs in two types: Temporal Summation: The rapid arrival of successive stimuli from a single presynaptic neuron to the postsynaptic neuron. The temporal factor is important here; when signals are repeated at short intervals, their effects accumulate without fading. And a strong response occurs in the postsynaptic neuron. Spatial Summation: The simultaneous stimulation of postsynaptic neurons by more than one presynaptic neuron from different points. In other words, the stimulating or inhibitory effects coming from different synapses combine at the same time to produce a stronger response in the postsynaptic cell.  Thank you