Lecture# 8 Sypanse.pptx for physical therapy

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1. Synapse

2. It is the junction between 2 neurons. It is not an anatomical continuation. But it is only physiological continuity between 2 nerve cells. A synapse, the junction where two neurons communicate, consists of  a presynaptic terminal, the synaptic cleft (the space between the two cells), and a postsynaptic terminal with receptor sites Presynaptic Terminal: This is the end of the axon of the sending neuron, containing vesicles filled with neurotransmitters. Synaptic Cleft: This is the small gap (about 20 nanometers) between the presynaptic and postsynaptic neurons. Postsynaptic Terminal: This is the receiving end of the next neuron, containing receptors that bind to the released neurotransmitters.

3. The information is transmitted in the central nervous system mainly in the form of nerve action potentials, called simply “nerve impulses,” through a succession of neurons, one after another. However, in addition, each impulse may be blocked in its transmission from one neuron to the next, may be changed from a single impulse into repetitive impulses, or may be integrated with impulses from other neurons to cause highly complicated patterns of impulses in successive neurons. All these functions can be classified as synaptic functions of neurons.

4. Synapses, the junctions between neurons, can be classified based on their mechanism of signal transmission (chemical or electrical) and the type of neurotransmitter they utilize, as well as the location of the synapse.

5. Classification of synapse: Synapse is classified by 2 methods Anatomical Classification Functional Classification

6. Anatomical Classification Usually synapse is formed by axon of one neuron ending on the cell body dendrite or axon of the next neuron. Depending upon ending of axon, synapse is further classified into: Axodendritic : Synapses where the axon terminal of one neuron connects with the dendrite of another neuron.  Axosomatic : Synapses where the axon terminal of one neuron connects with the cell body (soma) of another neuron.  Axoaxonic : Synapses where the axon terminal of one neuron connects with the axon of another neuron. 

7. Functional Classification Chemical Synapses: The most common type, where signals are transmitted via the release of neurotransmitters from the presynaptic neuron into the synaptic cleft, which then bind to receptors on the postsynaptic neuron.  Electrical Synapses: These transmit signals directly through gap junctions, allowing ions to flow directly between the cells, resulting in a faster transmission compared to chemical synapses. 

8. Based on Neurotransmitter Type : Synapses can be classified based on the type of neurotransmitter they use, such as: GABAergic:  Using GABA (gamma-aminobutyric acid) Dopaminergic:  Using dopamine Noradrenergic:  Using norepinephrine Cholinergic:  Using acetylcholine 

9. Functions of Synapse: Excitatory synapse , which transmit the impulses (excitatory function ) a type of synapse where the neurotransmitter released by the presynaptic neuron increases the probability of an action potential in the postsynaptic neuron, leading to a phenomenon known as an excitatory postsynaptic potential (EPSP ). The depolarization of the postsynaptic membrane due to the binding of excitatory neurotransmitters is called an EPSP.  Excitatory neurotransmitters, such as glutamate, epinephrine, and norepinephrine, bind to receptors on the postsynaptic neuron, causing the membrane potential to become more positive (depolarization). 

10. Inhibitory Function, which inhibit the transmission of impulses (inhibitory function) a specialized junction between neurons where the activity of one neuron reduces the likelihood of activity in an adjacent neuron, typically by initiating an inhibitory postsynaptic potential (IPSP).  Inhibitory synapses play a crucial role in regulating neuronal excitability and ensuring the balance of excitation and inhibition in the brain.  The influx of chloride ions causes the postsynaptic neuron to become hyperpolarized, making it less likely to fire an action potential. 

11. Properties of synaptic transmission One way conduction (Bell- M agendie law): The impulses are only transmitted in one direction in synapse i.e. from the presynaptic neuron to the postsynaptic neuron across the synapse At a given synapse, only one type of neurotransmitter is released, it may be excitatory or inhibitory.(DALE’s Law) later on it was found that in certain cases release of additional substances at a given synapse too occurs.

12. Synaptic Delay: During transmission of a neuronal signal from a presynaptic neuron to a postsynaptic neuron, a certain amount of time is consumed in the process of (1) Release of neurotransmitter ( 2) Passage of neurotransmitter from axon terminal to postsynaptic membrane (3 ) Action of the neurotransmitter to open the ionic channels in postsynaptic membrane. Normal duration of synaptic delay is 0.3 – 0.5 millisecond.

13. Fatigue of Synaptic Transmission. When excitatory synapses are repetitively stimulated at a rapid rate, the number of discharges by the postsynaptic neuron is at first very great, but the firing rate becomes progressively less in succeeding milliseconds or seconds. This is called fatigue of synaptic transmission . Depletion of acetylcholine occurs because of two factors: i . Soon after the action, acetylcholine is destroyed by acetylcholinesterase ii. Due to continuous action, new acetylcholine is not synthesized .

14. Electrical Properties : Synapses exhibit electrical properties, including excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs ). EPSPs depolarize the postsynaptic membrane, making it more likely to fire an action potential, while IPSPs hyperpolarize it, making it less likely to fire.

15. Summation : The effect of multiple synaptic inputs can be combined, this is called summation. When multiple presynaptic excitatory terminals are stimulated simultaneously or when a single presynaptic terminal is repeatedly stimulated, the effects are combined, leading to a larger EPSP. 

16. Effect of Hypoxia on Synaptic Transmission. Neuronal excitability is also highly dependent on an adequate supply of oxygen. Cessation of oxygen for only a few seconds can cause complete inexcitability of some neurons . This is observed when the brain’s blood flow is temporarily interrupted, because within 3 to 7 seconds, the person becomes unconscious.

17. Effect of Drugs on Synaptic Transmission. Many drugs are known to increase the excitability of neurons, and others are known to decrease excitability. For instance, caffeine, theophylline, and theobromine, which are found in coffee, tea, and cocoa, respectively, all increase neuronal excitability, presumably by reducing the threshold for excitation of neurons.

18. Effect of Acidosis or Alkalosis on Synaptic Transmission. Most neurons are highly responsive to changes in pH of the surrounding interstitial fluids. Normally, alkalosis greatly increases neuronal excitability. For instance, a rise in arterial blood pH from the 7.4 norm to 7.8 to 8.0 often causes cerebral epileptic seizures because of increased excitability of some or all of the cerebral neurons. This can be demonstrated especially well by asking a person who is predisposed to epileptic seizures to over breathe. The over breathing blows off carbon dioxide and therefore elevates the pH of the blood momentarily, but even this short time can often precipitate an epileptic attack. Conversely, acidosis greatly depresses neuronal activity; a fall in pH from 7.4 to below 7.0 usually causes a comatose state.

19. Convergence and Divergence of Synapse Synaptic Convergence: Multiple neurons synapse onto a single postsynaptic neuron. This allows for the integration of information from various sources, potentially enhancing sensitivity and the strength of signals. Examples include the convergence of rod cells in the retina to a single ganglion cell, increasing sensitivity to low light levels.

20. Synaptic Divergence: A single presynaptic neuron synapses onto multiple postsynaptic neurons. This allows a single neuron to influence a wider range of the nervous system, facilitating the coordination of responses. Examples include the divergence of signals from a sensory neuron to multiple motor neurons, allowing for a coordinated response to a stimulus.

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