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5. Nerves and Synapses.pdf

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PHGY 209
Erik Cook

Naveen Sooknanan McGill Fall 2011 Nerves and Synapses: The key idea of the nervous system is that it contains a central processing unit which allows the body to keep memory of the past Biologically speaking, you are not the same person you were 5 years ago, all your cells have turned over and you are now composed of completely new materials However, the brain is an incredibly complex central processing unit which allows to keep memory of the past The central nervous system is made up of various components which transfer information from one to another by chemical differences: The brain is the central unit The peripheral nervous system contains afferent fibers which transfer information to the brain from sensory nerves, and efferent fibers which carry signals from the brain to motor neurons, as well as autonomic fibers These categories are very unified; every part of the body belongs to the nervous system The system functions as a whole, and each cell exists as part of a larger system Neurons are the cells which make up the nervous system. The above picture is a dendrite found in the cerebellum of the brain. Neurons are very complex structures which make contact with each other in very specific ways Over 100 billion neurons make up the CNS They connect at points called synapses, of which there are over 100 billion just in the brain Although all cells are electrical to some extent, neurons are especially electrical and use their electrical properties in order to propagate information across the body. Although neurons are slightly different in different parts of the body, they all have the same major components The soma is the most cell-like component of the neuron and is where the nucleus and all organelles are o Essentially keeps the neuron alive The dendrites are branches off of the some which can attach to other neurons forming synapses o They receive information from nearby neurons o They can also branch many times, as seen in the cerebellar neuron The axon is a long strand coming directly out of the some o This is where information leaves one neuron and travels to others 1 Naveen Sooknanan McGill Fall 2011 o The section between the soma and the axon is called the initial segments and at the end of the axon is a swelling called the presynaptic terminal o An axon may be anywhere from a few millimeters long to more than 5 feet long Information propagates from the initial segment of one neuron, all the way down the axon, across the synapse, and to the dendrites of a nearby neuron This transport is caused by a polarity in the neuron which can change at any time A cell has a resting membrane potential of around -60mV to -70mV. This means that there is a net negative charge on the inside of a cell caused by the presence of more negative ions The actual potential energies on the inside and outside of the cell dont matter, its only the potential difference across the cell membrane that matters There are only a tiny fraction more negative ions in the cell than outside This electrical gradient is located across the entire membrane This tiny electrical gradient cant be measure chemically, but can be measured using electrical equipment Each ion has different concentration in and out of the cell Sodium and chloride ions are higher out of the cell then in the cell Potassium, and other anions are is higher in the cell and out of the cell Calcium is low outside the cell, but is virtually absent inside the cell o The cell actively pumps calcium out to keep the concentration almost 0 The difference in charge between in inside and outside of the cell is too small to see in this scale, but it does exist + A resting neuron is only permeable to K . As potassium leaves the cell freely, chloride tries to follow it but it cant cross the PM, causing a net accumulation of negative chloride ions inside the cell, contributing to the net negative charge of the cell Eventually, an equilibrium is reached where the concentration gradient pushing potassium out is equal to the electrical gradient pushing potassium back in This is why all cells have a resting membrane potential This equilibrium happens after only a very small amount of potassium leaves and re- enters the cell It is possible to predict exactly where the system will reach equilibrium with the Nernst equation o From this equation, it is derived that the ideal resting membrane potential for potassium is -90mV, which is pretty close to the resting membrane potential of a neuron. Leak potassium channels are a type of ion channel that always remain open in the neuron. These ion channels are specific to potassium ions and contribute to the net negative charge of the neuron Leak channels are the only open channels on the neuron at its 2 Naveen Sooknanan McGill Fall 2011 resting membrane potential They are located all around the neuron PM If these channels were the only ones present, the resting membrane potential of the neuron would be -90mV, but the neuron is also selectively permeable to other ions as well, making the resting potential stabilize around -70mV While sodium wants to have a resting membrane potential of +50mV, it doesnt have much of an effect on the total resting potential It does, however, exert enough effect to push 20mV worth of potassium out of the cell, creating the neurons actual resting membrane potential This the membrane potential depends on concentration gradients of ions and the PMs relative permeabilities to these ions The dominant ion, in this case potassium, makes the greatest contribution to the real membrane potential Under physiological conditions, the gradient changes, meaning that the permeability of the membrane must somehow change in order to the membrane potential to change Over time, these potentials would run down because potassium would leave the cell until it reaches equilibrium If this happens, the neuron would lose electrical properties and neuron would be useless The neuron in fact never reaches equilibrium, potassium is always being pushed out and sodium is constantly being pushed in To counter this leaking effect, sodium potassium pumps, which were described in transport mechanisms, are present on the cell and actively pump potassium back into the cell and sodium out of the cell to maintain the concentration gradients necessary for electrical activity. Axons propagate information from one neuron to another through a mechanism called an action potential. An AP is started at the initial segment and propagates down the length of the axon to the presynaptic terminal An action potential is a very narrow electrical signal caused by a sudden and strong depolarization of the cell membrane potential The AP therefore propagates down the axon as a wave of depolarization o This AP usually lasts about 2ms, which is very fast for a biological process In an AP, the membrane potential suddenly depolarizes from -70 to around +30mV only for 2ms In order for the initial segment to create an action potential, the membrane potential must depolarize to a specific value known as the threshold level (normally 50mV) The AP is an all or nothing event, if the threshold is not reached, the neuron will return to a resting state, is the 3 Naveen Sooknanan McGill Fall 2011 threshold is reached, then the segment suddenly depolarizes all the way to -30mV Different neurons have slightly different threshold values Since the cell at rest is not very permeable to sodium, a different type of ion channel must be responsible for this depolarization. The first one is the voltage gates sodium channel. The depolarization of the cell is caused by opening of these voltage gated channels o When these gates open, more sodium is allowed to enter the neuron which further increases the charge, thus opening more gates This is a positive feedback mechanism once the threshold is reached o An analogy can be rolling a ball up a hill: if the threshold (top) is not reached, then the ball will just roll back to the bottom, but once the threshold is reached, it will roll all the way to the other end by itself At rest, these gated channels are normally close. Then, during an AP, they begin to open one by one. After, they become blocked and inactivated, and to not reactivate until the neuron is repolarized Once the threshold is reached, it takes about ms in order to reach a +30 membrane potential There are much more gates sodium channels on the axon than leak potassium channels, so during an AP, sodium becomes the dominant ion durin
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