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Chapter 4

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Suzanne Erb

Chapter 4 – Excitability ad Chemical Signaling in Nerve Cells - All thoughts, emotions and behaviours come about because of biochemical and electrochemical processes that take place in specialized cells in the nervous system called neurons. - Drugs that affect these psychological variables do so because they alter these biochemical and electrochemical processes. - The nervous system is divided into two systems: o The Central Nervous System (CNS) which is composed of the brain, and the spinal cord o The Peripheral Nervous System (PNS) which is made up of all neurons outside the brain and spinal cord - Neurons are also mixed in with numerous other nonneuronal cellular elements called: o Glial cells which ensheath synaptic connections between neurons and are required for synapse formation and maintenance o Oligodendrocytes which wrap layers of myelin membrane around axons to insulate them for impulse conduction, which serve important metabolic and supportive functions. - It has been estimated that the human nervous system contains approximately 85 billion neurons. - Neurons act to transducer information about their physical and chemical environments, which means that they convert one form of energy into another form of energy or one type of signal into another type of signal. In addition, neurons transmit information, typically by generating electrical changes in one part of the cell, conducting these electrical changes to distant parts of the cell, then releasing chemical signals onto the neighbouring neuron. o They are chemically active, releasing signaling molecules known as neurotransmitters. The Neuron - Each neuron has numerous excitatory and inhibitory inputs or synapses, which regulate the frequency of action potentials produced by them. - The large arrows indicate the most common direction of information flow. - The main body of the neuron is called the soma, parts of which serve integrative functions in the communication of information. - Extensions from the soma are termed dendrites and axons. The enlarged region where the axon emerges from the soma is called the Axon Hillock. - Normally there are many dendrites extending from the soma, which serve as receivers for information from other neuron, and one axon, which serves as the pathway over which signals pass from the some to other neurons. - Dendrites tend to be relatively short, but axons can be quite long. Many axons have a coating called the myelin sheath, which is analogous to the insulation on a wire. Gaps in the myelin sheath where the axon comes into direct contact with the extracellular fluid, are called the nodes of Ranvier. The presence of these gaps allow for an increase in the rate of conduction down the axon. Near its end, the axon branches and at the tip of each branch is an enlargement called a terminal/varicosities. Chemicals found within the axon terminal can be released into an exceedingly small gap between the neurons, called a synaptic cleft, allowing the neuron to affect excitability of adjacent neurons. The point of functional connection between neurons is called a synapse and it consists of the pre-synaptic membrane of the axon terminal, the synaptic cleft and the post-synaptic membrane of the “target” neuron. Electrical Excitability of Neurons: The Resting Membrane Potential - Neurons are electrically active. Under baseline conditions, each neuron is said to be polarized; that is, they have a voltage difference between the inside and the outside of the cell that is known as the resting membrane potential. In neurons, this difference is approximately 70mV. Because the inside of the cell membrane is negative, this voltage is referred to as -70mV. The resting potential of individual neurons varies between -60 and -90mV. This electrical potential or charge is largely powered by sodium ions (Na ). + - Things that generate the resting potential: o The electrical characteristics of ions o The physical forces that drive the movements of ions o The characteristics of the nerve cell membrane, including both the lipid and protein components. - Several ions that need to be considered: o Sodium (Na )+ + o Potassium (K ) o Chloride (Cl) There are also large negatively charged groups on protein molecules which tend to be present in higher concentrations inside the membrane o Calcium (Ca )+ - Any positive ion is called a cation, while a negative is called an anion. - In addition, there are two main physical forced that we need to consider, which drive the movement of ions and other molecules. o Movement along a concentration gradient refers to the fact that molecules move from an area of high concentration to an area of low concentration. o An electrical gradient refers to ions being driven by electrical forces because like charges repel and opposites attract. - Characteristics of the neural membrane: o As reviewed in chapter 3, the nerve cell membrane consists of a phospholipid bilayer with imbedded proteins. The lipid portion of the bilayer acts as a barrier to the movement of ions or polar substances. o The protein components can include :  Enzymes – biological catalysts that promote biochemical reactions including neurotransmitter synthesis and metabolism.  Receptors – bind to neurotransmitters essentially acting as the initial detection device for the presence of a transmitter.  Channels – proteins that act as gates that can be opened or closed. When opened the channels allow for the passage of ions through the membrane. Channels are defined in terms of what ions they let though. Cation channels, anion channels, Na channels, K channels, CL channels and Ca channels, as well as defined by their gating mechanism (what opens and closes them).  Transporters – act as pumps that move substances across the membrane. o Each of the proteins can serve as a substrate for drug action. - Under baseline or resting conditions: o Cl and K channels are mostly open + o Na channels are virtually all closed o Na /K pump actively transports some K into the cell but transports more Na out of it. o These conditions lead to the generation of the resting membrane potential. It is negative on the inside because positively charged Na ions are pumped out, and these + ions are not allowed back in because the Na channels are closed. Thus, under the resting or baseline conditions, the membrane is impermeable to sodium ions. This established the inside of the membrane is relatively negative and the outside is relatively positive. o With more Na+ ions on the outside, the two forces that impinge upon the movement of ions are aligned and have the potential to act upon Na+ in the same direction. If the membrane were to suddenly become permeable to Na+, the sodium ions would rush into the cell driven by both the concentration gradient force and the electrical gradient force. Thus the resting or baseline condition represents an electrically unstable state because anything that increases the permeability to Na+ would discharge the capacitor (would allow sodium to flow through the membrane and make the inside of the membrane more positive). o This is how neurons change from the polarized resting or baseline state to become excited or depolarized. Electrical Excitability of Neurons: Excitation, inhibition and the Action Potential - Electrical conditions across the membrane can be recorded by electrodes, and when this is done, a variety of voltage or current changes can be measured. - Three types of electrical phenomena are commonly recorded from nerve cells. o Excitatory postsynaptic potentials (EPSPs) is a small transient change in the positive direction ie moves in a positive direction then returns to baseline. o Inhibitory postsynaptic potentials (IPSPs) is a small change in the negative direction ie the cell becomes more negative/hyperpolarized). o Action potentials (spikes/neuronal firing) is a rapid and dramatic movement in the positive direction, followed by a rapid restoration of the resting potential. o The term potential here refers to a voltage change, post synaptic refers to the convention that these are generally recorded from the postsynaptic membrane. - We say that EPSPs and IPSPs are propagated in a graded decremental fashion. They originate at one point and move outward in all directions across the surface of the membrane. They are said to be graded because they can vary in size, depending upon the magnitude of the stimulus, and decremental because they diminish in size as they travel out from the original point of stimulation. - EPSPs and IPSPs convey information about the magnitude of the chemical signal that a neuron is receiving. They are analogous to the ripples that occur when we throw a stone into a pond. - In contrast, Action Potentials are not graded or decremental. When the level of excitation is great enough to cross the threshold, an action potential is triggered and action potentials are considered to be all or none. When they occur, they typically maintain their size as they travel along the axonal membrane. Because they do not generally convey information based upon their size, action potentials encode information in terms of their frequency and overall pattern of activity. - Once generated, action potentials then travel down the axon, and in some cases this can be a very long distance. The speed of the action potential is related to two main factors: o The diameter of the axon (larger diameters conduct more rapidly) o Whether or not it is myelinated (myelinated conduct more rapidly) - Mechanisms that produce EPSPs and IPSPs are: o Ones that lead to excitation  GLU is the most common excitatory transmitter. It induces excitation by binding to a receptor known as N-methyl D-aspartate (NMDA) receptor. This receptor is linked to a cation channel. When GLU molecules bind to the binding site, it instigates the opening of the cation channel; positive ions flow through, and because one of the ions that can pass through is Na+, this represents an increase in permeability to Na+. The inward flow results in the inside of the membrane moving in the positive direction which is recorded as an EPSP. o Ones that lead to inhibition  GABA is the most common inhibitory transmitter. It can induce inhibition by binding to a receptor subtype known as GABAa. The GABAa receptor is linked to a Cl- channels and therefore when GABA molecules bind to the binding site, it instigates an opening of this chemically gated Cl- channel. As a result there is an inward flow which moves the voltage in the negative direction, and the change is recorded as an IPSP. o It is the different channels being opened that leads to either excitation or inhibition in each of these cases. - Action potentials are generated by a different mechanism that EPSPs and IPSPs. Under physiological conditions, action potentials are most frequently generated at the initial portion of the axon (the Axon Hillock). - Consider an action potential having two parts: o An ascending limb – when the voltage shoots up in the positive direction  It is instigated because of the opening of voltage-gated Na+ channels. An initial voltage change triggers the opening of these channels, which leads Na+ ions to go into the neuron, which leads to the membrane potential on the inside to shoot up in the positive direction. Meanwhile the membrane depolarization induces the opening of voltage-gated K+ channels. o A descending limb – wh
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