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Janelle Leboutillier

PSYB64: Introduction to Physiological Psychology Lecture 3/4: Cells of the Nervous System Overview  Neurons and Glia  Generation and Propagation of Action Potentials  Synapses o Electrical o Chemical  Neuromodulation Neurons and Glia  The Structure of Neurons o Neuron membranes separate intracellular fluid from extracellular fluid o The neural cytoskeleton provides structural support that maintains the shape of the neuron The Neural Membrane (Figure 3.2)  Neural membranes consist of double layers of phospholipid molecules  Embedded within the lipid layers are proteins that serve as ion channels and ion pumps  These structures open and close, controlling the movement of ions across the neural membrane.  Sample of a neuronal membrane from the axon which is not myelinated  Exposure to both the intracellular and extracellular fluid  Proteins found in membrane, they can open and close for ions The Neural Membrane  Membrane is a bi-layer composed largely of phospholipids and other lipids studded with proteins and other large molecules (forms part of the pores)  Ion Channels o Allow ions to diffuse down concentration gradient o Selective permeability to certain ions o Don’t require additional energy  Ion Pumps o Actively move ions against concentration gradient o Create ion concentration gradients o Require Energy Three Fiber Types Compose the Cytoskeleton of Neurons (Figure 3.3)  Microtubules provide means for transporting materials within the neuron  Neurofilaments provide structural support, whereas microfilaments may be involved with structural changes associated with learning.  Snapshot image of the neuron from the inside  Tubules that can be found that have added structures  3 types: microtubules, neurofilaments, and microfilaments  The main difference between them is due to their size  All 3 involved in transporting molecules, organelles, along them, either to the axon terminal at the end or back to the cell body  Can move things in both directions  Transport them toward the axon terminal = anterograde transport  Transport them toward the cell body = retrograde transport Tau Phosphorylation Leads to Cell Death (Figure 3.4)  Molecules of phosphate added to the tau protein lead  to neurofibrillary tangles and structural collapse. Neurons and Glia  Structural Features of Neurons o Cell body (soma) contains nucleus and other organelles o Dendrites – branches that serve as locations at which information from other neurons is received  Spines o Axons are responsible for carrying neural messages to other neurons  Vary in diameter and length  Many covered by myelin Abnormal Dendrites and Mental Retardation (Figure 3.7)  Example of dendrites  Possible to quantify the dendrites based on the different types of spines  Less developed, less complex dendrites on the right side, less info conveyed  Compared with the dendrites from a typical infant on the left, the dendrites from an infant with mental retardation are abnormally long and thin  Dominick Purpura has observed that the dendrites of children with mental retardation resemble the dendrites of the human fetus  The abnormal dendrites may cause retardation by failing to mature in response to environmental input. Axons and Dendrites (Figure 3.6)  Action potentials originate in the axon hillock and then travel the length of the axon to the axon terminal  The arrival of action potentials at the axon terminal signals the release of neurotransmitters from the synaptic vesicles  Molecules of neurotransmitter diffuse across the synaptic gap, where they interact with receptors embedded in the dendrite of the adjacent neuron. Structural Variations in Neurons 1. Unipolar 2. Bipolar 3. Multipolar Structural and Functional Classification of Neurons (Figure 3.8) 1. Unipolar o Single branch extending from the cell body 2. Bipolar o Two branches extending from the neural cell body: one axon and one dendrite 3. Multipolar o Many branches extending from the cell body; usually one axon and many dendrites  Most vertebrate unipolar and bipolar neurons are found in sensory systems, where they encode and transmit information from the outside world  Multipolar neurons typically serve as motor neurons, transmitting commands to glands and muscles, or as interneurons, providing bridges between sensory and motor neurons. Additional Examples Functional Variations in Neurons  Sensory Neurons o Specialized to receive information from the outside world (see, hear, touch)  Motor Neurons o Transmit commands from the CNS directly to muscles and glands (walk, run)  Interneurons o Act as bridges between the sensory and motor systems Table 3.1 Types of Glia The Generation of the Action Potential  Ionic Composition of the Intracellular and Extracellular Fluids 1) The difference between these fluids provides the neuron with a source of energy for electrical signaling 2) Differ from each other in the relative concentrations of ions they contain The Composition of Intracellular and Extracellular Fluids (Figure 3.12)  Extracellular fluid = similar to seawater  Composition of both fluids are not the same  Proteins = few on outside, more on the inside (they cannot cross)  Negative ions = chloride  Positive ions = sodium, potassium  Extracellular fluid is similar to seawater, with large concentrations of sodium and chloride but small concentrations of potassium  Intracellular fluid has a large concentration of potassium but relatively little sodium and chloride. Measuring the Resting Potential of Neurons (Figure 3.13)  Squid axon = large axon = easier to place an electrode inside the squid  Axons from invertebrates such as the giant squid can be dissected and maintained in a culture dish for study  Because these axons are so large in diameter, electrodes can be inserted into the intracellular fluid  If we compare the recordings from two electrodes, one located in the intracellular fluid of the axon and the other in the extracellular fluid, our recording shows that the inside of the cell is negatively charged relative to  the outside  The relatively larger number of negatively charged molecules in the intracellular fluid compared with the extracellular fluid is responsible for this difference  - 70mV (negative charge) The Generation of the Action Potential  The Movement of Ions 1) Diffusion is the tendency for molecules to distribute themselves equally within a medium  Doesn't require energy  Moves from high to low concentration  Spreads out evenly 2) Electrical force is an important cause of movement  Like electrical charges repel  Opposite electrical charges attract Diffusion and Electrical Force (Figure 3.14)  In a resting neuron, diffusion and electrical force balance each other to determine an equilibrium for potassium and chloride.  In contrast, both diffusion and electrical force act to push sodium into the neuron.  The large protein molecules found in the intracellular fluid cannot move through the membrane due to their size. Because  of their negative charge, they contribute to the relative negativity of the intracellular fluid.  K+ is higher inside the cell, move from inside to outside (diffusion will move it outside); inside is more negative;  Na+ is higher outside the cell, diffusion to balance to move them inside; electrical force will move it inside because it's positive and will be drawn to negative from the inside  Cl- diffusion will move it to the inside Resting and Action Potentials (Video) The Generation of the Action Potential  The Resting Potential o Membrane allows potassium to cross freely o Measures about -70mV o If potassium levels in extracellular fluid increase, resting potential is wiped out The Action Potential  Threshold o When recording reaches about -65mV  Channels open & close during action potential o Sodium (+) flows into neuron , potassium (+) flows out around the peak of the action potential  Refractory period o Recording returns to resting potential o Absolute (cannot stimulate neuron again) versus relative refractory periods (we can stimulate it)  The action potential is all-or-none So, why do we care about Aps?  According to his memoirs (Cook 1777, as cited by Kao, 1966), one of Cook’s sailors obtained a puffer fish from a local trader in New Caledonia. Cook’s two onboard biologists tried to talk him out of eating the fish, but Cook was after all the captain, and all three tasted the liver and roe. Cook wrote:  About three o’clock in the morning we found ourselves seized with an extraordinary weakness and numbness all over our limbs. I had almost lost the sense of feeling; nor could I distinguish between light and heavy bodies of such as I had strength to move, a quart pot full of water and a feather being the same in my hand.  Subsequent research in the 1800s showed that puffer toxin would bloc
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