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Psych170a-Lecture2-Neurons-Fall2008 - Psychology 170...

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Unformatted text preview: Psychology 170: Fundamentals of Neuroscience Structure and Functions of Neurons: Neurophysiology Reading: Carlson, Chapter 2 Cells of the Nervous System Lecture Overview Communication Within a Neuron Neurons Glia Communication Between Neurons The Resting Potential The Action Potential Neurotransmitters Postsynaptic Potentials Neural Integration Cells of the Nervous System What is the Nervous System? Central Nervous System (CNS) Includes the brain and spinal cord Peripheral Nervous System (PNS) Includes the nerves in the rest of the body The nervous system is composed of ~100 Different Classes of Neurons 1000 billion neurons Neurons can be classified into 3 different types, according to function: Sensory Neurons Receive sensory information (lights, sounds, etc) Motor Neurons Innervate and control the muscles Interneurons Lie between sensory and motor neurons (found only within the CNS) Neuron External Structure 1. 2. Contains the material that the cell needs for the life processes of the cell Processes that serve to receive incoming signals from other neurons 3. Process that transmits signal to another neuron (typically covered in myelin) 4. Site of neurotransmitter synthesis & chemical transmission to other cells Different Types of Neurons Multipolar Neurons Neurons that have 1 axon and many dendrites Bipolar Neurons Neurons that have 1 axon and 1 dendrite, usually at opposite ends of the soma Unipolar Neurons Neurons that have 1 process that leaves the soma and then divides into 2 branches, one that receives information and the other that transmits it these are also usually sensory neurons usually sensory neurons this is the most common in the CNS Bipolar Neuron Unipolar Neuron 1. Nucleus contains the nucleolus & the chromosomes that consist of DNA Endoplasmic reticulum (ER) serves as a reservoir for minerals and proteins and also traffics materials throughout the cell 3. 2. Mitochondria provide energy for the cell in the form of ATP 4. Golgi apparatus Synthesizes lipids and lysosomes, which contain enzymes that break down substances no longer needed by cells The Cell Nucleus & Protein Synthesis The nucleus contains the genetic material, called chromosomes When activated, the genes produce a single stranded copy of themselves called messenger ribonucleic acid (mRNA) The mRNA leaves the cell nucleus and interacts with a ribosome, which converts the mRNA into a peptide strand The peptide is then further modified into a protein, the basic building block of the cell In humans, the chromosomes consist of between 30,000100,000 functional genes 1) The process of mRNA production from genes is called transcription 2) The process of peptide formation from mRNA at the ribosome is called translation Transport Within Neurons The neuron's unique shape is determined by the cytoskeleton, a matrix of protein strands Since neurons are so long, there must be a Microtubles are the thickest of these protein strands system of transport within neurons This system is called axoplasmic transport Some neurons stretch from the base of the foot to the brain! Kinds of Axoplasmic Transport Anterograde transport process where substances are trafficked from the soma to the terminals along microtubles Kinesin is the protein that is responsible As fast as 500 mm per day! Retrograde transport trafficking from the terminals to the soma Dynein is the protein that is responsible Half as fast as anterograde Axoplasmic Transport There are 3 types of glia: Neurons compose roughly of the volume of the CNS; the rest consists of supporting cells called glia Astrocytes provide support and nutrients for neurons Oligodendrocytes provide support and myelin Microglia serve to scavenge dead neurons and debris Supporting Cells of the Nervous System Astrocytes receive glucose from capillaries and break it down to lactate Lactate is released so that neurons can take it up and utilize it Astrocytes also store glycogen Needed for the 1st stage of glucose metabolism Astrocytes Can be broken down to glucose and lactate during periods of high metabolic demand Oligodendrocytes Oligodendrocytes provide the axons of neurons in the CNS with a myelin sheath Insulates axon Speeds neural transmission Myelin, composed of 80% lipids and 20% proteins The myelin sheath is not continuous produced as the oligodendrocyte wraps itself around the axon contains gaps known as Nodes of Ranvier In the PNS, Schwann cells serve to make Schwann Cells myelin Schwann cells provide myelin for only 1 neuron Oligodendrocytes can serve many neurons Schwann cells also guide axons in the PNS during development and after damage Astrocytes in the CNS inhibit axon growth by forming scar tissue, which is why neurons in the CNS typically cannot regenerate after damage Communication Within A Neuron Neuronal Communication How do neurons communicate with other neurons? The Neuron's Resting Membrane Potential A giant squid axon is placed in sea water in a recording chamber A glass microelectrode is inserted into the axon The voltage measures 70 mV inside with respect to the outside this is called the neuron's resting membrane potential The neuron remains at its resting potential until it is perturbed If you stimulate the axon, the membrane potential will rise (become less negative) If you stimulate enough, a threshold of excitation will be reached: this is called depolarization The Action Potential This transient reversal of membrane potential is called the action potential ~4060 mV the membrane potential will temporarily reverse itself the inside of the cell will become positive with respect to the outside (around +40 mV) Inside Positive compared with Outside Inside Negative compared with Outside Maintaining The Resting Membrane Potential To understand what generates the action potential, we must understand what maintains the resting potential The resting membrane potential is a balance between two forces: Force of Diffusion between areas of different concentration; particles want to evenly distribute Electrostatic Force between charged particles; differently charged particles are attracted to each other The Neuron at Rest: It's All About Ions! 70 mV The only ion that feels both forces of concentration and charge is Na+!! The Na /K Pump + + Why doesn't Na+ enter the cell? Because it is being actively pumped out Na+/K+ transporter molecules are embedded in the membrane Heavy energy demand to move ions against their gradients the pumps require ~40% of the cell's energy!! The transporters pump sodium (Na+) out of the cell and pump potassium (K+) in 3 Na+ ions are pumped out for every 2 K+ ions pumped in Initiating the Action Potential: Sodium vs. Potassium! When the threshold of excitation is reached, the action potential is initiated by the opening of sodium ion channels Na+ rushes into the cell, following its concentration and charge gradients A short time later, potassium ion channels open K+ leaves the cell, repelled by the accumulation of Na+ inside The action potential obeys the Properties of the Action Potential allornone law either it occurs or it does not occur The AP is actively propagated down the axon toward the terminal It has a fixed velocity and amplitude It does not decline in strength Current can also move passively across the How Does the AP Propagate Down the Axon? neuron's membrane Disturbances of membrane potential can be carried along membrane by this passive transmission and this does not require the opening of ion channels This is called decremental conduction These signals degrade with time and distance Saltatory Conduction At each Node of Ranvier, the action potential is regenerated Beneath the myelin sheath, the signal is propagated passively High concentration of sodium channels This "jumping" of the AP from one Node of Ranvier to the next is called saltatory conduction, and has 2 advantages: it declines in strength, but not enough to prevent regeneration of the AP at the next Node Economy: Pumps need only work at the Nodes Speed: Passive conduction under myelin is very fast The action potential "jumps" down the axon over the Nodes of Ranvier Multiple Sclerosis Clinical disorder characterized by de myelination Thought to be an "autoimmune disorder", nerve conduction is slowed considerably in areas of the brain that are affected in which the immune system of the body becomes sensitized to the body's own myelin and periodically attacks it See pg. 544546 of text for further reading Communication Between Neurons The action potential is conducted along The Axon Terminal & The Synapse axon membrane to the axon terminal Within the axon terminal, there are multiple synaptic vesicles that contain neurotransmitters Neurotransmitters are released into the Chemical messengers that are released by the neuron synapse and bind to receptors on the other side of the synapse The neuron that sends signal is called the presynaptic neuron The neuron that receives signal is called the postsynaptic neuron There are multiple types of synapses depending on where the synapse is formed Different Types of Synapses Most axodendritic synapses occur on dendritic spines, small protrusions that occur on many important types of neurons in the brain Axodendritic A terminal of one neuron contacts a dendrite of another neuron (very common) Axosomatic A terminal of one neuron contacts the soma of another neuron Axoaxonic A terminal of one neuron contacts the terminal of another neuron A View of an Axodendritic Synapse Release of Neurotransmitter As the action potential arrives at the axon terminal, it opens calcium (Ca2+) channels The entry of Ca2+ ions into the terminal triggers the movement of the synaptic vesicles to the release zone they fuse with the membrane and release their contents This process is called exocytosis Ca2+ is more concentrated on the outside of the terminal Neurotransmitter diffuses across cleft to interact with receptors on the postsynaptic membrane The binding of synaptic vesicles at the release zone creates omega figures AP Ca2+ Neurotransmitter Release On The Postsynaptic Side.... Neurotransmitters diffuse across the synapse and interact with receptors on the other side Binding of the neurotransmitter to the receptor leads to a change in membrane potential by allowing ions to pass into the cell There are basically 2 types of receptors: Ionotropic these are opened directly by the binding of neurotransmitter Metabotropic these receptors bind neurotransmitter, and then open ion channels by generating a second messenger The net effect is the opening of an ion channel, but the effect is indirect (via a second messenger) The influx of ions through receptors in the Postsynaptic Potentials postsynaptic cell causes a small, transient change in the postsynaptic membrane potential These are not action potentials! There are general 2 types of changes that can occur: Excitatory postsynaptic potentials (EPSPs) these are changes that depolarize the cell (make it more excited and more likely to fire) Inhibitory postsynaptic potentials (IPSPs) these are changes that hyperpolarize the cell (make it less excited and less likely to fire) Postsynaptic potentials (PSPs) are Characteristics of Postsynaptic Potentials propagated passively along the membrane PSPs are determined by receptor type, not by their size is directly proportional to the amount of transmitter released they will degrade with time the neurotransmitter type There are generally 4 different types of ion channels that create PSPs These can create either EPSPs or IPSPs EXAM Question! A neuron _________when ____________: A) depolarizes; potassium moves from the inside to the outside of the cell B) hyperpolarizes; sodium ions move from the outside to the inside of the cell C) hyperpolarizes; chloride moves from the outside to the inside of the cell D) depolarizes; sodium moves from the inside to the outside of the cell E) hyperpolarizes; potassium moves from the outside to the inside of the cell EXAM Question! A neuron _________when ____________: A) depolarizes; potassium moves from the inside to the outside of the cell B) hyperpolarizes; sodium ions move from the outside to the inside of the cell C) hyperpolarizes; chloride moves from the outside to the inside of the cell D) depolarizes; sodium moves from the inside to the outside of the cell E) hyperpolarizes; potassium moves from the outside to the inside of the cell Termination of Postsynaptic Potentials Reuptake Reuptake Rapid removal of neurotransmitter from synaptic cleft to the presynaptic terminal, where it can be recycled or broken down This is an active mechanism that requires transporter molecules similar to the Na+/K+ pump Almost all neurotransmitters undergo reuptake Enzymatic Degradation An enzyme degrades the neurotransmitter into its component parts Neurons receive inputs from both excitatory neurons (that produce EPSPs) and inhibitory neurons (that produce IPSPs) EPSPs and IPSPs are added together in the postsynaptic cell as they propagate Neural Integration is the sum of all the PSPs as information travels from the dendrites to the axon hillock, where action potentials are triggered If at the axon hillock the sum of PSPs is greater than the threshold of excitation, then an action potential will be generated Neural Integration FIRE! DON'T FIRE! ...
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