How Neurons Send and Receive Signals TOPIC 3 Neural Conduction and Synaptic Transmission 1

INTRO  Communication within a neuron  Video: Passive and Active Transport across the Neural Membrane  Postsynaptic Potentials  Generation of Action Potentials  Conduction of Action Potentials  Action Potentials are Nondecremental and Slow  Communication between neurons  Changing Views on Dendritic Function  Synaptic Contacts and Transmission  Neurotransmitters and Receptors  Pharmacology of Synaptic Transmission 2

COMMUNICATION WITHIN A NEURON 3

Page 4  Afferent neurons relay messages from the sense organs and receptors—eyes, ears, nose, mouth, and skin—to the brain or spinal cord  Efferent neurons convey signals from the central nervous system to the glands and the muscles, enabling the body to move The Neuron

Page 5  There are three general types of neurons  Sensory neuron  A neuron that detects changes in the external or internal environment and sends information about these changes to the central nervous system.  Motor neuron  A neuron located within the central nervous system that controls the contraction of a muscle or the secretion of a gland.  Interneuron  A neuron located entirely within the central nervous system.  Interneurons carry information between neurons in the brain and between neurons in the spinal cord Cells of the Nervous System

Page 6  Three classifications of neurons  Multipolar neurons  A neuron with one axon and many dendrites.  Bipolar neurons  A neuron with one axon and one dendrite attached to its soma.  Unipolar neurons  A neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the central nervous system. Cells of the Nervous System

Page 7  Neurotransmitter  Chemical messengers that relay neural messages across the synapse  A chemical that is released into the synaptic cleft from a terminal button (axon) of a sending neuron, crosses a synapse, and binds to appropriate receptor sites on the dendrites or cell body of a receiving neuron, influencing the cell either to fire or not to fire;  Has an excitatory or inhibitory effect on another neuron.  Receptors  Protein molecules on the dendrite or cell body of a neuron that will interact only with specific neurotransmitters  Action of neurotransmitters  Excitatory  Influencing the neurons to fire  Inhibitory  Influencing neurons not to fire Neuron Basic Structure

Page 8  Internal structure  Membrane  A structure consisting principally of lipid molecules that defines the outer boundaries of a cell and also constitutes many of the cell organelles.  Cytoplasm  The viscous, semi-liquid substance contained in the interior of a cell.  Mitochondria  An organelle that is responsible for extracting energy from nutrients.

Page 9 INTERNAL STRUCTURE

Page 10  Internal structure  Adenosine triphosphate (ATP)  A molecule of prime importance to cellular energy metabolism; its breakdown liberates energy.  Nucleus  A structure in the central region of a cell, containing the nucleolus and chromosomes.  Chromosome  A strand of DNA, with associated proteins, found in the nucleus; carries genetic information.

Page 11  Internal structure  Deoxyribonucleic acid (DNA)  A long complex macromolecule consisting of two interconnected helical strands; along with associated proteins, strands of DNA constitute the chromosomes.  Gene  The functional unit of the chromosome, which directs synthesis of one or more proteins.  Microtubule  A long strand of bundles of protein filaments arranged around a hollow core; part of the cytoskeleton and involved in transporting substances from place to place within the cell.  Cytoskeleton  Formed of microtubules and other protein fibers, linked to each other and forming a cohesive mass that gives a cell its shape.

Page 12 THE NEURAL IMPULSE  Neural impulse – Brief electric surge that carries the neuron’s message  Ions – Charged particles that are moved across the cell membrane

Page 13  Measuring electrical potentials (Neural Impulses) of axons  Axons have two basic electrical potentials  Resting membrane potential  Action potential Communication Within a Neuron

MEMBRANE POTENTIAL  Measuring membrane potential  To learn how info sent from dendrites and soma of a neuron to its terminals, researchers study a neuron’s membrane potential  The membrane potential can change  Depolarization  Hyperpolarization  Threshold of excitation  Electrode  A conductive medium that can be used to apply electrical stimulation and record electrical potentials. 14

Page 15  Measuring electrical potentials of axons (see figure in next slide)  Microelectrode  A very fine electrode, generally used to record activity of individual neurons. Ie. Intracellular vs extracellular  Membrane potential  The electrical charge across a cell membrane; refers to the difference in electrical potential inside and outside the cell.  To measure a membrane potential  Oscilloscope  A laboratory instrument that is capable of displaying a graph of voltage as a function of time on the face of a cathode ray tube.

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Page 17  Measuring electrical potentials of axons  Resting membrane potential  When intracellular and extracellular electrodes are outside of a neuron, the difference between the electrical potentials at their tips is zero  When the intracellular electrode penetrates the neuron, the potential jumps to approximately -70 mV in the giant squid axon. This is the resting potential of a neuron.  Positively and negatively charged ions are distributed unequally on the tow sides of the neural membrane.  Thus, when membrane is polarized, it carries a charge  The membrane potential of a neuron at rest, about 270 millivolts  The resting membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials.

Page 18  Measuring electrical potentials of axons  Factors contributing to uneven distribution  Homogenizing (ie Random motion & concentration of gradients)  Electrostatic pressure like repels like, opposites attract  Membrane is selectively permeable  Sodium-potassium pumps

Page 19 When do membrane potential change?  Depolarization  Reduction (toward zero) of the membrane potential of a cell from its normal resting potential.  Hyperpolarization  An increase in the membrane potential of a cell, relative to the normal resting potential.  Threshold of excitation  The value of the membrane potential that must be reached to produce an action potential.

Page 20 Measuring electrical potentials of axons  Action potential  The brief electrical impulse that provides the basis for conduction of information along an axon.  The sudden reversal of the resting potential, which initiates the firing of a neuron.  **Threshold of excitation  The value of the membrane potential that must be reached to produce an action potential.

ACTION POTENTIAL  Is the technical term used to describe nerve impulse  Consists of a brief polarisation that spreads along an axon  Different from receptor potential (synaptic potential) in several aspects 21

ACTION POTENTIAL  Do not vary in amplitude or intensity  ‘All or nothing’ events – If the intensity of a stimulus fall below the neuron’s excitation threshold, nothing will happen.  Intensity of stimulus greater than threshold of excitation – does not matter whether it does so by small or large amount.  An action potential will be triggered!  Amplitude and frequency – same for any given cell.  THUS: to transmit info, - neuron vary the frequency of the action potentials – the number of AP that it transmit per second. 22

Page 23  Difference between a strong and weak stimulus  A weak stimulus may cause few neurons to fire and at a slow rate  A strong stimulus may cause thousands of neurons to fire at the same time and at hundreds of times per second

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Listening to Action Potentials  SCL: Passive and Active Transport across the Neural Membrane  nimations/transport1.html. nimations/transport1.html 26

FIGURE 4.2 The passive and active factors that influence the distribution of Na+, K+, and Cl- ions across the neural membrane. © 2011 Pearson Education, Inc. All rights reserved. 27