Chapter 48 Neurons Synapses and Signaling Neurons Nerve Cells that transfer information within the body Communication by neurons occurs through long distance electrical signals and short distance chemical signals Specialized structures allows neurons to receive transmit and regulate flow of information All neurons behave in the same manner what distinguish the type of information is the paths and connections the information takes Animals have a large group of neurons clustered into a BRAIN or simpler clusters called Ganglia 48 1 NEURON STRUCTURE AND ORGANIZTION REFLECT FUNCTION IN INFORMATION TRANSFER NEURON STRUCUTRE AND FUNCTION Most of the neurons organelles are located in the cell body A neuron usually has highly branched extensions called DENDRITES which receive signals from other neurons A Neuron has a single Axon an extension that transmits signals to other cells Axons can be really long and connect cells to transmit information at junctions called a SYNAPSE The region of each Axon branch that forms this specialized junctions called a synaptic terminal NEUROTRANSMITTERS chemical messengers pass information from the transmitting neuron to the receiving cell The transmitting neuron is the PRESYNAPTIC CELL while the cell that receives the signal is the POSTSYNAPTIC CELL muscle gland neuron cell Neurons require the support of GILAL CELLS which offer nourishment insulation and regulation of the EC Fluid GILA USUALLY OUTNUMBER NEURONS IN THE MAMMALIAN BRAIN INTRODUCTION TO INFORMATION PROCESSING Information processing occurs in three stages 1 Sensory Input a Detects change in environment 2 Integration analysis and interpretation 3 Motor Output a a Input processed Initiates response In complex animals specialized populations of neurons handle each stage of information processing Sensory Neurons transmit information about external stimuli Brain or Ganglia Neurons sensory input analyzed and interpreted Most neurons in the brain are INTERNEURONS which form inner circuits connecting neurons in the brain Neurons that extend out of processing centers generate output e g Motor Neurons transmit signals to muscle cells Neurons that carry out integration are organized in a CENTRAL NERVOUS SYSTEM CNS while the neurons that carry information into and out of the CNS form the PERIPHERAL NERVOUS SYSTEM Bundles of the axons of Neurons form Nerves Neurons that transmit information to target cells tend to have more branched axons Neurons that receive input have highly branched dendrites and synapses 48 2 ION PUMPS AND ION CHANNELS ESTABLISH THE RESTING POTENTIAL OF A NEURON In Neurons there is an unequal distribution of ions between the interior of the cell and the surrounding fluid which results in a partially negative charge inside compared to outside The difference in potential energy is called the MEMBRANE POTENTIAL For a resting neuron the membrane potential is called the RESTING POTENTIAL Inputs from other neurons or stimuli cause the changes in membrane potential FORMATION OF THE RESTING POTENTIAL Potassium and Sodium ions play essential roles in the formation of the resting potential In most neurons the concentration of K is higher inside while concentration of Na is higher outside The SODIUM POTASSIUM pump maintains the gradients and uses ATP to actively transport NA out and K in The Sodium Potassium pump results in a net export of positive charge but the difference in voltage is very small The reason there is a voltage difference is due to ION CHANNELS which allow diffusion of ions back and forth across the membrane and the carrying of electrical charge and any resulting movement if ions results in the formation of a membrane potential The concentration of ions across the membrane results in a chemical form of potential energy which is converted to electrical potential energy due to the permeability of certain ions Diffusion of Potassium ions through always open channels leads to a negative potential inside the cell which is a major source of the membrane potential Sodium ions cannot diffuse as much as Potassium ions because the channels are usually closed The attractive forces of the negative charges counterbalance the outflow of Potassium MODELLING THE RESTING POTENTIAL The outflow of Potassium ions proceeds until the chemical and electrical forces are in balance The Chloride ions cannot diffuse as easily as the potassium ions which results in a negative charge inside the cell Equilibrium Potential Eion is the magnitude of the membrane potential at equilibrium 48 3 ACTION POTENTIALS ARE THE SIGNALS CONDUCTED BY AXONS Changes in membrane potential occur because neurons contain GATED ION CHANNELS which open or close in response to a stimuli and leads to the alteration of the membrane potential HYPERPOLOARIZATION AND DEPOLARIZATION HYPERPOLARIZATION makes the inside of the membrane more negative and could occur due to the outflow of positive ions or inflow of negative ions and usually involves Potassium channels DEPOLARIZATION is the reduction of the magnitude of the membrane potential and usually involves opening gated sodium channels and the increase in the Sodium ions The flow of ions occurs due to stimuli which causes he gated ion channels to open or close GRADED POTENTIAL AND ACTION POTENTIALS The response to hyperpolarization or depolarization is simply a shift in the membrane potential and this is called a GRADED POTENTIAL which varies with the strength of the stimulus Graded potentials cause a small electrical current ACTION POTENTIAL is the result of a massive change in membrane voltage that occurs due to DEPOLARIZATION Action potentials can regenerate in adjacent regions of the membrane and spread along axons making them better suited for long distance transmittance Action potentials arise due to some of the ion channels being VOLTAGE GATED ION CHANNELS which open or close when changes in voltage reach a certain THRESHOLD The way this works is that sodium channels are activated due to depolarization which further opens more sodium channels and causes the Action potential and results in a positive feedback GENERATION OF ACTION POTENTIALS A CLOSER LOOK Depolarization opens both voltage gated sodium and potassium channels However the channels respond independently and sequentially Sodium channels open first initiating the action potential and eventually becomes inactivated until the resting potential is arrived at Potassium channels open more slowly but remain opened until the