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BIO204 Physio Lecture note 8.docx

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Ingo Ensminger

BIO204 Nov.24/2011 Chapter 45: Electric Signals in animals 45.1 Principles of electrical signalling  Nerve cells transmit info. Info they carry originates in sensory cells that are located in periphery of body. Sensory info inside eyes, ears, nose transmit streams of data about external env in form of signals.  A sensory receptor transmits the information it receives from the environment by means of a nerve cell called a sensory neuron.  In vertebrates, the sensory neuron sends the information to neurons in the brain or spinal cord via nerves—long, tough strands of nervous tissue, containing thousands of neurons.  The central nervous system (CNS), made up of the brain and spinal cord, integrates information from many sensory neurons.  Cells in the CNS called interneurons make connections between sensory neurons and motor neurons, which are nerve cells that send signals to effector cells in glands or muscles.  All of the components of the nervous system outside the CNS are part of the peripheral nervous system (PNS).  Sensory information from receptors in the PNS is sent to the CNS, where it is processed. Then a response is transmitted back to appropriate parts of the body.  Some information from a sensory neuron results in a reflex—a direct response to a signal that bypasses the brain. Anatomy of neuron  Most neurons have a cell body, dendrites, and one or more axons (Figure 45.3).  A dendrite receives electrical signals from the axons of adjacent cells.  The axon then sends the signal to the dendrites of other neurons.  The cell body, or soma, which includes the nucleus, integrates the incoming signals and generates an outgoing signal in the axon. An introduction to membrane potentials  A difference of electrical charge between any two points creates a difference in electrical potential, or a voltage.  Because ions are separated across a membrane and because they carry a charge, there is a separation of charge across the membrane called a membrane potential.  Membrane potentials are a form of electrical potential and are measured in millivolts . How is resting potential maintained?  When a neuron is not transmitting an electrical signal but is merely sitting in extracellular fluid at rest, its membrane has a voltage called the resting potential (Figure 45.4).  Neurons have a high intracellular concentration of K and low intracellular concentrations of Na and+ – Cl . +  In neurons, only K can cross the membrane easily along its concentration gradient. It does so via K channels.  Cross PM only in 3 ways:  Flowing along their electrochemical gradient through ion channel  Carried via membrane cotransporter with an ion that experiences strong electrochem gradient.  Pumped against electro chem gradient by membrane protein that hydrolyzes ATP.  Ion channels are pores in the membrane that allow only specific ions to pass through.  When a neuron is not actively involved in transmitting an electrical signal, the most common type of + channel that is open is one that admits K ions. +  Biologists say that the membrane is selectively permeable—it permits a much greater flow of K ions than of Cl or Na ions. + +  The K channels involved are sometimes called leak channels, because they allow K to leak out of the cell.  As K moves from the interior of the cell to
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