Lecture 21- ELECTRICAL SIGNALS IN ANIMALS.docx

ELECTRICALSIGNALS IN ANIMALS How is electrical signals in the heart and other parts of animal body affected in extreme cases, such as diving? • Animal movements are triggered by electrical signals conducted by nerve cells, or neurons. • Complex processes such as moving, seeing, and thinking are based on seemingly simple events: flows of ions across plasma membranes. • Neurons transmit electrical signals; muscles can respond to electrical signals by contracting. • There are two basic types of nervous systems: • The diffuse arrangement of cells called a nerve net, found in cnidarians (jellyfish, hydra, anemones) and ctenophores (comb jellies). • Acentral nervous system (CNS) that includes large numbers of neurons aggregated into clusters called ganglia. Types of Neurons in the Nervous System • Sensory receptors in the skin, eyes, ears, and nose transmit streams of data about the external environment. Sensory cells inside the body monitor conditions that are important in homeostasis, such as blood pH and oxygen levels. • Asensory 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 spinal cord or brain 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 (response) cells in glands or muscles. • All of the components of the nervous system outside the CNS are part of the peripheral nervous system (PNS). • In summary, 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. TheAnatomy of a Neuron • Most neurons have the same three parts: 1. Adendrite receives electrical signals from the axons of adjacent cells. 2. The cell body, or soma, which includes the nucleus, integrates the incoming signals and generates an outgoing signal. 3. The axon then sends the signal to the dendrites of other neurons. • Each neuron makes many connections with other neurons. Which structure generate electrical potential in cell membranes and how? - - Potassium and sodium leakchannels (more potassium than sodium channels)Na+ ‐K+‐ATPase pump resting potential: high intracellular concentration of K+ and proteins and low intracellular – concentration of Na+ and Cl . An Introduction to Membrane Potentials • Adifference of electrical charge between any two points creates a difference in electrical potential, or a voltage. • If the positive and negative charges on ions that exist on the two sides of a plasma membrane do not balance each other, the membrane will have an electrical potential. • When an electrical potential exists on either side of a plasma membrane, the separation of charges is called a membrane potential. • Membrane potentials are a form of electrical potential and are measured in millivolts (mV). • In neurons, membrane potentials are typically about 70–80 mV. • Membrane potentials are always expressed as inside-relative-to-outside. Electrical Potential, Currents, and Gradients • When a membrane potential exists, the ions on both sides of the membrane have potential energy. • If the membrane were removed, ions would spontaneously move from the area of like charge to the area of unlike charge—causing a flow of charge, called an electric current. • Ions move across membranes in response to concentration gradients as well as charge gradients. – The combination of an electric gradient and a concentration gradient is an electrochemical gradient. How Is the Resting Potential Maintained? • When a neuron is at rest in extracellular fluid, its membrane has a voltage called the resting potential. • This potential exists in part because neurons have a high intracellular concentration of K + and low intracellular concentrations of Na and Cl . – • The plasma membrane is selectively permeable; only certain substances can cross it. • Ions such as these can only cross plasma membranes efficiently in one of three ways: • • • • • • • • • • • Along their electrochemical gradient through an ion channel, a pore in the membrane that allows only specific ions to pass through. • Carried via a membrane cotransporter protein or antiporter protein. • Pumped against an electrochemical gradient by a membrane protein that hydrolyzes ATP. • The K Leak Channel • When a neuron is not transmitting an electrical signal, the type of membrane ion channel most likely to be open are those that admit K ions. + + • Resting neurons are most permeable to K ions, which cross the membrane easily along their concentration gradient. • The potassium channels involved are sometimes called leak channels, because they allow K to leak out of the cell. + + • As K moves out of the cell via K channels, the inside of the cell becomes more negatively charged relative to the outside. • Eventually, the membrane reaches a voltage at which there is equilibrium between the concentration gradient that moves K out and the electrical gradient that moves K in. + + This is called the equilibrium potential for K . • Even though Cl and Na cross the plasma membrane much less readily than does K , + each type of ion has its own equilibrium potential. The Role of the Na /K -ATPase • Na /K -ATPase imports K ions and exports Na ions, resulting in the concentration of K + + ions being higher inside the cell and Na being higher outside the cell. • This results in the inside of the neuron being negatively charged with respect to the extracellular environment. Thus, the neuron has a negative resti