Key Concepts Neurons are cells that transmit electrical signals used in communication Their plasma membranes carry a voltage called a membrane potential due to differences in the concentrations of ions on the inner and outer surfaces Action potentials are all or none changes in membrane potential that serve as electrical signals During an action potential an inflow of sodium ions is followed by an outflow of potassium ions 2011 Pearson Education Inc Key Concepts At synapses the electrical signal from a neuron triggers the release of a chemical signal a neurotransmitter When the neurotransmitter arrives at an adjacent neuron there is a change in that cell s membrane potential Most animals have a central nervous system CNS and a peripheral nervous system PNS PNS neurons receive sensory information and transmit it to the CNS for processing The CNS then sends signals to muscles glands or other tissues via PNS neurons 2011 Pearson Education Inc Introduction 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 2011 Pearson Education Inc Principles of Electrical Signaling Neurons transmit electrical signals muscles can respond to electrical signals by contracting There are two basic types of nervous systems 1 The diffuse arrangement of cells called a nerve net found in cnidarians jellyfish hydra anemones and ctenophores comb jellies 2 A central nervous system CNS that includes large numbers of neurons aggregated into clusters called ganglia 2011 Pearson Education Inc 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 A sensory receptor transmits the information it receives from the environment by means of a nerve cell called a sensory neuron 2011 Pearson Education Inc Types of Neurons in the Nervous System 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 2011 Pearson Education Inc Types of Neurons in the Nervous System 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 2011 Pearson Education Inc The Anatomy of a Neuron Most neurons have the same three parts 1 A dendrite 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 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc An Introduction to Membrane Potentials A difference 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 2011 Pearson Education Inc Units and Signs 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 tooutside 2011 Pearson Education Inc 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 2011 Pearson Education Inc 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 2011 Pearson Education Inc How Is the Resting Potential Maintained 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 1 Along their electrochemical gradient through an ion channel a pore in the membrane that allows only specific ions to pass through 2 Carried via a membrane cotransporter protein or antiporter protein 3 Pumped against an electrochemical gradient by a membrane protein that hydrolyzes ATP 2011 Pearson Education Inc 2011 Pearson Education Inc 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 2011 Pearson Education Inc The K Leak Channel 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 2011 Pearson Education Inc The Role of the Na K