Chapter Two Nerve Cells and Nerve Impulses

1. Chapter TwoNerve Cells and Nerve Impulses

2. Cells of the Nervous System Neurons and Glia The Structures of an Animal Cell Membrane-a structure that separates the inside of the cell from the outside Nucleus-the structure that contains the chromosomes Mitochondrion-structure where the cell performs metabolic activities Ribosomes-sites at which the cell synthesizes new protein molecules Endoplasmic reticulum-a network of thin tubes that transport newly synthesized proteins to other locations

3. Figure 2.3  The membrane of a neuronEmbedded in the membrane are protein channels that permit certainions to cross through the membrane at a controlled rate.

4. Figure 2.2  An electron micrograph of parts of a neuron from the cerebellum of a mouseThe nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnification approximately 3 23,000.

5. Cells of the Nervous System Neurons and Glia The Structure of a Neuron Dendrites-branching fibers that get narrower as they extend from the cell body toward the periphery; information receiver Dendritic spines-short outgrowths that increase the surface area available for synapses Cell body-contains the nucleus and other structures found in most cells Axon-thin fiber of constant diameter, in most cases longer then the dendrites; information-sender Myelin sheath-insulating material covering the axons; speed up communication in the neuron Presynaptic terminal-the point on the axon that releases chemicals

6. Figure 2.5  The components of a vertebrate motor neuronThe cell body of a motor neuron is located in the spinal cord. The various parts are not drawn to scale; in particular, a real axon is much longer in proportion to the size of the soma.

7. Cells of the Nervous System Neurons and Glia Terms associated with Neurons Motor neuron-receives excitation from other neurons and conducts impulses from its soma in the spinal cord to muscle of gland cells Sensory neuron-specialized at one end to be highly sensitive to a particular type of stimulation Local neuron-small neuron with no axon or a very short one Efferent axon-carries information away from the structure Afferent axon-brings information into a structure Intrinsic/interneuron-the cell’s dendrites and axon’s are entirely contained within a single structure

8. Figure 2.6  A vertebrate sensory neuronNote that the soma is located in a stalk off the main trunk of the axon. (As in Figure 2.5, the various structures are not drawn to scale.)

9. Figure 2.8  Cell structures and axonsIt all depends on the point of view. An axon from A to B is an efferent axon from A and an afferent axon to B, just as a train from Washington to New York is exiting Washington and approaching New York.

10. Cells of the Nervous System Neurons and Glia Glia-supportive cells in the nervous system Types Astrocytes-star-shaped glia that wrap around the presynaptic terminals of several axons Radial Glia-a type of astrocyte that guides the migration of neurons and the growth of their axons and dendrites during embryonic development Oligodendrocytes-located in the CNS and provide myelin sheaths for axons Schwann Cells-located in the PNS and provide myelin sheaths for axons

11. Figure 2.11 (a)  Shapes of some glia cells.Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among various neurons in an area. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well.

12. The Blood-Brain Barrier Why we need a blood-brain barrier To keep out harmful substances such as viruses, bacteria, and harmful chemicals How the blood-brain barrier works Tight Gap Junctions What can pass the blood-brain barrier Passive Transport-require no energy to pass Small uncharged molecules-oxygen and carbon dioxide Molecules that can dissolve in the fats of the capillary walls Active Transport-require energy to pass Glucose, amino acids, vitamins and hormones

13. Figure 2.13  The blood-brain barrierMost large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small uncharged molecules such as O2 and CO2 can cross; so can certain fat-soluble molecules. Active transport systems pump glucose and certain amino acids across the membrane.

14. Nourishment of Vertebrate Neurons Glucose-primary energy source for the brain Oxygen-needed to metabolize glucose Thiamine-necessary for the use of glucose

15. The Nerve Impulse The Resting Potential of the Neuron Resting potential-results from a difference in distribution of various ions between the inside and outside of the cell (-70mV) Measurement of the Resting Membrane Potential Microelectrodes Why a Resting Potential? Prepares neuron to respond rapidly to a stimulus

16. Figure 2.14  Methods for recording activity of a neuron(a) Diagram of the apparatus and a sample recording. (b) A microelectrode and stained neurons magnified hundreds of times by a light microscope. (Fritz Goro)

17. The Nerve Impulse The Forces Behind the Resting Potential Selective Permeability-the membrane allows some molecules to pass more freely than others The Forces Sodium-Potassium Pump-actively transports three sodium ions out of the cell while simultaneously drawing two potassium ions into the cell Concentration Gradients-difference in distribution for various ions between the inside and outside of the membrane Electrical Gradient-the difference in positive and negative charges across the membrane

18. Figure 2.16  The sodium and potassium gradients for a resting membraneSodium ions are more concentrated outside the neuron; potassium ions are more concentrated inside. However, because the body has far more sodium than potassium, the total number of positive charges is greater outside the cell than inside. Protein and chloride ions (not shown) bear negative charges inside the cell. At rest, very few sodium ions cross the membrane except by the sodium-potassium pump. Potassium tends to flow into the cell because of an electrical gradient but tends to flow out because of the concentration gradient. Animation

19. The Action Potential Important Definitions Hyperpolarization-increasing the negative charge inside the neuron Depolarization-decreasing the negative charge inside the neuron Threshold of excitation-Any stimulation beyond a certain level producing a sudden, massive depolarization of the membrane Action Potential-rapid depolarization and slight reversal of the usual polarization

20. Molecular Basis of the Action Potential Sodium channels open once threshold is reached causing an influx of sodium Potassium channels open as the action potential approaches its peak allowing potassium to flow out of the cell Cell overshoots resting membrane potential

21. Figure 2.17  The movement of sodium and potassium ions during an action potentialNote that sodium ions cross during the peak of the action potential and that potassium ions cross later in the opposite direction, returning the membrane to its original polarization.

22. The Action Potential The All-or-None Law The size, amplitude, and velocity of an action potential are independent of the intensity of the stimulus that initiated it.

23. The Action Potential The Refractory Potential Defined-During this time the cell resists the production of further action potentials Two Refractory Periods Absolute Refractory Periods The sodium gates are firmly closed The membrane cannot produce an action potential, regardless of the stimulation. Relative Refractory Periods The sodium gates are reverting to their usual state, but the potassium gates remain open. A stronger than normal stimulus can result in an action potential.

24. Propagation of the Action Potential Axon Hillock-where the action potential begins Terminal Buttons-the end point for the action potential The action potential flows toward the terminal and does not reverse directions because the area where the action potential just came from are still in refractory

25. The Myelin Sheath and Saltatory Conduction Myelin Sheaths increase the speed of neural transmission Nodes of Ranvier-Short area’s of the axon that are unmyelinated Saltatory Conduction-jumping action of actions potentials from node of Ranvier to node of Ranvier

26. Figure 2.20  Saltatory conduction in a myelinated axonAn action potential at the node triggers flow of current to the next node, where the membrane regenerates the action potential.

27. Signaling Without Action Potentials Depolarizations and hyperpolarizations of dendrites and cell bodies Small Local neurons-produce graded potentials (membrane potentials that vary in magnitude and do not follow the all-or-none law)