Chapter 44 Lecture 15 Neurons and Nervous System Dr. Alan McElligott

Neurons and Nervous Systems Aims: To examine the structure and function of the cells involved in the nervous system To understand the production of nervous signals

Neurons and Nervous Systems Aims: To examine the structure and function of the cells involved in the nervous system To understand the production of nervous signals These lecture aims form part of the knowledge required for learning outcome 3: Describe mechanisms for life processes (LOC3).

Essential reading Pages All of this chapter is useful, but only the contents of sections 44.1 and 44.2 will be examined. Neurons and Nervous Systems

44 Neurons and Nervous Systems 44.1 What Cells Are Unique to the Nervous System? 44.2 How Do Neurons Generate and Conduct Signals?

44.1 What Cells Are Unique to the Nervous System? Nervous systems have two categories of cells: Neurons generate and propagate electrical signals, called action potentials. Glial cells provide support and maintain extracellular environment.

44.1 What Cells Are Unique to the Nervous System? Neurons are organized into networks. Afferent neurons carry information into the system. Sensory neurons convert input into action potentials. Efferent neurons carry commands to effectors. Interneurons store information and help with communication in the system.

44.1 What Cells Are Unique to the Nervous System? Networks vary in complexity. Nerve net: simple network of neurons. Ganglia: neurons organized into clusters, sometimes in pairs. Brain: the largest pair of ganglia.

Figure 44.1 Nervous Systems Vary in Size and Complexity

44.1 What Cells Are Unique to the Nervous System? Central nervous system (CNS) – consists of cells found in brain and spinal cord Peripheral nervous system (PNS) – neurons and support cells found outside the CNS

Figure 44.2 Brains Vary in Size and Complexity

44.1 What Cells Are Unique to the Nervous System? Neurons pass information at synapses: The presynaptic neuron sends the message. The postsynaptic neuron receives the message.

44.1 What Cells Are Unique to the Nervous System? Most neurons have four regions: Cell body: contains the nucleus and organelles Dendrites: bring information to the cell body Axon: carries information away from the cell body Axon terminal: forms synapse at tip of axon

Figure 44.3 Neurons

Neurons Growing neurons

44.1 What Cells Are Unique to the Nervous System? Glial cells, or glia, outnumber neurons in the human brain. Glia do not transmit electrical signals but have several functions: Support during development Supply nutrients Maintain extracellular environment Insulate axons

44.1 What Cells Are Unique to the Nervous System? Oligodendrocytes produce myelin and insulate axons in the CNS. Schwann cells insulate axons in the PNS. Astrocytes contribute to the blood– brain barrier, which protects the brain.

Figure 44.4 Wrapping Up an Axon

44.2 How Do Neurons Generate and Conduct Signals? Action potentials are the result of ions moving across the plasma membrane. Ions move according to differences in concentration gradients and electrical charge. Membrane potential is the electric potential across the membrane. Resting potential is the membrane potential of a resting neuron.

44.2 How Do Neurons Generate and Conduct Signals? Voltage causes electric current as ions to move across cell membranes. Major ions in neurons: Sodium (Na + ) Potassium (K + ) Calcium(Ca 2+ ) Chloride (Cl – )

44.2 How Do Neurons Generate and Conduct Signals? Membrane potentials are measured with electrodes. The resting potential of an axon is –60 to –70 millivolts (mV). The inside of the cell is negative at rest. An action potential allows positive ions to flow in briefly, making the inside of the cell more positive.

Figure 44.5 Measuring the Resting Potential

44.2 How Do Neurons Generate and Conduct Signals? The plasma membrane contains ion channels and ion pumps that create the resting and action potentials. The sodium–potassium pump uses ATP to move Na + ions from inside the cell and exchanges them for K + from outside the cell. This establishes concentration gradients for Na + and K +.

Figure 44.6 Ion Pumps and Channels

44.2 How Do Neurons Generate and Conduct Signals? Ion channels in the membrane are selective and allow some ions to pass more easily. The direction and size of the movement of ions depends on the concentration gradient and the voltage difference of the membrane. These two forces acting on an ion are its electrochemical gradient.

44.2 How Do Neurons Generate and Conduct Signals? Potassium channels are open in the resting membrane and are highly permeable to K + ions. K + ions diffuse out of the cell along the concentration gradient and leave behind negative charges within the cell. K + ions diffuse back into the cell because of the negative electrical potential.

44.2 How Do Neurons Generate and Conduct Signals? The potassium equilibrium potential is the membrane potential at which the net movement of K + ceases. The Nernst equation calculates the value of the potassium equilibrium potential by measuring the concentrations of K + on both sides of the membrane.

Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 1)

Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 2)

Figure 44.7 Which Ion Channel Creates the Resting Potential? (Part 3)

44.2 How Do Neurons Generate and Conduct Signals? Ion channels and their properties can be studied by patch clamping. A patch clamp electrode is placed against the membrane and a seal forms with applied suction. Movement of ions and the opening and closing of ion channels are recorded as electric currents.

Figure 44.8 Patch Clamping

44.2 How Do Neurons Generate and Conduct Signals? Some ion channels are gated, and open and close under certain conditions. Voltage-gated channels respond to a change in the voltage across the membrane. Chemically-gated channels depend on molecules that bind or alter the channel protein. Mechanically-gated channels respond to force applied to the membrane.

44.2 How Do Neurons Generate and Conduct Signals? Gated ion channels change the resting potential when they open and close. The membrane is depolarized when Na + enters the cell and the inside of the neuron becomes less negative than when at rest. If gated K + channels open and K + leaves, the cell becomes more negative inside and the membrane is hyperpolarized.

Figure 44.9 Membranes Can Be Depolarized or Hyperpolarized

44.2 How Do Neurons Generate and Conduct Signals? Action potentials are sudden, large changes in membrane potential. Voltage-gated Na + and K + channels are responsible for action potentials. If a cell body is depolarized, voltage- gated Na + channels open and Na + rushes into the axon. The influx of positive ions causes more depolarization.

44.2 How Do Neurons Generate and Conduct Signals? A threshold is reached at 5–10 mV above resting potential.The influx of Na + is not offset by the outward movement of K +. Many voltage-gated Na + channels then open, the membrane potential becomes positive, and an action potential occurs. The axon returns to resting potential as voltage-gated Na + channels close and voltage-gated K + channels open.

Figure The Course of an Action Potential

44.2 How Do Neurons Generate and Conduct Signals? Voltage-gated Na + channels have a refractory period during which they cannot open. Na + channels have two gates: An activation gate is closed at rest but opens quickly at threshold. An inactivation gate is open at rest and closes at threshold but responds more slowly. The gate reopens 1–2 milliseconds later than the activation gate closes.

44.2 How Do Neurons Generate and Conduct Signals? Voltage-gated K + channels contribute to the refractory period by remaining open. The efflux of K + ions makes the membrane potential less negative than the resting potential for a brief period. The dip after an action potential is called hyperpolarization or undershoot.

44.2 How Do Neurons Generate and Conduct Signals? An action potential is an all-or-none event because voltage-gated Na + channels have a positive feedback mechanism that ensures the maximum value of the action potential. An action potential is self- regenerating because it spreads to adjacent membrane regions.

Figure Action Potentials Travel along Axons

44.2 How Do Neurons Generate and Conduct Signals? Myelination by glial cells increases the conduction velocity of axons. The nodes of Ranvier are regularly spaced gaps where the axon is not covered by myelin. Action potentials are generated at the nodes and the positive current flows down the inside of the axon.

44.2 How Do Neurons Generate and Conduct Signals? When the positive current reaches the next node, the membrane is depolarized and another axon potential is generated. Action potentials appear to jump from node to node, a form of propagation called saltatory conduction.

Figure Saltatory Action Potentials

Check out 44.1 RECAP, page CHAPTER SUMMARY, page RECAP, page 955, first 2 questions only 44.2 CHAPTER SUMMARY, page 962, See WEB/CD Activity 44.1 Self Quiz page 962: Chapter 44, questions 1, 2, 4, 5 and 6 For Discussion page 963: Chapter 44, question 1 Neurons and Nervous Systems

Key terms: action potential, axon, brain, cell body, central nervous system, depolarised, dendrites, efferent neurons, electrical gradient, ganglion (pl. ganglia), gated channel, glial cell (pl. glia), hyperpolarised, interneuron, membrane potential, myelin, Nernst equation, nerve cell, neuron, neurotransmitters, oligodendrite, postsynaptic, presynaptic, refractory period, resting potential, Schwann cells, sensory neuron, synapase, voltage, voltage-gated channels Neurons and Nervous Systems