Chapter 40 Neural Signaling

1. Chapter 40 Neural Signaling Communication

2. Nervous System General Scheme, fig 40-1 Stimulus excites a receptor (sensor) � Name some stimuli. Receptor sends info via sensory neuron (nerve cell) to an� Integration center (CNS) that processes info (interneurons) and sends instructions via a� Motor neuron to an effector (skeletal muscle, etc.) 2

4. Nervous System General Scheme cont�d The sensory or input pathway is also called the afferent pathway. The motor or output pathway is also called the efferent pathway. 4

5. Nervous System Neurons = nerve cells Soma or cell body with a large nucleus Dendrites � Axon � Axon hillock � spot where axon extends from soma. 5

7. Nervous System Neuroglia = helper cells that nourish and protect neurons. Examples: Microglia � Astrocytes � **Schwann cells (PNS) and oligodendrocytes (CNS) form myelin sheaths = insulation. 7

8. How Do Cells Generate Electrical Signals? Review: Atoms that lose electrons become positive ions Atoms that gain electrons become negative ions. Ions of opposite charges are attracted to each other. To separate them requires energy. Charge separation represents potential energy � like your car battery. 8

9. How Do Cells Generate Electrical Signals? Membrane potentials (electrical) In cells, different concentrations of ions on each side of a membrane produces an electrical potential or voltage. Ions must move through channels � 3 types: Ligand gated � Voltage gated � �Leak channels� � 9

10. How Do Cells Generate Electrical Signals? Resting Membrane Potential, fig 40-4 Inside the cell are proteins with negative charges and a high concentration of K+ but a low concentration of Na+. Outside the cell there is a high concentration of Na+ and low concentration of K+. 10

11. Resting Membrane Potential (RMP) K+ channels allow K to �leak� out of the cell down its concentration gradient. This causes the inside of the cell to have fewer positive ions than the outside. Also negatively charged proteins are trapped inside making the inside of the cell electrically negative relative to the outside. (RMP = -50 to -90 mV) This RMP is maintained by Na+/K+ ATPase. What does Na+/K+ ATPase do again? 11

14. How Do Cells Generate Electrical Signals? Because the charges on either side of the plasma membrane are different, cell membranes are said to be polarized. When the charge difference changes� and the potential decreases or becomes less negative, the membrane is said to be depolarized. and the potential increases or becomes more negative, the membrane is said to be hyperpolarized. 14

15. How Do Cells Generate Electrical Signals? Action Potential = rapid, transient depolarization, also called a nerve impulse. This occurs only at the axon hillock of a nerve cell. Sequence, fig 40-7. A stimulus causes a small depolarization by allowing Na+ to enter the cell through Na channels. If the depolarization reaches a certain voltage (=threshold potential), voltage-gated Na channels open. Na rushes into the cell causing a large, transient depolarization. 15

16. How Do Cells Generate Electrical Signals? Action Potential sequence cont�d At the peak of the large depolarization, the voltage gated Na+ channels close and voltage gated K+ channels open. K+ moves out of the cell and the membrane potential returns toward resting membrane potential = repolarization. K+ channels are slow to close and membrane potential falls below resting levels = hyperpolarization or undershoot. 16

17. 17

19. �Real Action Potential� Recording 19

20. How Do Cells Generate Electrical Signals? Membrane potential returns to resting levels. TOTAL TIME: 1 - 100 msec Refractory period � 20

21. How are Action Potentials Propagated? Action potentials take place at a discreet spot on the cell membrane. Neighboring areas have a different voltage and an electrical current to flow between the 2 spots � like a battery. Adjacent areas depolarize, voltage gated Na+ channels open and � the action potential sequence is repeated, fig 40.8. 21

23. How are Action Potentials Propagated? The process continues until the action potential moves over the entire cell. In large neurons with myelin sheaths, the action potential is regenerated at each Node of Ranvier, fig 40-9. Cause action potential to �jump� from node to node down the axon and move faster = saltatory conduction. Maintains strength of action potential. 23

25. How are Electrical Impulses Transmitted Between Cells? Connection = synapse Fluid filled gap or space between 2 neurons or between a neuron and another cell = synaptic cleft, fig 40-10. The neuron sending the impulse is the presynaptic cell, the cell receiving the impulse is the postsynaptic cell. The �message� is sent across the synapse via chemicals called neurotransmitters. 25

26. How are Electrical Impulses Transmitted Between Cells? As the action potential reaches the end of the presynaptic axon, Ca2+ enters the cell. Ca2+ causes the release of neurotransmitters into the cleft. Neurotransmitters cross the cleft and bind to receptors on the postsynaptic cell. 26

29. How are Electrical Impulses Transmitted Between Cells? The receptors on the postsynaptic cell are associated with ion channels or enzymes. When neurotransmitter binds, the channels open/close or the enzyme pathway is activated. Common Neurotransmitters, Table 40-2. Some are excitatory, some inhibitory, and some are both??? 29

30. Synaptic Integration Stimuli and neurotransmitters don�t always cause Action Potentials, fig 40-11. Some cause only small depolarizations = Some cause small hyperpolarizations = Summation = effect of all EPSPs and IPSPs on the membrane potential. i.e. can add to produce an action potential or NOT. 30