Chapter 8a Neurons: Cellular and Network Properties About this Chapter • • • • Organization of Nervous System Cells of the nervous system Electrical signals in neurons Cell-to-cell communication in the nervous system • Integration of neural information transfer Anatomically: Physiologically: Central: Afferent (Sensory) Brain Spinal Cord Peripheral: Nerves Receptors Ganglia receptors Efferent (Motor) somatic autonomic Sympathetic Parasympathetic Organization of the Nervous System Figure 8-1 The Neuron Model Neuron Input signal Dendrites • Dendrites receive incoming signals; axons carry outgoing information Integration Cell body Nucleus Axon hillock Axon (initial segment) Myelin sheath Presynaptic axon terminal Synapse Synaptic cleft Postsynaptic dendrite Output signal Postsynaptic neuron Figure 8-2 Anatomic and Functional Categories of Neurons Sensory neurons Somatic senses • Neurons can be classified according to function or structure Neurons for smell and vision Dendrites Neurons can be categorized by the number of processes and function Schwann cell Axon Pseudounipolar (a) Bipolar (b) Figure 8-3a-b Anatomic and Functional Categories of Neurons Interneurons of CNS Axon Dendrites Axon Anaxonic (c) Multipolar (d) Figure 8-3c-d Anatomic and Functional Categories of Neurons Efferent neuron Dendrites Axon Axon terminal Multipolar (e) Figure 8-3e Cells of NS: Glial Cells and Their Functions GLIAL CELLS • Glial cells provide physical and biochemical support for neurons. are found in Peripheral nervous system contains Satellite cells Schwann cells forms Myelin sheaths secrete Support Neurotrophic cell bodies factors (b) Glial cells and their functions Figure 8-5b (1 of 2) Cells of NS: Glial Cells and Their Functions GLIAL CELLS are found in Central nervous system contains Oligodendrocytes forms Microglia (modified immune cells) Astrocytes Ependymal cells act as Myelin sheaths Scavengers provide Substrates for ATP production help form Bloodbrain barrier secrete create take up K+, Neurotrophic water, factors neurotransmitters Source of neural stem cells Barriers between compartments (b) Glial cells and their functions Figure 8-5b (2 of 2) Amyotrophic Lateral sclerosis (ALS • ALS has been linked to a mutation on the gene coding for superoxide dismutase. • Microglia use reactive oxygen species (superoxides) to destroy, may lead to oxidative stress and neurodegeneration • A-myo-trophic comes from the Greek language. "A" means no or negative. "Myo" refers to muscle, and "Trophic" means nourishment–"No muscle nourishment." When a muscle has no nourishment, it "atrophies" or wastes away. Cells of NS: Glial Cells and Their Functions Ependymal cell Interneurons Microglia Capillary Astrocyte Myelin (cut) Axon Section of spinal cord Node Oligodendrocyte (a) Glial cells of the central nervous system Figure 8-5a Cells of NS: Schwann Cells • Sites and formation of myelin Nucleus Schwann cell wraps around the axon many times. Axon Schwann cell nucleus is pushed to outside of myelin sheath. Myelin consists of multiple layers of cell membrane. (a) Myelin formation in the peripheral nervous system Figure 8-6a Cells of NS: Schwann Cells Cell body 1–1.5 mm Node of Ranvier is a section of unmyelinated axon membrane between two Schwann cells. Schwann cell nucleus is pushed to outside of myelin sheath. Myelin consists of multiple layers of cell membrane. Axon (b) Each Schwann cell forms myelin around a small segment of one axon. Figure 8-6b Multiple Sclerosis Nystagmus - involuntary eye movement Electrical Signals: Nernst Equation • Describes the membrane potential that a single ion would produce if the membrane were permeable to only that ion • Membrane potential is influenced by • Concentration gradient of ions • Membrane permeability to those ions Electrical Signals: GHK Equation • Predicts membrane potential that results from the contribution of all ions that can cross the membrane Electrical Signals: Ion Movement • Resting membrane potential determined primarily by • K+ concentration gradient leak channels open • Cell’s resting permeability to K+, Na+, and Cl– • Gated channels control ion permeability • Mechanically gated • Pressure or stretch • Chemical gated • Ligands, NTs • Voltage gated • Membrane potential change • Threshold voltage varies from one channel type to another (minimum to open or close) Electrical Signals: Channel Permeability Table 8-3 Electrical Signals: Graded Potentials • Graded potentials decrease in strength as they spread out from the point of origin Figure 8-7 Electrical Signals: Graded Potentials • Subthreshold and (supra)threshold graded potentials in a neuron Figure 8-8a Electrical Signals: Graded Potentials Figure 8-8b Electrical Signals: Action Potentials 5 6 4 1 Resting membrane potential 2 Depolarizing stimulus 3 Membrane depolarizes to threshold. Voltage-gated Na+ channels open quickly and Na+ enters cell. Voltage-gated K+ channels begin to open slowly. 4 Rapid Na+ entry depolarizes cell. 5 Na+ channels close and slower K+ channels open. 6 K+ moves from cell to extracellular fluid. 7 K+ channels remain open and additional K+ leaves cell, hyperpolarizing it. 8 Voltage-gated K+ channels close, less K+ leaks out of the cell. 9 Cell returns to resting ion permeability and resting membrane potential. Threshold 3 1 2 7 8 9 Figure 8-9 (1 of 2) Electrical Signals: Action Potentials Figure 8-9 (2 of 2) Electrical Signals: Voltage-Gated Na+ Channels • Na+ channels have two gates: activation and inactivation gates Na+ ECF ICF Activation gate Inactivation gate (a) At the resting membrane potential, the activation gate closes the channel. Figure 8-10a Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10b Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10c Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10d Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10e Electrical Signals: Ion Movement During an Action Potential Figure 8-11 Electrical Signals: Refractory Periods Both Na+ channels channels closed open Na+ channels close and K+ channels open Na+ channels reset to original position while K+ channels remain open Na+ Na+ Both channels closed K+ and channels K+ K+ K+ Absolute refractory period Relative refractory period Ion permeability Membrane potential (mV) Action potential Na+ Excitability K+ High High Increasing Zero Time (msec) Figure 8-12 Electrical Signals: Coding for Stimulus Intensity Na+ and K+ [ ]’s change very little •1 in 100000 K+ leave to shift from +30 to 70mVolts • Na/K pump will re-establish, but neuron without pump can still 1000x Figure 8-13a Electrical Signals: Coding for Stimulus Intensity Figure 8-13b Electrical Signals: Trigger Zone • Graded potential enters trigger zone • Voltage-gated Na+ channels open and Na+ enters axon • Positive charge spreads along adjacent sections of axon by local current flow • Local current flow causes new section of the membrane to depolarize • The refractory period prevents backward conduction; loss of K+ repolarizes the membrane Electrical Signals: Trigger Zone Figure 8-14 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. 5 The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane. Refractory region Active region Inactive region Figure 8-15 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon Figure 8-15, step 1 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. Figure 8-15, steps 1–2 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. Figure 8-15, steps 1–3 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. Refractory region Active region Inactive region Figure 8-15, steps 1–4 Electrical Signals: Conduction of Action Potentials Trigger zone 1 A graded potential above threshold reaches the trigger zone. Axon 2 Voltage-gated Na+ channels open and Na+ enters the axon. 3 Positive charge flows into adjacent sections of the axon by local current flow. 4 Local current flow from the active region causes new sections of the membrane to depolarize. 5 The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane. Refractory region Active region Inactive region Figure 8-15, steps 1–5 Electrical Signals: Action Potentials Along an Axon Figure 8-16b Electrical Signals: Speed of Action Potential • Speed of action potential in neuron influenced by • Diameter of axon • Larger axons are faster • Resistance of axon membrane to ion leakage out of the cell • Myelinated axons are faster Electrical Signals: Myelinated Axons • Saltatory conduction Figure 8-18a Electrical Signals: Myelinated Axons Figure 8-18b Electrical Signals: Chemical Factors • Effect of extracellular potassium concentration of the excitability of neurons Figure 8-19a Electrical Signals: Chemical Factors Figure 8-19b Electrical Signals: Chemical Factors Figure 8-19c Electrical Signals: Chemical Factors video video2 Figure 8-19d