Neurons: Cellular and Network Properties Chapter 8a Neurons: Cellular and Network Properties

Organization of Nervous System Cells of the nervous system 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

Physiologically: Anatomically: Afferent (Sensory) receptors Efferent (Motor) somatic autonomic Sympathetic Parasympathetic Anatomically: Central: Brain Spinal Cord Peripheral: Nerves Receptors Ganglia

Organization of the Nervous System Figure 8-1

The Neuron

Dendrites receive incoming signals; axons carry outgoing information 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 Output signal Synaptic cleft Synapse Postsynaptic dendrite Postsynaptic neuron Figure 8-2

Anatomic and Functional Categories of Neurons Sensory neurons Somatic senses Neurons for smell and vision Neurons can be classified according to function or structure Dendrites Neurons can be categorized by the number of processes and function Schwann cell Axon Pseudounipolar Bipolar (a) (b) Figure 8-3a-b

Anatomic and Functional Categories of Neurons Interneurons of CNS Axon Dendrites Axon Anaxonic Multipolar (c) (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 provide physical and biochemical support for neurons. are found in Peripheral nervous system contains Satellite cells Schwann cells forms Myelin sheaths secrete Support cell bodies Neurotrophic factors (b) Glial cells and their functions Figure 8-5b (1 of 2)

Cells of NS: Glial Cells and Their Functions are found in Central nervous system contains Oligodendrocytes Microglia (modified immune cells) Ependymal cells Astrocytes forms act as Myelin sheaths Scavengers provide help form secrete take up create Blood- brain barrier Source of neural stem cells Barriers between compartments Substrates for ATP production Neurotrophic factors K+, water, neurotransmitters (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 Schwann cell nucleus is pushed to outside of myelin sheath. Nucleus Axon Myelin consists of multiple layers of cell membrane. (a) Myelin formation in the peripheral nervous system Schwann cell wraps around the axon many times. Sites and formation of myelin 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. Axon Myelin consists of multiple layers of cell membrane. (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 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. 5 6 K+ moves from cell to extracellular fluid. 7 K+ channels remain open and additional K+ leaves cell, hyperpolarizing it. 6 8 4 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 9 8 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 channels closed Na+ channels open Both channels closed Na+ channels close and K+ channels open Na+ channels reset to original position while K+ channels remain open Na+ Na+ and K+ channels K+ K+ K+ Absolute refractory period Relative refractory period Action potential Na+ Membrane potential (mV) Ion permeability K+ High High Excitability 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