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

About this Chapter How the nervous system is organized Nerve cell types and roles Excitability and electrical signals Graded and action potentials initiation and conduction Neurotransmitters and signal conduction cell to cell Modulation and integration of the signals Damage and diseases of the nerves

Organization of the Nervous System Rapid communication for homeostatic balance Emergent properties of intelligence & emotion Central Nervous system (CNS) Peripheral Nervous system (PNS)

Organization of the Nervous System Figure 8-1: Organization of the nervous system

A Typical Neuron Overview Dendrites Cell Body Axon Terminal Figure 8-2: Model neuron

Diverse Neuron Forms and Functions Pseudounipolar Bipolar Anaxionic Multipolar–CNS Multipolar–efferent

Diverse Neuron Forms and Functions Figure 8-3: Anatomic and functional categories of neurons

Metabolism and Synthesis in a Neuron Cell body site of energy generation and synthesis Axonal transport Vesicles – Fast axonal transport to terminal Retrograde to cell body Electrical depolarizations

Metabolism and Synthesis in a Neuron Figure 8-4: Axonal transport of membranous organelles

Glial Cell Functions Support neuron bodies, form myelin sheaths Barriers between compartments Scavenger/defense & metabolic assistance

Glial Cell Functions Figure 8-5: Glial cells and their functions

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Electrical Signals: Ionic Concentrations and Potentials Nernst & GHK Equations predict Membrane potential Cell concentration gradients [Na+, Cl- & Ca2+] higher in ECF [K+] higher ICF Depolarization causes electrical signal Gated channels control permeability

Electrical Signals: Ionic Concentrations and Potentials Table 8-2: Ion Concentrations and Equilibrium Potentials

Graded Potentials Incoming signals Vary in strength Lose strength over distance Are slower than action potentials (AP) Travels to trigger zone Subthreshold – Too weak No generation of AP Suprathreshold – generate AP

Graded Potentials Figure 8-7: Graded potentials decrease in strength as they spread out from the point of origin

Trigger Zone: Cell Integration and Initiation of AP Excitatory signal: depolarizes, reduces threshold Inhibitory signal: hyperpolarizes, increases threshold

Trigger Zone: Cell Integration and Initiation of AP Figure 8-8a: Subthreshold and suprathreshold graded potentials in a neuron

Trigger Zone: Cell Integration and Initiation of AP Figure 8-8b: Subthreshold and suprathreshold graded potentials in a neuron

Action Potential Stages: Overview "All or none" Signal does not diminish over distance

Action Potential Stages: Overview Figure 8-9: The action potential

Membrane & Channel Changes during an Action Potential Initiation Depolarization Signal peak Repolarization

Membrane & Channel Changes during an Action Potential Figure 8-10: Model of the voltage-gated channel Na+

Regulating the AP Positive feedback loop Absolute refractory period Relative refractory period

Figure 8-11: Ion movements during the action potential Regulating the AP Figure 8-11: Ion movements during the action potential

Regulating the AP Figure 8-12: Refractory periods

Frequency of Action Potentials Firing rate "Wave" of APs Proportional neurotransmitter (NT) release Stronger GP initiates more APs & more NT

Frequency of Action Potentials Figure 8-13: Coding for stimulus intensity

Conduction of Action Potentials Kinetic energy Depolarizes ahead Drives AP to terminal

Conduction of Action Potentials Figure 8-14a: Conduction of action potentials

Conduction of Action Potentials Figure 8-14b: Conduction of action potentials

Conduction of Action Potentials Figure 8-14c: Conduction of action potentials

Speed of Conduction Larger diameter faster conduction Myelinated axon faster conduction Saltatory conduction Disease damage to myelin Chemicals that block channels Alteration of ECF ion concentrations

Speed of Conduction Figure 8-16b: Axon diameter and speed of conduction

Speed of Conduction Figure 8-17: Saltatory conduction

Cell to Cell Conduction: the Synapse Electrical synapses: gap junctions Very fast conduction Example: cardiac muscle Chemical synapses Pre synaptic terminal Synthesis of Neurotransmitters Ca2+ releases Neurotransmitters Synaptic cleft Postsynaptic cell: Neurotransmitter receptors

Cell to Cell Conduction: the Synapse Figure 8-19: A chemical synapse

Synapse Mechanism Figure 8-20: Events at the synapse

Acetylcholine synthesis Figure 8-21: Synthesis and recycling of acetylcholine at the synapse

Neurocrines Neurotransmitters Neuromodulators Neurohormones

Neurocrines Table 8-4-1: Major Neurocrines

Neurocrines Table 8-4-2: Major Neurocrines

Multiple Receptors modify signal Amplification – depolarization Inhibition – hyperpolarization Duration Fast – channel opening Slow – protein synthesis

Multiple Receptors modify signal Figure 8-22: Fast and slow responses in postsynaptic cells

Inactivation of Neurotransmitters Recycled Enzyme degradation Diffuse away

Inactivation of Neurotransmitters Figure 8-23: Inactivation of neurotransmitters

Integration of Signals Information transfer at each exchange Signal can be lost Signal can be enhanced Divergence – one cell to many Convergence – many cells to one

Integration of Signals Figure 8-24a: Convergence and divergence

Integration of Signals Figure 8-24b: Convergence and divergence

Integration of Signals Figure 8-25: Locations of synapses on a postsynaptic neuron

Convergent Integration: Additive Summation Multiple excitatory GPs Temporal summation Additive strength at trigger zone

Convergent Integration: Additive Summation Figure 8-26a: Spatial summation

Convergent Integration: Inhibitory Summation Inhibitory GPs cancel strength of excitatory GP Signal at trigger too weak – no AP produced Figure 8-26b: Spatial summation

Nervous Tissue Development 100 billion neurons find their target Growth cones Follow growth factors, structural proteins Neurotropic factors – sustain new synapse "Use it or loose it"

Pathologies Synaptic transmission Drugs in ECF Disorders of ion balance Too much/too little NT release Examples: Parkinson's, schizophrenia, epilepsy, depression Nerve injury Limited regrowth Parallel nerves help some

Figure 8-31: Injury to neurons Pathologies Figure 8-31: Injury to neurons

Summary Organization and role of the nervous system: CNS & components of PNS Neuron and glial cell structure and function Electrical signals from waves of depolarization Graded potentials function and mechanism Action potentials function and mechanism

Summary Synapse: neurotransmitters, cell to cell communication Conduction, integration and modulation of the signals Development and pathologies of the nervous system