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

: Central: Brain Spinal Cord Peripheral: Nerves Receptors Ganglia

Physiologically

: Afferent (Sensory) receptors Efferent (Motor) somatic autonomic Sympathetic Parasympathetic

Organization of the Nervous System Figure 8-1

The Neuron

Model Neuron

• Dendrites receive incoming signals; axons carry outgoing information

Dendrites Cell body Synapse Presynaptic axon terminal Synaptic cleft Postsynaptic dendrite Input signal Nucleus Axon hillock Axon (initial segment) Myelin sheath Integration Output signal Postsynaptic neuron Figure 8-2

Anatomic and Functional Categories of Neurons

• Neurons can be classified according to function or structure

Sensory neurons Somatic senses Neurons for smell and vision Dendrites

Neurons can be categorized by the number of processes and function

Schwann cell Axon Pseudounipolar (a) (b) Bipolar Figure 8-3a-b

Anatomic and Functional Categories of Neurons Interneurons of CNS Axon Dendrites (c) Anaxonic (d) Axon Multipolar 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

are found in

• Glial cells provide physical and biochemical support for neurons.

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 GLIAL CELLS

are found in

Central nervous system

contains

Oligodendrocytes

forms

Myelin sheaths Microglia (modified immune cells)

act as

Scavengers Astrocytes Ependymal cells

provide help form secrete take up create

Substrates for ATP production Blood brain barrier (b) Glial cells and their functions Neurotrophic factors K + , water, neurotransmitters Source of neural stem cells Barriers between compartments 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 Myelin (cut) Axon Node Oligodendrocyte (a) Glial cells of the central nervous system Microglia Capillary Astrocyte Section of spinal cord 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 4 5 6 1 2 3 4 5 6 7 8 9 Resting membrane potential Depolarizing stimulus Membrane depolarizes to threshold.

Voltage-gated Na + and Na + channels open quickly enters cell. Voltage-gated K + channels begin to open slowly.

Rapid Na + entry depolarizes cell.

Na + K + channels close and slower channels open.

K + moves from cell to extracellular fluid.

K + channels remain open and additional K + leaves cell, hyperpolarizing it.

Voltage-gated K + less K + channels close, leaks out of the cell.

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 Inactivation gate 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 Na + and K + channels Both channels closed Na + channels open Na + Na + channels close and K + channels open K + K + Absolute refractory period Na + channels reset to original position while K + channels remain open K + Relative refractory period Both channels closed Action potential Na + K + High High Zero Time (msec) Increasing 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 + open and Na + channels 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 + open and Na + channels 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 + open and Na + channels 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 + open and Na + channels 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 + open and Na + channels 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

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Figure 8-19d