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:

BrainSpinal Cord

Peripheral:NervesReceptorsGanglia

Physiologically:Afferent (Sensory)

receptorsEfferent (Motor)

somaticautonomic

SympatheticParasympathetic

Figure 8-1

Organization of the Nervous System

The Neuron

Figure 8-2

Model Neuron

• Dendrites receive incoming signals; axons carry outgoing information

Dendrites

Cellbody

Nucleus

Axon hillock

Axon (initialsegment)

Myelinsheath

Synapse

Presynapticaxon terminal

Postsynapticneuron

Synapticcleft

Postsynapticdendrite

Integration

Outputsignal

Inputsignal

Figure 8-3a-b

Anatomic and Functional Categories of Neurons

• Neurons can be classified according to function or structure

Dendrites

Axon

Schwanncell

Somatic senses

Pseudounipolar Bipolar

Sensory neuronsNeurons for

smell and vision

(a) (b)

Neurons can be categorized by the number of processes and function

Figure 8-3c-d

Anatomic and Functional Categories of Neurons

Dendrites

Axon

Axon

Anaxonic Multipolar

Interneurons of CNS

(c) (d)

Anatomic and Functional Categories of Neurons

Figure 8-3e

Dendrites

Axon

Multipolar

Efferent neuron

(e)

Axonterminal

Figure 8-5b (1 of 2)

Cells of NS: Glial Cells and Their Functions

• Glial cells provide physical and biochemical support for neurons.

GLIAL CELLS

are found in

contains

forms

Peripheral nervous system

Myelin sheaths

Schwann cells

Neurotrophicfactors

Satellitecells

Supportcell bodies

secrete

(b) Glial cells and their functions

Cells of NS: Glial Cells and Their Functions

Figure 8-5b (2 of 2)

GLIAL CELLS

forms

contains

are found in

Central nervous system

AstrocytesOligodendrocytes Microglia (modifiedimmune cells)

provide help form secrete take up create

Scavengers

K+, water,neurotransmitters

Neurotrophicfactors

Blood-brain

barrierSubstrates for

ATP production

Ependymalcells

Barriersbetween

compartments

Source ofneural

stem cells

act as

Myelin sheaths

(b) Glial cells and their functions

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

Figure 8-5a

Interneurons

Astrocyte

Microglia

Oligodendrocyte

Capillary

Myelin(cut)

AxonNode

Section of spinal cord

(a) Glial cells of the central nervous system

Ependymalcell

Cells of NS: Schwann Cells

• Sites and formation of myelin

Figure 8-6a

Schwann cell nucleusis pushed to outside

of myelin sheath.

Nucleus

Axon

Myelin consistsof multiple layersof cell membrane.

(a) Myelin formation in theperipheral nervous system

Schwann cell wraps aroundthe axon many times.

Cells of NS: Schwann Cells

Figure 8-6b

Cell body

1–1.5 mm

Node of Ranvier is a section ofunmyelinated axon membranebetween two Schwann cells.

Schwann cell nucleusis pushed to outside

of myelin sheath.

AxonMyelin consists

of multiple layersof cell membrane. (b) Each Schwann cell forms myelin around

a small segment of one axon.

Multiple SclerosisNystagmus - 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

Figure 8-9 (1 of 2)

4

5

6

1 2

3

7 8 9

23

45

6

7

8

9

Threshold

Resting membrane potential

Depolarizing stimulus

Membrane depolarizes to threshold.Voltage-gated Na+ channels open quicklyand Na+ enters cell. Voltage-gatedK+ channels begin to open slowly.

Rapid Na+ entry depolarizes cell.

Na+ channels close and slowerK+ channels open.

K+ moves from cell to extracellularfluid.

K+ channels remain open andadditional K+ leaves cell, hyperpolarizing it.

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

Cell returns to resting ion permeabilityand resting membrane potential.

1

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

Figure 8-10a

ECF

ICFActivation

gateInactivationgate

Na+

(a) At the resting membrane potential, the activation gatecloses the channel.

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

Figure 8-12

Action potential

K+

K+ K+ K+

Absolute refractory period Relative refractory period

Na+

Na+

Na+ and K+

channels

Na+ channels close andK+ channels open

Na+ channels reset to original positionwhile K+ channels remain open

Na+

channelsopen

Bothchannels

closed

Bothchannels

closed

High

Zero

Increasing

High

Ion

perm

eabi

lity

Mem

bran

e po

tent

ial (

mV)

Exci

tabi

lity

Time (msec)

Electrical Signals: Coding for Stimulus Intensity

Figure 8-13a

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

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

Figure 8-15

1

Local current flow from the active region causes new sectionsof the membrane to depolarize.

Refractoryregion

Active region Inactive region

Trigger zone

Axon

Positive charge flows into adjacent sections of the axon by local current flow.

Voltage-gated Na+ channelsopen and Na+ enters the axon.

The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.

A graded potential above threshold reaches thetrigger zone.

2

3

4

5

Electrical Signals: Conduction of Action Potentials

Figure 8-15, step 1

1

Trigger zone

AxonA graded potential above threshold reaches thetrigger zone.

Electrical Signals: Conduction of Action Potentials

Figure 8-15, steps 1–2

1

Trigger zone

Axon

Voltage-gated Na+ channelsopen and Na+ enters the axon.

A graded potential above threshold reaches thetrigger zone.

2

Electrical Signals: Conduction of Action Potentials

Figure 8-15, steps 1–3

1

Trigger zone

Axon

Positive charge flows into adjacent sections of the axon by local current flow.

Voltage-gated Na+ channelsopen and Na+ enters the axon.

A graded potential above threshold reaches thetrigger zone.

2

3

Electrical Signals: Conduction of Action Potentials

Figure 8-15, steps 1–4

1

Local current flow from the active region causes new sectionsof the membrane to depolarize.

Refractoryregion

Active region Inactive region

Trigger zone

Axon

Positive charge flows into adjacent sections of the axon by local current flow.

Voltage-gated Na+ channelsopen and Na+ enters the axon.

A graded potential above threshold reaches thetrigger zone.

2

3

4

Electrical Signals: Conduction of Action Potentials

Figure 8-15, steps 1–5

1

Local current flow from the active region causes new sectionsof the membrane to depolarize.

Refractoryregion

Active region Inactive region

Trigger zone

Axon

Positive charge flows into adjacent sections of the axon by local current flow.

Voltage-gated Na+ channelsopen and Na+ enters the axon.

The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.

A graded potential above threshold reaches thetrigger zone.

2

3

4

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

Figure 8-19d

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