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Chapter 8a Neurons: Cellular and Network Properties

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• • • •

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