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Freberg_3e_PPT_ch03.pdf

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Cells of the Nervous System

Chapter Three

• Glia – Primary supporting cells of the CNS

• Macroglia (astrocytes, oligodendrocytes, Schwann cells)

• Microglia

• Neurons – Primary functioning cells of the CNS – Information processing and communication

Glia and Neurons

• Macroglia – Astrocytes, oligodendrocytes, Schwann cells

• Microglia

Glia Are Classified by Size

Astrocytes

Oligodendrocytes and Schwann Cells

• Phospholipid bilayer; ion channels/pumps

The Neural Membrane

• Microtubules, neurofilaments, and microfilaments

The Cytoskeleton of Neurons – Three Fiber Types

Tau Phosphorylation

• Site of synapses and organelles

The Neural Cell Body (Soma)

• Dendrites receive signals from adjacent neurons – Dendritic spines

• Axons transmit signals – Axon hillock – Myelination – Nodes of Ranvier – Axon terminal

Axons and Dendrites

Axons and Dendrites (cont’d.)

• Unipolar – Single branch extending from the cell body

• Bipolar – Two branches extending from the neural cell

body: one axon and one dendrite – von Economo neurons

• Multipolar – Many branches extending from the cell body;

usually one axon and many dendrites

Structural Variations in Neurons

• Sensory neurons – Specialized to receive information from the

outside world • Motor neurons

– Transmit commands from the CNS directly to muscles and glands

• Interneurons – Act as bridges between the sensory and

motor systems

Functional Variations in Neurons

Structural and Functional Classification of Neurons

Structural and Functional Classification of Neurons (cont’d.)

• An action potential is an electrical signal that begins the process of neural communication

• Ionic composition of the intracellular and extracellular fluids – Differs in the relative concentrations of ions

inside vs. outside the cell – The difference in ion composition between

these fluids provides the neuron with a source of energy for electrical signaling

Generating Action Potentials

• Voltage difference across the resting membrane = 70mV

• Extracellular environment is assigned a value of 0

• Therefore, the resting potential = -70mV

Resting Potential

The Composition of Intracellular and Extracellular Fluids

Measuring the Resting Potential of Neurons

• Diffusion – Molecules move from areas of high

concentration to areas of low concentration (along a concentration gradient)

• Electrostatic pressure – Like-signed ions repel each other – Opposite-signed ions move toward each other

The Generation of the Action Potential: The Movement of Ions

• Resting potential averages -70mV – Resting membrane is permeable to potassium – Some sodium leaks into the cell – Resting potential is maintained by controlling

the movement of potassium ions

Diffusion and Electrostatic Pressure

Diffusion and Electrostatic Pressure (cont’d.)

• Depolarization – Ion movement decreases the membrane

potential toward 0 mV • The membrane potential must reach the threshold

of about -65mV to produce an action potential

– When the threshold is reached, voltage-gated sodium ion channels open to allow sodium to flow into the neuron

– Voltage-gated potassium ion channels open near the peak of the action potential to allow potassium to flow out of cell

The Action Potential – All-or-None

• Once the cell returns to the resting level, it actually hyperpolarizes – Overshoots its target and becomes even

more negative than when at rest • Refractory period

– Membrane potential returns to resting potential

– Absolute versus relative refractory periods • The rate of neural firing varies to reflect

stimulus intensity

The Action Potential – All-or-None (cont’d.)

The Action Potential – The Sequence of Events

• The signal reproduces itself down the length of the axon

• Influenced by myelination – Propagation in unmyelinated axon requires

reproduction of the action potential at each successive axonal segment

– Propagation in myelinated axons requires reproduction of the action potential in the nodes of Ranvier: saltatory conduction

Propagating Action Potentials

Action Potentials Propagate Down the Length of the Axon

Propagation in Unmyelinated and Myelinated Axons

• The action potential is transmitted to the adjacent postsynaptic neuron at the synapse

Neurons Communicate at the Synapse

A Comparison of Electrical and Chemical Synapses

The Electrical Synapse

• Neurotransmitters are released from the presynaptic cell

• Neurotransmitters bind to postsynaptic receptor sites

• The chemical signal is then terminated

The Chemical Synapse

Exocytosis Results in the Release of Neurotransmitters

Ionotropic and Metabotropic Receptors

Methods for Deactivating Neurochemicals

• Small, local, graded potentials • Excitatory (EPSPs) or inhibitory (IPSPs)

Postsynaptic Potentials

Neural Integration Combines Excitatory and Inhibitory Input

• Axo-axonic synapses between an axon terminal and another axon fiber have a modulating effect on the release of neurotransmitter by the target axon

Neuromodulation