Chapter Three

Cells of the Nervous System

CHAPTER 3CELLS OF THE NERVOUS SYSTEM

Neurons and Glia

• The Structure of neurons– Neuron membranes separate intracellular fluid

from extracellular fluid– The neural cytoskeleton provides structural

support that maintains the shape of the neuron

Figure 3.2 The Neural Membrane

Figure 3.3 Three Fiber Types Compose the Cytoskeleton of Neurons

Figure 3.4 Tau Phosphorylation Leads to Cell Death

Neurons and Glia

• Structural Features of Neurons– Cell body (soma) contains nucleus and other

organelles– Dendrites – branches that serve as locations at

which information from other neurons is received– Axons are responsible for carrying neural

messages to other neurons• Vary in diameter and length• Many covered by myelin

Figure 3.5 The Neural Cell Body

Figure 3.6 Axons and Dendrites

Structural Variations in Neurons

• Unipolar– Single branch extending from the cell body

• Bipolar– Two branches extending from the neural cell body: one

axon and one dendrite

• Multipolar– Many branches extending from the cell body; usually one

axon and many dendrites

Figure 3.8 Structural and Functional Classification of Neurons

Functional 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

Glia

• Macroglia: Largest of the glial cells – Astrocytes– Oligodendrocytes– Schwann cells

• Microglia: Smallest of the glial cells

Table 3.1 Types of Glia

Figure 3.9 Astrocytes

Figure 3.10 Oligodendrocytes and Schwann Cells

The Generation of the Action Potential

• Ionic Composition of the Intracellular and Extracellular Fluids– The difference between these fluids provides the neuron

with a source of energy for electrical signaling– Differ from each other in the relative concentrations of

ions they contain

Figure 3.12 The Composition of Intracellular and Extracellular Fluids

Figure 3.13 Measuring the Resting Potential of Neurons

The Generation of the Action Potential

• The Movement of Ions– Diffusion is the tendency for molecules to distribute

themselves equally within a medium– Electrical force is an important cause of movement

• Like electrical charges repel• Opposite electrical charges attract

Figure 3.14 Diffusion and Electrical Force

The Generation of the Action Potential

• The Resting Potential– Membrane allows potassium to cross freely– Measures about -70mV– If potassium levels in extracellular fluid increase, resting

potential is wiped out

The Action Potential

• Threshold– When recording reaches about -65mV

• Channels open & close during action potential– Sodium flows into neuron , potassium flows out around

the peak of the action potential

• Refractory period– Recording returns to resting potential– Absolute versus relative refractory periods

• The action potential is all-or-none

Figure 3.15 The Action Potential

The Propagation of the Action Potential

• Propagation– Signal reproduces itself down the length of the neuron– Influenced by myelination

• Passive conduction = propagation in unmyelinated axon• Saltatory conduction = propagation in myelinated axon

Figure 3.16 Action Potentials Propagate Down the Length of the Axon

Figure 3.17 Propagation in Unmyelinated and Myelinated Axons

The Synapse

• Electrical synapses– Directly stimulate adjacent cells by sending ions across the

gap through channels that actually touch

• Chemical synapses– Stimulate adjacent cells by sending chemical messengers

• Neurotransmitter release• Neurotransmitters bind to postsynaptic receptor sites• Termination of the chemical signal• Postsynaptic potentials• Neural Integration

Table 3.2 A Comparison of Electrical and Chemical Synapses

Figure 3.19 The Electrical Synapse

Figure 3.21 Exocytosis Results in the Release of Neurotransmitters

Figure 3.22 Ionotropic and Metabotropic Receptors

Figure 3.23 Methods for Deactivating Neurotransmitters

Figure 3.24 Neural Integration Combines Excitatory and Inhibitory Input

Table 3.3 A Comparison of the Characteristics of Action Potentials, EPSPs and IPSPs

Neuromodulation

• Synapses between an axon terminal and another axon fiber– Axo-axonic synapses have modulating effect on the

release of neurotransmitter by the target axon• Presynaptic facilitation• Presynaptic inhibition

Figure 3.26 Synapses Between Two Axons Modulate the Amount of Neurotransmitter

Released