Discussion 2

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Physiology of Behavior

Twelfth Edition

Chapter 2

Structure and Functions of Cells of the Nervous System

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Chapter Outline

Cells of the Nervous System

Communication Within a Neuron

Communication Between Neurons

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Learning Objectives (1 of 4)

2.1 Contrast the location of the central and peripheral nervous systems.

2.2 Describe the structures of a neuron, including their general function.

2.3 Differentiate functions of supporting cells of the central and peripheral nervous systems.

2.4 Discuss the features and importance of the blood–brain barrier.

2.5 Compare neural communication in a withdrawal reflex with and without inhibition of the reflex.

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Learning Objectives (2 of 4)

2.6 Contrast the changes in electrical potential within a neuron when it is experiencing resting potential, hyperpolarization, depolarization, and an action potential.

2.7 Summarize the contributions of diffusion, electrostatic pressure, and the sodium–potassium pump to establishing membrane potential.

2.8 Summarize the series of ion movements during the action potential.

2.9 Describe the propagation of an action potential.

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Learning Objectives (3 of 4)

2.10 Describe the structures and functions of presynaptic cells that are involved in synaptic communication.

2.11 Describe the process of neurotransmitter release.

2.12 Contrast ionotropic and metabotropic receptors.

2.13 Compare the functions of EPSPs and IPSPs in postsynaptic cells.

2.14 Explain the roles of reuptake and enzymatic deactivation in terminating postsynaptic potentials.

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Learning Objectives (4 of 4)

2.15 Summarize the process of neural integration of EPSPs and IPSPs.

2.16 Differentiate between the locations and functions of autoreceptors and postsynaptic receptors.

2.17 Identify synapses other than those involved in neural integration.

2.18 Describe examples of nonsynaptic communication.

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Basic Divisions of the Nervous System

Central Nervous System (CNS)

Brain and spinal cord

Peripheral Nervous System (PNS)

Part of nervous system outside brain and spinal cord

Includes nerves attached to brain and spinal cord

Central Nervous System (CNS)

the brain and spinal cord

Peripheral Nervous System (PNS)

the part of the nervous system outside the brain and spinal cord, including the nerves attached to the brain and spinal cord

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Figure 2.1 The Central and Peripheral Nervous Systems

The central nervous system includes the brain and spinal cord. The peripheral nervous system includes all of the nerves that relay information between the central nervous system and the rest of the body.

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Figure 2.2 Sensory, Motor, and Interneurons

These three types of neurons relay information between the central and peripheral nervous systems. In this example, the person sees the glass of water and sensory nerves relay the sensory information toward the central nervous system. The motor output from the central nervous system allows the person to lift the glass to take a drink.

Sensory Neuron

Detects changes in external or internal environment and sends information about these changes to central nervous system

Motor Neuron

Located within central nervous system that controls contraction of a muscle or secretion of a gland

Interneuron

Located entirely within the central nervous system

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Figure 2.3 Parts of a Neuron

Figure 2.3, page 25

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Cells of the Nervous System

Basic Structure

Soma

Dendrite

Synapse

Axon

Terminal Buttons

Soma

the cell body of a neuron, which contains the nucleus

Dendrite

a branched, treelike structure attached to the soma of a neuron; receives information from the terminal buttons of other neurons

Synapse

a junction between the terminal button of an axon and the membrane of another neuron

Axon

the long, thin, cylindrical structure that conveys information from the soma of a neuron to its terminal buttons

Often covered in myelin (later)

Terminal Button

the bud at the end of a branch of an axon; forms synapses with another neuron; sends information to that neuron

Secrete neurotransmitter

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Figure 2.4 Overview of Structure and Synaptic Connections Between Neurons

The arrows represent the directions of the flow of information.

Figure 2.4 page 26

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Axoplasmic Transport

Neurotransmitter

Axoplasmic transport:

Propels substances within the axon

Anterograde: movement from soma to button

Retrograde: movement from button to soma

Neurotransmitter:

Released at button

Many types

Excites or inhibits receiving neuron

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Figure 2.6 Internal Structures of a Neuron

Fig 2.6 page 27

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Figure 2.7 Protein Synthesis

When a gene is active, a copy of the information is made onto a molecule of messenger RNA. The mRNA leaves the nucleus and attaches to a ribosome, where the protein is produced.

Figure 2.7 page 28

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Supporting Cells

Central Nervous System: Glia

Astrocytes (Figure 2.8, next slide)

Phagocytosis

Oligodendrocytes (Figure 2.9, upcoming)

Myelin Sheath

Microglia

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Figure 2.8 Structure and Location of Astrocytes

The processes of astrocytes surround capillaries and neurons of the central nervous system.

Figure 2.8 page 30

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Figure 2.9 Oligodendrocyte

An oligodendrocyte forms the myelin that surrounds many axons in the central nervous system. Each cell forms segments of myelin for several adjacent axons.

Figure 2.9 page 30

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Figure 2.10 Formation of Myelin

In the peripheral nervous system, an entire Schwann cell tightly wraps itself many times around an individual axon and forms one segment of the myelin sheath.

Peripheral Nervous System: Schwann cells; Myelin Sheath and Multiple sclerosis

Figure 2.10, page 31

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Blood–Brain Barrier

Semi-permeable barrier between the CNS and circulatory system helps to regulate the flow of nutrient rich fluid into the brain area Postrema

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Figure 2.11 The Blood–Brain Barrier

This figure shows that (a) the cells that form the walls of the capillaries in the body outside the brain have gaps that permit the free passage of substances into and out of the blood, and (b) the cells that form the walls of the capillaries in the brain are tightly joined.

The Blood-Brain Barrier

A semipermeable barrier between the CNS and circulatory system, which helps to regulate the flow of nutrient rich fluid into the brain

Area Postrema

A region of the medulla where the blood-brain barrier is weak. This allows toxins in the blood to stimulate this area, which initiates vomiting

Figure 2.11 page 32

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Cells of the Nervous System: Neurons

Types of Neurons

Multipolar Neuron

Bipolar Neuron

Unipolar Neuron

Multipolar Neuron – neuron with one axon and many dendrites attached to its soma.

Bipolar Neuron – neuron with one axon and one dendrite attached it its soma.

Unipolar Neuron – neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the central nervous system.

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Communication within a Neuron: An Overview

In the next two figures (and in subsequent figures that illustrate simple neural circuits) multipolar neurons are depicted in shorthand fashion as several-sided stars.

The points of these stars represent dendrites, and only one or two terminal buttons are shown at the end of the axon.

The sensory neuron in this example detects painful stimuli.

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Figure 2.12 A Withdrawal Reflex

The figure shows a simple example of a useful function of the nervous system. The painful stimulus causes the hand to pull away from the hot iron. Components of a Simple Withdrawal Reflex: Sensory Neuron, Motor Neuron, and Interneuron

Figure 2.2, page 34

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Figure 2.13 The Role of Inhibition

Inhibitory signals arising from the brain can prevent the withdrawal reflex from causing the person to drop the cup.

Inhibition in the Withdrawal Reflex: Brain input prevents the withdrawal reflex by decreasing activity of the motor neuron.

Figure 2.13 page 34

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Communication within a Neuron (1 of 3)

Measuring Electrical Potentials of Axons

Microelectrode: inserted into neuron to record electrical activity

Membrane Potential

Electrical charge across cell membrane

Resting Potential

Membrane potential of neuron not being altered by excitatory or inhibitory postsynaptic potentials

Electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell

Membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials, normally about -70 mV

Membrane Potential

the electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell

Oscilloscope

a laboratory instrument that is capable of displaying a graph of voltage as a function of time on the face of a cathode ray tube

Resting Potential

the membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials; approximately –70 mV in the giant squid axon

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Communication within a Neuron (2 of 3)

Measuring Electrical Potentials of Axons

Depolarization

Reduction (toward zero) of the membrane potential

Hyperpolarized

Increase in the membrane potential of cell

Electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell

Membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials, normally about -70 mV

Membrane Potential

the electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell

Oscilloscope

a laboratory instrument that is capable of displaying a graph of voltage as a function of time on the face of a cathode ray tube

Resting Potential

the membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials; approximately –70 mV in the giant squid axon

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Communication within a Neuron (3 of 3)

Force of Diffusion

Diffusion

Electrostatic Pressure

Sodium–Potassium Transporter

Diffusion

movement of molecules from regions of high concentration to regions of low concentration

Electrostatic Pressure

the attractive force between atomic particles charged with opposite signs or the repulsive force between atomic particles charged with the same sign

Sodium–Potassium Transporter

a protein found in the membrane

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Communication Within a Neuron The Membrane Potential: Balance of Two Forces

Intracellular Fluid Has Higher Levels of:

Organic Anions (A-)

Potassium Ions (K+)

Extracellular Fluid Has Higher Levels of:

Chloride Ions (Cl-)

Sodium Ions (Na+)

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Figure 2.15 Control of the Membrane Potential

The figure shows the relative concentration of some important ions inside and outside the neuron and the forces acting on them.

Figure 2.15, page 37

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The Sodium–Potassium Pump

These transporters are found in the cell membrane.

Figure from page 38 (not numbered in text)

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The Action Potential

Occurs when the cell is depolarized to the threshold of excitation and largely dependent upon movement of Na+ and K+ ions through special pores in the membrane called ion channels

Brief electrical impulse and provides basis for conduction of information along an axon

Threshold of Excitation: Value of membrane potential that must be reached to produce an action potential

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Figure 2.16 Ion Channels

When ion channels are open, ions can pass through them, entering or leaving the cell

Figure 2.16 page 39

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Figure 2.17 Ion Movements During the Action Potential

The image at the top shows the opening of sodium channels at the threshold of excitation, their refractory condition at the peak of the action potential, and their resetting when the membrane potential returns to resting potential.

Figure 2.17 page 40

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Conduction of an Action Potential

When an action potential is triggered, its size remains undiminished as it travels down the axon. The speed of conduction can be calculated from the delay between the stimulus and the action potential.

Conduction of the Action Potential: All-or-None Law

Figure from page 41 (not numbered in text)

All-or-None Law: Once the action potential begins, it proceeds without decrement to the terminal buttons

Rate Law: Variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires

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Figure 2.19 The Rate Law

The strength of a stimulus is represented by the rate of firing of an axon. The size of each action potential is always constant.

Conduction of the Action Potential: Rate Law

Figure 2.19, page 42

Rate Law: Variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires

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Figure 2.20 Saltatory Conduction

The figure shows propagation of an action potential down a myelinated axon.

Figure 2.20 page 42

Saltatory Conduction: Conduction of action potentials by myelinated axons; action potential appears to jump from one node of Ranvier to the next

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Communication Between Neurons: Structure of Synapses (1 of 2)

Synaptic transmission

The transmission of messages from one neuron to another across a synapse

Presynaptic to postsynaptic cell

Postsynaptic potential

Brief depolarizations or hyperpolarizations

Binding Site

Location on receptor protein to which ligand binds

Synapse and synaptic cleft: gap between neurons

Ligand: chemical that attaches to a binding site.

Neurotransmitters are naturally occurring ligands

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Communication Between Neurons: Structure of Synapses (2 of 2)

Dendritic spine

Small bud on the surface of a dendrite, with which a terminal button of another neuron forms a synapse

Presynaptic membrane

Membrane of terminal button

Adjacent to postsynaptic membrane and through which neurotransmitter is released

Postsynaptic membrane

Cell membrane opposite terminal button in synapse

Membrane of cell that receives message

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Figure 2.22 Details of a Synapse

This figure displays the structures of the synapse that are involved in synaptic communication.

Figure 2.22 page 45

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Figure 2.26 Ionotropic Receptors

The ion channel opens when a molecule of neurotransmitter attaches to the binding site. For purposes of clarity the drawing is schematic; molecules of neurotransmitters are actually much larger than individual ions.

Figure 2.26 page 47

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Figure 2.27 Metabotropic Receptors

When a molecule of neurotransmitter: G protein, Second messenger, and E.g. cyclic AMP binds with a receptor, a G protein activates an enzyme, which produces a second messenger (represented by black arrows) that opens nearby ion channels.

Metabotropic Receptor (meh tab oh trow pik)

Receptor that contains binding site for neurotransmitter; activates an enzyme that begins a series of events that opens an ion channel elsewhere in the membrane of the cell when a molecule of the neurotransmitter attaches to the binding site

G Protein

Protein coupled to a metabotropic receptor; conveys messages to other molecules when a ligand binds with and activates the receptor

Second Messenger

Chemical produced when a G protein activates an enzyme; carries a signal that results in the opening of the ion channel or causes other events to occur in the cell Figure 2.27 page48

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Communication between Neurons (1 of 3)

Synaptic Vesicle (vess i kul)

Small, hollow, beadlike structure found in terminal buttons

Contains molecules of neurotransmitter

Release Zone

Region of interior of the presynaptic membrane of synapse to which synaptic vesicles attach and release their neurotransmitter into synaptic cleft

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Communication between Neurons (2 of 3)

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP)

Inhibitory Postsynaptic Potential (IPSP)

Reuptake

Excitatory Postsynaptic Potential (EPSP)

Excitatory depolarization of postsynaptic membrane of synapse caused by liberation of neurotransmitter by terminal button

Inhibitory Postsynaptic Potential (IPSP)

Inhibitory hyperpolarization of postsynaptic membrane of synapse caused by liberation of neurotransmitter by terminal button

Reuptake

Reentry of neurotransmitter just liberated by terminal button back through its membrane, which terminates postsynaptic potential

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Communication between Neurons (3 of 3)

Termination of Postsynaptic Potentials

Reuptake

Enzymatic Deactivation

e.g. Acetylcholinesterase (AChE)

Enzymatic Deactivation

Destruction of neurotransmitter by enzyme after its release—for example, destruction of acetylcholine by acetylcholinesterase

Acetylcholinesterase (AChE) (a see tul koh lin ess ter ace)

Enzyme that destroys acetylcholine soon after it is liberated by terminal buttons, thus terminating postsynaptic potential

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Figure 2.28 Ionic Movements During Postsynaptic Potentials

Figure 2.28 page 49

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Figure 2.29 Reuptake

Molecules of a neurotransmitter that has been released into the synaptic cleft are transported back into the terminal button.

Figure 2.29 page 50

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Figure 2.30 Neural Integration

(a) If several excitatory synapses are active at the same time, the EPSPs they produce (shown in red) summate as they travel toward the axon, and the axon fires. (b) If several inhibitory synapses are active at the same time, the IPSPs they produce (shown in blue) diminish the size of the EPSPs and prevent the axon from firing.

Figure 2.30 page 51

Neural Integration

Process by which inhibitory and excitatory postsynaptic potentials summate and control rate of firing of neuron

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Autoreceptors

Postsynaptic receptors:

Detect presence of neurotransmitter in synaptic cleft

Autoreceptors:

Located on presynaptic neuron

Respond to neurotransmitter the neuron releases

Regulate internal processes

Other Types of Synapses

Axoaxonic

Presynaptic inhibition

Presynaptic faciliatation

Axodendritic

Dendrodendritic

Gap junction

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Figure 2.31 An Axoaxonic Synapse

The activity of terminal button A can increase or decrease the amount of neurotransmitter released by terminal button B.

Figure 2.31 page 52

Other Forms of Chemical Communication

Neuromodulators

Peptides

Hormones

Endocrine glands

Target cells

Steroid hormones

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Figure 2.32 Action of Steroid Hormones

Steroid hormones affect their target cells by means of specialized receptors in the nucleus. Once a receptor binds with a molecule of a steroid hormone, it causes genetic mechanisms to initiate protein synthesis.

Discussion 2

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