Objectives

Functions and Divisions of the Nervous System

1. List the basic functions of the nervous system.

2. Explain the structural and functional divisions of the nervous system.

Histology of Nervous Tissue

3. List the types of neuroglia and cite their functions.

4. Define neuron, describe its important structural components, and relate each to a functional role.

5. Differentiate between (1) a nerve and a tract, and (2) a nucleus and a ganglion.

6. Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems.

7. Classify neurons by structure and by function.

Membrane Potentials

8. Define resting membrane potential and describe its electrochemical basis.

9. Compare and contrast graded potentials and action potentials.

10. Explain how action potentials are generated and propagated along neurons.

11. Define absolute and relative refractory periods.

12. Define saltatory conduction and contrast it to continuous conduction.

The Synapse

13. Define synapse. Distinguish between electrical and chemical synapses by structure and by the way they transmit information.

14. Distinguish between excitatory and inhibitory postsynaptic potentials.

15. Describe how synaptic events are integrated and modified.

Neurotransmitters and Their Receptors

16. Define neurotransmitter and name several classes of neurotransmitters.

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Functions and Divisions of the Nervous System (p. 387; Figs. 11.1–11.2)The nervous system has three basic functions:

Figure 11.1 The nervous system’s functions.

1. Sensory input:

2. Integration:

3. Motor output:

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The _________________________________________(CNS) consists of the _____________________ and

_____________________________________ and is the integrating and control center of the

nervous system.

The _______________________________________________(PNS) is outside the central nervous

system. The PNS consists mainly of the _____________________________ (bundles of axons)

that extend from the brain and spinal cord.

Spinal nerves:

Cranial nerves:

The PNS has two functional divisions:

1. _____________________________________________________________________________

2. _____________________________________________________________________________

The _______________________________, or ___________________________________, division of the

peripheral nervous system carries impulses toward the central nervous system

from sensory receptors located throughout the body.

Somatic sensory fibers carry impulses from receptors in the ___________________,

__________________________________ muscles, and _________________________.

Visceral sensory fibers carry impulses from _____________________________ within

the ventral body cavity

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The _______________________________, or _________________________________, division of the

peripheral nervous system carries impulses from the central nervous system to

effector organs, which are muscles and glands.

The ____________________________________________________ consists of somatic motor

nerve fibers that conduct impulses from the CNS to skeletal muscles and

allow conscious (voluntary) control of motor activities.

The ____________________________________________________(ANS) is an involuntary

system consisting of visceral motor nerve fibers that regulate the activity of

smooth muscle, cardiac muscle, and glands.

o The ANS has two functional subdivision:

___________________________________________________________________________

___________________________________________________________________________

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Figure 11.2 Levels of organization in the nervous system.

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Histology of Nervous Tissue (pp. 387–395; Figs. 11.3–11.5; Table 11.1)

The nervous tissue is made up of two principle types of cells:

1. _____________________________________________________________________

2. _____________________________________________________________________

Neuroglia

Neuroglia, or _________________________ cells, are closely associated with neurons,

providing a protective and supportive network.

There are _______ types of neuroglia-___________ in the CNS and _____________ in the PNS.

Neuroglia in the CNS

_________________________________________regulate the chemical environment

around neurons and exchange between neurons and capillaries.

________________________________________ cells monitor health and perform defense

functions for neurons.

_______________________________________ cells line the central cavities of the brain

and spinal cord and help circulate cerebrospinal fluid.

______________________________________ wrap around neuron fibers, forming myelin

sheaths.

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Neuroglia in the PNS

_________________________________________ cells are glial cells of the PNS whose

function is largely unknown. They are found surrounding neuron cell bodies

within ganglia.

________________________________________cells, or neurolemmocytes, are glial cells

of the PNS that surround nerve fibers, forming the myelin sheath.

Figure 11.3 Neuroglia (a-d) The four types of neuroglia of the CNS. (e)

Neuroglia of the PNS.

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Neurons

Neurons are specialized cells that conduct messages in the form of

_______________________________________impulses throughout the body.

Special characteristics of neurons:

Extreme longevity

Amitotic

High Metabolic Rate

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Figure 11.4 Structure of a motor neuron.

Neuron Cell Body

The neuron cell body, also called the ____________________________ or soma, is the major

biosynthetic center containing the usual organelles (except for centrioles):

Spherical ___________________________ with a conspicuous nucleolus surrounded

by cytoplasm.

Free ribosomes and rough ER synthesize proteins and membranes.

The rough ER of the neuron body is called the_________________________

substance or _______________________ bodies.

Mitochondria are scattered among the other organelles.

Some contain pigments.

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The cell body is the focal point for the outgrowth of neuron processes during

embryonic development.

In most neurons, the plasma membrane of the cell body also acts as

_____________________________________________________ that receives information from other

neurons.

Most neuron cell bodies are located in the ___________________, where they are

protected by bones of the skull and vertebral column.

Clusters of cell bodies in the CNS are called ______________________, whereas those that

lie along the nerves in the PNS are called ________________________ (ganglion = knot of

string, swelling).

Neuron Processes

Neurons have armlike processes that extend from the cell body.

The brain and spinal cord (CNS) contain both neuron cell bodies and their

processes. The PNS consists chiefly of _________________________________________. Bundles

of neuron processes are called _________________________ in the CNS and

____________________________ in the PNS.

The two types of neuron processes are _____________________ and _________________________.

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Dendrites

• In motor neurons

– 100s of short, tapering, diffusely branched processes

– Same organelles as in body

• _____________________________________ (input) region of neuron

• Convey incoming messages toward cell body as ___________________________

potentials (short distance signals)

• In many brain areas fine dendrites specialized

– Collect information with ___________________________ spines

• Appendages with bulbous or spiky ends

The Axon: The Structure

• One axon per cell arising from __________________ hillock

– Cone-shaped area of cell body

• In some axon short or absent

• In others most of length of cell

– Some 1 meter long

• Long axons called ________________________________.

• Occasional branches (axon _________________________)

• Branches profusely at end (terminus)

• Can be 10,000 terminal branches

• Distal endings called axon terminals or terminal ___________________.

The Axon: Functional Characteristics

• Conducting region of neuron

• Generates nerve impulses

• Transmits them along ______________________________________ (neuron cell

membrane) to ___________________________________________.

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– Secretory region

– ________________________________ released into extracellular space

• Either excite or inhibit neurons with which axons in close

contact

• Carries on many conversations with different neurons at same time

• Lacks rough ER and Golgi apparatus

– Relies on cell body to renew proteins and membranes

– Efficient transport mechanisms

– Quickly decay if cut or damaged

Transport Along the Axon

• Molecules and organelles are moved along axons by motor proteins and

cytoskeletal elements

• Movement in both directions

– Anterograde—

• Examples: mitochondria, cytoskeletal elements, membrane

components, enzymes

– Retrograde—

• Examples: organelles to be degraded, signal molecules, viruses,

and bacterial toxins

Myelin Sheath

• Composed of myelin

– Whitish, protein-lipoid substance

• Segmented sheath around most long or large-diameter axons

– Myelinated fibers

• Function of myelin

– Protects and electrically insulates axon

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– Increases speed of nerve impulse transmission

• Nonmyelinated fibers conduct impulses more slowly

Myelination in the PNS

• Plasma membranes of myelinating cells have less protein

– No channels or carriers

– Good electrical insulators

– Interlocking proteins bind adjacent myelin membranes

• Myelin sheath gaps

– Gaps between adjacent Schwann cells

– Sites where axon collaterals can emerge

– Formerly called nodes of Ranvier

• Myelin sheath gaps between adjacent

Schwann cells

– Sites where axon collaterals can emerge

• Nonmyelinated fibers

– Thin fibers not wrapped in myelin; surrounded by Schwann cells but

no coiling; one cell may surround 15 different fibers

Figure 11.5 Nerve fiber myelination by Schwann cells in the PNS.

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Myelination in the CNS

• Formed by multiple, flat processes of oligodendrocytes, not whole cells

• Can wrap up to 60 axons at once

• Myelin sheath gap is present

• No outer collar of perinuclear cytoplasm

• Thinnest fibers are unmyelinated

– Covered by long extensions of adjacent neuroglia

• _____________________________ matter

– Regions of brain and spinal cord with dense collections of myelinated

fibers – usually fiber tracts

• _____________________________ matter

– Mostly neuron cell bodies and nonmyelinated fibers

Classification of Neurons

Neurons are classified both structurally and functionally.

Structural Classification

Neurons are grouped structurally according to the number of ______________________

extending from their cell body.

There are three structural classes of neurons:

________________________________ neurons have three or more processes.

o The major neuron type in the CNS.

________________________________ neurons have a single axon and dendrite.

o These rare neurons are found in some of the special sense organs.

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________________________________ neurons have a single process extending from

the cell body that is associated with receptors at the distal end.

o Distal (peripheral) process—associated with sensory receptor.

o Proximal (central) process-enters CNS.

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Functional Classification

This scheme groups neurons according to the ______________________________ in which

the nerve impulse travels relative to the central nervous system.

There are three functional classes of neurons:

_____________________________(or afferent) neurons conduct impulses

___________________ the CNS from receptors.

_____________________________(or efferent) neurons conduct impulses _____________

the CNS to effectors.

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________________________________________ (association neurons) conduct impulses

between sensory and motor neurons, or in CNS integration pathways.

Membrane Potentials (pp. 395–407; Figs. 11.6–11.15)

Basic Principles of Electricity

Some Definitions: Voltage, Resistance, Current

__________________________________ is a measure of the amount of difference in electrical

charge between two points, called the potential difference.

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The flow of electrical charge from point to point is called _____________________________,

and is dependent on voltage and ___________________________ (hindrance to current

flow).

Substances with high electrical resistance are _______________________________, and those

with low resistance are ________________________________.

Ohm’s Law gives the relationship between voltage, current, and resistance:

Ohm’s law tells us three things:

1.

2.

3.

In the body, electrical currents are due to the movement of ions across cellular

membranes.

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Role of Membrane Ion Channels

Plasma membranes contain membrane proteins that act as ion channels. Each of

these channels is ______________________________ as to the type of ion (or ions) it allows

to pass.

____________________________________ or nongated channels are always open.

There are three main types of gated channels:

Chemical gated (ligand-gated channels)

Voltage-gated channels

Mechanically gated channels

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Figure 11.6 Operation of gated channels.

When ion channels are open, ions diffuse across the membrane along their

electrochemical gradients, creating electrical currents.

According to Ohm’s law equation:

Voltage (V) =

Ions move along chemical _______________________________________ when they diffuse

passively from an area of their ___________________________ concentration to an area of

______________________ concentration.

Ions move along __________________________________________ when they move toward an

area of opposite electrical charge.

Electrochemical gradient =

The Resting Membrane Potential (pp. 397–398; Figs. 11.7–11.8)

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The membrane of a resting neuron is polarized, and the potential difference of this

polarity (approximately –70 mV) is called the resting membrane potential. The

resting membrane potential exists only across the membrane and is mostly due to

two factors: differences in ionic makeup of intracellular and extracellular fluids, and

differential membrane permeability to those ions.

Differences in Ionic Composition

The cytosol has a _________________________ concentration of Na+ and ______________________

concentration of K+ than extracellular fluid.

Anionic ____________________________ balance the cations inside the cell, while chloride

ions mostly balance cations outside of the cell.

Differences in Plasma Membrane Permeability

Potassium ions (K+) play the most important role in generating a resting membrane

potential, since the membrane is roughly ___________ times more permeable to K+

than Na+.

Potassium ions diffuse ________________ of the cell along their concentration gradient

much more easily than sodium ions can __________________ the cell along theirs.

K+ flowing out of the cells causes the cell to become more ____________________ inside.

Na+ trickling into the cell makes the cell just slightly more ____________________________

than it would be if only K+ flowed.

The sodium-potassium pump (Na+-K+ ATPase) stabilizes the resting membrane

potential by maintaining the concentration gradients for sodium and potassium.

________ Na+ are ejected from the cell

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________ K+ are transported back into the cell.

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Figure 11.8 Resting Membrane Potential

Membrane Potentials That Act as Signals (pp. 397–405; Figs. 11.8–11.15)

Neurons use changes in membrane potential as communication signals and can be

brought on by changes in membrane permeability to any ion, or alteration of ion

concentrations on the two sides of the membrane.

Changes in membrane potential can produce two types of signals:

Graded potentials

Action potentials

Relative to the resting state, potential changes can be ___________________________, in

which the inside of the membrane becomes less negative, or

_______________________________________________, in which the inside of the membrane

becomes more negatively charged.

Figure 11.9 Depolarization and hyperpolarization of the membrane.

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Graded Potentials

Graded potentials are short-lived _________________ changes in membrane potentials,

can either be depolarizations or hyperpolarizations, and are critical to the

generation of action potentials.

Graded potentials occurring on receptors of sensory neurons are called

____________________________ potentials, or ___________________________potentials.

Graded potentials occurring in response to a neurotransmitter released from

another neuron is called a ___________________________ potential.

Action Potentials

Action potentials, or nerve impulses, occur on axons and are the principle way

neurons communicate.

An action potential is a brief reversal of membrane potential with a total amplitude

(change in voltage) of about __________mV (from - ______mV to + ______mV).

Depolarization is followed by repolarization and often a short period of

hyperpolarization.

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An action potential (AP) is also called a _____________________________ and is typically

generated _______________________________________________________.

A neuron generates a nerve impulse only when adequately stimulated. This stimulus

changes the ____________________________ of the neuron’s membrane by opening specific

voltage-gated channels on the axon.

These channels open and close in response to changes in the membrane potential.

They are activated by local currents (_______________________________________________) that

spread toward the axon along the dendritic and cell body membranes.

In many neurons, the transition from local graded potential to long-distance action

potential takes place at the _____________________________________________.

Generation of an Action Potential

Generation of an action potential involves a transient increase in Na+ permeability,

followed by restoration of Na+ impermeability, and then a short-lived increase in

K+ permeability.

1. Resting state: _____________________________________________________________

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2. Depolarization: _____________________________________________________________

3. Repolarization: _____________________________________________________________

4. Hyperpolarization: _____________________________________________________________

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Figure 11.11 Action Potential

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Threshold and the All-or-None Phenomenon

A critical minimum, or threshold, depolarization is defined by the amount of influx

of Na+ that at least equals the amount of efflux of K+.

Action potentials are all-or-none phenomena: they either happen completely, in the

case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus.

Propagation of an Action Potential

If it is to serve as the neuron’s signaling device, an AP must be _________________________

along the axon’s entire length.

Figure 11.12 Propagation of an action potential (AP)

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• Na+ influx causes local currents

– Local currents cause depolarization of adjacent membrane areas in

direction away from AP origin (toward axon's terminals)

– Local currents trigger an AP there

– This causes the AP to propagate AWAY from the AP origin

• Since Na+ channels closer to AP origin are inactivated no new AP is generated

there

• Once initiated an AP is self-propagating

– In nonmyelinated axons each successive segment of membrane

depolarizes, then repolarizes

– Propagation in myelinated axons differs

Coding for Stimulus Intensity

All action potentials are alike and are independent of stimulus intensity

How does CNS tell difference between a weak stimulus and a strong one?

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Figure 11.13 Relationship between stimulus strength and action potential

frequency.

Refractory Period

The refractory period of an axon is related to the period of time required so that a

neuron can generate another action potential.

Absolute refractory period =

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Relative refractory period =

Figure 11.14 Absolute and relative refractory periods in AP.

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Conduction Velocity

The rate of impulse propagation depends on two factors:

1. _____________________________________________________________

2. _____________________________________________________________

Axons with larger diameters conduct impulses faster than axons with smaller

diameters.

Nonmyelinated axons conduct impulses relatively slowly, while myelinated axons

have a high conduction velocity.

Continuous conduction =

Saltatory conduction =

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Figure 11.15 Action Potential propogation in nonmyelinated and myelinated

axons.

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The Synapse (pp. 407–414; Figs. 11.16–11.19; Table 11.2)

A ____________________________________ is a junction that mediates information transfer

between neurons or between a neuron and an effector cell

Axodendritic synapses =

Axosomatic synapses =

Figure 11.17 Synapses. Axodendritic, axosomatic, and axoaxonal synapses.

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Neurons conducting impulses toward the synapse are _______________________________

neurons, and neurons carrying impulses away from the synapse are

__________________________________ neurons.

Electrical Synapses

Electrical synapses have neurons that are electrically coupled via protein channels

and allow direct exchange of ions from cell to cell

Chemical Synapses

Chemical synapses are specialized for release and reception of chemical

neurotransmitters

A typical chemical synapse is made up of two parts:

1.

2.

Synaptic cleft =

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Information Transfer Across Chemical Synapses

1. Action Potential arrives at axon terminal

2. Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal

3. Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis.

4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors

on the postsynaptic membrane.

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5. Binding of neurotransmitter opens ion channels, creating graded potentials.

6. Neurotransmitter effects are terminated.

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Figure 11.17 Chemical Synapse

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Synaptic Delay

Synaptic delay is related to the period of time required for release and binding of

neurotransmitters

Postsynaptic Potentials and Synaptic Integration (pp. 410–414; Figs. 11.18–

11.19; Table 11.2)

Neurotransmitters mediate graded potentials on the postsynaptic cell that may be

excitatory or inhibitory.

__________________________potentials on the postsynaptic cell occur when there is a net

influx of Na+ into the cell, and are called excitatory postsynaptic potentials (EPSPs).

_____________________potentials on the postsynaptic cell occur when there is an

increase in permeability to either K+ or Cl- and are called inhibitory postsynaptic

potentials (IPSPs).

Figure 11.8 Postsynaptic potential can be excitatory or inhibitory.

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Integration and Modification of Synaptic Events

Summation by the Postsynaptic Neuron

Summation by the postsynaptic neuron is accomplished in two ways: temporal

summation, which occurs in response to several successive releases of

neurotransmitter, and spatial summation, which occurs when the postsynaptic cell

is stimulated at the same time by multiple terminals.

Figure 11.19 Neural integration of EPSPs and IPSPs

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Synaptic Potentiation

Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or

continuously, resulting in an enhanced release of neurotransmitter.

Presynaptic Inhibition

Presynaptic inhibition results when another neuron inhibits the release of an

excitatory neurotransmitter from a presynaptic cell.

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Neurotransmitters and Their Receptors (pp. 414–421; Figs. 11.20–11.21;

Table 11.3)

Neurotransmitters fall into several chemical classes: acetylcholine, the biogenic

amines, amino acid derived, peptides, purines, and gases and lipids. (For a more

complete listing of neurotransmitters within a given chemical class

Functional classifications of neurotransmitters consider whether the effects are

excitatory or inhibitory and whether the effects are direct or indirect, including

neuromodulators that affect the strength of synaptic transmission.

There are two main types of neurotransmitter receptors: channel-linked receptors

mediate fast synaptic transmission and result in brief, localized changes, and G

protein–linked receptors mediate indirect transmitter action resulting in slow

synaptic responses

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