chapter 11 nervous system overview: focus on neurons

CHAPTER 11

NERVOUS SYSTEM OVERVIEW:

FOCUS ON NEURONS

FUNCTIONS OF

THE NERVOUS SYSTEM

Copyright © 2010 Pearson Education, Inc.

Figure 11.1 The nervous system’s functions.

Sensory input

Motor output

Integration

DIVISIONS OF

THE NERVOUS SYSTEM


Figure 11.2 Schematic of levels of organization in the nervous system.

Central nervous system (CNS)

Brain and spinal cord

Integrative and control centers

Peripheral nervous system (PNS)

Cranial nerves and spinal nerves

Communication lines between the

CNS and the rest of the body

Parasympathetic

division

Conserves energy

Promotes house- keeping functions during rest

Motor (efferent) division

Motor nerve fibers

Conducts impulses from the CNS

to effectors (muscles and glands)

Sensory (afferent) division

Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS

Somatic nervous

system

Somatic motor (voluntary)

Conducts impulses from the CNS to skeletal muscles

Sympathetic division

Mobilizes body systems during activity

Autonomic nervous

system (ANS)

Visceral motor (involuntary)

Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

Structure

Function

Sensory (afferent) division of PNS

Motor (efferent) division of PNS

Somatic sensory

fiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

Stomach Skeletal muscle

Heart

Bladder Parasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Quiz Q1:

All of these are functions of the nervous

system EXCEPT…

1) Sensation

2) Integration / decision making

3) Motor output

4) Carrying electricity

Quiz Q2:

The sympathetic nervous system is part of…

1) The central nervous system

2) The peripheral nervous system

Quiz Q3:

The somatic nervous system is…

1) Afferent (sensory)

2) Efferent (motor)

HISTOLOGY OF

THE NERVOUS SYSTEM

Neuroglia


Figure 11.3a Neuroglia.

(a) Astrocytes are the most abundant

CNS neuroglia.

Capillary

Neuron

Astrocyte


Figure 11.3b Neuroglia.

(b) Microglial cells are defensive cells in

the CNS.

Neuron

Microglial

cell


Figure 11.3c Neuroglia.

Brain or spinal cord tissue

Ependymal cells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinal

fluid-filled cavities.


Figure 11.3d Neuroglia.

(d) Oligodendrocytes have processes that form

myelin sheaths around CNS nerve fibers.

Nerve fibers

Myelin sheath

Process of oligodendrocyte


Figure 11.3e Neuroglia.

(e) Satellite cells and Schwann cells (which

form myelin) surround neurons in the PNS.

Schwann cells (forming myelin sheath)

Cell body of neuron Satellite cells

Nerve fiber

The cell type which produces cerebrospinal

fluid is the…

1) Astrocyte

2) Ependymal cell

3) Microglial cell

4) Oligodendrocyte

The cell type which is most abundant in the

CNS and which maintains neurons is the…

1) Astrocyte

2) Ependymal cell

3) Microglial cell

4) Oligodendrocyte

HISTOLOGY OF

THE NERVOUS SYSTEM

Neurons


Figure 11.4 Structure of a motor neuron.

Dendrites

(receptive regions)

Cell body

(biosynthetic center and receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon

(impulse generating and conducting region)

Axon hillock

Neurilemma Terminal branches

Node of Ranvier

Impulse direction

Schwann cell (one inter- node)

Axon terminals (secretory region)

Dendritic spine

Neuron cell body

(a)

(b)


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

(a) Myelination of a nerve

fiber (axon)

Schwann cell cytoplasm

Axon

Neurilemma

Myelin sheath

Schwann cell nucleus

Schwann cell plasma membrane

1

2

3

A Schwann cell

envelopes an axon.

The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.


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

Myelin sheath

Schwann cell cytoplasm

Neurilemma

(b) Cross-sectional view of a myelinated axon

(electron micrograph 24,000X)

Axon

HISTOLOGY OF

THE NERVOUS SYSTEM

Classification of Neurons

Structural vs. Functional


Table 11.1 Comparison of Structural Classes of Neurons (1 of 3)

A neuron which has one dendrite and one

Axon is a _______ neuron.

1) Bipolar

2) Multipolar

3) Unipolar

MEMBRANE POTENTIALS

MEMBRANE POTENTIALS

Basics of Electricity

MEMBRANE POTENTIALS

Role of membrane ion channels


Figure 11.6 Operation of gated channels.

(b) Voltage-gated ion channels open and close in response

to changes in membrane voltage.

Na+

Na+

Closed Open

Receptor

(a) Chemically (ligand) gated ion channels open when the

appropriate neurotransmitter binds to the receptor,

allowing (in this case) simultaneous movement of

Na+ and K

+.

Na+

K+

K+

Na+

Neurotransmitter chemical

attached to receptor

Chemical

binds

Closed Open

Membrane

voltage

changes

A channel which allows sodium to cross in

response to a change in membrane potential

is a...

1) Sodium leak channel

2) Potassium leak channel

3) Voltage-gated sodium channel

4) Voltage-gated potassium channel

True or false: the Na+/K+ pump (ATPase) is

an ion channel.

1) True

2) False

MEMBRANE POTENTIALS

Resting potential


Figure 11.7 Measuring membrane potential in neurons.

Voltmeter

Microelectrode

inside cell

Plasma

membrane Ground electrode

outside cell

Neuron

Axon


Figure 11.8 Resting Membrane Potential (1 of 4)

The concentrations of Na+ and K

+ on each side

of the membrane are different.

Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+ across the membrane.

The Na+ concentration is higher outside the cell.

The K+ concentration is higher inside the cell.

K+

(5 mM )

K+ (140 mM )

Outside cell

Inside cell

Na+ (140 mM )

Na+

(15 mM )



K+ loss through

abundant leakage channels establishes a negative membrane potential.

Suppose a cell has only K+ channels...

The permeabilities of Na+ and K

+ across the

membrane are different.

K+ leakage channels

Cell interior

–90 mV

K+ K+

K+ K+



Na+ entry through

leakage channels reduces the negative membrane potential slightly.

Now, let’s add some Na+ channels to our cell...

Cell interior

–70 mV Na+

K+ K+ Na+

K+ K+



Finally, let’s add a pump to compensate for leaking ions.

Na+-K

+ ATPases

(pumps) maintain the concentration gradients,

resulting in the resting membrane potential.

Cell interior –70 mV

Na+-K+

pump

Na+

K+ K+ Na+

K+ K+

True or false: the resting potential of a neuron

depends mostly on the potassium (K+)

concentration gradient.

1) True

2) False

MEMBRANE POTENTIALS

Depolarization and hyperpolarization


Figure 11.9a Depolarization and hyperpolarization of the membrane.

Depolarizing stimulus

Time (ms)

Inside

positive

Inside

negative

Resting

potential

Depolarization

(a) Depolarization: The membrane potential

moves toward 0 mV, the inside becoming

less negative (more positive).


Figure 11.9b Depolarization and hyperpolarization of the membrane.

Hyperpolarizing stimulus

Time (ms)

Resting

potential

Hyper-

polarization

(b) Hyperpolarization: The membrane

potential increases, the inside becoming

more negative.

If a neuron has a resting potential of -70mV,

then a change to -30 mV is called…

1) Depolarization

2) Resting potential

3) Hyperpolarization

MEMBRANE POTENTIALS

Graded potentials


Figure 11.10 The spread and decay of a graded potential.

Depolarized region

Stimulus

Plasma membrane

Distance (a few mm)

–70

Resting potential

Active area (site of initial depolarization)

(a) Depolarization: A small patch of the

membrane (red area) has become depolarized.

(b) Spread of depolarization: The local currents

(black arrows) that are created depolarize adjacent

membrane areas and allow the wave of

depolarization to spread.

(c) Decay of membrane potential with distance: Because current

is lost through the “leaky” plasma membrane, the voltage declines

with distance from the stimulus (the voltage is decremental).

Consequently, graded potentials are short-distance signals.

MEMBRANE POTENTIALS

Action potentials


Figure 11.11 Action Potential (1 of 5)

Action

potential

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

The big picture

1 1

2

3

4

Time (ms)

Threshold Me

mb

ra

ne

p

ote

ntia

l (m

V)



Action

potential

Time (ms)

1 1

2

3

4

Na+ permeability

K+ permeability

The AP is caused by permeability changes in

the plasma membrane

Me

mb

ra

ne

p

ote

ntia

l (m

V)

Re

lative

m

em

bra

ne

p

erm

ea

bility



Na+ Na+ Na+

Voltage-gated Na+ channels

The key players

Closed Opened Inactivated

Outside

cell

Inside

cell Activation

gate

Inactivation

gate



Voltage-gated K+ channels

The key players

Closed Opened

Inside

cell K+ K+

Outside

cell



Na+

Na+

Potassium

channel

Sodium

channel

1 Resting state

2 Depolarization

3 Repolarization

4 Hyperpolarization

The events

Activation

gates

Inactivation gate K+

K+

Na+

K+

Na+

K+


Figure 11.12 Propagation of an action potential (AP).

Voltage

at 2 ms

Voltage

at 4 ms

Voltage

at 0 ms

Recording

electrode

(a) Time = 0 ms. Action

potential has not yet

reached the recording

electrode.

(b) Time = 2 ms. Action

potential peak is at the

recording electrode.

(c) Time = 4 ms. Action

potential peak is past

the recording electrode.

Membrane at the

recording electrode is

still hyperpolarized. Resting potential

Peak of action potential

Hyperpolarization


Figure 11.13 Relationship between stimulus strength and action potential frequency.

Threshold

Action

potentials

Stimulus

Time (ms)


Figure 11.14 Absolute and relative refractory periods in an AP.

Stimulus

Absolute refractory

period Relative refractory

period

Time (ms)

Depolarization

(Na+ enters)

Repolarization

(K+ leaves)

After-hyperpolarization

ACTION POTENTIALS

“Ideal toilet analogy”

Action potentials…

1) Require voltage-gated sodium

channels

2) Can carry messages a long distance

3) Happen very quickly

4) All of the above

Action potentials…

1) Are local events

2) Last a long time

3) Can happen right after each

other immediately

4) Are caused by hyperpolarization

of the membrane

ACTION POTENTIALS

Propagation


Figure 11.15a Action potential propagation in unmyelinated and myelinated axons.

Size of voltage

Stimulus

(a) In a bare plasma membrane (without voltage-gated channels), as on a dendrite, voltage decays because current leaks across the membrane.


Figure 11.15b Action potential propagation in unmyelinated and myelinated axons.

Voltage-gated

ion channel

Stimulus

(b) In an unmyelinated axon, voltage-gated Na+ and K+

channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gates of channel proteins take time and must occur before voltage regeneration occurs.


Figure 11.15c Action potential propagation in unmyelinated and myelinated axons.

Stimulus

Myelin

sheath

Node of Ranvier

Myelin sheath

(c) In a myelinated axon, myelin keeps current in axons (voltage doesn’t decay much). APs are generated only in the nodes of Ranvier and appear to jump rapidly from node to node.

1 mm

Myelin sheaths speed up the rate of action

potential transmission by…

1) preventing decay of membrane potential.

2) reducing the number of action potentials

required.

3) making membranes depolarize faster.

4) 1 and 2 only

5) All of the above

SYNAPSES


Figure 11.16 Synapses.

Dendrites

Cell body

Axon

Axodendritic

synapses

Axosomatic synapses

Cell body (soma) of postsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomatic

synapses

(a)

SYNAPSES

Electrical

SYNAPSES

Chemical


Figure 11.17 Chemical Synapse (1 of 3)

1

2

3

Action potential

arrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causes

neurotransmitter-

containing synaptic

vesicles to release their

contents by exocytosis.

Ca2+

Synaptic vesicles

Axon

terminal

Mitochondrion

Postsynaptic

neuron

Presynaptic

neuron

Presynaptic

neuron

Synaptic

cleft

Ca2+

Ca2+

Ca2+

4 Neurotransmitter

diffuses across the synaptic

cleft and binds to specific

receptors on the

postsynaptic membrane.

Postsynaptic

neuron



5 Binding of neurotransmitter

opens ion channels, resulting in

graded potentials.

Ion movement

Graded potential



6 Neurotransmitter effects are terminated

by reuptake through transport proteins,

enzymatic degradation, or diffusion away

from the synapse.

Reuptake

Enzymatic

degradation

Diffusion away

from synapse

Electrical synapses…

1) Require neurotransmitters

2) Involve two adjacent cells that

don’t actually touch each other

3) Require calcium ions

4) Transmit action potentials via

gap junctions

Neurotransmitters…

1) Are released into a synaptic cleft

2) Need sodium ions to cause their

release

3) Are an electrical signal

4) Diffuse into the postsynaptic cell

SYNAPSES

Postsynaptic potentials


Figure 11.18 Postsynaptic potentials.

An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas- sage of Na+ and K+.

An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Time (ms)

(a) Excitatory postsynaptic potential (EPSP)

Threshold

Time (ms)

(b) Inhibitory postsynaptic potential (IPSP)

Threshold

Stimulus

Stimulus

SYNAPSES

Summation of postsynaptic potentials


Figure 11.19a Neural integration of EPSPs and IPSPs.

Threshold of axon of postsynaptic neuron

Excitatory synapse 1 (E1)


Inhibitory synapse (I1)

Resting potential

E1 E1

(a) No summation: 2 stimuli separated in time

cause EPSPs that do not add together.

Time

E1

Me

mb

ran

e p

ote

nti

al (m

V)


Figure 11.19b Neural integration of EPSPs and IPSPs.

(b) Temporal summation: 2 excitatory stimuli

close in time cause EPSPs that add together.

Time

0

–55

–70




E1 E1

E1

Me

mb

ran

e p

ote

nti

al (m

V)


Figure 11.19c Neural integration of EPSPs and IPSPs.




Me

mb

ran

e p

ote

nti

al (m

V)

E1 + E2

(c) Spatial summation: 2 simultaneous stimuli at

different locations cause EPSPs that add together.

Time

E1

E2

0

–55

–70


Figure 11.19d Neural integration of EPSPs and IPSPs.




Me

mb

ran

e p

ote

nti

al (m

V)

E1 + I1

Time

E1

I1 I1

(d) Spatial summation of EPSPs and IPSPs: Changes

in membane potential can cancel each other out.

0

–55

–70

NEUROTRANSMITTERS


Table 11.3 Neurotransmitters and Neuromodulators (1 of 6)


A Closer Look 11.1 Pleasure Me, Pleasure Me!

Normal

Abuser: 10 days without cocaine

Abuser: 100 days without cocaine

Which category of neurotransmitters do

epinephrine and norepinephrine fall under?

1) Acetylcholine

2) Biogenic amines

3) Peptides

4) Endocannabinoids

5) Amino acids

True or false: All neurotransmitters bind to

receptors on the surface of the post-synaptic

cell.

1) True

2) False

NEUROTRANSMITTERS

Mechanisms of neurotransmitter receptors


Figure 11.20a Direct and indirect neurotransmitter receptor mechanisms.

Ion flow blocked

Closed ion channel

(a) Channel-linked receptors open in response to binding

of ligand (ACh in this case).

Ions flow

Ligand

Open ion channel


Figure 11.20b Direct and indirect neurotransmitter receptor mechanisms.

1 Neurotransmitter

(1st messenger) binds

and activates receptor.

Receptor G protein

Closed ion

channel Adenylate cyclase Open ion

channel

2 Receptor

activates G

protein.

3 G protein

activates

adenylate

cyclase.

4 Adenylate

cyclase converts

ATP to cAMP

(2nd messenger).

cAMP changes

membrane permeability

by opening or closing ion

channels.

5b cAMP activates

enzymes.

5c cAMP activates

specific genes.

Active enzyme

GDP

5a

(b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic

AMP in this case) that brings about the cell’s response.

Nucleus

Whether direct or indirect, the function of most

neurotransmitters is to _________ in/on the

post-synaptic cell.

1) Stimulate cell division

2) Prevent action potentials

3) Open ion channels

NEURAL INTEGRATION

NEURONAL POOLS


Figure 11.21 Simple neuronal pool.

Presynaptic

(input) fiber

Facilitated zone Discharge zone Facilitated zone

NEURONAL POOLS

Types of circuits


Figure 11.22a Types of circuits in neuronal pools.


Figure 11.22b Types of circuits in neuronal pools.


Figure 11.22c-d Types of circuits in neuronal pools.


Figure 11.22e Types of circuits in neuronal pools.


Figure 11.22f Types of circuits in neuronal pools.

A neuronal circuit in which a single neuron

receives input from multiple other neurons,

then integrates the information to produce

a single output is a…

1) Diverging circuit

2) Converging circuit

3) Reverberating circuit

4) Parallel after-discharge circuit


Figure 11.23 A simple reflex arc.

1

2

3

4

5

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Stimulus

Response Spinal cord (CNS)

Interneuron


Figure 11.24 A neuronal growth cone.

Neuroblasts:

1) Proliferate

2) Differentiate &

Migrate

3) Connect

chapter 11 nervous system overview: focus on neurons

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