Chap 19 nervous system

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

THE NERVOUS SYSTEMChapter 19


• Overview: Command and Control Center

• The human brain

– Contains an estimated 100 billion nerve cells, or neurons

• Each neuron

– May communicate with thousands of other neurons


• Functional magnetic resonance imaging

– Is a technology that can reconstruct a three-dimensional map of brain activity


• The results of brain imaging and other research methods

– Reveal that groups of neurons function in specialized circuits dedicated to different tasks


• In vertebrates

– The central nervous system consists of a brain and dorsal spinal cord

– The peripheral nervous system (PNS) connects to the central nervous system (CNS)

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)


Information Processing

• Nervous systems process information in three stages

– Sensory input, integration, and motor output

Sensor

Effector

Motor output

Integration

Sensory input

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)


• Sensory neurons transmit information from sensors

– That detect external stimuli and internal conditions

• Sensory information is sent to the CNS

– Where interneurons integrate the information

• Motor output leaves the CNS via motor neurons

– Which communicate with effector cells


• The three stages of information processing

– Are illustrated in the knee-jerk reflex

Sensory neurons from the quadriceps also communicatewith intermediate neurons in the spinal cord.

The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.

The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.

4

5

6

The reflex is initiated by tapping

the tendon connected to the quadriceps

(extensor) muscle.

1

Sensors detecta sudden stretch in the quadriceps.

2 Sensory neuronsconvey the information to the spinal cord.

3

Quadricepsmuscle

Hamstringmuscle

Spinal cord(cross section)

Gray matter

White matter

Cell body of sensory neuronin dorsal root ganglion

Sensory neuron

Motor neuron

Intermediate neuron

(interneuron)


Neuron Structure

• Most of a neuron’s organelles

– Are located in the cell bodyDendrites

Cell body

Nucleus

AxonSignal direction

Synapse

Myelin sheath

Synapticterminals

Presynaptic cell Postsynaptic cell


• Most neurons have dendrons or dendrites

– Highly branched extensions that receive signals from other neurons

• The axon is typically a much longer extension

– That transmits signals to other cells at synapses

– That may be covered with a myelin sheath


• Neurons have a wide variety of shapes

– That reflect their input and output interactions

Figure 48.6a–c

Axon

Cell body

Dendrites

(a) Sensory neuron (b) Interneurons (c) Motor neuron


Supporting Cells (Glia)

• Glia are supporting cells

– That are essential for the structural integrity of the nervous system and for the normal functioning of neurons


• Schwann cells (in the PNS)

– Are glia that form the myelin sheaths around the axons of many vertebrate neurons

Myelin sheathNodes of Ranvier

Schwanncell Schwann

cellNucleus of Schwann cell

Axon

Layers of myelin

Node of Ranvier

0.1 µm

Axon

Figure 48.8


Transmission of Nerve Impulses

• Ion pumps and ion channels maintain the resting potential of a neuron

• Across its plasma membrane, every cell has a voltage

– Called a membrane potential

• The inside of a cell is negative

– Relative to the outside


• The membrane potential of a cell can be measured

APPLICATIONElectrophysiologists use intracellular recording to measure the

membrane potential of neurons and other cells.

TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.

Microelectrode

Referenceelectrode

Voltage recorder

–65 mV


The Resting Potential

• The resting potential

– Is the membrane potential of a neuron that is not transmitting signals


• In all neurons, the resting potential

– Depends on the ionic gradients that exist across the plasma membrane

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[Cl–]10 mM

[A–]100 mM

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM

–

–

–

–

–

+

+

+

+

+

Plasmamembrane


• The concentration of Na+ is higher in the extracellular fluid than in the cytosol

– While the opposite is true for K+


• Stimuli trigger a depolarization

– A reduction in the magnitude of the membrane potential

+50

0

–50

–100

Time (msec)0 1 2 3 4 5

Threshold

Restingpotential

Depolarizations

Me

mb

ran

e p

ote

ntia

l (m

V)

Stimuli

(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.


• A stimulus strong enough to produce a depolarization that reaches the threshold

– Triggers a different type of response, called an action potential

- 40 mV+50

0

–50

–100

Time (msec)0 1 2 3 4 5 6

Threshold

Restingpotential

Me

mb

ran

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l (m

V)

Stronger depolarizing stimulus

Actionpotential

(c) Action potential triggered by a depolarization that reaches the threshold.


• An action potential

– Is a brief all-or-none depolarization of a neuron’s plasma membrane

– Is the type of signal that carries information along axons


• Na+ channels and K+ channels

– Are involved in the production of an action potential

• When a stimulus depolarizes the membrane

– Na+ channels open, allowing Na+ to diffuse INTO the cell

– Which direction????

– Na+ diffuses INTO the cell


• As the action potential subsides

– K+ channels open, and K+ flows OUT of the cell

– Which direction??? K+ flows OUT

• A refractory period follows the action potential

– During which a second action potential cannot be initiated


• The generation of an action potential

– – – – – – – –

+ + + + + + + + + + + ++ +

– – – – – –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

+ +

– –

– –

+ +

– –

+ +

– –

+ +

– –

+ +

Na+ Na+

K+

Na+ Na+

K+

Na+ Na+

K+

Na+

K+

K+

Na+ Na+

5

1 Resting state

2 Depolarization

3 Rising phase of the action potential

4 Falling phase of the action potential

Undershoot

1

2

3

4

5 1

Sodiumchannel

Actionpotential

Resting potential

Time

Plasma membrane

Extracellular fluid ActivationgatesPotassium

channel

Inactivationgate

Threshold

Mem

bran

e po

tent

ial

(mV

)

+50

0

–50

–100

Threshold

Cytosol

Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respectto the outside.

The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative.

A stimulus opens theactivation gates on some Na+ channels. Na+

influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential.

The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained.

Both gates of the Na+ channelsare closed, but the activation gates on some K+ channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state.


Conduction of Action Potentials

• An action potential can travel long distances

– By regenerating itself along the axon


– +– + + + + +

– +– + + + + +

+ –+ – + + + +

+ –+ – + + + +

+ –+ – – – – –+ –+ – – – – –

– – – –– – – –

– –

– –+ +

+ +

+ ++ + – – – –

+ ++ + – – – –

– –– – + + + +– –– – + + + +

Na+

Na+

Na+

Actionpotential

Actionpotential

ActionpotentialK+

K+

K+

Axon

An action potential is generated as Na+ flows inward across the membrane at one location.1

2

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

3The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

• At the site where the action potential is generated

– An electrical current depolarizes the neighboring region of the axon membrane


Conduction Speed

• The speed of an action potential

– Increases with the diameter of an axon

• In vertebrates, axons are myelinated

– Also causing the speed of an action potential to increase

– 100 m/s

– non-myelinated axon – 0.5 m/s


• Action potentials in myelinated axons

– Jump between the nodes of Ranvier in a process called saltatory conduction

Cell body

Schwann cell

Myelin sheath

Axon

Depolarized region(node of Ranvier)

++ +

++ +

++ +

++

– –

– –

– –

–––

–

–

–


• Neurons communicate with other cells at synapses

• In an electrical synapse

– Electrical current flows directly from one cell to another via a gap junction

• The vast majority of synapses

– Are chemical synapses


• In a chemical synapse, a presynaptic neuron

– Releases chemical neurotransmitters, which are stored in the synaptic terminal

Postsynapticneuron

Synapticterminalof presynapticneurons

5 µ

m


• When an action potential reaches a terminal

– The final result is the release of neurotransmitters into the synaptic cleft

Presynapticcell

Postsynaptic cell

Synaptic vesiclescontainingneurotransmitter

Presynapticmembrane

Postsynaptic membrane

Voltage-gatedCa2+ channel

Synaptic cleft

Ligand-gatedion channels

Na+

K+

Ligand-gatedion channel

Postsynaptic membrane

Neuro-transmitter

1 Ca2+

2

3

4

5

6


Direct Synaptic Transmission

• The process of direct synaptic transmission

– Involves the binding of neurotransmitters to ligand-gated ion channels


• Neurotransmitter binding

– Causes the ion channels to open, generating a postsynaptic potential


• Postsynaptic potentials fall into two categories

– Excitatory postsynaptic potentials (EPSPs)

– Inhibitory postsynaptic potentials (IPSPs)


• After its release, the neurotransmitter

– Diffuses out of the synaptic cleft

– May be taken up by surrounding cells and degraded by enzymes


Summation of Postsynaptic Potentials

• Unlike action potentials

– Postsynaptic potentials are graded and do not regenerate themselves


• Since most neurons have many synapses on their dendrites and cell body

– A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

Figure 48.18a

E1 E1

Restingpotential

Threshold of axon ofpostsynaptic neuron

(a) Subthreshold, nosummation

Terminal branch of presynaptic neuron

Postsynaptic neuron E1

0

–70

Me

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• If two EPSPs are produced in rapid succession

– An effect called temporal summation occurs

Figure 48.18b

E1 E1

Actionpotential

(b) Temporal summation

E1

Axonhillock


• In spatial summation

– EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

Figure 48.18c

E1 + E2

Actionpotential

(c) Spatial summation

E1

E2


• Through summation

– An IPSP can counter the effect of an EPSP

Figure 48.18d

E1 E1 + II

(d) Spatial summationof EPSP and IPSP

E1

I


Indirect Synaptic Transmission

• In indirect synaptic transmission

– A neurotransmitter binds to a receptor that is not part of an ion channel

• This binding activates a signal transduction pathway

– Involving a second messenger in the postsynaptic cell, producing a slowly developing but long-lasting effect


Neurotransmitters

• The same neurotransmitter

– Can produce different effects in different types of cells


• Major neurotransmitters

Table 48.1


Acetylcholine

• Acetylcholine

– Is one of the most common neurotransmitters in both vertebrates and invertebrates

– Can be inhibitory or excitatory


Biogenic Amines

• Biogenic amines

– Include epinephrine, norepinephrine, dopamine, and serotonin

– Are active in the CNS and PNS


Amino Acids and Peptides

• Various amino acids and peptides

– Are active in the brain


Gases

• Gases such as nitric oxide and carbon monoxide

– Are local regulators in the PNS

Chap 19 nervous system

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Transcript of Chap 19 nervous system