System of the Organ The Nervous System 2 LIU Chuan Yong 刘传勇 Department of Physiology Medical...

System of the Organ

The Nervous System

2

LIU Chuan Yong

刘传勇

Department of Physiology

Medical School of SDU

Tel 88381175 (lab)

88382098 (office)

Email: liucy@sdu.edu.cn

Website: www.physiology.sdu.edu.cn

Characteristics of the Course

• Integration （整合）– From basic to clinical

• Combine with PBL, Laboratory and Clinical Clerkship

• In Bilingual (in English and Chinese)

Chapter 2

Elements of Cellular and Molecular Neuroscience

Reference

Neurones and Glial Cells

Review

Sections I

Neurone Excitability

Neuron Excitability

• Part 1 Facilitated Diffusion through Channel

• Part 2 Resting Membrane Potential

• Part 3 Action Potentials

• Part 4 Nerve Conduction

• Part 5 Synaptic Transmission

Part I Facilitated Diffusion through Channel

• Voltage gated channel

• Chemically gated channel

• Mechanically gated channel

Categories of Transport Across the Plasma Membrane

Cell membrane is selectively permeable to some molecules and ions.

Mechanisms to transport molecules and ions through the cell membrane:Non-carrier mediated transport.

Simple Diffusion （单纯扩散） :O2, CO2, steroids, H2O

Facilitated Diffusion （易化扩散） :Via Carrier: glucose, amino acidChannel

Voltage, Chemical and Mechanical gating channel

Active Transport 主动转运Primary active transportSecondary active transport

Facilitated diffusion through channels

Definition transport proteins complex

have watery spaces all the way through the molecule

allow free movement of certain ions or molecules.

channel proteins. Channel mediated diffusion.

Basic Structure of Voltage-Gated Calcium Channels

Cain & Snutch,2010

The alpha subunit is responsible for basic electrophysiological and pharmacological properties

The auxiliary subunits have regulatory roles

Principal one alpha subunit and several auxiliary subunits

Characteristics of the channels:

selectively permeable to specific substances (irons)

opened or closed by gates

Facilitated diffusion through channels

Classification of the channels (according to the gating mechanism):

Voltage gated channel

Chemically gated channel

Mechanically gated channel

Voltage-gated Channel

The molecular conformation of the gate responds to the electrical potential across the cell membrane

Voltage-gated (or dependent) channel.

Voltage-gated Na+ Channels Many flavors

nerves, glia, heart, skeletal muscle

Primary role:

action potential initiation

Multi-subunit channels (~300 kDa)

Skeletal Na+ Channel: 1 (260 kDa) and 1 (36kDa)

Nerve Na+ Channel: 112 (33 kDa)

gating/permeation machinery in 1 subunits

Three types of conformational states (close,

open or activation, inactivation) - each

controlled by membrane voltage

CO2H

Outside

NH2

+++

+++

+++

+++

Inside

Na+ Channel -Subunit Structure

I II III IV

RVIRLARIGRILRLIKGAKGIR+ + + + + + + +

IFM

IFM

- Inactivation “Gate” IVS4 Voltage Sensor

NH2

CO

2 H

I 异亮氨酸； F 苯丙氨酸； M 甲硫氨酸

Mechanism of voltage-gated ion channels work

movement of the voltage sensor generates a gating current

S4 transmembrane segment may be voltage sensor

pore formed by a nonhelical region between helix 5 and 6 (postulated to form sheets)

inactivation gate is in the cytoplasm

Molecular structure

• A large α subunits of 260 kDa and smaller βsubunits of 30–40 kDa

Tetrodotoxin (TTX ，河豚毒 ) selectively block the voltage-gated Na+ channel

Na+ Channel Conformations

Conductingconformation

Non-conductingconformation(s)

(at negative potentials)(shortly after more depolarized potentials)

Another Non-conductingconformation

(a while after moredepolarized potentials)

IFM IFM

IFM

Closed Open InactivatedOutside

Inside

Chemically-Gated Ion Channel

Some protein channel gates are opened by the binding of another molecule with the protein

causes a conformational change in the protein molecule that opens or closes the gate.

chemical gating.

chemically-gated (or dependent) channel

Ligand 配体 -Operated ACh （乙酰胆碱） Channels

Ion channel runs through receptor. Receptor has 5

polypeptide subunits that enclose ion channel.

2 subunits contain ACh binding sites.

Ligand-Operated ACh Channels

Channel opens when both sites bind to ACh.Permits diffusion of Na+

into and K+ out of postsynaptic cell.

Inward flow of Na+ dominates .Produces EPSPs.

Ligand-Operated ACh Channels

Mechanically-gated channel

Some protein channel gates may be opened by the mechanical deformation of the cell membrane. mechanically-gated channel.

It plays a very important role in the genesis of excitation of the hair cells

Organ of Corti

When sound waves move the basilar membrane it moves the hair cells that are connected to it,

but the tips of the hair cells are connected to the tectorial membrane the hair cell get bent .

There are little mechanical gates on each hair cell that open when they are bent.

K+ goes into the cell and Depolarizes the hair cell. (concentration of K+ in the endolymph is very high)

Water Channel

The structure of aquaporin

(AQP)

Part 2 Membrane Resting Potential

A constant potential difference across the resting cell membrane

Enable the cell ability to fire an action potential （动作电位）

Determine the basic signaling properties of neurons

Membrane Resting Potential

The membrane potential results from a separation of positive and negative charges across the cell membrane.

Membrane Resting Potential

excess of positive charges outside and negative charges inside the membrane

maintained because the lipid bilayer acts as a barrier to the diffusion of ions

an electrical potential difference

ranges from about 60 to 70 mV

Potentiometer

Concept of Resting Potential (RP)

A potential difference across the cell membrane at the rest stage or when the cell is not stimulated.

Property: constant or stable negative inside relative to the outside different in different cells.

Ion Channels

Two Types of Ion Channels

Gated

Non-Gated

Resting Membrane Potential

Na+ and Cl- are more concentrated outside the cell

K+ and organic anions (organic acids and proteins) are more concentrated inside.

Intracellular vs extracellular ion concentrations

Ion Intracellular Extracellular

Na+ 5-15 mM 145 mMK+ 140 mM 5 mMMg2+ 0.5 mM 1-2 mMCa2+ 10-7 mM 1-2 mMH+ 10-7.2 M (pH 7.2) 10-7.4 M (pH 7.4)

Cl- 5-15 mM 110 mM

Resting Membrane Potential

Potassium ions, concentrated inside the cell tend to move outward down their concentration gradient through nongated potassium channels

the relative excess of negative charge inside the membrane tend to push potassium ions out of the cell

Potassium equilibrium-90 mV

Resting Membrane Potential

• But what about sodium?• Electrostatic and Chemical forces act together on

Na+ ions to drive them into the cell • The Na+ channel close during the resting state

Na+ is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient

Na+ is driven into the cell by the electrical potential difference across the membrane.

Na+ electrochemical gradient

Equilibrium Potentials 平衡电位Theoretical voltage

produced across the membrane if only one kind of ion could diffuse through the membrane.

If membrane only permeable to K+, K+ diffuses until [K+] is at equilibrium.Force of electrical

attraction and diffusion are = opposite.

Calculating equilibrium potential

Nernst Equation

Allows theoretical membrane potential to be calculated for particular ion.Membrane potential that would exactly balance

the diffusion gradient and prevent the net movement of a particular ion.

Value depends on the ratio of [ion] on the 2 sides of the membrane.

Nernst equation

Equilibrium potential (mV) , Eion = lnRTzF

[C]o

[C]i

where,[C]o and [C]i = extra and intracellular [ion] R = Universal gas constant (8.3 joules.K-1.mol-1)T = Absolute temperature (°K)F = Faraday constant (96,500 coulombs.mol-1)z = Charge of ion (Na+ = +1, Ca2+ = +2, Cl- = -1)

For K+, with [K+]o = 4 mmol.l-1 and [K+]i = 150 mmol.l-1

At 37°C, EK = -97mV ENa = +60mv

1 10 100Extracellular potassium concentration (millimoles)

Membranepotential

(millivolts)

-130

-60

+10

(Red line shows valuesaccording to Nernst equation)

Experimental points

-70

5

[K+]o = 4 mmol.l-1

Resting Membrane Potential

less than Ek because some Na+ can also enter the cell.The slow rate of Na+ influx is accompanied by

slow rate of K+ outflux.Depends upon 2 factors:

Ratio of the concentrations of each ion on the 2 sides of the plasma membrane.

Specific permeability of membrane to each different ion.

ranges from - 65 to – 85 mV.

The Sodium-Potassium Pump钠 / 钾泵

• Dissipation of ionic gradients is ultimately prevented by Na+-K+ pumps

extrudes Na+ from the cell while taking in K

Resting Potential

Factors that affect resting potential

Difference of K+ ion concentration across the membrane

Permeability of the membrane to Na+ and K+.

Action of Na+ pump

Basic Electrophysiological Terms I:

Polarization 激化 : a state in which membrane is polarized at rest, negative inside and positive outside.

Depolarization 去极化 : the membrane potential becomes less negative than the resting potential (close to zero).

Hyperpolarization 超级化 : the membrane potential is more negative than the resting level.

Basic Electrophysiological Terms I:

Reverspolarization 反激化 : a reversal of membrane potential polarity.

The inside of a cell becomes positive relative to the outside.

Repolarization 复极 : restoration of normal polarization state of membrane.

the membrane potential returns from depolarized level to the normal resting membrane value.

Part 3 Action Potential 动作电位

Successive Stages:

(1) Resting Stage

(2) Depolarization stage

(3) Repolarization stage

(4) After-potential stage

(1)

(2) (3)

(4)

Concept

Action potential is a rapid, reversible 可逆转 , and conductive change of the membrane potential after the cell is stimulated.

Nerve signals are transmitted by action potentials.

Action Potential

Sequence

• Voltage-gated Na+ Channels open and Na+ rushes into the cell

Action Potential

Sequence

• At about +30 mV, Sodium channels close, but voltage-gated potassium channels open, causing an outflow of potassium, down its electrochemical gradient

Action Potential

Sequence

equilibrium potential of the cell is restored

Action Potential

Sequence

• The Sodium – Potassium Pump is left to clean up the mess…

Ion Permeability during the APIon Permeability during the AP

Figure 8-12: Refractory periods

Basic Electrophysiological Terms II (1)

Excitability 兴奋性 : The ability of the cell to generate the action potential

Excitable cells 可兴奋细胞 : Cells that generate action potential during excitation. in excitable cells (muscle, nerve, secretary cells), the

action potential is the marker of excitation.

Some scholars even suggest that in excitable cells, action potential is identical to the excitation.

Basic Electrophysiological Terms II (2)

Stimulus 刺激 : a sudden change of the (internal or external) environmental condition of the cell. includes physical and chemical stimulus. The electrical stimulus is often used for the

physiological research. Threshold 阈值 (intensity): the lowest or

minimal intensity of stimulus to elicit an action potential(Three factors of the stimulation: intensity,

duration, rate of intensity change)

Basic Electrophysiological Terms II (3)

Types of stimulus:

Threshold stimulus 阈刺激 : The stimulus with the intensity equal to threshold

Subthreshold stimulus 阈下刺激 : The stimulus with the intensity weaker than the threshold

Suprathreshold stimulus 阈上刺激 : The stimulus with the intensity greater than the threshold.

Action Potential Summary

Reduction in membrane potential (depolarization) to "threshold" level leads to opening of Na+ channels, allowing Na+ to enter the cell

Interior becomes positive The Na+ channels then close automatically

followed by a period of inactivation. K+ channels open, K+ leaves the cell and the

interior again becomes negative. Process lasts about 1/1000th of a second.

Properties of the Action Potential

“All or none” phenomenon with constant amplitude, time course and

propagation velocity in one cell.

Propagation Transmitted in both direction in a nerve

fiber

Squid giant axon

Gated channel states

CO2H

Outside

NH2

+++

+++

+++

+++

Inside

Na+ Channel -Subunit Structure

I II III IV

RVIRLARIGRILRLIKGAKGIR+ + + + + + + +

IFM

IFM

- Inactivation “Gate” IVS4 Voltage Sensor

NH2

CO

2 H

Voltage gated

But “ready” Not “ready”

Activation & Fast Inactivation

Sodium Activation and Inactivation Sodium Activation and Inactivation Variable vs VoltageVariable vs Voltage

Activation GateActivation Gate Inactivation gateInactivation gate

If resting potential depolarized by 15 – 20 mV, then activation gate opened with 5000x increase in Na+ permeability followed by inactivation gate close 1 ms later

Positive feedback loop

Reach “threshold”?

If YES, then...

Stimulation

Action potential initiation

S.I.Z.

Action potential termination

Threshold Potential 阈电位 plays a key role in the genesis of action potential.

a critical membrane potential level at which an action potential can occur.

dependent on the gating property of the voltage-gated Na+ channels.

most excitable cell ： about 15 to 20 mV less negative than the resting potential.

The threshold stimulus is just strong enough to depolarize the membrane to the threshold potential level, therefore it can cause an action potential.

Electrophysiological Method to Record Membrane Potential I

Voltage Clamp 电压钳

Th

e Nob

el Prize in

Ph

ysiology or Med

icine (1963)

“for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane”

Alan Lloyd Hodgkin Andrew Fielding Huxley

a method for maintaining Vm at any desired voltage level (FBA, Feedback Amplifier)

The voltage clamp

The Hodgkin-Huxley Model of Action Potential Generation

Triphasic response

Evidence for a Sodium Current

Remove extracellular sodium

Modern proof of

nature of currents

Use ion selective agents

Removing Na+ from the bathing medium, INa becomes negligible so IK can be measured directly.

Subtracting this current from the total current yielded INa.

Conductance of Na+ and K+ channels

Voltage-Dependence of Conductance

An action potential gNa increases quickly, but then inactivation kicks in and it decreases again.

gK increases more slowly, and only decreases once the voltage has decreased.

The Na+ current is autocatalytic. An increase in V increases gNa , which increases the Na+ current, and increases V, etc.

Hence, the threshold for action potential initiation is where the inward Na+ current exactly balances the outward K+ current.

"for their discoveries concerning the function of single ion channels in cells"

Th

e Nob

el Prize in

Ph

ysiology or Med

icine (1991)

Erwin Neher Bert Sakmann

Cytoplasm Ion channels

"Giga-seal"

Glassmicroelectrode

Suction

1 µm

Patch clamp 膜片钳 recording

Cell Membrane

100 ms

4 pA

Closed

Open

Single channel record

Voltage-dependent Channel Conductance

How channel conductances accumulate

Next page shows an idealized version

Inactivating Na+ channel currents

Part 3 Nerve Conduction

• Local Response and Summation

• Nerve Conduction

Local Response Definition:

a small change in membrane potential caused by a subthreshold stimulus

Properties: a graded potential Propagation: electronic conduction be summed by two ways

Spatial summation Temporal summation

Graded (local)

potential changes

2 x more chemical= 2 x more potential

change

Excitatory

Excitatory

Inhibitory

Time

Mem

bra

ne P

ote

nti

al (

mV

)

Spatial Summation

Spatial Summation

a

b

c

d

a

b

c

d

Excitatory

Excitatory

Inhibitory

Time

Mem

bra

ne P

ote

nti

al (

mV

)

Temporal & Spatial Summation

Temporal Summation

a

b

c

d

a

b

c

d

Distribution of channels

Leak channels everywhere

Axon Hillock(Trigger Zone)

Role of the Local Potential

Facilitate the cell. increase excitability of the stimulated

cell

Cause the cell to excite once it is summed to reach the threshold potential

Propagation of the Action Potential

Myelinated neuron of the central nervous system

Saltatory conduction 跳跃式传导 : The action potential jumps from node to node

Saltatory Conduction

Saltatory Conduction

The pattern of conduction in the myelinated nerve fiber from node to node

It is of value for two reasons: very fast conserves energy.

Factors that affect the propagation

Bioelectric properties of the membrane

Velocity and amplitude of membrane depolarization

Part 4 Synaptic Transmission

• Chemical synapse (Classical Synapse)– Predominates in the vertebrate nervous

system

• 2.Non-synaptic chemical transmission• 3.Electrical synapse

– Via specialized gap junctions– Does occur, but rare in vertebrate NS– Astrocytes can communicate via gap

junctions

Chemical Synapse

• Terminal bouton is separated from postsynaptic cell by synaptic cleft.

• Vesicles fuse with axon membrane and NT released by exocytosis.

• Amount of NTs released depends upon frequency of AP.

Non-synaptic chemical transmission

The postganglionic neurons innervate the smooth muscles.

No recognizable endplates or other postsynaptic specializations;

The multiple branches are beaded with enlargements (varicosities) that are not covered by Schwann cells and contain synaptic vesicles;

Fig. : Ending of postganglionic autonomic neurons on smooth muscle

In noradrenergic neurons, the varicosities are about 5m, with up to 20,000 varicosities per neuron;

Transmitter is apparently released at each varicosity, at many locations along each axon;

One neuron innervate many effector cells.

Fig. : Ending of postganglionic autonomic neurons on smooth muscle

Non-synaptic chemical transmission continued

Electrical Synapse• Impulses can be regenerated

without interruption in adjacent cells.

• Gap junctions:– Adjacent cells electrically

coupled through a channel.

– Each gap junction is composed of 12 connexin proteins.

• Examples:– Smooth and cardiac

muscles, brain, and glial cells.

•Electric current flow- communication takes place by flow of electric current directly from one neuron to the other

•No synaptic cleft or vesicles cell membranes in direct contact

•Communication not polarized- electric current can flow between cells in either direction

Electrical Synapses

Electrical Synapse Chemical SynapsePurves, 2001

The Chemical Synapse and Signal Transmission

• The chemical synapse is a specialized junction that transfers nerve impulse information from a pre synaptic membrane to a postsynaptic membrane using neurotransmitters and enzymes

Synaptic connections

• ~100,000,000,000 neurons in human brain

• Each neuron contacts ~1000 cells

• Forms ~10,000 connections/cell

• How many synapses?

•Neurotransmitter- communication via a chemical intermediary called a neurotransmitter, released from one neuron and influences another

•Synaptic cleft- a small gap between the sending (presynaptic) and the receiving (postsynaptic) site

Chemical Synapses

•Synaptic vesicles- small spherical or oval organelles contain chemical transmitter used in transmission

•Polarization- communication occurs in only one direction, from sending presynaptic site, to receiving postsynaptic site

Chemical Synapses

• Precursor transport

• NT synthesis

• Storage

• Release

• Activation

• Termination ~diffusion, degradation, uptake, autoreceptors

1. Synaptic Transmission Model

PresynapticAxon Terminal

PostsynapticMembrane

Terminal Button Dendritic

Spine

(1) Precursor Transport

_ _ _

NT

(2) Synthesis

enzymes/cofactors

(3) Storage

in vesicles

Synapse

Vesicles

NT

Terminal Button Dendritic

Spine

Synapse

Terminal Button Dendritic

Spine

(4) Release

Receptors

Synapse

Terminal Button Dendritic

Spine

AP

Ca2+

Exocytosis

Each vesicle contains one quanta of neurotransmitter (approximately 5000 molecules) – quanta release

(5) Activation

(6) Termination

(6.1) Termination by... Diffusion

(6.2) Termination by...Enzymatic degradation

(6.3) Termination by... Reuptake

(6.4) Termination by... Autoreceptors

A

Autoreceptors

• On presynaptic terminal

• Binds NT

same as postsynaptic receptors

different receptor subtype

• Decreases NT release & synthesis

• Metabotropic receptors

Synaptic Transmission

• AP travels down axon to bouton.• VG Ca2+ channels open.

– Ca2+ enters bouton down concentration gradient.

– Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs.

• Ca2+ activates calmodulin, which activates protein kinase.

• Protein kinase phosphorylates synapsins.– Synapsins aid in the fusion of synaptic vesicles.

Synaptic Transmission (continued)

• NTs are released and diffuse across synaptic cleft.

• NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane.

• Chemically-regulated gated ion channels open.– EPSP: depolarization.– IPSP: hyperpolarization.

• Neurotransmitter inactivated to end transmission.

2 EPSP and IPSP

(1)Excitatory postsynaptic

potential (EPSP)An AP arriving in the presynaptic terminal cause the release of neurotransmitter;The molecules bind and active receptor on the postsynaptic membrane;

(1)Excitatory postsynaptic

potential (EPSP)Opening transmitter-gated ions channels ( Na+) in postsynaptic- membrane;Both an electrical and a concentration gradient driving Na+ into the cell; The postsynaptic membrane will become depolarized(EPSP).

EPSP• No threshold.• Decreases resting

membrane potential.– Closer to threshold.

• Graded in magnitude.

• Have no refractory period.

• Can summate.

(2) Inhibitory postsynaptic potential (IPSP)

• A impulse arriving in the presynaptic terminal causes the release of neurotransmitter;

•The molecular bind and active receptors on the postsynaptic membrane open CI- or, sometimes K+ channels;

• More CI- enters, K+ outer the cell, producing a hyperpolarization in the postsynaptic membrane.

•(IPSPs):–No threshold.

–Hyperpolarize postsynaptic membrane.

–Increase membrane potential.

–Can summate.

–No refractory period.

3 Synaptic Inhibition

• Presynaptic inhibition:– Amount of

excitatory NT released is decreased by effects of second neuron, whose axon makes synapses with first neuron’s axon.

• Postsynaptic inhibition

Concept: effect of inhibitory synapses on

the postsynaptic membrane. Mechanism: IPSP, inhibitory interneuron Types:

Afferent collateral inhibition( reciprocal

inhibition)

Recurrent inhibition.

(1) Postsynaptic inhibition

1) Reciprocal inhibition

Activity in the afferent fibers from the muscle spindles (stretch receptors) excites (EPSPs) directly the motor neurons supplying the muscle from which the impulses come.

Postsynaptic inhibition

At the same time, inhibits (ISPSs) those motor neurons supplying its antagonistic muscles.

1) Reciprocal inhibition

The latter response is mediated by branches of the afferent fibers that end on the interneurons.

Postsynaptic inhibition

The interneurons, in turn, secrete the inhibitory transmitter (IPSP) at synapses on the proximal dendrites or cell bodies of the motor neurons that supply the antagonist.

Neurons may also inhibit

themselves in a negative feedback

fashion.

Each spinal motor neuron regularly

gives off a recurrent collateral that

synapses with an inhibitory

interneuron which terminates on

the cell body of the spinal neuron

and other spinal motor neurons.

The inhibitory interneuron to

secrete inhibitory mediator, slows

and stops the discharge of the

motor neuron.

2) Recurrent inhibition

Postsynaptic inhibition

Concept: the inhibition occurs at the

presynaptic terminals before the

signal ever reaches the synapse.

The basic structure: an axon-axon

synapse (presynaptic synapse), A

and B.

Neuron A has no direct effect on

neuron C, but it exert a

Presynaptic effect on ability of B

to Influence C.

The presynatic effect May decrease

the amount of neuro- transmitter

released from B (Presynaptic

inhibition) or increase it

(presynaptic facilitation).

(2) Presynaptic inhibition

AAB

C

A

C

B

The mechanisms:

• Activation of the presynaptic receptors increases CI- conductance,

to decrease the size of the AP reaching the excitatory ending,

reduces Ca2+ entry and consequently the amount of excitatory transmitter decreased.

Presynaptic inhibition

• Voltage-gated K+ channels are also opened, and the resulting K+ efflux also decreases the Ca2+ influx.

Excitatory Synapse

• A active

• B more likely to fire

• Add a 3d neuron ~

A B+

Presynaptic Inhibition

Excitatory Synapse

• Axon-axon synapse

• C is inhibitory ~

A B+

Presynaptic Inhibition

C

-

Excitatory Synapse

A B+

Presynaptic Inhibition

C

-

• C active

• less NT from A when active

• B less likely to fire ~

4 Synaptic Facilitation: Presynaptic and Postsynaptic

Excitatory Synapse

• A active

• B more likely to fire ~

A B+

(1) Presynaptic Facilitation

Excitatory Synapse

A B+

Presynaptic Facilitation

C

+

• C active (excitatory)• more NT from A when

active (Mechanism:AP of A is prolonged and Ca 2+ channels are open for a longer period.)

• B more likely to fire ~

(2) Postsynaptic facilitation: neuron that has been partially depolarized is more likely to undergo AP.

-65mv

- 70mv AT REST

Vm

Time

EPSP

+

-

• Depolarization

more likely to fire ~

Record here

+

5 Synaptic Integration

• EPSPs can summate, producing AP.– Spatial summation:

• Numerous PSP converge on a single postsynaptic neuron (distance).

– Temporal summation: • Successive waves of

neurotransmitter release (time).

(1) Spatial Summation

• The accumulation of neurotransmitter in the synapse due the combined activity of several presynaptic neurons entering the Area (Space) of a Convergent Synapse.

• A space (spatial) dependent process.

-65mv

- 70mv AT REST

vm

Time

+

-

SpatialSummation +

• Multiple synapses

+

(2) Temporal Summation

• The accumulation of neurotransmitters in a synapse due to the rapid activity of a presynaptic neuron over a given Time period.

• Occurs in a Divergent Synapse. (explain later)

• Is a Time (Temporal) dependent process.

-65mv

- 70mv AT REST

Vm

Time

+

-

TemporalSummation

+

Repeated stimulation same synapse ~

(3) EPSPs & IPSPs summate

• CANCEL EACH OTHER

• Net stimulation – EPSPs + IPSPs = net effects ~

- 70mv

+

-

-

EPSP

IPSP

+

6. Divergent and Convergent Synapse

Divergent Synapse

•A junction that occurs between a presynaptic neuron and two or more postsynaptic neurons (ratio of pre to post is less than one).

•The stimulation of the postsynaptic neurons depends on temporal summation).

Convergent Synapse

Presynaptic neurons

Postsynaptic neuron

•A junction between two or more presynaptic neurons with a postsynaptic neuron (the ratio of pre to post is greater than one).

•The stimulation of the postsynaptic neuron depends on the Spatial Summation.

Section 3 Neurotransmitters and Receptors

1. Basic Concepts of NT and receptor

Neurotransmitter: Endogenous signaling molecules that alter the behaviour of neurons or effector cells.

Neuroreceptor: Proteins on the cell membrane or in the cytoplasm that could bind with specific neurotransmitters and alter the behavior of neurons of effector cells

•Vast array of molecules serve as neurotransmitters

•The properties of the transmitter do not determine its effects on the postsynaptic cells

•The properties of the receptor determine whether a transmitter is excitatory or inhibitory

A neurotransmitter must (classical definition)

• Be synthesized and released from neurons• Be found at the presynaptic terminal• Have same effect on target cell when applied externally• Be blocked by same drugs that block synaptic transmission• Be removed in a specific way

Purves, 2001

Synaptic vesicles

• Concentrate and protect transmitter

• Can be docked at active zone

• Differ for classical transmitters (small, clear-core) vs. neuropeptides (large, dense-core)

Classical Transmitters (small-molecule transmitters)•Biogenic Amines

•Acetylcholine

•Catecholamines

•Dopamine

•Norepinerphrine

•Epinephrine

•Serotonin

•Amino Acids

•Glutamate

•GABA (-amino butyric acid)

•Glycine

•Neuropeptides

•Neurotrophins

•Gaseous messengers

–Nitric oxide

–Carbon Monoxide

•D-serine

Non-classical Transmitters

Peripheral Neurotransmitters

• Acetylcholine and Norepinephrine

• All preganglionic neurons are cholinergic

• Parasympathetic post ganglionic neurons are cholinergic

• Sympathetic post ganglionic neurons are adrenergic except

• Sympathetics innervating sweat glands, blood vessels in skeletal muscle, and piloerection muscles are cholinergic

Nerve Fibers that release Ach and NE in PNS

ACh AChSweatglands

Striatedmuscle

AChSOMATIC NERVOUS SYSTEM

HeartSm. mus.Glands

ACh AChParasympathetic

ACh E, NE

Ad. M.

HeartSm. mus.Glands

ACh NE

AUTONOMIC NERVOUS SYSTEM

Sympathetic

Neurotransmitters in CNS

• Major Excitatory and Inhibitory Neurotransmitters– Excitatory: glutamate– Inhibitory: γ-aminobutyric acid (GABA)

• Neuromodulators– NE, 5-HT, serotonin, peptides

• Gastrous Neurotransmitter– NO, CO, H2S

Neurotransmitter Co-existence (Dale principle)Some neurons in both the PNS and CNS produce both a classical neurotransmitter (ACh or a catecholamine) and a polypeptide neurotransmitter.

They are contained in different synaptic vesicles that can be distinguished using the electron microscope.

The neuron can thus release either the classical neurotransmitter or the polypeptide neurotransmitter under different conditions.

Purves, 2001

Agonist

A substance that mimics a specific neurotransmitter,

is able to attach to that neurotransmitter's receptor

and thereby produces the same action that the neurotransmitter usually produces.

Drugs are often designed as receptor agonists to treat a variety of diseases and disorders when the original chemical substance is missing or depleted.

Antagonist

Drugs that bind to but do not activate neuroreceptors,

thereby blocking the actions of neurotransmitters or the neuroreceptor agonists.

Receptor BReceptor A

• Same NT can bind to different -R

• different part of NT ~

NT

NT

Specificity of drugs

Drug ADrug B

Receptor BReceptor A

Five key steps in neurotransmission

• Synthesis

• Storage

• Release

• Receptor Binding

• Inactivation

Purves, 2001

Receptors determine whether:• Synapse is excitatory or inhibitory

– NE is excitatory at some synapses, inhibitory at others

• Transmitter binding activates ion channel directly or indirectly.– Directly

• ionotropic receptors• fast

– Indirectly• metabotropic receptors• G-protein coupled• slow

2. Receptor Activation

• Ionotropic channel– directly controls channel– fast

• Metabotropic channel– second messenger systems– receptor indirectly controls channel ~

(1) Ionotropic ChannelsneurotransmitterNTChannel

Ionotropic Channels

NT

Pore

Ionotropic Channels

NT

Ionotropic Channels

NT

(2) Metabotropic Channels

• Receptor separate from channel

• G proteins

• 2d messenger system– cAMP– other types

• Effects– Control channel– Alter properties of receptors– regulation of gene expression ~

(2.1) G protein: direct control

• NT is 1st messenger

• G protein binds to channel– opens or closes– relatively fast ~

G protein: direct control

RG

GDP

G protein: direct control

RG

GTP

Pore

(2.2) G protein: Protein Phosphorylation

Receptor

trans-ducer

primaryeffector

external signal: nt

2d messenger

secondary effector

Receptor

trans-ducer

primaryeffector

external signal: NT

2d messenger

secondary effector

GS

norepinephrine

cAMP

protein kinase

adrenergic -R

adenylylcyclase

G protein: Protein Phosphorylation

RG

GDP

AC

PK

G protein: Protein Phosphorylation

R

AC

PK

G

GTPATP

cAMP

G protein: Protein Phosphorylation

R

AC

PK

G

GTPATP

cAMP

P

Pore

(3) Transmitter Inactivation

• Reuptake by presynaptic terminal

• Uptake by glial cells

• Enzymatic degradation

• Presynaptic receptor

• Diffusion

• Combination of above

Neurotransmitter Reuptake by Na+ dependent (linked) transporters

Summary of Synaptic

Transmission

Purves,2001

Basic Neurochemistry

Receptor Desensitization

• Phosphorylation of the receptor and combined by arrestin

• Receptor endocytosis (internalization)

GPLR 的失敏：

例：肾上腺素受体被激活后， 10-15 秒 cAMP 骤增，然后在不到 1min 内反应速降，以至消失。

受体活性快速丧失（速发相） --- 失敏（ desensitization ）； 机制：受体磷酸化 受体与 Gs 解偶联， cAMP 反应停止并被 PDE 降解。 两种 Ser/Thr 磷酸化激酶： PKA 和肾上腺素受体激酶（ ARK ）， 负责受体磷酸化； 胞内协作因子扑获蛋白（ arrestin ） --- 结合磷酸化的受体，抑制其功能

活性（ arrestin 已克隆、定位 11q13 ）。 反应减弱（迟发相） --- 减量调节（ down-regulation ） 机制：受体 - 配体复合物内吞，导致表面受体数量减少，发现

arrestin 可直接与 Clathrin 结合，在内吞中起 adeptors 作用； 受体减量调节与内吞后受体的分选有关。

3. Some Important Transmitters

(1) Acetylcholine (ACh) as NT

Acetylcholine Synthesis

choline + acetyl CoA ACh + CoA

cholineacetyltransferase

Acetylcholinesterase (AChE)

• Enzyme that inactivates ACh.

– Present on postsynaptic membrane or immediately outside the membrane.

• Prevents continued stimulation.

The Life Cycle of Ach

Ach - Distribution

• Peripheral N.S.• Excites somatic skeletal muscle (neuro-muscular

junction)• Autonomic NS

Ganglia

Parasympathetic NS--- Neuroeffector junction

Few sympathetic NS – Neuroeffector junction

• Central N.S. - widespreadHippocampus

Hypothalamus ~

•ACh is both an excitatory and inhibitory NT, depending on organ involved.

–Causes the opening of chemical gated ion channels.

•Nicotinic ACh receptors:

–Found in autonomic ganglia (N1) and skeletal muscle fibers (N2).

•Muscarinic ACh receptors:

–Found in the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands .

Ach Receptors

Acetylcholine Neurotransmission

• “Nicotinic” subtype Receptor:– Membrane Channel for Na+ and K+

– Opens on ligand binding– Depolarization of target (neuron, muscle)– Stimulated by Nicotine, etc.– Blocked by Curare, etc.– Motor endplate (somatic) (N2), – all autonomic ganglia, hormone

producing cells of adrenal medulla (N1)

Acetylcholine Neurotransmission

• “Muscarinic” subtype Receptor: M1

– Use of signal transduction system

• Phospholipase C, IP3, DAG, cytosolic Ca++

– Effect on target: cell specific (heart , smooth muscle intestine )

– Blocked by Atropine, etc.– All parasympathetic target organs– Some sympathetic targets (endocrine sweat

glands, skeletal muscle blood vessels - dilation)

Acetylcholine Neurotransmission

• “Muscarinic” subtype: M2

– Use of signal transduction system• via G-proteins, opens K+ channels, decrease

in cAMP levels– Effect on target: cell specific– CNS – Stimulated by ?– Blocked by Atropine, etc.

Cholinergic Agonists

• Direct– Muscarine – Nicotine

• Indirect– AChE Inhibitors ~

Cholinergic Antagonists

• Direct

Nicotinic - Curare

Muscarinic - Atropine

Ligand-Operated ACh Channels

N Receptor

G Protein-Operated ACh ChannelM receptor

(2) Monoamines as NT

Monoamines

• Catecholamines –

Dopamine - DA

Norepinephrine - NE

Epinephrine - E

• Indolamines - Serotonin - 5-HT

Mechanism of Action ( receptor)

Epi1

G protein

PLC IP3

Ca+2

Norepinephrine (NE) as NT

• NT in both PNS and CNS.

• PNS: – Smooth muscles, cardiac muscle and glands.

• Increase in blood pressure, constriction of arteries.

• CNS:– General behavior.

Adrenergic Neurotransmission

1 Receptor– Stimulated by NE, E, – blood vessels of skin, mucosa, abdominal

viscera, kidneys, salivary glands – vasoconstriction, sphincter constriction, pupil

dilation

Adrenergic Neurotransmission2 Receptor

– stimulated by, NE, E, …..– Membrane of adrenergic axon terminals (pre-

synaptic receptors), platelets– inhibition of NE release (autoreceptor), – promotes blood clotting, pancreas decreased

insulin secretion

Adrenergic Neurotransmission

• 1 receptor– stimulated by E, ….– Mainly heart muscle cells, – increased heart rate and strength

Adrenergic Neurotransmission

• 2 receptor– stimulated by E ..– Lungs, most other sympathetic organs, blood

vessels serving the heart (coronary vessels),– dilation of bronchioles & blood vessels

(coronary vessels), relaxation of smooth muscle in GI tract and pregnant uterus

Adrenergic Neurotransmission

• 3 receptor– stimulated by E, …. – Adipose tissue, – stimulation of lipolysis

(3) Amino Acids as NT

• Glutamate acid and aspartate acid:– Excitatory Amino Acid (EAA)

• gamma-amino-butyric acid (GABA) and glycine:– Inhibitory AA

(4) Polypeptides as NT

• CCK:– Promote satiety following meals.

• Substance P:– Major NT in sensations of pain.

(5) Monoxide Gas: NO and CO

• Nitric Oxide (NO)– Exerts its effects by stimulation of cGMP.– Involved in memory and learning. – Smooth muscle relaxation.

• Carbon monoxide (CO):– Stimulate production of cGMP within neurons.– Promotes odor adaptation in olfactory neurons.– May be involved in neuroendocrine regulation in

hypothalamus.