The Synapse

Copyright © 2010 Pearson Education, Inc.

The Synapse

• A junction that mediates information transfer from one neuron:• To another neuron, or• To an effector cell (muscle, secretory…)

• Presynaptic neuron—conducts impulses toward the synapse (sending side)• Postsynaptic neuron—transmits impulses

away from the synapse (receiving side)


Types of Synapses

• Axodendritic—axon of one to dendrite of another• Axosomatic—axon of one to soma of another• Less common types:• Axoaxonic (axon to axon)• Dendrodendritic (dendrite to dendrite)• Dendrosomatic (dendrite to soma)

Copyright © 2010 Pearson Education, Inc. Figure 11.16

Dendrites

Cell body

Axon

Axodendriticsynapses

Axosomaticsynapses

Cell body (soma) ofpostsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomaticsynapses

(a)


Electrical Synapses

• Less common than chemical synapses• Neurons are electrically coupled (joined by gap

junctions)• Communication is very rapid, and may be

unidirectional or bidirectional• Are important in:• Embryonic nervous tissue• Some brain regions


Chemical Synapses

• Specialized for the release and reception of neurotransmitters• Typically composed of two parts • Axon terminal of the presynaptic neuron, which

contains synaptic vesicles • Receptor region on the postsynaptic neuron


Synaptic Cleft• Fluid-filled space between presynaptic and

postsynaptic neurons, found at chemical synapses• Transmission across the synaptic cleft• Is a chemical event (not electrical)

• Involves release, diffusion, and binding of neurotransmitters

• Provides unidirectional communication between neurons


Synaptic Transmission Summary• Action potential arrives at presynaptic neuron’s axon terminal

and opens voltage-gated calcium channels

• Calcium enters neuron terminal and causes synaptic vesicle fusion with cell membrane

• Neurotransmitter exocytosis occurs

• Neurotransmitter diffuses across cleft and binds to receptors on postsynaptic neuron

• Ion channels in membrane of post-synaptic cell open, causing excitation or inhibition (graded potential)

• Neurotransmitter diffuses away from receptors as it is broken down in the cleft and/or taken back up by pre-synaptic neuron


Action potentialarrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Postsynapticneuron

1

2

3

4

Figure 11.17, step 4

Copyright © 2010 Pearson Education, Inc. Figure 11.17, step 5

Ion movementGraded potential

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

5

Copyright © 2010 Pearson Education, Inc. Figure 11.17, step 6

Reuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Neurotransmitter effects are terminatedby reuptake through transport proteins,enzymatic degradation, or diffusion awayfrom the synapse.

6


Termination of Neurotransmitter Effects

• Neurotransmitter effect is terminated in a few milliseconds by• Degradation by enzymes• Reuptake by astrocytes or axon terminal • Diffusion away from synaptic cleft


Synaptic Delay• Neurotransmitter must be released, diffuse

across synapse, and bind to receptors• Synaptic delay = time needed to do this (0.3-5.0

ms)• Synaptic delay is the rate-limiting step of neural

transmission (in short neurons at least)


Postsynaptic Potentials

• Graded potentials• Strength determined by:• Amount of neurotransmitter released

• Time the neurotransmitter is in the area

• Types of postsynaptic potentials • EPSP—excitatory postsynaptic potentials

• IPSP—inhibitory postsynaptic potentials

Copyright © 2010 Pearson Education, Inc. Table 11.2 (1 of 4)





Excitatory Synapses and EPSPs

• Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions• Na+ influx is greater than K+ efflux, causing a net

depolarization• EPSP helps trigger AP at axon hillock if EPSP is

of threshold strength and opens the voltage-gated channels

Copyright © 2010 Pearson Education, Inc. Figure 11.18a

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas-sage of Na+ and K+.

Time (ms)(a) Excitatory postsynaptic potential (EPSP)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)


Inhibitory Synapses and IPSPs

• Neurotransmitter binds to and opens channels for K+ or Cl–

• Causes hyperpolarization (inside of cell becomes more negative)

• Reduces the postsynaptic neuron’s ability to produce an action potential

Copyright © 2010 Pearson Education, Inc. Figure 11.18b

An IPSP is a localhyperpolarization of the postsynaptic membraneand drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Time (ms)(b) Inhibitory postsynaptic potential (IPSP)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)


Integration: Summation

• One EPSP cannot induce an action potential

• EPSPs can sum to reach threshold

• IPSPs and EPSPs can cancel each other out


Integration: Summation

• Temporal summation

• One presynaptic neuron sends several or many impulses in a short time to the postsynaptic neuron

• Spatial summation

• Multiple presynaptic neurons stimulate the post-synaptic neuron simultaneously

Copyright © 2010 Pearson Education, Inc. Figure 11.19a, b

Threshold of axon ofpostsynaptic neuron

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

Resting potential

E1 E1 E1 E1

(a) No summation:2 stimuli separated in time cause EPSPs that do notadd together.

(b) Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.

Time Time

E1 E1

Copyright © 2010 Pearson Education, Inc. Figure 11.19c, d

E1 + E2 I1 E1 + I1

(d) Spatial summation ofEPSPs and IPSPs:Changes in membane potential can cancel each other out.

(c) Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.

Time Time

E1

E2 I1

E1


Neurotransmitters

• Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies

• 50 or more neurotransmitters have been identified

• Classified by chemical structure and by function


Chemical Classes of Neurotransmitters

• Acetylcholine (Ach)

• Released at neuromuscular junctions and some autonomic neurons

• Synthesized in the pre-synaptic neuron

• Degraded by acetylcholinesterase (AChE)



• Biogenic amines include:• Norepinephrine (NE)

• Epinephrine

• Serotonin

• Many others

• Broadly distributed in the brain

• Play roles in emotional behaviors and the biological clock



• Amino acids include:

• GABA—Gamma ()-aminobutyric acid

• Glutamate

• Many others


Functional Classification of Neurotransmitters• Excitatory (depolarizing) and/or inhibitory (hyperpol.)• Determined by receptor type on postsynaptic neuron

• GABA usually inhibitory

• Glutamate, epinephrine usually excitatory

• Acetylcholine

• Excitatory at neuromuscular junctions in skeletal muscle

• Inhibitory in cardiac muscle


Neurotransmitter Actions

• Direct action

• Neurotransmitter binds to channel-linked receptor and opens ion channels

• Promotes rapid responses

• Examples: ACh; glutamate and GABA at some of their synapses


Neurotransmitter Actions

• Indirect action

• Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular second messenger

• Promotes long-lasting effects

• Examples: Norepinephrine, epinephrine, serotonin; glutamate and GABA at some of their synapses

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The Synapse

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