Synapses and Integration

• Junction between two presynaptic and postsynaptic neurons

• There are two types of synapse:

1. Electrical Synapses: Two neurons connected by gap junctions

2. Chemical Synapses: Chemical messenger is transmitted across the junction separating the two neurons

Fig. 4-13, p. 85

Myelinatedaxon

Cell body ofpostsynaptic neuron

Synaptic inputs(presynaptic axon terminals)

Axon terminalsCell body ofpostsynaptic neuron

Axonhillock

Dendrites

See Figure4-14

Synapses and Neuronal Integration

• Signal at synapse either excites or inhibits the postsynaptic neuron

• Two types of synapses

– Excitatory synapses

– Inhibitory synapses

Fig. 4-14, p. 86

Fig. 4-14, p. 86

Synaptic knob(presynapticaxon terminal)

Chemically gatedreceptor-channelfor Na+, K+, or Cl–

Neuro-transmittermolecule

Voltage-gatedCa2+ channel

Ca2+

Postsynaptic neuron

Subsynapticmembrane

Synapticcleft

Synapticvesicle

Axon ofpresynapticneuron

Receptor forneurotransmitter

1

2

3

4

5

3

4

5

Neurotransmitters

• Vary from synapse to synapse

• Same neurotransmitter is always released at a particular synapse

• Quickly removed from the synaptic cleft

Fig. 4-15a, p. 87

Thresholdpotential

35

(a) Excitatory synapse

Mem

bra

ne

po

ten

tial

(m

V)

in p

ost

syn

apti

c n

euro

n

Activation of synapse

EPSP

+30

5 15 45

Time (msec)

25

–70

–50

0

(b) Inhibitory synapse

Time (msec)

Mem

bra

ne

po

ten

tial

(m

V)

in p

ost

syn

apti

c n

euro

n

IPSP

Activation of synapseThresholdpotential

355 15 4525

+30

–70

–50

0

Fig. 4-15b, p. 87

Basic Synaptic Integration

• Post synaptic membrane membrane potential is affected by neurotransmitters from several presynaptic neurons simultaneously– Effects of many neurotransmitters are added together

• Adding = summation– Temporal summation: one single presynaptics increases

frequency of activity to affect post-synaptic membrane

– Spatial summation: more than one presynaptic active simultaneously

Summation: more than one pre-synaptic action potential influences the polarity of the post-synaptic neuron

Po

stsy

nap

tic

mem

bra

ne

po

ten

tial

(m

V)

(d) EPSP-IPSPcancellation

Time (msec)

–70

(a) No(a) Nosummationsummation

(b) Temporalsummation (c) Spatial

summation+30

0

–50

Fig. 4-16, p. 89

c?cb

A

Recordedat“A”

Cannot look at a membrane figure out which type of summation is occurring; can only say “summation to threshold” occurred so an action

potential was generated at point A

Any one of the could be affecting what is recorded at “A”

Basic Synaptic Integration

• Convergence and Divergence

Fig. 4-17, p. 91

Advanced Neural Integration

• Neuromodulators

• Drugs

• Axon-axon transmission

Neuropeptides as Neuromodulators

• Neuropeptides are considered neuromodulators– don’t cause the formation of EPSP or IPSP, – bring about long term changes that subtly modulate, depress or

enhance the action of neurotransmitter at the synapse

• Large molecules consisting of from 2 to 40 amino acids– Synthesized in neuronal cell body in the endoplasmic reticulum

and Golgi complex– Packaged in large, dense-core vesicles present in axon terminal

Neuropeptides as Neuromodulators

– Release mechanism uncertain• Act as autocrine or paracrine agents

– Bind to presynaptic cell to affect amount of neurotransmitter released when an action potential occurs in that presynaptic

– Bind to post synaptic cell to affect number of receptors for the neurotransmitter released

Synaptic Drug Interactions

• Possible drug actions1. Altering the synthesis, axonal transport,

storage, or release of a neurotransmitter

2. Modifying neurotransmitter interaction with the postsynaptic receptor

3. Influencing neurotransmitter reuptake or destruction

4. Replacing a deficient neurotransmitter with a substitute transmitter

• Drug Examples– Cocaine: Blocks reuptake of neurotransmitter

dopamine at presynaptic terminals, so postsynaptic neuron is active longer

– Tetanus toxin: Prevents release of inhibitory neurotransmitter GABA, affecting skeletal muscles activation

– Strychnine: Competes with inhibitory neurotransmitter glycine at postsynaptic receptor site

Synaptic Drug Interactions

Presynaptic neuron involved in normal axon to dendrite synaptic transmission (a)

b. Presynaptic neuron involved in axon-axon transmission

Axon-axon transmission: neurotransmitter released from one presynaptic (b) binds with axon of another presynapticneuron.

Binding affects activity of recipient axon (a) next time it undergoes an action potential, normal post-synaptic neuron (c) is there by influenced

x

a

c

Axon-axon transmission• Presynaptic inhibition: normal axon is

inhibited from releasing neurotransmitter

• Presynaptic facilitation: normal axon releases more neurotransmitter

End of Basic Neuron Physiology

• On to chemical messengers in general and then hormones in detail