Synapse

Synapse

Chemical

Electrical

Neuromuscular Junction

What is a synapse?  A synapse is the junction between 2 neurones.  There is a very narrow gap of about 20nm between neurones called the synaptic cleft.  An action potential cannot cross the synaptic cleft, so nerve impulses are carried by chemicals called neurotransmitters.

A Synapse  Pre-synaptic neurone = neurone sending impulse  Post-synaptic neurone = neurone receiving impulse

Neurotransmitter

 Neurotransmitter is made by the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon.  The membrane of the postsynaptic neurone has chemicalgated ion channels called neuroreceptors. These have specific binding sites for neurotransmitters.

Synapse Brain consist of 100~130 billion neurons & place their meet & transmit AP known as synapse Type of synapse: 1. Electrical  involve Na+ 2. Chemical  involve neurotransmitter (NT) 1. excitatory synapse EPSP 2. inhibitory synapse IPSP

electrical synapse

Gap Junction

Protein channel

Chemical synapse

Types of synaptic 1. Axodendritic  axon & dendrite. 2. Axosomatic  axon & soma. 3. Axoaxonal axon & axon. 4. Denrodenritic dendrite & dendrite.

Type of Synapse 4 1 2 3

rest

NT release

Re-uptake

Transmission event at Pre-synapse 1 Impulse from somatic nervous system (A.P) 2  permeability of pre-synapse to Ca2+ 3 entry of Ca2+ into pre-synapse 4 Ca2+ will initial the phosphorylation of synapsin I than the synapsin I will dissociated from vesicles.  The vesicles will release the neurotransmitter into cleft synapse by exocytosis process.  At rest the synapsin I (protein) will bind strongly with vesicle that prevent dissociation of vesicle. 5. NT release into the cleft synapse 6. NT move to post-synaptic membrane and bind on the receptor.

Excitatory Synapse 7. NT bind to receptor cause depolarization known as Excitatory Post Synaptic Potential / EPSP cause by entry of Na+, Ca2+ &  K+ efflux

8. EPSP  reach the threshold value active Na+ & K+  AP transmit to next synapse  Excitatory synapse  EPSP & NT released known as excitatory NT (serotonin / 5-HT, Ach, Lglutamate, adrenalin) 9 Hydrolysis of NT  re-uptake into pre synapse to synthesis NT  stored in vesicles.

Inhibitory synapse 7. NT bind to receptor  membrane potential become hyperpolarisation  Inhibitory Post Synaptic Potential (IPSP)  cause by Cl- influx &  efflux of K+. 8. IPSP Never generate AP

NT known as inhibitory NT (Dopamine, GABA, lysine) 9. Hydrolysis of NT re-uptake  presynapse synthesis & stored in vesicles.

Excitatory Synapse (EPSP) & Inhibitory synapse (IPSP)

1

4 3

2

K+ Inhibitory synapse (IPSP)

8 IPSP  Influx of Cl- Cl(GABA) &  efflux K+ (Dopamine). 8

4 5

7

6

Excitatory synapse (EPSP) Serotonin (5HT), ACh, Glutamate 8 EPSP  influx Na+, Ca2+ & prevent efflux K+ 9. EPSP  AP

9

9. IPSP not generate AP

Excitatory synapse

Inhibitory synapse (IPSP)

EPSP  influx Na+,

Excitatory synapse (EPSP)

Ca2+ & decrease K+ efflux

Depolarization of post synaptic membrane potential  excitation

EPSP reach threshold  AP

NT: Serotonin (5-HT), ACh, Glutamate, NE.

Inhibitory Synapse (IPSP) IPSP  Influx Cl(GABA) &  efflux K+ (Dopamine).

Hyper polarization of post synaptic membrane potential inhibition

IPSP do not generate AP

Excitatory NT 5-HT

EPSP

Excitatory NT NE

Excitatory NT Glutamine

Inhibitory NT Dopamine

Slow IPSP Efflux(K+)

Fast IPSP influx (Cl-)

Inhibitory NT GABA

Characteristics of EPSP & IPSP 1. Local event & do not spread 2. No threshold and can be summed: I. temporal summation on same nerve when stimulate few times. ii. Spatial  summation on different nerve

Temporal summation

Spatial summation

EPSP

IPSP

Occlusion summation Stimulation at A

Stimulation at B

Stimulation at A & B AP 9+9 = 18

Subliminal Fringe Summation Stimulation at A

Stimulation at B

Stimulation at A & B AP 3+3 =6 but actual 9

3. Do not follow all or none event and can be graded (size of amplitude depend on strength of stimulant) 4. Ca2+ is need to release NT 5. Single EPSP can not dep. Post-neuron to achieve threshold to generate A.P. 6. Single IPSP can not hyperpolarisation on Post-neuron to inhibit A.P.

Characteristics of synaptic transmission

1. One-way  pre-neuron to post neuron. 2. Synaptic delay time need to release NT& allow NT bind at post-synapse to caused response. 3. Synaptic fatigue  failure of post-synapse in response to high rate of impulses (epilepsy stop due to exhausted of NT.

4. Post titanic facilitation  postsynaptic neuron can become more excitable when receive repetitive stimulation. 5. Permeability of membrane presynapse in increase release of NT. 6. Neuronal can be learn when the network been utilize repetively.

7. Drug effect: 1. Excitability of neuron increase  close to threshold caffeine, theophylline (tea) & theobromine (coco). 2.  Excitability of neuron cause by prevent inhibitory synapse  thus excitation is high & fast  If neuron firing repetitive will cause seizure. 3. Hypnotic & anesthetic drugs  threshold cause synapse less excite,  release & synthesis of NT. 8. Hypoxia   O2 supply to brain cause neuron death & Alkalosis (>PH 7.4) excitation  & Acidosis (