SYNAPSE & GRADED POTENTIAL

Prof. Dr. Ayisha QureshiMBBS, MPhil

Department of Physiology

SynapseDefinition:

A synapse is a region of functional contact and anatomical differentiation between two neurons.

ORIt is a point of contact between two adjacent neurons.

• Action potentials cannot cross the synaptic cleft present between 2 neurons.

• Nerve impulse is carried by neurotransmitters which transmit the nerve impulse from one nerve cell to the next across the synapse.

• The structure of synapse consists of: – presynaptic membrane (from where neurotransmitters in vesicles

are synthesized & released) – post synaptic membrane (has neuroreceptors in the membrane) – synaptic cleft

CLASSIFICATION OF SYNAPSES:

Classification

Chemical synapse

Electrical synapse Mixed synapse

Physiological/functional

Types of Synapses:

1. Chemical Synapse (transmission thru chemicals i.e. NT)2. Electrical Synapse • Impulse conducted without release of NT• Synaptic gap only 2-3 nm• No synaptic delay• Unidirectional & Bidirectional conduction3. Mixed Synapse i.e. having both electrical & chemical regions

CLASSIFICATION OF SYNAPSES:

Anatomical classification of Synapses:1. Axo-dendritic2. Axo-somatic3. Somato-dendritic4. Dendro-dendritic5. Somato-somatic6. Reciprocal7. Serial8. Triad

STRUCTURE OF A SYNAPSE:

Structure Of a SynapseSYNAPSE= Presynaptic terminal + Synaptic cleft + Postsynaptic terminal

• Presynaptic terminal: is the first part of the membrane and is usually themembrane of the Axon terminal (not always).

(The axon terminals are also called the bouton terminaux or synaptic knob.)

The synaptic knobs have synaptic vesicles that contain the NT(neurotransmitters). The NT is produced in the soma & conducted along theaxon (anterograde flow). The NT can be inhibitory or excitatory.• Synaptic cleft or gap: is app. 20nm. It is a non-anatomical continuity

between the post and pre-synaptic ends.• Postsynaptic terminal: is the name given to the last part of the synapse.

It is usually comprised of the dendrite or the cell body on which the axon synapses.

Mechanism Of Conduction of an Impulse in a chemical synapse

• action potential reaches the PRESYNAPTIC terminal↓

• voltage-gated Ca2+ channels open↓

• influx of Ca2+

↓• synaptic vesicles fuse with the pre-synaptic membrane (exocytosis)

↓ • neurotransmitters are released into SYNAPTIC TERMINAL cross it and diffuse to

the POST-SYNAPTIC terminal↓

• neurotransmitter binds to neuroreceptor on postsynaptic membrane↓

• causes Na+ channels to open, and Na+ flows into postsynaptic membrane↓

• Graded potential is initiated↓

• neurotransmitter is broken down by specific enzymes in the synaptic cleft.

Fate of the Neurotransmitter:

The NT dissociates from the Receptor & can have either of the 3 fates: • Enzymatic Degradation: A portion of it is inactivated by the

enzymes present in high concentration at the postsynaptic membrane.

• Re-uptake of remaining NT by Pre-synaptic neuron and Re-used.

• Diffusion into the blood stream.

Fate of Neurotransmitters:

POSTSYNAPTIC POTENTIAL CHANGES AT THE CELL

MEMBRANE

What happens when the NT attaches to the receptors on the

Postsynaptic membrane?

GRADED POTENTIAL

The kind of ion thatenters the Postsynapticterminal (whether Na, Clor K) will determine thekind of Graded potentialthat will be generated:

Excitatory or Inhibitory Postsynaptic Potential

GRADED POTENTIAL• Upon opening of the ligand gated channels, e.g. sodium channels, the

positive charge carried by the Na+ spreads as a wave of depolarization through the cytoplasm (much like the ripples created by a stone tossed into a pond).

• If the wave is strong enough, then the graded potential will lead to the generation of the action potential. If not, then the graded potential will automatically die off and NO action potential will be generated.

What are the differences between Action and Graded Potential?

DIFFERENCES B/W GRADED & ACTION POTENTIAL PROPERTY GRADED POTENTIAL ACTION POTENTIAL

Triggering eventStimulus by combination of NT

with receptor leading to change in permeability

Triggered by Dep. to threshold, usually by a graded potential or AP

Ion movement producingchange in Potential

Na+, K+, Cl- or Ca2+ by various means

Sequential movement of Na+

into & K+ out of the cell by voltage gated channels

Duration Varies with stimulus duration Constant

Direction of Pot. Change Can be Dep. Or Hyperpol. Always Depolarization

Location Usually dendrites & cell body Usually axon hillock & Trigger zone

Decremental in magnitude with distance

YES No

Summation YES NO

All or None Law NO YES

Refractory Period NO YES