Objectives
By the end of the lecture, students should be able to:
• Define a synapse
• Draw and label a synapse
• Describe the structure of a synapse
• Classify synapses
• Explain how a Graded Potential is generated
• Name the different types of Graded Potential
• Differentiate between Action & Graded Potential
Synapse
Definition:
A synapse is a region of functional contact and anatomical
differentiation between two neurons.
OR
It 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.
PHYSIOLOGICAL (FUNCTIONAL) CLASSIFICATION OF SYNAPSES:
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 conduction
3.Mixed Synapse i.e. having both electrical & chemical regions
STRUCTURE OF A SYNAPSE:
Structure Of a Synapse
SYNAPSE= Presynaptic terminal + Synaptic cleft + Postsynaptic terminal
•PRESYNAPTICTERMINAL:is the first part of the synapse and is usually the membrane of the Axonterminal (how ever, there can be exceptions!)
(The axon terminals are also called the bouton terminauxor synaptic knob.)
The synaptic knobs have synaptic vesicles that contain the NT (neurotransmitters). The NT is produced in the soma & transported along the axon by anterograde flow.
(Please note: The NT can be inhibitory or excitatory!)
•SYNAPTIC CLEFT OR GAP: is app. 20nm.
•POSTSYNAPTIC TERMINAL: is the name given to the final part of the synapse. It usually comprises of the dendrite or the cell body on which
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 neuroreceptoron 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-uptakeof remaining NT by Pre-synaptic neuron and Re-used.
•Diffusion into the blood stream.
POSTSYNAPTIC POTENTIAL CHANGES AT THE CELL MEMBRANE
What happens when the NT attaches to the receptors on the Postsynaptic membrane?
GRADED POTENTIAL
A Graded potential will be generated.
The kind of ion that enters the Post synapticterminal (whether Na, ClorK) will determine the kind of Graded potential that will be generated:
It can either be an Excitatory or Inhibitory Postsynaptic Potential
Types of Graded Potential
EPSP
(Excitatory postsynaptic potential)
•Opening of Na channels
•This causes an increased positivityin the neurons
•This excitation that leads to depolarization.
•The depolarization which is called an Excitatory Postsynaptic Potential (EPSP).
IPSP
(Inhibitory postsynaptic potential)
•Opening of Chloride channels/ opening of Potassium channels.
•Chloride influx or efflux of Potassium ions.
•Both cause increased negativity inside the neuron leading to repolarization
•The repolarization is called Inhibitory Postsynaptic Potential (IPSP).
GRADED POTENTIAL
EPSP
IPSP
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
