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Synapses
Synapses are neuron junctions which transmit the electrical impulses from one neuron
to another with no direct contact between two.
What is a neuron?
Neurons are the basic data processing units of the brain.
Each neuron receives electrical inputs from about 1000 other neurons.
Impulses arriving simultaneously are added together and, if sufficiently strong, lead to
the generation of an electrical discharge, known as an action potential (a 'nerve impulse').
The action potential then forms the input to the next neuron in the network.
What is brain made up of?
The bulk of the brain is made up of structural cells : Glial cells and Astrocytes.
Neuron cells are present in these cells that conduct electrical signals.
The average human brain contain 100 billion neurons.
Each connected to 1000 neurons and each connected by other 1000.
Totally, 106 pathways for a signal w.r.t. 1 neuron!!!
What do neurons look like?
Soma or cell body:
Dendrites: receive signals.
Axons: transmit signals, carry the action potential.
Myelin sheath: fatty acid surrounding the axon, allows rapid propagation of signals.
Synapses may present between: axon and axon
axon and dendrites
axon and other cell bodies.
How do neurons work?
Membrane Potential:
Difference in concentration of ions on
opposite side of plasma cell membrane.
Defined as (Vinterior − Vexterior)
Arises due to ion channels and ion pumps
embedded in the membrane, maintains the
different ion concentration.
At equilibrium, outer side is more +ve than
inner side. (Resting potential)
(-70mV to -80mV)
Deviation from resting potential: opening or
closing of ion channels.
Depolarization if interior side become +ve, (ex: if sodium ions move inside)
Hyperpolarization if exterior side become more +ve. (ex: if potassium ions move outside)
Action Potential:
The transient switch in the membrane potential.
Action Potenital [www.keepvid.com].mp4
generation of action potential.swf
As the membrane potential is increased, sodium ion channels open, allowing the
entry of sodium ions into the cell. This is followed by the opening of potassium ion channels
that permit the exit of potassium ions from the cell.
Thus, there is first an influx of sodium ions (leading to massive depolarization)
followed by a rapid efflux of potassium ions from the neuron (leading to repolarisation).
Above critical threshold, the feedback of sodium current activates even more channels to
open, thus the cell ‘fires’ producing the action potential.
How does the axon potential propogates along the axon?
propagation of action potential.swf
When an action potential depolarizes the membrane, the leading edge activates other
adjacent sodium channels. This leads to another spike of depolarization the leading edge
of which activates more adjacent sodium channels ... etc. Thus a wave of depolarization
spreads from the point of initiation.
Action potentials move in a specific direction. This is because sodium channels have a
refractory period following activation, during which they cannot open again.
Role of Myelin:
Acts as insulator around the axon, preventing the dissipation of the depolarization wave.
It greatly increases the rate of transmission.
Synapses:
Pre-synaptic neuron: signal sending neuron
Post-synaptic neuron: receiving neuron.
Communication between neurons is chemical process.
• When an action potential reaches a synapse, voltage gated ion channels cell membrane are
opened allowing an influx of calcium ions into the pre-synaptic terminal…
ie, depolarization of the membrane.
• This causes a small 'packet' of a chemical neurotransmitter to be released into a
small gap between the two cells, known as the synaptic cleft.
• The neurotransmitter diffuses across the synaptic cleft and interacts with specialized
proteins called receptors that are embedded in the post-synaptic membrane.
• These receptors are ion channels that allow certain types of ions (charged atoms) to pass
through a pore within their structure.
• The pore is opened following interaction with the neurotransmitter allowing
an influx of ions into the post-synaptic terminal, which is propagated along the dendrite
towards the soma.
Synaptic Transmission.swf
Key step:
Binding of neurotransmitter with receptors which affects the behavior of post-synaptic cells.
Synaptic strength: change is membrane potential resulting from the activation of postsynaptic
receptors.
• Neurotransmission can be either:
excitatory, it increases the possibility of the post-synaptic neuron firing an action potential, or
inhibitory, where it reduces the likelyhood of an action potential being generated.
• Influx of +ve ions or efflux of –ve ions -----> excitatory
Influx of –ve ions or efflux of +ve ions -----> inhibitory
• Whether the result of synaptic transmission will be excitatory or inhibitory depends on
the type of neurotransmitter used and the ion channel receptors they interact with.
• Excitatory synaptic transmission uses a neurotransmitter called L-glutamate, interacts
with glutamate receptor channels in post synaptic neuron, permeable to sodium ions.
• Inhibitory synaptic transmission uses a neurotransmitter called GABA, interacts
with GABA receptor channels in post synaptic neuron, permeable to chloride ions.