7120 Lecture Notes - Lecture 3: Reflex Arc, No Brain, Motor Neuron
21 Jun 2018
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LECTURE 3: Synapses
- The synapse
oNeurons communicate by transmitting chemicals at junctions; called synapses
Charles scott sherrington in 1906 to describe the gap between neurons
Behavioural observation
oSherrington
Looked at how neurons communicate with each other by studying reflexes
in a process known as a reflex arc
Eg leg flexion reflex; a sensory neuron excites a second neuron;
which excites a motor neuron, which excites a muscle
No brain involvement for the reflex; just conscious awareness
- Reflexes
oSherringtons observations
Reflexes are slower than conduction along an axon
Several weak stimuli presented at slightly difference times or slightly
different locations produce a stronger reflex than a single stimulus
As on set of muscles become excited, another set relaxes
Flexor muscles contracted, extensor muscle relax etc
- Speed of conducton
oHe found a difference in the speed of conduction in a reflex arc from previously
measured action potentials
He believed the difference must be accounted for by the time it took for
communication between neurons
Evidence validated the idea of the synapse
- Synaptic delay
o
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- Temporal summation
oHe observed that repeated stimuli over a short period of time produced a stronger
response
Temporal summation
Repeated stimuli can have a cumulative effect and can produce a
nerve impulse when a single stimuli is too weak
- Excitatory postsynaptic potentials
oPresynaptic neuron; neuron that delivers the synaptic transmission
oPostsynaptic neuron; neuron that receives the message
oExcitatory postsynaptic potential EPSP; graded potential that decays over time and
space
oThe cumulative effect of EPSPs are the basis for temporal and spatial summation
Multiple EPSPS over time or space
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- Spatial summation
oSherrington also noticed that several small stimuli in a similar location produced a
reflex when a single stimuli did not
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Synaptic input from several locations can have a cumulative effect and
trigger a nerve impulse
- Spatial and temporal summation
oSpatial and temporal summation – critical for brain functioning
oEach neuron receives many incoming axons that frequently produce synchronized
responses
- Inhibitory synapses
oHe noticed that during the reflex that occurred, the leg of a dog that was pinched
retracted while the other three legs were extended
Suggested that an interneuron in the spinal cord sent an excitatory message
to the flexor muscles of one leg and an inhibitory message was sent to the
other three legs
Extensors of other legs flexing so that the dog doesn’t fall over
- Inhibitory postsynaptic potentials
oIPSP – temporal hyperpolarization of a membrane
Serves as an active ‘brake’ that suppresses excitation
Movement away from 0
- Wiring of synapses
o
oGreen; excitatory. Red; inhibitory
oBrief excitation; then inhibition
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Document Summary
The synapse: neurons communicate by transmitting chemicals at junctions; called synapses. Charles scott sherrington in 1906 to describe the gap between neurons. Looked at how neurons communicate with each other by studying reflexes in a process known as a reflex arc. Eg leg flexion reflex; a sensory neuron excites a second neuron; which excites a motor neuron, which excites a muscle. No brain involvement for the reflex; just conscious awareness. Reflexes are slower than conduction along an axon. Several weak stimuli presented at slightly difference times or slightly different locations produce a stronger reflex than a single stimulus. As on set of muscles become excited, another set relaxes. Speed of conducton: he found a difference in the speed of conduction in a reflex arc from previously measured action potentials. He believed the difference must be accounted for by the time it took for communication between neurons.
