Bio Psychology – Chapter 3
The concept of the synapse
o Neurons communicate by transmitting chemicals at junctions, called
“synapses”
In 1906, Charles Scott Sherrington coined the term synapse to
describe the specialized gap that existed between neurons
Sherrington’s discovery was a major feat of scientific
reasoning
Properties of synapses
o Sherrington investigated how neurons communicate with each other
by studying reflexes (automatic muscular responses to stimuli)
Example: Leg flexion reflex: a sensory neuron excites a
second neuron, which excites a motor neuron, which excites a
muscle
Reflex arc: the circuit from sensory neuron to muscle
o Sherrington observed three important points about reflexes:
Reflexes are slower than conduction along an axon
Several weak stimuli presented at slightly different times or
slightly different locations produces a stronger reflex than a
single stimulus does
As one set of muscles relaxes, another set becomes excited
o Sherrington observed 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
o
o Sherrington 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
Presynaptic neuron: neuron that delivers the synaptic
transmission
Postsynaptic neuron: neuron that receives the message
Excitatory postsynaptic potential (EPSP): graded
potential that decays over time and space (depolarization)
The cumulative effect of EPSPs are the basis for temporal and
spatial summation
, Sherrington also noticed that several small stimuli on a similar
location produced a reflex when a single stimuli did not
This led to the idea of spatial summation or that synaptic
input from several locations can have a cumulative effect and
trigger a nerve impulse
o Spatial summation is critical to brain functioning
Each neuron receives many incoming inputs from different
axons that frequently produce synchronized responses
Temporal summation and spatial summation ordinarily occur
together
The order of a series of axons influences the results
o
o Sherrington also noticed that during the reflex that occurred, the leg
of a dog that was pinched retracted while the other three legs were
extended
He 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
o This led to the idea of inhibitory postsynaptic potential or the
temporary hyperpolarization of a membrane
An ISPS occurs when synaptic input selectively opens the
gates for positively charged potassium ions to leave the cell
or negatively charged chloride ions to enter the cells
Serves as an active “brake” that suppresses excitation
Relationship Among EPSP, IPSP, and Action Potentials
o Sherrington assumed that synapses produce on and off responses
o Synapses vary enormously in their duration of effects
o The effect of two synapses at the same time can be more than
double the effect of either one, or less than double
The concept of the synapse
o Neurons communicate by transmitting chemicals at junctions, called
“synapses”
In 1906, Charles Scott Sherrington coined the term synapse to
describe the specialized gap that existed between neurons
Sherrington’s discovery was a major feat of scientific
reasoning
Properties of synapses
o Sherrington investigated how neurons communicate with each other
by studying reflexes (automatic muscular responses to stimuli)
Example: Leg flexion reflex: a sensory neuron excites a
second neuron, which excites a motor neuron, which excites a
muscle
Reflex arc: the circuit from sensory neuron to muscle
o Sherrington observed three important points about reflexes:
Reflexes are slower than conduction along an axon
Several weak stimuli presented at slightly different times or
slightly different locations produces a stronger reflex than a
single stimulus does
As one set of muscles relaxes, another set becomes excited
o Sherrington observed 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
o
o Sherrington 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
Presynaptic neuron: neuron that delivers the synaptic
transmission
Postsynaptic neuron: neuron that receives the message
Excitatory postsynaptic potential (EPSP): graded
potential that decays over time and space (depolarization)
The cumulative effect of EPSPs are the basis for temporal and
spatial summation
, Sherrington also noticed that several small stimuli on a similar
location produced a reflex when a single stimuli did not
This led to the idea of spatial summation or that synaptic
input from several locations can have a cumulative effect and
trigger a nerve impulse
o Spatial summation is critical to brain functioning
Each neuron receives many incoming inputs from different
axons that frequently produce synchronized responses
Temporal summation and spatial summation ordinarily occur
together
The order of a series of axons influences the results
o
o Sherrington also noticed that during the reflex that occurred, the leg
of a dog that was pinched retracted while the other three legs were
extended
He 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
o This led to the idea of inhibitory postsynaptic potential or the
temporary hyperpolarization of a membrane
An ISPS occurs when synaptic input selectively opens the
gates for positively charged potassium ions to leave the cell
or negatively charged chloride ions to enter the cells
Serves as an active “brake” that suppresses excitation
Relationship Among EPSP, IPSP, and Action Potentials
o Sherrington assumed that synapses produce on and off responses
o Synapses vary enormously in their duration of effects
o The effect of two synapses at the same time can be more than
double the effect of either one, or less than double
