Chapter 3 - synapses Flashcards
How does neurons communicate with one another ?
Neurons communicate by transmitting chemicals at junctions, called “synapses”
What did sherrington do ?
Reflexes : automatic muscular responses to stimuli (book)
Investigated how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc
reflec arc: the circuit from sensory neuron to muscle response.
Example
Leg flexion reflex: a sensory neuron excites a second neuron, which excites a motor neuron, which excites a muscle
The term “synapse” was coined by Charles Scott Sherrington (who had physiologically demonstrated that communication between one neuron and the next differs from communication along a single axon) in 1906 to describe the specialized gap that existed between neurons
Sherrington’s discovery was a major feat of scientific reasoning
Sherrington’s observations
Sheerrington strapped a dog into a harness above the ground pinched one of the dog’s feet. After a short delay, the dog flexed (raised) the pinched leg and extended the others.
Sheerington observed several properties of reflexes suggesting special processes at the junctions between neurons :
- Reflexes are slower than conduction along an axon
- Several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus
- As one set of muscles becomes excited, another set relaxes
Sherrington found a difference in the speed of conduction in a …
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
When Sheerington’s pinched a dog’s foot, the dog flexed that leg after a short delay. During that delay, an impulse had to travel up an axon from the skin receptor to the spinal cord, and then an impulse had to travel from the spinal cord back down the leg to a muscle.
He had thus concluded that something was slowing down the conduction through the reflex and he inferred that the delay must occur where one neuron communicates with another.
Sherington and temporal summation ?
Sherrington observed that repeated stimuli over a short period of time produced a stronger response
Thus, the idea of temporal summation (summation over time)
Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak
for example, A light pinch of the dog’s foot did not evoke a reflex but a few rapidly repeated pinches did.
He surmised that a single pinch produced a synaptic
transmission less than the threshold for the postsynaptic neuron (the cell that receives the message)
With a rapid succession of pinches , each adds its effects to what remained from the previous ones, until the combination exceeds the threshold of the postsynaptic neuron, producing an action potential.
Presynaptic neuron
neuron that delivers the synaptic transmission
Postsynaptic neuron
neuron that receives the message
Excitatory postsynaptic potential (EPSP):
depolarization is a graded potential.
graded depolarization that decays over time and space
The cumulative effect of EPSPs are the basis for temporal and spatial summation
Sherrington and spatial summation
Sherrington also noticed that
spatial summation = summation over space.
Synaptic inputs from separate locations combine their effects on a neuron.
He again began with a pinch too weak to elicit a reflex.
Instead of pinching one point twice, he pinched two points at once. Together, these 2 pinches produce a reflex.
several small stimuli in a similar location produced a reflex when a single stimuli did not
Thus, the idea of spatial summation
Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse
this is due to that the 2 points activated 2 sensory neurons , whose axons converged onto one neuron in the spinal cord.
spatial summation is critical for …
Spatial summation is critical to brain functioning
Each neuron receives many incoming axons that frequently produce synchronized responses
Does temporal summation and spatial summation occur separately or together ?
Temporal summation and spatial summation ordinarily occur together
The order of a series of axons influences the results
Sherrington and inhibitory synapses ?
Sherrington 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
Inhibitory Postsynaptic Potential (IPSP)
Thus, the idea of inhibitory postsynaptic potential (IPSP)—the temporary hyperpolarization of a membrane
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
Sherrrington and duration of synapses ?
Sherrington assumed that synapses produce on and off responses
Synapses vary enormously in their duration of effects
The effect of two synapses at the same time can be more than double the effect of either one, or less than double
Spontaneous Firing Rate
The periodic production of action potentials despite synaptic input
EPSPs increase the number of action potentials above the spontaneous firing rate
IPSPs decrease the number of action potentials below the spontaneous firing rate
who did The Discovery of Chemical Transmission at Synapses
German physiologist Otto Loewi
The first to convincingly demonstrate that communication across the synapse occurs via chemical means
