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
Neurotransmitters
chemicals that travel across the synapse and allow communication between neurons
Chemical transmission predominates throughout the nervous system
Otto Loewi’s experiment
Found that stimulating one nerve released something that inhibited heart rate, and stimulating a different nerve released something that increased heart rate
Realized that he was collecting and transferring chemicals, not loose electricity
He did this by stimulating the vagus nerve to a frog’s heart , which had decreased its heartbeat. When he transferred fluid from that heart to another frog’s heart, he observe a decrease in its heartbeat.
Same thing had happened when he raised the heartbeat of one frog.
The Sequence of Chemical Events at the Synapse
The major sequence of events allowing communication between neurons across the synapse
The neuron synthesizes chemicals that serve as neurotransmitters
(small NT in the axon terminal and neuropeptides in the cell body.)
Action potentials travel down the axon
Released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron (the entry of calcium in the presynaptic terminal allows for the release of NT.)
The neurotransmitter molecules separate from their receptors
The neurotransmitters may be taken back into the presynaptic neuron for recycling or diffuse away
Some postsynaptic cells may send reverse messages to slow the release of further neurotransmitters by presynaptic cells
How do neurons synthesize neurotransmitters ?
Neurons synthesize neurotransmitters and other chemicals from substances provided by the diet
Acetylcholine synthesized from choline found in milk, eggs, and nuts
Tryptophan serves as a precursor for serotonin
Catecholamines contain a catechol group and an amine group (epinephrine, norepinephrine, and dopamine)
Storage of Transmitters
Vesicles: tiny spherical packets located in the presynaptic terminal where neurotransmitters are held for release
MAO (monoamine oxidase):
breaks down excess levels of some neurotransmitters
since in some cases, neurons apparently accumulate excess levels of a NT.
ex: neurons that release serotonin , dopamine or epinephrine contains this enzyme. (book)
Exocytosis:
bursts of release of neurotransmitter from the presynaptic terminal into the synaptic cleft
Triggered by an action potential
Release and Diffusion of Transmitters
Transmission across the synaptic cleft (20–30 nm wide) by a neurotransmitter takes fewer than 0.01 ms
Most individual neurons release at least two or more different kinds of neurotransmitters
Neurons may also respond to more types of neurotransmitters than they release
