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
Activating Receptors of the Postsynaptic Cell
The effect of a neurotransmitter depends on its receptor on the postsynaptic cell
Transmitter-gated or ligand-gated channels are controlled by a neurotransmitter
Ionotropic Effects
Occurs when a neurotransmitter attaches to receptors and immediately opens ion channels
Most effects:
Occur very quickly (sometimes less than a millisecond after attaching) and are very short lasting
Rely on glutamate or GABA
Most of the brains excitatory ionotropic synapse use the neurotransmitter glutamate.
while for inhibitory ionotropic synapse use the NT GABA. (opens chloride gates)
ionotropic : vision, hearing
the brain needs it to be rapid, quickly changing information.
Metabotropic Effects and Second Messenger Systems
Occur when neurotransmitters attach to a receptor and initiate a sequence of slower and longer lasting metabolic reactions than ionotropic effects.
Metabotropic synapses use many neurotransmitters (large variety of NT unlike ionotropic) such as dopamine, norepinephrine, serotonin, and sometimes glutamate and GABA
When neurotransmitters attach to a metabotropic receptor, it bends the receptor protein that goes through the membrane of the cell
Bending allows a portion of the protein inside the neuron to react with other molecules
note : An ionotropic synapse has effects localized to one point in the membrane , whereas a metabotropic synapse, by way of its second messenger, influence activity in much or all of the cell and over a longer time.
Metabotropic events include such behaviors as taste, smell, and pain
G-Proteins
G-protein activation: coupled to guanosine triphosphate (GTP), an energy storing molecule
Increases the concentration of a “second-messenger”
The second messenger communicates to areas within the cell
May open or close ion channels, alter production of activating proteins, or activate chromosomes
Neuropeptides
Metabotropic effects utilize a number of different neurotransmitters
Neuropeptides are often called neuromodulators
Release requires repeated stimulation for it to be release.
Released peptides trigger other neurons to release same neuropeptide
Diffuse widely and affect many neurons via metabotropic receptors
Drugs That Act by Binding to Receptors
Many hallucinogenic drugs distort perception
Chemically resemble serotonin in their molecular shape (e.g., LSD)
Stimulate serotonin type 2A receptors (5-HT2A) at inappropriate times or for longer duration than usual, thus causing their subjective effect
Nicotine stimulates acetylcholine receptors
Opiate Drugs and Endorphins
Opiates attach to specific receptors in the brain
The brain produces certain neuropeptides now known as endorphins—a contraction of endogenous morphines
Opiate drugs exert their effects by binding to the same receptors as endorphins
Inactivation and Reuptake of Neurotransmitters
Neurotransmitters released into the synapse do not remain and are subject to either inactivation or reuptake
During reuptake, the presynaptic neuron takes up most of the neurotransmitter molecules intact and reuses them
Transporters are special membrane proteins that facilitate reuptake
examples of inactivation and reuptake
Serotonin is taken back up into the presynaptic terminal
Acetylcholine is broken down by acetylcholinesterase into acetate and choline
Excess dopamine is converted into inactive chemicals
COMT: enzymes that convert the excess into inactive chemicals
Amphetamine and cocaine
Stimulate dopamine synapses by increasing the release of dopamine from the presynaptic terminal
Methylphenidate (Ritalin)
Also blocks the reuptake of dopamine but in a more gradual and more controlled rate
Often prescribed for people with ADD; unclear whether Ritalin use in childhood makes one more likely to abuse drugs as an adult
Negative feedback in the brain is accomplished in two ways
Autoreceptors: receptors that detect the amount of transmitter released and inhibit further synthesis and release
Postsynaptic neurons: respond to stimulation by releasing chemicals that travel back to the presynaptic terminal where they inhibit further release
Cannabinoids
The active chemicals in marijuana that bind to anandamide or 2-AG receptors on presynaptic neurons or GABA
When cannabinoids attach to these receptors, the presynaptic cell stops sending
In this way, the chemicals in marijuana decrease both excitatory and inhibitory messages from many neurons
Electrical Synapses
A few special-purpose synapses operate electrically
Faster than all chemical transmissions
Gap junction: the direct contact of the membrane of one neuron with the membrane of another
Depolarization occurs in both cells, resulting in the two neurons acting as if they were one
Hormones
Chemicals secreted by a gland or other cells that is transported to other organs by the blood where it alters activity
Produced by endocrine glands
Important for triggering long-lasting changes in multiple parts of the body
Proteins and Peptides
Composed of chains of amino acids
Proteins are longer chains; amino acids are shorter
Attaches to membrane receptors where they activate second messenger systems
The Pituitary Gland and the Hypothalamus
Attached to the hypothalamus and consists of two distinct glands
Anterior pituitary: composed of glandular tissue
Hypothalamus secretes releasing and inhibiting hormones that control anterior pituitary
Posterior pituitary: composed of neural tissue
Hypothalamus produces oxytocin and vasopressin, which the posterior pituitary releases in response to neural signals
Maintaining Hormonal Levels
The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system
Example: TSH-releasing hormone and thyroid hormone levels
