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Chapter 2 Lecture
Modified: 2020-09-03
Nervous transmission is an electrochemical process. Chapter 2 concentrates on the activity of the synapse. For another look at this information see my general psychology pages at: peace.saumag.edu or edkardas.com (both pages are identical)
Neurons communicate by transmitting chemicals at junctions, called “synapses”
The term was coined by Charles Scott Sherrington in 1906 to describe the specialized gap that existed between neurons
Sherrington’s discovery was a major feat of scientific reasoning (done behaviorally with dogs)
Sherrington
Investigated how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc.
He discovered that reflexes are mediated by the spinal cord (read p. 43 to see how)
I usually demonstrate the startle reflex in class by making an unexpected loud noise. Then, I do it over and over. The reflex only happens when the noise is unexpected. Can't do that online, sorry!
Example
Leg flexion reflex: a sensory neuron excites a second neuron, which excites a motor neuron, which excites a muscle
Notice that the sensory neurons ENTER the spinal cord on the dorsal (back) side and that the motor neurons EXIT the spinal cord on the ventral (stomach) side.
Sherrington’s observations
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
The reflex impulse is mediated at the spinal cord level (that's why you may be surprised to see your foot jerk at the doctor's office when your patellar reflex is tested)
Sherrington 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 (the delay was caused by the actions taking place at the synapse)
Before you go further check out this summary page:Integration
Sherrington observed that repeated stimuli over a short period of time produced a stronger response
Thus, the idea of temporal summation
Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak.
Think of this as the postsynaptic neuron saying, 'I'm not going to wake up the boss (e.g., the CNS) unless this is really important.'
Presynaptic neuron: neuron that delivers the synaptic transmission
Postsynaptic neuron: neuron that receives the message.
Note: these terms are relative to a particular synapse. As the impulse travels down a series of neurons then the postsynaptic neuron becomes the presynaptic neuron for the NEXT synapse
Excitatory postsynaptic potential (EPSP): graded depolarization that decays over time and space
The cumulative effect of EPSPs are the basis for temporal and spatial summation.
Think of an EPSP synapse as one that will make the nervous transmission CONTINUE
Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not
Thus, the idea of spatial summation.
Think of spatial summation as several presynaptic neurons delivering EPSPs at the same time. The postsynaptic neuron will now be more likely to fire (continue the nerve impulse) that if only one neuron had fired.
Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse
Spatial summation is critical to brain functioning
Each neuron receives many incoming axons that frequently produce synchronized responses
Temporal summation and spatial summation ordinarily occur together
The order of a series of axons influences the results
Undergraduate Video of both types of summation (but imagine that during temporal summation the pencil is hitting the balloon with the same intensity)
Notice the All-or-None activity at 3 and 4 above. Also notice the IPSP at 5 (see below for more about IPSPs)
Notice the temporal summation at upper left and the spatial at bottom. The postsynaptic neuron analyzes those inputs and then produces an EPSP to the next neuron (on the right). Below, we'll look at a neuron receiving EPSPs and IPSPs simultaneously.
Here's another 'but wait, there's more' moment: look at the order of the summations above. When the EPSPs arrive as on the left, more depolarization takes place.
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
Muscles are paired (flexors and extensors). Here, notice the global response to the right paw's contraction of the flexors (the remaining three paws extensors contract).
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.
Basically, an IPSP put a stop to (inhibits) nerve conduction, but that still conveys information!
Think of the following example from a typicalwestern movie. Two cowpokes, surrounded by night, are listening tothe distant drums. Then one says to the other, 'Listen'. The otherreplies, 'I don't hear anything'. The first answers, 'I know.' Thedrums have stopped, and that conveys information, does it not? So,excitation and inhibition have the property of vastly increasing theinformation capacity of the nervous system because a decrease or anincrease in the rate of nerve conduction conveys information.
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
In the diagram above EPSPs are in green and IPSPs are in red. In this case transmission will occur at the dentrite. Notice that any given neuron will respond or not depending on the input it receives. Dennett labels this as 'armies of idiots' meaning that our complex behavior is the result of billions of neurons firing or not firing in a complicated manner.
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.
Meaning that all neurons 'check themselves out' constantly
Click here for a summary of action WITHIN neurons and here for a summary of action BETWEEN neurons
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
The great majority of synapses rely on chemical processes
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
The major sequence of events allowing communication between neurons across the synapse
The neuron synthesizes chemicals that serve as neurotransmitters
Action potentials travel down the axon
Released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron
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
So, nerve conduction is an electro-chemical process
Amino acids | Glutamate, GABA, glycine, asparate, maybe others |
A modified amino acid | Acetylcholine |
Monoamines (also modified from amino acids) | indoleamines: serotonin Catecholamines: dopamine, norepinephrine, epinephrine |
Neuropeptides (chains of amino acids) | Endorphins, substance P, neuropeptide Y, many others |
Purines | ATP, adenosine, maybe others |
Gases | NO (nitric oxide), maybe others |
Table 2.1 Neurotransmitters (p. 52)
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)
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
Exocytosis: bursts of release of neurotransmitter from the presynaptic terminal into the synaptic cleft
Triggered by an action potential
a) mouse synapse (electron micrograph), b) axon terminals at soma (electron micrograph)
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
The effect of a neurotransmitter depends on its receptor on the postsynaptic cell
Transmitter-gated or ligand-gated channels are controlled by a neurotransmitter
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
a) at rest, b) attached to receptor
Occur when neurotransmitters attach to a receptor and initiate a sequence of slower and longer lasting metabolic reactions
Metabotropic synapses use many neurotransmitters 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
Metabotropic events include such behaviors as taste, smell, and pain
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
Metabotropic effects utilize a number of different neurotransmitters
Neuropeptides are often called neuromodulators
Release requires repeated stimulation
Released peptides trigger other neurons to release same neuropeptide
Diffuse widely and affect many neurons via metabotropic receptors
Neuropeptides | Neurotransmitters | |
Place synthesized | Cell body | Presynaptic terminal |
Place released | Mostly from dendrites, also cell body and sides of axon | Axon terminal |
Released by | Repeated depolarization | Single action potential |
Effect on neighboring cells | They release the neuropeptide too | No effect on neighbors |
Spread of effects | Diffuse to wide area | Effect mostly on receptors of the adjacent postsynaptic cell |
Duration of effects | Minutes | Milliseconds to seconds |
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
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
Opioids are synthetic opiates
Powerful analgesics
Analgesia vs Anesthesia
Analgesic: drug or substance that lowers pain levels
Anesthetic: drug or substance that lowers: touch sensitivity, hot and cold sensitivity, and pain (Novocain or Xylocain)
I once had a small tumor cut out of my right bicep as an outpatient. The surgeon injected my arm and picked up the scalpel. I said, 'wait.' But he said with xylocaine you won't feel a thing, it's fast acting. I watched as he made a 2.5' incision, fluid and blood came out, but I felt nothing.
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
Curare
Curare, a South American poison, has its effect because it inhibitsthe action of an enzyme, cholinesterase. The effects of curare aremuscular and respiratory paralysis. You may infer then, that thevoluntary muscles and the lungs are controlled by theneurotransmitter, acetylcholine, abbreviated ACh. Curare does notaffect neurons that are not controlled by ACh. Curare kills bycausing all of the neurons that control breathing to be stuck open.Many of the nerve agents that the allied armed forces were concernedabout during the Gulf War work in a similar fashion to curare.The movie, 'The Emerald Forest,' a true story about a child abducted and raised by Brazilian Indians, contains a scene in which the child and his adoptive father go monkey hunting. They find a monkey, and carefully prepare a blowgun dart by dipping it in the curare, which they carry on a pouch around their necks. They shoot the monkey, and hit it on the foot; then they wait. As the poison takes effect, the monkey stiffens, then falls to the ground. The two hunters then walk up to the monkey and kill it. Presumably, the monkey is able to see them as they walk up and then hit it on the head.
If care is taken to keep the lungs working by artificial means, then curare will not be fatal. Before the widespread use of anesthetics, curare was routinely used in surgery. Patients were kept alive by artificial lungs until the curare dose wore off. The curare was used not as anesthetic but as a means of immobilizing the patient while the surgery took place.
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 (see figure below)
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
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
A few special-purpose synapses operate electrically
Faster (and different) 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
Chapter 2: Patterns And Relationsmr. Mac's Page Numbering
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
Slower acting integrative system
Days (e.g., 28) to years (0 to 18)
Organ | Hormone | Hormone Functions (Partial) |
Hypothalamus | Various releasing hormone | Promote/inhibit release of hormones from pituitary |
Anterior pituitary | Thyroid-stimulating hormone Luteinizing hormone Follicle-stimulating hormone ACTH Prolactin Growth hormone | Stimulates thyroid gland Stimulates ovulation Promotes ovum maturation (female), sperm production (male) Increases Steroid hormone production by adrenal gland Increases milk production Increases body growth |
Posterior pituitary | Oxytocin Vasopressin | Uterine contractions, milk release, sexual pleasure Raises blood pressure, decreases urine volume |
Pineal | Melatonin | Sleepiness; also role in puberty |
Adrenal cortex | Aldosterone Cortisol | Reduces release of salt in the urine Elevated blood sugar and metabolism |
Adrenal medulla | Epinephrine, norepinephrine | Similar to actions of sympathetic nervous system |
Pancreas | Insulin Glucagon | Helps glucose enter cells Helps convert stored fats into blood glucose |
Ovary | Estrogens and progesterone | Female sexual characteristics and pregnancy |
Testis | Testosterone | Male sexual characteristics and pubic hair |
Kidney | Renin | Regulates blood pressure, contributes to hypovolemic thirst |
Fat cells | Leptin | Decreases appetite |
Chapter 2: Patterns And Relationsmr. Mac's Pages
Composed of chains of amino acids
Proteins are longer chains; amino acids are shorter
Attaches to membrane receptors where they activate second messenger systems
Attached to the hypothalamus and consists of two distinct glands (aka the “master gland”
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
The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system
Example: TSH-releasing hormone and thyroid hormone levels.
Example: Keeping the house warm. Here, the thermostat plays the role of the pituitary gland, and the furnace the role of an endocrine gland. Note how negative feedback keeps the temperature of the house within a narrow range. The pituitary accomplishes the same pattern.
Positive feedback also occurs in behavior, economics, and house fires. In positive feedback the response grows and grows until it crashes. Example
Behavior: think of a concert and an encore. We saw Fleetwood Mac in N. Little Rock a few years ago. No one clapped strongly at the the end and the band did not play an encore. The NEXT night in Los Angeles, during the encore, Christine McVie, emerged through a trapdoor in the floor, and played. As you might imagine, the crowd went wild (aka, positive feedback). Eventually, though, the band has to leave and the audience has to go home.
Economics: think of a fad, Beanie Babies, baseball cards, or stock prices. When everyone wants to get one of those items, the price goes up and up. At some point, people begin to realize they are paying too much and the price crashes.
House fires: This is why there are no positive feedback thermostats! That thermostat would only say 'make it hotter.' You see the results. The house burns down and slowly cools.
Oh, btw, human sexual behavior often exhibits positive feedback. I'll let you figure that out for yourselves. Suffice it to say, there will be a point where 'enough' will be heard.