Biological Psychology: Synapses

transcript

Chapter 2

Synapses

2.1 The Concept of the Synapse

• 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

The Properties of Synapses

• Sherrington – Investigated how neurons communicate with

each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc

• Example – Leg flexion reflex: a sensory neuron excites a

second neuron, which excites a motor neuron, which excites a muscle

The Relationship Among a Sensory Neuron, Intrinsic Neuron, and Motor Neuron

Three Important Points About Reflexes

• Sherrington’s observations– Reflexes are slower than conduction along an

– 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

Difference in the Speed of Conduction

• 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

Sherrington’s Evidence for Synaptic Delay

Temporal Summation

• 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

Excitatory Postsynaptic Potential (EPSP)

• 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

• The cumulative effect of EPSPs are the basis for temporal and spatial summation

Spatial Summation, Part 1

• Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not

• Thus, idea of spatial summation – Synaptic input from several locations can have

a cumulative effect and trigger a nerve impulse

Recordings From a Postsynaptic Neuron During Synaptic Activation

Spatial Summation, Part 2

• 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

Temporal and Spatial Summation

The Effects of Summation

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

Antagonistic Muscles

Inhibitory Postsynaptic Potential (ISPS)

• Thus, the idea of inhibitory postsynaptic potential (ISPS) – 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

Sherrington’s Inference of Inhibitory Synapses

Relationship Among EPSP, IPSP, and Action Potentials

• 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

A Possible Wiring Diagram for Synapses

Wiring Diagram for an “A or B” Response

Wiring Diagram for an “A and B” Response

Wiring Diagram for an “A and B if not C” Response

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

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

Module 2.2 Chemical Events at the Synapse

• 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

Nerves Send Messages by Releasing Chemicals

The Sequence of Chemical Events at the Synapse, Part 1

• 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 Sequence of Chemical Events at the Synapse, Part 2

– 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

Some Major Events in Transmission At a Synapse

Table 2.1 Neurotransmitters

Types of Neurotransmitters, Part 1

Amino acids Glutamate, GABA, glycine, asparate, maybe others

A modified amino acid Acetylcholine

Monoamines (also modified from amino acids)

indoleamines: serotoninCatecholamines: 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

Types of Neurotransmitters, Part 2

• 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)

Pathways in the Synthesis of Transmitters

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

• Exocytosis: bursts of release of neurotransmitter from the presynaptic terminal into the synaptic cleft– Triggered by an action potential

Anatomy of a Synapse

Release and Diffusion of Transmitters

• Transmission across the synaptic cleft by a neurotransmitter takes fewer than 0.01 microseconds

• 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

The Acetylcholine Receptor

Metabotropic Effects and Second Messenger Systems, Part 1

• 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

Metabotropic Effects and Second Messenger Systems, Part 2

• 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

Sequence of Events at a Metabotropic Synapse

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

– Released peptides trigger other neurons to release same neuropeptide

– Diffuse widely and affect many neurons via metabotropic receptors

Distinctive Features of Neuropeptides

Neuropeptides Other 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

Spread of effects Diffuse to wide area Effect mostly on receptors of the adjacent postsynaptic cell

Duration of effects Many minutes Less than a second to a few seconds

Drugs that Act by Binding to Receptors

• Many hallucinogenic drugs distort perception– Chemically resemble serotonin in their

molecular shape

– 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, Part 1

• 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

Inactivation and Reuptake of Neurotransmitters, Part 2

• 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

Stimulant Drugs

• 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 from the Postsynaptic Cell

• 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

Effects of Some Drugs at Dopamine Synapses

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

A Gap Junction for an Electrical Synapse

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

Location of Some Major Endocrine Glands

A Selective List of Hormones

Organ Hormone Hormone Functions (Partial)Hypothalamus Various releasing hormone Promote/inhibit release of hormones from pituitary

Anterior pituitary Thyroid-stimulating hormoneLuteinizing hormoneFollicle-stimulating hormoneACTHProlactinGrowth hormone

Stimulates thyroid glandStimulates ovulationPromotes ovum maturation (female), sperm production (male)Increases Steroid hormone production by adrenal glandIncreases milk productionIncreases body growth

Posterior pituitary OxytocinVasopressin

Uterine contractions, milk release, sexual pleasureRaises blood pressure, decreases urine volume

Pineal Melatonin Sleepiness; also role in puberty

Adrenal cortex AldosteroneCortisol

Reduces release of salt in the urineElevated blood sugar and metabolism

Adrenal medulla Epinephrine, norepinephrine Similar to actions of sympathetic nervous system

Pancreas InsulinGlucagon

Helps glucose enter cellsHelps 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

Proteins and Peptides

• Composed of chains of amino acids

• 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

Location of the Hypothalamus and Pituitary Gland in the Human Brain

Pituitary Hormones

Negative Feedback in the Control of Thyroid Hormones

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

Biological Psychology: Synapses

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