An Introduction to Brain and Behavior Third Edition CHAPTER How Do Neurons Communicate and Adapt? 5...

An Introduction to Brain and Behavior Third Edition

CHAPTER

How Do Neurons Communicate and Adapt?

5

PowerPoints prepared by: Paul Smaldino, UC Davis, Department of Psychology

Bryan Kolb & Ian Q. Whishaw

Kolb & Whishaw, An Introduction to Brain and Behavior, Third

Edition - Chapter 5

Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 5

How Do Neurons Communicate and Adapt?

• A Chemical Message

• Varieties of Neurotransmitters

• Neurotransmitter Systems and Behavior

• Role of Synapses in Three Kinds of Learning and Memory


A Chemical Message

• Otto Loewi (1921)– Frog heart experiment

– Role of the the vagus nerve and the neurotransmitter acetylcholine (ACh) in slowing heart rate

• Acetylcholine– The first neurotransmitter discovered in the PNS

and CNS; activates skeletal muscles in the somatic nervous system and may excite or inhibit internal organs in the autonomic nervous system


A Chemical Message

Otto Loewi’s Subsequent research

• Epinephrine (EP, or adrenaline)– Chemical messenger that acts as a hormone to

mobilize the body for fight or flight during times of stress and as a neurotransmitter in the central nervous system

• Norepinephrine (NE, or noradrenaline)– Neurotransmitter found in the brain and in the

parasympathetic division of the autonomic nervous system


A Chemical Message

• Neurotransmitter– Chemical released by a neuron onto a target

with an excitatory or inhibitory effect

– Outside the CNS, many of these chemicals circulate in the blood stream as hormones

HypothalamusPituitaryGland

HormonesTarget Organs

and Glands


A Chemical MessageStructure of Synapses

• Electron Microscope– Projects a beam of electrons through a very thin

slice of tissue

– Varying structure of the tissue scatters the beam onto a reflective surface where it leaves an image, or shadow, of the tissue

– Much better resolution than the light microscope

– 1950s: Revealed the structure of a synapse for the first time



• Synaptic Vesicle– Organelle consisting of a membrane structure

that encloses a quantum of neurotransmitter

• Synaptic Cleft– Gap that separates the presynaptic membrane

from the postsynaptic membrane

• Chemical Synapse– Junction where messenger molecules are

released when stimulated by an action potential



• Presynaptic Membrane– Membrane on the transmitter - output side of

a synapse

• Postsynaptic Membrane– Membrane on the transmitter - input side of a

synapse

• Storage granule– Membranous compartment that holds several

vesicles containing a neurotransmitter



• Majority of synapses in the mammalian nervous system are chemical, but there are others

• Electrical Synapse (a.k.a. gap junction)– Fused presynaptic and postsynaptic

membrane that allows an action potential to pass directly from one neuron to the next

• Chemical synapses allow more flexibility in neuron-to-neuron communication


A Chemical MessageNeurotransmission in Four Steps

The neurotransmitter must be• Synthesized and stored in the axon terminal

• Transported to the presynaptic membrane and released in response to an action potential

• Able to activate the receptors on the target-cell membrane located on the postsynaptic membrane

• Inactivated, or it will continue to work indefinitely



Step 1: Synthesis and StorageNeurotransmitters are derived in two general ways

• Synthesized in the Axon Terminal– Building blocks from food are pumped into cell via

transporters: protein molecules embedded within the cell membrane

• Synthesized in the Cell Body– According to instructions contained in the DNA

– Transported on microtubules to axon terminal



Step 2: Neurotransmitter Release• At the terminal, the action potential opens

voltage-sensitive calcium (Ca2+) channels

• Ca2+ enters the terminal and binds to the protein calmodulin forming a complex

• Complex causes some vesicles to empty their contents into the synapse, and others to get ready to empty their contents



Step 3: Receptor-Site Activation• After being released, the neurotransmitter

diffuses across the synapse and activates receptors on the postsynaptic membrane

• Transmitter-Activated Receptors– Protein embedded in the membrane of a cell that has a

binding site for a specific neurotransmitter



Step 3: Receptor-Site ActivationNeurotransmitter may

• Depolarize the postsynaptic membrane causing excitatory action on the postsynaptic neuron

• Hyperpolarize the postsynaptic membrane causing inhibitory action on the postsynaptic neuron

• Initiate other chemical reactions that modulate either the excitatory or inhibitory effect, or influence other functions of the receiving neuron



Step 3: Receptor-Site Activation• Autoreceptors

– “Self-receptor” on the presynaptic membrane that responds to the transmitter that the neuron releases

• Quantum (pl. quanta)– Quantity, equivalent to the contents of a single

synaptic vesicle, that produces a just observable change in postsynaptic electric potential



Step 4: Deactivation of the NeurotransmitterAccomplished In At Least Four Ways

• Diffusion away from synaptic cleft

• Degradation by enzymes in the synaptic cleft

• Reuptake into the presynaptic neuron for subsequent re-use

• Taken up by neighboring glial cells


A Chemical MessageExcitatory and Inhibitory Messages

Type I Synapse

• Excitatory

• Typically located on dendrites

• Round vesicles

• Dense material on membranes

• Wide cleft

• Large active zone

Type II Synapse

• Inhibitory

• Typically located on cell body

• Flat vesicles

• Sparse material on membranes

• Narrow cleft

• Small active zone


A Chemical MessageEvolution of Complex Neurotransmission

Systems

• Chemical transmission may have had its origins in the feeding behavior of single-celled creatures– Digestive juices are secreted onto prey via

exocytosis

– Prey is captured via endocytosis

• This process parallels the use of neurotransmitters for communication


Varieties of Neurotransmitters

• About 50 different kinds have been identified

• Some are inhibitory at one location and excitatory at another

• More than one neurotransmitter may be active at a single synapse

• No simple one-to-one relationship between a single neurotransmitter and a single behavior


Varieties of NeurotransmittersFour Criteria for Identifying

Neurotransmitters1. The chemical must be synthesized in the neuron

or otherwise be present in it

2. When the neuron is active, the chemical must be released and produce a response in a some target

3. The same response must be obtained when the chemical is experimentally placed on the target

4. A mechanism must exist for removing the chemical from its site of action after its work is done


Acetylcholine and the Renshaw Loop


Varieties of NeurotransmittersThree Classes of Neurotransmitters

1. Small-molecule transmitters

2. Peptide transmitters

3. Transmitter gases



1. Small-Molecule Transmitters

• Class of quick-acting neurotransmitters

• Synthesized from dietary nutrients and packaged ready for use in axon terminals




• Examples:– Acetylcholine (ACh)

– Amines: Dopamine (DA), Norepinephrine (NE), Epinephrine (EP), Serotonin (5-HT)

– Amino Acids: Glutamate (Glu), Gamma-aminobutyric acid (GABA), Glycine (Gly)




• Acetylcholine Synthesis – Choline

– Acetate

• Breakdown of Acetylcholine– Enzyme: Acetylcholinesterace (AChE)

Two important enzymes: • Acetyl coenzyme A • Choline acetyltransferace (ChAT)




• Sequential Synthesis of Three Amines




• Rate-Limiting Factor– Any enzyme that is in limited supply, thus

restricting the pace at which a chemical can be synthesized

– Example: Tyrosine hydroxylase in amine synthesis




• Amino Acid Transmitters– Glutamate: main

excitatory transmitter

– GABA: main inhibitory transmitter



2. Peptide Transmitters

• Neuropeptide– A multifunctional chain of amino acids that act as a

neurotransmitter

– Synthesized from mRNA on instructions from the cell’s DNA

– Do not bind to ion channels; do not have direct effects on the voltage of the postsynaptic membrane



2. Peptide Transmitters

• Examples:– Opioids

– Neurohypophyseals

– Secretins

– Insulins

– Gastrins

– Somatostatins

– Corticosteroids



3. Transmitter Gases

• Synthesized in cell, as needed

• Easily crosses cell membrane

• Examples:– Nitric Oxide (NO)

– Carbon Monoxide (CO)


Varieties of NeurotransmittersTwo Classes of Receptors

• Ionotropic Receptor– Embedded membrane protein with two parts

• A binding site for a neurotransmitter

• A pore that regulates ion flow to directly and rapidly change membrane voltage



• Metabotropic Receptor– Embedded membrane protein with a binding site for a

neurotransmitter but no pore

– Linked to a G protein that can affect other receptors or act with second messengers to affect other cellular processes

• G protein – Belongs to a family of guanyl-nucleotide-binding proteins

coupled to metabotropic receptors that, when activated, bind to other proteins



• Second Messenger– A chemical that carries a message to initiate a

biochemical process

– Activated by a neurotransmitter (the first messenger)• Examples

– Alter ion flow in a membrane channel

– Formation of new ion channels

– Production of new proteins


Neurotransmitter Systems and BehaviorNeurotransmission in the Somatic Nervous

System• Cholinergic Neuron

– Neuron that uses acetylcholine (ACh) as its main neurotransmitter

– Excites skeletal muscles to cause contractions

• Nicotinic ACh Receptor– Ionotropic receptor at which acetylcholine and

the drug nicotine act to open an pore and allow the flow of ions through the receptor pore


Neurotransmitter Systems and BehaviorTwo Activating Systems of the Autonomic

Nervous System

• Cholinergic neurons from the CNS control both divisions1. Sympathetic (fight-or-flight response)

2. Parasympathetic (rest-and-digest response)

• Norepinephrine is also involved in the fight-or-flight response


Neurotransmitter Systems and BehaviorFour Activating Systems in the Central

Nervous System• Activating System

– Neural pathways that coordinate brain activity through a single neurotransmitter

– Cell bodies are located in a nucleus in the brainstem and their axons are distributed through a wide region of the brain

– Four Systems• Cholinergic, Dopaminergic, Noradrenergic, and

Serotonergic



Nervous System• Alzheimer’s disease

– Degenerative brain disorder that first appears as progressive memory loss and later develops into generalized dementia

– Involves cholinergic system

• Schizophrenia– Behavioral disorder characterized by delusions,

hallucinations, disorganized speech, blunted emotion, agitation or immobility, and a host of other symptoms

– Involves dopaminergic system



Nervous System• Major depression

– Mood disorder characterized by prolonged feelings of worthlessness and guilt, the disruption of normal eating habits, sleep disturbances, a general slowing of behavior, and frequent thoughts of suicide

– Involves noradrenergic system



Nervous System

• Obsessive-Compulsive Disorder– Behavioral disorder characterized by compulsively

repeated acts (e.g., hand washing) and repetitive, often unpleasant, thoughts (obsessions)

– Involves serotonergic system


Role of Synapses in Three Kinds of Learning and Memory

• Learning– Relatively permanent change in behavior that

results from experience

• Neuroplasticity– The nervous system’s potential for change

that enhances its ability to adapt

– Required for learning and memory



• Hebb Synapse– “When the axon of cell A is near enough to

excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”

– D. O. Hebb, 1949



Habituation Response

• Habituation– Learning behavior in which a response to a

stimulus weakens with repeated stimulus presentations

– Example:• Gill withdrawal response in the marine snail Aplysia

californica


Role of Synapses in Three Kinds of Learning and MemorySensitization Response

• Sensitization– Learning behavior in which the response to a

stimulus strengthens with repeated presentations of that stimulus because the stimulus is novel or stronger than normal

– For example, after habituation has occurred



Long-Term Potentiation and Associative Learning

• Associative Learning– Linkage of two or more unrelated stimuli to elicit

a behavioral response

• Long-term Potentiation (LTP)– In response to stimulation at a synapse,

changed amplitude of an excitatory postsynaptic potential that lasts for hours to days or longer

– Plays a part in associative learning




• A strong burst of electrical stimulation applied to the presynaptic neuron produces an increase in the amplitude of the EPSP in the postsynaptic neuron

• First recorded in the hippocampus by Bliss and Lømø in 1973– Field Potential:

• EPSPs from many neurons; recorded with extracellular electrodes




Neurochemistry of LTP

• Glutamate acts on two different types of receptors on the postsynaptic membrane– AMPA (alpha-amino-3-hydroxy-5-methylisoa-

zole-4-proprionic acid)• Normally responds to glutamate

– NMDA (N-methyl-D-aspartate)• Doubly gated channels

• Normally blocked by magnesium (Mg2+) ions





• Two events must occur close together in time for NMDA receptors to open– Depolarization of postsynaptic membrane,

which displaces Mg2+ from pore (strong electrical stimulus)

– Activation by glutamate from the presynaptic neuron (weak electrical stimulus)

– Strong and weak stimuli have been paired





• After NMDA receptors open, Ca2+ enters the postsynaptic neuron, altering the postsynaptic neuron:– Increased responsiveness of AMPA receptors

to glutamate

– Formation of new AMPA receptors

– Retrograde messengers that trigger more glutamate release from presynaptic neuron




• Long-Term Depression (LTD)– Another form of synaptic plasticity– Neuron becomes less active in response to

repeated stimulation– Involves NMDA receptors – Requires Ca2+ entry:

• Decreased responsiveness of AMPA receptors• Decreased numbers of AMPA receptors



Learning As a Change in Synapse Number• What neural processes underlie the

persistent, long-term changes of learning?– Ca2+ enters postsynaptic neuron and activates

a second messenger (e.g., cyclic AMP)– cAMP alters gene expression in nucleus,

which physically alters synapse:• Structural changes in the synapse

– Dendritic spines

• Formation or loss of synapses

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