brain

Introduction to Brain and Behavior

  • Authors: Bryan Kolb, Ian Q. Whishaw, G. Campbell Teskey

  • Focuses on how neurons communicate and adapt, covering:

    • A chemical message

    • Varieties of neurotransmitters and receptors

    • Neurotransmitter systems and behavior

    • Adaptive role of synapses in learning and memory

A Chemical Message

  • Otto Loewi (1921): Conducted an experiment with frog hearts

    • First isolation of a chemical messenger

    • Demonstrated the role of the vagus nerve and neurotransmitter acetylcholine (ACh) in slowing heart rate

  • Acetylcholine (ACh):

    • First neurotransmitter discovered in both the peripheral nervous system and central nervous system (CNS)

    • Activates skeletal muscles in the somatic nervous system (SNS)

    • May excite or inhibit internal organs in the autonomic nervous system (ANS)

Research Focus 5-1: The Basis of Neural Communication in a Heartbeat

  • Heart rate adjusts to match energy expenditure and body’s nutrient and oxygen needs

  • Loewi's demonstration showed that the frog’s vagus nerve contains a chemical that signals the heart to slow down.

  • Later, Loewi identified two messenger chemicals:

    • Excitatory message: signals a speed up

    • Inhibitory message: signals a slow down

Experiment 5-1: How Does a Neuron Pass on a Message?

  • Loewi’s complementary experiments led to discoveries regarding other neurotransmitters:

    • Epinephrine (EP, or adrenaline):

    • Acts as a hormone to prepare the body for fight or flight during stress

    • Also serves as a neurotransmitter in the CNS

    • Norepinephrine (NE, or noradrenaline):

    • Neurotransmitter found in the brain and in the parasympathetic division of the autonomic nervous system

    • Accelerates heart rate in mammals

Neurotransmitters

  • Neurotransmitter:

    • Chemical released by a neuron onto a target, exerting an excitatory or inhibitory effect

    • Outside the CNS, many neurotransmitters circulate in the bloodstream as hormones

    • Slower action than that of neurotransmitters

  • Loewi's work sparked extensive research into the number of neurotransmitters

    • Actual number of transmitters: 60 confirmed; 200 posited

Structure of Synapses

  • Discovery of synaptic structure aided by electron microscopy in the 1950s

  • Neurotransmitters are packaged in vesicles at the end terminal of axons

  • Chemical synapse:

    • Junction where messenger molecules (neurotransmitters) are released from one neuron to excite or inhibit another

    • Most synapses in mammals are chemical

Chemical Synapse

  • Depicted in illustrations to demonstrate synaptic interactions

Clinical Focus 5-2: Parkinson Disease

  • James Parkinson initially described the condition; named Parkinson's disease by Jean-Martin Charcot

  • Key findings in understanding its neural basis:

    • Degeneration of the substantia nigra opposite the symptomatic side

    • Symptoms manifest when dopamine (DA) levels drop below 10% of normal in the basal ganglia

    • Experiments in rats indicated that selectively destroying dopamine-containing neurons produced Parkinsonian symptoms, affirming dopamine's role in the substantia nigra to basal ganglia pathway

Terms to Learn: Structure of Chemical Synapses

  • Presynaptic membrane: Axon terminal

  • Postsynaptic membrane: Dendritic spine

  • Synaptic cleft: Space between presynaptic and postsynaptic membranes

  • Tripartite synapse: Involves pre- and postsynaptic neurons and supporting glial cells

  • Synaptic vesicle: Contains neurotransmitters, located in presynaptic area

  • Storage granule: Presynaptic structure for holding neurotransmitters

  • Postsynaptic receptor: Receives neurotransmitters, located on postsynaptic neuron

  • Anterograde synaptic transmission: From presynaptic to postsynaptic

  • Transporter: Protein that moves neurotransmitters in the neuron

Neurotransmission in Five Steps

  • Steps of anterograde synaptic transmission:

    1. Neurotransmitter synthesis occurs within the neuron

    2. Neurotransmitter is packaged and stored in vesicles at the axon terminal

    3. During an action potential, neurotransmitter is released into the synaptic cleft

    4. Neurotransmitter binds to receptors on the postsynaptic membrane

    5. Neurotransmitter is degraded or removed to stop its action

Steps 1 and 2: Neurotransmitter Synthesis, Packaging, and Storage

  • Neurotransmitters derive from two main pathways:

    • Synthesized in axon terminal: Using dietary building blocks via transporters

    • Synthesized in cell body: Following DNA instructions (peptide transmitters), then transported to axon terminal

  • Origins of neurotransmitters include:

    • Peptide

    • Lipid

    • Gaseous

    • Ion

Step 3: Neurotransmitter Release

  • At the axon terminal, action potential opens voltage-sensitive calcium (Ca²⁺) channels

  • Ca²⁺ enters and binds to calmodulin forming a complex that causes vesicles to release neurotransmitter contents

  • Exocytosis: Process of transmitting neurotransmitters into the synaptic cleft

Step 4: Receptor-Site Activation

  • After neurotransmitter release, it diffuses across the synaptic cleft to activate postsynaptic receptors

  • Types of transmitter-activated receptors:

    • Ionotropic receptors: Directly control ion channels

    • Metabotropic receptors: Indirectly influence cellular activity through second messengers

    • Autoreceptors: Regulate neurotransmitter release

    • Quantum: The quantal release of neurotransmitters

Step 5: Neurotransmitter Inactivation

  • After signaling, neurotransmitters are removed from receptors and synaptic cleft through:

    1. Diffusion: Moving away from receptor sites

    2. Degradation: Enzymatic breakdown

    3. Reuptake: Neurotransmitter is reabsorbed into presynaptic neuron

    4. Astrocyte uptake: Supported by glial cells

The Versatile Synapse

  • Synapses exhibit considerable variation in type, location, structure, and function

  • This diversity allows synapses to be a flexible biological communication system

    • Transmitters can influence neuron actions by connecting to dendrites, cell body, or axon

Electrical Synapses

  • Gap junction: Contains connexin proteins connecting adjacent cell membranes to form hemichannels

  • Allows ions to pass directly between neurons

    • Regulated gates can open or close, facilitating rapid communication

    • Can enable dual chemical and electrical transmission (mixed synapse)

Excitatory and Inhibitory Messages

  • Excitatory synapse characteristics:

    • Typically located on dendrites

    • Round vesicles with dense material on membranes

    • Wider synaptic clefts

    • Larger active zones

  • Inhibitory synapse characteristics:

    • Found mainly on cell body

    • Flat vesicles with less dense material on membranes

    • Narrow synaptic clefts

    • Smaller active zones

Research Focus 5-3: Dendritic Spines; Small but Mighty

  • Dendritic spines: Key structures in learning and memory

    • Can act independently and perform structural changes

    • Impairments can lead to cognitive disabilities and Alzheimer’s disease

Evolution of Complex Neurotransmission Systems

  • Chemical communication may have originated from single-celled organisms’ feeding behaviors

    • Juices secreted via exocytosis to capture prey (similar to neurotransmitter release)

Quiz Questions

  • The neurotransmitter that accelerates the heart rate in mammals is called:

    • a. epinephrine

    • b. norepinephrine (correct)

    • c. acetylcholine

    • d. bradycardia

  • A chemical with an excitatory or inhibitory effect when released by a neuron onto a target is called:

    • a. molecule

    • b. neurotransmitter (correct)

    • c. impulse

    • d. messenger

  • Synaptic structure was first revealed in the 1950s, using:

    • a. electron microscopy (correct)

    • b. digital X-rays

    • c. chemotherapy

    • d. gamma radiation

Varieties of Neurotransmitters and Receptors

  • Extensive diversity in neurotransmitters and receptors allowing versatile interactions

    • Some neurotransmitters can have both excitatory and inhibitory effects depending on their site of action

    • Interaction among neurotransmitters at a single synapse may enhance overall potency

Criteria for Identifying Neurotransmitters

Identification of New CNS Neurotransmitters

  • Staining techniques: Identify specific chemicals in living tissues

  • Stimulating: Use of microelectrodes on neuronal targets

  • Collecting: Preserve tissue in saline to detect neuronal communication

Renshaw Loop

  • All motor-neuron axons from the spinal cord use acetylcholine to communicate

  • Each may have an axon collateral within the spinal cord synapsing on an interneuron, which then synapses back on the motor neuron's cell body

  • Toxin example: Strychnine blocks the Renshaw loop leading to increased motor neuron activity, risking respiratory failure

Broadening the Term Neurotransmitter

  • Expanded definition includes chemicals that:

    1. Transmit messages affecting postsynaptic voltage

    2. Alter synaptic structure

    3. Communicate retrograde signals to influence presynaptic activity

Classes of Neurotransmitters

  • Small-molecule transmitters: Quick-acting, synthesized in axon terminals

  • Peptide transmitters

  • Lipid transmitters

  • Gaseous transmitters

  • Ion transmitters

Small-Molecule Transmitters

  • Fast-acting neurotransmitters synthesized in axon terminals from nutrient sources

  • Table of Best-Known Small-Molecule Neurotransmitters:

    • Acetylcholine (ACh)

    • Amines:

    • Dopamine (DA)

    • Norepinephrine (NE)

    • Epinephrine (EP)

    • Serotonin (5-HT)

    • Amino Acids:

    • Glutamate (Glu)

    • Gamma-aminobutyric acid (GABA)

    • Glycine (Gly)

    • Histamine (H)

    • Purines:

    • Adenosine

    • Adenosine triphosphate (ATP)