A&P: ch 11 Synapses and Neurotransmitters

part: 3/3

Synapses

• What is the function of synapses?

  • What are the two types of synapses?

• Transmit signals between cells

  • Electrical synapses & chemical synapses

• How do electrical synapses transmit signals?

• How do chemical synapses transmit signals?

• Through gap junctions

• By releasing neurotransmitters across the synaptic cleft

Anatomy of a Chemical Synapse

• What usually forms the presynaptic neuron?

• What structures are located at axon terminals?

• An axon

• Synaptic vesicles containing neurotransmitters

• What is the synaptic cleft?

  • What does the postsynaptic cell contain?

    • What are the three possible locations of the postsynaptic structure?

• The small space between cells

  • Neurotransmitter receptors

    • Dendrite, cell body, or axon

• What is an axodendritic synapse?

  • What is an axosomatic synapse?

    • What is an axo axonal synapse?

• A synapse between an axon and dendrite

  • A synapse between an axon and cell body

    • A synapse between two axons

Signal Transfer Across the Synapse

• What activates voltage-gated Ca+ channels?

  • What happens when Ca+ enters the presynaptic terminal?

    • What process releases neurotransmitters into the synaptic cleft?

      • How do neurotransmitters cross the synaptic cleft?

• An action potential

  • Synaptic vesicles fuse with the presynaptic membrane

    • Exocytosis

      • Diffusion

• What do neurotransmitters bind to on the postsynaptic membrane?

  • Binding of neurotransmitters opens what type of channels?

    • What happens when chemically-gated ion channels open?

• Specific protein receptors

  • Chemically-gated ion channels

    • Ions flow into the postsynaptic cell

Postsynaptic Potentials

What are the two types of postsynaptic potentials?

EPSPs and IPSPs

• What does EPSP stand for?

• What ion movement occurs during an EPSP?

  • What effect does an EPSP have on the inside of the cell?

    • Does an EPSP depolarize or hyperpolarize the membrane?

• Excitatory postsynaptic potential

• Na+ influx

  • Makes it more positive

    • Depolarize

• What does IPSP stand for?

• What ion movements occur during an IPSP?

  • What effect does an IPSP have on the inside of the cell?

    • Does an IPSP depolarize or hyperpolarize the membrane?

• Inhibitory postsynaptic potential

• Cl− influx or K+ efflux

  • Makes it more negative

    • Hyperpolarize

Signal Strength

• What determines synaptic signal strength?

• Synaptic signal strength is directly proportional to what two factors?

• Amount of neurotransmitter released and time neurotransmitter remains in the synapse

• Amount of neurotransmitter released and duration in the synapse

Termination of the Signal

• What are the four ways synaptic signals terminate?

• What is reuptake?

  • What causes neurotransmitter degradation?

    • What happens during diffusion termination?

• Cessation of neurotransmitter release, reuptake, degradation by enzymes, and diffusion

• Neurotransmitters are taken back up by surrounding cells

  • Enzymes

    • Neurotransmitters diffuse out of the synapse

Integration of Synaptic Events

Summation

• What is temporal summation?

  • Why might temporal summation fail?

• Combining stimuli from events occurring close in time

  • If the time interval between signals is too great

• What is spatial summation?

  • Why can spatial summation cancel out signals?

• Combining stimuli from events occurring close physically

  • Excitatory and inhibitory signals may combine

Synaptic Potentiation

• What is synaptic potentiation?

  • What increases during synaptic potentiation?

• Repeated use of a synapse enhances its ability to excite a postsynaptic neuron

  • Ca+ concentration in the presynaptic terminal

Presynaptic Inhibition

• What is presynaptic inhibition?

  • Which type of inhibition directly inhibits neurotransmitter release?

• Inhibitory neurotransmitter prevents release of excitatory neurotransmitter from another neuron

  • Presynaptic inhibition

Postsynaptic Inhibition

• What is postsynaptic inhibition?

  • Which type of inhibition directly inhibits development of an action potential?

• Inhibitory neurotransmitter negates excitatory neurotransmitter effects in the postsynaptic cell

  • Postsynaptic inhibition

Neurotransmitters Overview

• Approximately how many neurotransmitters have been identified?

  • Can the same neuron release different neurotransmitters?

    • What determines which neurotransmitter may be released?

• More than 50

  • Yes

    • Different stimulation frequencies

Acetylcholine (ACh)

• Where is acetylcholine released?

  • What toxin inhibits acetylcholine release?

    • What blocks acetylcholine receptors on muscle cells?

• Neuromuscular junctions and the CNS

  • Botulism toxin

    • Curare

• What disease destroys acetylcholine receptors?

  • What enzyme degrades acetylcholine?’

    • inhibits acetylcholinesterase?

• Myasthenia gravis

  • Acetylcholinesterase

    • Nerve gas and insecticides like malathion

Biogenic Amines

Norepinephrine

• What is the function of norepinephrine?

  • What drugs enhance norepinephrine release?

    • What blocks norepinephrine degradation?

• Prepares the body for action

  • Amphetamines

    • Antidepressants and cocaine

Dopamine

• What is dopamine associated with?

  • Dopamine deficiency is associated with what disease?

    • Increased dopamine is associated with what disorder?

• Feel-good and reward-motivated behavior

  • Parkinson’s disease

    • Schizophrenia

Amino Acid Neurotransmitters

• What is the principal inhibitory neurotransmitter of the brain?.

  • What is the principal inhibitory neurotransmitter of the spinal cord?

    • What is the principal excitatory neurotransmitter of the nervous system?

• GABA

  • Glycine

    • Glutamate

Peptides

What are endorphins?

Natural opiates that inhibit pain

Neurotransmitter Receptors

Ionotropic Receptors

• What are ionotropic receptors?

• Do ionotropic receptors produce rapid or slow responses?

• Ligand-gated ion channels

• Rapid responses

Metabotropic Receptors

• What are metabotropic receptors?

  • Do metabotropic receptors produce rapid or slow responses?

• G-protein coupled receptors

  • Slower responses

Specific Synapses: Cholinergic Synapse

• What neurotransmitter is used at cholinergic synapses?

• What type of receptors are found at cholinergic synapses?

  • What ion movement occurs at cholinergic synapses?

    • Are cholinergic synapses excitatory or inhibitory?

• Acetylcholine

• Chemically-gated ion channel receptors

  • Net influx of Na+

    • Excitatory

GABA-ergic Synapse

• What neurotransmitter is used at GABA-ergic synapses?

  • What type of receptors are found at GABA-ergic synapses?

  • What ion movement occurs at GABA-ergic synapses?

    • Are GABA-ergic synapses excitatory or inhibitory?

• GABA

  • Chemically-gated ion channel receptors

  • Net influx of Cl−

    • Inhibitory

Adrenergic Synapse

• What neurotransmitter is used at adrenergic synapses?

  • What type of receptors are found at adrenergic synapses?

  • What type of response occurs at adrenergic synapses?

• Norepinephrine

  • G-protein coupled receptors

  • Delayed response with amplification

Neuronal Pools

• What are neuronal pools?

  • What is the function of a diverging circuit?

• Functional groups of neurons

  • Amplification

What is the function of a converging circuit?

Concentrating signals

What is the function of a reverberating circuit?

Oscillating signals

What is the function of a parallel after-discharge circuit?

Produces a burst of impulses

ch 11 Nerve Physiology: Membrane Ion Channels

1. What are non-gated ion channels also called?

2. Are leakage channels always open or closed?

3. Which ions commonly move through leakage channels?

4. What are aquaporins?

1. Leakage channels

2. Always open

3. K+, Na+, and water through aquaporins

4. Water leakage channels

Gated Ion Channels

• What are the three types of gated ion channels?

• What do mechanically gated ion channels respond to?

• Give examples of stimuli for mechanically gated channels.

• Mechanical, chemical, and voltage-gated channels

• Physical forces

• Touch, pressure, and sound

• What do chemically gated ion channels respond to?

  • Give examples associated with chemically gated channels.

• Ligands

  • Smell, taste, and somatic motor pathways

• What do voltage-gated ion channels respond to?

• Voltage-gated channels are important for what process?

• Changes in membrane potential

• Action potentials

Activity of Gated Ion Channels

1. What happens when gated ion channels open?

2. What drives ion diffusion through channels?

   1. What does ion flow across the membrane create?

   2. Ion flow across the membrane can generate what major event?

1. Rapid diffusion of ions occurs

2. Concentration gradients and charge gradients

   1. Electrical current

   2. Action potential

Resting Membrane Potential

• What is resting membrane potential?

  • What causes resting membrane potential?

• Which ion concentration is highest inside the cell (ICF)?

• Which ion concentration is highest outside the cell (ECF)?

• The electrical potential across the membrane of a resting cell

  • Differences in ion concentrations across the membrane

• Potassium (K+)

• Sodium (Na+)

• What is the resting membrane potential of a neuron?

  • Is the outside of the membrane more positive or less positive at rest?

    • Is the inside of the membrane more positive or less positive at rest?

• Approximately -70 mV

  • More positive

    • Less positive

• What term describes the membrane at resting membrane potential?

  • What is depolarization?

  • What is hyperpolarization?

• Polarized

  • A membrane potential greater than -70 mV

  • A membrane potential less than -70 mV

1. What causes rapid leakage out of the cell?

2. What causes slow leakage into the cell?

1. K+ leakage

2. Na+ leakage

• What maintains the resting membrane potential?

  • How many sodium ions does the sodium-potassium pump move out?

    • How many potassium ions does the sodium-potassium pump move in?

• Sodium-potassium pump

  • Three Na+ out

    • Two K+ in

• What can change membrane potential?

  • Ex?

• Changes in membrane permeability and ion concentration gradients

  • depolarizing

  • hyperpolarizing

Graded Potentials

• What are graded potentials also called?

• What do graded potentials reflect?

  • Can graded potentials be strong or weak?

• Local potentials

• Stimulus strength

  • Yes

• What are the two strength categories of graded potentials?

• What determines whether a graded potential reaches threshold?

• Subthreshold and suprathreshold

• Strength at the trigger zone (axon hillock)

Why do graded potentials lose strength over distance?

Current leak and cytoplasmic resistance

• What is an inhibitory graded potential?

• What is an excitatory graded potential?

• Hyperpolarization

• Depolarization

Action Potentials

• What type of response is an action potential?

• What must be reached to trigger an action potential?

  • Do action potentials travel short or long distances?

• All-or-none response

• Threshold

  • Long

• Where do action potentials travel from and to?

• Why are action potentials high speed?

• Axon hillock to axon terminals

• Sequential ion channel opening

Generating an Action Potential

During resting potential, are Na+ and K+ gates open or closed?

Closed

1. What causes depolarization to begin?

2. What happens when threshold is reached?

3. During depolarization, which ion flows into the axon?

4. What happens to the inside of the axon during depolarization?

5. Why does depolarization stop?

1. A graded potential reaching threshold

2. Voltage-activated sodium gates open quickly

3. Na+

4. It becomes positive

5. Voltage inactivation gates close

• What happens during repolarization?

  • During repolarization, which ion flows out of the axon?

    • What happens to the inside of the axon during repolarization?

• Voltage-gated K+ gates open

  • K+

    • It becomes negative again

• What causes hyperpolarization?

  • What happens to membrane potential during hyperpolarization?

• K+ gates remain open

  • It becomes more negative than resting potential

Propagation of an Action Potential

• How does an action potential propagate?

  • What allows action potentials to move in one direction?

• It is self-propagating in one direction

  • Sequential opening and closing of Na+ gates

Refractory Periods

• What is the refractory period?

  • What occurs during the absolute refractory period?

    • What occurs during the relative refractory period?

• The period when a new action potential cannot be initiated

  • Sodium gates are inactivated

    • The membrane is hyperpolarized

Signal Strength

• Do all action potentials have the same strength?

  • How does a strong stimulus create a stronger signal?

• Yes

  • By increasing the frequency rate of action potentials

Conduction Velocity

• What two factors affect conduction velocity?

• How does axon diameter affect conduction speed?

  • How does myelination affect conduction speed?

    • What type of conduction occurs in myelinated axons?

• Axon diameter and myelination

• Larger diameter axons conduct faster

  • It increases conduction speed

    • Saltatory (salutatory) conduction