PSYC 170 - Summer 2013 - Professor Claffey

Notes: Synapse

Overview

Conversion from an _____________ signal to a ________________ signal
    - electrical signal is the ______________________
    - chemical signal is the ______________________

Presynaptic - refers to ______________________ that sends/receives the signal
Postsynaptic - refers to ________________ that sends/receives the signal

________________________________(PSP)
    the membrane potential on the dendrite that is affected by activity in the synapse

Source: faculty.washington.edu/chudler/synapse.html

Directed and non-directed synapses
- depends on proximity between neurotransmitter and receptor

Neurotransmitter

The unit of communication between neurons

________________ neurotransmitters
- cause the post-synaptic neuron to depolarize/hyperpolarize, more/less likely to fire
- Excitatory Postsynaptic Potential (EPSP)

________________ neurotransmitters
- cause the post-synaptic neuron to depolarize/hyperpolarize, more/less likely to fire
- Inhibitory Postsynaptic Potential (IPSP)

Neurotransmitters will only influence the cells that have a receptor for it

synapse steps

Lifecycle of neurotransmitter

Synthesized in cell

Packed into ________________

________________ - released into synapse
    arriving action potential causes influx of Ca++ ions, which trigger exocytosis

Interact with ________________
     a molecule in the post synaptic membrane that reacts to neurotransmitter
     sometimes there are presynaptic receptors (autoreceptor)

Removed from synapse
     - reuptake transport
     - enzymatic degradation
     - diffusion

Receptors

Ionotropic

Permits the flow of ions
Ligand-gated, as opposed to voltage-gated channels in action potential
Neurotransmitter can cause channel to open or close
For example:
if Na+ channels are opened, cause EPSP/IPSP
if K+ channels are opened, cause EPSP/IPSP
Fast/short acting

Metabotropic

G-protein-coupled receptor
Subsequently can effect
- ion channels
- 2nd messenger
2nd messenger can alter DNA expression, having effects on cell functioning
Slow/long acting

Integration

A depolarization at one synapse is typically not enough to trigger an action potential
Each neuron can receive signals from 1000s of synapses
Integration - combining many different signals to determine if an action potential occurs

Temporal Integration
depolarizations that _________________________ may combine to cause an action potential

Spatial Integration
depolarizations that __________________________ may combine...

Types of neurotransmitters

More than 100 identified neurotransmitter substances

________________ (small NT)
    Molecules that make up proteins, found in our diet
    Most common neurotransmitters for fast acting, directed synapses

    ___________________ - main excitatory transmitter, sensory & learning systems

    ___________________ - main inhibitory transmitter

________________ (small NT)
    Not a typical synapse NT
    Release NT broadly throughout brain
    Produced by cells in the brain stem
    More likely to modify "global" function/state of the brain

    ___________________ - motor function and reward

    ___________________ - (adrenaline, adrenergic)

    ___________________- (nor-adrenaline, nor-adrenergic) - brain arousal, mood, hunger, sleep

    ___________________ - mood, temperature, aggression, sleep

______________________ (small NT)
    Neurons are called cholinergic
    Location
        Neuromuscular junction
        Throughout autonomic system (parts of our nervous system NOT under voluntary control)
        In parts of central nervous system - cortical arousal, memory

_________________________ (large NT)
    Full proteins made up of many amino acids
    >100 neuropeptides
    Many have role as both neurotransmitter and hormone
    Typically act at metabotropic receptors
    Examples: endorphins, enkephalin (another opioid), Neuropeptide Y (food intake)
    Long lasting effects

Drug action

Antagonist - decrease the effect of a neurotransmitter
Agonist - increase the effect of a neurotransmitter

Receptor agonists
Nicotine - stimulates acetylcholine receptors

Receptor blockers
Botox - nicotinic receptor antagonist

Reuptake blockers
Cocaine, amphetamines - impairs dopamine transporter

Source: http://www-scf.usc.edu/~uscience/adderall_abuse.html

Gap Junctions

A direct connection between the cytosol (internal fluid) of two cells
Faster than a chemical synapse
Doesn't require neurotransmitters
Not common in the brain, but extensive in heart muscle and retina

Synaptic Plasticity

Changes in connections between neurons can produce changes in behavior

"Neurons that fire together, wire together"
- paraphrasing Donald Hebb's 1949 theory of memory

Long Term Potentiation (LTP)

Typically studied in hippocampus

Process

Put a STIMULATING electrode in the upstream (presynaptic) neuron
Put a RECORDING electrode in the downstream (post-synaptic) neuron
Record the reaction of the downstream neuron to a single upstream stimulation
Provide high-frequency, high-intensity stimulation to upstream neuron (no measurement)
Record the reaction of the downstream neuron to a single upstream stimulation

Increased response in downstream neuron
Can be measured months later
Requires pre- and post- synaptic neurons to both have action potentials

Induction

Source: http://employees.csbsju.edu/ltennison/PSYC340/learning.htm

LTP Requires ___________ receptors
    NMDA receptors allow Ca++ to enter
        Ca++ causes changes in cell functioning, such as creating new receptors

    NMDA receptors are excited by ________________
        and require post-synaptic neuron to already be partially ______________

NMDA are able to detect the co-occurrence of

______________________ and ______________________ depolarization

Maintenance/Expression

The post-synaptic neuron does not strengthen connections with all pre-synaptic neurons,
only across synpases that were depolarized

LTP requires protein synthesis

Neuron is making new ____________ to "build" or strengthen the synapse

Relation to behavior

LTP is a model for memory, and memory is probably "like" LTP,
but LTP is an artificial technique done in a laboratory setting

Rats learning a maze task fail to learn if NMDA receptors are blocked (Morris et al 1986)

Rats learning an avoidance task produce new AMPA receptors, the same receptors produced by LTP. Also, in vitro synapse that were potentiated during training could not be further induced by LTP. (Whitlock et al, 2006)

Animals undergoing fear conditioning show evidence of LTP-like processes (Rogan et al, 1997)

Learning in a Sea Slug

Aplysia

A sea slug with 200 neurons studied by Eric Kandel
Has a gill which it will withdraw if its siphon is touched

These are two examples of synaptic changes that explain an animal's behavior

Habituation

Source: http://michaeldmann.net/mann18.html

If the siphon is repeatedly touched, the gill withdrawal reflex disappears
With repeated activation, the neurons contain less dopamine and release less dopamine each time
Eventually gill withdrawal stops

A "short term" change in synaptic activity that does NOT require building new proteins

Sensitization

Source: http://michaeldmann.net/mann18.html

If the tail is shocked at the same time the siphon is touched:
    Gill is more vigorously withdrawn to a siphon touch
     Siphon has become "more sensitive"

Facilitating interneuron releases serotonin onto sensory neuron
    Causes the sensory neuron to release more neurotransmitter when the siphon is stimulated
    Causes motor neuron to react more vigorously

A "short term" change in synaptic activity that does NOT require building new proteins
    (can be made "long term" with more training)