Neurophysiology II:
The Synapse
Synapse Defined
Space between adjacent neurons!
Relays information from one neuron
to another!
Neuron  Neuron
Neuron  Muscle
Neuron  Gland
Axodendritic Synapses
Most common synapse type in body
How do synapses work?
ELECTRICAL SYNAPSES:
Allow ion flow from AP to continue from
neuron to neuron
Transmission of information is extremely
rapid!
Allow for synchronized neuron activity!
Electrical Synapses continued
Isolated in the body!
CNS:
Sleep arousal
Concentration
Emotion/memory centers
Embryonic Nervous Tissue:
Predominant synapse type!
Will develop into…
CHEMICAL SYNAPSES!
Release chemicals across synapse from neuron to
neuron (or muscle, gland)
Chemicals are known as:
N E U R O T R A N S M I T T E R S!
CHEMICAL SYNAPSES ARE:
THE MOST COMMON TYPE OF
SNYAPSE IN THE BODY!
Composed of Three Major Areas:
Axon terminal on Presynaptic Neuron!
Synaptic Cleft (the space!)
Receptor region on Postsynaptic Neuron!
Vesicles of
neurotransmitter
Axon
terminal
presynaptic
neuron
Synaptic
cleft
postsynaptic neuron
Receptors
Neuron functioning is…
AN
ELECTROCHEMICAL
EVENT!!
The “Electro” Part:
Generation of Action Potential along
axon
AP will “fizzle out” at axon terminal!
The “Chemical” Part
Neurotransmitters are:
1. Released from presynaptic axon terminal
2. Diffused across synaptic cleft
3. Bound to receptors on postsynaptic cell
How the “Chemical” Part Works…
The Players:
NEUROTRANSMITTER
(stored in vesicles)
CALCIUM
The Steps:
1. AP reaches axon terminal
2. Voltage-gated Calcium Channels
open!
3. Ca++ rushes into axon terminal
membrane!
(will get pumped back out later)
4. Influx of Calcium causes vesicles
(containing neurotransmitter) to fuse
with axon terminal membrane
5. Exocytosis occurs!
6. Neurotransmitter released into
synaptic cleft
7. Neurotransmitter diffuses across
synaptic cleft
8. Neurotransmitter binds to receptor
proteins on membrane of
postsynaptic neuron.
9. Ion channels on postsynaptic
membrane open!
10. Membrane permeability changes
11. EPSP (Excitatory Post-Synaptic
Potential) or IPSP (Inhibitory PostSynaptic Potential) begins!
EPSP vs.
IPSP
Determined by type of
neurotransmitter released
EPSP neurotransmitter stimulates
the neuron to depolarize at the axon
(AP occurs!)
(What axon channels will open to
do this?)
IPSP  neurotransmitter inhibits
depolarization at the axon (NO AP!)
Instead…
neurotransmitter causes
hyperpolarization at axon!
K+ channels open  K+ moves out!
Cl- channels open Cl- moves in!
Now that it’s over…
HOW DO YOU
STOP THE EFFECTS OF
THE
NEUROTRANSMITTER?
3 OPTIONS!
1.Neurotransmitter is broken down by
enzymes
2. Neurotransmitter is
Reabsorbed/recycled by presynaptic
cell
3. Neurotransmitter diffuses out of
synapse
ACh=neurotransmitter
E= Enzyme
Summary of
Neurotransmitters…
chemical messengers
present in terminal and discharged upon
stimulation
produce membrane potential changes on
postsynaptic cell
naturally removed from synapse
most cells make and respond to more than
one neurotransmitter
Over 50 chemicals are
known to function as
NEUROTRANSMITTERS!
BUT 1 chemical is the
most well known!
ACETYLCHOLINE
1st neurotransmitter idenitified
Used at neuromuscular junctions
Excitatory to skeletal muscles!
(EPSP)
Effect : muscle contraction
Abbreviated ACh
ACh=neurotransmitter
E= Enzyme
ACETYLCHOLINESTERASE
ENZYME USED TO BREAK DOWN
ACh bound to post synaptic
receptors!
Abbreviated AChE
NERVE AGENTS
Other Neurotransmitters
NAME
Norepinephrine
Dopamine
Seratonin
Histamine
Endorphins
GABA
EFFECT
Feelings of Euphoria
Feelings of Euphoria
Sleep State
Inflammation
Pain Inhibitor
Major IPSP Effect!
Cholinergic synapses
Use acetylcholine as NT
Parasympathetic Division
Adrenergic Synapses
Use noradrenaline as NT
Sympathetic Division
Endorphins
vs. Enkephalins
“The Pain Killers”
Bind to
postsynaptic
receptors
Block calcium
channels in
presynaptic neuron
Parkinsons Disease
Causes of Parkinsons
Dopamine deficiency!
Results in contraction of antagonistic
muscles!
Results in uncontrolled shaking