Download Synaptic Transmission

PHYSIOLOGY 1
LECTURE 14
SYNAPTIC TRANSMISSION
SYNAPTIC TRANSMISSION

Objectives: The student should know
– 1. The types of synapses, electrical and
chemical
– 2. The structure and function of synapses
– 3. Neurotransmitters, types, synthesis and
removal
SYNAPTIC TRANSMISSION
Synapses are one of the means by
which excitable cells communicate with
one another. Nerve to nerve synapse,
nerve to muscle - neuromuscular
junction, and nerve to gland neuroglandular junction
 There are two general types of
synapses

– Electrical Synapses (Gap Junctions)
– Chemical Synapses (Neurological)
SYNAPTIC TRANSMISSION
TYPES of SYNAPSES
Electrical Synapses  1. Characteristics of electrical synapses

– a. Gap junctions
– b. Cytoplasmic continuity - direct ionic
pathway cell to cell
– c. No delay in transmission of the AP
– d. Can be unidirectional or bidirectional
– e. Locations - cell membrane to cell
membrane protein interconnection
SYNAPTIC TRANSMISSION
TYPES of SYNAPSES

2. Types of Electrical Synapses (2)
– a. Non-rectifying (bidirectional) electrical
synapses - open channel between cells
– b. Rectifying (unidirectional) electrical
synapse - one way traffic
SYNAPTIC TRANSMISSION
Electrical Synapse


Nonrectifying
Electrical Synapse This is a simple gap
junction which when
open connects cells
cytosol to cytosol.
Ions can move
freely back and forth
between cells,
hence, action
potentials.
SYNAPTIC TRANSMISSION
Electrical Synapses


Rectifying Electrical
Synapse Because of the
nature of the
proteins making up
these gap junctions
ions may only move
in one direction Example - From cell
A to Cell B but not
the reverse
SYNAPTIC TRANSMISSION
Electrical Synapses


Cytoplasmic
Continuity Gap junctions allow
closely aligned cells
to communicate
cytoplasm to
cytoplasm by
exchanging ions and
other dissolved
particles up to about
1500 mol. weight.
SYNAPTIC TRANSMISSION
Chemical Synapses
B. Chemical Synapses

Important characteristics -- 1. one way transmission
 2. time delay
 3. exocytosis
 4. diffusion
 5. receptor activation
SYNAPTIC TRANSMISSION
Chemical Synapses

Anatomy of a Synapse
– 1. Neuron structures - dendrites, soma
(body), axon, terminal end or bouton, and
axon hillock
– 2. Synapse
 a. Axosomatic synapse
 b. Axodendritic synapse
 c. Axoaxonic synapse (always
inhibitory)
SYNAPTIC TRANSMISSION
Chemical Synapses
SYNAPTIC TRANSMISSION
Chemical Synapses
SYNAPTIC TRANSMISSION
Chemical Synapses

B. Pattern of Chemical Synaptic Transmission
– 1. Distinct separation of presynaptic and
postsynaptic cells
– 2. Can synapse on dendrites, soma, or axon
– 3. Nerves usually synapse with more than one
nerve normally many nerves
– 4. AP in presynaptic cell converted to chemical
signal
– Postsynaptic membrane receives chemical signal
and membrane potential is altered - Postsynaptic AP
may or may not occur
SYNAPTIC TRANSMISSION
Chemical Synapses

C. Characteristics of a Chemical Synapse
– 1. One way signal conduction
– 2. Synaptic delay
– 3. Transmitter can alter the conductance of the
postsynaptic membrane to Na+, K+, and CL– 4. A change in membrane conductance alters
membrane potential
– 5. Central tendency – postsynaptic nerves near the
center of the axonal ending cone have a greater
chance of passing on an action potential
SYNAPTIC TRANSMISSION
Chemical Synapses
5. AP is NOT produced at the synapse
a. change in membrane potential is conducted
electronically (local graded response) across
the soma
 b. membrane threshold is lower at axon hillock
 c. AP will be generated IF the sum of all inputs
to the cell causes the membrane potential to
reach threshold

SYNAPTIC TRANSMISSION
Chemical Synapses

D. Synaptic Transmission
– 1. Presynaptic action potential
 a. Effect at the bouton  Opening of voltage gated Ca++ channels  Influx of Ca++ at the axon terminal
 Ca++ activates calmodulin which in turn
activates a protein kinase which phosphorylates
the tethering proteins holding the
neurotransmitter vesicles causing release
SYNAPTIC TRANSMISSION
Chemical Synapses
– b. Quanta release of
neurotransmitter
1. Each vesicle contains nearly the
same number of neurotransmitters
 2. For each presynaptic cellular action
potential nearly the same number of
vesicles are released
 3. Therefore, we have about the same
amount of neurotransmitter released for
each presynaptic action potential Quanta Release

SYNAPTIC TRANSMISSION
Chemical Synapses
SYNAPTIC TRANSMISSION
Chemical Synapses
2. Synapse
 Transmitters diffuse from the
presynaptic membrane to the
postsynaptic membrane passing
through the basement membrane.

SYNAPTIC TRANSMISSION
Chemical Synapses

3. Postsynaptic membrane
– a. Contains receptors for the
neurotransmitters
– b. Binding of the neurotransmitter causes
a change in the postsynaptic membrane
potential (EPSP or IPSP)
– c. No single excitatory or inhibitory input
can bring the soma membrane to threshold
SYNAPTIC TRANSMISSION
Chemical Synapses
SYNAPTIC TRANSMISSION
Chemical Synapses

4. Receptors
– a. Excitation
 1) opening of Na+ channels
 2) depressed conduction through CL- or
K+ channels
– b. Inhibition
 1) opening Cl- channels
 2) Increased conduction through K+
channels
SYNAPTIC TRANSMISSION
Chemical Synapses

5. Summation
– a. If the momentary sum of all of the
EPSP bring the the axon hillock membrane
to threshold it will fire an action potential
– b. Summation is the nerve process of
integrating various inputs (decision making
process - To fire an AP or not to fire an AP)
SYNAPTIC TRANSMISSION
Chemical Synapses

Summation (Cont.)
– c. Spacial summation
 1) Occurs when two or more inputs arrive
simultaneously
 2) Two inputs are added and two EPSP inputs
will depolarize the membrane twice as much as a
single input
 3) one IPSP + one EPSP = zero postsynaptic
membrane change
SYNAPTIC TRANSMISSION
Chemical Synapses

Summation (Cont.)
– d. Temporal summation
– 1) two or more action potentials in a single
presynaptic neuron occurring in rapid succession
cause the postsynaptic membrane to depolarize or
hyperpolarize more than it would with a single input
– 2) High enough excitatory presynaptic firing rate
(frequency) could cause the generation of an action
potential in the postsynaptic nerve
SYNAPTIC TRANSMISSION
Chemical Synapses
SYNAPTIC TRANSMISSION
Chemical Synapses
Nerve to Nerve Synapse  Nerves have thousands of other nerves
connecting to them. The signal from any one
nerve generally does not generate an action
potential in the postsynaptic neuron unless the
presynaptic nerve fires very rapidly (Temporal
Summation). Normally it requires several
nerves firing at the same time to generate an
action potential in the postsynaptic neuron
(Spaceal Summation).

SYNAPTIC TRANSMISSION
Chemical Synapses

Furthermore, while some presynaptic
neurons are stimulatory others are
inhibitory so the end physiological
response in the postsynaptic neuron is
determined by the relative numbers of
stimulatory neurons versus inhibitory
neurons firing at any one time.
SYNAPTIC TRANSMISSION
Chemical Synapses
Neurotransmitters

Neurotransmitters are divided into two
groups depending on the rate of action
– A. Small-molecule - rapidly acting
transmitters (usually open ion channels)
 1. Acetylcholine
 2. Amines
 3. Amino acids
 4. NO
Neurotransmitters

B. Neuropeptides - action is slow
(usually act on DNA or through second
messenger systems) - Released in very
small quantities but effect is very potent
– 1. Opioids
– 2. GI peptides
– 3. Hypothalamic and pituitary peptides
Neurotransmitters

C. Fate of released neurotransmitters
– 1. Neuropeptides - diffusion and
enzymatic hydrolysis
– 2. Small molecule transmitters
 1) diffusion
 2) enzymatic hydrolysis
 3) re-uptake into the presynaptic
terminal
 4) Bind to receptor than degradation
(enzymatic hydrolysis)
SUMMARY
1. What are synapses, junctions?
 2. What are the types of synapse and how do
they differ?
 3. What is the structure of synapses?
 4. What are the types of neurotransmitters?
 5. What are the possible fates of
neurotransmitters?
