Download NEURONS COMMUNICATE WITH OTHER CELLS AT SYNAPSES

NEURONS
C O M M U N I C AT E W I T H
OTHER CELLS AT
SYNAPSES
34.3
NEURONS COMMUNICATE WITH
OTHER CELLS AT SYNAPSES
• Neurons communicate with other neurons or target cells at synapses.
• Chemical synapse: a very narrow space between cells (synaptic cleft) that an action potential
cannot cross
– When an action potential arrives at the end of the presynaptic cell, a neurotransmitter is released
that diffuses across the space.
CHEMICAL SYNAPSES ARE MOST COMMON,
BUT ELECTRICAL SYNAPSES ALSO EXIST
• Neurotransmitters diffuse across the
synaptic cleft very rapidly (short
distance).
• They bind to receptors on the
postsynaptic cell membrane, which
generates another action potential or
other change.
• Neurotransmitters are quickly removed
from the cleft—to end signal
transmission—by enzymatic breakdown,
uptake by other neurons or glial cells, or
reuptake by the presynaptic cell.
ELECTRICAL SYNAPSE
• Electrical synapse: cells are joined by gap junctions where the cytoplasm is continuous;
signals cross with essentially no delay
– They occur where very fast, invariant signal transmission is needed, such as neurons that control
escape swimming in some fish.
– Also occur where many cells must be stimulated to act together, such as fish electric organs.
VERTEBRATE NEUROMUSCULAR JUNCTION
IS A MODEL CHEMICAL SYNAPSE
• Neuromuscular junctions: chemical synapses between motor neurons and skeletal muscle
cells.
• The axon of the presynaptic cell branches close to the muscle cell, creating several axon
terminals (boutons) that synapse with the muscle cell.
NEUROMUSCULAR JUNCTIONS
• An action potential causes voltage-gated Ca+ channels to open in the presynaptic membrane,
allowing Ca+ to flow in.
• This induces release of the neurotransmitter acetylcholine (ACh):
– ACh is stored in vesicles that fuse with the cell membrane to release ACh into the cleft by
exocytosis.
CONCEPT 34.3 NEURONS COMMUNICATE
WITH OTHER CELLS AT SYNAPSES
• ACh diffuses across the cleft and
binds to receptors on the
postsynaptic cell.
• These receptors allow Na+ and K+
to flow through, and the increase in
Na+ depolarizes the membrane.
• If it reaches threshold, more Na+
voltage-gated channels are activated
and an action potential is generated.
• Synaptic Transmission
• Neurons and Synapses
• Put some Ach into it!
MANY NEUROTRANSMITTERS ARE KNOWN
• Three categories of neurotransmitters:
• Amino acids—glutamate, glycine, and
γ-aminobutyric acid (GABA)
– Biogenic amines include acetylcholine, dopamine, norepinephrine, and serotonin
– A variety of peptides (strings of amino acids)
MANY NEUROTRANSMITTERS ARE KNOWN
• In the brain, a postsynaptic neuron
may have chemical synapses with
hundreds or thousands of presynaptic
neurons, which may use different
neurotransmitters.
• Receptors for a given
neurotransmitter on the postsynaptic
cell may be of different types with
different actions.
• This complexity in synapse function
helps explain the complexity of brain
function.
SYNAPSES CAN BE FAST OR SLOW
DEPENDING ON THE NATURE OF RECEPTORS
• Two broad classes of receptors are recognized, they are fast or slow
• Neurotransmitter receptors:
• Ionotropic receptors are ligand-gated ion channels—cause changes in ion movement; response is
fast and short-lived.
• Metabotropic receptors are G protein-linked receptors that produce second messengers that
induce signaling cascades; responses are slower and longer-lived.
Khan Academy Video
FAST SYNAPSES PRODUCE POSTSYNAPTIC
POTENTIALS THAT SUM TO DETERMINE ACTION
POTENTIAL PRODUCTION
• Excitatory synapses produce graded membrane depolarizations called excitatory
postsynaptic potentials (EPSPs); shift membrane potential towards threshold.
• Inhibitory synapses shift membrane potential away from threshold; produce graded
membrane hyperpolarizations called inhibitory postsynaptic potentials (IPSPs).
FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS
THAT SUM TO DETERMINE ACTION POTENTIAL
PRODUCTION
• Each EPSP or IPSP is usually
less than 1 mV, and
disappears in 10–20
milliseconds.
• They are graded potentials,
typically produced at
synapses on dendrites and
the cell body.
• They affect membrane
potential at the axon hillock,
where action potentials are
generated.
• Summation of the graded
potentials is both temporal
(must be present at the same
time), and spatial.
FAST SYNAPSES PRODUCE POSTSYNAPTIC
POTENTIALS THAT SUM TO DETERMINE ACTION
POTENTIAL PRODUCTION
• The postsynaptic cell sums the
excitatory and inhibitory input.
• Summation determines whether the
postsynaptic cell produces action
potentials.
• If the sum of EPSPs and IPSPs at the
axon hillock is great enough to reach
threshold, an action potential is
produced.
SYNAPTIC PLASTICITY
• Synaptic plasticity: synapses in an individual can undergo long-term changes in functional
properties and physical shape during the individual’s lifetime.
• This may be one of the major mechanisms of learning.
– Experiences at one time in life produce long-term changes in synapses, so that future experiences
are processed by the nervous system in altered ways.
SYNAPTIC PLASTICITY
• Sea hares (mollusks) pull their gills inside when certain parts of the body are touched:
• They withdraw their gills more vigorously if they have previously been exposed to a noxious
agent (sensitization).
• The synapses between the sensory neurons and the motor neurons for gill withdrawal are
functionally strengthened—more neurotransmitter is released per impulse.
• The postsynaptic cell is thus excited to a greater degree.
SYNAPTIC PLASTICITY
• In mammals, the hippocampus is associated
with spatial learning and memory formation.
• In studies of mice brains, when a circuit is
repeatedly stimulated, the postsynaptic
structures physically grow and the synapses
strengthen functionally.
• The postsynaptic receptor molecules
increase, increasing response.
• Synaptic plasticity has been shown to
depend on second messengers, altered
protein synthesis, and altered gene
transcription.
SYNAPTIC PLASTICITY
• In studies of mice brains, when a circuit is repeatedly stimulated, the postsynaptic structures
physically grow and the synapses strengthen functionally.
• The postsynaptic receptor molecules increase, increasing response.
• Synaptic plasticity has been shown to depend on second messengers, altered protein synthesis,
and altered gene transcription.
Synaptic Plasticity 1
Brain Repair - TedEd