Download 4. Nervous System: Synapses

Synapses
Synapse
• Location where information is transmitted
from presynaptic neuron to postsynaptic
neuron- gap where axon terminal meets
dendrite of next neuron
• Neurotransmitter molecules released into
this space
• Synapses can be electrical or chemical
• Electrical synapse: electrical current flows
between 2 cells
– Via Gap Junction
– Cell membranes are
continuous
– in cardiac muscle
and some smooth
muscle cells
• Chemical synapse
– Neurotransmitters: chemicals that move from one
cell to the next to carry the signal across a space
to the next cell
• Examples:
acetylcholine
serotonin
norepinephrine
histamine
dopamine
3 parts of synapse:
• 1. synaptic knob—bulge at end of one axon
terminal of presynaptic neuron
• 2. synaptic cleft-tiny (25 nm) gap between
two neurons
• 3. plasma membrane of post synaptic
neuron– usually at the dendrite or cell
body- contains protein receptors
Synaptic transmission
• Action potential itself cannot cross cleft
• Instead neurotransmitters released from
knob, attach to receptors in next neuron,
and cause response in next neuron
Neurotransmitters
• Two categories:
– Excitatory: cause depolarization in postsynaptic
neuron (stimulate next neuron)
– Inhibitory: cause hyperpolarization of
postsynaptic membrane that inhibits/stops the
potential from moving on
• Drugs can be inhibitory or excitatory
• Endorphins are inhibitory- block pain
Neurotransmitters
• Neurotransmitters cause ion channels to
open
– Some channels let Na+ ions in  depolarization
– Other channels let K+ ions out and Cl- in 
hyperpolarization that inhibits action potential
Summation
• Summation: amount released by one knob
won’t start AP in next neuron—may need
several working together or “rapid fire” of
repeated stimulation= summation
• Does all sensory information received by
sensory neurons get transmitted to conscious
part of brain?
Summation…
• Much sensory information gets stopped at
synapses where threshold was not reached
• Summation= mechanism for making decisions
about what info is important to move on
Synapses and Memory
• Theory: info is stored in form of increased flow
of info at synapses in particular pathways
• Certain neurotransmitters and structural
changes in synapses may affect short vs.
intermediate vs. long term memories
• Ex: more synapses and receptors are built
when we create long term memories