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Chapter 45: Synapses Transmission of Nerve Impulses
Between Neurons
Chad Smurthwaite & Jordan Shellmire
The Chemical Synapse The most common type of synapse
used for signal transmission in the
central nervous system
Anatomy of a Chemical Synapse
Soma - Main body of a neuron
Axon - Extensions from the soma
into a peripheral nerve that
leaves the spinal cord
Dendrites: Numerous branching
projections from the soma
Transmission of Signals
The first neuron converts the electrical signal into a chemical one
The first neuron secretes a neurotransmitter to stimulate the second neuron
The neurotransmitter is sent across the synaptic cleft
Where it binds with proteins in the membrane of the second neuron that stimulates the neuron
It can excite, inhibit, or modify its sensitivity in some other way
One way conduction
This allows for signals to be directed towards specific goals
Presynaptic Terminals
Mitochondria provide ATP for the synthesis of
neurotransmitters
Transmitter vesicles contain transmitter
substances that are released into the
synaptic cleft via exocytosis
The release of neurotransmitters is caused
when an action potential depolarizes the
presynaptic membrane, opening voltage-
gated calcium channels
This activates proteins that promote the fusion
of the vesicles to the membrane,
Which of the following voltage-gated ion channels open in response to an action potential in the presynaptic terminal? A. Potassium B. Sodium C. Calcium D. Chloride
Postsynaptic Potentials
A change in the membrane
potential of the postsynaptic cell
caused by the action of
neurotransmitters released by
the presynaptic cell
Postsynaptic Potentials
Excitatory (EPSP)
Open Na+ channels (influx)
Inhibit K+ & Cl- channels
Begin to depolarize membrane
May lead to action potentials
Inhibitory (IPSP)
Open K+ (outflow) and Cl- (inflow) channels
Hyperpolarizes membrane
Inhibits ability to generate action potentials
Fig 45-9
Fig 45-11
Summation
Spatial
Additive effect of stimuli from various axons
Transmission of an impulse by simultaneous
or nearly simultaneous stimulation of two
or more presynaptic neurons
-Two separate neurons firing separated by
space
Temporal
Additive effect of successive stimuli from an axon
Transmission of an impulse by rapid
stimulation of one or more presynaptic
neurons
-Same neuron firing, separated by time
Fig 45-10
True or False Temporal Summation occurs when a second
postsynaptic potential (excitatory or inhibitory) arrives
before the membrane has returned to its resting level.
Characteristics of Synaptic Transmission
Characteristics of synaptic transmission
Fatigue
When synapses are repetitively stimulated at a rapid
rate, the response of the postsynaptic neuron
diminishes over time, and the synapse is said to be
fatigued
Mainly a result of INCREASED BUILD-UP of CALCIUM
in the synaptic bouton and an inability to replenish
the supply of NT agent rapidly
Effect of PH
More acidic pH values -->
DECREASE excitability
More basic pH values --->
INCREASE neuronal activity
Characteristics of synaptic transmission Effect of hypoxia
Neuronal excitability is highly dependent on an
adequate supply of oxygen. Cessation of oxygen
for only a few seconds can cause complete
inexcitability of some neurons. This is observed
when the brain’s blood flow is temporarily
interrupted, because within 3 to 7 seconds, the
person becomes unconscious.
A decrease in the supply of OXYGEN -->diminishes
synaptic activity
Effect of anesthetics
Most anesthetics increase the neuronal membrane
threshold for excitation and thereby decrease
synaptic transmission at many points in the
nervous system. Because many of the anesthetics
are especially lipidsoluble, it has been reasoned
that some of them might change the physical
characteristics of the neuronal membranes,
making them less responsive to excitatory agents.
Types of Neurotransmitters
Group 1: Small, Rapidly Acting
General mode of action is to either alter ion channel conductance OR stimulate
receptor-activated enzyme systems
Synthesized in the cytosol of presynaptic terminals and stored in secretory
vesicles
Examples: Acetylcholine, biogenic amines, amino acid derivatives, and nitric
oxide
Acetylcholine
Location: All neuromuscular junctions, many CNS neurons, preganglionic
neurons of the ANS, and postganglionic neurons of PSNS, some SNS
Action: Typically excitatory, some inhibitory effects in PSNS
Biogenic Amines
Dopamine
Location: Midbrain
Action: Usually inhibitory
Norepinephrine
Location: Many CNS neurons, also in most postganglionic neurons of SNS
Action: Excitatory or inhibitory depending on the target
Serotonin
Location: Brainstem
Action: Inhibitory
Amino Acids & Derivatives
Glutamate
Location: Secreted by presynaptic terminals in many of the sensory
pathways entering the CNS
Action: Fast excitatory synapses of brain, fast-pain fibers in spinal cord,
plays a role in strokes
Nitric Oxide
Location: Nerve terminals in brain related to long-term behavior and memory
Action: Synthesized as needed (not stored)
Readily diffuses through membranes
Doesn’t significantly directly alter membrane potential
Modifies intracellular metabolic activity of postsynaptic neurons to affect
neuronal excitability
Group 2: Neuropeptides
Often hormones or releasing/inhibiting factors
Characteristics:
More potent than fast acting transmitters
Smaller quantities produced
Actions more prolonged
Synthesis: Synthesized in neuron cell bodies, packaged by Golgi apparatus and
transported down the axon to the terminal
Then stored/exocytosed in secretory vesicles
Clearance of Neurotransmitters
Enzymatic Degradation (example - acetylcholine)
Vesicles are Recycled - Once they release their neurotransmitters, they
temporarily become part of the membrane. The vesicle portion then recedes
back into the presynaptic terminal and pinches off to form a new vesicle
The new vesicle still contains the appropriate materials for synthesizing new
transmitter substances