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INTRODUCTION
TO
NEUROBIOLOGY
LECTURE 9
CH 7
THE NERVOUS SYSTEM: NEURONS
AND SYNAPSES
• Students must review the beginning of Chapter 7 on
their own.
• See Intro to Neurobiology “A” lecture for assistance;
there may be questions on this section on the
exams.
• This lecture begins with “Electrical Activity in
Axons” Section 7.2 (but you are required to know
7.1, which is Anatomy).
Fig. 7.11 - Observing Depolarization and Hyperpolarization
- Oscilloscope
- Measures the potential
difference
- - 70 mV = resting
(neuron), polarized
- Depolarization =
+ charge moves in
(excitatory)
- Hyperpolarization =
neg. charge moves
in. (inhibitory)
- Repolarization = return
to resting.
Click HERE
Ion Gating in Axons
Gated channels – gates open
or close in response to stimuli.
Closing step 1) ball and chain,
step 2) fully closed.
K+ Channels can be voltagegated or non-gated.
Na+ Channels are always
voltage-gated
Depolarization of an Axon
All-or-None Law
Coding for Stimulus Intensity: the code is not amplitude; the
code is frequency!
Note the threshold
increase. A heavier
weight activates axons
with higher thresholds
RECRUITMENT – more and more axons become activated
Refractory Period: time when axon is not responsive to a 2nd stimulus
The absolute refractory period occurs during the action
potential. Na+
channels are
inactive (not just
closed).
Relative
refractory period
can be overcome
by a strong stimulus.
(while K+ diffuses
outward).
Each action
potential remains a separate, all-or-none event.
Cable Properties of Neurons
--The ability of
neurons to
conduct charges
through their
cytoplasm
--Poor due to
internal
resistance
-- and charge
leaks out
Conduction of Nerve
Impulses
When an action potential
occurs at a given point on
a neuron membrane,
voltage- gated Na+
channels open as a wave
down the length of the
axon.
The action potential at one
location serves as the
depolarization stimulus for
the next region of the
axon
Saltatory Conduction in a Myelinated Neuron
Myelin provides insulation, improving the speed of action
potential. Nodes of Ranvier allow Na+ and K+ to cross the
membrane every 1−2 mm. Na+ ion channels are concentrated
at the nodes. Action potentials “leap” from node to node.
The Synapse
A synapse is the functional connection
between a neuron and the cell it is signaling
a.In the CNS, this second cell will be another
neuron.
b.In the PNS, the second cell will be in a muscle
or gland; often called myoneural or
neuromuscular junctions
Otto Loewi (1921)
CHEMICAL SYNAPSES
• Discovered Vagusstoff, a
chemical secreted by the
vagus nerve (acetylcholine).
• EXPERIMENT:
Acetylcholine (ACh)
ACh is a neurotransmitter that
directly opens ion channels
when it binds to its receptor.
a. In some cases, ACh is
excitatory, and in other cases
it is inhibitory, depending on
the organ involved
b. Excitatory in some areas of
the CNS, in some autonomic
motor neurons, and in all
somatic motor neurons
c. Inhibitory in some autonomic
motor neurons
GAP JUNCTIONS: Electrical Synapses
• Electrical synapses occur in smooth muscle and cardiac
muscle, between some neurons of the brain, and between
glial cells.
• Stimulation causes phosphorylation or dephosphorylation
of connexin proteins to open or close the channels
Chemical Synapses
Vesicles are
produced in
the Golgi
and stored at
the end of an
axon (terminal
boutons)
The synaptic
cleft is very
small, 10nm,
and the
presynaptic
and postsynaptic cells are held close together by cell
adhesion molecules (CAMs).
Release of Neurotransmitter
The neurotransmitter (ligand) binds to a receptor in the postsynaptic membrane
Summary of Neurotransmitter Action
7.4 Acetylcholine
Acetylcholine (ACh)
ACh is a neurotransmitter
that can have different
effects on different tissues.
a.In some cases, ACh is
excitatory, and in other
cases it is inhibitory,
depending on the organ
involved
b.How does this happen?
THIS IS AN EARLIER SLIDE!
Two Types of Acetylcholine (Cholinergic) Receptors
a.Nicotinic ACh receptors
1)Can be stimulated by nicotine
Found on the motor end plate of skeletal muscle
cells, in autonomic ganglia, and in some parts of
the CNS
b.Muscarinic ACh receptors
1)Can be stimulated by muscarine
(from poisonous mushrooms)
2)Found in CNS and plasma membrane of smooth
and cardiac muscles and glands innervated by
autonomic motor neurons
BINDING OF ACETYLCHOLINE (LIGAND) TO ITS RECEPTOR
ACETYLCHOLINE
CAN BIND TO
ITS RECEPTOR,
WHICH IS ALSO
AN ION CHANNEL
Notice that 2
acetylcholines must bind
Graded nature of EPSPs
Question?
What would happen if a person had a disease
that caused a deactivation of nicotinic ACh
receptors located in the motor end plates* of
skeletal muscle cells?
Discuss this with your neighbor.
*motor end plates are the part of the muscle
cell that is activated by neurons.
MUSCARINIC Ach Receptors Require G-Proteins
Binding of acetylcholine opens K+ channels in some tissues (IPSP) or closes K+ channels
in others (EPSP).
1)
2)
Heart: K+ channels open, creating IPSPs  heart rate slowed
Smooth muscle: K+ channels close, creating EPSPs  smooth muscles contract
Agonists and Antagonists
Agonists: drugs that can stimulate a receptor
a)Nicotine for nicotinic ACh receptors
b)Muscarine for muscarinic ACh receptors
Antagonists: drugs that inhibit a receptor
a)Atropine from plants, is an antagonist for
muscarinic receptors.
b)Curare (from plants) is an antagonist
for nicotinic receptors. Clinically,
curare is used as a muscle relaxant.
Lethal in high doses; poison dart use.
Acetylcholinesterase (AChE)
• AChE is an enzyme that
inactivates ACh activity
shortly after it binds to the
receptor.
• Hydrolyzes ACh into acetate
and choline, which are
taken back into
the presynaptic cell
for reuse.
Action of Acetylcholinesterase (AChE)