Download Neuronal Function

Nervous System
• Brain, spinal cord, efferent and afferent neurons
• Pattern of information flow:
Receptor
Afferent path
Integration
Efferent Path
Central Nervous
System (CNS)
• Main cell types are neurons and glial cells
Effect
Typical Arrangement of
Neural Connections
• Neurons communicate via
electrical signaling
• They are excitable
• Structurally the soma (cell
body) has an extensive ER and
prominent nucleoli
• Long appendages or processes:
• Dendrites (receive info)
• Axons (deliver info); some
are covered by myelin
A collection of axons is
called a NERVE
Types of glial cells:
CNS =
oligodendrocytes,
astrocytes,
microglia,
ependymal cells
PNS = Schwann
cells, satellite cells
Myelin acts as an insulator and inhibits
ion movement in the axonal membrane
that it surrounds.
Neurons as Excitable Tissue
• Excited by altering the resting membrane potential (-90 mV)
• Depolarize
• Hyperpolarize
• Most changes in membrane potential occur through the opening or closing of certain ion
channels (they are voltage-gated or gated via second messenger system activation).
Ion
Intracellular
(mM)
Extracellular
(mM)
Na+
2
140
Cl-
10
105
Ca2+
10-8
2.5
K+
150
5
Proteins (-)
65
2
• Gates can be chemically opened by
neurotransmitters
• Gates can be opened via signal transduction
mechanisms linked to neurotransmitter binding to
receptor
• Gates can be opened by stretch, pressure, etc.
Stimulus = anything that can cause the opening or closing of gated
channels in a neuronal membrane
What happens to
the resting
membrane
potential of the
membrane
adjacent to the
site of Na+
entry?
How about
here?
The axon hillock (trigger zone) is sensitive to
changes in ion concentration and is the site at
which an action potential is initiated.
An action potential is a self-propagating
depolarization of the axonal membrane that
initiates at the hillock and runs to the axon
terminus without diminishing in strength.
All action potentials are the same “size”.
What determines whether an action potential will occur or not?
If the graded potential doesn’t
change the resting membrane
potential enough, the signal
from the stimulus will die out
and the neuron will not respond
with an action potential.
The amount of change in
membrane potential necessary to
generate an action potential is
called a threshold stimulus.
Action Potential = depolarization along the axon
1
3
2
If the trigger area of the axon reaches threshold, the influx of Na+
and the generation of the action potential will be repeated over and
over again in one direction, at each segment of membrane, down the
axon.
What will happen at this
area of membrane?
What will happen at this area of
membrane?
One portion of the membrane has just been depolarized and is relatively insensitive to
changes in cation concentration. It is said to be refractory to stimulus. Downstream
membrane is at resting potential, and can be influenced by cation influx.
Saltatory conduction in myelinated neurons
Action potentials
cause the release
of
neurotransmitter
from the
presynaptic axon
terminus
What will happen to the resting membrane potential if the
activation gate is opened?
How could a cell open this activation gate?
Strength of stimulus determines neuronal response
EPSP
mV
time
time
mV
IPSP
time
Neurotransmitter activity is stopped by: diffusion away from the synapse, transport into cells
(glial or back into presynaptic neuron), or degradation by specific enzymes.
What is the response in the post-synaptic neuron?
What will determine whether this postsynaptic neuron will
respond?
A
B
Red neuron is releasing
serotonin which causes an
IPSP. The neuron is firing at
70 APs/sec
Neuron A is releasing
dopamine, causing and EPSP.
The neuron is firing at 40
APs/sec
Neuron B is releasing
acetylcholine to create an
EPSP. It is firing at 20
APs/sec.
What will the outcome be in
the postsynaptic cell?
Transmitter Molecule
Derived From
Site of Synthesis
Acetylcholine
Choline
CNS, parasympathetic nerves
Serotonin
5-Hydroxytryptamine (5-HT)
Tryptophan
CNS, chromaffin cells of the gut, enteric cells
GABA
Glutamate
CNS
Glutamate
CNS
Aspartate
CNS
Glycine
spinal cord
Histamine
Epinephrine
Norpinephrine
Dopamine
Adenosine
Histidine
hypothalamus
Tyrosine
adrenal medulla, some CNS cells
Tyrosine
CNS, sympathetic nerves
Tyrosine
CNS
ATP
CNS, periperal nerves
Neurotrans.
Types of
receptors
Mode of action
Result in
postsynaptic
cell
Target
Acetylcholine
Nicotinic
Muscarinic
Opens ion channels
EPSP
CNS neurons; skeletal muscle
Serotonin
Two main
classes;
multiple
subclasses
G-protein coupled
receptors; both AC
and IP3/DAG
Depends on
receptor type
Platelet aggregation, smooth muscle
contraction, satiety, vomiting
GABA
GABA-A
GABA-B
Receptor Cl- channel
G-linked K+ channel
 Receptor
G-protein linked to
cAMP
IPSP
b receptor
G-protein linked to
cAMP
EPSP
D1, D2, D3,
D4, and D5
G-protein linked to
cAMP, direct channel
opening, cAMP to
K+ channel opening
EPSP and IPSP
Norepinephrine
Dopamine
Throughout CNS and in retina
IPSP in all cases
Relaxes smooth muscles of gut,
bronchial tree, and vessels to skel.
muscle
Increases rate and strength of cardiac
contraction; excites smooth muscle in
vessels
D1-3 are located in the striatum of the
CNS, and the basal ganglia
D3-5 play a role in mood, psychosis
and neuroprotection