Transcript action potential

The Nervous System
Chapter 8 – Overview and
Neural Tissue
The Nervous system has three major functions:
 Sensory – monitors internal & external
environment through presence of receptors
 Integration – interpretation of sensory information
(information processing); complex (higher order)
functions
 Motor – response to information processed
through stimulation of effectors
 muscle contraction
 glandular secretion
General Organization of the nervous system

Two Anatomical Divisions

Central nervous system (CNS)



Brain
Spinal cord
Peripheral nervous system (PNS)



All the neural tissue outside CNS
Afferent division (sensory input)
Efferent division (motor output)


Somatic nervous system
Autonomic nervous system
General Organization of the nervous system
Brain & spinal
cord
Histology of neural tissue
Two types of neural cells in the nervous system:
 Neurons - For processing, transfer, and storage
of information
 Neuroglia – For support, regulation & protection
of neurons
Neuroglia (glial cells)
CNS neuroglia:
• astrocytes
• oligodendrocytes
• microglia
• ependymal cells
PNS neuroglia:
• Schwann cells (neurolemmocytes)
• satellite cells
Astrocytes
• create supportive
framework for neurons
• create “blood-brain
barrier”
• monitor & regulate
interstitial fluid surrounding
neurons
• secrete chemicals for
embryological neuron
formation
• stimulate the formation of
scar tissue secondary to
CNS injury
Oligodendrocytes
• create myelin sheath
around axons of neurons
in the CNS. Myelinated
axons transmit impulses
faster than unmyelinated
axons
Microglia
• “brain macrophages”
• phagocytize cellular
wastes & pathogens
Ependymal cells
• line ventricles of brain &
central canal of spinal cord
• produce, monitor & help
circulate CSF
(cerebrospinal fluid)
Schwann cells
• surround all axons of neurons in
the PNS creating a neurilemma
around them. Neurilemma allows
for potential regeneration of
damaged axons
• creates myelin sheath around
most axons of PNS
Satellite cells
• support (structurally &
functionally) groups of cell bodies
of neurons within ganglia of the
PNS
Neuron structure
of Ranvier
•Most axons of the nervous system are
surrounded by a myelin sheath
(myelinated axons)
•The presence of myelin speeds up
the transmission of action potentials
along the axon
•Myelin will get laid down in segments
(internodes) along the axon, leaving
unmyelinated gaps known as “nodes
of Ranvier”
•Regions of the nervous system
containing groupings of myelinated
axons make up the “white matter”
•“gray matter” is mainly comprised of
groups of neuron cell bodies, dendrites
& synapses (connections between
neurons)
Anatomical organization of neurons
Neurons of the nervous system tend to group together into
organized bundles
The axons of neurons are bundled together to form nerves in
the PNS & tracts/pathways in the CNS. Since most axons
are myelinated, these regions will look white in appearance
(“white matter”)
The cell bodies of neurons are clustered together into
ganglia in the PNS & nuclei/centers in the CNS. These
parts are not myelinated, therefore will look gray in
appearance (“gray matter”)
Neural Tissue Organization
Figure 8-6
Classification of neurons
Structural classification based on number of processes coming
off of the cell body:
Multipolar neuron
• multiple dendrites & single axon
• most common type
Bipolar neuron
• two processes coming off
cell body – one dendrite &
one axon
• only found in eye, ear &
nose
Unipolar neuron
• single process coming
off cell body, giving rise to
dendrites (at one end) &
axon (making up rest of
process)
Classification of neurons
Functional classification based on type of information &
direction of information transmission:
• Sensory (afferent) neurons –
• transmit sensory information from receptors of PNS towards the CNS
• most sensory neurons are unipolar, a few are bipolar
• Motor (efferent) neurons –
• transmit motor information from the CNS to effectors
(muscles/glands/adipose tissue) in the periphery of the body
• all are multipolar
• Association (interneurons) –
• transmit information between neurons within the CNS; analyze inputs,
coordinate outputs
• are the most common type of neuron (20 billion)
• are all multipolar
Reflex arc
(p. 283-286)
Reflex – a quick, unconscious response to a stimulus to protect
or maintain homeostasis. e.g. stretch reflex, withdrawal reflex
Reflex arc – neural pathway involved in the production of a
reflex. Structures include:
• receptor
• sensory neuron
• integrating center (brain or spinal cord; may or may
not involve association neurons (interneurons))
• motor neuron
• effector
Stretch reflex
- simplest type of
reflex
- no association
neuron involved
Figure 8-29
Simplified Withdrawal reflex
Figure 8-28
Neuron Function
Neurons at rest have an unequal distribution of charged ions inside/outside
the cell, which are kept separate by the plasma membrane
• more Na+ ions outside
• more K+ ions inside
• large negatively charged proteins &
phosphate ions inside
The sum of charges
makes the outside of
the membrane
positive, & the inside
of the membrane
negative
Because of the difference of ionic charges
inside/outside the cell, the membrane of the resting
neuron is “polarized”
The difference in charges creates a potential
electrical current across the membrane known as
the “membrane potential (transmembrane
potential)”
At rest, the transmembrane potential can also be referred to as
the “resting membrane potential” (RMP)
The RMP of a neuron = -70mV
For ions to cross a cell membrane, they must go through
transmembrane channels
“leakage channels” – open all the time, allow for
diffusion
“gated channels” – open & close under specific
circumstances (e.g. voltage changes)
Because Na+ & K+ can move through leakage channels of
nerve cells, the resting membrane potential is maintained
by the sodium-potassium exchange pump
When a stimulus is applied to a resting neuron, gated
ion channels can open
If a stimulus opens gated K+ channels, positive charges
leave cell  membrane potential becomes more negative
(-70mV  -90mV)
This change in membrane potential is known as
hyperpolarization
When a stimulus causes Na+ gates open, Na+ diffuses into the
cell
This changes the electrical charge inside the cell membrane,
bringing it away from its RMP of -70mV toward 0mV
This change in membrane potential is known as depolarization
If a stimulus only affects Na+ gates at a specific site of the
axon, the depolarization is small & localized only to that
region of the cell. This is known as a graded potential
But if the stimulus reaches a certain level (threshold level),
voltage controlled Na+ gates will begin to open in sequence
along the length of the axon. The depolarization will
propagate along the entire surface of the cell membrane
This propagated change in the membrane potential is known
as an action potential (nerve impulse)
Action potentials
• APs involve the movement of Na+ ions into the cell (causing
depolarization of the membrane), followed immediately by K+
ions moving out of the cell through voltage controlled K+ gates
(causing repolarization of the membrane), that propagates
down the length of the cell
• APs are due to voltage changes that open & close gated Na+
& K+ channels within excitable cells
• Only nerve cells & muscle cells are excitable, i.e. can generate
APs.
• Once an AP begins, it will propagate down the entire cell at a
constant & maximum rate. This is known as the “all or none”
principle
Action Potential
Conduction
Depolarization to threshold
Activation of voltageregulated sodium channels
and rapid depolarization
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
Transmembrane potential (mV)
+30
DEPOLARIZATION
3
REPOLARIZATION
0
2
_ 60
_ 70
The return to normal
permeability and resting state
Threshold
1
4
Resting
potential
REFRACTORY PERIOD
0
1
2
Time (msec)
3
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-8
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Depolarization to threshold
• Nerve cell at rest (RMP= -
Sodium ions
70mV)
• Stimulus applied to cell
Local
current
Transmembrane potential (mV)
+30
• Na+ gates at axon hillock
cause localized
depolarization (graded
potential)
DEPOLARIZATION
• If stimulus is strong enough,
flow of Na+ ions into cell
reach threshold level
triggering opening of voltage
gated Na+ channels &
formation of an action
potential (nerve impulse)
0
_ 60
_ 70
Threshold
1
Resting
potential
0
1
2
Time (msec)
3
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-8
2 of 5
Depolarization to threshold
Activation of voltageregulated sodium channels
and rapid depolarization
Sodium ions
Local
current
Potassium ions
Transmembrane potential (mV)
+30
• Once threshold is reached,
Na+ will quickly diffuse into the
cell causing a rapid
depolarization of the membrane
(- 70 mV  0 mV  +30 mV)
DEPOLARIZATION
0
2
_ 60
_ 70
• this depolarization will spread
to adjacent parts of the
membrane, activating more
voltage controlled Na+ gates in
succession
Threshold
1
Resting
potential
0
1
2
Time (msec)
3
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-8
3 of 5
Depolarization to threshold
Activation of voltageregulated sodium channels
and rapid depolarization
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
Transmembrane potential (mV)
+30
DEPOLARIZATION
3
REPOLARIZATION
0
2
_ 60
_ 70
Threshold
1
Resting
potential
Figure 8-8
4 of 5
0
1
2
Time (msec)
3
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
• When the transmembrane
potential reaches +30mV, Na+
gates will close & K+ gates will
open
• K+ will quickly exit cell resulting
in repolarization of membrane &
return to resting state
Depolarization to threshold
Activation of voltageregulated sodium channels
and rapid depolarization
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
Transmembrane potential (mV)
+30
DEPOLARIZATION
3
REPOLARIZATION
0
2
_ 60
_ 70
The return to normal
permeability and resting state
Threshold
1
4
Resting
potential
REFRACTORY PERIOD
0
1
2
Time (msec)
3
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-8
5 of 5
Propagation of an Action Potential
Continuous propagation
(continuous conduction)
Involves entire membrane
surface
 Proceeds in series of
small steps (slower)
 Occurs in unmyelinated
axons (& in muscle cells)

Figure 8-9(a)
Propagation of an Action
Potential
Saltatory propagation
(saltatory conduction)
 Involves patches of
membrane exposed at
nodes of Ranvier
 Proceeds in series of
large steps (faster)
 Occurs in myelinated
axons
Figure 8-9(b)
 Action
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Potential Propagation
Neurophysiology:
Continuous
Saltatory Propagation
Neurophysiology: Action
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Potential
“The Big Picture”
 “Information” travels within the nervous
system primarily in the form of
propagated electrical signals known as
action potentials.
 An action potential occurs due to a rapid
change in membrane polarity
(depolarization followed by repolarization)
 Depolarization is due to the influx of
sodium ions (Na+); repolarization is due to
the efflux of potassium ions (K+)
Conduction across synapses
In order for neural control to occur, “information” must
not only be conducted along nerve cells, but must
also be transferred from one nerve cell to another
across a synapse
Most synapses within the nervous system are
chemical synapses, & involve the release of a
neurotransmitter
The Structure of a Typical Synapse
Figure 8-10
Events at a Typical Synapse
Extracellular Ca2+ enters the synaptic
cleft triggering the exocytosis of ACh
An action potential arrives and
depolarizes the synaptic knob
PRESYNAPTIC
NEURON
Synaptic vesicles
Action potential
EXTRACELLULAR
FLUID
ACh
ER
Synaptic
knob
Ca2+
Synaptic
cleft
Ca2+
AChE
CYTOSOL
Chemically regulated
sodium channels
POSTSYNAPTIC
NEURON
ACh is removed by AChE
(acetylcholinesterase)
ACh binds to receptors and depolarizes
the postsynaptic membrane
Initiation of
action potential
if threshold
is reached
Propagation of
action potential
(if generated)
Na2+ Na2+
Na2+
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Na2+ Receptor
Na2+
Figure 8-11
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An action potential arrives and
depolarizes the synaptic knob
PRESYNAPTIC
NEURON
Synaptic vesicles
Action potential
EXTRACELLULAR
FLUID
ER
Synaptic
knob
AChE
CYTOSOL
POSTSYNAPTIC
NEURON
• An action potential arrives &
depolarizes the synaptic knob
(end bulb)
• Before repolarization can
occur, Ca+2 gates open & Ca+2
diffuses into end bulb
• Repolarization occurs
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-11
2 of 5
Extracellular Ca2+ enters the synaptic
cleft triggering the exocytosis of ACh
An action potential arrives and
depolarizes the synaptic knob
PRESYNAPTIC
NEURON
Synaptic vesicles
Action potential
EXTRACELLULAR
FLUID
ACh
ER
Synaptic
knob
Ca2+
Synaptic
cleft
Ca2+
AChE
CYTOSOL
POSTSYNAPTIC
NEURON
Chemically regulated
sodium channels
• Ca+2 causes the synaptic vessicles to fuse with the end
bulb membrane causing the exocytosis of the
neurotransmitter
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 8-11
3 of 5
Extracellular Ca2+ enters the synaptic
cleft triggering the exocytosis of ACh
An action potential arrives and
depolarizes the synaptic knob
PRESYNAPTIC
NEURON
Synaptic vesicles
Action potential
EXTRACELLULAR
FLUID
ACh
ER
Synaptic
knob
Ca2+
Synaptic
cleft
Ca2+
AChE
CYTOSOL
Chemically regulated
sodium channels
POSTSYNAPTIC
NEURON
• The neurotransmitter diffuses
across the synaptic cleft &
binds to its receptors on the
post synaptic membrane,
causing an effect on the post
synaptic cell
ACh binds to receptors and depolarizes
the postsynaptic membrane
Initiation of
action potential
if threshold
is reached
Na2+ Na2+
Na2+
Na2+ Receptor
Na2+
Figure 8-11
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The effect on the post synaptic neuron will depend on
whether the neurotransmitter released is
 Excitatory (e.g. Ach, norepinephrine (NE))
 Inhibitory (e.g. seratonin, GABA)
Excitatory neurotransmitters cause Na+ gates to open in the post
synaptic membrane  depolarization (impulse conduction)
Inhibitory neurotransmitters cause K+ or Cl- gates to open in the
post synaptic cell  hyperpolarization (no impulse conduction)
 The effects of neurotransmitters on the post
synaptic neurons are usually short lived because
most neurotransmitters are rapidly removed from the
synaptic cleft by enzymes or reuptake
Play - Neurophysiology: Synapse