Transcript Chapter 48 Presentation

Chapter 48
Nervous Systems
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Nerve Systems
 A neuron is a nerve cell, and there are
100 billion in the brain.
 Except for sponges, all animals have
some type of nervous system. The thing
that sets them apart is their
organization.
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Nerve Systems
 Simple animals have
nerve systems
classified in nerve
nets-very diffuse
organization.
 Example: Cnidarian
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Nerve Systems
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 Increasing in their
complexity, nerve
nets are also
associated with
nerves.
 These assist with
more complex
movements.
 Example: Sea
stars
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Nerve Systems
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 Nerve systems with
greater complexity
involve cephalization.
 This included the
clustering of neurons
in the head and
bilaterally symmetrical
bodies. These are
simple CNS’s.
 Example: Planarians
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Nerve Systems
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 The more complex brains
as well as ventral nerve
cords and clusters of
nerve cells called ganglia
are seen in more
complex invertebrates.
 These systems have a
peripheral nervous
system that connects
with the CNS.
 Example: Annelids
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Nerve Systems
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 The structure of nerve
system organization is
closely related to
function.
 For example: molluscs
are slow moving and
don’t have a very highly
organized nervous
system.
 Example: Clams and
Chitons
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Nerve Systems
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 Fast moving molluscs
such as the
cephalopods have
more highly organized
nervous systems.
 Example: Squids and
Octupi
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Nerve Systems
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 Vertebrates have a
CNS consisting of a
brain and spinal cord
running along the
dorsal side of the
body, along with
nerves and ganglia
comprising the PNS.
 Example: Salamander
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Nerve Systems
 Information processing by the nervous
system consisting of 3 stages:
 1. Sensory input
 2. Integration
 3. Motor output
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Nerve Systems
 These three stages are handled by
specialized neurons.
 1. Sensory neurons transmit information
from sensors that detect external stimuli and
internal conditions.
 2. Interneurons integrate and analyze
sensory input.
 3. Motor output leaves the CNS via motor
neurons which communicate with effector
cells eliciting a change.
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Form Fitting Function
 The organelles of a neuron are located in
the cell body. Two extensions arise from
the cell body:
 1. Axons--longer, transmit signals.
 2. Dendrites--highly branched, receive
signals.
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Form Fitting Function
 Near its end, an axon divides into several
branches, each ending in a synaptic
terminal.
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Form Fitting Function
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 A synapse is the site
of communication
between one synaptic
terminal and another.
 Neurotransmitters
transmit the signal
from a pre-synaptic
cell to a post-synaptic
cell.
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Supporting Cells of the
Nervous System
 Glia are the supporting cells of the
nervous system.
 There are several different types, among
them are:




1.
2.
3.
4.
Schwaan cells
Oligodendrocytes
Radial glia
Astrocytes.
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1. Schwaan Cells
 Schwaan cells are associated with the PNS as
are glia, and they form myelin sheaths
around the axons of many vertebrate
neurons.
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2. Oligodendrocytes
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 Oligodendrocytes are
associated with the CNS
and do the same thing as
Schwaan cells.
 The myelin sheath
generated by these cells
forms an insulation
blanket. This aids in
nerve conduction.
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3. Radial Glia
 In an embryo, radial glia form tracks
along which newly formed neurons
migrate from the neural tube during
development.
 Radial glia and astrocytes act as stem
cells and give rise to new neurons and
glia.
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4. Astrocytes
 These provide structural
support, regulate extracellular
ion concentrations and
neurotransmitter
concentrations.
 They are involved in dilating
blood vessels, increasing blood
flow to neurons, and they
facilitate information transfer.
 They induce tight junction
formation in the course of
development of the CNS
helping form the blood-brain
barrier.
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Potential Difference
 A typical cell has a potential difference across
the membrane of -60 to -80mV. This is the
resting membrane potential.
 The membrane voltage at equilibrium is
calculated using the Nernst equation. It is
called the equilibrium potential, (Eion).
 Eion = 62mV(log([ion]outside/[ion]inside))
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The Nernst Equation
 Eion = 62mV(log([ion]outside/[ion]inside))
 This equation applies to any membrane
that is permeable to a single type of ion.
 All you need to know is the ion
concentration inside and outside of the
membrane.
 A minus sign indicates the inside is more
negative than the outside.
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Membrane Potential
 This is the basis of nearly all electrical
signals in the nervous system.
 The membrane potential can change
from its resting value when the
membrane’s permeability to a particular
ion changes.
 Na+, K+, Ca2+, and Cl- all play major roles in
nerve signal transmission.
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Ion Channels
 When ion channels are always open, they
are said to be ungated.
 Gated ion channels switch open and closed
to one of three kinds of stimuli:
 Stretch gated ion channels sense stretch.
 Ligand gated ion channels open and close in
response to specific signals.
 Voltage gated ion channels open and close due
to changes in membrane potential.
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Ion Channel Stimulation
 Stimulating gated ion channels can trigger
hyperpolarization or depolarization.
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Ion Channel Stimulation
 Hyperpolarization
results in an
increased
magnitude of
membrane
potential--The
inside of the
membrane
becomes more
negative.
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Ion Channel Stimulation
 Depolarization
reduces the
magnitude of
the membrane
potential--the
inside becomes
less negative.
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Ion Channel Stimulation
 In most neurons,
depolarizations are
graded up to a
certain threshold.
 Once a stimulus
has reached a
threshold, an
action potential is
triggered.
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Action Potentials
 Action potentials are all or none. They
carry signals over a long distance along
axons. They are very brief, and can thus
be generated at a high frequency.
 Both Na+ and K+ voltage-gated ion
channels are involved in the production
of an action potential.
 Both open by depolarization of the
membrane. Na+ opens 1st, K+ 2nd.
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Action Potentials
• Na+ channels have 2 gates--an activation gate
and an inactivation gate. Both must open for
Na+ to get through.
 1. At resting potential, the
activation gate is closed,
inactivation gate is open. (For
Na+).
 Depolarization rapidly opens the
activation gate and slowly closes the
inactivation gate.
 For K+, the activation gate is closed at
resting potential.
 Depolarization slowly opens the gate.
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Action Potentials
 2. When a stimulus
depolarizes the
membrane, the
activation gates open
on some channels
allowing some Na+ in.
 Na+ influx causes
depolarization opening
more activation gates
and so on (positive
feedback).
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Action Potentials
 3. When the
threshold is crossed,
this positive
feedback cycle
brings the
membrane potential
close to ENa
(equilibrium
potential) during the
rising phase.
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Action Potentials
 4. ENa is not reached:
 -Activation gates close
most Na+ channels
halting Na+ influx.
 -K+ activation gates
open causing efflux of
K+ decreasing the
membrane potential.
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Action Potentials
 5. Undershoot occurs
as too much K+ leaves
the cell. Eventually,
K+ activation gates
close and the
membrane returns to
its membrane resting
potential.
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Action Potentials
 The refractory period occurs when the Na+
channels remain closed and prevent the
triggering of another action potential.
 This is what prevents the backflow of a
stimulus.
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Action Potentials
 Myelinated axons help to increase the diameter
of the nerve and thereby increase the speed at
which the impulse is propagated.
 It also contributes to saltatory conduction which
is where the action potential appears to jump
from node to node along the axon.
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Action Potentials--Synapses
 When action potentials reach the ends of
axons, they contribute one of 2 general
mechanisms of information transfer.
 1. Electrical synapse.
 2. Chemical synapse.
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Synapses--Electrical
 1. Electrical synapses contain gap
junctions which allow electric current to
flow from cell to cell.
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Synapses--Chemical
 2. Chemical synapses
make up the vast
majority of synapses.
 They involve the
release of chemical
neurotransmitters
from the pre-synaptic
neurons via synaptic
vesicles.
 The synaptic vesicles
interact with the
dendrites of a postsynaptic neuron.
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Action Potentials
 The diffusion of neurotransmitter through
the synaptic cleft has a change on the
post-synaptic neuron, either direct or
indirect.
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Action Potentials
 When the neurotransmitter binds directly
to the post-synaptic membrane and
opens a channel, ions can diffuse across
the membrane in a process called direct
synaptic transmission.
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Action Potentials
 In indirect synaptic transmission, a
neurotransmitter binds to a receptor that
is not part of an ion channel.
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Action Potentials
 This involves activation of a signal transduction
pathway involving a second messenger in the
post-synaptic cell.
 These have an overall slower effect than direct
transmission, but they last longer.
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