Download Invertebrate nervous systems:

Invertebrate nervous system organization -Nervous system structure reflects the body plan organization
Invertebrate nervous systems:
cnidarians (anemones, medusoid jellyfish)
flatworms/roundworms (planaria, nematodes)
segmented worms (polychaetes, oligochaetes, hirudinae)
arthropods (crustacea and insects)
mollusks (snails, octopods/squid)
pre-vertebrates: hemichordates, cephalochordates
agnathans (lamprey)
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Cnidarians
Flatworms
nerve net - little or no
collecting of neurons into
ganglia, no anterior “brain”
With the appearance
of bilateral symmetry:
came cephalization
= concentation of
neurons into ganglia
at the anterior end
http://cas.bellarmine.edu/tietjen/
Research/RobotAnimalModels.htm
Do have neural “plexi” (sing.
plexus), loose collections of
neurons analogous to the
enteric nervous system in
vertebrates
With the appearance of
segmentation: segmental
“ganglia”
Leech
serotonin
immunoreactivity
single cell
dye filling
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nerves to
periphery
intersegmental
connectives
segmental ganglion
Arthropods -- fusion and specialization of segments accompanied
By f & s of ganglia
supraesophogeal
subesophogeal
thoracic
abdominal
www.st-andrews.ac.uk/ ~wjh/jumping/motorsys.htm
thoracic
abdominal
With the elaboration of sophisticated sense
organs at the anterior (head) end of the
animal there is an elaboration and
expansion of the circumesophegeal
ganglia. Supraesoph. dominated by sensory
processing circuits.
With the specialization of thoracic
segments for locomotion there is
specialization of thoracic ganglia.
Abdominal ganglia serve motor, sensory
and reproductive functions of the
abdominal segments. Successively more
fusion of segmental ganglia from crustacea
to insects
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Gastropod mollusks
Tritonia diomedea
Tritonia festiva
supra and subesophogeal
ganglia and pleural gangia
1 mm
Buccal ganglia
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Vertical lobe
retina
optic nerves
optic lobe
Lateral view of octopus brain
…. octopi live 1-2 yrs….. what a waste?
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Spontaneous activity
Procerebral lobe of
Limax maximus – 100,000
small neurons
odor
Leech
Locust
AP
P
Crayfish
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Giant glial cells in leech ganglion
Processes of packet
glia surrounding neural
somata
http://www.uni-kl.de/FB-Biologie/AG-Deitmer/Confocal/confocal_gallery.htm
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Isolation of individual neurons from the Aplysia CNS.
Lovell P , Moroz L L Integr. Comp. Biol. 2006;46:847-870
© The Author 2006. Published by Oxford University Press on behalf of the Society for Integrative
and Comparative Biology. All rights reserved. For permissions please email:
[email protected].
http://www.youtube.com/watch?v=V6H01cUSpfQ
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Two ways to construct central pattern generators
persistent excitation = driver
+
_
+ The half-centre oscillator
_
The neurogenic leech heart (a rhythm generator)
All inhibitory synapses– where does the excitatory drive come from?
Neurogenic leech heart
Hyperpolarize 1 HN
Hyperpolarize an HN…. note the re-setting of the phase
This capacity to “reset” rhythm is a property of neurons
that are central elements in central pattern generators.
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Box 16C The autonomy of central pattern generators: evidence from the lobster stomatogastric
ganglion (Part 1)
(photo Marie Suver MIT)
Pyloric Dilator --
C. Goldsmith, C. Staedele Illinois State U
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Box 16C
Autonomy of central pattern generators: evidence from the lobster stomatogastric ganglion
PD neuron activity patterns
Concept:
Polymorphic networks
One set of physical
connections can
generate different
outputs depending on
neuromodulation
Diffuse networks of neurons mixed in with body tissues
Formation of ganglia
Bilateral symmetry and concentration of sensory structures at one end leads to cephalization
Body segmentation is accompanied by segmention of nervous system = segmental ganglia
Specialization and fusion of segmental ganglia reflecting specialization of segments
Molluscs – continue process of fusing ganglia into larger “brains”
Limitations to the invertebrate plan…. Diffusion of O2 and nutrients into a thick tissue…
Terms to know and be able to explain:
nerve net
cephalization
ventral nerve cord
ganglion/ganglia
commissure
connective
nerve trunk/nerve
segmental ganglion/ganglia
central pattern generator
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BIRD
ucumberland.edu
LAMPREY
FISH
HUMAN
FROG
Very early in its evolution, the vertebrate brain underwent modifications
that set the stage for later evolutionary trends.
Briefly, the modifications were these:1.The hindbrain became divided into a ventral
portion, called the medulla oblongata, a dorsal portion, the cerebellum, and the
anterior pons. The medulla became specialized as a control center for some
autonomic and somatic pathways concerned with vital functions (such as breathing,
blood pressure, and heartbeat) and as a connecting tract between the spinal cord and
the more anterior parts of the brain. The pons is above the medulla and also acts as a
connecting tract. The cerebellum enlarged and became a structure concerned with
balance, equilibrium, and muscular coordination.
Fish
Frog
Lamprey (agnathan)
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2.The midbrain became specialized as the optic lobes, visual centers
associated with the optic nerves.
3.The forebrain became divided into an anterior portion consisting of the
cerebrum, with its prominent olfactory bulbs, and a posterior portion
consisting of the thalamus and hypothalamus.
During the course of vertebrate evolution, there have been few
changes in the hindbrain, though the cerebellum has become larger
and more complex in many animals. The major evolutionary
change has been the steady increase in size and importance of
the cerebrum, with a corresponding decrease in relative size and
importance of the midbrain (see Figure below and next slide).
Bird
You
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Increase in size of forebrain
Decrease in relative size of midbrain
Increased function for forebrain (cerebrum), e.g. more visual
processing, more somatosensory processing, more
associational processes.
Increase in the number of layers of neurons in the cerebrum
(cortex) addition of 6-layered neo-cortex to pre-existing 3layered archi-cortex (hippocampus and pyriform, ==
olfactory cortex)
Increase in size of cerebellum (a complex motor-learning/
sensory integration area).
Large expansion of “associational” higher processing areas
in neocortex
Folding of neocortex (upon itself and infolding) to
accommodate increased area within the cranial cavity
Expansion of area devoted to specialized functions in some
species, e.g. audition in dolphins and owls, somatosensory
areas in blind, burrowing moles etc.
Cool brain facts
A cubic mm of neocortex contains
90,000 neurons
400 meters of dendrites
4 km of axons
About 7x108 synapses
The most cell dense part of the brain is the granule layer of
cerebellum (not neocortex) with about 7x106 neurons per
cubic mm, each of which is about 7-8µm diameter.
Human surface area of cortex – 2500 cm2
Surface area of 3/4 of a sphere with 7 cm radius = 460 cm2
Folding of cortex = 5 to 6-fold increase in area.
(brain volume is 140cm3 which is equivalent to a sphere with 7 cm radius, 75% of this is neocortex so
roughly assume area is 75% of the area of a 7cm radius sphere)
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Brain regions expand to meet computational demands:
e.g. in weakly electric
fish the cerebellum
devoted to generating
and interpreting electric
field signals = biggest
part of weakly electric
fish brain.
Sensory inputs are usually organized so that information from
adjacent areas is processed by neurons located near each other
in brain regions all along the processing chain.
Somatotopy in the
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The amount of brain tissue devoted to processing information
varies depending upon the amount of information that is needed to
be processed.
peripheral afferents
somatosensory subdivisions
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Expansion of forebrain is not equally distributed – expansion
occurred mainly in so-called “association” areas
Frog
Neocortex
Hippocampus
Mouse
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Frog
Frog Cerebrum
Neocortex
= 3 layered “Archicortex”
Hippocampus
Mouse
neocortex
Hippocampus
Projection neurons - long axons
“Principal” or
Projection Neurons
Mouse
6 “layers”
appeared early in
mammalian
evolution (Triassic
Jurassic)
Front Neurosci. 2015; 9:
162. doi: 10.3389/fnins.
2015.00162
E.g. Cortical pyramidal
neurons
Action potential
output
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INTERNEURONS
short axons -- mainly local connections
Spiny stellate
Basket neuron
Chandelier
Axon
Axon
100µm
Mouse
Axon
Inhibitory
(GABA)
Excitatory
(glutamate)
Neural communication….exciting!
Next section of course: Chapters 2-5.
Where does neural ‘electricity’ come from?
active transport and passive diffusion….
Nernst, Goldman-Hodgkin-Katz
Active vs passive properties of neurons
Signal transmission by axons,
Signal transfer by synapses
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The simple myotatic reflex circuit illustrates several points about the
functional organization of neural circuits (Fig. 1.7)
•  PN01050.JPG
Sherrington
Relative frequency and timing of action potentials in different
components of the myotatic reflex (Fig.1.8)
•  PN01060.JPG
Extracellular electrical recordings
+
+
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Intracellularly Recorded Reponses Underlying the Myotatic Reflex (Fig. 1.9)
•  PN01072.JPG
Intracellular recordings
To prepare for exams: Be able to describe the processes involved in signaling in a simple
reflex circuit like this. How are electrical signals converted to chemical and then back to
electrical? What causes delays in the transfer of signals? What would slow down or speed up
signaling? What determines the sign (inhibition vs excitation) of the synaptic potentials. Why
are “inhibitory” potentials “inhibitory”
Neuro-electricity is due to
ionic currents that are
generated by the diffusion
of 4 essential ions….
Na+, K+, Ca2+, Cl-
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Figure 2.2 Recording passive and active
electrical signals in a nerve cell
Figure 2.1 Types of neuronal electrical signals
Receptor
potential
“passive”
Synaptic
potential
“passive”
Action
potential
“regenerative
” = “active”
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Where does the non-zero voltage difference, which we call the
membrane potential come from?
It is produced by maintaining different concentrations of
charged molecules (mainly ions) inside and outside a selectively
permeable cell surface membrane (the plasmalemma = cell
membrane).
TOTAL 490-600 mM
TOTAL 185 mM
TOTAL 1030 mM
TOTAL 262 mM
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TOTAL 490-600 mM
TOTAL 185 mM
TOTAL 1030 mM
TOTAL 262 mM
Figure 2.3 Transporters and channels move ions
across neuronal membranes
What is the role of pumps in setting membrane potentials?
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Permeable only to K+
….. at 18 °C
If we have more of one ion on one side of a membrane then we
have a concentration gradient and a charge gradient = possible
energy sources – BUT there is only an actual energy source if ion
is out of its ‘equilibrium condition’.
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Permeability is needed to “harvest” the energy
stored in the concentration gradient. No permeability
no “voltage” will be generated
Seems like we could have energy stored in in the form of
voltage differences (batteries) or concentration
differences…. In fact cells have both kinds of energy
storage mechanisms = electrochemical gradients
How much--- what “direction”. ?? etc.
For our questions about how neurons will generate time
variant electrical signals we first need to know how
much potential electrical energy is stored due to the
concentration gradient for EACH ionic species.
That way we know what kind of voltage changes (large/
small, positive/negative) we could produce if we
allowed a particular ion to move (flow as electrical
current) down its energy gradient….
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for unequal distribution of charged molecules across a membrane:
A general equation to relate stored electrical energy to stored
chemical energy derives from equating chemical to electrical
energy
Electrical “work” = zFV and
Chemical “work” = RT*ln[X]1/[X]2
at equilibrium, the chemical work that can be done by a
concentration difference of ions is equal to the negative of
the electrical work that goes into separating the charges.
F= Faraday’s constant (96,000 Coulombs/mole),
R = universal gas constant (8.314 VCK-1 mol)
z = valence of charge (e.g. +1)
At equilibrium chemical and electrical forces are equal and
opposite
When the energy stored in the electrical gradient (V) = the
negative of the energy stored in the chemical gradient then the
forces cancel and net diffusion is zero:
zFV = - RT*ln ([X]1/[X]2)
Thus: V = -(RT/zF)*ln ([X]1/[X]2) or
V = +(RT/zF)*ln ([X]2/[X]1) :: {ln a/b = -ln (b/a)}
This particular voltage difference where the forces balance is
called the Equilibrium potential for ion x
Also written as Veq(x) or Vx or Ex
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At equilibrium chemical and electrical forces are equal and
opposite
When the energy stored in the electrical gradient (V) = the
negative of the energy stored in the chemical gradient then the
forces cancel and net diffusion is zero:
zFV = - RT*ln ([X]1/[X]2)
Thus: V = -(RT/zF)*ln ([X]1/[X]2) or
V = +(RT/zF)*ln ([X]2/[X]1) :: {ln a/b = -ln (b/a)}
This particular voltage difference where the forces balance is
called the Equilibrium potential for ion x
Also written as Veq(x) or Vx or Ex
Ex == Equilibrium potential of ion x (ALSO known as the reversal
potential)
It can be calculated as the “Nernst” potential for ion X. By convention
we use outside over inside which gives us V of the inside relative to the
outside
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Ex == Equilibrium potential of ion x (ALSO known as the reversal
potential)
It can be calculated as the “Nernst” potential for ion X. By convention
we use outside over inside which gives us V of the inside relative to the
outside
Nernst: Ex = (RT/zF) *ln ( [X]outside / [X]inside)
Note -- we can use log10 or ln. Since ln(a) = 2.303log10(a):
Ex = ((2.303*RT)/zF) *log10 ( [X]outside / [X]inside)
Ex is measured “inside” with respect to “outside”.
Therefore “[X]outside is the numerator in our equation
-----------------------------------------------------------------------z is ion’s valence (+1, +2, -1, -2 etc)
T = temperature in °Kelvin (273 + °K = °C)
2.303 RT/F = .058 Volts (58 mV) @ 18 °C;
= .062 Volts (62 mV) @ 37°C
= .056 Volts (56 mV) @ 10°C
Notice that the voltage change is not accompanied by a
measureable change in concentration….. Only a small
fraction of the total ions in the compartments have to
re-distribute to create the gradient across the thin
membrane
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Figure 2.7 Resting membrane potential is determined PRIMARILY by the K+ concentration gradient –
WHY???
at a T° of 18C
of course!
Why does the real data
tail off at low [K]out?
Dead man walking……
Practical applications of Nernst…..
Sodium pental or
(barbituate)
pentobarbital
Pancurium bromide (paralytic)
Potassium chloride
(heart stopper)
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Recall Nernst equation (for one ionic species)
Veq = Eion =
+(2.303RT/zF)* log ([ion1]o/ +[ion1]i)
Recall Nernst equation (for one ionic species)
Veq = Eion =
+(2.303RT/zF)* log ([ion1]o/ +[ion1]i)
REAL NEURONS have more than just one permeant ion species
Goldman-Hodgkin Katz (GHK) equation
Vm =
+(2.303RT/zF)* log
(
( Pion1[ion1]o +Pion2[ion2]o)
( Pion1[ion1]i +Pion2[ion2]i)
)
( “P’ refers to the relative permeabilty of the ion species )
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Where does ionic permeability come from?
What is conductance and how does it relate to permeability
in the GHK equation?
The tricky thing for many is understanding the difference
between conductance and current…
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Figure 2.5 Membrane potential influences ion fluxes
The membrane potential of a neuron can change from
the “resting” potential to a new potential
whenever:
a)  permeability of the membrane changes for one or
more of the ionic species that is not at equilibrium
inside/outside the neuron. -- permeability can be
changed quickly -- thus electrical changes can
occur quickly
b) the concentration of an ionic species to which the
membrane has permeability is changed. If this
occurs it is usually much slower than (a
c) The activity of an electrogenic pump increases or
decreases. V will change slowly
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