Nervous System & Nervous Tissue
BIO 231
Anatomy and Physiology I
The Structure of Nervous
Systems
•
Parts
of the Nervous System
–
Central
Nervous System (CNS)
•
Brain
& nerve cord (spinal cord in humans)
•
Mixture
of processing regions & nerve
–
Peripheral
Nervous System (PNS)
•
Nerves
that connect to CNS & any
nerve cell bodies outside the CNS (ganglion)
•
nervous system
•
Somatic
nervous system
–
(Voluntary
nervous system)
Neurons
•
Cell
body - soma
•
Neurites
- slender outgrowths
–
Axon -
conveys information from soma
–
Dendrites
- conveys information toward the soma
Neuron Classification
•
Monopolar
neurons - single neurite extends - this may branch
•
Bipolar
neurons - two neurites (one axon & one dendrite)
•
Multipolar
neurons - single axon & many dendrites extending from soma
Types of Neurons
•
Sensory
neurons - respond to sensory stimuli - afferent
•
Motor
neurons - deliver output to muscle and glands - efferent
•
Interneurons
- act in making
–
Local -
confined to one region of brain or spinal cord
–
Projection
neuron - send branches to some distant site
Classes of Neurons
Classes of Neurons
Supporting Cells
•
Glial
cells or neuroglia
•
Two to
five times more glial cells than nerve cells in the human nervous system
•
Have
resting membrane potential just like neurons
•
Do not
have any electrical response
•
Many
resemble nerve cells - all lack an axonal like structure
Types of Glial Cells
•
Schwann
Cells - myelin peripherally
•
Oligodendrocytes
- myelin centrally
•
Microglia
-
•
Astrocytes
- regulate external environment
•
Ependymal
Cells - line ventricles
•
Satellite
Cells - in autonomic ganglia
Types of Glial Cells
Myelin
•
Membrane
tightly wrapped several times around an axon & some long dendrites
•
An
electrical insulator
•
Increases
the speed of action potential propagation
•
Myelin
sheath forms when glial cell wraps itself around an
•
The
myelin sheath is not continuous but has breaks or nodes (Nodes of Ranvier)
between the individual cells that compose the sheath
Schwann Cells - Peripheral
Myelination
Oligodendrocytes - Central
Myelination
Definitions
•
Voltage
- measure of the potential energy generated by separation of charge - measured
in volts & milivolts
•
Current
- flow or movement of
•
Resistance
- hindrance to the flow of charge
•
Ohm’s
law
Resting Membrane Potential
•
The
membranes of all nerve cells have
an electrical charge on them due to of positive charges on the outside of
the membrane and negative charges on the membrane.
•
This
separation of charge gives rise to the membrane potential Vm
Resting Membrane Potential
•
At rest
when a neuron is not signaling, it is positively charged on the outside
and charged on the inside.
•
All
signaling of neurons involves in the potential difference across the membrane away form the rest value.
•
In most
neurons Vm = -60 to -95 mV (inside is more negative).
Charge Distribution of a
Resting Neuron
What Causes The Resting
Membrane Potential?
The resting membrane potential results from an distribution of ions across a selectively
permeable membrane.
Ionic Distribution - Squid
& Cat
Passive Movement of Ions
•
Ions
diffuse across the membrane only through intramembranous protein pores called
•
Each
channel is selective for the type of ion it allows to pass, based on size, ,
and hydration energy of the ion
Membrane Channels
Establishing Membrane
Potential
The normal ionic gradient across a cell
membrane to Na+, K+, Cl-, and organic ions
could be maintained and resting membrane potential would be generated if the
membrane were permeable only to K+ ions.
Membrane Potential with K+
Permeability
•
1.
Again we start with a cell whose membrane is only permeable to K+ such as glial
cell membranes.
–
The
positively charged K+ would tend to diffuse out of the cell
–
This is
because of the concentration gradient
•
2. As
the K+ ions diffuse out - the cell becomes slightly more negative on
the inside than on the
•
3. This
internal negativity results from the net excess of non-permeable anions that are left behind, inside
the cell, when K+ diffuses out
Membrane Potential with K+
Permeability
•
What
happens - a cloud of charge tends
to form on each side of their membrane due to electrostatic attraction between
excess cations on the outside and
anions on the inside.
•
The
progressive buildup of positive charge outside the cell and the negative charge
inside the cell slowly begins to the contained movement of K+
ions
Membrane Potential with K+
Permeability
•
Hence
there exists an interaction between the two opposing forces.
–
1. The
force of the concentration
gradient which tends to push K+ out
–
The
force due to the charge separation which results in an electrical potential
difference - that makes the of the cell membrane positive in
relation to the inside of the cell
•
Here
there exists an equilibrium across the membrane - there is no net flux or flow
into and out of the cell
•
The
voltage at which this occurs is called the K+ equilibrium potential
(Ek)
–
At
20° C EK =
-75 mV
Hodgkin & Keynes
Experiment
Conclusions of Hodgkin
& Keynes
•
Membrane
permeable to more than just potassium
•
Subsequent
intracellular studies - membrane permeable to Na+
•
Both
conc & electrical gradients tend to drive potassium into the cell
•
Relatively
little Na+ enters
•
Permeability
(P) for Na+ is very low
Consequences of Sodium
Entry
•
1. As
(+) charge enters - inward Na+ current
•
2. This
inward current reduces membrane
•
3. Vm
< EK
•
4.
The K+ concentration gradient is
now able to drive K+ out
•
Such
that INa + -IK
•
Thus we
have a steady state not a true
AT Rest
•
No net
fluxes of charge across the membrane
V rest is neither EK or ENa
Squid: EK
= -75 mV
ENa
= + 55 mV
At EK No
net flux of K+
Net
influx of Na+
At ENa No net flux of Na+
Net
efflux of K+
In the Squid: Steady state results when the two fluxes are equal and opposite
-- -60mV
Role of Chloride Ions
•
Very
permeable at rest
•
Cl-
not pumped in most cells
•
Thus
free to diffuse in or out
•
ECl-
= V rest
•
In some
cells Cl- is pumped
•
Outward
directed pump
•
Thus Cl-
leaks in and is pumped
•
ECl- more negative than V rest
Nerve Impulses or Action
Potentials
An action potential is a brief (»1mSec) all or nothing, regenerative
electrical potential that propagates along the length of an axon or muscle
fiber.
Action Potential
Characteristics of the
Active Conductance Changes
•
Na +
- Voltage sensitive & Time
dependent
•
K+
- Voltage sensitive & time dependent
Active Conductances
Phases of an Action
Potential
Active Sodium Conductance
•
m gates
- sensitive
•
h gates
- time dependent
–
closed
- inactivated (refractory)
Threshold
Axons
and muscle fibers must be depolarized 15-20 mV from rest to evoke an
action potential
Refractory Periods
•
Absolute
- lasts up to 2mS
•
Relative
- 1-10 mS
Refractory Periods
Propagation of APs
•
Unmyelinated
axons & muscle fibers
–
Local
response
–
For
conduction to continue - passive spread to next piece of membrane
–
Reaches
threshold - New AP
–
Carried
internally by - billiard ball effect
–
K+
tries to leak out next avail channel - depolarizes membrane
Propagation of APs
Propagation of APs
•
Myelinated
Axon
–
Myelin
good insulator
–
Very
rapid internal spread - but some K+ leaks out
–
Would
eventually run down
–
Nodes
of Ranvier - 0.5 - 5 mm
–
Recharging
- active channels present
–
Saltatory
conduction
Saltatory Conduction
The Synapse
•
Functional
site of contact between two neurons or a neuron & a muscle or gland.
•
Types
–
Bridged
- electrical
–
Unbridged
-
Main Types of Synaptic
Transmission
•
Electrical
transmission
–
Fast,
no transmitter & no postsynaptic receptors
•
Chemical
transmission
–
Slight
delay (0.5 -1.0 mSec), neurotransmitter as intermediary, transmitter binds with
postsynaptic receptor
Chemical Synaptic
Transmission
•
Excitatory
- probability that postsynaptic cell
produces an AP
•
Inhibitory
- decreases probability that postsynaptic cell produces an AP
•
Neuromodulatory
- slow changes - usually in metabolic activity or electrical properties of
postsynaptic cell
Electrical Transmission
•
Zone of
apposition is bridged by protoplasmic channels
•
Physical
gap - 2nM - 1/10 normal cell spacing
•
Channels
composed of connexin
•
Create
hexagonal subunits to make a channel
–
Intercellular
channels
•
Where -
motor neurons that control extraocular eye muscles
Gap Junction
Chemical Transmission
•
Pre
& post synaptic cells not in continuity
•
Separated
by synaptic cleft (Centrally -
30nM)
•
Pre
& post cells - membrane
•
Presynaptic
terminal
•
Synaptic
vesicles - Neurotransmitter
Chemical Transmission
•
Transmission
accomplished by chemical transmitter
•
Synaptic
delay - 0.3 to 1 mSec
–
Secretory
–
Diffusion
time
–
Binding
& activation of receptor at postsynaptic cell
–
Unidirectional
transmission
Neuromuscular Junction
Transmitter Release
•
AP
arrives synaptic terminal
•
Triggers
Ca++ entry via voltage Ca++ channels
•
Ca++
promotes the fusion of synaptic vesicles with the postsynaptic membrane
•
Transmitter
is released from synaptic vesicles
•
Diffuses
across the synaptic cleft
•
Binds
to postsynaptic receptors
•
Ion
channels open - postsynaptic response
Chemical Transmission
Postsynaptic Responses
•
If a depolarization -
EPSP
–
Due to influx of Na+
& slight efflux of K+
•
If hyperpolarization -
–
Due to increased K+
–
Due to increased Cl -
EPSPs and IPSPs
EPSPs to Action Potentials
EPSPs to Action Potentials
Interactions between IPSPs
and EPSPs
Acetylcholine
•
Nicotinic
ACh Receptors
•
Two
binding sites - channel
•
Mostly
Na+ & a little K+ - Depolarization
•
Ligand-activated
channels (directly gated)
Acetylcholine
•
Muscarinic
ACh receptors
•
One
binding site - transmitter binding - activates a G protein
•
G
protein interacts with channel protein - Efflux of potassium
–
IPSP
results
Termination of Transmission
•
Diffusion
•
Enzymatic
degradation
•
Reuptake
of whole of transmitter
–
Most
common mechanism
Monoamines
•
Dopamine
•
Epinephrine
(hormone)
•
Norepinephrine
•
Serotonin
•
Monoamine
Receptors
–
Work
through 2nd messengers (cAMP)
Amino Acid Transmitters
•
Glutamic
Acid (glutamate) - excitatory
•
Aspartic
acid (aspartate) - excitatory
•
Glycine
-
•
GABA -
inhibitory
Opioid Transmitters
•
Come
from large precursors - via
cleavage
•
1.
Prodynorphine
–
Dynorphin
A, Dynorphin B, b
neoendorphin
•
2.
Proenkephalins
–
Met-enkephalin
–
Leu-
•
3. POMC
- pro-opiomelanocortin
–
ACTH, b Lipotrophin, b endorphin, Clip -
cortiotropin-like-peptide
Nitric Oxide
•
NO -
first gas known as a regulatory molecule
•
produced
by nitric oxide synthetase from AA, L-Arginine
•
In
blood vessels - local tissue regulator
•
Smooth
muscle relaxes - vessels
Nitric Oxide
•
In
macrophages - kills
•
PNS
& CNS - diffuses across presynaptic membrane - use c-GMP as second messenger
- post synaptically
•
PNS -
GI tract, penis, respir. passages & cerebral blood vessels