Transcript Chapter 11 - Nervous Tissue

Chapter 11
Fundamentals of the Nervous System
and Nervous Tissue
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
The Nervous System
 The Nervous System is the rapid control
system of the body
 There are two anatomical divisions to the
Nervous System:
 The Central Nervous System (CNS)
 The Peripheral Nervous System (PNS)
 They work together as a single coordinated whole
The Functions of the Nervous System
 There are three
interconnected
functions:
 sensory input
 from millions of
specialized
receptors
 receive stimuli
 integration
 process stimuli
 interpret stimuli
 motor output
 cause response
 at many effector
organs
Organization of the Central Nervous System
 the Brain and Spinal Cord
 process & integrate information,
store information, determine
emotions
 initiate commands for muscle
contraction, glandular secretion
and hormone release (regulate
and maintain homeostasis)
 connected to all other parts of
the body by the Peripheral
Nervous System (PNS)
Organization of the Peripheral NS
 anatomical connections


spinal nerves are connected to the
spinal cord
cranial nerves are connected to
the brain
 two functional subdivisions


sensory (afferent) division
 somatic afferents - skin,
skeletal muscle, tendons,
joints
 special sensory afferents
 visceral afferents - visceral
organs
motor (efferent) division
 motor (efferent) neurons
 muscles/glands
Organization of the PNS (continued)
 motor (efferent) division
has two parts:
 Somatic Nervous System
(SNS)
 voluntary motor neurons
 output to skeletal muscles
 Autonomic Nervous System
(ANS)
 involuntary visceral motor
neurons
 output to smooth muscle,
cardiac muscles and to glands
 two cooperative components:
•
•
sympathetic division
parasympathetic division
Autonomic Nervous System
 Sympathetic
Division – for
muscular exertion
and for “fight or
flight” emergencies
 Parasympathetic
Division – for
metabolic/
physiologic
“business as usual”
(“feed or breed”)
Nervous Tissue
 Review the microanatomy of nervous tissue in lab
and in the PPT with audio: CH11 Histology of
Nervous Tissue
 Nerve cell physiology is primarily a cell membrane
phenomenon
 Information transmission differs between dendrites
and axons
Neuron Processes - Dendrites




short, tapering, highly branched extensions of the soma
not myelinated
contain some cell organelles
receptive—initiate and transmit graded potentials (not action
potentials) to the cell body
Neuron Processes - Axons
 A single process that transmits
action potentials from the
soma
 Originates from a cone-shaped
“axon hillock”
 May be long (1 meter) or short
(<1 mm)
 long axons called nerve fibers
 Up to 10,000 terminal branches
 each with an axon terminal
that synapses (joins) with a
neuron or an effector (muscle or
gland cell)
Axons (continued)
 Axoplasm: the cytoplasm of the axon
 Axolemma: the cell membrane of the
axon, specialized to initiate and conduct
action potentials (nerve impulses)
 initiated at the axon hillock (trigger zone),
travels to the axon terminal
 causes release of neurotransmitter from terminal
 neurotransmitters can excite or inhibit
 transfers a control message to other neurons or
effector cells
Histology of Neurons – Myelin Sheath
lipid-rich, segmented covering on axons
most larger, longer axons are myelinated
dendrites are never myelinated
myelin protects & electrically insulates
the axon
 increases the speed of nerve impulses




 myelinated fibers conduct impulses 10-150x
faster than unmyelinated fibers
 150 m/sec vs. 1 m/sec
Myelinating Cells
 neurolemmocytes (Schwann cells)
in the Peripheral NS
 oligodendrocytes in the Central NS
Myelination
 occurs during fetal development
and the first year of life
 each myelinating cell wraps
around an axon up to 100
times, squeezing its cytoplasm
and organelles to the periphery
 myelin sheath: multiple layers of
the cell membrane
 neurolemma (sheath of
Schwann): outer layer containing
the bulk of the cytoplasm and cell
organelles
Myelinated and Unmyelinated Axons
 Myelinated Fibers
 Myelin sheath
 neurofibril nodes
(Nodes of Ranvier)
periodic gaps in the myelin
sheath between the
neurolemmocytes
 Unmyelinated Fibers
 surrounded by
neurolemmocytes but
no myelin sheath present
 neurolemmocytes may
enclose up to 15 axons
neurolemmocytes guide regrowth of
(unmyelinated fibers)
neuron processes after injury
Myelination In the Central NS
 Gray matter - unmyelinated cell bodies & processes
 White matter – myelinated processes in various fiber
tracts
Classification of Neurons
 Structural: based on the number of
processes extending from the cell body
 Functional: based on the direction
(location) of nerve impulses
We will focus on functional classification
Afferent (= Sensory) Neurons
 afferent = towards CNS
 nerve impulses from
specific sensory receptors
(touch, sight, etc.) are
transmitted to the spinal
cord or brain (CNS)
 afferent neuron cell bodies
are located outside the
CNS in ganglia
Efferent (= Motor) Neurons
 efferent = away from
CNS
 nerve impulses from CNS
(brain and spinal cord)
are transmitted to
effectors (muscles,
endocrine and exocrine
glands)
 efferent neuron cell
bodies are located inside
the CNS
Association Neurons (= Interneurons)
 carry nerve impulses
from one neuron to
another
 99% of the neurons
in the body are
interneurons
 most interneurons
are located in the
CNS
Neurophysiology - Definitions
 voltage
 the measure of potential energy generated by
separated charges
 always measured between two points – the inside
versus the outside of the cell
 referred to as a potential - since the charges (ions)
are separated there is a potential for the charges
(ions) to move along the charge gradient
Neurophysiology - Definitions
 current
 the flow of electrical charge from one point to
another
 in the body, current is due to the movement of
charged ions
 resistance
 the prevention of the movement of charges (ions)
 caused by the structures (membranes) through
which the charges (ions) have to flow
Neurophysiology - Basics
 Cell interior and exterior have different chemical
compositions
 Na+/K+ ATPase pumps change the ion concentrations
 a semi-permeable membrane allows for separation of ions
 Ions attempt to reach electrochemical equilibrium
 two forces power the movement of ions
 individual ion concentrations (chemical gradients)
 net electrical charge (overall charge gradient)
 the balance between concentration (chemical) gradients
and the electrical gradient known as the electrochemical
equilibrium
 the external voltage required to balance the concentration
gradient is the equilibrium (voltage) potential
Neurophysiology - Membrane Ion Channels
 regulate ion movements
across cell membrane
 each is specific for a
particular ion or ions
 many different types
 may be passive (leaky)
 may be active (gated)
 gate status is controlled
 gated channels are
regulated by signal
chemicals or by other
changes in the membrane
potential (voltage
potential)
Resting Membrane Potential (RMP)
 electrical charge gradient
associated with outer cell
membrane
 present in all living cells
 the cytoplasm within the cell
membrane is negatively
charged due to the charge
disequilibrium concentrations of
cations and anions on either
side of the membrane
 RMP varies from about -40 to
-90 millivolts (a net negative
charge inside relative to a net
positive charge outside the cell)
Resting Membrane Potential (cont.)
 RMP is similar to a battery
 stores an electrical charge and can release the charge
 2 main reasons for this:
 ion concentrations on either side of the plasma membrane
are due to the action of the Na+/K+ ATPase pumps
 primarily, Na+ and Cl- are outside; the membrane is polarized
 primarily, K+, Cl-, proteins- and organic phosphates- are inside
 plasma membrane has limited permeability to Na+ and K+
ions
Resting Membrane Potential (cont.)
 Resting conditions
 Na+/K+ ATPase pumps 3 Na+ ions out and 2 K+
ions in per ATP hydrolysis – opposing their
concentration gradients
 concentration gradient drives Na+ to go into the cell
 concentration gradient drives K+ to go out of the cell
 if the cell membrane were permeable to Na+ and
K+ ions, then Na+ and K+ ions would diffuse along
their electrical and chemical gradients and would
reach equilibrium
 if the cell was at equilibrium in terms of ion
concentrations and charge, their would be no
potential energy available for impulse transmission
Resting Membrane Potential (cont.)
 Neuron Membrane at rest is polarized
 the cytoplasm inside is negatively charged relative
to the outside
 the net negative charge in the cytoplasm attracts
all cations to the inside
 some Na+ leaks in, despite limited membrane
permeability
 Na+-K+ ATPase keeps working to pump 3 Na+ ions out
and 2 K+ ions in, opposing the two concentration
gradients (for Na+ and K+)
Resting Membrane Potential (cont.)
 Here is the electrochemical gradient at rest:
the resting potential
Membrane Potentials As Signals
 cells use changes in
membrane potential
(voltage) to exchange
information
 voltage changes occur by two
means:
1. changing the membrane
permeability to an ion; or
2. changing the ion concentration
on either side of the membrane
 these changes are made by
ion channels
 passive channels – leaky: K+
 active channels:
•
•
chemically gated – by
neurotransmitters
voltage gated
Types of Membrane Potentials
 graded potentials
 graded = different
levels of strength
 dependent on
strength of the
stimulus
 action potentials
 in response to graded
potentials of
significant strength
 signal over long
distances
 all or nothing
Types of Membrane Potentials
 graded potentials and
action potentials may be
either:
 hyperpolarizing
 increasing membrane
polarity
 making the inside more
negative
 depolarizing
 decreasing membrane
polarity
 making the inside less
negative = more positive
Properties of Action Potentials
 a nerve impulse (action potential) is generated
in response to a threshold graded potential
 depolarization
 change in the membrane polarization
 stimuli reach a threshold limit and open voltagegated Na+ channels
 Na+ ions rush into the cell  down the Na+
concentration and electrical gradients
 the cytoplasm inside the cell becomes positive
 reverses membrane potential to +30 mV
 local anesthetics prevent opening of voltagegated Na+ channels - prevent depolarization
Sequence of Events in Action Potentials
1. Resting
membrane
potential
Sequence of Events in Action Potentials
2. Depolarization
a) stimulus
strength reaches
threshold limit
b) voltage gated
Na+ channels
open
c) Na+ flows into
the cytoplasm
d) More V-gated
Na+ channels
open
[positive feedback]
Sequence of Events in Action Potentials
3. Repolarization
a) voltage gated
K+ channels
open
b) voltage gated
Na+ channels
close
Sequence of Events in Action Potentials
4. Hyperpolarization
a) gated Na+
channels are
reset to closed
b) membrane
remains
hyperpolarized
until K+
channels close,
causing the
relative
refractory period
Repeat the process:
Sequence of Events in Action Potentials
1. Resting
membrane
potential
Sequence of Events in Action Potentials
2. Depolarization
a) stimulus
strength reaches
threshold limit
b) voltage gated
Na+ channels
open
c) Na+ flows into
the cytoplasm
d) More V-gated
Na+ channels
open
[positive feedback]
Sequence of Events in Action Potentials
3. Repolarization
a) voltage gated
K+ channels
open
b) voltage gated
Na+ channels
close
Sequence of Events in Action Potentials
4. Hyperpolarization
a) gated Na+
channels are
reset to closed
b) membrane
remains
hyperpolarized
until K+
channels close,
causing the
relative
refractory period
The All-or-None Principle
 stimuli/neurotransmitters arrive and open
some of the chemically-gated Na+ channels
 if stimuli reach the threshold level 
depolarization occurs
 voltage-gated Na+ channels open
 an Action Potential is generated which is constant and
at maximum strength
 if stimuli do not reach the threshold level 
nothing happens
Repolarization
 Re-establishing the resting membrane polarization
state
 threshold depolarization opens Na+ channels
 Na+ ions flow inward, making the cell interior more positive
 a few milliseconds later, K+ channels also open
 K+ channels open more slowly and remain open longer
 K+ ions flow out along its concentration and charge
gradients
 carries positive (+) charges out, making the cell interior more
negative (-)
 Ion movements drive the membrane potential back toward
resting membrane potential value
 Na+/K+ ATPase continue pumping ions, adjusting levels
back to resting equilibrium levels
 hyperpolarization – briefly the exterior of the membrane
is more negative than resting potential voltage level
Refractory Periods
 Absolute Refractory Period
 the time period during
which second AP cannot
be initiated
 due to closure of voltagegated Na+ channels
 the voltage-gated Na+
channels must be reset
before the membrane can
respond to the next
stimulus
Many physiologists
consider this to be the
start of the absolute
refractory period
Refractory Periods
 Relative Refractory Period
 The time period during
which a second AP can be
initiated with a
suprathreshold stimulus
 K+ channels are open, Na+
channels are closed
 the membrane is still
hyperpolarized
Propagation of an Action Potential
 the movement of an Action Potential down an
unmyelinated axon
 a local electrochemical current, a flow of charged ions
 influx of sodium ions
 attraction of positive charges for negative area of membrane
nearby
 depolarizes nearby membrane – opening V-gated Na+ channels
Propagation of an Action Potential
 destabilizing the adjacent membrane makes the
Action Potential self-propagating and self-sustaining
 the Action Potential renews itself at each region of
the membrane – a relatively slow process because
so much is happening at the molecular level
Conduction Velocity
 physical factors may influence impulse
conduction
 heat increases conduction velocity
 cold decreases conduction velocity
 2 structural modifications can increase impulse
velocity:
 increase neuron diameter - decreases resistance
 insulate the neuron - myelin sheath
 myelinated fibers may conduct as rapidly as 150 m/sec
 unmyelinated may conduct as slowly as 0.5 m/sec
Saltatory Conduction
 not a continuous region to region depolarization
 instead, a “jumping” depolarization
 myelinated axons transmit an Action Potential differently
 the myelin sheath acts as an insulator preventing ion flows in and out
of the membrane
 neurofibral nodes (node of Ranvier) interrupt the myelin sheath and
permit ion flows at the exposed locations on the axon membrane
 the nodes contain a high density of voltage-gated Na+ channels
Saltatory Conduction
 in a myelinated fiber, the ionic current flows in at
each node and travels through the axoplasm to the
next node
 each node depolarizes in sequence, renewing the
Action Potential at that node
 the Action Potential jumps to next node very rapidly
 energy efficient – the membrane only has to
depolarize and repolarize at the nodes
 less Na+/K+ ATPase activity is required, therefore,
less energy is required
The Synapse
 Function
 there must be a means
of communication
between each neuron
and the next target cell
 the synapse is the
connection
 Organization
 presynaptic neuron
 postsynaptic neuron
 separated by synaptic cleft
The Two Types of Synapses
 (1) electrical synapses
 gap junctions – found in cardiac muscle and in some
smooth muscle tissues
 direct, rapid electrochemical connections between neurons
 may be bidirectional; useful for coordinated contraction
 rare in adults
 (2) chemical synapses
 specialized for synthesis, release, reception and removal of
neurotransmitters
 neurotransmitters
 chemical signal molecules released from a presynaptic neuron
 function to open or close chemically-gated ion channels
 effect membrane permeability and membrane potential
Action of a Chemical Synapse
 Presynaptic Events
 an action potential reaches the axon terminal and
depolarizes the terminal
 voltage gated Ca2+ channels open; Ca2+ ions enter the axoplasm
 neurotransmitter is released by exocytosis
 neurotransmitter molecules diffuse across the cleft
Action of a Chemical Synapse (cont.)
 Postsynaptic Events
1) the neurotransmitters bind to specific
postsynapticreceptors
2) gated ion channels open as a result
3) neurotransmitter molecules are eliminated quickly
a) degraded by extracellular enzymes in the synapse, with the
products re-uptaken and recycled by the axon terminal
b) diffuse away from the synapse to the blood circulation
Postsynaptic Potentials
 EPSP
 excitatory postsynaptic
potential
 provides a small local
depolarization
 generally results from
opening Na+ channels
 IPSP
 inhibitory postsynaptic
potential
 provides a small local
hyperpolarization
 generally results from
opening K+ or CL- channels
Summation of Postsynaptic Potentials
 temporal – rapid repeated stimulation from 2 or
more presynaptic neurons
 spatial – simultaneous stimulation at 2 or more
different places on the neuron by presynaptic
neurons
 EPSPs and IPSPs counteract each other
End Chapter 11
The Nernst Equation
Ex= RT ln [X]out
zF
[X]in
EX= Equilibrium potential of ion X in volts
R = gas constant
T = temperature in kelvins
z = charge of each ion
F = Faraday’s constant (96,500 coulombs/gram-equivalent charge
[X] = ion concentration
At 38°C, (the standard temperature of many mammals) &
converting ln:
Ex=
61
z
log
[X]out
[X]in
The Goldman-Hodgkin-Katz Equation
+]
+]
-]
RT
P
[K
+
P
[Na
+
P
[Cl
K
out
Na
out
Cl
in
ENa,K,Cl=
log
F
PK[K+]in + PNa[Na+]in + PCl[Cl-]out
PERMEABILITY CHANGES DEPENDING UPON NEURON STATUS
At rest:
PK:PNa:PCl=1/0.04/0.45
At Action Potential Peak:
PK:PNa:PCl=1/20/0.45
EX= Equilibrium potential of all ions in volts
R = gas constant
T = temperature in kelvins
F = Faraday’s constant (96,500 coulombs/gram-equivalent charge
The Goldman-Hodgkin-Katz Equation
+]
+]
-]
RT
P
[K
+
P
[Na
+
P
[Cl
K
out
Na
out
Cl
in
ENa,K,Cl=
log
F
PK[K+]in + PNa[Na+]in + PCl[Cl-]out
At rest:
PK:PNa:PCl=1/0.04/0.45
Ion Species
Extracellular
(mM)
Intracellular (mM)
K+
5
150
Na+
150
15
Cl-
120
10