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Visual Anatomy & Physiology
First Edition
Martini & Ober
Chapter 11
Nervous System I
Lecture 18
1
Lecture Overview
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Overview of the NS
Review of nervous tissue
Functions of the Nervous System (NS)
Histology and Structure of the NS
Classification of Neurons
Neurophysiology
Nerve impulse transmission
Synaptic transmission
2
Function of the Nervous System
• The nervous system is a coordination and
control system that helps the body maintain
homeostasis. It
– Gathers information about the internal and
external environment (sense organs, nerves)
– Relays this information to the spinal cord and
the brain
– Processes and integrates the information
– Responds, if necessary, with impulses sent via
nerves to muscles, glands, and organs
3
Overview of the Nervous System
4
Divisions of the Nervous System
Figure from: Hole’s Human A&P, 12th edition, 2010
Know all of these
subdivisions of
the nervous
system
5
Major subdivisions
CNS
PNS
Nervous Tissue
• found in brain, spinal cord, and peripheral nerves
• conduction of nerve impulses
• basic cells are neurons
• sensory reception and motor impulses
• neuroglial cells are supporting cells
6
Figure from: Hole’s Human A&P, 12th edition, 2010
Neuron Structure
(soma)
rER
*Initial segment (origin of nerve
impulses)
Identify/label the
structure/parts of a
neuron shown here.
State the function of
dendrites, the cell
body, axons, initial
segment, and
synaptic knobs
8
Figure from: Hole’s Human A&P, 12th edition, 2010
Neuron function and Nerve Connections
Figure from: www.erachampion.com
(fast)
(slow)
The major functions of a neuron are to
1) collect input from other neurons
2) integrate the signals
3) send (or not) an appropriate type of signal to neurons it synapses with
9
Structural Classification of Neurons
Figure from: Hole’s Human A&P, 12th edition, 2010
Bipolar
• two processes
• sense organs
Unipolar
• one process
• ganglia
Multipolar
• many processes
• most neurons of
CNS
**Classification is based on the number of processes
coming directly out of the cell body
10
Functional Classification of Neurons
Sensory Neurons
• afferent, ascending
• carry impulse to CNS
• most are unipolar
• some are bipolar
Interneurons
• link neurons
• integrative
• multipolar
• in CNS
Motor Neurons
• efferent, descending
• multipolar
• carry impulses away
from CNS
• carry impulses to
effectors
Figure from: Hole’s Human A&P, 12th edition, 2010
Notice the directionality – one-way
11
Neuroglia (glia = glue)
Figure from:
Martini,
Anatomy &
Physiology,
Prentice Hall,
2001
Know the information contained in the Table on following slide.
12
Summary Table of Neuroglia
Name of Cell
Location
Function(s)
Satellite Cells
Ganglia of PNS
Regulate microenvironment
of neurons
Astrocytes
CNS
Regulate microenvironment
of neurons;
scar tissue in CNS
Schwann Cells
PNS
Myelination of axons;
structural support for nonmyelinated axons
Oligodendrocytes
CNS
Myelination of axons;
structural framework
Microglia
CNS
Phagocytes of the CNS
Ependymal Cells
CNS
Assist in producing and
controlling composition of
CSF
13
Intro to Cell Response/Signaling
• How does a neuron “know” its being
stimulated?
• When stimulated by multiple inputs, how
does a neuron “know” whether it should
send a nerve impulse (action potential) or
not?
Answer: Changes in cellular ionic composition
But recall that ions are ‘hydrated’ and cannot pass
through a cell membrane. How do they pass from
outside to inside or from inside to outside? CHANNELS
14
Intro to Transmembrane Potential
For activation of neurons, ions need to pass from one side of
the cell membrane to the other. Would that happen if there
was an equal concentration of those ions on both sides of the
membrane? NO!
Therefore, it is necessary to have the cell in a ready state to
let ions flow from one side of the membrane to the other
when the time is right.
This requires that an UNEQUAL concentration of ions exist
BEFORE the cell decides its time to move ions from one
place to the other.
HOW IS THIS DONE?
15
Intro to Transmembrane Potential
• Positive and negative charges attract
• Whenever + and – charges are held apart, a
potential difference exists
• Size of a potential difference is measured in
Volts (V) or millivolts (mV = Volt/1,000)
• If + and – charges are allowed to flow
toward one another, a current develops
• The opposition to a current is called
resistance
Let’s look at the origin of the membrane potential…
16
A Cell With Zero Electrical Membrane Potential
Extracellular
1 K+
20 Na+
21 Cl- (major extracellular anion)
Leak
channel
Total
charge = 0
Intracellular
19 K+
1 K+
21 protein- (major intracellular anion)
1 Na+
Where would K+ tend to go? Why? What stops it?
Where would Na+ tend to go? Why? What stops it?
17
Establishing Resting Membrane Potential
Extracellular
1 K+
1 K+
+ +
Intracellular
- - 19 K+
21 Cl-
+ + +
+
1 K+
1
20 Na+
Na+
-
21 protein-
This process continues until the inside of the cell
membrane is about -70 mV (negative) relative to
the outside of the cell membrane – the
membrane is said to be polarized.
The INSIDE
of the cell
membrane is
now negative
relative to the
outside
mainly due to
the outward
flow of K+
ions.
18
Transmembrane Potential
A potential difference of -70 mV exists in the
resting neuron due to the electrochemical
gradient – membrane is polarized
-3 mV
• inside is
negative
relative to
the outside
• *polarized
membrane
due to
distribution
of ions
• Na+/K+ATPase
pump
19
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Membrane Channel Proteins
• Passive channels are ALWAYS open
– Also called ‘leak’ channels
– Passive K+ channels always allow K+ through
• Active (gated) channels open or close in response to
signals
– Mechanical – respond to distortion of membrane
– Ligand-gated (Chemical)
• Binding of a chemical molecule, e.g., ACh on MEP
• Present on dendrites, soma, sometimes on axons
– Voltage-gated
• Respond to changed in electrical potential
• Found on excitable membranes, e.g., axons, sarcolemma
20
Mechanically-gated Channels
From: http://www.ionchannels.org/content/images/3-01.jpg
21
Ligand-gated Channels
From: http://en.wikipedia.org/wiki/Ligand-gated_ion_channel
22
Voltage-gated Channels
----
From:
http://courses.cm.utexas.edu/jrobertus
/ch339k/overheads-2.htm
++++
23
Changes in Membrane Potential
0
• If membrane potential becomes more positive than its
resting potential, it has depolarized (Movement of ? charges causes this?)
• A membrane returning to its resting potential from a
depolarized state is being repolarized (Movement of ? charges causes this?)
• If membrane potential becomes more negative than
its resting potential, it has hyperpolarized
24
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Action Potentials
• Action potential = nerve impulse (neuron must reach
THRESHOLD before an action potential occurs)
• Begins at initial segment of axons (high density of voltageregulated Na+ channels)
• all-or-none (think: finger on a gun’s trigger)
• Does not weaken with distance
• refractory period
• absolute - time when threshold stimulus does not start another
action potential (Na+ channels inactivated)
• relative – time when stronger threshold stimulus can start
another action potential (Na+ channels restored, K+ channels begin
closing)
25
Action Potentials Begin at the Initial Segment
Figure from: Hole’s Human A&P,
12th edition, 2010
Ligand-gated
Na+ channels
Voltage-gated
Na+ channels
Action potential begins
here, in the initial segment.
Note the high number of
voltage-gated channels.
26
Threshold and Action Potential
Figure from: Hole’s Human A&P, 12th edition, 2010
= Nerve impulse
What causes
depolarization?
Repolarization
Influx of
Na+
Efflux of K+
Depolarization
What causes
repolarization?
Threshold
Steps in Action Potential:
1) depolarization 2) repolarization 3) hyperploarization
4) return to resting potential
27
How does a neuron ‘know’ when to fire?
Any one neuron
receives many
THOUSANDS of
inputs from other
neurons.
Not all of these will
make the neuron
generate a nerve
impulse.
How does this work?
29
Figure from: Hole’s Human A&P, 12th edition, 2010
Local (Graded) Potential Changes
• Caused by various stimuli
• chemicals
• temperature changes
• mechanical forces
• Cannot spread very far (~ 1 mm max) – weaken rapidly
• Uses ligand-gated Na+ channels
• On membranes of many types of cells including epithelial cells,
glands, dendrites and neuronal cell bodies
• General response method for cells
• Can be summed (so that an action potential threshold is
reached; change in membrane potential  stimulus strength
• Starting point for an action potential
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*
Channels on Neurons
Figure from: Hole’s Human A&P,
12th edition, 2010
Note the high number
of ligand-gated
channels on the
dendrites and soma;
this is where graded
(local) potentials
occur
Ligand-gated
Na+ channels
Voltage-gated
Na+ channels
31
Refractory Period
Figure from: Hole’s Human A&P, 12th edition, 2010
ARP = Absolute Refractory Period
RRP = Relative Refractory Period
Influx of Na+
(Depolarization)
Efflux of K+ (Repolarization)
Great
summary
graphic to
know for
the exam!
Threshold
ARP
RRP
32
Action Potential Membrane Changes
Figure from: Hole’s Human A&P, 12th edition, 2010
Step1: Depolarization
to threshold
Step 2: Na+ channels
open
Step 3: Na+ channels
close, K+ channels
open
Step 4: Return to
normal polarization
Action potentials use voltage-gated Na+ channels
33
Action Potentials
Figure from: Hole’s Human A&P, 12th edition, 2010
Shown at left is an
example of continuous
propagation (~ 1m/s)
What keeps the action
potential going in ONE
DIRECTION, and not
spreading in all
directions like a graded
potential?
Figure from: Saladin,
Anatomy & Physiology,
McGraw Hill, 2007
Absolute refractory
period of the previously
depolarized segment.
34
Action Potential
Myelination of Axons
Figure from: Hole’s Human A&P, 12th edition, 2010
White Matter
• contains myelinated
axons
Gray Matter (CNS)
• contains
unmyelinated
structures
• cell bodies, dendrites
Smaller
axons in PNS
In CNS, myelin is
produced by ?
Oligodendrocytes
35
Saltatory (Leaping) Conduction
Figure from: Hole’s Human A&P, 12th edition, 2010
Myelin acts as an insulator and increases the resistance
to flow of ions across neuron cell membrane
(fast)
Ions can cross membrane only at nodes of Ranvier
Impulse transmission is up to 20x faster than in non-myelinated nerves.
Myelinated axons are primarily what makes up white matter.
36
Regeneration of A Nerve Axon
Figure from: Hole’s Human A&P, 12th edition, 2010
Growth = 3-4 mm/day
Damage to cell body usually cannot be repaired –
neurons lack centrioles and usually cannot divide
37
The Chemical Synapse
Figure from: Hole’s Human A&P, 12th edition, 2010
Nerve impulses pass
from neuron to
neuron at synapses in
about 0.5 msec
Are there any other
kind of synapses
besides chemical ones?
38
Chemical Synaptic Transmission
Neurotransmitters (ntx) are
released when impulse reaches
synaptic knob
You should understand this process
This may or may not
release enough ntx to bring the
postsynaptic neuron to threshold
Chemical neurotransmission may
be modified
Ultimate effect of a ntx is
dependent upon the properties of
the receptor, not the ntx
How is the neurotransmitter
neutralized so the signal doesn’t
continue indefinitely?
39
Figure from: Hole’s Human A&P, 12th edition, 2010
Neurotransmitters
Table from: Hole’s Human A&P, 12th edition, 2010
*
*
*
Neuromodulators: Influence release of ntx or the
postsynaptic response to a ntx, e.g., endorphins, enkephalins
40
Postsynaptic Potentials
EPSP
• excitatory postsynaptic potential
• depolarizes membrane of postsynaptic neuron
• action potential of postsynaptic neuron becomes more
likely
IPSP
• inhibitory postsynaptic potential
•hyperpolarizes membrane of postsynaptic neuron
• action potential of postsynaptic neuron becomes less likely
Both of these act by changing the resting membrane
potential; either de- or hyperpolarizing it
41
Summation of EPSPs and IPSPs
Figure from: Hole’s Human A&P, 12th edition, 2010
• EPSPs and IPSPs are
added together in a
process called
summation
• Summation can be
temporal or spatial
• More EPSPs lead to
greater probability of
action potential
42
Review
• There are two major divisions of the
nervous system
– CNS – Brain and spinal cord
– PNS – Cranial and spinal nerves
• The nervous system has three general
functions
– Sensory
– Integrative (associative)
– Motor
44
Review
• Neurons are the impulse-transmitting cells
of the nervous system
–
–
–
–
Dendrites
Soma (cell body)
Axon
Initial segment
• Larger axons of peripheral nerves are
myelinated
– Schwann cells (PNS)
– Myelin and nodes of Ranvier
– Increase conduction speed (saltatory)
45
Review
• Neurons can be classified according to
function or structure
– Structural; bi-, uni-, and multipolar
– Functional; sensory, associative (interneurons),
motor
• CNS Neuroglia support and nourish neurons
– Astrocytes; support, ion regulation, blood-brain
barrier
– Oligodendrocytes; myelination in CNS, growth
factors
– Microglia; support and phagocytosis
– Ependyma; line ventricles, choroid plexuses,
regulate composition of CSF
46
Review
• Important terms in nerve impulse transmission
– Resting potential
• Na+ / K+ and Na+/K+ Pump
–
–
–
–
–
–
–
Local potential and summation
Action potential
Hyperpolarization and depolarization
All-or-none response
Refractory period
Saltatory conduction
See Table 10.3 in Hole (good to know!)
47
Review
• Communication between nerves and/or
effectors takes place at the synapse
– EPSP and IPSP
– Neurotransmitters mediate synaptic transmission
(Acetylcholine, Norepinephrine)
• Convergence and divergence
48