Bio 211 Lecture 18

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Mariebs Human Anatomy and Physiology Marieb w
Hoehn
Chapter 11 Fundamentals of the Nervous System and
Nervous Tissue Lecture 18
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Lecture Overview

• 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

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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

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Divisions of the Nervous System
Figure from Holes Human AP, 12th edition, 2010
Know all of these subdivisions of the nervous
system
CNS
PNS
Major subdivisions
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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
Figure from Holes Human AP, 12th edition, 2010
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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
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Structural Classification of Neurons
Figure from Holes Human AP, 12th edition, 2010

• Multipolar
• many processes
• most neurons of CNS

• Bipolar
• two processes
• sense organs

• Unipolar
• one process
• ganglia

Classification is based on the number of
processes coming directly out of the cell body
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Functional Classification of Neurons

• Sensory Neurons
• afferent, ascending
• carry impulse to CNS
• most are unipolar
• some are bipolar

Figure from Holes Human AP, 12th edition, 2010

• Interneurons
• link neurons
• integrative
• multipolar
• in CNS

• Motor Neurons
• efferent, descending
• multipolar
• carry impulses away from CNS
• carry impulses to effectors

Notice the directionality one-way
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Neuroglia (glia glue)
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
Know the information contained in the Table on
following slide.
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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 non-myelinated 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
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Transmembrane Potential
A potential difference of -70 mV exists in the
resting neuron due to the electrochemical
gradient membrane is polarized

• inside is negative relative to the outside
• polarized membrane
• due to distribution of ions
• Na/K-ATPase pump

-3 mV
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
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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 (Chemically-gated)
• 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

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Mechanically-gated Channels
From http//www.ionchannels.org/content/images/3-
01.jpg
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Ligand-gated Channels
From http//en.wikipedia.org/wiki/Cell_surface_re
ceptor
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Voltage-gated Channels
- - - -
From http//courses.cm.utexas.edu/jrobertus/ch339
k/overheads-2.htm

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Changes in Membrane Potential
0

• If membrane potential becomes more positive than
  its resting potential, it has depolarized
• A membrane returning to its resting potential
  from a depolarized state is being repolarized
• If membrane potential becomes more negative than
  its resting potential, it has hyperpolarized

(Movement of ? charges causes this?)
(Movement of ? charges causes this?)
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
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Action Potentials

• Action potential nerve impulse (neuron must
  reach THRESHOLD before an action potential
  occurs)
• Begins at initial segment of axons (high density
  of voltage-regulated Na channels)
• all-or-none (think finger on a guns 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)

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Action Potentials Begin at the Initial Segment
Figure from Holes Human AP, 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
Na channels.
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Threshold and Action Potential
Figure from Holes Human AP, 12th edition, 2010
Nerve impulse
Repolarization
What causes depolarization?
Influx of Na
Efflux of K
Depolarization
What causes repolarization?
Threshold
Steps in Action Potential 1) depolarization
2) repolarization 3) hyperpolarization 4)
return to resting potential
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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?
Figure from Holes Human AP, 12th edition, 2010
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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 Holes Human AP, 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
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The Way Graded Potentials Work
0 mV
LARGE Graded potential has caused enough membrane
depolarization to generate an action potential!
Na-K-ATPase pumps kick in to restore -70mV
Threshold
Na-K-ATPase pumps kick in to restore -70mV
Larger Graded potential
-70 mV
Small Graded potential
Influx of Na from graded potential is so large
and so fast that Na-K-ATPase pumps are
overwhelmed and cannot restore -70mV resting
potential
More sustained opening of ligand gated Na
channel
Brief opening of ligand gated Na channel
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Refractory Period
Figure from Holes Human AP, 12th edition, 2010
Great summary graphic to know for the exam! (Who
knows, you might even have to label a couple of
things on this diagram. ?)
ARP Absolute Refractory Period
RRP Relative Refractory Period
Influx of Na (Depolarization)
Efflux of K (Repolarization)
Threshold
ARP
RRP
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Action Potential Membrane Changes
Figure from Holes Human AP, 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
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Action Potentials
Figure from Holes Human AP, 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?
Absolute refractory period of the previously
depolarized segment.
Figure from Saladin, Anatomy Physiology,
McGraw Hill, 2007
Action Potential
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Myelination of Axons
Figure from Holes Human AP, 12th edition, 2010

• White Matter
• contains myelinated axons
• Gray Matter (CNS)
• contains unmyelinated structures
• cell bodies, dendrites

In CNS, myelin is produced by ?
Smaller axons in PNS
Oligodendrocytes
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Saltatory (Leaping) Conduction
Figure from Holes Human AP, 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.
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Regeneration of A Nerve Axon
Figure from Holes Human AP, 12th edition, 2010
Growth 3-4 mm/day
Damage to cell body usually cannot be repaired
neurons lack centrioles and cannot divide
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Chemical Synaptic Transmission
You should understand this process
Neurotransmitters (ntx) are released when impulse
reaches synaptic knob 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 doesnt continue
indefinitely?
Figure from Holes Human AP, 12th edition, 2010
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Chemical Synaptic Transmission
Neurotransmitters (ntx) are released when impulse
reaches synaptic knob 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 doesnt continue
indefinitely?
You should understand this process and be able to
diagram it.
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Neurotransmitters
Table from Holes Human AP, 12th edition, 2010


Neuromodulators Influence release of ntx or the
postsynaptic response to a ntx, e.g., endorphins,
enkephalins
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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
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Summation of EPSPs and IPSPs
Figure from Holes Human AP, 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

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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

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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)

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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

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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!)

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Review

• Communication between nerves and/or effectors
  takes place at the synapse
• EPSP and IPSP
• Neurotransmitters mediate synaptic transmission
  (Acetylcholine, Norepinephrine)
• Convergence and divergence