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UNIT 3 The Human Body: Control and integration
MODULE 11
THE NERVOUS SYSTEM: NERVOUS TISSUE
AND NEUROPHYSIOLOGY
_________________________________________________________________________________
11.1 Function and organisation of the nervous system
11.2 Histology of nervous tissue
11.3 Neurophysiology - membrane potentials
11.4 Neurophysiology - the synapse
11.5 Neurophysiology - post synaptic events
11.6 Basic concepts of neural integration
11.7 Review exercises
The nervous and endocrine systems are the bodys major regulating and
coordinating systems.
These systems enable the body to function effectively and respond to
changes in the internal and
external environments, i.e. as discussed in Module 1, they regulate and
coordinate other body systems
in order to maintain homeostasis. Of course the nervous system is
responsible for more than just
maintaining homeostasis. The brain is also the centre for many other
complex functions such as
consciousness, memory, thinking, personality, language, behaviour and
emotions.
The next six modules investigate various aspects of the nervous system
and the endocrine
system. We will begin with a discussion of the nervous system. In this
first module you will
examine the organisation of the nervous system and the structure and
function of the cells that
comprise nervous tissue. In later modules you will examine different
parts of the nervous
system in more detail.
I hope you enjoy studying the fascinating but complex human nervous
system.
11.1 Function and organisation of the
nervous system
Time for completion about 1 hour
Objectives
After completing this section of the module you should be able to :
- State the basic functions of the nervous system;
- Explain the way the nervous system is organised into divisions;
Reading
pp. 362-364 of your textbook
Learning activities
 Figure 11.1 in your textbook has a table
explaining the organisation of the nervous system.
When studying this information ensure that you look at the interaction
between the levels of the
nervous system. You will find it very useful in future modules to have a
clear understanding of this
organisation and the flow of information through the various levels of
the nervous system.
1. Match the phrase in the column on the left with the
correct description on the column
on the right.
| Sensory division |
Regulates cardiac and smooth muscle and
glands under resting conditions |
| Motor division |
Consists of nervous tissue outside of the
brain and spinal cord |
| Central nervous system |
Conducts impulses from the central
nervous system to the rest of the body |
| Autonomic nervous system |
"Flight or fight" response |
| Sympathetic nervous system |
Controls cardiac muscle, glands and
smooth muscle under all conditions |
| Parasympathetic nervous system
|
Consists of the brain and spinal cord
|
| Peripheral nervous system |
Conducts impulses from central nervous
system to skeletal muscle |
| Somatic nervous system |
Conducts impulses from sensory receptors
to the central nervous system |
11.2 Histology of nervous tissue
Time for completion about 2 hours
Objectives
After completing this section of the module you should be able to :
- Recognise that nervous tissue consists of neurons and supporting cells
and that each
of these has a different function;
- State the name, location, structure and function of the six
types of supporting cells
(neuroglia) in nervous tissue;
- Identify the major features of a neuron as the cell body,
axon, synaptic knob and
dendrites and discuss the function of each;
- Explain the role of the myelin sheath and discuss how it is
formed in the central and
peripheral nervous systems;
- Classify neurons both functionally and structurally;
- Define a nerve, a nerve tract, a nucleus and a ganglion;
- Discuss the structure and function of the three groups of
neurons defined by the
functional classification system;
Reading
pp. 364-372 of your textbook
Learning activities
1 Draw a sketch of a typical myelinated motor neuron and
label the cell body, axon,
axon hillock, synaptic knob, axolemma, myelin sheath, nodes of
Ranvier, dendrites and
Schwann cell. State the function of each of these structures.
Also label the direction of
travel of the nerve impulse
2 Draw a sketch of a typical
unmyelinated sensory neuron and label the cell body,
axon, synaptic knob, axolemma and dendrites.
State the function of each of these
structures. Label the direction of travel of
the nerve impulse.
- Are motor neurons examples of multipolar, bipolar or unipolar
neurons?
4 Are sensory neurons usually examples of multipolar, bipolar or
unipolar neurons?
5. What type of molecules are moved in the anterograde direction
in the neuron?
- What type of molecules are moved in the retrograde direction in the
neuron?
7. What is the function of the myelin
sheath surrounding neurons?
| The importance of myelin to normal
conduction of impulses along a neuron can be illustrated by a disease
such as multiple sclerosis. Multiple sclerosis is one of many diseases
that result in a loss of the myelin from the neurons in the central
nervous system. In multiple sclerosis (MS), nerve impulses traveling
through the central nervous system suddenly meet a disruption in their
pathway. Some impulses make it to their destinations; others don't.
Multiple sclerosis involves repeated episodes of inflammation of
nervous tissue in any area of the central nervous system (brain and
spinal cord). The location of the inflammation varies from person to
person and from episode to episode. The inflammation destroys the
covering of the nerve cells in that area (myelin sheath). This leaves
multiple areas of scar tissue (sclerosis) along the covering of the
nerve cells.
The resulting symptoms such as muscular paralysis
and loss of sensation attest to the functional significance of myelin.
Symptoms vary because the location and extent of each attack varies.
There is usually a stepwise progression of the disorder, with episodes
that last days, weeks, or months alternating with times of reduced or
no symptoms (remission). Recurrence (relapse) is common. The exact
cause of the inflammation associated with multiple sclerosis is
unknown. Geographic studies indicate there may be an environmental
factor. Multiple sclerosis has a higher incidence in northern Europe,
northern United States, southern Australia, and New Zealand than in
other areas of the world. There seems to be a familial tendency toward
the disorder, with higher incidence in certain family groups than in
the general population. An increase in the number of immune cells in
the body of a person with MS indicates that there may be a type of
immune response that triggers the disorder. The most frequent theories
about the cause of multiple sclerosis include a virus-type organism,
an abnormality of the genes responsible for control of the immune
system, or a combination of both factors.
Currently there is no treatment that will alter the
demyelination process, but much can be done to manage the symptoms and
the acute exacerbations that occur in multiple sclerosis |
| At this point you can reflect on what
you have learned about cells in nervous tissue.
There are two types of cells in nervous tissue -
neuroglial cells and neurons. The six different types of
neuroglial cells all have different structures enabling them to perform
specific functions but essentially they are all known as support cells.
The neurons or nerve cells are the cells that are able to conduct
messages. Neurons have different shapes but all have a cell body,
dendrites and an axon with a synaptic knob. Many
neurons have a myelin sheath around the axon formed by a
Schwann cell in the peripheral nervous system or an
oligodendrocyte in the central nervous system. The myelin sheath
increases the rate at which a message is transmitted along an axon.
Neurons can be classified according to their shape or
according to their function. You will probably find for your purposes
that the functional classification is most useful. The three groups in
this classification system are the sensory, motor and
association neurons. The sensory neurons transmit information from
sensory receptors in the periphery (in the skin, in visceral organs,
joints etc.) to the central nervous system. Motor neurons transmit
information from the central nervous system to skeletal muscles in the
periphery. Association neurons are found in the central nervous system
and conduct impulses within the central nervous system. |
 Have a break before continuing
11.3
Neurophysiology - membrane potentials
Time for completion about 2 hours
Objectives
After completing this section of the module you should be able to :
- Discuss how the resting membrane potential is maintained across a cell
membrane;
- State the meaning of the terms: depolarization, repolarization and
hyperpolarization;
- Explain where and how a graded potential is produced;
- State the function of an action potential;
- Explain where and how action potentials are generated and propagated
along unmyelinated neurons;
- Explain how action potentials are propagated along myelinated neurons;
- Explain the effect of axon diameter and the myelin sheath on the speed
of conduction of an action potential;
Reading
pp. 373-383 of your textbook
Learning activities
| Are you finding this material difficult
to understand? If you are, do not despair. In my experience when
introduced to neurophysiology many students sweat blood and tears as
they grapple with the new concepts. But they do make it in the end. I
think the first step is to create a framework in which to fit all the
information. And as usual you need to be sure that you do not lose
yourself in all the detail.
Very broadly you need to understand that neurons like
all cells have a resting membrane potential i.e. the fluid inside of the
membrane has a negative charge compared to the fluid
outside of the membrane. This charge difference across
the membrane of a neuron can be reversed in order to create a signal in
the neuron. There are two types of signals possible: a graded potential
and an action potential. Each has a different function. You will examine
the function of graded potentials in a later section.
Action potentials are the mechanism by which signals
are transmitted along the axon of a neuron. The rate and method of
transmission depends on whether or not the neuron is myelinated. In
later sections of the module you will discover what causes an action
potential to be generated initially at the axon hillock and how the
action potential or signal that is transmitted along the axon is passed
onto another neuron. Keep this broad picture in mind has you fill in
some detail. |
1. Match the terms in the left hand column with the descriptions in the
right hand column.
| Sodium ions |
Period of repolarisation of the neuron
during which it cannot respond to a second stimulus |
| Depolarisation |
Highest concentration of this ion is
found outside the resting cell |
| Repolarisation |
Is equal to about -70mV |
| Refractory period |
The way that neurons transmit messages
over a long distance |
| Potassium ions |
The membrane potential of the neuron
becomes more negative |
| Action potential |
Period during which sodium ions move into
the cell |
| Hyperpolarisation |
Highest concentration of this ion is
found inside the cell |
| Resting membrane potentia |
Period during which potassium ions
diffuse out of the cell |
- Where are axon potentials generated in a motor neuron?
in a sensory neuron?
.
- What is the difference between a graded potential and an action
potential?
- Draw yourself a sketch of an action potential and label the resting
membrane
potential, the threshold point, the
depolarisation phase, the repolarisation phase and
the refractory period (absolute plus relative). Also show
the period during which sodium
gates are open and the period during which potassium gates are
opened.
Ask yourself if you really understand the meaning and significance of
these terms.
5 In your own words explain the meaning of the following
phrases?
Threshold of an action potential
All-or-none phenomenon as applied to an action potential
Refractory period
6 Why is it important that an action potential is only generated
when threshold is reached?
7 Why is it important that there is a refractory period during an
action potential?
8 Briefly, what is the essential difference between conduction of
action potentials in
unmyelinated and myelinated neurons?
What is the functional significance of this difference?
.
9 Name the two factors that will increase the speed of
conduction of an action potential
in a neuron.
.
| Hypernatremia is
a condition in which the sodium concentration in the extracellular
(i.e. fluid outside the body cells) fluid increases. This causes the
extracellular fluid to become hypertonic. Now think back to
Module 2 on the cell. What will happen to the cells if they are
surrounded by hypertonic fluid rather than the normal isotonic
fluid?
If you said that an osmotic gradient would be
created across the plasma membrane such that fluid would move out of
the cell by osmosis you would be correct. Congratulations. Remember
fluid moves from an area of low solute concentration (high water
concentration) to high solute concentration (low water
concentration). The cells in the body become dehydrated. In the
brain this causes neurologic symptoms such as intense thirst,
fatigue, weakness, lethargy, restlessness, agitation, seizures and
if severe, eventually coma. Some of these symptoms are due to the
high sodium in the extracellular fluid changing the concentration
difference across the cell membrane making it easier to generate an
action potential. Consequently the neurons are more easily excited
and are said to be irritable. |
Have a break before continuing
11.4 Neurophysiology - the synapse
Time for completion about 2 hours
Objectives
After completing this section of the module you should be able to :
- Describe the important features of a chemical synapse;
- Explain the events that occur during the transmission of information
across a chemical synapse;
- State the function of neurotransmitters and recognise that there are a
number of different neurotransmitters and different chemical classes of
neurotransmitters;
- Explain that neurotransmitters can either be excitatory or inhibitory
in function;
Reading
- 383-386, 389-395 of your textbook
Learning activities
The concepts related to chemical
synapses should be familiar to you as you came across
them in the previous module on muscle tissue. Of course at the
neuromuscular junction there is
a neuron and a muscle fibre whereas in the nervous system the synapse is
between a
presynaptic neuron and a postsynaptic neuron. However the principle steps
involved in
synaptic transmission are very similar.
Please note from the objectives that you do not need to know a great deal
of detail about
the chemistry of neurotransmitters.
- Draw a sketch of a chemical neural synapse and label the axon of
the presynaptic
neuron, the synaptic vesicles, the synaptic cleft,
the membrane of the postsynaptic neuron and the
receptor for the neurotransmitter
2. List the steps or draw a diagram or flowchart to summarise
the steps that occur
as information is transferred across a chemical synapse by a
neurotransmitter.
3. Explain why the effects of neurotransmitter binding are very brief.
| Many drugs exert their
effects through their action at the synapse. An example is Prozac,
an antidepressant drug that is a very popular drug at present.
It blocks the reuptake of serotonin by the
presynaptic terminal at synapses in the central nervous system.
What effect would this have on the levels of serotonin in the
synapse? Can you see how this treatment fits in with the theory
that depression results from a decreased level of serotonin in the
brain? |
Have a break before continuing
11.5
Neurophysiology - post synaptic events
Have a break before continuing
11.5 Neurophysiology - post synaptic events
Time for completion about 1 hour
Objectives
After completing this section of the module you should be able to :
- Distinguish between the effect of excitatory and inhibitory
neurotransmitters in terms of the type of postsynaptic potentials they
produce;
- Briefly describe how events at the synapse are integrated and
modified;
Reading
- 386-389 of your textbook
Learning activities
- What is responsible for the generation of IPSPs and EPSPs?
.
- Where are EPSPs and IPSPs generated?
..
- In terms of the generation of action potentials in postsynaptic
neurons what is the
difference between EPSPs and an IPSPs?
4 Where are action potentials generated?
.
5 Your answers to questions 2 and 4 should be different. So how do
EPSPs and IPSPs influence the generation of action potentials?
| Keep in mind the essential concept
that emerges from this section of the neurophysiology. Excitatory
and inhibitory neurotransmitters released from pre-synaptic neurons
cause EPSPs and IPSPs in the dendrites of the postsynaptic neuron. This
allows many different neurons to influence the activity of a single
neuron. Whether or not an action potential is generated in the
postsynaptic neuron will depend on the integration of all the IPSPs and
the EPSPs generated in the dendrites. |
11.6 Basic
concepts of neural integration
Time for completion about 1 hour
Objective
After completing this section of the module you should be able to :
- Understand that in the nervous system neurons are organised into
neuronal pools in order to facilitate integration;
- Understand that neuronal pools are functional groups of neurons
organised into neural circuits;
- Briefly describe the four types of neural circuits;
- Briefly distinguish between serial and parallel patterns of neural
processing;
Reading
- 395-399 of your textbook
Learning activities
| Reading the material on neural
integration should give you a feel for how all the neurons in the
nervous tissue can act together to perform the complex tasks required of
our nervous systems. It is very easy when studying action potentials,
EPSPs and IPSPs etc. to loose sight of the complexity of the connections
in the nervous system. So in this section of the module it is much more
important that you recognise how complexity can be achieved through
synapses than remember the details of each type of circuit. Do not
forget that the concepts introduced in the last sections of the module
on the synapse and EPSPs and IPSPs explain the nuts and bolts of how
excitatory and inhibitory neurons function together in neuronal pools
and circuits. |
11.7
Review Exercise Review Exercises
1. The autonomic nervous system is divided into which two divisions?
A. Afferent and efferent
B. Sympathetic and parasympathetic
C. Voluntary and involuntary
D. Central and peripheral
2. The myelin of myelinated neurons of the central nervous system is
produced by:
A. Oligodendrocytes
B. Schwann cells
C. Microglial cells
D. Ependymal cells
3. The cells that form the blood-brain barrier are known as:
A. Oligodendrocytes
B. Astrocytes
C. Microglial cells
D. Ependymal cells
4. The branching fibres on the neuron that are specialised to receive
impulses are
called the:
A. Axons
B. Cell bodies
C. Dendrites
D. Synaptic knobs
5. The neuroglial cells are the cells in the nervous tissue that
conduct impulses.
A. True
B. False
6. How is the resting cell membrane potential of -70mV best
interpreted?
A. The inside of the cell is -70mV
B. The outside of the cell is -70mV
C. The outside of the cell is 70mV more negative than the inside
D. The inside of the cell is 70mV more negative than the outside
7. Which of the following increase the rate at which an action
potential is conducted
along a neuron?
A. Increasing diameter of a neuron
B The presence of a myelin sheath
C Decreasing diameter of the neuron
D. Both A and B are correct
E. Both A and C are correct
8. During repolarisation of a neuron:
A. Potassium ions move into the neuron
B. Potassium ions move out of the neuron
C. Sodium ions move into the neuron
D. Sodium ions move out of the neuron
9. The somatic nervous system innervates skeletal muscle while the
autonomic
system innervates smooth and cardiac muscle tissue.
A. True
B. False
10. When a neuron reaches threshold and an action potential is
generated:
A. Voltage regulated sodium gates open and sodium moves into the cell
B. Voltage regulated potassium gates open and potassium moves into
the cell
C. Voltage regulated sodium gates close
- Voltage regulated potassium gates close
11. When the action potential arrives at the end of an axon it is:
A. Moderately weaker than the initial action potential
B. Greatly weaker than the initial action potential
C. Precisely identical to the initial action potential
D. Moderately greater than the initial action potential
12. A concentration of neuron cell bodies outside the central nervous
system is
referred to as a ganglion.
A. True
B. False
13. An excitatory postsynaptic potential is characterised by:
A. Increased permeability of the postsynaptic membrane to sodium
ions
B. Always producing an action potential in the neuron
C. Not being capable of summation, either spatial or temporal
D Only producing one action
potential in the postsynaptic neuron
14. Correctly order the following events that occur when an action
potential reaches
the synaptic knob at the end of the axon.
- Neurotransmitter diffuses across synaptic cleft.
- Ion channels open in postsynaptic membrane.
- Vesicles fuse with the cell membrane of the presynaptic cell.
- Calcium ions diffuse into the presynaptic terminal.
- Neurotransmitter binds to postsynaptic receptors
A. 3,1,5,2,4
B. 1,5,2,3,4
C. 3,4,1,2,5
D. 4,3,1,5,2
15. Summation occurs when:
A. A neurotransmitter combines with a receptor on a postsynaptic
neuron
B. An IPSP reaches threshold level
C. Subthreshold EPSPs are added together.
D. Subthreshold IPSPs are added together
16. A neuron that transmits information from the central nervous system
to muscle is
called a sensory neuron.
A. True
B. False
17. Synaptic innervation of a number of cells by one neuron is an
example of:
A. Divergence
B. Facilitation
C. Self-excitation
D. Convergence
18. In broad terms explain how an action potential in one neuron can
influence the
development or not of an action potential in the neuron it synapses on.
- Explain in broad terms how the generation of an action potential in a
neuron is dependent
on the activity in neurons that synapse on its dendrites.
20 Explain how neurotransmitters are inactivated.
ANSWERS
1b 2a 3b 4c 5b 6d 7d 8b 9a 10a 11c 12a 13a 14d 15c 16b 17a
18. When an action potential traveling down a neuron reaches the end of
the neuron a neurotransmitter is released into the synaptic gap. The
neurotransmitter diffuses across the synaptic gap and interacts with a
receptor on the postsynaptic neuron. The effect of the neurotransmitter
and hence the neuron it is released from depends on whether the
neurotransmitter is excitatory or inhibitory. Excitatory neurotransmitters
result in the generation of EPSPs in the postsynaptic membrane and hence
will contribute to exciting the postsynaptic neuron. Inhibitory
neurotransmitters result in hyperpolarisation of the postsynaptic membrane
and hence will contribute to inhibiting the firing of the postsynaptic
neuron
19.Many neurons may synapse on the dendrites of a neuron which in this
case will be the postsynaptic neuron. Some of these neurons will release
inhibitory neurotransmitters and some will release excitatory
neurotransmitters. Whether or not an action potential is generated in the
postsynaptic neuron depends on the balance between the excitatory and
inhibitory inputs.
20.The effects of neurotransmitters on the postsynaptic membrane do not
last forever. The action of the neurotransmitter can be terminated in
three ways:
- Enzymes degrade the neurotransmitter
- The neurotransmitter is taken up from the synaptic cleft into the
presynaptic neuron where it can be used again.
- The neurotransmitter diffuses away from the synapse and hence ceases
to effect the postsynaptic membrane.
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