|
|
|
|
|
1) Neuron Anatomy
Neurons are the structural units of nervous
tissue. They are highly specialized to transmit nerve impulses
from one part of the body to another. Although neurons differ
structurally, they have many identifiable features in common.
All have a cell body from which slender processes extend.
Although neuron cell bodies are typically found in the brain and
spinal cord in clusters called nuclei, occasionally they reside
in ganglia (collections of neuron cell bodies outside the CNS).
They make up the gray matter of the nervous system. Neuron
processes running through the CNS form tracts of white matter;
outside the CNS they form the peripheral nerves.
The neuron cell body contains a large round nucleus surrounded
by cytoplasm. The cytoplasm is riddled with darkly staining
structures called Nissl bodies. Nissl bodies, a type of rough
endoplasmic reticulum, are involved in the metabolic activities
of the cell. |
|
|
FIGURE 1.1 Neuron |
|
According to the traditional scheme, neuron processes
that conduct electrical currents
toward
the cell body are called dendrites,
and those that carry impulses
away from
the nerve cell body are called axons.
Neurons have only one axon but may have many dendrites,
depending on the neuron type.
|
FIGURE 1.2
Neuron Anatomy |
In general, a neuron is excited by other
neurons when their axons release neurotransmitters close to its
dendrites or cell body. The electrical current generated travels
across the cell body and down the axon. The axon begins at the
cell body and ends in many small structures called synaptic
knobs. These knobs store the neurotransmitter chemical in tiny
vesicles. Each synaptic knob is separated from the cell body or
dendrites of the next neuron by a tiny gap called the synaptic
cleft. Thus, although they are close, there is no actual
physical contact between neurons. When an impulse reaches the
synaptic knobs, some of the synaptic vesicles rupture and
release neurotransmitter into the synaptic cleft. The
neurotransmitter then diffuses across the synaptic cleft to bind
to membrane receptors on the next neuron, initiating the nerve
impulse.
|
Many drugs can
influence the transmission of impulses at synapses.
Some, like caffeine, are stimulants which decrease the
receptor neuron’s threshold and make it more irritable.
Others block transmission by binding competitively with
the receptor sites or by interfering with the release of
neurotransmitter by the synaptic knob. As might be
anticipated, some of these drugs are used as painkillers
or tranquilizers.
Most long nerve
fibers are covered with a fatty material called myelin,
and such fibers are referred to as myelinated fibers.
Axons in the peripheral nervous system are typically
heavily myelinated by special cells called Schwann
cells, which wrap
themselves tightly around the axon. This wrapping is the
myelin sheath. The peripheral part of the Schwann cell
and its plasma membrane is referred to as the
neurilemma. The myelin sheath is formed by many
individual Schwann cells; thus gaps in the sheath are
formed called nodes of Ranvier.
Within the brain and
spinal cord, myelination is accomplished by glial cells
called oligodendrocytes. These glial cell sheaths do not
exhibit the neurlolemma seen in fibers myelinated by
Schwann cells. Myelin insulates the fibers and greatly
increases the speed of neurotransmission by neuron
fibers. |
FIGURE 1.3 Schwann Cells
|
|
**Using
the images given and your text, sketch
a portion of a myelinated nerve
fiber, illustrating two or three nodes of Ranvier.
Label the
Axon,
Myelin sheath,
Nodes of Ranvier, Neurilemma
|
FIGURE 1.5
Cross section of Axon
|
|
FIGURE 1.4
Cross section of Axon
|
2) Neuroglia Types
Use Figures 1.6, 1.7, 1.8 and
1.9 in
performing the sketches listed below
** Sketch
the following neuroglia types and indicate their functions
within the CNS: Ependymal
Cells, Oligodendrocytes, Astrocytes, Microglia
 |
 |
|
FIGURE 1.6
Microglial Cell |
FIGURE 1.7
Astrocyte |
|
|
|
 |
 |
|
FIGURE 1.8
Oligodendrocyte |
FIGURE 1.9
Ependymal cells
|
|
|
|
3) Spinal Cord Smear Figure 1.10 is a prepared slide of
teased nerve tissue, which has large, easily identifiable
neurons. The neuron represented here was photographed under high
power(400X).
** Sketch
the neuron cell and label the following:
Cell
Body, Nucleus, Nissl Bodies, Dendrites, Axon
|
 |
| |
FIGURE 1.10
Neuron
|
|
4) Neuron
Classification
Neurons may be classified on the
basis of structure or of function. Figures 1.11 and 1.12 show both schemes.
**Make
a sketch of the three types of Neurons depicted in
Figure 1.11
and label the following:
Multipolar,
Bipolar, Unipolar
|
FIGURE 1.11
Neuron classification |
Basis of Structure
Structurally, neurons may be
defined according to the number of processes
attached to the cell body. In unipolar neurons,
one short process extends from the cell body.
Functionally, only the most distal portions of
the peripheral process act as dendrites; the
rest acts as an axon along with the central
process. Most neurons that conduct impulses
toward the central nervous system are unipolar.
Bipolar neurons
have two processes (one axon and one dendrite)
attached to the cell body. This neuron type is
quite rare, typically found only as part of the
eye, ear, and olfactory mucosa.
Many processes
issue from the cell body of multipolar neurons,
all classified as dendrites except for a single
axon. Most neurons in the brain and spinal cord
and those, whose axons carry impulses away form
the CNS fall into this multipolar category.
|
|
FIGURE 1.12
Neuron Functions
|
Basis of Function
Neurons carrying impulses
from the sensory receptors in the internal
organs or in the skin are termed sensory, or
afferent, neurons. The dendritic endings of
sensory neurons are equipped with specialized
receptors that are stimulated by specific
changes in their immediate environment. The cell
bodies of sensory neurons are found in ganglion
outside the CNS, and these neurons are typically
unipolar. Neurons carrying activating impulses
from the CNS to the viscera or body muscles and
glands are termed motor, or efferent, neurons.
Motor neurons are for the most part multipolar.
Their cell bodies are always located within the
CNS and they are multipolar neurons
structurally. |
|
|
|
5) Structure of a Nerve
A nerve is a bundle of neuron
fibers wrapped in connective tissue coverings
and that extends to and from the CNS and
visceral organs or structures such as skeletal
muscles, glands, and skin.
Within a nerve, each fiber is
surrounded by a connective tissue sheath called
an endoneurium, which insulates it from the
other neurons adjacent to it. Groups of fibers
are bound by a coarser connective tissue, called
the perineurium, which form bundles of fibers
called fascicles. Finally, all the fascicles are
bound together by a tough, fibrous connective
tissue sheath called the epineurium. The entire
bundle forms the cord-like nerve.
Blood vessels and
lymphatic vessels serving the fibers also travel
within a nerve.
Nerves are classified
according to the direction in which they
transmit impulses. Nerves carrying both sensory
(afferent) and motor (efferent) fibers are
called mixed nerves. All spinal nerves are mixed
nerves. Nerves that carry only sensory processes
are referred to as sensory, or afferent, nerves.
Some of the cranial nerves are pure sensory
nerves, but the majority are mixed nerves. The
ventral roots of the spinal cord, which carry
motor fibers, are considered motor or efferent
nerves.
A prepared cross section of a peripheral nerve is shown below in
Figure 1.14 and Figure 1.15. Use these figures
to sketch the image asked for below.
|
FIGURE 1.13
Nerve Anatomy
|
|
**Sketch and identify the following
nerve structures:
nerve
fibers, myelin sheaths, fascicles, endoneurium, perineurium,
epineurium |
FIGURE 1.15
Nerve XS sketch
|
|
FIGURE 1.14
Nerve XS Microscopic Image
|
|
|
6) Neuron
Anatomy
Quiz
|
**Identify the
numbered parts of the neuron
given below. List your answers and include with your lab report.
Item #8 is asking what moves in the direction of the
arrow?
|
FIGURE 1.16
Neuron anatomy
|
|
7) Neuron Function
**Answer the following
questions (at the bottom of the page) about nerve impulses.
This site has an animation of a
nerve impulse through a nerve cell.
http://normandy.sandhills.cc.nc.us/psy150/neuron.html
|
 
FIGURE 1.17
Nerve impulse animation
|
The diagram seen here
represents an axon terminal on the left and a dendritic process
on the right separated by a synaptic cleft. When an impulse
reaches the end of an axon, it triggers the formation of
synaptic vesicles at that terminal. Synaptic vesicles are
specialized vacuoles that contain neurotransmitters such as
acetylcholine. The vesicles transport the neurotransmitters to
the end of the axon and release them into the synaptic cleft.
These neurotransmitters attach to receptor sites on the cell
membrane of the receiving neuron. When enough receptor sites are
filled, the firing threshold of the receiving neuron is reached
and a depolarization event is triggered.
Before the neuron depolarizes, it
is held steady in its resting potential. This potential, which
is achieved by maintaining a relatively high concentration of
sodium ions outside of the cell membrane, represents an
approximately -70 milli-volts discrepancy between the negatively
charged interior and positively charged exterior. As
neurotransmitters attach to the receptor sites and overcome the
firing threshold, small molecular gates open along the cell
membrane allowing the sodium ions to rapidly flood the neuron.
This sudden change in polarity from the influx of positive ions
triggers an action potential that moves like a wave down the
axon triggering another nerve, muscle cell, etc.
At this point, a different series
of molecular gates open which allows potassium ions to rush out
of the neuron. The potassium ions, which have a positive charge
as well, create a negatively charged cell interior by their
absence. This event stops the depolarization process. The sodium
ions are pumped more slowly to the cell exterior by active
transport, resulting in the fully restored resting potential
once again.
Neuron Function Questions
1) Describe some differences
between the axon and dendrite of a cell.
2) Where are the synaptic vesicles formed?
3) What is the name of the space between two neurons?
4) What is the collective name of the chemicals stored in the
vesicles?
5) Where is the highest concentration of Na+ ions, inside the
axon or outside?
6) How many milli-volts is a normal resting potential?
|
|
END
OF LAB 1 |
|
|
|
|
|
|
|
|
|
|
|
|
|
Page 3 |
|
|
|