THE NERVOUS
SYSTEM: NEURAL
TISSUE
Cells in Nervous Tissue
• Neurons
• Neuroglia
Neuroglia (Glia)
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about half the volume of cells in the CNS
smaller than neurons
5 to 50 times more numerous
do NOT generate electrical impulses
divide by mitosis
2 types in PNS
– Schwann cells
– Satellite cells
• 4 types in the CNS
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Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
Astrocytes
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Largest of glial cells
Star shaped with many processes projecting from the cell body
Help form and maintain blood-brain barrier
Provide structural support for neurons
Regulate the chemical/ion environment for generation of nerve impulse
Regulate nutrient & ion concentrations for neuron survival
Take up excess neurotransmitters
Oligodendrocytes
• Most common glial cell
type
• Each forms myelin
sheath around the axons
of neurons in CNS
• Analogous to Schwann
cells of PNS
• Form a supportive
network around CNS
neurons
Microglia
• few processes
• derived from mesodermal cells
that also give rise to monocytes
and macrophages
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Small cells found near blood vessels
Phagocytic role - clear away dead cells
protect CNS from disease through phagocytosis of microbes
migrate to areas of injury where they clear away debris of
injured cells - may also kill healthy cells
Ependymal Cells
• epithelial cells arranged in a
single layer
• range in shape from cuboidal
to columnar
• line ventricles of the brain & central canal of spinal cord
• produce & circulate the cerebrospinal fluid (CSF)
• CSF = colorless liquid that protects the brain and SC against
chemical & physical injuries, carries oxygen, glucose and other
necessary chemicals from the blood to neurons and neuroglia
Cells of the CNS
PNS: Satellite Cells
• Flat cells surrounding PNS neuronal bodies
• hold the cell bodies together to form a ganglion
PNS: Schwann Cells
• produces part of the myelin sheath surrounding an axon in the
PNS
• contributes regeneration of PNS axons
Cells of the PNS
Neurons
•have the property of electrical excitability - ability to produce
action potentials or nerve impulses in response to stimuli
Representative Neuron
http://www.horton.ednet.ns.ca/sta
ff/selig/Activities/nervous/na1.ht
m
1. cell body or soma
-same components of a typical eukaryotic cell
-e.g. nucleus, Golgi, mitochondria
-Nissl bodies -rough ER & ribosomes for protein synthesis
-cytoskeleton of neurofilaments and microtubules to give neuron it’s
shape and to move neurotransmitters to the terminals
Neurons
2. Cell processes =
dendrites (little trees)
- the receiving or input
portion of the neuron
-short, tapering and
highly branched
-surfaces specialized
for contact with other
neurons
3. Cell processes = axon
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conducts nerve impulses away from cell
body to another neuron
joins the cell body at a cone-shaped
elevation = axon hillock
nerve impulse arises at a region of the
axon hillock = trigger zone
cytoplasm = axoplasm
plasma membrane = axolemma
side branches = collaterals arise from the
axon
axon and collaterals end in fine processes
called axon terminals
swollen tips called synaptic end bulbs
contain vesicles filled with
neurotransmitters
Classification of Neurons
• neurons can be classified based on:
– their shape – e.g. multipolar, bipolar, unipolar
– who identified them – e.g. Purkinje
– function
• Sensory (afferent) neurons
– transport sensory information from skin, muscles, joints, sense organs &
viscera to CNS
• Motor (efferent) neurons
– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons
– connect sensory to motor neurons
– 90% of neurons in the CNS
The Nerve Impulse: Terms to know
• membrane potential = electrical voltage difference measured across the
membrane of a cell
– results from the build-up of negative ions in the cytosol along the inside of the neuron’s
PM
– the outside of the PM becomes more positive
– this difference in charge can be measured as potential energy – measured in millivolts
• resting membrane potential = membrane potential of a neuron measured when it
is unstimulated
– ranges from -40 to -90 mV
The Nerve Impulse: Terms to know
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polarization – change in membrane potential
1. depolarization – increase in MP away from resting
2. hyperpolarization – decrease in MP away from resting
3. repolarization – “return to resting membrane potential”
Ion Channels
• ion channels in the PM of neurons and muscles contributes to their
excitability
• when open - ions diffuse down their concentration gradients
• some ion channels are permanently open – non-gated channels
• some ion channels possess gates to open and close them – gated
channels
• two types: ligand gated & voltage gated
Ion Channels
1. Leakage (non-gated) or Resting channels: are always open, contribute to the resting
potential
-nerve cells have more K+ than Na+ leakage channels
-so K+ leak channels contribute more to resting membrane potential than Na+ leak
channels
-leaking ions are pumped back to where they belong
2. Gated channels: open and close in response to a stimulus
A. voltage-gated: open in response to change in voltage - participate in the AP
B. ligand-gated: open & close in response to particular chemical stimuli
(hormone, neurotransmitter, ion)
C. mechanically-gated: open with mechanical stimulation
Action Potential
• Resting membrane potential
is -70mV
• AP triggered when the
membrane potential reaches
a threshold usually -55 MV
• if the membrane potential
exceeds that of threshold
 Action Potential
• action potential = nerve impulse
• takes place in two stages: depolarizing phase (more positive) and repolarizing
phase (more negative - back toward resting potential)
• followed by a hyperpolarizing phase or refractory period in which no new AP
http://www.blackwellpublish
can be generated
ing.com/matthews/channel.
html
Action Potential
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1. neuron is at resting membrane
potential (resting MP)
2. neuron binds neurotransmitters via
ligand-gated sodium channels
3. channels open & Na diffuses into
neuron = depolarization
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4. if neuron depolarizes enough &
reaches Threshold  Action
Potential (AP)
5. 1st stage of AP – opening of voltagegated Na channels
large diffusion of Na+ ions into neuron
= BIG depolarization
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membrane potential goes from negative
to positive
6. closing of VGNa channels & opening of voltage-gated K channels
7. BIG outflow of potassium through VGK channels = repolarization
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inside of neuron (i.e. MP) becomes
more positive
inside of neuron (MP) becomes more negative
8. closing of VGK channels BUT so much K+ has diffused out – neuron’s MP goes past resting and
hyperpolarizes
9. neuron is hyperpolarized – no new AP can be generated with a normal stimulus
10. all voltage-gated channels closed, Na/K pump “resets” ion distribution to resting situation
Continuous versus Saltatory Conduction
• Continuous conduction
(unmyelinated fibers)
– action potential spreads continuously
over the surface of the axolemma
– as one section of the axon is
depolarized, the membrane potential
of the next section is depolarized
toward threshold
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter45/animations.html#
Saltatory Conduction
• Saltatory conduction
-depolarization happens only at Nodes
of Ranvier - areas along the axon that
are unmyelinated and where there is a
high density of voltage-gated ion
channels
-action potential “jumps” from node
to node
http://www.blackwellpublishing.com/matthews/actionp.html
Synapses
• Synapse: Site of intercellular communication between 2 neurons or between a
neuron and an effector (e.g. muscle – neuromuscular junction)
• Permits communication between neurons and other cells
– Initiating neuron = presynaptic neuron
– Receiving neuron = postsynaptic neuron
• You can classify a synapse according to:
– 1. the action they produce on the post-synaptic neuron – excitatory or
inhibitory
– 2. the mode of communication – chemical vs. electrical
Synapses
• Electrical
– Direct physical contact between cells required
– Conducted through gap junctions
– Two advantages over chemical synapses
• 1. faster communication – almost instantaneous
• 2. synchronization between neurons or muscle fibers
– e.g. heart beat
Chemical Synapse
• Most common type of synapse
– Membranes of pre and
postsynaptic neurons do not touch
– Space = Synaptic cleft
• Most are axon terminal  dendrite
• Some are axon terminal  axon
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect3.dcr
Chemical Synapse
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the AP cannot travel across the cleft – release of neurotransmitters
1. arrival of action potential in the synaptic end bulb
2. opening of voltage-gated calcium channels – influx of Ca2+ into end bulb
3. docking of synaptic vesicles with NTs with plasma membrane – release of NTs into
synaptic cleft
4. binding of NT to ligand-gated channels – channels open
5. diffusion of Na+ ions into post-synaptic membrane
6. depolarization of post-synaptic neuron – if the NT is excitatory
7. depolarization to threshold  Action Potential
• if the neurotransmitter is an inhibitory
NT - then the post-synaptic neuron
will hyperpolarize rather than
depolarize
• NO ACTION POTENTIAL!!!
http://www.blackwellpublishing.com/matthews/nmj.html
The Neuromuscular Junction
• the motor neuron’s synaptic terminal is
in very close association with the muscle
fiber
• distance between the bulb and the folded
sarcolemma = neuromuscular junction
•neurotransmitter released = acetylcholine
https://www.youtube.co
m/watch?v=7wM5_aUn2
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