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Chapter 12:

Neural Tissue
An Introduction to Neural Tissue
The nervous system includes all the neural tissue in the body
...
They perform all
the communication, information processing, and control functions of the nervous system
...
They separate and
protect the neurons
...


12-1
The nervous system has anatomical and functional divisions
We can look at the nervous system from anatomical and functional perspectives
...


Central Nervous System (CNS): consists of the brain and spinal cord
...


Functions of the CNS:
Integrating:
processing:
coordinating sensory data and motor commands:

Peripheral Nervous System (PNS): all the neural tissue outside the CNS
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Chapter 12:

Nerves (peripheral nerves)) bundles, with associated blood vessels and connective tissues
...

Spinal Nerves) nerves that attach to the spinal cord
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Afferent division) (to bring in) brings sensory info to the CNS from receptors in peripheral
tissues and organs
...


Efferent division) (to bring out) carries motor commands from the CNS to muscles, glands, and
adipose tissue
...


That has both somatic and autonomic components:
•

Somatic Nervous System: controls skeletal muscle contractions
...

o The ANS includes the sympathetic division and a parasympathetic division, which
commonly have antagonistic (opposite) effects
...
Multipolar neurons are the most common type in the CNS
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Chapter 12:

The Cell Body
Cell body) or soma, contains a large, round nucleus with a prominent nucleolus
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The cytoskeleton contains neurofilaments
and neurotubules, which are similar to intermediate filaments and microtubules of other types of cells
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Dendrites and Axons
Dendrites) slender, sensitive processes, that extend out from the cell body
...
Dendritic spines are the finger-like studs at the ends of dendrites
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Axoplasm) or cytoplasm of the axon, contains neurofibrils, neuraltubules, small vesicles,
lysosomes, mitochondria, and various enzymes
...
In the CNS
it may be exposed to the interstitial fluid or, as we’ll see, it may be covered by the
cellular processes of neuroglia
...
Of the axon in a multipolar neuron, that joins the cell body to
a…

Axon hillock) thickened region joined by the interstitial fluid
...
They enable a single neuron to communicate
with several other cells
...
They in term end at the
axon terminals
...

Every synapse involves 2 cells:
1
...
Postsynaptic cell: which receives the message

The synaptic cleft separates the two cells

Neurotransmitters) chemicals released into the synaptic cleft
...


Neuromuscular junction) a synapse between a neuron and a muscle cell
Neuroglandular junction) where a neuron controls or regulates the activity of a sectory
(gland) cell
...

Presynaptic membrane) where neurotransmitters are released
Postsynaptic membrane) where receptor for neurotransmitters are
...
They run on ATP

Axoplasmic transport) what the movement of material between the cell body (soma) and the
axon terminal is called
...


Antergrade flow) the flow of materials from the cell body to the axon terminal, carried by kinesin
↑ at the same time ↓

Retrograde flow) other substances are transported from the axon terminal toward the cell body,
carried by dynein
...

❖ Anaxonic: are small and have numerous dendrites, but no axons
...

❖ Bipolar neurons: have 2 distinct processes 1 dendrite that branches exclusively into
dendritic branches at its distal tip, and 1 axon---with cell body between the two
...

They occur in special sense organs
...

❖ Unipolar neurons: the dendrites and axon are continuous—basically, fused---and the cell
body lies of to one side
...
Most sensory
neurons of the PNS are unipolar
...
The longest are from the toe tips to the spinal cord
...
They are most common
neurons in the CNS
...
Length is similar to that of unipolar neurons
...
Sensory neurons (10million or so)
2
...
Interneurons

Sensory neurons)

or afferent neurons form the afferent division of the PNS, deliver info from
sensory receptors to the CNS
...


(Ganglion) a collection of neuron cell bodies in the PNS)
...
Sensory neurons are either processes of specialized sensory
neurons or cells monitored by sensory neurons
...

Sensory receptors are broadly categorized into 3 groups:
•

Interoceptors: monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and
provides sensations of distension (stretch), deep pressure, and pain
...


•

Proprioceptors: monitor the position and movement of skeletal muscles and joints
...
They carry instructions from the CNS to peripheral effectors in a
peripheral tissue, organ, or organ system
...
Remember the efferent systems are the
SNS and the ANS
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We have conscious
control over the SNS
...

We do not have conscious control over the activities of the ANS
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The axons of visceral motor neurons in the CNC innervate a 2 nd
set of visceral neurons in peripheral autonomic ganglia
...


To distinguish between afferent and efferent, think of the SAME principle:

S- sensory
A- afferent
M- motor
E- efferent
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Chapter 12:

Preganglionic fibers) what axons extending to from the CNS to an autonomic ganglion are called
...


Interneurons)

are located between sensory and motor neurons
...

Most are located within the brain and spinal cord, but some are in autonomic ganglia
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They also play a part in the higher functions such as memory, planning, and learning
...

The organization of neural tissue in the CNS differs from that in the PNS, primarily because the
CNS has a greater variety of neuroglial cell types
...
Ependymal cells
2
...
Oligodendrocytes
4
...
This fluid, which circulates
continuously, provides a protective cushion and transports dissolved gases, nutrients, wastes, and other
materials
...

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Chapter 12:

Central canal) the narrow passageway in the spinal cord
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Ependyma) forming of cuboidal to columnar epithelia, from ependymal cells that line the central
canal and ventricles
...

There are 3 types of Ependymal cells:
•

Ependymocytes: have motile cilia that aid in the circulation of CFS and also microvilli
...


•

Tanycytes: specialized, non-ciliated ependymal cells with microvilli on their apical surfaces
...
It is thought that they transport substances between the CSF and the brain
...

Functions of astrocytes:
•

Maintaining the Blood-Brain Barrier>Neural tissue must be physically and biochemically isolated from
the general circulation, because hormones, amino acids, or other chemicals in the blood can alter neuron
function
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•

Creating a 3D Framework for the CNS>Astrocytes are packed with microfilaments that extend across
the breadth of the cell and its processes
...


•

Repairing damaged neural tissue>Stabilize the tissue to prevent further damage

•

Guiding Neuron Development>Growth and interconnection of developing neurons
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Chapter 12:

Oligodendrocytes
Oligodendrocytes) like astrocytes they have slender cytoplasmic extensions, but the bodies are
smaller, with fewer processes
...
Many oligodendrocytes cooperate in forming myelin sheath along the length of an axon
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Internodes) what the fairly large areas of the axon that are wrapped in myelin are called
...


Nodes) the small gaps of a few micrometers that separate adjacent internodes
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White matter) what the glossy white regions of myelinated axons are called, primarily because of
lipids in the myelin
...


Tip
The overall color of the CNS is related to its structure and function
...

White matter has a whole lot of myelinated axons and whisks nerves impulses
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Their slender processes have
many fine branches
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Microglia appear only in embryonic
development, originating from mesodermal stem cells that produce monocytes and macrophages
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Neuroglia of the PNS
Recall that the cell bodies of neurons in the PNS are clustered in masses called ganglia
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Satellite cells: surround neuron cell bodies in ganglia
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Schwann cells: or neurilemma cells, either form a thick, myelin sheath or indent folds of
plasma membrane around the peripheral axons
...


Neuroilemma) what the outer surface of schwann cells that cover neurons are called
...
The key to recovery seems to be events in the axon
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Limited regeneration can occur in the CNS, but the situation is more complicated because (1)
many more axons are likely to be involved, (2) astrocytes produce scar tissue that can inhibit axon
growth across the damaged area, and (3) astrocytes release chemicals that block the regrowth of axons
...
All neural
activities begin with a change in the resting membrane potential of a neuron

•

Graded potential a typical stimulus produces a temporary, localized change in the resting
membrane potential
...
This is an
electrical impulse that is propagated (spread) along the surface of the axon and does not
diminish as it moves away from the source
...


•

Synaptic activity then produces graded potentials in the plasma membrane of the
postsynaptic cell
...


The extracellular fluid (ECF) and intercellular fluid (cytosol) differ
greatly in ionic composition: (ECF) contains high concentrations of

(Na+) and

chloride ions (Cl-), whereas the cytosol contains high concentrations of potassium ions (K+)
and neg charged proteins
...


Cells have selective permeable membranes:

3
...

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Chapter 12:

Passive forces Acting across the Plasma Membrane
The passive forces acting across the plasma membrane involve both chemical and electrical
gradients
...


Current) a movement of oppositely charges ions that eliminates a potential difference
...
If
resistance is high, the current is small, because few ions can cross the membrane
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Electrochemical gradient) can either reinforce or oppose the chemical gradient for each ion
...


Intracellular concentration of potassium ions is high, whereas the extracellular concentration is
very low
...


Tips & Tricks:
To remember the relative distributions of ions across the resting membrane, associate

Negative with iNside and pOsitive with the Outside
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An electrochemical gradient is a form of potential energy
...
This important for establishing normal resting membrane potential
...

Each gated channel can be in 1 of 3 stages:
1
...
open (active)

or

3
...
Voltage-gated channels

Open or close when they bind specific chemicals
...
These
channels are most abundant on the dendrites and
cell bodies of a neuron, the area where where most
synaptic communication occurs
...
They are characteristic of
areas of excitable membrane, a membrane
capable of generating and propagating an action
potential
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Mechanically gated channels

Open or close in response to physical
distortion of the membrane surface
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Chapter 12:

Gated Potentials
Gated Potentials) or local potentials, are changes in the membrane potential that cannot spread
far from the site of stimulation
...

The degree of depolarization decreases with distance away from the stimulation site
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Repolarization) the process of restoring the normal resting membrane potential after
repolarization
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Table 12-2

Graded Potentials

Graded potentials, whether depolarizing or hyperpolarizing, share 4 basic characteristics:
1
...


2
...
Due to local currents
...


The graded change in the membrane potential may involve either depolarization or hyperpolarization
...


4
...


12-5
An action potential is an electrical event
Action potentials) are propagated changes in the membrane potential that, once initiated, affect
an entire excitable membrane
...

The signal travels like a line of falling dominoes
...
The threshold for an axon
is typically between -60mV and -55mV, corresponding to a depolarization of 10 to 15mV
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This is because a
given stimulus either triggers a typical action potential, or none at all
...


Generation of Action Potentials
Steps:
1
...


activation of voltage-gated sodium channels and rapid depolarization

3
...


closing of voltage-gated potassium channels

The Refractory Period
The Refractory Period) the plasma membrane does not respond normally to additional
depolarizing stimuli from the time an action potential begins until the normal resting membrane
potential has stabilized
...
4-1
...
When the membrane cannot respond to
further stimulation from the moment the voltage-gated sodium channels open at threshold until sodium
channel inactivation ends, because all the voltage-gated sodium channels are already open or are
inactivated
...


The Role of the Sodium-Potassium Exchange Pump
In an action potential, depolarization results from the influx of Na+, and repolarization involves
the loss of K+
...


Propagation) the event takes place over and over as it spreads down the line

Continuous Propagation
Continuous propagation) how an action potential moves along in an unmyelinated axon
...

The “wave” moves but the people remain in place
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Chapter 12:

12-6
Axon diameter, in addition to myelin, affects propagation speed
Myelin greatly increase the propagation speed of action potentials
...


Type A fibers: are the largest myelinated axons, diameters range from 4-20, these fibers
carry action potentials at speeds of up to 120 meters per sec, or 268mph
(carries sensory info)

2
...
Their
propagation speeds average around 18 meters per sec, or roughly 40mph (

3
...
These axons propagate
action potentials at the leisurely pace of 1 meter per sec, or a mere 2mph

12-7
At synapses, communication occurs among neurons or between
neurons and other cells
In the nervous system, messages move from one location to another in the form of action
potentials (nerve impulses) along axons
...


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Chapter 12:

General Properties of Synapses
A synapse may be electrical, with direct physical contact between cells, or chemical, involving
a neurotransmitter
...


Connexons) proteins that bind the position of integral membranes
...


Chemical Synapses
Chemical synapses) are far more variable than electrical synapse, because the cells are not
directly coupled
...


Classification of neurotransmitters:
•

Excitatory neurotransmitters: cause depolarization and promote the generation of action potentials

•

Inhibitory neurotransmitters: cause hyperpolarization and suppress the generation of action
potentials

The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the
receptor, not the nature of the neurotransmitter
...
(the neuromuscular
junction is an example)
it is released at
1
...


at many synapses in the CNS

3
...


at all neuromuscular and neuroglandular junctions in the parasympathetic division of the ANS

Events at a Cholingergic Synapse:
1
...
extracellular calcium ions enter the axon terminal, triggering the

exocytosis of Ach:
3
...
ACh is removed by AChE:

AchE is breaks down molecules of Ach (by hydrolysis) into acetate and choline
...
2-0
...


Synaptic Fatigue
Results from intensive stimulation, resynthesis, and transport mechanisms that may not keep up
with the demand for neurotransmitter

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Chapter 12:

12-8

Neurotransmitters and neuromodulators have
various functions
The Activities of Other Neurotransmitters
Major categories of neurotransmitters include:
•

Biogenic amines

•

Amino acids

•

Neuropeptides

•

Dissolved gases

•

and a variety of other compounds

Some Neurotransmitters:
•

Norepinephrine: released by adrenergic synapses
...
Appears to reduce anxiety
...

These substances are typically

Neuropeptides) small peptide chains synthesized and released by the axon terminal
...


endorphines

2
...


endomorphins

4
...

In general, neurotransmitters:
1
...


trigger responses that involve a number of steps and intermediary compounds

3
...


can be released alone or along with a neurotransmitter

How Neurotransmitters and Neuromodulators Work
Functionally, neurotransmitters and neuromodulators fall into 1 of 3 groups:
1
...


2
...

G protein) an enzyme complex coupled to a membrane receptor
...
lipid-soluble gases that exert their effects inside the cell:

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Chapter 12:

12-9
Individual neurons process information by integrating excitatory
and inhibitory stimuli
A single neuron may receive information across thousands of synapses
...
This the simplest level of information processing
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Excitatory postsynaptic potential) is a graded depolarization caused by the arrival of a
neurotransmitter at the postsynaptic membrane
...


Summation
Summation) individual EPSPs combining process, which integrates the effect of all graded
potentials that affect 1 portion of the plasma membrane
...


Spatial summation) occurs when simultaneous stimuli applied at different locations have a
cumulative effect on the membrane potential
...


Summation of EPSPs and IPSPs
The antagonism between IPSPs and EPSPs is important in cellular information processing
...


Presynaptic Inhibition and Presynaptic Facilitation
Axoaxonic synapse) axon-to-axon
...

Presynaptic inhibition) release of GABA inhibits the opening of voltage-gated calcium
channels in the axon terminal
...


The Rate of Generation of Action Potentials
In the nervous system, complex information is translated into action potentials that are
propagated along axons
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It produces paralysis by
preventing Ach from binding to the postsynaptic membrane (motor end plates) of skeletal
muscle fibers

dysthymia: a form of clinical depression
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The nerve cells become damaged and
killed by the overactivation of receptors

neuroblastoma: malignant tumor composed of neuroblasts, most commonly in the adrenal gland; most common
cancer in infancy

neuropathy: condition that causes tingling, numbness, and/or pain in parts of the body, notably the hands and feet
neurotoxin: a compound that disrupts normal nervous system function by interfering with the generation or
propagation of action potentials
...
Resulting in gradual deterioration of neurons due to the
buildup of metabolic by = products and the release of lysosomal enzymes

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