Fundamentals of the Nervous
System and Nervous Tissue
Chapter 12
Introduction




The nervous system is the master
controlling and communicating system of
the body
It is responsible for all behavior
Along with the endocrine system it is
responsible for regulating and
maintaining body homeostasis
Cells of the nervous system communicate
by means of electrical signals
Nervous System Functions

The nervous system has three overlapping functions



Gathering of sensory input
Integration or interpretation of sensory input
Causation of a response or motor output
Introduction

Sensory input


Integration


The nervous system has millions of sensory
receptors to monitor both internal and
external change
It processes and interprets the sensory input
and makes decisions about what should be
done at each moment
Motor output

Causes a response by activating effector
organs (muscles and glands)
Organization


There is only one,
highly integrated
nervous system
Basic divisions of the
nervous system


Central Nervous
Systems
Peripheral Nervous
System
Organization

In order to discuss the nervous in smaller
portions, for convenience the nervous
system is divided into two parts

The central nervous system
• Brain and spinal cord
• Integrative and control centers

The peripheral nervous system
• Spinal and cranial nerves
• Communication lines between the CNS and the
rest of the body
Organization of the Nervous System
Organization

The peripheral nervous system has two
fundamental subdivisions

Sensory (afferent) division
• Somatic and visceral sensory nerve fibers
• Consists of nerve fibers carrying impulses to the
central nervous system

Motor (efferent) division
• Motor nerve fibers
• Conducts impulses from the CNS to effectors
– (glands and muscles)
Organization of the Nervous System
Organization

The motor division of the peripheral
nervous system has two main subdivisions

The Somatic motor
• Voluntary motor
• Conducts impulses from the CNS to skeletal muscle

The Visceral motor
• Involuntary motor
• Conducts impulses from the CNS to cardiac muscles,
smooth muscles, and glands
• Equivalent to the autonomic nervous system (ANS)
Organization of the Nervous System
Peripheral Nervous System
Organization of the Nervous System

Somatic sensory



The sensory receptors that are spread widely
throughout the outer tube of the body
These include the many senses experienced
on the skin and in the body wall, such as
touch, pain, pressure, vibration and
temperature
Proprioception provides feedback from the
stretch of the muscles, tendons and joint
capsules - your “body sense”
Organization of the Nervous System

Somatic sensory


The special somatic senses are receptors are
more localized and specialized
The special senses include; sight, hearing,
balance, smell and taste.
Organization of the Nervous System

Visceral sensory



The general visceral senses include stretch,
pain, and temperature which can be felt
widely in the digestive and urinary tracts,
reproductive organs, and other viscera
Sensations such as hunger and nausea are
also general visceral sensations
The chemical senses such as taste and smell
are considered by some as special visceral
senses
Organization of the Nervous System

Somatic motor



The general somatic motor is part of the
PNS that stimulates contraction of the
skeletal muscle in the body
Also referred to as voluntary nervous system
Skeletal muscles are widely distributed
throughout the body, and therefore there is
no special somatic motor category
Organization of the Nervous System

Visceral motor


The general visceral motor part of the PNS
regulates the contraction of smooth and
cardiac muscle and secretion by the body’s
many glands
General visceral motor neurons make up the
autonomic nervous (ANS) which controls the
function of the visceral organs
Organization of the Nervous System

Visceral motor


Because we generally have no voluntary
control over such activities as the pumping
of the heart and movement of food through
the digestive tract
The ANS is also called the involuntary
nervous system
Organization of ANS

The autonomic nervous system has two
principle subdivisions

Sympathetic division
• Mobilizes body systems during emergency
situations

Parasympathetic division
• Conserves energy
• Promotes non-emergency functions


The two subdivisions bring about opposite
effects on the same visceral organs
What one subdivision stimulates, the other
inhibits
Nervous Tissue


The nervous system consists mostly of
nervous tissue whose cells are densely
packed and tightly intertwined
Nervous tissue is made up just two main
types of cells



Neurons - the excitable cells that transmit
electrical signals
Neuroglia - nonexcitable supporting cells
that surround and wrap the neurons
Both cell types develop from the same
embryonic tissues: neural tube and crest
The Neuron



The human body contains many billions
of neurons which are the basic structural
units of the nervous system
Neurons are highly specialized cells that
conduct electrical signals from one part
of the body to another
These signals are transmitted along the
plasma membrane in the form of nerve
impulses or action potentials
Neurons


Neurons are the structural units of the
nervous system
Neurons are highly specialized cells that
conduct messages in the form of nerve
impulses from one part of the body to
another
The Neuron

Special characteristics of neurons


They have extreme longevity. Neurons can
and must function over a lifetime
They do not divide
• As fetal neurons assume their roles as
communication links in the nervous system, they
lose their ability to undergo mitosis
• Cells cannot be replaced if destroyed - Some
limited exceptions do exist in the CNS as neural
stem cells have been identified
The Neuron

Special characteristics of neurons


They have an exceptionally high metabolic
rate requiring continuous and abundant
supplies of oxygen and glucose
Neurons cannot survive for more than a few
minutes without oxygen
The Neuron


Neurons are typically large, complex cells
Neurons vary in their structure but they
all have two fundamental components


Neuron cell body
One or more processes
The Cell Body



The cell body of the neuron is also called
a soma
The cell bodies of different neurons vary
widely in size (from 5 to 140 m in
diameter)
However, all consist of a single nucleus
surrounded by cytoplasm
The Cell Body

Typically large, complex cells, neurons
have the following structures

Cell body
•
•
•
•

Nuclei
Chromatophilic (Nissl) bodies
Neurofibrils
Axon hillock
Cell processes
• Dendrites
• Axon
• Myelin sheath or neurilemma
The Cell Body

Cell Body





Nuclei
Chromatophilic
(Nissl) bodies
Neurofibrils
Axon hillock
Neuron Processes




Dendrites
Axons
Myelin sheaths
Axon terminals
The Cell Body

In all but the
smallest neurons,
the nucleus is
spherical and
clear and contains
a nucleolus near
its center
The Cell Body


The cytoplasm contains
all the usual cellular
organelles with the
exception of centrioles
(not needed in amitotic
cells) as well as Nissl
bodies
These cellular
organelles continually
renew the membranes
of the cell
The Cell Body


Neurofibrils are
bundles of
intermediate
filaments that run
in a network
between
chromatophilic
bodies
These filaments
keep the cell from
being pulled apart
when it is subjected
to tensile forces
The Cell Body

The cell body is the
focal point for the
outgrowth of the
neuron processes
during embryonic
development
The Cell Body

In most neurons,
the plasma
membrane of the
cell body acts as a
receptive surface
that receives
signals from other
neurons
The Cell Body


Most neuron cell
bodies are located
within the CNS
However, clusters of
cell bodies called
ganglia (singular
ganglion) lie along
the nerves in the
PNS
Neuron Processes


Bundles of neuron processes in the CNS
are called tracts
Bundles of neuron processes in the PNS
are called nerves
Neuron
Processes


Armlike processes
extend from the cell
bodies of all
neurons
There are two types
of processes


Dendrites
Axons
Motor
neuron
Neuron
Processes


The cell processes
of neurons are
described here
using a motor
neuron
Motor neurons
represent a typical
neuron, but sensory
neurons differ from
the typical pattern
Motor
neuron
Dendrites



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Dendrites are short, tapering, diffusely
branching extensions from the cell body
Motor neurons have hundreds of
dendrites clustering close to the cell body
Dendrites function as receptive cites
providing an enlarged area for the
reception of signals from other neurons
By definition, dendrites conduct electrical
signals toward the cell body
Dendrites


Dendritic spines
represent areas of
close contact with
other neurons
These electrical
signals are not nerve
impulses but are
short distance
signals call graded
potentials
Axons



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Each neuron has
only one axon
The axon arises
from the cone
shaped axon hillock
It narrows to form
a slender process
that stays uniform
in diameter the rest
of its length
Length varies;
short or absent to 3
feet in length
Axons


Each axon is called
a nerve fiber
Axons are impulse
generators and
conductors that
transmit nerve
impulses away from
the cell body
Axons


Chromatophilic bodies (Nissl) and the
Golgi apparatus are absent from the axon
and the axon hillock
Axons lack ribosomes and all organelles
involved in protein synthesis, so they
must receive their proteins from the cell
body
Axons



Neurofilaments, actin microfilaments,
and microtubules are especially evident
in axons, where they provide structural
strength
These cytoskeletal elements also aid in
the transport of substances to and from
the cell body as the axonal cytoplasm is
recycled and renewed
The movement of substances along axons
is called axonal transport
Axons



The axon of some neurons are short, but
in others it can be extremely long
Motor neurons in the CNS have axons
that must reach to the musculature that
they control that might be 3-4 feet away
Any long axon is called a nerve fiber and
travels in a group of fibers composing a
nerve
Axons


Although axons
branch far less
frequently than
dendrites,
occasional branches
do occur along their
length
These branches,
called axon
collaterals, extend
from the axon at
almost right angles
Axons



Axons branch
profusely at its
terminus
Ten thousand of
these terminal
branches per
neuron is not
unusual
These branches end
in knobs called
axon terminals or
boutons
Axon


A nerve impulse is typically generated at
the axon’s initial segment and is
conducted along the axon to the axon
terminals, where it causes the release of
chemicals called neurotransmitters into
the extracellular space
The neurotransmitters excite or inhibit
the neurons or target organs with which
the axon is in close contact
Axon



Axon diameter varies considerably
among the different neurons of the body
Axons with larger diameters conduct
impulses faster than those with smaller
diameters
Neurons follow the law of physics: The
resistance to the passage of an electrical
current decreases as the diameter of any
“cable” increases
Synapses


The site at which
neurons
communicate is
called a synapse
Most synapses in
the nervous system
transmit
information through
chemical
messengers
Synapses

Some neurons in
certain areas of the
CNS transmit
signals electrically
through gap
junctions
Synapses

Because signals pass
across most
synapses in one
direction only,
synapses determine
the direction of
information flow
through the nervous
system
Synapses

The neuron that
conducts signals
toward a synapse is
called the
presynaptic neuron;
the neuron that
transmits signals
away from the
synapse is called the
postsynaptic neuron
Synapses

Most neurons in the CNS function as both
presynaptic (information sending) and
postsynaptic (information receiving)
neurons, getting information from some
neurons and dispatching it to others
Synapses



There are two main
types of synapses
Most synapses occur
between the axon
terminals of one
neuron and the
dendrites of another
neuron
These are called
axondendritic
synapses
Synapses


Many synapses also
occur between
axons and neuron
cell bodies
These synapses are
called axosomatic
synapses
Synapses


Synapses are
elaborate cell
junctions
This section shows
axodendritic
synapses because its
structure is
representative of
both types of
synapses
Synapses


Structurally
synapses are
elaborate cell
junctions
At the typical
axodendritic
synapse the
presynaptic axon
terminal contain
synaptic vesicles
Synapses


Synaptic vesicles
are membrane
bound sacs filled
with molecular
neurotransmitters
These molecules
transmit signals
across the synapse
Synapses

Mitochondria are
abundant in the
axon terminal as
the secretion of
neurotransmitters
requires a great
deal of energy
Synapses


At the synapse, the
plasma membranes
of the two neurons
are separated by a
synaptic cleft
On the under
surfaces of the
opposing cell
membranes are
dense materials; the
pre- and postsynaptic densities
Synapses


When an impulse travels along the axon
of the presynaptic neuron, it signals the
synaptic vesicles to fuse with the
presynaptic membrane at the presynaptic
density
The fused area then ruptures releasing
neurotransmitter molecules to diffuse
across the synaptic cleft and bind to the
postsynaptic membrane at the post
synaptic density
Synapse

The binding of the two membranes
changes the membrane charge on the
postsynaptic neuron, influencing the
membrane’s ability to generate a nerve
impulse
Signals Carried by Neurons



Neurons carry information via electrical
signals called nerve impulses, or action
potentials
Signals are relayed from neuron to
neuron via chemical neurotransmitters
In essence an impulse is a reversal of
electrical charge that travels rapidly
along the neuronal membrane
Signals Carried by Neurons

In a resting (un-stimulated) neuron, the
membrane is polarized which means that the
inner cytoplasmic side is negatively charged
with respect to its outer, extracellular side
Signals Carried by Neurons

In addition, the concentration of potassium
ions (K+) is higher inside the neuron and the
concentration of sodium ions (Na+) is higher
outside the neuron
Signals Carried by Neurons


When a neuron is stimulated the permeability
of the plasma membrane changes at the site of
the stimulus, allowing Na+ ions to rush in.
As a result, the inner face of the membrane
becomes less negative or depolarized
Signals Carried by Neurons

If the stimulus initiating the depolarization is
strong enough, the membrane at the axon’s
initial segment is depolarized, so that it is
positively charged inside the axon and negatively
charged outside
Signals Carried by Neurons


Once begun, this depolarization occurs all along
the axon length
It is this wave of charge reversal that constitutes
the nerve impulse
Signals Carried by Neurons

The impulse travels rapidly down the entire
length of the axon without decreasing in strength
Signals Carried by Neurons

After the impulse has passed the membrane
repolarizes itself
Signals Carried by Neurons


Neurons in the body receive stimuli either
directly from the environment or from
signals received at synapses
In signals received at synapses
neurotransmitters released by presynaptic
neurons alter the permeability of the
postsynaptic membrane to certain ions
Signals Carried by Neurons


Synapses that result in an influx of positive
ions into the postsynaptic neuron depolarize
the neuron’s membrane and bring the
neuron closer to impulse generation
These synapses are called excitatory
synapses because they stimulate the
postsynaptic neuron
Signals Carried by Neurons



Other synapses increase membrane
polarization, making the external surface of
the postsynaptic cell even more positive
than it was
This makes the postsynaptic cell less likely
to generate a nerve impulse
These types of synapses are called inhibitory
synapses because they reduce the ability of
the postsynaptic neuron to generate a nerve
impulse
Signals Carried by Neurons

Thousands of excitatory and inhibitory
synapses act on every neuron, competing to
determine whether or not that neuron will
generate an impulse
Classification of Neurons



Neurons can be classified structurally or
functionally
Neurons are grouped structurally
according to the number of processes that
extend from the cell body
By this classification there are three types
of neurons;



Multipolar
Bipolar
Unipolar
Classification of
Neurons



Multipolar - many
processes extend from cell
body, all dendrites except
one axon
Bipolar - Two processes
extend from cell, one a
fused dendrite, the other
an axon
Unipolar - One process
extends from the cell body
and forms the peripheral
and central process of the
axon
Classification of Neurons


Multipolar neurons usually have a single axon
and many dendrites
This type of neuron constitutes 99% of the
neurons in the body
Classification of Neurons




Multipolar
neurons have
more than two
processes
Most common
type in humans
Major neuron of
the CNS
Some neurons
lack an axon
Classification of Neurons


Bipolar neurons have two processes that extend
from opposite sides of the cell body
This rare type of neuron occurs in the special
sensory organs
Classification of Neurons



Bipolar neurons are
found only in special
sense organs where they
function as receptor
cells
Examples include those
found in the retina of
the eye, inner ear, and
epithelium of the
olfactory mucosa
They are primarily
sensory neurons
Classification of Neurons

Unipolar neurons have a short, single process
that emerges from the cell body and divides like
a “T” into two long branches
Classification of Neurons




Unipolar neurons have a single
process that emerges from the
cell body
The central process (axon) is
more proximal to the CNS and
the peripheral is closer to the
PNS
Unipolar neurons are chiefly
found in the ganglia of the
peripheral nervous system
Function primarily as sensory
neurons
Functional Classification


The functional classification scheme
groups neurons according to the direction
in which the nerve impulse travels
relative to the CNS
Based on this criterion there are three
types of neurons



Sensory neurons
Motor neurons
Interneurons
Functional Classification
Sensory
Neurons


These afferent
neurons make up the
sensory division of the
PNS
They transmit
impulses toward the
CNS from sensory
receptors in the PNS
Sensory Neurons


Sensory neurons have their cell bodies in
ganglia outside of the CNS
The single (unipolar) process is divided into the
central process and the peripherial process
Sensory Neuron



The central process is clearly an axon
because it carries a nerve impulse and
carries that impulse away from the cell
body which meet the criteria which
define an axon
The peripheral by contrast carries nerve
impulses toward the cell body which
suggests that it is a dendrite
However, the basic convention is that the
central process and the peripheral
process are parts of a unipolar neuron
Motor
Neurons



Neurons that carry
impulses away from
the CNS to effector
organs (muscles and
glands) are called
motor or efferent
neurons
Upper motor
neurons are in the
brain
Lower motor
neurons are in PNS
Motor Neurons


Motor neurons are multipolar and their cell
bodies are located in the CNS (except autonomic)
Motor neurons form junctions with effector cells,
signaling muscle to contract or glands to secrete
Interneuron or Association Neurons




These neurons lie
between the motor
and sensory neurons
Form complex
neural pathways
Confined to CNS
Make up 99.98% of
the neurons of the
body and are the
principle neuron of
the CNS
Interneuron Neurons


Almost all interneurons are multipolar
Interneurons show great diversity in the size and
branching patterns of their processes
Interneurons


The Pyramidal cell
is the large neuron
found in the
primary motor
cortex of the
cerebrum
The Purkinje cell is
from the
cerebellum
Supporting Cells

All neurons associate closely with nonnervous supporting cells called neuroglia

Support cells of the CNS
•
•
•
•

Astrocytes
Microglial
Ependymal
Oligodendrocyte
Support cells of the PNS
• Schwann cells
• Satellite cells
Supporting Cells


While each support cell has a unique
specific function, in general these cells
provide a supportive scaffolding for
neurons
In addition, they all cover nonsynaptic
parts of the neurons thereby insulating
the neurons and keeping the electrical
activities of adjacent neurons from
interfering with each other
Supporting Cells in the CNS





Like neurons, glial cells have branching
processes and a central cell body
Neuroglia can be distinguished from
neurons by their much smaller size and
darker staining nuclei
They outnumber neurons in the CNS by a
ratio of 10 to 1
Make up half of the mass of the brain
Unlike neurons, glial cells divide
throughout one’s lifetime
Astrocytes




Star shaped
Most abundant type
of glial cell
Radiating projections
cling to neurons and
capillaries, bracing
the neurons to their
blood supply
Astrocytes play a role
in exchanges of ions
between capillaries
and neurons
Astrocytes



Astrocytes take up and
release ions to control
the environment around
neurons
Concentrations of ions
must be kept within
narrow limits for nerve
impulses to be
generated & conducted
Astrocytes recapture
and recycle potassium
ions and released neurotransmitters
Astrocytes


Astrocytes contact both the neuron and
the capillary in order to sense when the
neuron are highly active and releasing
large amounts of neurotransmitters
(glutamate)
Astrocytes then extract blood sugar from
the capillaries they contact to obtain the
energy they need to fuel the process of
glutamate uptake
Astrocytes


Astrocytes also are involved with synapse
formation in developing neural tissue,
produce molecules necessary for neural
growth (brain-derived trophic factor
BDTF) and propagate calcium signals
that may be involved in memory
Understanding the role of these abundant
glial cells in neural functioning is an area
of ongoing research
Microglial



Smallest and least
abundant type of
neuroglial cell
The elongated cells
have relatively long
“thorny” processes
They are phagocytes,
the macrophages of
the CNS
Microglial


Microglial derive
from blood cells and
migrate to the CNS
during embryonic and
fetal development
Microglial engulf
invading
microogranisms and
injured or dead
neurons
Microglial


When invading microorganisms are present
or damaged neurons
have died, the microglial transforms into a
special type of macrophage that protects
the CNS by
phagocytizing the
microorganisms or
neuronal debris
Important because
cells of the immune
system can enter CNS
Ependymal


CNS tissue originates in
the embryo as a hollow
neural tube and retains
a central cavity
throughout life
Form a simple
epithelium that lines
the central cavity of the
spinal cord and brain
Ependymal


Forms a fairly
permeable barrier
between cerebrospinal
fluid of those cavities
and the cells of the CNS
Ependymal cells bear
cilia that helps circulate
the cerebrospinal fluid
Oligodendrocytes



Fewer branches than
astrocytes
Cells wrap their
cytoplasmic
extensions tightly
around the thicker
neurons in the CNS
Produce insulating
coverings called
myelin sheaths
Neuroglia in the PNS

There are two supporting cells in the PNS



Satellite cells
Schwann cells
These cells are similar in type and differ
mainly in location
Satellite Cells


Somewhat flattened satellite cells surround cell
bodies within ganglia
Thought to play some role in controlling the
chemical environment of neurons with which
they are associated, but function is largely
unknown
Schwann Cells



Surround and form myelin sheaths around the
larger nerve fibers in PNS
Similar to the oligodendrocytes of CNS
Schwann cells are vital to peripheral nerve
fiber regeneration
Myelin Sheaths


Myelin sheaths are produced by oligo
dendrocytes in the CNS and Schwann
cells in the PNS
Myelin sheaths are segmented structures,
each composed of the lipoprotein myelin
and surround the thicker axons of the
body
Myelin Sheaths

Each segment of myelin consists of a plasma
membrane of a supporting cell rolled in
concentric layers around the axon
Myelin Sheaths

Myelin sheaths form an insulating layer that…
 Prevents the leakage of electrical current from
the axon
 Increases the speed of impulse conduction
 Makes impulse propagation more energy
efficient
Myelin Sheaths in PNS


Myelin sheaths in the PNS are formed by
Schwann cells
Myelin sheaths develop during the fetal
period and continue to develop during
the first year of postnatal life
Myelin Sheaths in the PNS


In forming, the cells
indent to receive the
axon and then wrap
themselves around the
axon repeatedly in a
jellyroll fashion
Initially loose, the
wrapping eventually
squeeze the cytoplasm
outward between cell
membrane layers
Myelin Sheaths in the PNS



When the process is complete many
concentric layers of Schwann cell plasma
membrane wrap the axon in tightly
packed coil of membranes
The nucleus and most of the cytoplasm of
the Schwann cell end up just external to
the myelin layers
This external material is called the
neurilemma
Myelin Sheaths - PNS


Because the adjacent Schwann cells along a
myelinated axon do not touch one another,
there are gaps in the myelin sheath
These gaps called the Nodes of Ranvier, occur
at regular intervals about 1mm apart
Myelin Sheaths - PNS

In myelinated axons, nerve impulses do not
travel along the myelin-covered regions of the
axonal membrane, but instead jumps from the
membrane of one Node of Ranvier to the next
greatly increasing impulse conduction
Myelin Sheaths in the PNS


Only thick, rapidly conducting axons are
sheathed in myelin
Thin, slowly conducting axons lack a
myelin sheath and are called
unmyelinated axons
Myelin Sheaths in the PNS


In unmyelinated axons
the Schwann cells
surround the axons but
do not wrap around
them in concentric
layers of membrane
A single Schwann cell
can partly enclose 15 or
more unmyelinated
axons with each in a
separate tubular recess
on the surface of the
cell
Myelin
Sheath



Myelin increases
the speed of
transmission of
nerve impulses
Myelinated axons
transmit nerve
impulses rapidly;
150 meters/second
Unmyelinated
axons transmit
quite slowly; 1
meter/second
Myelin Sheaths in the PNS

Unmyelinated axons
are found in portions
of the autonomic
nervous system as well
as in some sensory
fibers
Myelin Sheaths of the PNS


Electron micrograph
of an unmyelinated
axon
Note the tubular
tunnels that separate
the axons
Myelin Sheaths in the CNS


Oligodendrocytes form the myelin sheaths in
the brain and spinal cord
Each oligodendrocyte has multiple processes
that coil around several different axons
Myelin
Sheaths - PNS

The nucleus of
the cell and most
of the cytoplasm
end up just
external to the
myelin layers
Myelin Processes - PNS


Myelin sheaths are associated only with
axons and their collaterals as these are
impulse conducting fibers and need
insulation
Dendrites which carry only graded
potentials are always unmyelinated
Myelin
Sheaths - PNS


When the wrapping
process is complete
many concentric layers
wrap the axon
Plasma membranes of
myelinating cells have
less protein which makes
them good electrical
insulators
Myelin
Sheaths - PNS


Because the adjacent
Schwann cells do not
touch one another
there are gaps in the
myelin sheath
These gaps, called
nodes of Ranvier,
occur at regular
intervals about 1 mm
apart
Myelin
Sheaths - PNS

Since the axon is only
exposed at these
nodes nerve impulses
are forced to jump
from one node to the
next which greatly
increases the rate of
impulse conduction
Myelin Sheaths - PNS



Schwann cells that
surround but do not coil
around peripheral
fibers are considered
unmyelinated
A single Schwann cell
can partly enclose 15 or
more axons
Each ends occupying a
separate tubular recess
CNS Axons




Oligodendrocytes form
the CNS myelin sheaths
In contast to Schwann
cells, oligodendrocytes
can form the sheaths of
as many as 60 processes
at one time
Nodes are spaced more
widely than in PNS
Axons can be
myelinated or
unmyelinated
CNS Axons


Regions of the brain containing dense
collections of myelinated fibers are
referred to as white matter and are
primarily fiber tracts
Gray matter contains mostly nerve cell
bodies and unmyelinated fibers
Graded Potential



In humans, natural stimuli are not applied
directly to axons, but to dendrites and the
cell body which constitute the receptive
zone of the neuron
When the membrane of this receptive zone
is stimulated it does not undergo a polarity
reversal
Instead it undergoes a local depolarization
in which the inner surface of the
membrane merely becomes less negative
Graded Potential



This local depolarization is called a graded
potential which spreads from the receptive
zone to the axon hillock (trigger zone)
decreasing in strength as it travels
If this depolarizing signal is strong enough
when it reaches the initial segment of the
axon, it acts as the trigger that initiates an
action potential in the axon
Signals from the receptive zone determine
if the axon will fire an impulse
Synaptic Potential


Most neurons in the body do not receive
stimuli directly from the environment but
are stimulated only by signals received at
synapses from other neurons
Synaptic input influences impulse
generation through either excitatory or
inhibitory synapses
Synaptic Potential

In excitatory synapses, neurotransmitters
released by presynaptic neurons alter the
permeability of the postsysnaptic
membrane to certain ions, this
depolarizes the postsynapatic membrane
and drives the postsynaptic neuron
toward impulse generation
Synaptic Potential


Inhibitory synapses cause the external
surface of the postsynaptic membrane to
become even more positive, thereby
reducing the ability of the postsynaptic
neuron to generate an action potential
Thousands of excitatory and inhibitory
synapses act on every neuron, competing
to determine whether or not that neuron
will generate an impulse
Neural Integration



The organization of the nervous system is
hierarchical
The parts of the system must be
integrated into a smoothly functioning
whole
Neuronal pools represent some of the
basic patterns of communication with
other parts of the nervous system
Neuronal Pools


Note: The illustrations presented are a
gross oversimplification of an actual
neuron pool
Most neuron pools consist of thousands of
neurons and include inhibitory as well as
excitatory neurons
Neuronal Pools

Neuronal
pools are
functional
groups of
neurons that
process and
integrate
incoming
information
from other
sources and
transmit it
forward
One incoming presynaptic fiber synapses with
Several different neurons in the pool. When
Incoming fiber is excited it will excite some
Postsynaptic neurons and facilitate others.
Neuronal Pools


Neurons most
likely to generate
impulses are those
most closely
associated with the
incoming fiber
because they
receive the bulk of
the synaptic
contacts
These neurons are
in the discharge
zone
Discharge Zone
Neuronal Pools


Neurons farther
away from the
center are not
excited to threshold
by the incoming
fiber, but are
facilitated and can
easily brought to
threshold by stimuli
from another source
The periphery of the
pool is the
facilitated zone
Facilitated
zone
Types of Circuits



Individual neurons in a neuron pool send
and receive information and synaptic
contacts may cause either excitation or
inhibition
The patterns of synaptic connections in
neuronal pools are called circuits and
they determine the functional capabilities
of each type of circuit
There are four basic types of circuits

Diverging, converging, reverberating, and
parallel discharge circuits
Diverging Circuits


In diverging circuits
one incoming fiber
triggers responses in
ever-increasing
numbers of neurons
farther and farther
along in the circuit
Diverging circuits are
often called
amplifying circuits
because they amplify
the response
Diverging Circuits



These circuits are
common in both sensory
and motor systems
Input from a single
receptor may be relayed
up the spinal cord to
several different brain
regions
Impulses from the brain
can activate a hundred
neurons and thousands
of muscle fibers
Converging Circuits



The pattern of
converging circuits is
opposite to that of
diverging circuits
Common in both motor
and sensory pathways
In these circuits, the
pool receives inputs
from several
presynaptic neurons,
and the circuit as a
whole has a funneling or
concentrating effect
Converging Circuits

Incoming stimuli
may converge from
many different areas
or from the same
source, which results
in strong stimulation
or inhibition
Reverberating (oscillating) Circuits


In reverberating
circuits the incoming
signal travels through a
chain of neurons, each
of which makes
collateral synapses with
neurons in the previous
part of the pathway
As a result of this
positive feedback, the
impulses reverberates
through the circuit
again and again
Reverberating (oscillating) Circuits



Reverberating circuits
give a continuous signal
until one neuron in the
circuit is inhibited and
fails to fire
These circuits are
involved in the control
of rhythmic activities
such as the sleep-wake
cycle and breathing
The circuits may
oscillate for seconds,
hours, or years
Parallel After-Discharge Circuits


The incoming fiber
stimulates several
neurons arranged in
parallel arrays that
eventually stimulate
a common output cell
Impulses reach the
output cell at
different times,
creating a burst of
impulses called an
after discharge that
may last 15 ms after
initial input ends
Parallel After-Discharge Circuits


This circuit has no
positive feedback and
once all the neurons
have fired, circuit
activity ends
These circuit may be
involved with
complex problem
solving activities
Patterns of Neural Processing





Processing of inputs in the various circuits
is both serial and parallel
In serial processing, the input travels along
a single pathway to a specific destination
In parallel processing, the input travels
along several different pathways to be
integrated in different CNS regions
Each pattern has its advantages
The brain derives its power from its ability
to process in parallel
Serial Processing



In serial processing the whole system
works in a predictable all-or-nothing
manner
One neurons stimulates the next in
sequence, producing a specific, anticipated
response
Reflexes are examples of serial processing
but there are others
Parallel Processing



In parallel processing inputs are
segregated into many different pathways
Information delivered by each pathway is
dealt with simultaneously by different
parts of neural circuitry
During parallel processing several aspects
of the stimulus are processed


Barking dog
The same stimulus can hold common or
unique meaning to different individuals
Parallel Processing


Parallel processing is not repetitious
because the circuits do different things
with more information
Each parallel path is decoded in relation
to all the others to produce a total picture
of the stimulus
Parallel Processing



Even simple reflex arcs do not operate in
complete isolation
As an arc moves through an association
neuron this activates parallel processing
of the same input at higher brain levels
The reflex arc may cause you to pull away
from a negative stimulus while parallel
processing of the stimulus initiates
problem solving about what need to be
done
Parallel Processing




Parallel processing is extremely
important for higher level mental
functioning
An integrated look at the whole problem
allows for faster processing
Parallel processing allows you to store a
large amount of information in a small
volume
This allows logic systems to work much
faster
Reflexes



Reflexes are rapid, automatic responses
to stimuli, in which a particular stimulus
always causes the same motor response
Reflex activity is stereotyped and
dependable
Some your are born with and some you
acquire as a consequence of interacting
with your environment
Reflex Arcs


Reflex arcs are simple chains of neurons
that explain our simplest, reflective
behaviors and determine the basic
structural plan of the nervous system
Reflex arcs are responsible for reflexes,
which are defined as rapid, automatic
motor responses to stimuli
Reflex Arcs


Reflexes that involve the contraction of
skeletal muscle are referred to as somatic
reflexes
Reflexes that involve the contraction of
smooth muscle, cardiac muscle, or glands
are referred to as visceral reflexes
Serial Processing: A Reflex Arc

Reflexes occurs over neural pathways called reflex
arcs that contain five essential components





Receptor
Sensory neuron
CNS integration center
Motor neuron
Effector
Reflex Arcs



The receptor, sensory neuron, motor
neuron, and effector are all relatively
straightforward components
When considering the integration center
associated with reflex arcs, it is
important to understand that the number
of synapses involved can vary
The simplest reflex arcs involve only one
synapse in the CNS while others involve
multiple synapses and interneurons
Reflex Arcs

At the top is a reflex arc, at the left is a
monosynaptic reflex and on the right is a poly
synaptic reflex
Reflex Arcs

The monosynaptic reflex has only one synapse
and no interneuron, while the polysynaptic has
multiple synapses and an interneuron
Reflex Arcs - Monosynaptic


This is the simple
knee-jerk reflex
The impact of the
hammer on the
patellar tendon
stretches the
quadriceps muscles
Reflex Arcs - Monosynaptic


Stretching activates a
sensory neuron
that directly activates
a motor neuron in the
spinal cord,
which then signals the
quadriceps muscle to
contract
This contraction
counteracts the
original stretching
caused by the hammer
Reflex Arcs - Monosynaptic




Many skeletal muscles of the body can be
activated by monosynaptic stretch reflexes
These reflexes help maintain equilibrium
and upright posture
In these postural muscles, sensory neurons
sense the stretching of muscles that occurs
when the body begins to sway
Motor neurons activate muscles that
adjust the body’s position to prevent a fall
Reflex Arcs - Monosynaptic


Because stretch reflexes contain just one
synapse monosynaptic reflexes are the
fastest of all reflexes
They are used in the body to maintain
balance and equilibrium where speed of
adjustment is essential to keep from
falling
Reflex Arcs - Polysynaptic


Polysynaptic reflexes
are the more common
reflexes in the body
In these reflexes, one
or more interneurons
are part of a reflex
pathway between the
sensory and motor
neurons
Reflex Arcs - Polysynaptic


Most of the simple
reflex arcs in the
body contain a single
interneuron and
therefore have a total
of three neurons
Since there are two
synapses joining the
three neurons they
are referred to as
polysynaptic
Reflex Arcs - Polysynaptic


Withdrawal reflexes
by which we pull
away from danger are
three-neuron reflexes
Pricking a finger with
a tack initiates an
impulse in the
sensory neuron,
which activates the
interneuron in the
CNS
Reflex Arcs - Polysynaptic

The interneuron
signals the motor
neuron to contract
the muscle that
withdraws the hand
from the negative
stimulus
Reflex Arcs - Polysynaptic


The three neuron reflex arc are of special
importance in the science of neuroanatomy
Three neuron reflex arcs reveal the
fundamental design of the entire nervous
system
Design of the Nervous System

Three neuron reflex arcs from the basis of the
structural plan of the nervous system
Design of the Nervous System

Note that the cell bodies of the sensory neurons lie
outside the CNS in sensory ganglia and that their
central processes enter the dorsal aspect of the cord
Design of the Nervous System

In the CNS the cell bodies of most interneurons lie
dorsal to those of the motor neurons and the long
axons exit the ventral aspect of the spinal cord
Design of the Nervous System

The nerves of the PNS consist of the motor axons
plus the long peripheral process of the sensory
neurons
Design of the Nervous System

These motor and sensory nerve fibers extend
throughout the body to reach the peripheral
effectors and receptors
Design of the Nervous System



Even though reflex arcs determine its
basic organization, the human nervous
system is obviously more complex than a
series of simple reflex arcs
To appreciate its complexity, we must
expand our conception of interneurons
Interneurons include not only the intermediate neurons of reflex arcs, but also
all the neurons that are entirely confined
within the CNS
Design of the Nervous System


The complexity of the CNS arises from
the organization of the vast numbers of
interneurons in the spinal cord and brain
into complex neural circuits that process
information
The complexity of the CNS results from
long chains of interneurons that are
interposed between each sensory and
motor neuron
Design of the Nervous System

Although tremendously oversimplified, the information depicted is a useful way to conceptualize the
organization of neurons in the CNS
Design of the Nervous System



The CNS has distinct regions of gray and
white matter that reflect the arrangement
of its neurons
The gray matter is a gray colored zone that
surrounds the hollow cavity of the CNS
It is H-shaped in the spinal cord, where its
dorsal half contains cell bodies of
interneurons and its ventral half contains
cell bodies of motor neurons
Design of the Nervous System


Gray matter is a site where neuron cell
bodies are clustered
Specifically, gray matter is a mixture of
neuron cell bodies, dendrites, and short
unmyelinated axons
Design of the Nervous System



White matter which contains no neuron
cell bodies but millions of axons
Its white color comes from the myelin
sheaths around many of the axons
Most of these axons ascend from the
spinal cord to the brain or descend from
the brain to the spinal cord, allowing
these two regions of the CNS to
communicate with each other
Design of the Nervous System


White matter consists of axons running
between different parts of the CNS
Within the white matter, axons traveling
to similar destinations form axon bundles
called tracts
Nervous Tissue Development


During the embryonic period, which spans
8 weeks, the embryo goes from zygote to
blastocyst, to two layer embryo, to three
layer embryo
The embryo upon reaching three layers
begins to form the neural tube from which
will differentiate the brain and spinal cord
Nervous Tissue Development

The nervous system develops from the
dorsal section of the ectoderm, which
invaginates to form the neural tube and
the neural crest
Nervous System Development


The walls of the
neural tube begin as
a layer of
neuroepithelial cells
become the CNS
These cells divide,
migrate externally,
and become
neuroblasts (future
neurons) which
never again divide
Nervous System Development


These cells divide,
migrate externally,
and become
neuroblasts (future
neurons) which
never again divide
They cluster as
future interneurons
and motor neurons
Nervous System Development

Just external to the
neuroepithelium, the
neuroblasts cluster
into alar and basal
plates
Nervous System Development


Dorsally, the
neurons of the alar
plate become
interneurons
Ventrally, the
neuroblasts of the
basal plate become
motor neurons and
sprout axons that
grow out to the
effector organs
Nervous System Development


Axons that sprout
from the young
interneurons form
the white matter by
growing outward the
length of the CNS
These events occur
in both the spinal
cord and the brain
Nervous System Development


Most of the events described take place in
the second month of development, but
neurons continue to form rapidly until
the about the sixth month
At the sixth month neuron formation
slows markedly, although it may continue
at a reduced rate into childhood
Nervous System Development



Just before neuron formation slows, the
neuroepithelium begins to produce astrocytes
and oligiodendrocytes
The earliest of these glial cells extend outward
from the neuroepithelium and provide
pathways along which young neurons migrate
to reach their final destination
As the division of its cells slows, the
neuroepithelium becomes the ependymal layer
Nervous System Development



Sensory neurons do
not arise from the
neural tube but from
the neural crest
This explains why
the cell bodies of the
sensory neurons lie
outside the CNS
Sensory neurons also
stop dividing during
the fetal period
Nervous System Development


Sensory neurons cell
bodies develop
outside the CNS in
the neural crest
Sensory neurons also
stop dividing during
the fetal period
Nervous System Development



Neuroscientists are actively investigating
how forming neurons “hook up” with
each other during development
As the growing axons elongate at growth
cones, they are attached by chemical
signals from other neurons called
neurotrophins
At the same time, the receiving dendites
send out thin, extensions to reach the
approaching axons to form synapses
Nervous System Development

Which synaptic connections are made,
and which persist, are determined by two
factors;


The amount of neurotrophin initially
received
The degree to which a synapse is used after
being established
Nervous System Development



Neurons that make “bad” connections
are signaled to die via apoptosis
Of the neurons formed during the
embryonic period, about two-thirds die
before birth
This initial overproduction of neurons
ensures that all necessary neural
connections will be made and that
mistaken connections will be eliminated