The Nervous System
How do you feel?
Function of Nervous System
Sensory.
– Integrative.
– Motor .
–
Divisions of the Nervous System
Nerve Tissue
(1)
(2)
Neurons
Neuroglia
Neuroglia
 Neuroglia
cells do not conduct
nerve impulses, but instead, they
support, nourish, and protect the
neurons. They are far more
numerous than neurons and, unlike
neurons, are capable of mitosis.
Major types of glia
–
–
–
–
–
Astrocytes.
Microglia .
Ependymal cell.
Oligodendrocytes.
Schwann cell
Neurons

Cell Body

Dendrites
Axon

Cell Body

In many ways, the cell body is similar to
other types of cells. It has a nucleus with
at least one nucleolus and contains many
of the typical cytoplasmic organelles. It
lacks centrioles, however. Because
centrioles function in cell division, the
fact that neurons lack these organelles is
consistent with the amitotic nature of the
cell.
Dendrites

Dendrites are usually, but not always, short and
branching, which increases their surface area to
receive signals from other neurons. The number
of dendrites on a neuron varies. They are called
afferent processes because they transmit
impulses to the neuron cell body. There is only
one axon that projects from each cell body. It is
usually elongated and because it carries
impulses away from the cell body, it is called an
efferent process.
Axon

An axon may have infrequent branches called axon
collaterals. Axons and axon collaterals terminate in
many short branches or telodendria. The distal ends of
the telodendria are slightly enlarged to form synaptic
bulbs. Many axons are surrounded by a segmented,
white, fatty substance called myelin or the myelin
sheath. Myelinated fibers make up the white matter in
the CNS, while cell bodies and unmyelinated fibers
make the gray matter. The unmyelinated regions
between the myelin segments are called the nodes of
Ranvier.
There are several differences between
axons and dendrites:
Axons
Take
information away from
the cell body
Smooth Surface
Generally only 1 axon per cell
No ribosomes
Can have myelin
Branch further from the cell
body
Dendrites
Bring
information to the cell
body
Rough Surface (dendritic
spines)
Usually many dendrites per
cell
Have ribosomes
No myelin insulation
Branch
near the cell body
Classification of Perikayron


Based on number of branches
Unipolar - single stalk from the cell body
–


Most insect’s neurons
Bipolar - bears an axon and a single branch,
or unbranched dendrite
Mutlipolar - an axon and several branched
dendrites
Reflex Arc

Neurons are often arranged in a pattern called a reflex arc. Basically, a
reflex arc is a signal conduction route to and from the CNS. The most
common form of reflex arc is the three-neuron arc. It consists of an
afferent neuron, and an efferent neuron. Afferent or sensory, neuron
conducts signals to the CNS from sensory receptors in the PNS.
Efferent neurons, or motor neurons, conduct signals from the CNS to
effectors. An effector is muscle tissue. Intra neurons conduct signals
from afferent neurones toward or to motor neurons in its simplest
form, a reflex arc consists of an afferent neurons and an efferent
neuron, this is called a two neuron arc . In essence, a reflex arc is a
signal conduction route from receptors to the CNS and out to
effectors. By now you should recognize that the reflex arc is an
example of the information pathway.
Synapse

1.
2.
3.
The synapse is a small gap separating neurons.
The synapse consists of:
a presynaptic ending that contains
neurotransmitters, mitochondria and other cell
organelles,
a postsynaptic ending that contains receptor sites
for neurotransmitters and,
a synaptic cleft or space between the presynaptic
and postsynaptic endings.
Action Potential
Resting Membrane Potential
Resting Membrane Potential
Resting Membrane Potential
Initiation of an Action Potential
(Extracellular Fluid)
Na+
K+
Na+
K+
Org-
Na+
Na+
Cl-
Cl-
K+
Na+
K+
Na+
K+
(Neuronal Cytoplasm,
positively charged)
K+
Na+
K+
K+
Na+
Org-
Na+
Org-
K+
Org-
(negatively charged)
Cl-
K+
Initiation of an Action Potential
Propagation of an Action Potential
1 An action
potential is
initiated
2
Structure
Action potential &
reaches synapticOperation
terminal
of the
Synapse
3 Synaptic vesicles
release
neurotransmitter
4 Receptor binds
neurotransmitter &
opens ion channel
Signaling Stimulus Intensity
Gentle
(a)
Touch
Moderate
Pressure
Strong
Pressure
2
1
(b)
2
1
(c)
2
1
1
Fires slowly
2
Silent
1
Fires more rapidly
2
Silent
1
Fires even faster
2
Fires slowly
Mechanisms of Neural Activity
 Electrical
signals and membrane potentials
–
Resting potential
–
Action potential
 Communication
between neurons
–
Synapses
–
Neurotransmitters and ion gradients
–
Excitatory and inhibitory potentials
 The
body's many neurotransmitters
Electrical Events during an
Action Potential
Recorded Potential
(millivolts)
80
Action Potential
Fluid
40 Extracellular
(uncharged)
0
2
Resting Threshold
Potential
1
-40
3
EPSP
-80
IPSP
Time (milliseconds)
The Neuron Maintains
Ionic Gradients
Ionic pumps keep
some ions inside:
• Potassium ions
• Large organic
ions
Other ions kept
outside:
• Sodium ions
• Chloride ions
Na+
Cl-
Cl- Na+
K+
K+
Cl-
Na+
Na+
K+
K+ K+
Na+
K+
Na+
Cl-
Cl-
ClK+
An Unstimulated Neuron
(Extracellular Fluid)
Na+
Cl-
Na+
Cl-
K+
Na+
Na+
Cl-
Sodium
Channel
(closed)
Potassium
Channel
Cl-
K+
Org-
K+
Org-
K+
(Neuronal Cytoplasm, negatively charged)
OrgOrg-
OrgK+
Ganglion (Brain)


Aggregation or mass of nervous material comprising
the ventral nerve cord
Four divisions of Ganglion
–
–
–
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Neural Lamella - outer sheath of connective tissue
(noncellular)
Perineurium - inner cellular part of outer sheath beneath
neural lamella and probably secretes neural lamella
Perikaryon - cell bodies, located outside of neuropile
Tracts of fibers - groups of axons running parallel
Ganglion



Neuropile - composed of intermingled fibrous
processes and neurotically elements
Nerve - a bundle of axon
Types of Neurons: classified according to function
–
–

Sensory (Afferent) - usually bipolar and located peripherally
Motor (efferent) - unipolar have the cell body in a ganglion
and axon extending to an effector muscle or gland
Association (internucial) - have cell body in a
ganglion and may synapse with one or more
association neurons
Types of Neurons: classified according to
function

Sensory (Afferent) - usually bipolar and located
peripherally
–

Motor (efferent)
–
–

Dendrites associated with a sensory structure
Unipolar have the cell body in a ganglion and axon extending to an
effector muscle or gland cell body lacks dendrites
Bundles of axons form the motor nerves that activates muscles
Association (internucial)
–
–
Have cell body in a ganglion and may synapse with one or more
association neurons
May synapse with other interneurons both sensory & motor
neurons axons form Giant Axons that run the length of the insect’s
body serves as rapid conduction system for Alarm reactions
The Central Nervous System

Brain - located in the head above the esophagus
–

Brain to body size/volume--e.g. diving beetle 1:4200; bee
1:174
Protocerebrum - innervates the ocelli, compound
eyes, and contains 2 groups of association neurons
(Mushroom bodies)
–
–
Optice lobes --extension of the protocerebrum
Deutocerebrum - innervates the antennae


Two (2) lobes that is separated by esophagus
Tritocerebrum - innervates labrum, makes up
circumesophageal connectives, foregut located
below deutocerebrum, and 2 lobes separated by
esophage
Circumesophageal Connections (CNS)

Subesophageal Ganglion
–

Thoracic Ganglia
–

Typically 3 segmental ganglia, each containing sensory and motor
centers; each with 5 or 6 neurons
Abdominal Ganglia
–

Formed out of 3 ganglia, innervates sense organs and muscles of
the mouthparts, salivary glands, and neck
Eight in apterygote insects, 7 in dragonfly, 5-6 grasshopper, and
several in adult flies
Caudal ganglion
- compound, innervates genitalia,- controls copulation and ovipositor
– Longitudinal connections made up of axon and supporting cells
The Visceral Nervous System

Sympathetic with 3 separate systems
–
–

Ventral Sympathetic
–
–

Associated with the Double Ventral Nerve Cord
Single median nerve arise from segmented ganglion and divides
into 2 lateral nerves
Cadual Sympathetic
–

Stomodeal - associated with the brain, aorta, and proctodeum
It arises during embryogeny from the dorsal or dorsal and lateral
walls of stomodaem and becomes connected to the brain
Associated with the posterior segment of abdomen
Structures of the Stomodeal nervous system
–
Frontal Ganglion - located on the dorsal midsection of the foregut
in front of the brain
VNS






Recurrent Nerve - arises from the frontal ganglion and extends
posterior to the
brain
Hypocerebral Ganglion - variation in different insects--the recurrent
nerve ends with the hyopcerebral ganglion
Ventricular or Gastric Nerves - continues to form a ventricular ganglion
Ventricular Ganglion –
Neuroglands associated with the Stomodeal System
–
Corpora Cardiace - hormonal in function therefore part of the endocrine
system


Forward part of the hyopcerebral system
Corpora Allata - secretes juverite hormone (during larva and other
stages)—inhibits molting
The Sense Organs


The mechanical Senses: touch, pressure, vibration,
ect or mechanical distortions
Stimuli arising from
within organisms, maintains proper orientation of
body parts in relationship to others
Three principal types
–
Hair (Sensillum) Sensilla - the simplest of the tactile
receptors



ttichoid- a seta provides with a nerve cell
Campaniform Sensellium - similar to the hair
sensillium, but there is no seta the
nerve lies under a domelike cuticular area
Sense Organs

Scolopophorous Organ - made up of a bundle of
sensory cells
–
–
Subgenual Organs - generally located at the proximal end
of the Tibia
Johnston’s Organs - in the second antennal segment

–

Sound receptor in some insects
Tympanal Organs - hearing sensilla
Stretch - attached connective tissue or muscle;
registering the tension of such tissue, esp. in
abdomen
Chemical Senses



Taste and smell--Principal different--taste is detected
by contact and Smell is detected from a distance
These sensilla generally consists of a group of
sensory nerve cells whose distal
Processes form a bundle extending to the body
surface
–
Location of taste organ (Gustatory)



Principally on the mouth parts
On antennae-bees, ants and wasp
Tarsi--Lepidoptera, and Diptera
Chemical Sense
–
Locations of the Olfactory organs



Principally--antennae
Some cases--Palps and possibly Tarsi
Sensilla of 4 types commonly associated
with chemical receptor neurons
–
–
–
–
Trichorid
Basiconic
Coeloconic
Plate organ
Auditory Organs


Insect detect airborne sounds by means of 2 types of
sensilla
Vibrations of substrate are detected by sudgenual
organ (not airborne)
–
Hair sensilla


–
–
–
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Hairs of the second antennal segment of mosquitoes
(Johnston organ)
Tympanal Organ - sensory cells are attached to or very near
the tympanic membranes
Number of cells-1 or 2 to several hundred
Location
Short horned Grasshoppers (Acrididae)- 1st abdominal
segment
Long horned Grasshoppers (Tettigoniidae) tibia
Auditory Organs



Crickets (Gryillidae)- when present--on the
proximal end of the front tibia
Some Moths - dorsal surface of the
metathorax
Cicadas - 1st abdominal segment
Organs of Vision

Photoreceptors -if the cuticular parts of a sensillium
is Translucent may well function as photoreceptor-many larvae have such--but not called eyes
–
Two types of eyes

Simple (Ocelli) - two types of ocelli
–
Dorsal ocelli - simple round corneal lens in adults
– Stemmata - lateral ocelli of larvae, number 6-10
– Structure of ocelli
 Single corneal len --cornea-convex
 Corneagenous cells - transparent and secrete the cornea
 Retina - composed of several to many light sensitive cells
Organs of Vision



Rhabdoms - located in the outer part of the
retinal cells basal part of the retina often
pigmented
Lateral ocelli of larvae different from Dorsal
ocelli of adult in that the larvae ocelli have
crystalline lens below the corneal lens
Believed that ocelli reacts to changes in light
intensity, but does not perceive the form of
light
Organs of Vision

Compound eye - most adults have compound eyes,
but not necessarily simple eyes
–
–
Ommatidia - visual units up to several thousands in 1
compound
Divisible into 2 parts

Dioptric responsible for collection of light
–

Hexagonal Corneal lens - collectively forming the cornea
–

Reaction apparatus
Individually from the facets
Crystalline Cone - composed of translucent material
Vision


Corneal cell - 2 surrounding the cone are pigmented
on the periphery of cone cells
Retinular sensory cells - 8, beneath the
corneagenous cells
–
–


Striated portion forms an axial rhabdom
Each forming an axon going to the brain
Epidermal pigment cells - form a sheath around
sensory cells
Ommatidia of Diurnal Insects - usually surround by
pigments
–
Type of vision - mosaic