The Nervous
System
Functions, Structures and the Classification of Neurons
http://www.alz.org/brain/01.asp
FEATURES
• Composed of unique tissue which is both
responsive and conductive
(messages can travel at least 200 mph)
• Pinpoint control
(targets specific tissues in body)
• The most intricately organized matter on
Earth
(1cm3 Brain = Several million neurons)
Organs of the Nervous System
• Central Nervous System (CNS)
– Brain & Spinal Cord
• Peripheral Nervous System
(PNS)
– Cranial Nerves & Spinal Nerves
Nervous System
PNS
CNS
Motor
Sensory
(efferent neurons)
*messages from CNS
to muscle or glands
(afferent neurons)
*incoming messages to CNS
*senses external and internal
environment
Somatic
Effectors: skeletal
muscles:
conscious control
Parasympathetic
Autonomic
Effectors: smooth
muscles, cardiac,
glands: unconscious
control
*fight orSympathetic
flight
*Maintains homeostasis: self-maintenance
fxns (calming affect)
•Arousal or energy generation
*NT: AcH
•Increase heart &respiratory
rate
*NT: norepinephrine
Structural Classification
1. Multipolar
a.
b.
Many processes/extensions but only one is an AXON
Abundant in CNS
2. Bipolar
a.
b.
? Extensions; one is still the axon
Abundant in eyes, nose and ears
3. Unipolar
a.
? Extension; forms central and peripheral processes
General Functions of the Nervous
System
1. Sensory
2. Integrative
3. Motor
FUNCTIONS
• Sensory Input - Conduction of signals from receptors
to integration center (eyes)
• Integration - Process by which info. from environment
is interpreted & associated with appropriate response
of the body
– Most occurs in CNS (brain & spinal cord)
• Motor Output - Conduction of signals from CNS to
effector cells (muscles or glands) that actually carry
out response to stimulus
Integrative Function
• CNS
• Type of neuron it occurs in: interneuron
• Stimulated by: NT-neurtotransmitter
– Released by a sensory neuron
• Receptive portion: dendrites
• Involves interpretation of an incoming sensory
impulse
• Begins a motor impulse from dendrites down axon
into terminal end of the axon
Sensory Input Function
• PNS
• Type of neuron: sensory neuron (afferent)
• Sensory Receptors(receptive portion) detect changes
occurring in their surroundings at end of dendrites
• Stimulated by: light, temp change, etc
– Once stimulated, sensory receptors transmits a sensory
impulse to the CNS
• Sensory impulse is carried on a sensory neuron from
dendrites through cell body down axon to synaptic
knobs
Motor Function
• PNS
• Type of neuron it occurs in: motor neuron (efferent)
– involves the response of a body part
• Motor impulses are carried from CNS to responsive
body parts called effectors
• Stimulated by: NT released by interneurons
• Effectors
– Muscles (contract)
– Glands (secrete a hormone)
Cells of the Nervous System
• There are 2 main classes of cells:
• A. Neurons - conduct messages
• Common Features
– Cell body (contains nucleus & other organelles…no centrioles???)
– Dendrites (conveys signal to cell body)
– Axons (conduct messages away from cell body)
– Myelin Sheath (insulation layer composed of Schwann Cells)
– Synaptic Terminal (relays signals to other cells by releasing
neurotransmitters)
– Synapse (site of contact between terminal & target cell)
– Nodes of Ranvier (gaps in myelin, voltage channels)
•
B. Supporting Cells - protects, insulates, and assists neurons
(Outnumber neurons 10-50 times)
• Do not conduct impulses, but are essential for the integrity
of nervous system
• Supporting Cells are called “Glial Cells” (glue)
•
1.
Several Types
Astrocytes - encircles capillaries in brain, restricts passage of substances
into brain
2.
Oligodendrocytes - form insulating myelin sheaths around axons in
neurons of CNS
3.
Schwann Cells - form insulating myelin sheaths around axons in neurons
of Peripheral nervous system
–
–
Myelin Sheaths - jelly roll composed of lipid (poor conductor)
Multiple Sclerosis = degenerative disease of myelin sheaths (What symptoms
would you expect to see?)
Histology of the Neuron
1. Cell Body
2. Dentrites
3. Axon
a.
b.
c.
d.
e.
One per neuron
Long, thin processes with uniform diameter
carry impulses away from cell body
Many fine extensions at end called collaterals
Axon ending= at ends of collaterals contain
synaptic knob (comes in contact with receptive
surface of another cell)
Histology of the Neuron
Different types of Axons
1. Axons in PNS
a. Large and surrounded by myelin sheath (made of
lipids)
b. Contains Schwann Cells
i. Myelinated nerve fibers/ produce the myelin
ii. Between each Schwann cell is a space called Nodes of
Ranvier
2. Small axons
a. Lack myelin sheath
b. However all axons are associated with Schwann
cells
Histology of the Neuron
3. Axons in CNS
a. Myelin is produced by oligodendrocyte NOT
schwann cells
b. Myelinated nerve bundles are referred to as
white matter
c. Gray matter
a. Unmyelinated nerve fibers
Histology of the Neuron
Regeneration of Nerve Fibers
1. Injury to cell body: dead neuron
2. Injury to axon: possible regeneration
Histology of the Neuron
Neuroglial Cells
accessory cells of the nervous system form the
supporting network for neurons (nerve glue)
1 PNS= Schwann
2 CNS= 4 types
•
Oligodendrocytes
o
•
Astrocytes
o
•
Nourishes neurons
Microglia
o
•
Looks like an eye balls; produces myeline
Looks like spider; phagoctosis
Ependyal cells
o
o
Epithelial like layer
Functions to line spaces in CNS
Classification of the Neuron
• Neurons of classified based on
– Function
– Structure
Functional Classification
1. Sensory neurons
a.
b.
c.
PNS
Afferent neurons: carry sensory impulses from sensory
receptors to CNS
Location: skin and sense organs
2. Interneurons
a.
b.
c.
CNS
Links other neurons together
“Association”
3. Motor
a.
b.
c.
PNS
Efferent neurons; carry motor impulses away fron CNS and
to effectors
Effectors: muscles and glands (involuntary and voluntary
controls)