Introduction to the Nervous
System
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Nervous Tissue &
Homeostasis
• excitable characteristic of nervous tissue
allows for generation of nerve impulses
(action potentials) that provide
communication & regulation of most body
tissue.
• together with endocrine system: responsible
for maintaining homeostasis
Differences in Nervous &
Endocrine Control of
Homeostasis
NERVOUS
• rapid responder
• action potentials
ENDOCRINE
• slow, prolonged
response
• releases hormones
Structures of the Nervous
System
• total mass of 2 kg (~3% of total body
mass)
• Skull
• Spinal Cord
• Spinal Nerves
• Cranial Nerves
• Ganglia
• Enteric Plexus
• Special Senses & other Sensory
Receptors
Functions of the Nervous System
• 3 basic functions:
1. Sensory
2. Integrative
3. Motor
Sensory Function
• sensory receptors detect internal &
external stimuli
• sensory (afferent) neurons carry this
sensory information to spinal cord &
brain thru cranial & spinal nerves
Integrative Function
• integrate: process
• nervous system takes information from
sensory neurons & processes that
information, analyzes it, stores some of it
& makes decisions for appropriate
responses
• served by interneurons (connect 1
neuron to another neuron
• Perception:
– conscious awareness of sensory stimuli
– occurs in brain
Motor Function
• served by motor (efferent) neurons
• carry info from brain/spinal cord 
effectors (muscle or gland) thru
cranial or spinal nerves
• results in muscles contraction or
gland secreting
Organization of the Nervous
System
Histology of the Nerrvous
System
• 2 cell types
1. Neurons
2. Neuroglia
Neurons
• nerve cells that possess electrical
excitability:
– ability to respond to a stimulus & convert
it into an action potential
– stimulus: any change in environment
that is strong enough to initiate an action
potential
Action Potential
• electrical signal that propagates along
surface of neurolema (membrane)
– begins & travels due to movement of ions
between interstitial fluid & inside of
neuron thru specific ion channels
– once begun it travels rapidly @ constant
strength
Parts of a Neuron
• contains
cytoplasm,
typical
Parts ofnucleus,
Neuron:
Cell Body
organelles,
• + Nissl bodies clusters of RER
– make materials for:
• growth of neuron
• regenerate damaged axons in PNS
Nerve Fiber
• general term for any neuronal process
or extension that emerges from cell
body
• most neurons have 2:
1. Dendrites
2. Axons
• “little trees”Dendrites
• input portion of neuron
• usually, short, tapering, highly
branched
• their cytoplasm contains Nissl bodies,
mitochondria
Axon
• propagates action potentials 
– another neuron
– muscle fiber
– gland cell
Parts of an Axon
• joins cell body @ cone-shaped
elevation: axon hillock
• part of axon closest to hillock = initial
segment
• jct of axon hillock & initial segment
where action potential arises so is
called the trigger zone
Parts of an Axon
• axoplasm: cytoplasm of an axon
• axolemma: plasma membrane of axon
• axon collaterals: side branches along
length of axon (most @ 90°)
• axon terminals: axon divides into
many fine processes
Synapse
• site of communication between 2
neurons or between a neuron & effector
cell
• synaptic end bulbs: tips of some axon
terminals swell into bulb-shaped
structures
• synaptic vesicles: store neurotransmitter
– many neurons have >1 neurotransmitter,
each with different effects on postsynaptic
cell
Axonal Transport
• 2 types:
– for moving materials from cell body  axon
terminals
1. slow
– 1-5 mm/d
– replenishes new axoplasm to developing or
regenerating axons
2. fast
– 200 – 400 mm/d
– moves materials to/from cell body
•
organelles or membranes needed in axon terminal
Types of Neurons
Functional Classification
• Sensory
• Interneurons
• Motor
Structural Classification
• use # processes
extending from cell
body
1. Multipolar neurons
2. Bipolar neurons
3. Unipolar neurons
Multipolar Neurons
• several dendrites with 1 axon
• includes most neurons in brain &
spinal cord
Bipolar Neuron
• 1 main dendrite & 1 axon
• retina, inner ear, olfactory area of
brain
Unipolar Neuron
• are sensory neurons that begin in
embryo as bipolar
• during development axon & dendrite
fuse then divide into 2 branches (both
have characteristic structure & function
of an axon)
• 1 branch ends with dendrites (out of
CNS)
• 2nd branch ends in axon terminal (in
CNS)
• cell bodies of most found in ganglia
Unipolar Neuron
Pyramidal Cells
• in cerebral cortex of brain
Neuroglia (Glia)
• ~50% vol of CNS
• “glue”
• do not generate or propagate action
potentials
• multiply & divide in mature nervous
systems
• glioma:
– brain tumors derived from glial cells
– very malignant, grow rapidly
Glial Cells of the CNS
1.
2.
3.
4.
ASTROCYTES
OLIGODENDROCYTES
MICROGLIA
EPENDYMAL CELLS
Astrocytes
• star-shaped
• largest & most numerous of glial cells
• functions:
1. physically support neurons
2. assist in blood-brain-barrier (bbb)
3. in embryo: regulate growth, migration,
&interconnections between neurons
4. help maintain appropriate chemical
environment for propagation of action
potentials
Oligodendrocytes
• “few trees”
• smaller & fewer branches than
astrocytes
• Functions:
1. form & maintain myelin sheath on
axons in CNS
2. 1 oligo. myelinates many axons
Microglia
• small cells with slender processes
giving off many spine-like projections
• function:
1. phagocytes
– remove cellular debris made during
normal development
– remove microbes & damaged nervous
tissue
Ependymal Cells
• single layer of cuboidal to columnar cells
• ciliated & have microvilli
• function:
1. line ventricles of brain & central canal
of spinal cord
2. produce, monitor, & assist in
circulation of cerebrospinal fluid (CSF)
3. form bbb
Neuroglial Cells of the PNS
• Schwann cells
• Satellite cells
Schwann Cells
• functions:
1. myelinate axons in PNS
– 1 Schwann cell myelinates 1 axon
2. participate in axon regeneration
Satellite Cells
• flat cells that surround cell bodies of
neurons in PNS ganglia
• functions:
1. structural support
2. regulate exchange of materials
between neuronal cell bodies &
interstitial fluid
Myelination
• myelin sheath: made up of
multilayered lipid & protein (plasma
membrane) covering
• function:
1. electrically insulates axon
2. increases speed of nerve impulses
Myelinated & Unmyelinated
Axons
Nodes of Ranvier
• gaps in myelin sheath
• 1 Schwann cell wraps axon between
nodes of Ranvier
Myelin
• amount increases from birth to
maturity
• infant‘s responses slower & less
coordinated as older child or adult in
part because myelination is a work in
progress thru infancy
Demyelination
• loss of myelin sheath
• see in disorders:
– multiple sclerosis
– Tay-Sachs
– side effect of radiation therapy &
chemotherapy
Gray Matter of the Nervous
System
• contains:
– neuronal cell bodies
– dendrites
– unmyelinated axons
– axon terminals
– neuroglia
White Matter of the Nervous
System
• composed of:
– myelinated axons