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1/1/2016
1
Neurons
Chapter 11
The Nervous System
The Nervous System
The nervous system is responsible for all of our behaviors, memories, & movements
The nervous system…
• Cooperates with endocrine system
• Senses environment
• Responds to changes in environment
• Maintains homeostasis
Functions of the Nervous System
1. Sensory input
– Information gathered by sensory receptors about internal and external changes
2. Integration
– Interpretation of sensory input
3. Motor output
– Activation of effector organs (muscles and glands) produces a response
Works through rapid and specific
electrical and chemical signals
to produce immediate responses
Sensory input
Motor output
Integration
Organization
• Central Nervous System
– Brain
– Spinal Cord
Organization
• Peripheral Nervous System
– Afferent (sensory)
– Efferent (motor)
• Somatic
• Autonomic
– Sympathetic
– Parasympathetic
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Copyright 2009 John Wiley & Sons, Inc. 7
Major Structures of the Nervous System
Central nervous system (CNS)
Brain and spinal cord
Integrative and control centers
Peripheral nervous system (PNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Parasympathetic
division
Conserves energy
Promotes house-
keeping functions
during rest
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Sensory (afferent) division
Somatic and visceral sensory
nerve fibersConducts impulses from
receptors to the CNS
Somatic nervous
system
Somatic motor
(voluntary)Conducts impulses
from the CNS to
skeletal muscles
Sympathetic division
Mobilizes body
systems during activity
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Somatic sensory
fiber
Visceral sensory fiber
Motor fiber of somatic nervous system
Skin
StomachSkeletal
muscle
Heart
BladderParasympathetic motor fiber of ANS
Sympathetic motor fiber of ANS
Histology of Nervous Tissue
• Two principal cell types1. Neurons
• Excitable cells that transmit electrical signals
2. Accessory cells (neuroglia)• Non-excitable supporting cells
� Only about 20% empty space!
Neurons and Nerves
• Neurons
– Individual nerve cells
• Nerves
– Parallel bundles of neurons carrying
impulses in the PNS (called tracts in the CNS)
• Types– Sensory (afferent)
– Motor (efferent)
– Association (interneurons)
Nerves
� Endo = within
� Peri = next to; around
� Epi = above
Neurons
• Special characteristics of neurons
– Long lived
– Amitotic
– High metabolic rate
– Excitable
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The Structure of a Neuron
• Body
• Dendrites
• Axon (only one)
Copyright © 2010 Pearson Education, Inc.
Figure 11.4 Structure of a motor neuron.
Dendrites
(receptiveregions)
Cell body
(biosynthetic centerand receptive region)
Nucleolus
Nucleus
Nissl bodies
Axon
(impulsegeneratingand conductingregion)
Axon hillock
NeurilemmaTerminalbranches
Node of Ranvier
Impulsedirection
Schwann cell(one inter-node)
Axon terminals(secretoryregion)
Dendriticspine
Neuron cell body
(a)
(b)
The Structure of a Neuron
• Unipolar neurons– Found mostly in ANS, although some in CNS
The Structure of a Neuron
• Pseudounipolar neurons– Found chiefly in PNS as sensory neurons
The Structure of a Neuron
• Bipolar neurons– Rare, but found in the special sense organs (retina, olfactory mucosa)
The Structure of a Neuron
• Multipolar neurons– Most common type in humans (>99%)
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Accessory Cells
• Neuroglia
• About 50% of cellular mass in nervous tissue
• Do not conduct impulses
• Most retain capacity to divide
• Different types found in PNS and CNS
Accessory Cells
• Neuroglia of the CNS
1. Astrocytes - support
2. Oligodendrocytes - myelin
3. Microglial cells - defense
4. Ependymal cells - CSF
Accessory Cells
• Astrocytes
– Most abundant of CNS neuroglia
– Contact blood vessels
– Blood Brain Barrier (BBB) regulates passage of
molecules
Figure 11.3a
Astrocytes are the most abundant of
CNS neuroglia. Provide most structural support
Capillary
Neuron
Astrocyte
Accessory Cells
• Oligodendrocytes
– Branched cells
– Processes wrap around CNS neurons
• Form insulating myelin sheaths
Figure 11.3d
Oligodendrocytes have processes that form
myelin sheaths around CNS nerve fibers.
Nerve
fibers
Myelin sheath
Process of
oligodendrocyte
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Myelin insulates neuronal axons, much the way
insulation protects wires.
Accessory Cells
– Ependymal cells
• Vary in shape from squamous to columnar
• Many are ciliated
• Line the central cavities of the brain and spinal column
• Form cerebrospinal fluid
• Separate the CNS interstitial fluid from the cerebrospinal fluid
in the cavities
Figure 11.3c
Brain or
spinal cord
tissue
Ependymal
cells
Fluid-filled cavity
Ependymal cells line cerebrospinal
fluid-filled cavities.
Accessory Cells
• Microglia– Small, ovoid cells with thorny processes
– Migrate toward injured neurons
– Phagocytize microorganisms and neuronal debris• Why do you think this would be so important?
Add these to your list!
Figure 11.3b
(b) Microglial cells are defensive cells in
the CNS.
Neuron
Microglial
cell
Accessory Cells
• Neuroglia of PNS
1. Satellite cells
• Surround neuron cell bodies in the PNS
2. Schwann cells
• Surround peripheral nerve fibers and form myelin sheaths
– Axon + myelin = nerve fiber
• Vital to regeneration of damaged peripheral nerve fibers
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Figure 11.3e
Satellite cells and Schwann cells (which
form myelin) surround neurons in the PNS.
Schwann cells
(forming myelin sheath)
Cell body of neuronSatellite
cells
Nerve fiber
Accessory Cells
• Schwann cells cont.
– Myelin sheath
– Neurilemma
– Nodes of Ranvier
Figure 11.5a
(a) Myelination of a nerve
fiber (axon)
Schwann cell
cytoplasm
Axon
Neurilemma
Myelin sheath
Schwann cellnucleus
Schwann cell
plasma membrane
1
2
3
A Schwann cell
envelopes an axon.
The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
The Schwann cell
cytoplasm is forced from
between the membranes.
The tight membrane
wrappings surrounding
the axon form the myelin
sheath.
Multiple Sclerosis
• Autoimmune disease of young adults
• Variety of clinical signs
• No known cure
Antibodies produced in multiple sclerosis attack
myelin made by oligodendrocytes, and lead to
demyleination in the CNS.