The Nervous System
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Lab 8B-BIO 105
Divisions
• Central nervous system (CNS)
– Brain and spinal cord (dorsal body cavity)
– Integration and command center
• Interprets sensory input and dictates motor output
• Peripheral nervous system (PNS)
– Portion of nervous system outside CNS;
brings information from body
– Consists mainly of nerves that extend from
brain and spinal cord
• Spinal nerves to and from spinal cord
• Cranial nerves to and from brain
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Peripheral Nervous System (PNS)
• Two functional divisions
– Sensory (afferent) division
• Somatic sensory fibers—convey impulses from
skin, skeletal muscles, and joints to CNS
• Visceral sensory fibers—convey impulses from
visceral organs to CNS
– Motor (efferent) division
• Transmits impulses from CNS to effector organs
– Muscles and glands
• Two divisions
– Somatic or Voluntary nervous system
– Autonomic nervous system
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Motor Division of PNS: Somatic Nervous System
• Voluntary nervous system
– Conscious control of skeletal muscles
• Conducts impulses from CNS to skeletal
muscle
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Motor Division of PNS: Autonomic Nervous
System
• Involuntary nervous system
– Regulates smooth muscle, cardiac muscle,
and glands
–
Visceral motor nerve fibers
• Two functional subdivisions which work in opposition
to each other
• Sympathetic
• Parasympathetic
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Histology of Nervous Tissue
• Two principal cell types
– Neurons (nerve cells)—excitable cells that
transmit electrical signals
– Neuroglia or Glia – small cells that surround,
wrap, and generally support neurons
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• Neuroglial cells:
– Provide a supportive scaffolding for neurons
– Insulate neurons
– Guide young neurons so they can make
proper connections
– Promote health and growth
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Neuroglia
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Astrocytes (CNS)
Microglial cells (CNS)
Ependymal cells (CNS)
Oligodendrocytes (CNS)
Satellite cells (PNS)
Schwann cells (PNS)
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Astrocytes
• Most abundant, highly branched cell
• Cling to neurons, synaptic endings, and covers
capillaries
• Functions include
– Support neurons and help maintain position
– Play role in exchanges between capillaries and
neurons; help with nutrient supply
– Control chemical environment around neurons
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Microglial Cells
• Small cells with processes that monitor
neurons
• Migrate toward injured neurons
• Can transform to phagocytize
microorganisms and neuronal debris
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Ependymal Cells
• May be ciliated
– Cilia help CerebroSpinalFluid circulate
throughout CNS
• Line central cavities of brain and spinal column
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Myelin Sheath
• Whitish, protein-lipoid segmented sheath around
most long or large-diameter axons
– Myelinated fibers
• Function of myelin
– Protects and electrically insulates axon
– Increases speed of nerve impulse transmission
• Nonmyelinated fibers conduct impulses more slowly
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Oligodendrocytes
• Branched cells that wrap CNS nerve fibers and
form insulating myelin sheaths
• Can wrap up to 60 axons at once
• Have Nodes of Ranvier (gaps in myelin)
•
No neurilemma
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Satellite Cells and Schwann Cells: in PNS
• Schwann cells
– Surround all peripheral nerve fibers and form
myelin sheaths
– Wrap around axon in jelly roll fashion
– One cell forms one segment of myelin sheath
• Similar function as oligodendrocytes
– Vital to regeneration of damaged peripheral nerve
fibers
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• Neurilemma
– Outermost layer of Schwann cell containing
nucleus and most of cytoplasm
– Essential for axon healing and repair
• Nodes of Ranvier
– Myelin sheath gaps between adjacent
Schwann cells
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Neurons
– Structural units of nervous system--form gray
matter of CNS
– Highly specialized cells that conduct impulses
– Extreme longevity
– High metabolic rate—requires continuous supply
of oxygen and glucose
– All have cell body, axon and one or more
dendrites
• Plasma membrane functions in:
– Electrical signaling
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Neuron Cell Body (Perikaryon or Soma)
• Contains nucleus and nucleolus
• Major biosynthetic center
– Synthesizes proteins, membranes, and other
chemicals
– Rough ER is very active; also called nissl bodies
• Synthesizes neurotransmitters
• Plasma membrane receives information from
other neurons
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Dendrites
– short, tapering, diffusely branched processes
– Increase surface area of cell so can receive
messages
• Receptive (input) region of neuron
•
Convey incoming electrical signals toward cell body as graded potentials (short distance signals)
not action potentials
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The Axon
• Generates and transmits nerve impulses along
axolemma (neuron cell membrane) to axon terminal
– Neurotransmitters released into extracellular space
• Either excite or inhibit neurons with which axons are in close
contact
• Carries on many conversations with different neurons at
same time
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Structural Classification of Neurons
• Grouped by number of processes
• Three types
– Multipolar – 3 or more processes
• 1 axon, other processes dendrites
• Most common; major neuron in CNS
– Bipolar – 2 processes
• Rare, e.g., Retina and olfactory mucosa
– Unipolar – 1 short process
– One process –sensory or afferent receptor
– Other process – motor or efferent branch
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Functional Classification of Neurons
• Grouped by the direction in which impulse travels in
relation to CNS
• Sensory or afferent
– Transmit impulses from sensory receptors toward CNS
– Almost all are Unipolar
• Motor or efferent
– Carry impulses away from CNS to effectors
– Multipolar
• Interneurons (association neurons)
– Lie between motor and sensory neurons
– Shuttle signals through CNS pathways; most are entirely within
CNS
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Structure of a Nerve
• Bundle of myelinated and non-myelinated axons
enclosed by connective tissue
• Cell bodies lie in CNS or in ganglia near CNS
• Connective tissue coverings include
– Endoneurium—loose connective tissue that
encloses axons and their myelin sheaths
– Perineurium—coarse connective tissue that
bundles fibers into fascicles
– Epineurium—tough fibrous sheath around a
nerve
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LABWORK
1. Explain organization of nervous system.
2. Describe neuroglia and process of myelin
formation.
3. Describe and identify parts of a neuron and the
different classifications of neurons on models
and microscope slides.
4. Describe and identify structure of a nerve
(microscope slides and models).
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