Download Nervous System The master controlling and communicating system

Nervous System
The master controlling and communicating system
of the body
3 overlapping Functions
Sensory receptors to monitor changes
inside/outside the body e.g. sensory input
Processes or interprets sensory input,
e.g. Integration
Motor output – response to stimuli
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Nervous System
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Figure 11.1
Organization of the Nervous System
Divided into 2 parts: CNS and PNS
Central nervous system (CNS)
Consists of brain and spinal cord
Integration and command center of the nervous system
Peripheral nervous system (PNS)
Consists of nerves (bundles of axons) that extend from the
brain & spinal cord.
Spinal nerves: carry impulses to and from spinal cord
Cranial nerves: carry impulses to and from brain
Serve as lines of communication
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Peripheral Nervous System (PNS): Two
Functional Divisions
Sensory (afferent) division (“carrying toward”)
E.g. Sensory afferent fibers – carry impulses from
skin, skeletal muscles, and joints to the brain
E.g. Visceral afferent fibers – transmit impulses
from visceral organs to the brain
Motor (efferent) division (“carrying away”)
Transmits impulses from the CNS to effector
organs (muscels and glands)
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Motor Division: Two Main Parts
Somatic nervous system
Somatic motor nerve fibers (axons) that conduct impulses from the CNS to
skeletal muscles
Voluntary nervous system (conscious control of skeletal muscle)
Autonomic nervous system (ANS)
Visceral motor nerve fibers that regulate the activity of smooth muscle, cardiac
muscle, and glands
Involuntary nervous system
Consists of 2 subdivisions:
i) sympathetic
ii) parasympathetic
i and ii usually work in opposition of each other, e.g. one
stimulates, while the other inhibits
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Histology of Nervous Tissue
The two principal cell types of the nervous system
are:
Neurons – the excitable nerve cells that transmit
electrical signals
Supporting cells – smaller cells that surround and
wrap the more delicate neurons
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Supporting Cells: Neuroglia (“Nerve Glue”)
Often called glial cells
Tend to have a smaller size and dark staining nuclei
Outnumber neurons in the CNS 10:1
Make up ½ the mass of the brain
6 types of glial cells: 4 in CNS, 2 in PNS
All 6 types;
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
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Neuroglia in the CNS
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Astrocytes
Most abundant, versatile, and highly branched glial
cells
Radiating processes cling to neurons and their
synaptic endings, and cover capillaries
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Astrocytes
Functionally, they:
Support and brace neurons
Anchor neurons to their nutrient supplies
Guide migration of young neurons
Aid synapse formation
“Mop up” leaked K+ and recapture/recycle
released neurotransmitters
Connected by gap junctions, astrocytes can
themselves respond to nearby nerve impulses.
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Astrocytes
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Figure 11.3a
Microglia Cells
Microglia – small, ovoid cells with spiny processes
Processes touch nearby neurons
Migrate toward neurons when they are in trouble and
“transform” into macrophage-like cell that phagocytize
microorgansims and neural debris
Important because cells of the immune system are denied access
to the CNS
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Ependymal Cells
Often ciliated
They line the central cavities of the brain and spinal cord
forming a barrier between cerebrospinal fluid and the tissue
fluid bathing the cells of the CNS
Range in shape from squamous to columnar
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Oligodendrocytes, Schwann Cells, and
Satellite Cells
Oligodendrocytes – branched cells that wrap
around CNS nerve fibers producing insulating
coverings called myelin sheaths.
Schwann cells (neurolemmocytes) – surround
fibers of the PNS
Satellite cells surround neuron cell bodies with
ganglia
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Oligodendrocytes, Schwann Cells, and
Satellite Cells
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Figure 11.3d, e
Neuroglia in the PNS
2 types of neuroglia in the PNS:
i) Satellite cells: surround neuron cell bodies in the
PNS. Unknown function
Schwann cells: form myelin sheaths around larger
neurons of the PNS
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Neurons (Nerve Cells)
Structural units of the nervous system
Conduct nerve impulses from one part of the body to another
Characteristics:
Extreme longevity- can function for up to 100 years!
Amitotic- lose ability to divide. Can not be replaced
Exceptions: olfactory and hippocampal (memory) cells
High metabolic rate. Require abundant supplies of O2 & glucose
Neurons all have cell bodies that project processes and the plasma
membrane is the site of electrical signalling
PLAY
InterActive Physiology ®:
Nervous System I, Anatomy Review, page 4
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Neurons (Nerve Cells)
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Figure 11.4b
Nerve Cell Body (Perikaryon or Soma)
Contains the usual organelles (e.g. nucleus, nucleolus, etc…) but has no
centrioles (hence its amitotic nature)
Is the major biosynthetic center
Membrane synthesis and free ribosomes & rough E.R. are most
developed in the cell body
Golgi apparatus is highly developed
Some neuron cell bodies are pigmented (lipofuscin)
Has well-developed Nissl bodies (rough ER)
Most neuron cell bodies are located in the CNS where they are protected
by the bones of the skull and vertebral column
Contains an axon hillock – cone-shaped area from which axons arise
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Processes
Armlike processes extend form the cell body
CNS contain both neuron cell bodies and processes
PNS contain only neuron processes
Bundles of processes are called tracts in the CNS
and nerves in the PNS
There are two types of neuron processes: axons
and dendrites
Axons and dendrites differ in their structure and
function of their plasma membranes
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Dendrites of Motor Neurons
Short, tapering, and diffusely branched processes
Hundreds of dendrites clustering close to cell body
All organelles found in the cell body are also found in the dendrites
They are the main receptive, or input, regions of the neuron
Large surface area to receive signals from other neurons
Possess dendritic spines which are close contact (synapses) with other
neurons
Convey in coming messages toward the cell body
Message are usually not action potentials (nerve impulses) but
rather short distance graded potentials
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Axons: Structure
Each neuron has a single axon which arises from the cell body at the
axon hillock then narrow to form a process w/ consistent diameter the
rest of its length
Some axons are 3-4’ long (e.g. lumbar region to great toe)
Long axons are called nerve fibers
If an axon branches, the branches are called axon collaterals and extend
at right angles to the axon
Axons usually branch profusely at their terminus (e.g. 10,000 branches is
common per neuron)
Ends of each branch are called axon terminals, synaptic knobs, or
boutons
The axon is the conducting region of the neuron
It generates nerve impulses and transmits them away from the cell body
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Axons: Function
In motor neurons, the nerve impulse is generated at the junction of the
axon hillock and axon (trigger zone) and conducted along the axon to the
axon terminals which are the secretory regions of the neuron
When impulses reach the terminals, neurotransmitters are released into
the extracellular space
Neurtotransmitters either excite or inhibit neurons (effector cells) with
which the axon is in close contact
Axon has the same organells in the cell body as the dendrites, but it lacks
Nissel bodies and Golgi apparatus (protein synthesis)
Thus,
1) axon depends on cell body for protein and membrane renewal
2) Efficient transport mechanisms to transport these items
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Axons: Function
Movement along axons occurs in two ways
Anterograde movement:
Movement toward axonal terminal
Include replenishing sources from the cell body to the
outlying axon
Retrograde movement:
Movement toward the cell body
E.g. organelles moving back to the cell body for
recycling, communication to the cell body of axon
health, signaling molecules being brought to the cell
body
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Myelin Sheath
Whitish, fatty (protein-lipoid), segmented sheath
around most long axons
It functions to:
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
Axons are always myelinated
Dendrites are always unmyelinated
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Myelin Sheath and Neurilemma: Formation
Formed by Schwann cells in the PNS
Schwann cells:
Cytoplasm is squeezed from between the membrane layers
Possess few proteins in their membranes (good insulators)
Envelopes an axon in a trough
Encloses the axon with its plasma membrane
Has concentric layers of membrane that make up the myelin
sheath
Neurilemma – remaining nucleus and cytoplasm of a
Schwann cell
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Myelin Sheath and Neurilemma: Formation
PLAY
InterActive Physiology ®:
Nervous System I, Anatomy Review, page 10
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.5a–c
Nodes of Ranvier (Neurofibral Nodes)
Gaps in the myelin sheath between adjacent
Schwann cells
They are the sites where axon collaterals can
emerge
PLAY
InterActive Physiology ®:
Nervous System I, Anatomy Review, page 11
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Unmyelinated Axons
A Schwann cell surrounds nerve fibers but coiling
does not take place
Schwann cells partially enclose 15 or more axons
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Axons of the CNS
Both myelinated and unmyelinated fibers are
present
Myelin sheaths are formed by oligodendrocytes
Nodes of Ranvier are widely spaced
There is no neurilemma
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Regions of the Brain and Spinal Cord
White matter – dense collections of myelinated
fibers
Gray matter – mostly soma and unmyelinated
fibers
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Neuron Classification
Classified structurally & functionally
Structural classifications: (grouped according to the # of processes extending
from the cell body)
Structural:
Multipolar: three or more processes
Most common neuron type (99%)
Major type in the CNS
Bipolar: two processes (axon and dendrite)
Axon and dendrite extend from opposite sides of the cell body
Rare, found in special sense organs, e.g. retina, olfactory mucosa
Unipolar: single, short process emerging from cell body
Process divides “T”-like into central & peripheral processes
Found mainly in ganglia to the PNS
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Neuron Classification
Functional: (groups neurons according to the direction in which the nerve impulse travels
relative to the CNS)
Sensory (afferent): transmit impulses from sensory receptors in the skin or internal organs
toward or into the CNS
All are unipolar
Cell bodies are located in sensory ganglia outside of the CNS
Only most distal parts act as receptor sites, with long peripheral processes (e.g.
again, the great toe)
Motor (efferent): carry impulses away from the CNS to the effector organs (e.g. muscles,
glands)
Multipolar
Cell bodies are located in the CNS
Interneurons (association neurons): lie between sensory & motor neurons in neural
pathways
Confined to the CNS
99% of neurons in the body
multipolar
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Comparison of Structural Classes of Neurons
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Table 11.1.1
Comparison of Structural Classes of Neurons
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Table 11.1.2
Comparison of Structural Classes of Neurons
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Table 11.1.3
Neurophysiology
Neurons are highly irritable (responsive to stimuli)
Action potentials, or nerve impulses, are:
Electrical impulses carried along the length of
axons
Always the same regardless of stimulus
The underlying functional feature of the nervous
system
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