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Nervous System – General
Nervous System Structure
cells, tissues and organs of body are all working for
organisms survival
every cell in our body responds to stimuli by changing
its metabolism in one way or another
need to integrate all body activities for homeostasis
but cells of the nervous system are highly specialized
for receiving stimuli and conducting impulses to
various parts of the body
need good communication and control:
Nervous System
Endocrine System
in humans, these nerve cells have become organized
into the most complex and least understood of the
body’s systems
Neuroendocrine
System
receptor ! integration ! effector
CNS:
General Functions of the Nervous System
1. receive and process sensory information from
internal and external environment
PNS:
cranial nerves
spinal nerves
two major cell/tissue types in Nervous System:
2. maintain homeostasis by transmitting the
appropriate responses through muscles and glands
neurons – impulse conduction
communicates by:
electrochemical impulses (=nerve impulses)
cell-to-cell chemicals (=neurotransmitters)
3. Integrate rapid reflex responses with slower
hormonal responses
~1 Trillion neurons
generally no mitosis
4. generate complex neural pathways of all higher
brain functions:
neuroglia (=glial cells) – support, protection,
insulation, aid in function of neurons
self awareness
thinking, learning
speech, communication
emotions
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
brain
spinal cord
[need specialized cells because of unique sensitivity of
neurons to their environment]
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10-50 times more neuroglia than neurons
some mitosis
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
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two types; axons and dendrites
Dendrites
Neurons
shorter; branching
highly specialized to:
receptor regions
respond to stimuli
conduct messages in the form of nerve impulses
! each neuron receives info from dozens to 10’s of
1000’s of other neurons
generally don’t divide after birth
specialized for information collection
!live up to 100 years
(eg. dendritic spines)
thinly insulated
very high metabolic rate:
convey messages toward cell body
require glucose, can’t use alternate fuels
= graded potentials (not nerve impulses)
require lots of O2 – only aerobic metabolism
can’t survive more than a few minutes without
O2
large surface area for reception of signals
from other neurons
all neurons have cell body and 1 or more processes
contain all organelles (except nucleus) as in cell body
Axons
cell body:
each neuron has a single axon
contains: most cytoplasm
nucleus
most organelles
no centrioles (don’t divide)
neurofibrils
long, slender process
up to 3-4 feet long
(eg. motor neuron of toe)
processes:
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
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axons may give off a few collateral branches
(=axon collaterals)
sensory neurons in CNS are multipolar
b. interneurons (association)
sometimes with branching axon collaterals
in CNS
where integration occurs
99% of neurons in body
lots of variation in structure
thick insulation
at terminus, axon branches profusely
c. motor neurons (efferent)
(up to 10,000 branches)
outside CNS
multipolar
all cell bodies of somatic and some autonomic are inside CNS
each branch ends in enlarged bulb
= synaptic knob (=axonal terminal)
2. Structure
has all organelles except rough ER
! gets proteins via microtubules and
microfilaments
a. unipolar (=pseudounipolar)
difference between nerve and neuron:
single short process that splits into two longer
processes that together act as an axon
neuron = individual nerve cell
originate as bipolar neurons
nerve = bundle of axons outside CNS surrounded
by layers of connective tissue
functionally one long fiber carries impulse that bypasses cell
body
the dendrites are considered the receptive branches at the
beginning of the single process
neurons can be classified by:
1. function
2. structure (# of processes)
especially in ganglia of PNS
most are sensory neurons
1. Function
b. bipolar
a. sensory neurons (afferent)
2 processes; 1 axon, 1 dendrite
only in some sense organs
outside CNS, almost all are unipolar, a few are bipolar
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
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Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
5. Schwann cells
eg. olfactory cells in nose, some retinal neurons, sensory
neurons of inner ear
1. Astrocytes
c. multipolar
have numerous branches producing a starlike
shape
!3 processes; 1 axon, many dendrites
most common
largest and most abundant type
most neurons in CNS (interneurons)
! comprise >90% of the tissue in some parts of the brain
also some sensory and some motor neurons in PNS
astrocytes cover the entire brain surface and most
of the nonsynaptic regions of the neurons in
the gray matter of CNS
d. anaxonic neurons
have multiple dendrites but no axons
do not produce action potentials
also most functionally diverse type
found in brain and retina
form supportive framework for nervous tissue
in very general terms, shape is related to
function:
sensory neurons
interneurons
motor neurons
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direct the formation of tight webs of cells around
brain’s capillaries
many unipolar
=blood/brain barrier
& bipolar
because of “irritability” of nervous tissue and
sensitivity to 02, glucose etc neurons are
isolated into their own “fluid compartment”
mostly multipolar
Neuroglia
1. astrocytes
2. microglia
3. ependymal cells
4. oligodendrocytes
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
this blockage of free exchange between
capillaries and tissues is unique for nervous
tissue
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! prevents sudden and extreme fluctuations in
composition of tissue fluid in CNS
ciliated cells ! resemble cuboidal epithelium
line ventricles and spinal canal
! protects irreplaceable neurons from damage
help to produce and circulate CerebroSpinal Fluid
capillaries in brain are much less leaky than
normal capillaries
4. Oligodendrocytes
(CNS)
smaller cells, fewer (up to 15) processes
! tight junctions: materials must pass
through cells
clustered around nerve cell bodies
astrocytes form an additional layer around
these capillaries to further restrict
exchange
each process reaches out to nerve fiber and wraps
around it to produce myelin sheath (electrical
insulation) around neurons in CNS
[myelin=fatty substance]
! astrocytes help regulate flow into CSF
myelin (in CNS and PNS) can be:
small molecules (O2 , CO2 , alcohol) diffuse rapidly
thick = “myelinated fibers”, “white matter”
thin = “unmyelinated fibers”, “gray matter”
larger molecules penetrate slowly or not at all
2. Microglia (CNS)
small macrophage cells
in inflamed or degenerating brain tissue they
carry out phagocytosis of microbes and cellular
debris
3. Ependymal Cells (CNS)
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
(oligodendroglia)
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5. Schwann Cells (PNS)
Multiple Sclerosis
autoimmune disease possibly triggered by a virus in genetically
susceptible individuals
oligodendrocytes and myelin sheaths of CNS deteriorate and are
replaced by hardened scar tissue
occur esp between 20-40 yrs of age
nerve fibers are severed
& myelin sheaths in CNS are gradually destroyed
! short circuits; loss of impulse conduction
affects mostly young adults
common symptoms:visual problems, muscle weakness, clumsiness
eventual paralysis
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
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Synapses
found only in PNS
meeting point between a neuron and any other cells
= synapse
form a segmental wrapping around nerve fibers
each segment is produced by 1 Schwann cell
gaps between cells = Nodes of Ranvier
neurons generally are not directly connected to
each other but are separated by a small gap
form neurilemma and myelin sheath in PNS
neurons
synapses are the functional connection between
neurons and a few other cells (eg. muscles, glands)
outermost coil of Schwann cell with most of
cytoplasm & organelles forms neurilemma
CNS:
neuron ! neuron
PNS:
! only in PNS neurons
sensory cell!neuron
neuron ! neuron
neuron ! muscle fiber [=neuromuscular jct]
neuron ! gland [=neuroglandular jct]
neuron ! epithelial cells
! plays essential role in regeneration of
cut or injured neurons
[CNS neurons don’t regenerate]
each neuron synapses with 1000 – 10,000 axonal
terminals
! ~1 quadrillion synapses in human brain
at birth brain has ~50 trillion synapses
! 1 month later has 1 quadrillion synapses
at synapse the electrical signal is converted to a
chemical signal that must diffuse across the
synapse to have an effect on the next neuron
synapses make neural integration possible
Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4
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! each synapse is a “decision making” device that determines
whether the second cell will respond to the signal from the
first
General Function of a Synapse:
1. nerve impulse reaches end of axon at synapse and
triggers release of chemical (=neurotransmitter)
! exocytosis
2. NT diffuses across synapse and binds to receptor
proteins in cell membrane of target cell
3. triggers response in target cell
whole process takes 0.3 – 5.0 ms
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