Download nervous system - Doctor Jade Main

NERVOUS SYSTEM
FUNCTION
• communication
• regulates & coordinates
organ systems
Endocrine System
• communication system
• shares commonalities &
differences with nervous
system
• endocrine system
– slow to make changes
– changes longer lasting
– adjusts metabolic
operations of other
systems
– growth, maturation, sexual
development, pregnancy
& response to chronic
environmental stress
• nervous system
– faster changes
– short lived changes
– temporary modifications
Neural Tissue
• 3% of total body weight
• most complex organ system
• vital to life
• needed for awareness &
appreciation of life
• basic functional unit-neuron
– specialized to carry
messages
• half the volume of nervous
tissue is glial cells or
neuroglia
– supporting cells
– without glial
cellsneurons would not
function
Nervous System Composition
• Brain
• Spinal cord
• Receptors of
sense organs
• Nerves
– connects organs &
links nervous
system with other
systems
Anatomical Divisions
Central Nervous System
• brain & spinal cord
– center of integration
and control
Peripheral Nervous System
• nervous system outside
brain * spinal cord
• consists of:
– 31 Spinal nerves
» Carry info to &
from spinal cord
– 12 Cranial nerves
» Carry info to &
from brain
Peripheral Nervous System
– Sensory Nervous System
• Sensory & Motor Neurons
– Sensory-conduct impulses from somatic
receptors to CNS
– Motor-conduct impulses from the CNS to
skeletal muscles
– Autonomic Nervous System
• Sensory Neurons
– conducts impulses from autonomic sensory
receptors in visceral organs to the CNS, smooth
muscles, cardiac muscle and glands
Division of Automatic Nervous
System
• Sympathetic
Division
• Parasympathetic
division
• antagonistic
actions
• Sympathetic
– usually speeds
processes
• Parasympathetic
– usually slows
processes
Neurons
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functional unit of nervous system
come in many shapes and sizes
most common-multipolar type
has cell body, perikaryon
or soma surrounded by plasma
membrane
large nucleus containing a
nucleolus
cytoskeleton comprised of
neurofibrils that extend into
extensions of cell
mitochondria, golgi bodies, free &
fixed ribosomes & endoplasmic
reticulum (ER) are also present
clusters of rough ER & free
ribosomes form nissl bodies
lack centrioles
– cannot divide & cannot be replaced
– stem cells exist but are inactive
except in the nose & hippocampus
Neuron Structure
• Dendrites
– highly branched processes
extending from soma
– each branches into fine
processes- dendritic spines
– receive information
• Axons
– long processes
– take information away or send
information to other cells
– capable of propagating electrical
impulses or action potentials
• cytoplasm of axon axoplasmsurrounded by axolemma
• base of axon is initial segment
(tigger zone)
• attached to nerve cell at thickened
region-axon hillock
Axon
• may branch along their length
– side branches or collaterals
allow neurons to communicate
with several cells at same time
• Axon & collaterals ends in fine
extensions or telodendria or axon
terminals which end in synaptic
knobs-filled with synaptic vesicles
• Axons maybe encased in a myelin
sheath
• Between each section of myelin isnode of Ranvier
• Where myelin is locatedinternodes
• Myelinated axons are able to
transmit nerve impulses faster than
non-myelinated axons
Neuron Function
Synapse
• synaptic knob of neuron that is
sending the messagepresynaptic cell makes contact
with postsynaptic cell
• presynaptic cell is always a
neuron
– sends messages
• postsynaptic cell can be
another nerve cell, muscle cell,
gland or an adipocyte
– receives messages
• when neuron meets another
neuron,
• synapse can be on a dendriteaxodendritic,
• cell body-axosomatic
• axon-axoaxonic
Synapses
• small gap-synaptic cleft
separates pre & post
synaptic membranes
• neurons communicate with
other cells with
neurotransmitters
• packaged in synaptic
vesicles found in synaptic
knob
• released when action
potential is propagated
down axon of presynaptic
cell
• postsynaptic side of
synapse contains receptors
Neuron Classification
• Can be classified by structure or
function
• Structure
–relationship of dendrites to cell
body and axon
• Function
–what the neuron does
Structural Classification
Bipolar
– two distinct processes-dendrite and
axon
– cell body lies in between
– found-retina & olfactory epithelium
Unipolar or pseudounipolar
– axon & dendrite are continuous or
fused
– cell body lies off to one side
– initial segment lies where dendrites
converge and the rest of the process
carries an action potential and is
therefore considered the axon
– Dorsal root ganglion cells & sensory
neurons of PNS
Multipolar
– most abundant & most common
– possess two or more dendrites and one
axon
Functional Classification
• Sensory or Afferent
• Motor or Efferent
• Interneuron
Sensory or Afferent Neurons
• cell bodies found in peripheral
sensory ganglia-collection of
nerve cell bodies in PNS
• deliver information from sense
receptors to CNS
• often these are unipolar neurons
• there are about 10 X 106 sensory
neurons in the body
Motor or Efferent Neurons
• cell bodies in CNS
• axons travel away from CNS
to peripheral effectors
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• there are 0.5 X10
• mainly mulitpolar neurons
Interneuron
• most abundant type of neurons by
function-most are multipolar
• 20 billion
• often termed association neurons
• most found in brain & spinal cord
• distribute sensory information &
coordinate motor activity
• one or more can be found between a
sensory & a motor neuron allowing for
reflexes
REFLEX
Neuroglia-Glial Cells
• comprise one half of nervous
tissue
• needed for proper functioning of
nerve cells
• different types are found in CNS &
ANS
Ependymal Cells
line spinal cord &
ventricles
form an ependyma
or an epithelia
cell layer
produce CSF
patches of cilia on
apical surface
help circulate
CSF
Astrocytes
largest & most numerous glial cell in
CNS
create a 3-dimensional supportive
framework for CNS
secrete nerve growth factors to
promote growth & synapse
formation
maintain composition of tissue fluid
cells have extensions or perivascular
feet
contact blood capillariesstimulate
them to
form a tight seal called blood-brain
barrier
• serves to control exchange of
blood products with the brain
– neural tissue must be
physically & biochemically
isolated from general
circulation
Oligodendrocytes & Microglia
Oligodendrocytes
– have slender cytoplasmic
extensions that wrap
around other nerve fibers
forming myelin sheath
– speeds action potential
propagation
– presence of myelin makes
axons appear
whitetermed white matter
Microglia
– least numerous & smallest
neuroglila cell of CNS
– migrate through tissues
engulfing cellular debris, waste
products & pathogens
Schwann Cells
• form sheaths around
peripheral axons of
neurons in PNS
• cell spirals outward as it
wraps nerve fiberending
in thick outer coilneuilemma
• nerve fiber is much longer
than one cell can reachtherefore-many Schwann
cells are needed to
complete myelin sheath
• gaps between cells arenodes of Ranvier
• myelin covered segments
are-internodes
Satellite Cells
• surround neuron
cell bodies in
ganglia of PNS
• regulate exchange
of materials
between nerve cell
bodies & interstitial
fluid
Neurophysiology
• neural activity
begins as a change
in resting
membrane
potential (RMP)
• refers to difference
in electrical charge
between inside &
outside of the cell
membrane
Forces Maintaining Resting
Membrane Potential
• Cl- & Na+ are high
outside
• K+ & negatively charged
proteins are high inside
• inside of cell is negative
with respect to outside
• there are more positive
charged ions outside & a
slight excess of
negatively charged ions
inside
Resting Membrane Potential
• positive & negative
charges attract
• held apart by permeability
of membrane & by active
transport mechanisms
• these conditions set up a
potential difference
– stored energy
– stretched spring
• RMP = potential
difference of an
undisturbed neuron
• measured in millivolts
• -70 mV or -0.07V
CELL MEMBRANE PROPERTIES
• concentrations differences
due to differences in
permeability of cell
membrane to ions & to
active transport
mechanisms
– If membrane was freely
permeable, distribution
of chemicals would
become even
– cell membranes are
selectively permeable
– Ions enter & leave only
through ion channels
TRANSPORT MECHANISMS
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ion movement occurs by leak
channels
easier for K to diffuse out of a cell
than for Na to enter
always open
allows for slow diffusion of K+ out of
the cell & Na into the cell
– to maintain resting potential,
cell must kick Na out & bring K
in
– requires energy or ATP
– Na/K exchange pump uses
carrier protein, Na-K ATPase to
push 2 K+ in for every 3 Na+
pumped out
Na/K ATPase provides energy to
pump ions by splitting a phosphate
group from ATPADP
Sodium is ejected as quickly as it
enters
– keeps resting potential
Sodium-Potassium Pump
Electrochemical Gradient
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sum of electrical & chemical forces acting across a
cell membrane
For Na+
– chemical gradient pushes sodium into cell
– electrical gradient pulls sodium into cell
For K+
– chemical gradient drives K out of cell
– electrical gradient opposes K leaving cell
• K is attracted to negative charges on
inside of cell membrane and repelled by
positive charges outside membrane
electrochemical gradients for K & Na are primary
factors affecting resting potential
refers to stored or potential energy like water
behind a dam
can think of a cell membrane as a dam
– even a small opening will release water under
tremendous pressure
any stimulus increasing membrane permeability to
Na or K will produce sudden & dramatic ion
movement
Stimulusopens Na channelsNa rushes in
stimulus opens doorelectrochemical gradient
does the rest
Ion Channels
• any stimulus increasing membrane
permeability to Na or K will produce sudden
& dramatic ion movement
• changes in RMP occur when channels open
to allow these ions into or out of the neuron
• Leak
• Ligand-gated
• Mechanically gated
• Voltage gated
Leak Channels
–always open
–depend on
electrochemical
gradient
Gated Channels
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Ligand-Gated
– open or close when specific chemicals bind
– most abundant
– found on dendrites & cell bodies of neurons
Voltage Gated
– open or close in response to changes in membrane potential
– restricted to axons of excitable membranes
– most important type
– each has two independently functioning gates: an activation gate-closed in resting
membrane & opens with proper chemical stimulationNa can enter
– inactivation gate-when closedNa stops coming in
Mechanically Gated
– open or close in response to physical distortion of membrane surface
– found on dendrites of sensory neurons especially receptors for touch, pressure,
vibration
GATED CHANNEL
Local Potentials
• any stimulus that opens a gated channel
disrupts resting potential produces a
temporary, localized change in resting
membrane potentialgives rise to a
graded potential
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Local Potentials
when a gated Na channel opens
Na ions enter cell due to attraction to negative charges inside cell & because of
the higher concentration of Na outside the cell
entrance of Na shifts membrane potential to 0 mV
any shift of resting potential toward 0 (or becoming more positive) is called
depolarization
degree of depolarization decreases with distance from opened channel
local currents produce changes that cannot spread far from area of stimulation
cytosol resists ion movement & some entering Na can move back across
through Na channels
at a distance from Na entry pointeffect on membrane potential is
undetectable –decremental conduction
Local Potentials
• local potentials are graded
• spread of current down axon depends on stimulus
• stronger stimulusfurther local potential can be
propagated
• maximum change in membrane potential is
proportional to size of initial stimulus
– determined by number of open Na channels
more open Na channelsmore Na
entersgreater area affected greater
degree of depolarization
• can be excitatory or inhibitory
Opening K Channels
• opening gated K channel has opposite effect
on membrane potential
• as rate of K outflow increases & interior of
cell loses positive ionscell is said to be
hyperpolarized
– membrane potential becomes more negative
• Repolarization
– when stimulus is removed
– cell returns to normal resting potential
Action Potential Generation
• neurons receive information as graded potentials
at dendrites, cell bodies & synaptic terminals
• if graded potentials are large enough action
potential begins electrical impulse is
propagated across surface of membrane and
then down axon to synapse
Action Potential Generation
• when sodium ions enter
cell membrane
depolarizes local
potential begins
• local potential must rise to
a value termed threshold
(about -55mV) for
anything to happen
• Threshold-minimum
voltage needed to open
voltage-regulated gates
• once threshold is reached
neuron fires or produces
an action potential
All-or-None Principle
• Once stimulus depolarizes neuron to
thresholdneuron fires at maximum
voltage
• if threshold is not reachedneuron does
not fire
• above threshold values do not produce
stronger action potentials
• Subthreshold values do not produce an
action potential
Action Potential Steps
• Step 1: Resting State & Depolarization to threshold
• Step 2: Activation of Na channels & Rapid
Depolarization
• Step 3: Inactivation of Na channels & Activation of
K channels-Repolarizaton
• Step 4: Hyperpolarization & return to normal
permeability
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Action Potential Steps
Step 1: Depolarization to threshold
Step 2: Activation of Na channels & Rapid Depolarization
– at threshold voltage-regulated Na gates open quickly sodium rushes into the cellrapid
depolarization
– membrane potential changes from -70mV to more positive value
Step 3: Inactivation of Na channels & activation of K channels
– as membrane potential passes 0 mV, sodium gates are inactivatedbegin to close
– by the time they all close and Na inflow ceases voltage peaks at about +35mV Na
channels close & voltage regulated K channels open
– both electrical & chemical gradients favor movement of K into cell
– sudden loss of positive changes (K+) shifts membrane potential back toward resting
levelrepolarization begins
K gates remain open longer than Na gatesK leaves the cell than Na enters causes
membrane potential to be more negative than original RMP-membrane hyperpolarized
Step 4: Return to normal permeability
Refractory Periods
• from time action potential begins
until normal resting potential is
reestablished, membrane cannot
respond to stimuli
• Refractory Period
• if second stimulus is applied
<0.001 second after the first, will
not trigger an impulse
• membrane cannot respond
• all voltage regulated Na channels
are open or inactivated
• Absolute Refractory period
• Relative refractory period
• time when another action potential
can occur if membrane is
sufficiently depolarized
– requires a larger than normal
stimulus
Action Potential Propagation
• once threshold has been reachedchange in
electrical potential passively spreads along axon
to adjacent regions of membrane from area to
area in series of steps
• at each stepmessage is repeated
• because same events occur over & over this is
called propagation
Propagation Types
• Continuous
–Unmyelinated fibers
• Saltatory Conduction or
Leaping
–Myelinated fibers
Continuous Propagation
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unmyelinated fibers
begins at initial segment of axon
Step 1: membrane potential becomes positive briefly
Step 2: local current develops & spreads in all directions depolarizing
adjacent parts of membrane
– continues in a chain reaction
Steps 3-4: more distant parts of membrane are affected
– action potential moves forward
– cannot reverse
– because previous segment is still in absolute refractory period
speed of propagation is about 2mph which can be increased to 300mph with
myelination-300mph
Saltatory Conduction
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myelinated axon
axolemma is wrapped in myelin sheath
makes continuous conduction impossible
– myelin increases resistance to ion flow
– ions cross best at nodes
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Depolarization occurs only at nodes
– action potential begins at initial segment which produces a local current that skips
internode & depolarizes closest node
– action potential jumps from node to nodesalatory propagation
– moves message faster
– uses less energy
Propagation Review
Nerve Fiber Types
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Type A fibers
– largest & myelinated
– send impulses at 300mph
– carry sensory information to CNS
about body position, balance,
touch and pressure from skin
– motor neurons to skeletal muscles
Type B fibers
– smaller & myelinated
– send impulses at 40mph
Type C fibers
– Unmyelinated
– <2um in diameter
– carry impulses at 2mph
Type B & C carry information to CNS
about temperature, pain, gentle touch
& pressure
carry information to smooth muscle,
cardiac muscle, glands & other
peripheral effectors
Synaptic Activity
• messages are conveyed
from neuron to neuron
via synapses
• presynaptic cell
converges on a
postsynaptic cell
• where pre & post
synaptic cells meet is
synapse
• there are electrical &
chemical synapses
Electrical Synapses
• found in both CNS & PNS
• pre & post synaptic membranes are locked at gap junctions
• integral membrane proteins or connexions possess pores
allowing for ion passage
• changes in membrane potential of one cell produces a
local current in the other since cells share common
membrane
Chemical Synapses
• neurons are not directly
connected
– there is a gap-synaptic cleft
• action potential is transferred
with small chemicalsneurotransmitters
• In electrical synapses action
potential is always propagated
– need not be so in chemical
synapses
• post synaptic cell can be
adjusted to respond more or
less
Excitatory & Inhibitory Action
Potentials
• some neurotransmitters are
excitatory
– depolarize postsynaptic cell
promoting an action potential
• other neurotransmitters are
inhibitory
– hyperpolarize postsynaptic
membranesuppress action
potential
• effect depends on post synaptic
cell receptors -not
neurotransmitter
• ACHdepolarizes most post
synaptic membranes but inhibits
post synaptic membranes in
heart
• synapses can be categorized as
to type of neurotransmitter that is
released at the presynaptic
terminal
Synapse Types Based on
Neurotransmitter
• Cholinergic synapses
– release ACH
– found-all neuromuscular junctions, CNS, at all
neuron-neuron synapses in PNS, & at all
neuroglandular junctions in the
parasympathetic ANS
• Adrenergic synapses
– release norepinephrine
– found in brain & ANS
Other Neurotransmitters
• Gaba or gamma aminobutyric acid
– inhibitory neurotransmitter
– synapses termed GABA-ergic synapses
• Dopamine
– found in CNS
– can be inhibitory or excitatory
– inadequate amounts are found in those with
Parkinson’s disorder
• Serotonin
– found in CNS
– inadequate amounts have been linked to
depression
Release of Neurotransmitter
• Step 1: Action potential arrives at synaptic
knobdepolarizes itopens voltage regulated Ca channels
• Step 2: extracellular Ca enters synaptic knobtriggers
exocytosis of NT into synaptic cleft
• Step 3: NTdiffuses across cleft and binds to receptors on
post synaptic cell which depolarizes postsynaptic membrane
and increases Na permeability
• If enough NT is released post synaptic membrane
depolarizes or hyperpolarizes
Synaptic Delay
• there is a delay of about
0.5ms between arrival of
action potential at synaptic
knob & effect on
postsynaptic membrane
• corresponds to time needed
for Ca influx &
neurotransmitter release
• fewer number of
synapses shorter total
synaptic delay faster
response
• fastest response is a reflex
• has just one synapse
Removal of Neurotransmitter
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Diffusion
Neurotransmitter diffuses out of the synapse
Uptake by Cells
Neuron may reabsorb (reuptake) the NT
Enzymatic Degradation
ACHE-acetylcholinesterase decomposes
ACHacetate + choline
Neural Integration
• Neurons process, store & recall information
• more synapses a neuron hasgreater its
information-processing ability
• neural integration is based on postsynaptic
potentials produced by neurotransmitters
• there are two types
– excitatory post synaptic potentials (EPSP)
– inhibitory post synaptic potentials (IPSP)
EPSPS & IPSPS
• EPSPs
– occur due to opening of
chemically regulated
membrane channels that
depolarize cell
membrane
– graded
– only affect area
immediately surrounding
synapse
• IPSPs
– hyperpolarize post
synaptic membrane
– some due to opening of
chloride gates
– others due to opening of
K channels
– graded and local
Neural Integration
• One EPSP or IPSP
will not result in an
action potential
• Summation is
responsible for
integrating EPSPs &
IPSPs in post
synaptic neurons
• Individual EPSPs &
IPSPs sum in
several ways
Temporal &Spatial Summation
• Temporal summation
– addition of stimuli in rapid
succession
– occurs at single synapse
– every time action potential
arrives vesicles discharge
ACH into synaptic cleft
– each time an action potential
arrivesmore chemically
regulated channels open
degree of depolarization
increases
• Spatial summation
– simultaneous stimuli at different
locations
– multiple synapses are active
simultaneously
– action potentials have
cumulative effect on membrane
potential
Neural Pools
• neurons usually work
in large groups-neural
pools
• groups-a few hundred
to a few thousand
• function depends on
how neurons are
connected- neural
circuit
Neural Circuits
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Simple Series Circuit
Diverging Circuit
Converging Circuit
Reverberation Circuit
Parallel Circuit
Simple Series Circuit
• presynaptic neuron stimulates a single
post synaptic neuron
• which then stimulates another
• and so on and so on
Diverging Circuit
• one nerve fiber
branches & synapses
with several
postsynaptic cells
• each postsynaptic
cells synapses with
several more
Converging Circuit
• several
neurons
synapse on
same post
synaptic
neuron
Reverberation Circuit
• neurons stimulate each other in a liner sequence
• one of the neurons sends an axon collateral back to
first neuron in circuit
– type of positive feedback
• each time a neuron fires it stimulates itself to refire
• once circuit is activatedit continues to stimulate itself
Parallel After Discharge Circuits
• Presynaptic neuron stimulates several
groups of neurons
• each group reconverge on a single
postsynaptic neuron
• differing number of synapses between first
& last neuron causes impulses to have
varying synaptic delays
• last neuron exhibits multiple EPSPs or
IPSPs.
• complex mental processing such as math