Download Nervous Tissue

NERVOUS SYSTEM
Maintenance of homeostasis
Major regulatory and control center
FUNCTIONS


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Sensing changes (INPUT)
Processing information
(INTEGRATING)
Responding (OUTPUT)
SEAT OF ALL MENTAL ACTIVITY
• CONSCIOUSNESS
• MEMORY
• THINKING
Divisions of the
Nervous System
Central Nervous System
Peripheral Nervous System
Central Nervous System



Involves the brain and spinal cord
Part of the dorsal body cavity
Contains gray and white matter,
and has fluid filled spaces
Peripheral Nervous
System
Cranial and Spinal nerves
 Afferent Nervous System - input
 Efferent Nervous System output

• somatic (voluntary) NS
• autonomic (involuntary) NS
•sympathetic and
parasympathetic divisions
Cells of the Nervous
System
Neuroglial Cells
Neurons
Neuroglial Cells

Found in CNS
• Astrocytes
• Star-shaped;
most abundant;
anchor to blood
vessels
• Microglia –
• Protect
Neuroglial Cells

Found in CNS
• Oligodendrocytes
• Produce
insulating myelin
sheath in the CNS
• Ependymal
• Line cavities
• Circulate
cerebrospinal
fluid
Neuroglial Cells
 Found
in PNS
• Schwann cell
•Form myelin sheath in PNS
cells
• Satellite cell
•Surround cell
•Function relatively unknown
Neuron Structure
Cell body (Soma)
 Processes

• Dendrites
• Receiving branches
• Axon
• Sending branch
Classifications of
Neurons
Structure
Function
Area served
Neurons by structure
Pseudounipolar (unipolar) – one
process
 Bipolar – one dendrite & one axon
 Multipolar
• 3 or more processes; most
common
• 1 axon and 2 or more dendrites

Neurons by function



Afferent neurons - sensory neurons
Efferent neurons - motor or
secretory neurons
Association neurons - interneurons
or intermediate neurons
Neurons by the area
served

Visceral neurons
• serving the internal organs
• visceral afferent, visceral efferent

Somatic neurons
• serving the body wall
• somatic afferent, somatic efferent
Neuron Characteristics




Excitable
Conductive
Transmits
information
Cannot
reproduce

Regeneration
• cell body intact
• presence of
neurolemma
sheath
• regeneration
tunnel aligned
• very, little scar
tissue
Nerves



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Bundles of nerve cell processes in
the PNS
Link the PNS to the CNS
Sensory, motor, or mixed
Connective tissue
• epineurium, perineurium, endoneurium

Tract - bundles of nerve fibers in the
CNS
White / Gray Matter

White matter contains myelinated
processes
• PNS - myelinated nerves
• CNS - myelinated tracts

Gray matter- consists of cell
bodies and unmyelinated
processes
• Nuclei- collection of cell bodies (CNS)
• Ganglia - cell bodies outside CNS
Neurophysiology

Basic electrical principles
• Voltage – measures potential difference
between to charges
• Resistance – hindrance to charge flow
• Insulators – resist charge flow
• Conductors – allow charge flow
• Ohm's law: Current = Voltage/Resistance
• Ions – charged atoms flow across
membranes
Ion channels


Passive channels – always open
Active (gated) channels.
• Chemically-gates channels – open
when combined with appropriate
neurotransmitter
• Voltage-gates channels – open in
response to changes in membrane
potential
Establishing the resting
membrane potential.


The neuron membrane is positively charged
on the outside and negatively
charged on the inside.
Inequality of charged particles occurs only
at membrane. The total number
of positive and negative ions and molecules
inside and outside the cell are equal.
Distribution of charged
particles




Sodium ions (Na+)
Potassium ions (K+)
Chloride ions (Cl-)
Negatively charged
proteins (A-)
Factors that help
establish the RMP.

Differences in the resting permeability
of the plasma membrane to Na+ and
K+ ions –

ATPase, Na+ ion/K+ ion pump

Impermeability of the plasma
membrane to proteins –
Membrane potentials
that act as signals

Communication in neurons and
muscle cells involves changing
the membrane potential.
Factors that change
membrane potential.



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Changing the permeability of the
plasma membrane to any ion.
Changing the concentration of ions
across the plasma membrane.
Depolarization - membrane potential
decreases
Hyperpolarization – membrane
potential increases
Graded membrane
potential.



Short-lived depolarization or
hyperpolarization of the plasma
membrane.
Caused by opening of gated ion channels
in the plasma membrane.
Magnitude of the change in potential is
directly related to intensity of the
stimulus.
Action potential



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Rapid reversal of membrane
potential.
Occurs only in neurons and muscle
cells.
Also called a nerve impulse in a
neuron.
Only axons can generate an action
potential.
Steps in generating an
action potential.


Resting membrane potential.
• Voltage-gated Na+ channels closed.
• Voltage-gated K+ channels closed.
Depolarization.
• Opening voltage-gated Na+ channel in the
axon.
• Influx of Na+ ions results in
depolarization of the axonal membrane.
• Closing of Na+ channels stops the influx
of Na+ ions.
Repolarization.
• Open voltage-gated K+ channel in the
axon.
• Efflux of K+ ions results in repolarization
of the axonal membrane.
• Membrane potential moves back to RMP.
• Na+/K+ ATP pump helps re-establish the
Na+ and K+ ion concentrations inside and
outside the neuron.
Propagation of the
action potential.


An action potential travels away
from its point of origin.
An action potential is selfpropagating. (domino effect)
• Changes in membrane potential in one
section stimulates depolarization in
next section of the membrane
Threshold and the all-ornone phenomenon 


All action potentials are the same
regardless of the strength of
stimulus.
Strong stimuli lead to more action
potentials during a time frame.
Weak stimuli lead to fewer action
potentials during a time frame.
Refractory period.

Absolute refractory period • Time period in which no impulse
can be generated

Relative refractory period –
• Time period in which only a strong
stimulus will generate an impulse
Conduction velocities of
axons 

Speeds up to 100 m/s or more
Influenced by
• Axon diameter: larger = faster
• Myelinated versus unmyelinated
axons
• Saltatory Conduction = impulse
leaps from one node of Ranvier to
the next
Synapse = junctions
between neurons

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Axodendritic – axon to dendrite
Axosomatic - axon to cell body
Axoaxonic - axon to axon
Dendrodendritic – dendrite to dendrite
Presynaptic neurons – conducts to the
synapse
Postsynaptic neuron – conducts away
from the synapse
Synapses
Electrical synapse direct connections allow current to
flow from one cell to the next
Chemical synapse –
use chemical neurotransmitters to
conduct impulses across the
synapse
Parts of a chemical
synapse



Presynaptic neuron's axon terminal
contains synaptic vesicles containing
a neurotransmitter.
Synaptic clefts - Fluid-filled space
between the pre- and postsynaptic
neurons.
Postsynaptic neuron has receptors for
neurotransmitters released from the
synaptic vesicle.
Information flow across
a chemical synapse.


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Action potential opens calcium channels in
the presynaptic membrane.
Synaptic vesicles fuse with the membrane
of the axon terminal and neurotransmitter
is released into the synaptic cleft.
Neurotransmitter binds with receptors on
the postsynaptic membrane.
Ions channels open leading to the
depolarization or hyperpolarization of the
postsynaptic membrane.
Termination of the
neurotransmitter effect.



Neurotransmitter degraded by an
enzyme.
Neurotransmitter taken up by the
presynaptic terminal.
Diffusion of the neurotransmitter
from the synaptic cleft.
Postsynaptic potentials.

Excitatory postsynaptic potential
(EPSP).
• Causes depolarization of postsynaptic
membrane.
• Opens channels allowing Na+ and K+ ions
to cross the membrane.

Inhibitory postsynaptic potential
(IPSP).
• Causes hyperpolarization of postsynaptic
membrane.
• Opens K+ or chloride channels allowing
one of both of these ions to cross the
membrane.
Modification of synaptic
events.

Summation = effects add up
• Temporal summation – rapid
stimulation
• Spatial summation – stimulation
from mutiple presynaptic
terminals

Synaptic potentials – continued
use increases ability to excite
the postsynaptic membrane
Neurotransmitters 

Acetylcholine Biogenic amines.
• Catecholamines.
• Dopamine • Norepinephrine • Epinephrine • Indolamines.
• Serotonin • Histamine -
Neurotransmitters


Amino acids.
• Gamma amino
butyric acid • Glutamate • Glycine • Aspartate Peptides.
• Substance P • Endorphins • Enkephalins -

Novel
messengers
• ATP
• NO
• CO
Neurotransmitters by
function.

Excitatory - cause depolarization

Inhibitory – cause hyperpolarization
SENSORY SYSTEMS
Levels of Sensation


Sensation - is the arrival of a
sensory impulse to the brain
Perception - is the
interpretation of the sensation
Sensory Receptors

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

Simple receptors - General senses
Complex receptors - Special senses
Selectivity
Types by location
• Exteroceptors - outside
• Interoceptors – visceral (more general)
• Proprioceptors – musculoskeletal
(more specific
Receptors classified by
stimulus



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Mechanoreceptor – touch, pressure,
vibration, etc.
Photoreceptor - light
Thermoceptor - temperature
Chemoreceptor - chemicals
Nociceptor – damage / pain
Cutaneous Sensation

Tactile sensations
•
•
•
•
•

touch
pressure
vibration
cold, heat
pain
Crude / Discriminative Touch
Types of Tactile
Receptors
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Meissner’s corpuscles – light touch
Hair root plexuses – light touch
Merkel discs – light touch
Pacinian corpuscles – deep pressure
Itch/tickle
Thermoreceptors - heat
Nociceptors - pain