Transcript Graded Potential - wquerryeducation

Nervous System
& Tissue
• Terms & function
• Neurophysiology
• Synapse & membrane
potential
• Impulse conduction &
processing
• Clinical application
Organizatio
n of the
Nervous
System
Types of Neural Tissue
Neuroglial cell
Functions of the
Nervous System
1. Sensory Input:
sensory receptors gather
information & deliver it to
the CNS
2. Integration:
sensory information used
to create
• sensations
• memory
• thoughts
• decisions
effector organ
3. Motor Output:
decisions are acted
upon & impulses are
carried to effectors
Structure of
a Neuron
• Neurons are excitable
cells that generate &
carry electrical signals
• Structures include:
cell body (soma)
neurofibrils
Nissel bodies
dendrites
axon hillock
axon
axoplasm
axolemma
Structural Classification of Neurons
Functional
Classification
Sensory Neurons
• afferent
• carry impulse to CNS
• unipolar
Interneurons
• link neurons
• multipolar
• only in CNS
Motor Neurons
• multipolar
• carry impulses away
from CNS to effectors
Neuroglial (Glial): support cells of
the nervous system
PNS
Satellite
cells
CNS
Schwann
cells
Oligodendrocytes Astrocytes Microglia Ependymal
phagocytic
barrier b/t
compartments
Myelin sheaths
Regulate chemical
environment
Support for
CNS
BBB
Neurotrophic
factors
Regulate
ion [ ]
Myelination of
Axons via Schwann
Cells in PNS
White Matter:
• myelinated axons
Gray Matter:
• unmyelinated
structures, cell bodies
& dendrites
Peripheral Neuroglial Cells
Four Types of Glial Cells in CNS
Oligodendrocyte
Gated Channels
Control Ion
Permeability
• Nongated (leak) channels
• Chemically gated (ligand)
• Voltage-gated
• Mechanically gated
Resting Membrane Potential
• Electrical disequilibrium: resulting
from uneven distribution of ions
across the cell membrane
• Concentration gradient: based
on 4 ion species: Ca2+, Na+ , Cl- ,
K+ & organic anions A-
• Membrane permeability: K+ is
the principal ion contributing to
the resting membrane potential
- necessary for electrical excitability: neurons, skeletal, cardiac
& smooth muscle
• Resting membrane potential maintained by Na+/K+ pump
Graded Potential: rapid change in membrane
potential propagated along length of cell
Sub-threshold depolarization
Threshold depolarization
Action Potential
Refractory Periods
(graded)
The AP is an All-Or-None Phenomenon
Comparison of Graded Potential & Action Potential
Graded Potential
Action Potential
Type of signal
input signal
conduction signal
Where occurs
dendrites & cell body
trigger zone of axon
Type of channels
mechanical, chemical
voltage
voltage-gated
Ions involved
usually Na+, Cl-, Ca2+
Na+ & K+
Type of signal
depolarizing/hyperpolar
depolarizing
Strength of signal
depends on initial
stimulus
All-or-none phenomena
What initiates signal entry of ions via channel above threshold graded
potential
Other characteristics no threshold, can be
summed, localized D
refractory period, travel
long distances
Concept Check
- Match the ion movement with the type of graded potential it will
create.
(a) Na+ entry
(b) Cl- entry
(c) K+ exit
(d) Ca2+ entry
1. depolarizing
2. hyperpolarizing
- Pyrethrin insecticides disable inactivation gates of Na+ channels
so that they permanently remain open. In neurons poisoned with
pyrethrins what would you predict would happen to the
membrane potential – explain?
Conduction of a
Nerve Impulse in an
Unmyelinated Axon
• Propagation of Action
Potential (AP):
- Nerve impulse: wave of
AP moving along the axon
- AP in one region
stimulates the adjacent region
(self-propagating)
- impulse always propagates
away from its point of origin
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
Saltatory Conduction in a Myelinated Axon
Events Leading to Propagation of Nerve Impulse
(graded)
(trigger zone)
Conduction Velocity
• Speed at which an AP moves
along a nerve or muscle fiber:
1. electrical properties of
cytoplasm
2. PM that surrounds the fiber
3. axon diameter
• Myelin alters electrical properties of the nerve fiber  in
conduction velocity
-
the effective membrane resistance
permits reflexes to avoid dangerous stimuli
abnormal myelin formation contributes to learning & mental disorders
demyelinating disorders: MS, Tay-Sachs & diabetic neuropathy
Concept Check
• Why is conduction of action potentials faster in
myelinated then in unmyelinated axons?
• Comparing graded potentials & action potentials (AP)
which is bigger, travels farthest and which initiates the
other?
• If an axon receives 2 stimuli close together in time,
only one AP occurs – why?
Multiple Sclerosis
Symptoms
• blurred vision
• numbness/ coordination
• disoriented/forgetfulness
• fatigue
• incontinence
• leads to paralysis
Treatments
• no cure
• b interferon (anti-viral drug)
• steroids
• physical therapy
Causes
• inflammatory response to
oligodendrocytes
• myelin destroyed in
different areas of CNS
(demyelination)
• hard scars (scleroses) form
• nerve impulses blocked
• muscles do not receive
innervation
• related to autoimmune,
pathogens, genetics, damage
to BBB
Synapse
• Junction mediates transfer
of information from one
neuron to the next or from
one neuron to an effector
cell
Neuron of a marine slug
Electrical & Chemical
Synapses
• Gap junctions: protein
channels interconnect the
cytoplasm of adjacent neurons
• Gated ion channels:
channels that open or close &
influence membrane permeability (membrane potential)
- presynapitc terminal
- synpatic cleft
- postsynaptic terminal
- neurotransmitters
http://www.bishopstopford.com/faculties/science/arthur/synapse.swf
Neurotransmitters & Representative Actions
Excitatory &
Inhibitory
Neurotransmission
• Examples:
- cholinergic synapse
- adrenergic synapse
- GABA-ergic synapse
• Cessation of signal
• Neuromodulators
Synaptic Potentials
• Functions to help trigger/inhibit AP distally at axonal
hillock (trigger zone)
- EPSP: excitatory postsynaptic potential
• graded
• depolarizes postsynaptic neuron
• AP of postsynaptic neuron becomes more likely
- IPSP: inhibitory postsynaptic potential
• graded
• hyperpolarizes postsynaptic neuron
• AP of postsynaptic neuron becomes less likely
Summation of EPSPs & IPSPs
• Summation: EPSPs
& IPSPs added together
• The integrated sum
determines whether an
AP results
• More EPSPs lead to
greater probability of
AP
Trigger
zone
Impulse Processing: Neuronal Pools
Convergence:
• neuron receives input from
several neurons
• impulses represent stimuli from
different types of sensory receptors
• allows nervous system to collect,
process, & respond to stimuli
• neuron sums impulses from
different sources
Divergence
• 1 neuron sends impulses
to several neurons
• Amplifying an impulse
• e.g.: impulse from single
neuron in CNS may be
amplified to activate
enough motor units needed
for muscle contraction
Injury to Neurons