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The Physical Basis of Neuronal Function
Major communication systems coordinate
parts of animals body
1.Neuronal system: Rapid & Short Burst
2.Endocrine system: Slow & Persistent
Nervous System
• CNS (central nervous system):
– Brain
– Spinal cord
• PNS (peripheral nervous system):
– Cranial nerves (from brain)
– Spinal nerves (from spinal cord)
• 2 Types of cells in nervous system:
– Neurons
– Supporting cells
Neurons
Neurons (nerve cell): major players in the nervous
system; are specialized cells that receive information,
process information and transmit the information to other
cells;communicate information using combination of
electrical and chemical signals.
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•
•
•
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Structural and functional units of the nervous system.
Respond to physical and chemical stimuli.
Produce and conduct electrochemical impulses.
Release chemical regulators.
Can not divide by mitosis.
Neurons
Most neurons are morphologically asymmetric cells
3 Principal regions:
• Soma (Cell body):
– Contains the nucleus, metabolic
maintenance
• Dendrites:
– Provide receptive area.
– Transmits electrical impulses to
cell body.
• Axon:
– Conducts impulses away from
cell body.
Classification of Neurons
•Functional:
– Based upon direction
impulses conducted.
– Sensory or afferent:
• Conduct impulses
from receptors to
CNS.
– Motor or efferent:
• Conduct impulses out
of CNS to effector.
– Association or
interneurons:
• Located entirely
within the CNS.
Typically, a region of neuron
membrane called spike initiation
zone integrates signals from many
input neurons to determine whether
the neuron will initiate its own
signals, called action potential.
Neurons transmit electrical signals
without loss of signal strength (active
electrical properties)
Neurons possess various type of ion
channels
Information is typically carried
through a neuronal circuit via
electrical action potential
alternating with chemical
siganls
1.Afferent neurons
2.Synapses
3.Efferent neurons
4.Interneurons
5.Neuronal circuit
6.Presynaptic
7.Postsynaptic
Membrane potential (Vm): The electronic potential measured
from within a cell relative to the potential of the extracellular
fluid, which is by convention considered to be zero; the potential
difference across a membrane.
Resting potential (Vrest): The normal membrane potential of a
cell at rest.
Electrochemical Potential
• Theoretical voltage produced across the membrane if only 1 ion
could diffuse through the membrane.
• Magnitude of difference in charge on the 2 sides of the
membrane.
Membrane Potential
• Proteins and phosphates are negatively
charged at normal cellular pH.
• These anions attract positively charged
cations that can diffuse through the
membrane pores.
• Membrane more permeable to K+ than Na+.
– Concentration gradients for Na+ and K+.
• Na+/ K+ATP pump 3 Na+ out for 2 K+ in.
• All contribute to unequal charge across the
membrane.
Resting Membrane Potential
• The normal membrane potential of a cell at
rest, lies -30 and -100 mV.
• Cell membrane is selectively permeable to
some ions
• Unequal distribution of inorganic ions
between cell interior and cell exterior.
• Any ions that cannot cross the membrane
have no effect on the membrane potential.
Fig. 3-21, p.96
Nernst Equation
• Membrane potential
that would exactly
balance the diffusion
gradient and prevent
the net movement of a
particular ion.
• Equilibrium potential
for K+ = - 90 mV.
• Equilibrium potential
for Na+ = + 60 mV.
Fig. 3-23, p.99
Voltage Gated Ion Channels
• Changes in membrane
potential caused by ion
flow through channels.
• Specific ion channels
for Na+ and K+.
• Passive channels are
always open.
• Voltage gated channels
open in response to
change in membrane
potential.
Membrane Permeability
• AP followed by
increase in Na+
diffusion.
• After short delay,
increase in K+
diffusion.
Action Potential (AP; nerve impulse,
spike):
Transient all-or-none reversal of a
membrane potential produced by
regenerative inward current in
excitable membranes.
Three key elements for AP
1. Active transport
2. Unequal distribution of ions
3. Electrochemical gradient
Overshoot:
The reversal of membrane potential during action potential;
the period of time during which cell becomes inside-positive.
After-hyperpolarization (undershoot):
The transient period at end of of action potential when Vm is
more negative than Vrest.
Electrical Activity of Axons
• Depolarization:
– Potential difference
reduced (become more
positive).
• Repolarization:
– Return to resting
membrane potential
(become more negative).
• Hyperpolarization:
– More negative than
resting membrane
potential.
Action Potentials (AP)
• Stimulus causes depolarization to threshold.
• Voltage gated (VG) Na+ channels open.
– Electrical and chemical (electrochemical)
gradients inward.
– + feedback loop.
• VG K+ channels open.
– Electrochemical gradients outward.
– - feedback loop.
• Changes in membrane potential constitute
AP.
Action Potentials (AP)
• Once AP completed, Na+ / K+ ATPase pump
extrude Na+, and recover K+.
• All or none:
– When threshold reached, maximum potential
change occurs.
• Coding for Intensity:
– Increased frequency of AP indicates greater
stimulus strength.
Conduction of Nerve Impulses
1. Cable properties:
–
–
Ability of neuron to transmit charge through
cytoplasm.
High internal resistance.
2. An AP does not travel down the entire
axon.
3. Each AP is a stimulus to produce another
AP in the next region of membrane with
VG channels.
Refractory Periods
• Absolute refractory
period:
– Axon membrane is
incapable of producing
another AP.
• Relative refractory
period:
– Axon membrane can
produce another action
potential, but requires
stronger stimulus.
Threshold
stimulus: The
minimum stimulus
energy necessary to
produce a detectable
response or an allor-none response
50% of the time.
Accommodation
Temporary increase in threshold that develops
during the course of a stimulus
Phasic response:
The response of a neuron that,
when stimulated continuously
by a current of constant
intensity, accommodates rapidly
and generates action potentials
only during the beginning of
the stimulus period
Tonic response:
The response pattern of
neurons that, when stimulated
continuously by a current of
constant intensity,
accommodate slowly and fire
repetitively with gradually
decreasing frequency.
Molecular structure of voltage-gated K+ channel
•Oligomeric complex of four monomeric a-subunits
•Four b-units associated with each a-subunit
•Each a-subunit (70 kD) has six transmembrane domains
•S4 is voltage sensor
Molecular structure of voltage-gated Na+ channel
•One large a-protein (~260 kD)
Molecular Structures of Potassium Channel
Molecular Structures of Sodium Channel