Transcript Electrical Activity of a Membrane Resting Potential

CHAPTER
An Introduction to Brain
and Behavior Third Edition
4
How Do Neurons Transmit
Information?
Bryan Kolb & Ian Q. Whishaw
PowerPoints prepared by: Paul Smaldino, UC Davis, Department of Psychology
How Do Neurons Transmit
Information?
• Searching for Electrical Activity in the
Nervous System
• Electrical Activity of a Membrane
• How Neurons Integrate Information
• Into the Nervous System and Back Out
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
The Basics: Electricity and Electrical
Stimulation
• Electricity
– A flow of electrons from a body that contains a
higher charge (more electrons) to a body that
contains a lower charge (fewer electrons)
• Negative Pole
– The source of electrons; higher charge
• Positive Pole
– Location to which electrons flow; lower charge
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
The Basics: Electricity and Electrical
Stimulation
• Electrical Potential
– An electrical charge measured in volts; the
ability to do work through the use of stored
potential electrical energy
• Volt
– A measure of a difference in electrical potential
• Voltmeter
– A device that measures the difference in
electrical potential between two bodies
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electron
- - - -- -
Current
Negative pole
(higher charge)
-
- Positive pole
(lower charge)
Difference = Electrical Potential (volts)
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Early Clues that Linked Electricity and
Neuronal Activity
Electrical Stimulation Studies
• Galvani (18th Century)
– Electrical current applied to a dissected nerve
causes the muscle connected to the nerve to
twitch; concluded that electricity flows along
the nerve
– Electrical Stimulation
• Passing an electrical current from the tip of an
electrode through brain tissue, resulting in changes
in the electrical activity of the tissue
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Early Clues that Linked Electricity and
Neuronal Activity
Electrical Stimulation Studies
• Fritsch and Hitzig (Mid-19th Century)
– Electrical stimulation of the neocortex causes
movement (arms and legs)
• Bartholow (1874)
– First report of human brain stimulation
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Early Clues that Linked Electricity and
Neuronal Activity
Electrical Recording Studies
• Caton (Early 19th Century)
– First to attempt to measure electrical currents
of the brain using a voltmeter and electrodes
on the skull
• Electroencephalogram
– Electrical brain graph that records electrical
activity through the skull or from the brain and
represents graded potentials of many neurons
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Early Clues that Linked Electricity and
Neuronal Activity
Electrical Recording Studies
• von Helmholtz (19th Century)
– Flow of information in the nervous system is
too slow to be a flow of electricity
• Nerve conduction: 30-40 meters/second
• Electricity: 3 x 108 meters/second
• It is not the charge but the wave that
travels along an axon (Bernstein, 1886)
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Tools for Measuring a Neuron’s
Electrical Activity
Giant Axon of the Squid
• Much larger in diameter than human axons
– Humans: 1 to 20 micrometers
– Squid: Up to 1 millimeter (1000 micrometers)
• Easier on which to perform experiments
– Used by Hodgkin and Huxley in the
1930s and 1940s
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Tools for Measuring a Neuron’s
Electrical Activity
The Oscilloscope
• A device that serves as a sensitive
voltmeter
• Used to record voltage changes on an
axon
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
Tools for Measuring a Neuron’s
Electrical Activity
Microelectrodes
• A set of electrodes small enough to place
on or into an axon.
• Can be used to:
– Measure a neuron’s electrical activity
– Deliver an electrical current to a single
neuron (stimulation)
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
How the Movement of Ions Creates
Electrical Charges
• Cations
– Positively charged ions
• Examples: Sodium (Na+), potassium (K+)
• Anions
– Negatively charged ions
• Examples: Chloride (Cl-), protein molecules (A-)
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
How the Movement of Ions Creates
Electrical Charges
• Diffusion
– Movement of ions from an area of higher
concentration to an area of lower
concentration through random motion
• Concentration Gradient
– Differences in concentration of a substance
among regions of a container that allows the
substance to diffuse from an area of higher
concentration to an area of lower
concentration
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Searching for Electrical Activity in
the Nervous System
How the Movement of Ions Creates
Electrical Charges
• Voltage Gradient
– Difference in charge between two regions
that allows a flow of current if the two regions
are connected
• Opposite charges attract
• Similar charges repel
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Resting Potential
• Resting Potential
– Electrical charge across the cell membrane in
the absence of stimulation
– A store of negative energy on the intracellular
side relative to the extracellular side
– Approximately -70 mV
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Resting Potential
• Four charged particles take part in
producing the resting potential
– Sodium (Na+) and chloride (Cl-)
• Higher concentration outside cell
– Potassium (K+) and large proteins (A-)
• Higher concentration inside cell
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Resting Potential
• Maintaining the Resting Potential
– Large A- molecules cannot leave cell: make
inside negative
– Ungated channels allow K+ and Cl- to move
into and out of cell more freely, but gated
sodium channels keep out Na+ ions
– Na+-K+ pumps extrude Na+ from intracellular
fluid and inject K+
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Graded Potentials
• Graded Potential
– Small voltage fluctuation in the cell membrane
– Restricted to the vicinity on the axon where ion
concentrations change
– Can be hyperpolarization or depolarization
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Graded Potentials
• Hyperpolarization
– Increase in electrical charge across a membrane
(more negative)
– Usually due to the inward flow of chloride ions or
outward flow of potassium ions
– Tetraethylammonium (TEA)
• Depolarization
– Decrease in electrical charge across a membrane
(more positive)
– Usually due to the inward flow of sodium
– Tetrodotoxin
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
The Action Potential
• Action Potential
– Large, brief reversal in polarity of an axon
– Lasts approximately 1 millisecond (ms)
• Threshold Potential
– Voltage on a neural membrane at which an action
potential is triggered
– Opening of Na+ and K+ voltage-sensitive
channels
– Approximately −40 mV relative to extracellular
surround
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
The Action Potential
• Voltage-Sensitive Ion Channels
– Gated protein channel that opens or closes only
at specific membrane voltages
– Sodium (Na+) and potassium (K+)
– Closed at membrane’s resting potential
– Na+ channels are more sensitive than K+
channels and therefore open sooner
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
The Action Potential
• Absolute Refractory Period
– The state of an axon in the repolarizing period
during which a new action potential cannot be
elicited (with some exceptions) because gate 2 of
sodium channels, which is not voltage-sensitive, is
closed
• Relative Refractory Period
– The state of an axon in the later phase of an action
potential during which increased electrical current is
required to produce another action potential
– Potassium channels are still open
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
The Nerve Impulse
• Nerve Impulse
– Propagation of an action potential on the
membrane of an axon
– Refractory periods create a single, discrete
impulse that travels only in one direction
– Size and shape of action potential remain
constant along the axon
• All-or-none law
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Saltatory Conduction and Myelin Sheaths
• Myelin
– Produced by oligodendroglia in the CNS and
Schwann cells in the PNS
– Speeds up neural impulse
• Node of Ranvier
– Part of an axon that is not covered by myelin
– Tiny gaps in the myelin sheath
– Enables saltatory conduction
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Electrical Activity of a Membrane
Saltatory Conduction and Myelin Sheaths
• Saltatory Conduction
– Saltare: “to dance” (Latin)
– Propagation of an action potential at successive
nodes of Ranvier
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
How Neurons Integrate Information
Excitatory and Inhibitory Postsynaptic
Potentials
• Excitatory Postsynaptic Potential (EPSP)
– Brief depolarization of a neuron membrane in
response to stimulation
– Neuron is more likely to produce an action
potential
• Inhibitory Postsynaptic Potential (IPSP)
– Brief hyperpolarization of a neuron membrane
in response to stimulation
– Neuron is less likely to produce an action
potential
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
How Neurons Integrate Information
Summation of Inputs
EPSPs and IPSPs Are Summed
• Temporal Summation
– Pulses that occur at approximately the same
time on a membrane are summed
• Spatial Summation
– Pulses that occur at approximately the same
location on a membrane are summed
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Temporal Summation
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Spatial Summation
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
How Neurons Integrate Information
Summation of Inputs
• The Role of Ions in Summation
– The influx and efflux of ions is what is being
summed
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
How Neurons Integrate Information
Voltage-Sensitive Channels and The
Axon Hillock
• The Axon Hillock
– Junction of cell body and axon
– Rich in voltage-sensitive channels
– Where EPSPs and IPSPs are integrated
– Where action potentials are initiated
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Into the Nervous System and Back Out
How Sensory Stimuli Produce Action
Potentials
• Several different sensory modalities
– Visual, auditory, tactile, chemical (taste and
olfaction)
• Many different types of sensory receptors
– Ion channels on their cell membranes
– Example: Stretch-sensitive receptors
• Ion channel on a tactile sensory neuron that activates in
response to stretching of the membrane, initiating a
nerve impulse
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Into the Nervous System and Back Out
How Nerve Impulses Produce Movement
• Motor neurons generate action potentials in
muscle cells to make them contract
• End plate
– On a muscle, the receptor–ion complex that is
activated by the release of the neurotransmitter
acetylcholine from the terminal of a motor
neuron
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Into the Nervous System and Back Out
How Nerve Impulses Produce Movement
• Acetylcholine
– The first neurotransmitter discovered in the
peripheral and central nervous systems
– Activates skeletal muscles
• Transmitter-sensitive channel
– Receptor complex that has both a receptor
site for a chemical and a pore through which
ions can flow
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4
Kolb & Whishaw, An Introduction to Brain and Behavior, Third Edition - Chapter 4