Transcript Chapter 2: Introduction to Physiology of Perception

THE ROLE OF NEURONS
IN PERCEPTION
Basic Question
• How can the messages sent by neurons
represent objects in the environment?
Basic Brain Structure
• The brain has modular organization
• The sensory modalities have primary receiving areas
• Vision - occipital lobe
• Audition - temporal lobe
• Tactile senses - parietal lobe
• Frontal lobe coordinates information received from the
senses and decides what response to make.
Temporal, Parietal, Occipital Lobes receive and process
incoming information.
Frontal Lobe decides how to respond to information,
what action to take.
CNS and PNS
KW 1-2
Neuron
KW 3-3
Neurons: Transduction &Transmission
• Key components of neurons:
• Cell body
• Dendrites
• Axon or nerve fiber
• Receptors - specialized neurons that respond
to specific kinds of energy
Types of
Neurons
KW 3-7
Neuron (info
flow)
KW 3-6
Transduction
• Transduction is the
transformation of one form
of energy to another.
• Dam uses moving water to
generate electricity.
• The “job” of sensory
receptors.
• Environmental energy into
neural energy.
The neuron on the left that receives stimuli from the
environment has a receptor in place of the cell body.
The neuron on the right consists of a cell body,
dendrites, and an axon, or nerve fiber.
Vision
Hearing
Touch
Smell
Taste
Each of these receptors is specialized to transduce a
type of environmental energy into neural energy.
*’s indicate the place on the receptor where the
stimulus acts to begin the process of transduction.
Neural Communication
Cell body end
of axon
Direction of neural impulse: toward axon terminals
Squid and axon
K&W 4-5
Microelectrodes
KW 47
Recording Neural Signals
• Microelectrodes are used to record from single
neurons.
• Recording electrode is inside the nerve fiber.
• Reference electrode is outside the fiber.
• Difference in charge between them is -70 mV
• This negative charge of the neuron relative to its
surroundings is the resting potential.
Resting Cell Charges
KW 4-10
Basics of Neural Signals
• Neurons are surrounded by a solution
containing ions.
• Ions carry an electrical charge.
• Sodium ions (Na+) - positive charge
• Chlorine ions (Cl-) - negative charge
• Potassium ions (K+) - positive charge
• Electrical signals are generated when such ions
cross the membranes of neurons.
• Membranes have selective permeability.
Figure 2.8 A nerve fiber, showing the high concentration of sodium
outside the fiber and potassium inside the fiber. Other ions, such as
negatively charged chlorine, are not shown.
Fig. 2-17, p. 43
Recording Neural Signals - continued
• Electrical signals or action potentials occur when:
• permeability of the membrane changes
• Na+ flows into the fiber making the neuron more positive
• K+ flows out of the fiber making the neuron more negative
• This process travels down the axon in a propagated
response
Figure 2.7 (a) When a nerve fiber is at rest, there is a
difference in charge of -70 mV between the inside and
the outside of the fiber. This difference is measured by
the meter on the left; the difference in charge measured
by the meter is displayed on the right.
b) As the nerve impulse, indicated by the red band,
passes the electrode, the inside of the fiber near the
electrode becomes more positive. This positivity is the
rising phase of the action potential.
(
C
D
(C) As the nerve impulse moves past the electrode, the
charge inside the fiber becomes more negative. This is
the falling phase of the action potential.
(D) Eventually the neuron returns to its resting state.
Phases of the action potential
K&W
Properties of Action Potentials
• Action potentials:
• propagate their response.
• remain the same size regardless of stimulus intensity.
• increase in rate of firing to increase in stimulus intensity.
• show spontaneous activity that occurs without stimulation.
Soft
Medium
Strong
Response of a nerve fiber to (a) soft, (b) medium, and (c) strong
stimulation. Increasing the stimulus strength increases both the rate
and the regularity of nerve firing in this fiber.
(a) A signal traveling down
the axon of a neuron
reaches the synapse at
the end of the axon.
(b) The nerve impulse
causes the release of
neurotransmitter
molecules from the
synaptic vesicles of the
sending neuron.
(c) The neurotransmitters
fit into receptor sites and
cause a voltage change in
the receiving neuron.
Synaptic Transmission of Neural Impulses
• Neurotransmitters are:
• released by the presynaptic neuron from vesicles.
• received by the postsynaptic neuron on receptor sites.
• matched like a key to a lock into specific receptor sites.
• used as triggers for voltage change in the postsynaptic
neuron.
Steps
KW 5-5
VIDEO: Synaptic Transmission
Importance of Excitation
• Excite cells
• Bring about activity
• Sensation felt
• Muscle moved
Excitation must be balanced
• Nervous system can’t
run on just excitation
• Sometimes better not
to respond
• Role on inhibition
• Calm down the
nervous system
Role of Inhibition
• Provides break for
the nervous system
• Lowers activity levels
• Keeps the brain from
over-excitation, as in
epilepsy
Types of Neurotransmitters
• Excitatory transmitters - cause depolarization
• Neuron becomes more positive
• Increases the likelihood of an action potential
• Inhibitory transmitters - cause hyperpolarization
• Neuron becomes more negative
• Decreases the likelihood of an action potential
Figure 2.12 (a) Excitatory
transmitters cause
depolarization, an increased
positive charge inside the
neuron.
(b) Inhibitory transmitters
cause hyperpolarization, an
increased negative charge
inside the axon. The charge
inside the axon must reach
the dashed line to trigger an
action potential.
E strong
I none
E medium I weak
E medium I medium
E weak
I medium
E none
I strong
Effect of excitatory (E) and inhibitory (I) input on the firing rate of a
neuron. The amount of excitatory and inhibitory input to the neuron is
indicated by the size of the arrows at the synapse. As inhibition
becomes stronger relative to excitation, firing rate decreases, until
eventually the neuron stops firing.
Convergence
• Neurons occur in networks.
• Blend and share information.
• Like a merge on a highway.
• Many lanes into fewer lanes.
• Funnel is another example.
Neural Circuits
• Groups of neurons connected by excitatory
and inhibitory synapses
• A simple circuit has no convergence and only
excitatory inputs.
• Input into each receptor has no effect on the output
of neighboring circuits.
• Each circuit can only indicate single spot of
stimulation.
No convergence
Left: A circuit with no convergence.
Right: Response of neuron B as we increase the number of receptors
stimulated.
Neural Circuits - 2
• Convergent circuit with only excitatory
connections
• Input from each receptor summates into the next
neuron in the circuit.
• Output from convergent system varies based on
input.
• Output of circuit can indicate single input and
increases output as length of stimulus increases.
Convergence with Excitation added
Neuron B now receives inputs form all of the receptors, so increasing
the size of the stimulus increases the size of neuron B’s response.
Neural Circuits - 3
• Convergent circuit with excitatory and
inhibitory connections
• Inputs from receptors summate to determine output
of circuit.
A cell
decides to
fire
Democracy of Cells
K&W 4-
Control over heart
• Sympathetic 
excites
• Parasympathetic 
inhibits
• Work together to
control heart