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 right consists of a cell body, dendrites, and an axon, or nerve fiber.

The neuron on the left that receives stimuli from the environment has a receptor in place of the cell body.

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.

Vision Hearing Touch Smell Taste

Neural Communication

Cell body end of axon

Direction of neural impulse: toward axon terminals

Squid and axon

K&W 4-5

Microelectrodes

KW 4-7

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) 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.

C

D

Phases of the action potential

K&W 4-14

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.

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.

Soft

Medium

Strong

(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.

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.

E strong I none

E medium I weak

E medium I medium

E weak I medium

E none I strong

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.

Left: A circuit with no convergence. Right: Response of neuron B as we increase the number of receptors stimulated.

No convergence

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.

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.

Convergence with Excitation added

Neural Circuits - 3

• Convergent circuit with excitatory and inhibitory connections• Inputs from receptors summate to determine output

of circuit.

A cell decides to fire

K&W 4-21

Democracy of Cells

Control over heart

• Sympathetic excites

• Parasympathetic inhibits

• Work together to control heart