1 Psychology 304: Brain and Behaviour Lecture 11

1

Psychology 304: Brain and Behaviour

Lecture 11

2

From a classmate ....

An amusing YouTube clip regarding brain structures:

http://www.youtube.com/watch?v=fh5hjbQWQ78&feature=related

3

The Cells of the Nervous System and The Generation of Electrochemical Neural Signals

1. What are glial cells? (continued)

2. What is the neuron’s resting potential?

3. What causes a neuron to produce an action potential?

4

2. explain how the resting potential of a neuron is maintained.

By the end of today’s class, you should be able to:

3. distinguish between EPSPs, IPSPs and action potentials.

1. discuss glial-mediated neural regeneration.

4. describe the electrochemical changes that trigger an action potential.

5

What are glial cells? (continued)

• Oligodendrocytes do not facilitate neural regeneration.

• Schwann cells do facilitate neural regeneration.

6

Myelination of Axons in the CNS by Oligodendrocytes vs. Myelination of Axons in the

PNS by Schwann Cells

7

• A neuron’s membrane potential refers to the difference in electrical charge between the inside and

the outside of the cell.

What is the neuron’s resting potential?

• The membrane potential of a resting neuron is about -70 mV (-50 to -80 mV). Thus, the resting neuron is “polarized.”

8

• Resting neurons are polarized due to the distribution of ions around the neuron’s membrane.

• Sodium ions (Na+), potassium ions (K+), chloride ions (Cl-) and negatively charged protein ions are distributed

unevenly across the neuron’s membrane.

• The ratio of negative to positive charges is greater inside the resting neuron than outside.

9The Resting Neuron

10

• Two processes maintain the unequal distribution of ions across the membrane of resting neurons:

1. The differential permeability of the membrane to ions (most permeable to K+ and Cl-; least permeable to negatively charged protein ions).

2. The action of sodium-potassium pumps (continually exchange three Na+ ions inside the neuron for two K+ ions outside of the neuron).

11

A Sodium-Potassium Pump in a Neuron Membrane

12

What causes a neuron to produce an action potential?

• A neuron produces an action potential or “fires” when it generates and conducts an electrochemical signal.

• A neuron receives electrochemical signals from thousands of adjacent neurons, in the form of “synapses” onto the dendrites or cell body of the target neuron.

13

Electron Micrograph of Synaptic Contact

14

• The terminal buttons release chemicals or neuro-transmitters that bind to receptors on the dendrites or cell body of the target neuron.

• The neurotransmitters can excite or inhibit the target neuron.

15

• Neurotransmitters that excite the target neuron depolarize its membrane, producing excitatory postsynaptic potentials (EPSPs). EPSPs increase the likelihood that the target neuron will fire.

• Neurotransmitters that inhibit the target neuron hyper-polarize its membrane, producing inhibitory postsynaptic potentials (IPSPs). IPSPs reduce the likelihood that the target neuron will fire.

16

• The EPSPs and IPSPs are conducted to an area adjacent to the axon hillock and integrated.

• If the integrated sum of the EPSPs and IPSPs is sufficient to depolarize the membrane to the threshold of activation (-40 to -65mV), an action potential is generated.

17

Neural Integration

18

• An action potential is a momentary reversal of the membrane potential from a highly negative value (e.g.,

-70mV) to a highly positive value (e.g., +50 mV).

19

The Cells of the Nervous System and The Generation of Electrochemical Neural Signals

1. What are glial cells? (continued)

2. What is the neuron’s resting potential?

3. What causes a neuron to produce an action potential?