Modules 3 & 4 represent biological psychology. Why this approach?
Because everything psychological is simultaneously biological.
The brain: Composed of 2 kinds of cells:
-- GLIAL CELLS, which provide nourishment and structural support
-- NEURONS, which do the active processing
Basic structure of a neuron:
DENDRITES MYELIN SHEATH (increases speed of action
potential)
END BULBS
(OF THE AXON)
CELL BODY AXON
SELECTIVELY (or SEMI-) PERMEABLE
CELL MEMBRANE (not in book)
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<more detail here than your book has>
ACTION POTENTIAL - an electrochemical voltage (+50 mV) which is initially generated in the cell body, and which moves down the axon to its end bulbs.
- different from normal household electricity (which involves mov't of electrons) -- neural voltage changes happen much more slowly.
- voltage generated by the movement of ions (especially K+, Na+, Cl-, but
others too) through GATES (a.k.a. "pores") in the semi-permeable membrane.
These channels can open or close to let ions through, or keep them out.
In its RESTING STATE, a neuron has a charge differential of about -70mV.
When a neuron produces an action potential, the gates at one part of its membrane open, which DEPOLARIZES that area (i.e. makes the charge differential move toward, and then past, zero). This depolarization causes the gates in the adjacent areas of the cell membrane to open which depolarizes that area, etc., etc.
Once the action potential (approx. +50 mV) has been generated, other channels open and a special structure called the “sodium-potassium pump” reestablish the resting potential.
However, before the resting potential is reestablished, there is a brief interval called a REFRACTORY PERIOD (actually a hyperpolarization), during which it's very difficult to get the neuron to fire again.
Graphically, these changes in voltage look like this:
millivolts (mV)
action potential
^
+50 |
|
|
0 |-----------------------------------
|
|
|
-70 |
|------------------------------------> time
resting refractory
potential period
Neurons follow the ALL-OR-NONE law -- either the neuron fires, or it doesn't; it doesn't generate half of an action potential, for instance.
When does the neuron fire? In effect, the cell body acts as a kind of decision-making device that bases its decision on incoming signals from other neurons.
These incoming signals can individually be of one of two types:
EXCITATORY INFLUENCE: an incoming signal that makes it MORE likely that the neuron will fire, or
INHIBITORY INFLUENCE: an incoming signal that makes it LESS likely that the neuron will fire.
Basically, the cell body adds up these influences, balancing one against the other, and if the result exceeds a certain threshold, then the neuron fires.
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How do neurons communicate with one another? In other words, how do they exert either excitatory or inhibitory influence over one another?
Answer: Generally, via NEUROTRANSMITTER ACTIVITY over SYNAPSES.
SYNAPSE: a tiny gap between one of a transmitting neuron‘s end bulbs, and the dendrite of the receiving neuron.
NEUROTRANSMITTER: a special molecule (about 75 are known so far) that’s released out of an end bulb of one neuron and that attaches to the receiving neuron.
Basically, here's how it works:
(1) An action potential is generated down the axon of a neuron. Then, it reaches an end bulb.
(2) The end bulb contains tiny bubbles called VESICLES of neurotransmitter molecules. When the action potential reaches these vesicles, it causes their neurotransmitter molecules to be physically forced into the synapse.
(3) The neurotransmitter molecules travel quickly across the fluid-filled synaptic gap to the cell membrane of the dendrite of the receiving neuron.
(4) The neurotransmitter molecules chemically attach to RECEPTOR SITES in the in the cell membrane of the receiving neuron -- in much the same way that a key fits into the lock made for it.
(5) This action produces either an excitatory or inhibitory influence, depending upon the particular neurotransmitter.
(6) Special enzymes detach the neurotransmitter, and allow it to be reabsorbed by the transmitting neuron. This re-absorption called REUPTAKE.
Graphically, it looks like this:
Action Potential
Vesicles (containing neurotransmitter molecules)
Synapse
Receptor sites on dendrite of next neuron
End bulb
Some neurotransmitters:
ACETYLCHOLINE (ACh) - a neurotransmitter associated with muscular control (among other things).
ENDORPHINS -- (ENDOgenous moRPHINE) -- A group of neurotransmitters associated especially with PAIN and PLEASURE responses.
- function in the same areas of the brain as opiates do
- associated with "runner's high," sexual afterglow, cases
of injury with little sensation of pain.
Sometimes, toxins can interfere with how neurotransmitters work.
- Alcohol is chemically similar to a neurotransmitter called GABA, and mimics its activity.
- Cocaine heightens DOPAMINE activity by blocking its reuptake.
- CURARE -- occupies & block acetylcholine receptor sites, producing paralysis.
Actually, only a few molecules can get from the bloodstream to the brain because of THE BLOOD-BRAIN BARRIER -- a kind of filtration mechanism.
Finally, three classes of neurons in the body:
AFFERENT (a.k.a., SENSORY) NEURONS -- carry messages from the body’s sensors toward the central nervous system (brain & spinal cord)
INTERNEURONS -- the neurons in the central nervous system that do the actual processing.
EFFERENT (a.k.a, MOTOR) NEURONS carry messages from the central nervous system to the body’s muscles.
Most interneurons are in the brain itself, but simple
REFLEXES are processed by interneurons in the spinal cord (this increases
the speed of our simple reflexes).