General Biology 101
The Nervous System II
Diagram of synapse (lecture handout) with the following labels:
A. Mitochondria (make ATP)
B. Synaptic vesicles of pre-synaptic neuron
C. Synaptic gap
D. Receptor sites on post-synaptic cell (neuron, muscle or gland)
SYNAPTIC transmission
1. Neurotransmitter is synthesized by the neuron from amino acids in diet.
2. Neurotransmitter is stored (packaged) in synaptic vesicles.
3. Each action potential releases neurotransmitter from vesicles into the synaptic gap.
4. Neurotransmitter molecules diffuse across the gap.
5. Neurotransmitter combines with receptor sites on the post-synaptic cell.
6. Sodium gates are opened in the post-synaptic cell, which fires if depolarization is > firing threshold.
7. Deactivating enzymes in gap remove any neurotransmitter they find on receptor sites.
8. Sodium gates on post-synaptic cell close again.
9. Reuptake of unused neurotransmitter in gap by pre-synaptic cell.
SOME MECHANISMS OF DRUG ACTION
(at excitatory and inhibitory synapses)|
At excitatory synapses |
Step |
Mechanism |
|
Tranquilizer A |
2 |
Blocks packaging of NT into synaptic vesicles; if NT can’t be transported to gap, it can’t be released |
|
Tranquilizer B |
5 |
Structure like the NT, so competes with NT at receptor sites, but without opening Na+ gates |
|
Amphetamines ("speed") |
6 |
Structure so similar to NT that it opens Na+ gates |
|
Nerve gas, antidepressants, insecticides |
7 |
Block the action of deactivating enzyme |
|
Cocaine |
9 |
Blocks reuptake of NT from gap, so more left in gap to stimulate the post-synaptic cell |
|
At inhibitory synapses |
|
|
|
Valium (reduces anxiety) |
6 |
Helps inhibitory NT bind to post-synaptic cell |
|
LSD |
|
Structure similar to inhibitory NT; competes with NT for receptor sites without opening Na+ gates |
PERIPHERAL PROCESSING OF VISUAL INPUT
A. Image of object focused on a corresponding area of retina by cornea and lens
B. 3-layered retina = photoreceptors--> bipolar cells --> ganglion cells, whose axons make up the optic nerve. Because 125 million photoreceptor cells converge into the 1 million ganglion cell, the brain does not get a "point-by-point" (TV-like) picture of the retina.
C. Neurons in the human visual cortex have "receptive fields"; that is, the neuron fires only if the image falling on the retina produces a certain pattern of light in its region of the retina; these neurons act like edge "detectors", with specific positions, orientations and motions.
D. Optical illusions = brain puts pieces together according to the rules it has learned to expect from the brain’s past experience with the visual world.
E. Costs and benefits of peripheral analysis
1. decrease number of (metabolically costly) neurons needed
2. increase speed of processing and response
3. can be fooled by unnatural stimuli
LATERAL SPECIALIZATION IN THE CEREBRAL HEMISPHERES
Sperry's experiments with "split-brain" patients
Observation 1: Patient's R-hand doesn't recognize an object that L-hand has just handled, and vice versa. Explanation: Sensory-motor connections are contralateral.
Observation 2: Patient can name an object held in his R-hand but not in his L-hand. Explanation: Language control is in L-hemisphere only.
Observation 3: If flash two different words simultaneous in the R and L visual field:
(a) R-hand recovers object named in R-visual field, and can speak the object's name, and
(b) L-hand recovers object named in the L-visual field, but can not say the name.
Explanation: (a) Connections are contralateral between brain and visual fields, and
(b) only L-hemisphere has language control.
Observation 4: Flash a split face on screen, then ask patient to point out the face he saw from among an array of several faces. (a) If told to do this using L-hand, patient points quickly, without speaking. (b) If told to use R-hand, patient gives a running verbal commentary and identifies face by process of elimination or other reasoning.
Explanation: R and L hemispheres (a) process sensory information differently, and (b) remember different things about the same sensory input. Namely:
|
Left hemisphere |
Right hemisphere |
|
analytical, logical |
intuitive |
|
remembers namable features |
recognizes "gestalt" (whole) |
|
language control |
mute (silent) |
Normally the two hemispheres are connected via the 200,000 axons of the corpus callosum,
so their functions are integrated.