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Chapter 13

PSY290H5 Chapter Notes - Chapter 13: Synaptic Plasticity, Behavioral Enrichment, Biological Neural Network

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Alison Fleming

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Chapter 13: Memory, Learning, and Development
Part II Neural Mechanisms of Memory
- Neuroplasticity: also called neural plasticity. The ability of the nervous system to change in response to experience
or the environment
- Neuroplasticity is found in virtually all animals, indicating that it is an ancient and vital product of evolution
Memory Storage Requires Neuronal Remodelling
- Charles Sherrington speculated that synaptic alterations might be the basis of learning
- Most theories of learning focus on plasticity of the structure and function of synapses
- Synaptic changes that may store information can be
measured physiologically; the changes can be presynaptic,
postsynaptic, or both
- They can include changes in the amount of
neurotransmitter released and/or changes in the number
or sensitivity of the postsynaptic receptors, resulting in
larger (or smaller) postsynaptic potentials
- Inhibiting inactivation of the transmitter (by altering
reuptake or enzymatic degradation) can produce a similar
- Synaptic activity can also by influenced by inputs from
other neurons, causing extra depolarization or
hyperpolarization of the axon terminals and therefore
changes in the amount of neurotransmitter released
- Long-term memories may require changes in the nervous
system so substantial that they can be directly observed
- New synapses can form (or old synapses may die back) as a result of use; training can also lead to the
reorganization of synaptic connections
- Simply living in a complex environment, with its many opportunities for new learning, produces pronounced
biochemical and anatomical changes In the brains of rats
- In standard studies of environmental enrichment, rats are randomly assigned to one of three housing conditions
1. Standard condition (SC): animals are housed in small groups in standard lab cages. This is the typical
environment for laboratory animals
2. Impoverished condition (IC): animals are housed individually in standard lab cages
3. Enriched condition (EC): animals are housed in large social groups in species cages containing various toys and
other interesting features. This condition provides enhanced opportunities for learning perceptual and motor
skills, social learning, and so on.
- A variety of changes in the brain were linked to environment enrichment. For example, compared with IC animals:
EC animals have a heavier, thicker cortex, especially in somatosensory and visual cortical areas
EC animals have more dendritic branches on cortical neurons, and many more dendritic spines on those
EC animals have larger cortical synapses, consistent with the storage of long-term memory in cortical areas
through changes in synapses and circuits
EC animals have more neurons in the hippocampus because newly generated neurons live longer
EC animals show enhanced recovery from brain damage

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- Standard condition (SC): the usual environment for laboratory rodents, with a few animals in a cage and adequate
food and water, but no complex stimulation
- Impoverished condition (IC): also called isolation condition. An environment for laboratory rodents in which each
animal is housed singly in a small cage without complex stimuli
- Enriched condition (EC): also called complex environment. An environment for laboratory rodents in which animals
are group-housed with a wide variety of stimulus objects
- At the neuronal level, even species that are only remotely related likely share the same basic cellular processes
for information storage
- Invertebrate nervous systems have relatively few neurons; because these neurons are arranged identically in
different individuals, it is possible to construct detailed neural circuit diagrams for particular behaviours and study
the same few neurons in multiple individuals
- Non-associative learning: a type of learning in which presentation of a particular stimulus alters the strength or
probability of a response. It includes habituation
- Habituation: a form of non-associative learning in which an organism becomes less responsive following repeated
presentations of a stimulus
- To be true habituation, the decreased response can’t be due to a failure of the sensory system to detect the
stimulus or due to an inability of the motor system to respond
- Scientists discovered how the sea slug Aplysia learns to habituate to a stimulus
- If you squirt water at the slug’s siphon, the animal protectively retracts its gill
- But with repeated stimulation, the animal retracts the gill
less and less, as it learns that the stimulation represents no
danger to the gill
- Short-term habituation Is caused be changes in the synapse
between the sensory cell that detects the squirt of water
and the motoneuron that retracts the gill
- As less transmitter is released at this synapse, the gill
withdrawal in response to the stimulation slowly fades
- The number a size of synapses can also vary with training in
- For example, if an Aplysia is tested in the habituation
paradigm over a series of days, each successive day the
animal habituates gaster than it did the day before
- This phenomenon represents long-term habituation (as
opposed to short-term habituation), and in this case there
is a reduction in the number of synapses between the
sensory cell and the motoneuron
- Success at studying the more complicated mammalian brain came when researchers probed simple associative
learning: classical conditioning of the eye-blink reflex
- When a puff of air is aimed at the cornea of a rabbit the animal reflexively blinks
- The eye-blink reflex can be classically conditions
- Over several trials, if the air pus (US) immediately follows an acoustic tone (CS), a simple conditioned response
(CR) develops rapidly: the rabbit comes to blink when the tone is sounded
- The neural circuit of the eye-blink reflex is also simple, involving cranial nerves and some interneurons that
connect their nuclei

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- Sensory fibers from the cornea run along cranial nerve V (the trigeminal nerve) to its nucleus in the brainstem
- From there, some interneurons send axons to synapse on other cranial nerve motor nuclei (VI and VII), which in
turn activate the muscles of the eyelids, causing the blink
- Destruction of the hippocampus and the rest of the medial temporal lobes has little effect on the conditioned
eye-blink response in rabbits
- Instead, researchers found that a cerebellar circuit is both necessary and sufficient for eye-blink conditioning
- The trigeminal (cranial nerve V) pathway that carries information about the corneal stimulation (the US) to the
cranial motor nuclei also sends axons to the brainstem
- These brainstem neurons, in turn, send axons called climbing fibers to synapse on cerebellar neurons
- The same cerebellar cells also receive information about the auditory CS by a pathway through the auditory nuclei
- So information about the US and CS converges at the cerebellum
- After conditioning, the occurrence of the CS the tone has an enhanced effect on the cerebellar neurons, so
they now trigger an eye blink even in the absence of an air puff
Synaptic Plasticity Can Be Measured in Simple Hippocampal Circuits
- Modern ideas about synaptic plasticity have their origins in the theories of Donald Hebb, who proposed that
when a presynaptic and a postsynaptic neuron were repeatedly activated together, the synaptic connection
between them would become stronger and more stable
- “the cells that wire together fire together”
- These Hebbian synapses could then act together to store memory traces
- Hebbian synapses: a synapse that is strengthened when it successfully drives the postsynaptic cell
- Researchers in the 1970s discovered an impressive form of neuroplasticity in the hippocampus that appeared to
confirm Hebb’s theories about synaptic changes
- In experiments like theirs, electrodes are placed within the rat hippocampus, positioned so that the researchers
can stimulate a group of presynaptic axons and immediately record the electrical response of a group of
postsynaptic neurons
- Normal, low-level activation of the presynaptic cells produces stable and predictable excitatory postsynaptic
potentials (EPSPs)
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