Human Physiology/The Nervous System

10

a structured record of facts, concepts, and skills that we have acquired. The information in the semantic memory is

derived from our own episode memory, such as that we can learn new facts or concepts from experiences.

There are three main activities that are related to long term memory:

1. Storage

2. Deletion

3. Retrieval

Information for short term memory is stored in long term memory by rehearsal. The repeated exposure to a stimulus

or the rehearsal of a piece of information transfers it into long term memory. Experiments also suggest that learning

is most effective if it is distributed over time. Deletion is mainly caused by decay and interference. Emotional factors

also affect long term memory. However, it is debatable whether we actually ever forget anything or whether it just

sometimes becomes increasingly difficult to retrieve it. Information may not be recalled sometimes but may be

recognized, or may be recalled only with prompting. This leads us to the third operation of memory, information

retrieval.

There are two types of information retrieval:

1. Recall

2. Recognition

In recall, the information is reproduced from memory. In recognition the presentation of the information provides the

knowledge that the information has been seen before. Recognition is of lesser complexity, as the information is

provided as a cue. However, the recall may be assisted by the provision of retrieval cues which enable the subject to

quickly access the information in memory.

Long-term Potentiation

. Long-term potentiation (LTP) is the lasting enhancement of connections between two neurons that results from

stimulating them simutaneously. Since neurons communicate via chemical synapses, and because memories are

believed to be stored within these synapses, LTP and it's opposing process, long-term depression, are widely

considered the major cellular mechanisms that underlie learning and memory. This has been proven by lab

experiments. When one of the chemicals involved (PKMzeta, it will be discussed later) is inhibited in rats, it causes

retrograde amnesia with short term memory left intact (meaning they can't recall events from before the inhibitor was

given).

By enhancing synaptic transmission, LTP improves the ability of two neuron, one presynaptic and the other

postsynaptic, to communicate with one another across a synapse. The precise mechanism for this enhancement isn't

known, but it varies based on things like brain region, age and species. This will focus on LTP in the CA1 section of

the hippocampus, because that's what is well known.

The end result of LTP is a well established neural circuit that can be called upon later for memory.

LTP in the CA1 hippocampus is called NMDA receptor-dependent LTP. It has four main properties.

• Rapid induction

LTP can be rapidly induced by applying one or more brief, high-frequency, stimulus to a presynaptic cell.

• Input specificity

Once induced, LTP at one synapse does not spread to other synapses; rather LTP is input specific. LTP is only

propagated to those synapses according to the rules of associativity and cooperativity.

• Associativity

Associativity refers to the observation that when weak stimulation of a single pathway is insufficient for the

induction of LTP, simultaneous strong stimulation of another pathway will induce LTP at both pathways.

• Cooperativity