We would like to tackle the very delicate subject of memory, firstly by proposing a review of the available knowledge, and then by providing explanations of how neurons work. This part therefore deals with the theory. The 'tips' section provides practical advice on how to maintain your memory.
Memory is subject to physiological aging. Dr. Crook has run several Memory Assessment Clinics in the US for the past 20 years. Dr. Crook has also led a memory task force with the US National Institutes of Health. The work of these institutes has pioneered a non-pathological definition of Age Associated Memory Impairment (A.A.M.I.) validated as a clinical entity in geriatrics. This definition describes "memory loss that may occur in healthy older people in the later years of life".
There is now considerable evidence (Crook, Wesnes) of age-related declines in attention, working memory (the ability to temporarily hold information in mind), and secondary memory (the ability to retrieve previously learned information). In healthy, active individuals, these deficits may begin in late middle age and may become more pronounced in the 6th, 7th and 8th decades.
Databases have been developed from several series of studies analysing populations of between 1000 and 2500 people aged between 18 and 90 years. These studies involve batteries of tests focusing on the state of memory in daily life and have made it possible to establish a normative tool for measuring and comparing memory capacities by age group.
Data from neuropsychological tests simulating an effort of memory abilities in everyday life: the memory of people's names immediately or one hour after their presentation. Ability on this type of memory exercise declines with age.
Dr Crook also recognised the potential importance of such declines in everyday life: "The effects of secondary memory decline are non-trivial and could severely compromise the ability of individuals to function in intellectual activities and employment situations. Behavioural deficits resulting from memory impairment could extend beyond simulated tests of memory in everyday life, such as name and face recognition after introduction ... memory is a key determinant of fluid intelligence ... remembering is an essential component of problem solving, concept formation and insightful decision making. The practical impact of memory impairment could be profound and widespread in some individuals. "More information is available in the studies tab.
Memory is an incredibly complex process that involves many specialised areas in the brain: visual area, auditory area, encoding areas, storage areas, recall area etc...
Nerve cells organised in networks continuously exchange signals for the encoding, storage and recall of information. We will focus on the cellular aspect of signal exchange between neurons. Generally speaking, we can assume that memory is based on the passage of chemical and electrical information across a cell membrane.
Flexible, fluid and dynamic cell membranes are essential:
- the conditions for stable cell life, with the membranes delimiting the inside and outside of the cells.
- cell metabolism: tens of thousands of chemical reactions take place on both sides of the membranes. This intense cellular life requires the optimal functioning of the receptors associated with the membranes.
Diagram of a neuron
In interaction with its environment, the neuron receives and emits information of an electrical or chemical nature.
Diagram of a cell membrane
The phospholipid bilayer is shown in blue. The interior of the cell is shown in yellow. The trans-membrane receptors are shown in purple and green. The electrical or chemical messengers.
Phospholipids are essential components of all cell membranes. These constituents have a double polarity: a pole of attraction and a pole of repulsion towards water molecules. Phospholipids are assembled in two layers to form a network called the "lipid bilayer", making the membrane impermeable.
Detail of a cell membrane
The trans-membrane receptors are embedded in the membrane.
Ageing, stress and oxidation damage these delicate membranes. This alteration, associated with ageing, leads to a degradation of cognitive abilities and memory. With age, the cell membranes of neurons tend to become rigid, impairing exchanges between neurons.
The loss of membrane quality (shown below) leads to :
- Incorrect positioning of message receptors within the membranes.
- Less good transmission of information between neurons.
Drawing of a damaged membrane
When the quality of the phospholipid bilayer is impaired and low in phosphatidylserine, the trans-membrane receptor (in purple) is out of alignment. Consequence: loss of quality in the reception of the message.
We have diagrammed the effect of ageing on the neurons involved in memory. The natural tendency is to slow down the exchange processes.
In the background :
Representation of networks of neurons interacting with each other at the level of synaptic contacts. The synapses are represented in the manner of "suction cups"!
In the foreground: in blue, detail of a transmitter neuron, with :
-top: its electrically conductive fibre (axon) and then its synaptic saterminisin, described as a mushroom or flower bud, with the release of neurotransmitters (de-escalated into bags of blue beads) captured by the receiving neuron.
-bottom: detail of the membrane of a receptor neuron. The membrane phospholipids are disorganised. Low phosphatidylserine content (phospholipid species shown in red, near the purple receptors)
Consequence: loss of reliability in the reception of the message.
A weakening of exchanges between neurons is observed, particularly at the level of chemical messengers, the neurotransmitters:
Details of synaptic transmission
The different neurotransmitter species (chemical messengers) are represented as green or blue bead bags.Release of the message by the transmitter (pre-synaptic) neuron and reception by the post-synaptic neuron.
The decline in memory associated with ageing is associated with a loss of efficiency in the transmission of several neurotransmitters.
1. ACETYLCHOLINE, a neuromediator essential to many functions, including memory.
2. DOPAMINE, a neuromediator, mood regulator that controls physical sensation and movement, and is involved in memory.
3. GLUTAMATE, an essential neuromediator for learning and memory.