Until relatively recently it was thought that connections between nerve cells, once formed, were fixed wired, i.e. could not change.

However, there is increasing evidence that connectivity within neural networks can show long term changes as a result of

• alterations in the effectiveness of synaptic transmission,
• the formation or removal of neural connections, and
• the production in some areas of the brain of new nerve cells (neural stem cells).

Synaptic plasticity is the change in effectiveness of synaptic transmission as a consequence of synaptic activity. This may be due to changes in the amounts of transmitter released at the synapse, or to a change in the density of post-synaptic receptors; changes in the levels of second messengers such as calcium concentration appear to be involved in these processes. One example is the tenderness and hypersensitivity of the area around a skin injury which is due to increased synaptic transmission from that area of skin within the dorsal horn. Another is the process of laying down of memories within the hippocampus of the brain.

The formation or removal of neural connections is another mechanism that involves changes in synaptic effectiveness. During development an excess of peripheral neurones grow to contact their targets, and approximately 50% of these fail to make significant contacts and die as a consequence - by a process of neuronal cell death. As a result of this process the peripheral targets can regulate the size of the peripheral innervation. During development of frogs, for example, the tadpole stage has a tail that is innervated from the spinal cord; later the tail regresses, and the nerves that innervate of these targets die off. The neurotrophic theory suggests that the peripheral tissues secrete local mediators, neurotrophins, that are transported retrogradely along axons, and are used to inform the neuronal cell body of the presence of effective contacts with the target tissues. When the contact is lost, the neurones undergo apoptosis.

Neural Stem Cells

Adult neurogenesis is the process of generating new neurons in adults, well after CNS development has ceased. This production of new neurones and glial cells occurs in the sub-ventricular zone (SVZ) of the hypothalamus, in the hippocampus and cerebellum in most mammalian species.

There is much interest in the the possibility of using these stem cells in treatments of neurological disease, and significant developments are awaited. Recent research using stem cells include (a) the injection of neural stem cells into the eye to treat some forms of blindness (b) the injection of stem cells into the damaged spinal cord to treat paralysis, and (c) injection of neural stem cells into the brains of patients with Parkinson's disease, to treat neurodegeneration.

Plasticity in the visual cortex

In the CNS, there is ample evidence that the structure and function of neurones of the visual cortex are influenced by the visual environment during early post-natal life, and that these changes have behavioural correlates in the adult animal. On example is seen when kittens are raised in an environment of vertical stripes; the visual cortex fails to develop cells that respond to horizontal stripes, and the adult animals do not recognise horizontal edges in their environment. It is known that the plastic changes in the visual cortex occur during a critical period during development; whether plastic changes in connectivity can also occur in adults is under investigation.

Plasticity in the Motor Cortex

There is much interest in neuroplasticity within the nervous system of adults. Motor skills that are learned are known to involve changes in connectivity within the CNS. One example is the effects of regular practice amongst violinists, whose fingers of the left hand require delicate control. It is known that professional violinists have a reorganisation of their motor cortex in the right hemisphere, such that a greater area is given over to the fingers of the left hand. This reorganisation take place at cortical level, and the exact mechanism is unknown.

Plasticity in Hearing and Speech

In most adults the areas of the brain concerned with speech recognition and vocalisation are in the left hemisphere. However in young children who sustain damage to the left hemisphere, the right side can take over these functions. There are examples of children who have had surgical removal of epileptic foci from the left hemisphere, who develop normal speech functions using the right hemisphere.

This suggests that the neuroplasticity in the right hemisphere takes over functions that are more commonly found in the left hemisphere.