The word neuroplasticity breaks down as neuro for “neuron”, the nerve cells in our brain and nervous system. Plastic is for “changeable, malleable, modifiable.” Neuroplasticity refers to the brain’s ability to change in response to experience. The brain does this by strengthening the connections between some nerve cells while weakening the connections between others. This is how the brain stores memories and learns. Two principles govern brain plasticity:
• ‘Nerve cells that fire together wire together’ and
• ‘Use it or lose it’.
‘Nerve cells that fire together wire together’ means that two events can become strongly connected if they occur at the same time. For example, a toddler touching a hot stove for the first time activates both nerve cells that process the visuals of a stove-top and nerve cells that experience burning pain. These two previously unconnected events become permanently wired together in the brain via nerve cell branches.
‘Use it or lose it’ is most apt during certain windows of development. It is why it is much easier to learn particular skills or behaviours at certain ages. We don’t see Olympic gymnasts starting at age 12 or concert musicians beginning at age 25. Not unlike the toddler, a porn-watching teen connects external objects with his innate circuit for sexual excitement. Adolescence is the time to learn about sexuality. The nerve cells involved in surfing the internet and clicking from scene to scene fire together with those for sexual excitement and pleasure. His limbic system is just doing its job: touching stove = pain; surfing porn sites = pleasure. Ceasing an activity helps weaken the associations.
Our brain is part of an extended nervous system. It has around 80 billion nerve cells or neurons.
Neurons are similar to other cells in the body because:
1. Neurons are surrounded by a cell membrane.
2. Neurons have a nucleus that contains genes.
3. Neurons contain cytoplasm, mitochondria and other organelles.
4. Neurons carry out basic cellular processes such as protein synthesis and energy production.
However, neurons differ from other cells in the body because:
1. Neurons have specialised cell parts called dendrites and axons. Dendrites bring electrical signals to the cell body and axons take information away from the cell body.
2. Neurons communicate with each other through an electrochemical process.
3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).
Neurons are the messenger cells in the nervous system. Their function is to transmit messages from one part of the body to another. They constitute about 15% of the cells in the brain. The other approximately 85% are glial cells. These are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the central nervous system and peripheral nervous system. Glial cells do the maintenance such as cleaning up dead cells and repairing others.
The neuron or nerve cell has a cell body that contains the nucleus with DNA material. It also contains proteins that change shape as they adapt to the input of information from elsewhere.
Neurons can also be classified by the direction that they send information.
• Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system.
• Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands.
• Interneurons: send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system.
Together the three sorts of neurons form what we think of as ‘grey matter’. When the axon, which can be very long or short, is insulated by a white fatty substance (myelination), this allows the signals to pass along more rapidly. This coating is what is often referred to as ‘white matter’. Dendrites which receive information do not get myelinated.
Electrical and chemical signals
Our neurons carry messages in the form of electrical signals called nerve impulses or action potentials. To create a nerve impulse, our neurons have to be excited enough, because of a thought or an experience, to send a wave firing down the length of the cell to excite or inhibit the neurotransmitters at the end point of the axon. Stimuli such as light, sound or pressure all excite our sensory neurons.
Information can flow from one neuron to another neuron across a synapse. As the neurons don’t actually touch each other, the synapse contains a small gap separating neurons. Neurons each have anywhere between 1,000 and 10,000 connections or ‘synapses’ with other neurons. So when a nerve impulse reaches the end of one neuron, a neurotransmitter is released. It moves from this neuron across the junction or synapse and excites or inhibits the next neuron.
If there is a decline in either the amount of neurochemical (e.g. dopamine) or number of receptors, the message becomes harder to pass on. Higher levels of neurochemicals or receptors translate into a stronger message or memory pathway.