Neurological Changes and Nervous System Structure

Cellular Foundations: Neurons and Communication

At the microscopic level, the nervous system relies on specialized cells called neurons. A neuron consists of a cell body (soma), branching dendrites that receive signals, and an axon that transmits signals away from the cell.

Signal Transmission and the Synapse

Communication between neurons occurs at a junction called a synapse. One neuron can influence the next based on the information it receives, much like a telephone line. For a signal to travel successfully, the neuron must reach a specific threshold to trigger an action potential—a change in electrical voltage that allows the impulse to move along the axon.

To ensure these signals travel quickly and efficiently, many axons are wrapped in a myelin sheath. Myelin acts as insulation; if this sheath is damaged—as seen in neurodegenerative diseases like Multiple Sclerosis (MS)—the quality and speed of the signal are severely compromised.

Exam Tip: Remember the "A" rule for fiber types: Afferent = Arriving at the brain (sensory); Efferent = Exiting the brain (motor).

Neurodevelopment: From Proliferation to Pruning

The brain does not emerge fully formed; it undergoes a rigorous sequence of developmental stages.

1. Cell Proliferation and Migration: Early in development, nerve cells increase rapidly. By birth, the brain has reached about 44% of its adult weight.
2. Synaptogenesis: This is the creation of new connections (synapses) between neurons.
3. Myelination: As myelin develops, the brain's size can double, and the speed of information processing increases significantly.
4. Pruning and Cell Death: Around age 10, the brain undergoes a "natural elimination" process. The brain identifies the strongest, most efficient connections and gets rid of the pathways that are no longer needed. This process of removing excess neurons and synapses is a normal, healthy part of development.

Brain Plasticity and Critical Periods

During these early years of "exuberant connectivity," the brain possesses high plasticity—the ability to change and reorganize itself in response to experience. This leads to critical periods, or "windows of opportunity," where specific experiences are most effective at forging brain circuitry.

For example, in motor development, the circuits for involuntary reflexes are wired prenatally, but the circuits for posture and general movement (housed in the cerebellum) are most open during the first two years. Finer muscle control and timing have a window that stays open from birth until about age 9. If a child misses these windows—for instance, due to lack of physical activity or environmental deprivation—it becomes much harder to develop those specific functions later, as plasticity declines with age.

Brain Lateralization

While the brain works as a whole, it also exhibits lateralization, meaning the two hemispheres have specialized functions.

• Left Hemisphere: Generally associated with language, logic, and sequential processing.
• Right Hemisphere: Specialized for nonverbal tasks, such as visuospatial functions, music, awareness, and map reading.

Note that the control is crossed: the right hemisphere controls the muscular activity of the left side of the body, and vice versa.

Aging and Neurodegeneration

As we age, the nervous system undergoes several structural and functional changes. While the brain may lose about 7% of its weight from young adulthood, it often maintains much of its functional ability through reorganization.

However, advanced aging is characterized by:

• Neuron Loss: A gradual shrinking and withering of dendrites and axons.
• Decreased Connectivity: A delay in the growth of new axons and a decrease in myelination.
• Reduced Excitability: As neurons become less excitable, they require a larger stimulus to reach the threshold for an action potential.
• Synaptic Delay: An estimated twofold increase in the time it takes for signals to cross synapses.

By age 60, many individuals begin to experience a decline in motor skills, which can be caused by natural aging or secondary factors such as diet, alcohol, or neurodegenerative diseases.