Structures and Functions of the Nervous System

The Peripheral Nervous System (PNS)

The PNS includes all neuronal structures located outside the CNS. This includes the 12 pairs of cranial nerves and the various spinal nerves. While spinal nerves always contain both sensory (afferent) and motor (efferent) fibers, some cranial nerves may be purely sensory or purely motor.

The PNS is further categorized by function, specifically through the Autonomic Nervous System (ANS), which governs involuntary functions such as heart rate and glandular activity. The ANS is split into two opposing branches:

1. Sympathetic Nervous System: Often associated with the "fight or flight" response.
2. Parasympathetic Nervous System: Generally responsible for "rest and digest" functions.

Cellular Foundations: Neurons and Neuroglia

The functionality of the nervous system relies on two distinct types of cells: neurons and neuroglia.

Neurons and Signal Transmission

Neurons are the functional units of the nervous system. They are characterized by three main properties: excitability (generating impulses), conductivity (transmitting impulses), and the ability to influence other cells.

Communication occurs through a process called an action potential—an electrical impulse that travels along the axon of a neuron. When this impulse reaches the end of the nerve fiber, it must cross a gap to reach the next cell. This junction is known as a synapse. To bridge this gap, the neuron releases a chemical agent called a neurotransmitter, which carries the signal across the synaptic cleft to the next neuron, muscle, or gland.

Neuroglia

While neurons handle the "messages," neuroglia (or glial cells) provide the essential infrastructure. They provide support, nourishment, and protection to the neurons, ensuring the environment is optimal for electrical signaling.

Protection and Defense Mechanisms

Because the nervous system is so vital, it is protected by several layers of physical and physiological barriers.

• The Meninges: These are three layers of protective membranes that surround both the brain and the spinal cord.
• Cerebrospinal Fluid (CSF): Circulating within the subarachnoid space, CSF provides cushioning for the brain and spinal cord, carries nutrients, and allows for fluid shifts between the cranial and spinal cavities.
• The Blood-Brain Barrier: This is a physiological barrier formed by specialized capillaries in the brain. It prevents many substances that are normally allowed into the body's tissues from entering the sensitive brain tissue.
• Skeletal Support: The bony skull protects the brain from external trauma, while the vertebral column protects the spinal cord and supports the head.

Clinical Assessment and Diagnostics

In clinical practice, assessing the nervous system involves evaluating mental status, cranial nerve function, motor strength, coordination, sensory perception, and reflexes.

To move beyond physical observation, healthcare providers use electrographic studies to measure electrical activity:

• Electroencephalography (EEG): Records the electrical activity of the surface cortical neurons of the brain.
• Electromyography (EMG): Records the electrical activity associated with the innervation of skeletal muscles.

Exam Tip: When distinguishing between the CNS and PNS, remember that the CNS is the "processor" (Brain and Spinal Cord), while the PNS is the "wiring" (Nerves and Ganglia) that connects the processor to the rest of the body.

As humans age, the nervous system undergoes changes, such as a loss of neurons in the brainstem, cerebellum, and cerebral cortex. These changes can lead to declines in memory, vision, hearing, and reaction time, making careful neurological assessment essential in older populations.