1.1 The Nervous System: CNS versus PNS

The nervous system is the command center of the body in charge of controlling every aspect of human behavior, from movement to autonomic responses and how we react to external cues as our senses are stimulated by the environment and experiences we go through. It is composed of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). While the CNS consists of the brain and the spinal cord (SC), the PNS consists of the cranial nerves that arise from the brain and the spinal nerves that emanate from the spinal cord. These two systems work together to collect information from inner brain compartments or the external environment, integrate this information into distinct brain areas, and direct specific responses that lead to our behavior. However, if examined separately, the CNS is the control center for many of our daily functions including the regulation of movement, touch, memory, vision, speech production and comprehension, and personality, among many other things. The PNS, on the other hand, is recognized for connecting the muscles and sensory receptors of the human body with the brain. Modulation of critical involuntary autonomic functions, such as respiration, cardiac function, digestion, blood circulation, and body temperature, among others, is also maintained by the PNS. For efficiency of this involuntary regulation, the autonomic system is divided into “sympathetic” and “parasympathetic” systems. While the sympathetic system is considered to prepare our body for the “fight or flight” response, the parasympathetic system keeps our body at a “resting state”, hence displaying an opposing role (Figure 1).

At the cellular level, the brain’s basic unit is the neuron. Neurons are pre-synaptic or post-synaptic, depending on whether they are sending or receiving information, respectively. The transmission of information from one neuron to another occurs through specialized connections known as “synapses”. Like other cell types, neurons display a complex morphology that contains a cell body, numerous dendrites and one long terminal that forms the axon (Figure 2). Effective synaptic communication between neurons occurs via dendrites and their connections to a post-synaptic neuron’s dendrites (axo-dendritic), cell body (axo-somatic) or axons (axo-axonal). Neurons are heterogeneous and they can connect to one single neuron or multiple neurons via synaptic knobs. Depending on the axonal terminal’s length, neurons may contact proximal neurons and glands or distal muscles.

The brain also contains a variety of glial cells, also known as neuroglial cells. They outnumber neurons in the brain by 3:1, but this ratio varies depending on the brain region being examined. For example, a higher number of glial cells locate in the cerebral cortex when compared to the cerebellum, which displays less glial cells. The main role of glial cells during brain development is to provide axonal guidance cues to direct neuronal migration and axonal extension, to buffer chemicals and ions that surround neurons and more importantly, to control the speed of action potentials by insulating neuronal axons with myelin. In the CNS, the glia cells myelinating neurons are oligodendrocytes, while in the PNS they are Schwann cells. Other types of glia cells include microglia, astrocytes, and ependymal cells, which serve to phagocytose debris surrounding neurons, regulate blood flow by being part of the blood brain barrier and maintain homeostasis of the cerebrospinal fluid (CSF), respectively.

Efficiency of neuronal synaptic transmission from the brain to the rest of the body or vice versa occurs via the SC, that extends from the distal end of the brainstem at the top of the neck to the pelvic region. Meningeal layers covering the SC (pia, arachnoid, and dura matter) extend all the way from the brain and serve to protect these structures from mechanical trauma by providing a continuous passageway through which the CSF can flow down the SC (Figure 3).

Protected by the vertebra column, the SC divides into cervical (C1-C7), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5) and coccygeal (C1-C4). It is enlarged in the cervical and lumbar regions in correlation with the density of spinal nerves that irradiate out of the spinal cord to provide input and output to the upper and lower limbs. Together, with the twelve cranial nerves that originate in the brain, these spinal nerves comprise the PNS. A particular distinction is that although the 31 spinal nerves arise bilaterally within 31 SC segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal), and are named according to the vertebra level at which they exit the vertebra via the intervertebral foramen, at the cervical level, spinal nerves will arise above the vertebra level, but at the thoracic level, thoracic spinal nerves will arise below the vertebra level (example: C7 spinal nerve over C7 vertebra; C8 spinal nerve over T1 vertebra; T1 spinal nerve over T2 vertebra).