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CNS Overview

The human nervous system has evolved over the last 600 million years into one of the most complex systems in human anatomy. Comprised of trillions of neural circuits traveling from the brain to distal effector organs, it supports higher cognition, behavioral identities, musculoskeletal mobility, and the spectrum of functions in between.1 The human nervous system has been traditionally divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is anatomically defined as the brain and spinal cord. These structures are protected by highly fortified bony structures called the calvarium and vertebrae, respectively. All other nervous tissue then falls into the category of the PNS which is further subdivided into the somatic and autonomic nervous systems (Chapter 5).

The CNS is wrapped in three layers of meninges that form a protective sheath around the highly delicate nervous tissue. The outermost layer is the dura matter, the middle is the arachnoid, and the delicate inner membrane is the pia mater. Together these three membranes function to provide a protective “shock absorption” barrier, deliver oxygen/nutrients to the tissue, and remove chemical waste from the tissue.2 Within the CNS, groups of neurons that share a similar function are called nuclei. Furthermore, the CNS can be divided into gray matter which is composed mostly of neuron cell bodies and white matter which is composed mostly of myelin-rich neuron axons.3

Three terms have classically distinguished the neural modality, or type of information being transmitted.

  1. Direction of information flow

    • Afferent: Sensory neurons that transmit a stimulus to the CNS from receptors outside of the CNS. One example is the dorsal root ganglion (DRG) which is the major conduit of afferent information flow for all vertebral levels.

    • Efferent: Motor neurons that transmit a stimulus from the CNS to muscle and secretory effector cells. One example is the spinal motor neurons which signal for muscles to fire.

  2. Anatomical distribution of information flow

    • Visceral: Neurons that carry stimuli to or from internal organs as well as embryologically resurrected structures of the branchial arch. One example is the carotid body chemoreceptors.

    • Somatic: Neurons that carry stimuli to or from nonvisceral anatomy. The two major examples are neurons innervating the skin and musculature.

  3. Embryological origin of the structure being innervated

    • Special: Neurons that carry stimulus to or from “special” visceral and somatic structures. One example is the special visceral neurons which send information to or from branchial arch derived anatomy such as the pharyngeal muscles. The second example is special somatic neurons, which only transmit sensory information from organs of special sense (e.g., retina, gustatory receptors, cochlea).

    • General: Nonspecial group neurons carrying stimulus to or from both visceral and somatic structures.

Understanding the basic components of nervous system anatomy and organization is quintessential to grasping the ...

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