General Structure of the Nervous System

The components of the human nervous system weigh about 2.04 kg (4.5 lb) in total, which accounts for approximately 3% of the entire body weight. The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). The system is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system.

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Overview

The nervous system is separated into the central and peripheral nervous systems. In addition, the sympathetic, parasympathetic, and enteric nervous systems act as parts of the autonomic nervous system.

Anatomical subdivision

  • CNS: brain and spinal cord
  • Peripheral nervous system (PNS):
    • Somatic nervous system:
      • Sensory
      • Motor
    • ANS:
      • Sympathetic system
      • Parasympathetic system
      • Enteric nervous system

Functional subdivision

  • Sensory function: comprises sensory or afferent neurons
  • Integrative function: comprises interneurons
  • Motor function: comprises motor or efferent neurons

Central Nervous System

The CNS is composed of the brain, brain stem, cerebellum, and spinal cord. Each of these structures is covered by meninges and CSF.

Composition

The CNS is contained within the skull and spinal column. The components of this system include:

  • Cerebral cortex
  • Spinal cord
  • Brain stem
  • Cerebellum

Protective covering

The meninges provide a protective covering to the brain and spinal cord. The meninges are composed of:

  • Dura mater
  • Arachnoid mater
  • Pia mater

Cerebrospinal fluid

  • Clear, colorless liquid containing a small amount of protein
  • Provides buoyancy and protection for the brain
  • Regulates chemical concentration of neurotransmitters and metabolic waste

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Peripheral Nervous System

The PNS is composed of neural components that extend from the aspects of the CNS. The 2 divisions of the PNS are the somatic nervous system (SNS) and the ANS.

Somatic nervous system

The SNS is composed of cranial nerves and spinal nerves that are located outside of the CNS. There are 12 cranial and 31 spinal nerve pairs:

  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal

The somatic nervous system is composed of:

  • Dorsal roots:
    • Carry sensory information
    • Afferent fibers
  • Ventral roots:
    • Carry motor information
    • Efferent fibers
  • Dorsal root ganglia: location of sensory neurons (cell bodies)
  • Interneurons
  • Mechanoreceptors
Anatomical landmarks of the spinal cord general structure of the nervous system

Anatomical landmarks of the spinal cord

Image by Lecturio.

The somatic nervous system is involved in:

  • Voluntary muscular movement
  • Reflex arcs: 
    • Involuntary
    • Monosynaptic or polysynaptic, according to the number of synapses

Autonomic nervous system

The ANS monitors and controls the functions of internal organs. The ANS is composed of the parasympathetic and sympathetic nervous systems. The nerve fibers of the ANS originate in the CNS and synapse with ganglia in the periphery.

  • Sympathetic nervous system: fight-or-flight response
  • Parasympathetic nervous system: rest-and-digest processes

Clinical Relevance

  • Upper motor neuron syndrome: collection of deficits that can occur after damage to an upper motor neuron (e.g., cerebrovascular accident). Symptoms include weakness, decreased motor control, altered muscle tone, a decrease or increase in the baseline level of muscle activity, decreased endurance, exaggerated deep tendon reflexes, spasticity, and clonus. 
  • Lower motor neuron syndrome: collection of deficits that can occur after damage to a lower motor neuron (e.g., trauma or impingement). Signs and symptoms may include paralysis or paresis, muscle atrophy, areflexia, and fibrillations. 
  • Amyotrophic lateral sclerosis (ALS): also known as Lou Gehrig disease. Amyotrophic lateral sclerosis is a sporadic or inherited neurodegenerative disease of the upper motor neurons (UMNs) and the lower motor neurons (LMNs). This disease is characterized by the coexistence of UMN and LMN signs and symptoms that lead to weakness and difficulty speaking and swallowing. The diagnosis is made clinically. Management is supportive and symptomatic, with a poor prognosis and low 5-year survival.

References

  1. Brotman, R.G., Moreno-Escobar, M.C., Joseph, J., Pawar, G. (2021). Amyotrophic lateral sclerosis. StatPearls. http://www.ncbi.nlm.nih.gov/books/NBK556151/
  2. Huppert L.A., & Dyster T.G. (Eds.). (2021). Diseases & pathophysiology in neurology. In: Huppert’s Notes: Pathophysiology and Clinical Pearls for Internal Medicine. McGraw-Hill.
  3. Brown, R.H., Jr. (2018). Amyotrophic lateral sclerosis and other motor neuron diseases. In: Harrison’s Principles of Internal Medicine, 20th ed. McGraw-Hill. 
  4. Elman, L.B., McCluskey, L. (2021). Clinical features of amyotrophic lateral sclerosis and other forms of motor neuron disease. UpToDate. https://www.uptodate.com/contents/clinical-features-of-amyotrophic-lateral-sclerosis-and-other-forms-of-motor-neuron-disease
  5. Ratti, E. (2015). Motor Neuron Diseases. DeckerMed Medicine. Retrieved August 8, 2021, from http://dx.doi.org/10.2310/PSYCH.6266

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