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Anatomy of the peripheral nervous system: an overview
This article provides an overview of the anatomy of the peripheral nervous system, highlighting the functional relationship between the peripheral nervous system and the central nervous system, describing the structure and classification of neurons, through to the process and function of myelination.
The nervous system consists of two main parts – the central nervous system (CNS) and the peripheral nervous system (PNS):1,2
Figure 1. The nervous system. Adapted from Eunice Kennedy Shriver National Institute of Child Health and Human Development, 2018 and Jessen, 2004.
The PNS consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.3
The cranial nerves emerge from the brain stem and mediate sensation as well as the motor control of the face, head, and neck.3 The spinal nerves emerge from points along the spinal cord, mediating the sensation and motor control of the trunk and limbs.3
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Figure 2. Diagram of the major components of the CNS and PNS and their functional relationship. Adapted from Purves et al. 2001.
Stimuli from the environment convey information to processing circuits within the brain and spinal cord, which in turn interpret their significance and send signals to peripheral effectors that move the body and adjust the workings of its internal organs.
Neurons
Nerves comprise of neurons – electrically excitable cells that process and transmit information through electrical and chemical signals.5
Neurons are classified according to the direction of the impulses that they conduct:4
- In afferent/sensory neurons, impulses travel towards the CNS from sensory receptors
- In efferent/motor neurons, impulses travel away from CNS to effector organs
In efferent neurons, two types of signals occur – somatic signals, which are voluntary and control the skeletal muscles, and autonomic signals, which are involuntary and control body function.4
Figure 3. Structure of a typical neuron.1,5,6 Adapted from Eunice Kennedy Shriver National Institute of Child Health and Human Development, 2018; National Institute of Neurological Disorders and Stroke, 2019; and Brain facts, 2018.
Myelination and Schwann cells
The myelin sheath is made up of concentric rings of cellular cytoplasmatic membrane.7,8 They provide insulation for the axon, prevent the escape of action potential, and enable faster conduction.7,8
In the PNS, myelination is preceded by invasion of the nerve bundle by Schwann cells, rapid multiplication of these cells and segregation of the individual axons by Schwann cell processes. Smaller axons (≤1 μm), which will remain unmyelinated, are segregated; several may be enclosed in one cell, each within its own pocket, similar to the structure shown in the figure below. Large axons (≥1 μm) destined for myelination are enclosed singly, one Schwann cell per axon per internode. These cells line up along the axons with intervals between them; the intervals become the nodes of Ranvier.
Figure 4. Myelin formation in the PNS.7 Adapted from Siegel et al. 1999.
A: The Schwann cell has surrounded the axon, but the external surfaces of the plasma membrane have not yet fused in the mesaxon. B: The mesaxon has fused into a five-layered structure and spiraled once around the axon. C: A few layers of myelin have formed but are not compacted completely. Note the cytoplasm trapped in zones where the cytoplasmic membrane surfaces have not yet fused. D: Compact myelin showing only a few layers for the sake of clarity. Note that Schwann cell cytoplasm forms a ring both inside and outside of the sheath.
Before myelination, the axon lies in an invagination of the Schwann cell (Fig. 4A). The plasmalemma of the cell then surrounds the axon and joins to form a double-membrane structure that communicates with the cell surface. This structure, called the mesaxon, elongates around the axon in a spiral fashion (Fig. 4). Thus, formation of myelin topologically resembles rolling up a sleeping bag; the mesaxon winds about the axon, and the cytoplasmic surfaces condense into a compact myelin sheath and form the major dense line. The two external surfaces form the myelin intraperiod line.
References
- Eunice Kennedy Shriver National Institute of Child Health and Human Development. What are the parts of the nervous system? 2018. Available at: https://www.nichd.nih.gov/health/topics/neuro/conditioninfo/parts. Accessed February 2022.
- Jessen KR. Glial cells. Int J Biochem Cell Biol 2004;36(10):1861–7. doi:10.1016/j.biocel.2004.02.023.
- National Cancer Institute. The Peripheral Nervous System. Available at: https://training.seer.cancer.gov/anatomy/nervous/organization/pns.html. Accessed February 2022.
- Purves D, et al., eds. Neuroscience. 2nd ed, Sunderland, MA: Sinauer Associates; 2001. Neural Systems. Available at: https://www.ncbi.nlm.nih.gov/books/NBK11061/. Accessed February 2022.
- National Institute of Neurological Disorders and Stroke. Brain Basics: The Life and Death of a Neuron. 2019. Available at: https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Life-and-Death-Neuron. Accessed February 2022.
- Brain Facts: A Primer on the Brain and Nervous System. Society for Neuroscience. 2018. Available at: https://www.brainfacts.org/the-brain-facts-book. Accessed February 2022.
- Morell P, et al. The Myelin Sheath. In: Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999. Available at: https://www.ncbi.nlm.nih.gov/books/NBK27954/. Accessed February 2022.
- Biga LM, et al. Anatomy & Physiology (adapted by Oregon State University). Available at: https://openstax.org/details/books/anatomy-and-physiology. Accessed February 2022.
