Peripheral Nervous System

Structure

The peripheral nervous system is itself classified into two systems: the somatic nervous system and the autonomic nervous system. Each system contains 2 components:

• The afferent arm consists of sensory (or afferent) neurones running from receptors to the CNS. Afferent nerves detect the external environment via receptors for external stimuli such as pressure or temperature etc. Afferent nerves exist in both the somatic and autonomic nervous systems as both can use sensory signals to alter their activity.
• The efferent arm consists of motor (or effector) neurones running from the CNS to the effector organ. Effector organs can either be muscles or glands.

Somatic and Autonomic Nervous Systems

The somatic nervous system of the PNS is responsible for voluntary, conscious control of skeletal muscles (effector organ). Its afferent arm links sensory receptors on the body surface or deeper within it with relevant processing circuits, whereas the efferent arm directly controls skeletal muscles using motor nerves.

The autonomic (visceral) nervous system control the visceral functions of the body and acts largely unconsciously.  These visceral functions include the regulation of heart rate, digestion, salivation, urination, digestion, and many more. The afferent (sensory) arm of this system includes receptors that monitor the arterial pressure, levels of carbon dioxide and oxygen in the blood or the chemical composition of the content of the gastrointestinal tract. The efferent arm of this system can be further subdivided into the parasympathetic (PSNS) and sympathetic (SNS) components, which control numerous smooth muscles and glands.

The enteric nervous system is classified as a separate component of the autonomic nervous system and is sometimes even considered a third independent branch of the PNS.

By Nervous_system_organization_fr.svg: JDifoolderivative work: Looie496 (Nervous_system_organization_fr.svg) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)

Fig 1
Diagram showing the components that make up the somatic nervous system

Sympathetic and Parasympathetic Nervous Systems

The SNS and PSNS are sub-divisions of the autonomic nervous system.  The autonomic nervous system has a unique structure as it employs a sequential two-neurone efferent pathway. Hence, the preganglionic neurone must first travel to and synapse upon a ganglion, a collection of neuronal cell bodies in the PNS. A ganglion then gives rise to a postganglionic neurone which innervates the target organ.

Sympathetic Nervous System

The SNS is responsible for the fight or flight response of the body and originates from the thoracolumbar segments of the spinal cord. It incorporates short preganglionic neurones and long postganglionic neurones.

The preganglionic neurones use acetylcholine as a neurotransmitter while the postganglionic neurones use noradrenaline. The exception to this rule is the innervation of the sweat glands and chromaffin cells of the adrenal medulla, which are cholinergic as they use acetylcholine as a neurotransmitter.

Another exception is the chromaffin cells of the adrenal medulla. They act as a modified sympathetic ganglion without the postganglionic neurones.  Hence, the activation of chromaffin cells via preganglionic cells leads to the release of two neurotransmitters: adrenaline and to lesser extent noradrenaline, directly into the bloodstream.

The actions mediated by SNS are most apparent when the body is faced with stressful situations. It is designed to mobilise energy stores, allowing us to cope with the stress and increase our chances of survival.

Parasympathetic Nervous System

The PSNS is responsible for the rest and digest actions of the body. It originates from craniosacral segments of the spinal cord. This system consists of long preganglionic neurones and short postganglionic neurones. Both preganglionic and postganglionic neurones use the neurotransmitter acetylcholine.

Summary of SNS and PSNS

The sympathetic and parasympathetic pathways have very similar structures but with a few key differences. The table below shows a comparison of these 2 systems.

By Geo-Science-International [public domain], accessed via Wikimedia Commons

Fig 2
Functions of the sympathetic and parasympathetic nervous systems

Enteric Nervous System

This subdivision of PNS is embedded within the lining of the gastrointestinal (GI) tract so it can directly control the functions of the GI tract. It consists of two plexuses:

• Myenteric (Auerbach’s) plexus:
  • Located between the circular and longitudinal layers of muscularis externa
  • Responsible for increasing the tone of gut
  • Controls velocity and intensity of contractions
• Submucosal (Meissner’s) plexus:
  • Located in the submucosal layer
  • Responsible for secretions and absorption in the gut
  • Controls local muscle movements as well

The PSNS also stimulates the enteric nervous system to increase function. Similarly, the SNS may inhibit the enteric function. Therefore, defecation is not possible during fight or flight mode.