Peripheral nervous system

Peripheral nerves

The workhorse of the peripheral nervous system are the peripheral nerves. Each nerve consists of a bundle of many nerve fibers (axons) and their connective tissue coverings. Each nerve fiber is an extension of a neuron whose cell body is held either within the grey matter of the CNS or within ganglia of the PNS. The comparable structure of the CNS is called a ‘tract’.

Peripheral nerves that carry information towards the CNS are called afferent or sensory neurons, while the ones transmitting impulses from the CNS are known as efferent or motor neurons.

Afferent neurons transmit a variety of impulses from sensory receptors/sense organs. They carry general sensations like touch, pain, temperature and position in space (proprioception). Some also transmit more special, sensorial information like the special senses of smell, vision, hearing and balance. In contrast, efferent neurons bring general nervous information towards effector organs, like skeletal muscles, visceral organs and glands. They are responsible for initiating voluntary and involuntary motor functions, such as muscle contraction and gland secretion.

Nerves can also be classified as ‘cranial’ or ‘spinal’ according to where they exit the CNS. Cranial nerves emerge from the cranium (brain/brainstem) whilst spinal nerves leave the CNS via the spinal cord. There are 12 pairs of cranial nerves and 31 spinal nerve pairs, giving a total of 43 paired nerves forming the basis of the peripheral nervous system.

To learn more about the structure of peripheral nerves and clarify the ‘afferent vs efferent’ difference, take a look at the following:

Somatic nervous system

The somatic, voluntary, nervous system is responsible for providing sensory and motor innervation to skin, muscles and sensory organs. In other words, it carries sensations from the body (pain, touch, temperature, proprioception) and innervates skeletal muscles that are under conscious, or voluntary control. In addition, the somatic nervous system is involved in spinal reflexes, an example being the withdrawal reflex. This helps you to instantly pull away your hand when touching a hot object.

Both cranial and spinal nerves contribute to the somatic nervous system. Cranial nerves provide voluntary motor control and sensation to the head and face. Spinal nerves, as we mentioned previously, supply the trunk and limbs. The posterior rami travel backwards to supply the vertebral column, vertebral muscles and skin of the back whilst the anterior rami supply the limbs and anterior trunk. The majority of anterior rami combine to form nerve plexuses from which many major peripheral nerves stem. The exception to this is the anterior rami of the thoracic region which travel relatively independently from one another without forming plexuses, as the intercostal and subcostal nerves of the trunk. 

Nervous plexuses, formed by the anterior rami of spinal nerves, are as follows:

• C1-C4 form the cervical plexus
• C5-T1 combine into the brachial plexus
• T12-L4 form the lumbar plexus
• L4 - S4 combine into the sacral plexus

The lumbar and sacral plexuses can also be combined as one lumbosacral plexus. Each nerve plexus gives out several peripheral nerves, which carry sensory and motor fibers to and from their respective target structures. 

You can find more information about each nervous plexus in the following study units:

Let's now turn our attention back to spinal nerves. If you recall, spinal nerves represent the initial source of a peripheral nerve. Well, a single spinal nerve along with its relevant peripheral nerves and the specific region of skin and muscle supplied, share an embryological origin. For this reason, the area of skin supplied by the sensory fibers of a single spinal nerve (and cranial nerve V) can be mapped. This is represented in the form of dermatomes. Despite overlap between adjacent spinal nerves, the innervation pattern is presented as evenly spaced horizontal or longitudinal patterns across the entire body, as illustrated in the following dermatome map:

Clinicians will understand that it is important to differentiate between dermatomal/segmental innervation (spinal nerves) and cutaneous innervation provided by individual peripheral nerves. More clarifications about dermatomes are given next:

Similar to dermatomes, the group of muscles supplied by the motor fibers of a single spinal nerve can also be mapped. This is represented in the form of myotomes.

Autonomic nervous system

Last but not least, we have reached the autonomic division of the peripheral nervous system (ANS). It is involuntary in nature, meaning that we have no conscious control over it. The ANS is responsible for providing sensory and motor innervation to smooth muscles, blood vessels, glands, and internal organs. As such, it provides a coordinated regulation of visceral and glandular functions, playing a role in maintaining homeostasis.

The autonomic nerves are also peripheral in nature, so the general structure of a peripheral nerve discussed earlier still applies. However, there is one caveat; all autonomic nerves synapse with a sympathetic or parasympathetic ganglion. The portion of the nerve before the ganglion is referred to as preganglionic and carries the impulse towards the cluster of cell bodies. The portion located from the ganglion onwards is called postganglionic and carries the impulse away from the cell bodies. 

The ANS has three major branches: sympathetic, parasympathetic and enteric. The sympathetic division prepares the body for dealing with periods of increased physical activity through actions such as regulating blood vessels (often, but not always, vasoconstriction), dilating pupils, increasing heart rate and blood pressure, and decreasing peristalsis. The parasympathetic division helps the body to conserve energy, it has ‘rest and digest’, feeding, and breeding functions. This is carried out by actions that slow the cardiovascular system, stimulates gland secretion and increases peristalsis. The PNS is also involved in sexual arousal and lacrimation (crying). The enteric nervous system (ENS) lies within the walls of the gastrointestinal tract and consists of the myenteric (Auerbach) and submucosal (Meissner) plexuses. They work together to control peristalsis within the digestive system. This system is often described as the second brain because it acts independently while only being influenced by impulses from the ANS.

Related articles

Articles within this topic:

• Dermatomes
• Dorsal root ganglion
• Myotomes
• Superior hypogastric plexus