Peripheral nervous system
In this article, we’ll discuss the peripheral nervous system and its divisions, as well as the peripheral nerves. The peripheral nervous system (PNS) consists of all the nerves branching out of the brain and spinal cord (the central nervous system, CNS). If you imagine the CNS as the main highway, then the PNS forms all the connecting secondary roads. These allow electrical impulses to travel to and from the furthest regions, or periphery, of the human body.
The PNS is built almost entirely from nerves. There are two main types; spinal nerves and cranial nerves. Functionally, the PNS can be divided into the autonomic and somatic nervous systems. Both of these can be further subdivided; the former into sympathetic and parasympathetic arms and the latter into sensory and motor divisions.
It might sound complicated, but it’s not. Let’s take a closer look at all of the above terms in order to better understand the divisions of the nervous system.
|Definition||A nervous system division composed of all the neural tissue found outside the cranial vault and vertebral canal.|
Peripheral nerves (spinal nerves, cranial nerves, autonomic nerves)
Autonomic nervous system (ANS) - involuntary part in control of cardiac, smooth and glandular cells. It consists of sympathetic and parasympathetic divisions.
Somatic nervous system (SNS) - voluntary part in control of skeletal muscles and processing of somatic sensation.
|Function||Transmits motor and sensory information between the central nervous system and peripheral body tissues.|
- Peripheral nerves
- Somatic nervous system
- Autonomic nervous system
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:
The first set of peripheral nerves are the twelve cranial nerves: olfactory (CN I), optic (CN II), oculomotor (CN III), trochlear (CN IV), trigeminal (CN V1, CN V2, CN V3), abducens (CN VI), facial (CN VII), vestibulocochlear (CN VIII), glossopharyngeal (CN IX), vagus (CN X), spinal accessory (CN XI), and hypoglossal (CN XII) nerves.
Cranial nerves are peripheral nerves that mainly innervate anatomical structures of the head and neck. The exception to this is the vagus nerve, which also innervates various thoracic and abdominal organs. Cranial nerves originate from specific nuclei located in the brain. They leave the cranial cavity through foramina and project to their respective target structure. Cranial nerves are divided into three groups according to the type of information carried by their fibers:
Are you curious to find out more about the cranial nerves? Then take a look at the following resources that present the information in an easy to digest and visual way.
The second set of peripheral nerves are spinal nerves, of which there are 31 pairs: eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal. Their numbering relates to the vertebral column exit level; cervical spinal nerves are numbered according to the vertebra located below, while all the rest according to the vertebra situated above.
Each spinal nerve starts as several rootlets that unite to form two main roots. The anterior root carries motor fibers from neurons whose cell bodies are located in the anterior horn of the spinal cord. The posterior root transports sensory fibers from neurons which have their cell bodies in the dorsal root ganglion. In the thoracic and upper lumbar region, the anterior root also carries autonomic fibers from preganglionic sympathetic neurons whose cell bodies are located in the spinal cord’s lateral horn. The anterior and posterior roots subsequently join to form the spinal nerve proper that carries mixed (sensory, motor, autonomic) fibers.
Spinal nerves leave the vertebral column through the intervertebral foramina located between two successive, adjacent vertebrae. Each spinal nerve then divides into two branches called posterior/dorsal and anterior/ventral rami. Both rami carry mixed fibers. The posterior rami travel backward and divide into branches that supply post-vertebral structures. The anterior rami supply the skin and muscles of the limbs and anterior trunk.
Immediately after the division of the spinal nerve into the two rami, smaller communicating fibers branch out. These white and grey rami communicantes establish a connection between spinal nerves and the two sympathetic trunks of the autonomic nervous system that run along the length of the vertebral column. Note that grey rami communicantes exist at all levels of the spinal cord, whilst white rami are only found at T1-L2 levels.
If you want more detail about spinal nerves take a look below:
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.
|Cervical plexus||Lesser occipital nerve
Greater auricular nerve
Transverse cervical nerve
Other smaller branches, for instance the nerves to the rhomboids and serratus anterior muscles
Other smaller branches such as; dorsal scapular, long thoracic, suprascapular nerve, nerve to subclavius, lateral & medial pectoral, medial cutaneous of arm & forearm, superior & inferior subscapular, thoracodorsal
Lateral cutaneous nerve of the thigh
|Sacral plexus||Superior gluteal nerve
Inferior gluteal nerve
Sciatic nerve (common fibular and tibial divisions)
Posterior cutaneous nerve of the thigh (perforating cutaneous)
Other smaller branches such as; nerve to piriformis, nerve to quadratus femoris and inferior gemellus, nerve to obturator internus
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 it's 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.
The preganglionic fibers of the sympathetic nerves leave the spinal cord through the T1 to L2 anterior roots, entering the corresponding spinal nerve. The fibers then travel through the white rami communicantes to paravertebral ganglia of the sympathetic trunks, located on either side of the vertebral column.
Some fibers synapse here, while others travel through it without synapsing, exiting the sympathetic trunks as the splanchnic nerves (greater, lesser, least, lumbar, sacral). These splanchnic nerves synapse closer to their target organs in prevertebral ganglia called celiac, aorticorenal, and mesenteric (superior and inferior). The postganglionic fibers then project onto their target structures either directly, or by returning through the grey ramus communicantes and following the path of spinal nerves throughout the body. Target organs include blood vessels, sweat glands, arrector pili, the iris, and internal organs. One example target organ of the sympathetic nervous system is the adrenal gland. Sympathetic nervous system activity stimulates the release of epinephrine/adrenaline via the sympathetic-adrenal medullary system.
For more information about the sympathetic nervous system, take a look at the following article and video:
The parasympathetic nervous system is divided into cranial and sacral outflows. The preganglionic fibers of the cranial outflow exit the brainstem within the oculomotor, facial, glossopharyngeal, and vagus cranial nerves. They synapse in the ciliary, pterygopalatine, otic, submandibular, and enteric ganglia. The postganglionic fibers ultimately innervate salivary glands of the head, iris and ciliary muscles of the eye and, in the case of the vagus nerve, thoracic and abdominal viscera.
The preganglionic fibers of the sacral outflow are much more restricted, only exiting the spinal cord through the anterior roots of the S2-S4 spinal nerves. They travel with the pelvic splanchnic nerves, ultimately innervating the pelvic viscera (descending colon, sigmoid colon, rectum, bladder, penis/clitoris).
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