Peripheral Nervous System
The peripheral nervous system consists of nerves that supply our muscles with power, and our skin with sensation. In short, it is at the very centre of what enables us to survive and thrive as humans. Although it can seem daunting to discover and decipher so many different names, it is all well structured, and is easily learnt once basic rules are memorized and applied. In this article we will explore the anatomy and clinical relevance of the nerves that enable us to perform our daily lives, and respond to our environment appropriately.
Within a spinal nerve you will find a dorsal sensory component and a ventral motor component. The dorsal horns of the spinal cord reflect the inflow of the sensory component, and the ventral horns reflect the outflow of the motor component. Both sensory and motor nerve fibres run within each spinal nerve.
Each mixed spinal nerve will leave at its respective level, via the intervertebral foramen. These nerves then divide into a ventral and dorsal rami. The anterior rami are responsible for the majority of innervation, including the limbs, abdomen and cervical regions. Those of spinal nerves C1 - C4 combine with each other to form the branches of the cervical plexus. The posterior rami of each spinal nerve supplies the skin and muscles of the back.
Head and Neck: Cervical plexus
Locating the cervical plexus in a clinical setting is relatively simple. You should place your fingers halfway along the sternocleidomastoid muscle. Deep to this is the plexus. The motor branches are deep to the sensory branches.
The motor branches travel anteriorly and medially, in contrast with the sensory branches that travel posteriorly.
The phrenic nerve arises from the anterior rami of C3-C5. It provides motor innervation to the diaphragm. The relaxed position of the diaphragm is domed, so the nerve acts to flatten it, and therefore expands the volume of the thorax. This in turn, lowers the pressure in the lungs, thus drawing in external air.
The nerve travels down the front of the anterior scalene muscle, and enters the thorax . Once the nerve enters the thorax, it travels anterior to the lung root. The vagus nerve travels posterior to the lung root. It innervates the diaphragm from below (C4 is the major root).
Learning tip: A good memory aid for the roots of the phrenic nerve is: C3,4,5 keeps the diaphragm alive.
Nerves to Geniohyoid and Thyrohyoid
Both the geniohyoid and the thyrohyoid move the airway. The C1 root innervates the geniohyoid (moves the hyoid anteriorly and superiorly), and the thyrohyoid (depresses the hyoid and elevates the larynx). Both these nerves will run along side the hypoglossal nerve (cranial nerve number 12) before reaching their respective muscles.
The ansa cervicalis (goose’s neck) is a loop of nerves, formed by the superior nerve root C1, and the inferior nerve root C2-3. The loop gives rise to four muscular branches that innervate the following muscles:
- Superior belly of the omohyoid muscle
- Inferior belly of omohyoid muscle
These infrahyoid muscles act to depress the hyoid bone; an important function for swallowing and speech.
There are four sensory branches of the cervical plexus. These supply the skin of the neck, upper thorax, scalp and ear. All four nerves enter the skin at the middle of the posterior border of the sternocleidomastoid. This area is known as the nerve point of the neck (Erb’s point), and is targetted when performing a superficial cervical plexus nerve block.
Greater Auricular Nerve
It is the largest ascending branch of the cervical plexus and arises from the C2 and C3 roots. It provides sensation to the skin over the parotid area and the external ear. The nerve also communicates with the auricular branch of the vagus nerve and the posterior auricular branch of the facial nerve (this nerve supplies a few small muscles around the ear).
Transverse Cervical Nerve
This nerve arises from C2 and C3. To reach its target, it will run around the back of sternocleidomastoid, and supply sensation to the anterior aspect of the neck. It will pierce the deep cervical fascia , and then provide branches to supply superior and inferiorly, and reach the skin of the upper sternum and neck.
Lesser Occipital Nerve
This nerve is derived from the C2 nerve root. This nerve passes around the accessory nerve (cranial nerve 11), and passes superiorly, close to the posterior border of the sternocleidomastoid. It supplies sensation to the scalp that lies posterosuperior to the ear, and frequently communicates with the lateral branches of the greater auricular nerve.
These are three nerves that arise from the C3 and C4 roots. They form beneath the posterior border of sternocleidomastoid, and supply sensation to the skin over pectoralis major and deltoid muscles, as well as the superior and posterior aspect of the sternoclavicular joint and shoulder.
Upper Limb: Brachial Plexus
The brachial plexus supplies the muscles of and relays sensation from the upper limb. It is formed by the ventral rami of C5-T1. It has numerous branches that we will discuss below.
The posterior cord is formed from the unification of branches of the upper, middle and lower trunks. It gives rise to the upper and lower subscapular nerves (both from C5-6) which both supply subscapularis (a medial shoulder rotator). Only the lower supplies teres major.
The next branch is the thoracodorsal nerve (C6-C8) that supplies latissimus dorsi. This muscle is an adductor, extensor and internal rotator of the shoulder.
The axillary nerve (C5-6) supplies teres minor and deltoid muscles, and also relays sensation from the regimental badge area of the shoulder.
The termination and largest branch of the posterior cord is the radial nerve (C5-T1), which supplies all posterior muscles of the arm, and relays sensation from the posterior aspect of the arm. It also supplies brachioradialis, an elbow flexor; hence individuals with musculocutaneous nerve injuries can still flex their elbows due to this muscle. It leaves the axilla via the triangular interval, and runs in the spiral groove of the humerus. Compression of the axilla often affects this nerve, leading to wrist drop.
The medial cord gives rise to the medial cutaneous nerve of the arm (C8-T1) and of the forearm (C8-T1), as well as the ulnar nerve (C8-T1). The latter supplies flexor carpi ulnaris, the ulnar head of flexor digitorum profundus and the majority of intrinsic hand muscles (all the interossei and medial two lumbricals ). The ulnar nerve also supplies sensation to the medial 1½ fingers on their palmar and dorsal surfaces.
The medial and lateral cords unite to form the median nerve (C5-T1), which supplies all the wrist flexors (apart from those supplied by the ulnar nerve) as well as the muscles of the thenar eminence and lateral two lumbricals. It also relays sensation from the palmar 3½ fingers and the nail beds of these fingers dorsally. The commonest clinical problem related to this nerve is carpal tunnel syndrome, where the nerve is compressed below the flexor retinaculum due to flexor tendon overuse or tendinous oedema/swelling.
This is the plexus of the lower limb. It is formed by the ventral rami of L1-L4. A good acronym to remember the branches is: ‘I Twice Get Lunch On Fridays’
This gives you the following branches:
Iliohypogastric (L1) : It supplies motor innervation to transversus abdominis and internal oblique muscles. It also supplies the skin over part of the gluteal region and pubis.
Genitofemoral (L1-L2): It has a genital branch that runs within the spermatic cord and provides motor innervation to the cremaster muscle. It also relays sensation from the external genitalia. The femoral branch supplies sensation to a small patch of skin on the upper medial thigh.
Lateral Femoral cutaneous (L2-L3): This relays sensation from the lateral aspect of the thigh.
Obturator (ventral divisions of L2-L4): It supplies motor innervation to the muscles of the medial/adductor compartment.
Femoral (dorsal divisions of L2-L4): It supplies the muscles of the anterior compartment of the thigh, as well as sensation over the thigh via the medial and intermediate cutaneous nerves of the thigh.
The lower limb rotates 180 degrees during early development of the limbs in utero. As a result, the femoral nerve (which innervates the anterior compartment of the thigh) arises from the posterior divisions of the lumbar plexus, and the obturator nerve arises from the anterior divisions.
For the same reason, we can discuss why the lumbar nerves should innervate the leg muscles at all. They are not lumbar structures. Again, the embryology of the lower limb shows us that the quadriceps femoris are muscles that have migrated from the lumbar region (the trunk) down to the thigh.
The sacral plexus is fiendishly difficult to memorize. In general terms, it supplies the muscles of the gluteal region (gluteal muscles, short external rotators of the hip) and also the pelvic sphincters.
The superior gluteal nerve arises from L4-S1, and innervates gluteus medius and minimus, and tensor fascia lata.
The inferior gluteal nerve arises from L5-S2, and innervates gluteus maximus. Other nerves innervate the short external rotators of the hip and these include:
- the nerve to piriformis (L5, S1-S2, piriformis divides the greater sciatic foramen into two openings)
- the nerve to obturator internus (L5-S1)
- the nerve to superior gemellus (L5-S1)
- the nerve to inferior gemellus (L5-S1)
- the nerve to obturator externus (L3-L4)
- the nerve to quadratus femoris (L5-S1)
They originate from the brainstem and brain, but these twelve pairs of nerves are in fact part of the peripheral nervous system.
- Olfactory nerve (sense of smell): It enters the skull via the cribriform plate.
- Optic nerve (sight): It enters the skull via the optic canal.
- Oculomotor: The name offers a clear clue as to its function. It moves the eye by innervating four of the six extraocular muscles. These are inferior rectus, superior rectus, medial rectus and inferior oblique. It also innervates levator palpebrae superioris that elevates the upper eyelid, and constrictor pupillae, which constricts the pupil. It leaves the skull via the superior orbital fissure.
- Trochlear nerve:It moves the eye down and out and innervates the superior oblique muscle. It leaves the skull via the superior orbital fissure.
- Trigeminal nerve: (V1- Ophthalmic, V2- Maxillary, V3- Mandibular, sensation to face and innervation of chewing muscles i.e. masseter, medial and lateral pterygoids and temporalis). V1 enters via the superior orbital fissure, V2 enters via the foramen rotundum, and V3 enters via the foramen ovale.
- Abducens nerve: The name again clearly indicates the function of this nerve. It moves the eye laterally by innervating the lateral rectus and leaves the skull via the superior orbital fissure.
- Facial nerve: It innervates the muscles of facial expression, i.e. it moves the face. It has a special branch called the chorda tympani, which gives the sense of taste to the anterior 2/3 of the tongue. It also gives off the nerve to stapedius, which dampens down loud sounds by stabilizing the stapes. It also innervates the lacrimal gland , and submandibular and sublingual glands. The motor branch leaves the skull via the stylomastoid foramen.
- Vestibulocochlear nerve: It has a vestibular branch (balance) and a cochlear branch (hearing). This nerve does not leave the skull and terminates in the inner ear.
- Glossopharyngeal nerve: (taste from the posterior 1/3 of the tongue, sensation to pharynx i.e. afferent limb of the gag reflex, innervation of one muscle, stylopharyngeus). It leaves the skull via the large J shaped jugular foramen.
- Vagus nerve: Parasympathetic innervation to the whole body down to the splenic flexure. The recurrent laryngeal nerve also innervates the muscles that move the larynx, airway and enable us to change the pitch and volume of our voices. It is also the motor part of the gag reflex. It leaves the skull via the jugular foramen.
- Accessory nerve: It innervates the sternocleidomastoid and trapezius muscles. It leaves the skull via the jugular foramen.
- Hypoglossal nerve:It innervates all tongue muscles (except palatoglossus, which is innervated by the vagus nerve). It leaves the skull via the hypoglossal canal.