Peripheral Nervous System Flashcards
Parasympathetic Innervation of Head and Neck
The parasympathetic fibres begin in the central nervous system. The nerves supplying the head and neck are situated within four nuclei, located within the brainstem. Each nucleus is associated with a cranial nerve (the oculomotor, facial, glossopharyngeal and vagus nerves) – these nerves carry the parasympathetic fibres out of the brain.
After leaving the brain, the parasympathetic fibres from each nuclei synapse in a peripheral ganglion (a collection of neurone cell bodies outside the CNS). These ganglia are typically located near to the target viscera. From the ganglia, post-ganglionic parasympathetic fibres continue to the organs in the head and neck, providing parasympathetic innervation.
There are four parasympathetic ganglia located within the head – ciliary, otic, pterygopalatine and submandibular. They receive fibres from the oculomotor, facial and glossopharyngeal nerves (the vagus nerve only innervates structures in the thorax and abdomen).
Ciliary Ganglion
The ciliary ganglion is located within the bony orbit. It is situated anteriorly to the superior orbital fissure, between the lateral rectus muscle and the optic nerve.
Pre-ganglionic fibres: The ciliary ganglion is supplied by fibres from the Edinger-Westphal nucleus (associated with the oculomotor nerve).
Post-ganglionic fibres: The parasympathetic fibres leave the ganglion via the short ciliary nerves. These fibres continue into the orbit to innervate structures of the eye.
Target Organs: The post-ganglionic fibres from the ciliary ganglion innervate the sphincter pupillae (contracts the pupil) and the ciliary muscles (accommodates for near vision).
In addition, two sets of nerve fibres pass through the ciliary ganglion without synapsing:
Sympathetic nerves from the internal carotid plexus – innervate the dilator pupillae muscle
Sensory fibres from the nasociliary nerve (a branch of the ophthalmic division of the trigeminal nerve) – innervate the cornea, ciliary body and iris.
Pterygopalatine Ganglion
The pterygopalatine ganglion (also known as sphenopalatine) is the largest of the four parasympathetic ganglia. It is located within the pterygopalatine fossa – a space located inferiorly to the base of the skull, and posteriorly to the maxilla.
Pre-ganglionic fibres: The pterygopalatine ganglion is supplied by fibres from the superior salivatory nucleus (associated with the facial nerve). These fibres travel within the greater petrosal nerve and the nerve of the pterygoid canal to reach the ganglion.
Post-ganglionic fibres: The parasympathetic fibres leave the ganglion by hitch-hiking on branches of the maxillary nerve (derived from the trigeminal nerve).
Target organs: The post-ganglionic fibres from the pterygopalatine ganglion provide secretomotor innervation to lacrimal gland, mucous glands of posterosuperior nasal cavity, nasopharynx, and the palate.
Sympathetic fibres from the internal carotid plexus and sensory branches from the maxillary nerve pass through the pterygopalatine ganglion without synapsing.
Submandibular Ganglion
The submandibular ganglion is located inferiorly to the lingual nerve, from which it is suspended.
Pre-ganglionic fibres: The ganglion is supplied by fibres from the superior salivatory nucleus (associated with the facial nerve). These fibres are carried within a branch of the facial nerve, the chorda tympani. This nerve hitch-hikes along the lingual branch of the mandibular nerve to reach the ganglion.
Post-ganglionic fibres: Fibres leave the ganglion and travel directly to the submandibular and sublingual glands.
Target Organs: Secretomotor innervation to the submandibular and sublingual salivary glands.
Sympathetic fibres from the facial artery plexus pass through the submandibular ganglion. They are thought to innervate glands in the base of the oral cavity.
Otic Ganglion
The otic ganglion is located inferiorly to the foramen ovale, within the infratemporal fossa. It is medial to the mandibular branch of the trigeminal nerve.
Pre-ganglionic fibres: The ganglion is supplied by fibres from inferior salivatory nucleus (associated with the glossopharyngeal nerve). Parasympathetic fibres travel within a branch of the glossopharyngeal nerve, the lesser petrosal nerve, to reach the otic ganglion.
Post-ganglionic fibres: The parasympathetic fibres hitchhike along the auriculotemporal nerve (branch of the mandibular division of the trigeminal nerve) to provide secretomotor innervation to the parotid gland.
Target Organs: The post-ganglionic fibres from the otic ganglion provide innervation to the parotid gland.
Sympathetic fibres from the superior cervical chain pass through the otic ganglion. They travel with the middle meningeal artery to innervate the parotid gland.
Summary Head and Neck Parasymp
Sympathertic Innervation to Head and Neck
The sympathetic fibres begin in the spinal cord. They originate from the thoracic region (T1-6), and therefore need to ascend to reach the structures in the head and neck.
After leaving the spinal cord, the fibres enter the sympathetic chain. This structure spans from the base of the skull to the coccyx, and is formed by nerve fibres and ganglia (collections of nerve cell bodies). There are three ganglia within this chain that are of interest – thesuperior, middle and inferior cervical ganglia. The sympathetic fibres synapse with these ganglia, with post ganglionic branches continuing into the head and neck.
Each of the three ganglia are related to specific arteries in the head and neck. The post-ganglionic fibres hitch-hike along these arteries (and their branches) in order to reach their target organs.
Superior Cervical Ganglion
The superior cervical ganglion is located posteriorly to the carotid artery, and anterior to the C1-4 vertebrae. Several important post-ganglionic nerves originate from here:
Internal carotid nerve – hitch-hikes along the internal carotid artery, forming a network of nerves. Branches from the internal carotid plexus innervate structures in the eye, the pterygopalatine artery and the internal carotid artery itself.
External carotid nerve – hitch-hikes along the common and external carotid arteries, forming a network of nerves. It innervates the smooth muscle of the arteries.
Nerve to pharyngeal plexus – combines with branches from the vagus and glossopharyngeal nerves to form the pharyngeal plexus.
Superior cardiac branch – contributes to the cardiac plexus in the thorax.
Nerves to cranial nerves II, III IV, VI and IX.
Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C1-C4.
Middle Cervical Ganglion
The middle cervical ganglion is absent in some individuals. When present, it is located anteriorly to the inferior thyroid artery and the C6 vertebra. Its postganglionic fibres are:
Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C5 and C6.
Thyroid branches – travel along the inferior thyroid artery, distributing fibres to the larynx, trachea, pharynx and upper oesophagus.
Middle cardiac branch – contributes to the cardiac plexus in the thorax.
Inferior Cervical Ganglion
The inferior cervical ganglion is situated anteriorly to the C7 vertebra. It is occasionally fused with the first thoracic vertebrae, forming the cervicothoracic ganglion. There are three post-ganglionic fibres that arise from this ganglion:
Gray rami communicantes – distributes sympathetic fibres to the anterior rami of C7, C8 and T1.
Branches to the subclavian and vertebral arteries – innervates the smooth muscle present in the arteries.
Inferior cardiac nerve – contributes to the cardiac plexus in the thorax.
Summary Symp Head and Neck
Cervical Plexus
The cervical plexus gives rise to numerous branches which supply structures in the head and neck. They can broadly be divided into two groups – muscular branches and sensory branches.
Phrenic Nerve
The phrenic nerve arises from the anterior rami of C3-C5. It provides motor innervation to thediaphragm.
After arising from the cervical plexus, the nerve travels down the surface of the anterior scalene muscle, and enters the thorax. In the thoracic cavity, the nerve descends anteriorly to the root of the lung to reach the diaphragm.
A good memory aid for the roots of the phrenic nerve is C3,4,5 keeps the diaphragm alive.
Nerves to Geniohyoid and Thyrohyoid
The C1 spinal nerve gives rise to nerves to the geniohyoid (moves the hyoid bone anteriorly and upwards, expanding the airway) and the thyrohyoid (which depresses the hyoid bone and elevates the larynx).
These nerves travel with the hypoglossal nerve to reach their respective muscles.
Other Muscular Branches
Several other minor branches arise from the nerve roots to supply muscles of the neck and back:
C1-C2: Rectus capatis anterior and lateralis
C1-C3: Longus capatis
C2-C3: Prevertebral muscles and sternocleidomastoid
C3-C4: Levator scapulae, trapezius and scalenus medius
The middle and anterior scalenus muscles also receive innervation directly from the cervical plexus.
Spinal Nerves
The spinal nerves C1 – C4 form the basis of the cervical plexus.
At each vertebral level, paired spinal nerves leave the spinal cord via theintervertebal foramina of the vertebral column.
Each nerve then divides into anterior and posterior nerve fibres. The cervical plexus begins as the anterior fibres of the spinal nerves C1, C2, C3 and C4.
These fibres combine with each other to form the branches of the cervical plexus.
Sensory Branches
The cutaneous branches of the cervical plexus supply the skin of the neck, upper thorax, scalp and ear. These nerves all 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 utilised when performing a cervical plexus nerve block.
Greater Auricular Nerve
The greater auricular nerve is formed by fibres from C2 and C3 roots. It provides sensation to the external ear and the skin over the parotid gland. It is the largest ascending branch of the plexus.
The nerve also communicates with the auricular branch of the vagus nerve and theposterior auricular branch of the facial nerve (which innervates some small muscles around the ear).
Transverse Cervical Nerve
The transverse cervical nerve is also formed by fibres from C2 and C3. It curves around the posterior aspect of the sternocleidomastoid, and supplies sensation to the anterior neck. The nerve then pierces the deep cervical fascia and then gives branches that pass superiorly and inferiorly to supply the anterolateral skin of the neck and upper sternum.
Lesser Occipital Nerve
The lesser occipital nerve is derived solely from the C2 root. It supplies cutaneous sensation to the posterosuperior scalp, and commonly communicates with the posterior branch of the greater auricular nerve.
After its formation, the nerve curves around the accessory nerve, and passes superiorly, close to the posterior border of the sternocleidomastoid.
Supraclavicular Nerves
The supraclavicular nerves are a group of nerves formed from the C3 and C4 roots.
They arise from the behind the posterior border of sternocleidomastoid, and provide sensation to the skin overlying the suprascapular fossa and upper thoracic region and sternoclavicular joint.
Formation of Spinal Nerves
The spinal nerves are mixed nerves that originate from the spinal cord, forming the peripheral nervous system.
Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.
The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:
Posterior rami – supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.
Anterior rami – supplies nerve fibres to much of the remaining area of the body, both motor and sensory.
The nerve roots L2-S5 arise from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina.
Brachial Plexus
The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and enters the upper arm.
The plexus is formed by the anterior rami (divisions) of the cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.
Musculocutaneous Nerve
Roots: C5, C6, C7.
Motor Functions: Innervates the brachialis, biceps brachii and corocobrachialis muscles.
Sensory Functions: Gives off the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral portion of the posterior forearm.
Axillary Nerve
Roots: C5 and C6.
Motor Functions: Innervates the deltoid, teres minor and the long head of the triceps brachii.
Sensory Functions: Gives off the superior lateral cutaneous nerve of arm, which innervates the inferior region of the deltoid (“regimental badge area”).
Median Nerve
Roots: C6 – C8 and T1.
Motor Functions: Innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbrical muscles that move the index and middle fingers.
Sensory Functions: Gives off the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.
Radial Nerve
Roots: C5-C8 and T1.
Motor Functions: innervates the triceps brachii, and the extensor muscles in the posterior compartment of the forearm.
Sensory Functions: Innervates the posterior aspect of the arm and forearm, and the posterior, lateral aspect of the hand.
Ulnar Nerve
Roots: C8 and T1.
Motor Functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus.
Sensory Functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and associated palm area.
Minor Branches of Brachial Plexus
Iliohyogastric Nerve
The iliohypogastric nerve is the first major branch of the lumbar plexus. It runs to the iliac crest, across the quadratus lumborum muscle of the posterior abdominal wall. It then perforates the transversus abdominis, and divides into its terminal branches.
Roots: L1 (with contributions from T12).
Motor Functions: Innervates the internal oblique and transversus abdominis.
Sensory Functions: Innervates the posterolateral gluteal skin in the pubic region. (Tip: an easy way to remember that the IlioHypogastric comes before the IlioInguinal is that H comes before I in the alphabet!)
Ilioinguinal Nerve
The ilioinguinal nerve follows the same anatomical course as the larger iliohypogastric nerve. After innervating the muscles of the anterior abdominal wall, it passes through the superficial inguinal ring to innervate the skin of the genitalia and middle thigh.
Roots: L1.
Motor Functions: Innervates the internal oblique and transversus abdominis.
Sensory Functions: Innervates the skin on the upper middle thigh. In males, it also supplies the skin over the root of the penis and anterior scrotum. In females, it supplies the skin over mons pubis and labium majus.
GenitoFemoral Nerve
After leaving the psoas major muscle, the genitofemoral nerve quickly divides into a genital branch, and a femoral branch.
Roots: L1, L2.
Motor Functions: The Genital branch innervates the cremasteric muscle.
Sensory Functions: The Genital branch innervates the skin of the anterior scrotum (in males) or the skin over mons pubis and labium majus (in females). The Femoral branch innervates the skin on the upper anterior thigh.
Lateral Cutaneous Nerve of Thigh
This nerve has a purely sensory function. It enters the thigh at the lateral aspect of the inguinal ligament, where it provides cutaneous innervation to the skin there.
Roots: L2, L3
Motor Functions: None.
Sensory Functions: Innervates the anterior and lateral thigh down to the level of the knee.
Obturator Nerve
Roots: L2, L3, L4.
Motor Functions: Innervates the muscles: obturator externus, pectineus, adductor longus, adductor brevis, adductor magnus, gracilis.
Sensory Functions: Innervates the skin over the medial thigh.
Femoral Nerve
Roots: L2, L3, L4.
Motor Functions: Innervates the muscles: Illiacus, pectineus, sartorius, all the muscles of quadriceps femoris.
Sensory Functions: Innervates the skin on the anterior thigh and the medial leg.
Sacral Plexus
The anterior rami of the S1-S4 spinal roots (and the lumbosacral trunk) divide into several cords. These cords then combine together to form the five major peripheral nerves of the sacral plexus.
These nerves then descend down the posterior pelvic wall. They have two main destinations:
Leave the pelvis via the greater sciatic foramen – these nerves enter the gluteal region of the lower limb, innervating the structures there.
Remain in the pelvis – these nerves innervate the pelvic muscles, organs and perineum.
Superior Gluteal Nerve
Roots: L4, L5, S1.
Motor Functions: Innervates the gluteus minimus, gluteus medius and tensor fascia latae.
Sensory Functions: None.
A useful memory aid for the major branches of the sacral plexus is ‘Some Irish Sailor PestersPolly’. This stands for Superior Gluteal, Inferior Gluteal, Sciatic, Posterior cutaneous nerve of thigh, Pudendal
Inferior Gluteal Nerve
Roots: L5, S1, S2.
Motor Functions: Innervates gluteus maximus.
Sensory Functions: None.
Sciatic Nerve
Nerve Roots: L4-S3.
Motor: Innervates the muscles of the posterior thigh, and the hamstring portion of the adductor magnus. Indirectly innervates (via its terminal branches) the muscles of the leg and foot.
Sensory: No direct sensory function. Indirectly innervates (via its terminal branches) the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot.
Posterior Femoral Cutaneous
The posterior cutaneous nerve of thigh leaves the pelvis via the greater sciatic foramen, entering the gluteal region inferiorly to the piriformis muscle. It descends deep to the gluteus maximus and runs down the back of the thigh to the knee.
Roots: S1, S2, S3
Motor Functions: None
Sensory Functions: Innervates the skin on the posterior surface of the thigh and leg. Also innervates the skin of the perineum.
Pudendal Nerve
This nerve leaves the pelvis via the greater sciatic foramen, then re-enters via the lesser sciatic foramen. It moves anterosuperiorly along the lateral wall of the ischiorectal fossa, and terminates by dividing into several branches.
Roots: S2, S3, S4
Motor Functions: Innervates the skeletal muscles in the perineum, the external urethral sphincter, the external anal sphincter, levator ani.
Sensory Functions: Innervates the penis and the clitoris and most of the skin of the perineum.
(Tip – an easy way to remember the functions of the pudendal nerve is S2, S3, S4 keeps poo off the floor!)
Tibial Nerve
The tibial nerve is a branch of the sciatic nerve, and arises at the apex of the popliteal fossa. It travels through the popliteal fossa, giving off branches to muscles in the superficial posterior compartment of the leg. Here, the tibial nerve also gives rise to branches that contribute towards the sural nerve, which innervates the posterolateral aspect of the leg.
The tibial nerve continues its course down the leg, posterior to the tibia. During its descent, it supplies the deep muscles of the posterior leg.
At the foot, the nerve passes posteriorly and inferiorly to themedial malleolus, through a structure known as the tarsal tunnel. This tunnel is covered superiorly by the flexor retinaculum. Within this tunnel, branches arise from the tibial nerve to supply cutaneous innervation to the heel
Immediately distal to the tarsal tunnel, the tibial nerveterminates by dividing into sensory branches, which innervate the sole of the foot.
Sensory Functions of Tibial Nerve
In the popliteal fossa, the tibial nerve gives off cutaneous branches. These combine with branches from the common fibular nerve to form the sural nerve. This sensory nerve innervates the skin of the posterolateral side of the leg and the lateral side of the foot.
The tibial nerve also supplies all the sole of the foot via three branches:
Medial calcaneal branches: These arise within the tarsal tunnel, and innervate the skin over the heel.
Medial Plantar Nerve: Innervates the plantar surface of the medial three and a half digits, and the associated sole area.
Lateral Plantar Nerve: Innervates the plantar surface of the lateral one and a half digits, and the associated sole area.
Common Fibular Nerve
Nerve roots: L4 – S3
Motor: Innervates the short head of the biceps femoris directly. Also supplies (via branches) the muscles in the lateral and anterior compartments of the leg.
Sensory: Innervates the skin over the upper lateral and lower posterolateral leg. Also supplies (via branches) cutaneous innervation to the skin of the anterolateral leg, and the dorsum of the foot.
Sensory Functions of Common Fibular
There are two cutaneous branches that arise directly from the common fibular nerve as it moves over the lateral head of the gastrocnemius.
Sural commicating nerve: This nerve combines with a branch of the tibial nerve to form the sural nerve. The sural nerve innervates the skin over the lower posterolateral leg.
Lateral sural cutaneous neve: Innervates the skin over the upper lateral leg.
In addition to these nerves, the terminal branches of the common fibular nerve also have a cutaneous function:
Superficial fibular nerve: Innervates the skin of the anterolateral leg, and dorsum of the foot (except the skin between the first and second toes).
Deep fibular nerve: Innervates the skin between the first and second toes.
