Peripheral Nervous System

Learning & Test Objectives

  1. Ganglia:

    • Types and functions.
    • Anatomical distribution.
  2. Autonomic nervous system:

    • Anatomical distribution of the sympathetic division.
    • Anatomical distribution of the parasympathetic division.
    • Functional coordination of both systems and their outcomes on organs.
  3. Spinal & Cranial nerves:

    • The 12 cranial nerves, Cervical plexus, Intercostal nerves, Brachial plexus, Lumbar Plexus, Sacral plexus.
    • Origin from segments/site at the CNS.
    • Type of carried fibres.
    • Course and distribution.
    • Dermatomes as well cutanous innervation.
    • Exerted function and lesion dysfunctional outcome.
    • Anatomical bases of the reflex arch, including muscle spindle and tendon organ.

Description of the test

The test is held by the general rules of written tests (see Continuous Testing – Organisation).

The test on the peripheral nervous system is in written format and follows the general rules for written tests (see Continuous testing – organisation). The main topics of the test are the spinal and cranial nerves and the autonomic nervous system. Students are required to know the origin, course, branches and structures innervated by each cranial and spinal nerve. It is necessary for the student to know the spinal cord segments of each spinal nerve. In particular students should know the spinal segments that form the phrenic nerve and the main nerves of the trunk (median, ulnar, radial, femoral, obturator and sciatic nerves). For the cranial nerves it is necessary to know the type of fibres found in each nerve and their functions. In this test it is not necessary to know the brain stem nuclei that give rise to the cranial nerves. For the autonomic system it is necessary to know the main divisions, the ganglia, the main nerves and the structures they innervate. The test can also contain questions on the general structure of the peripheral nervous system, dermatomes, head zones, peripheral nerve fields (areae nervinae), topographic spaces where nerves run (e.g. axilla, common tendinous ring, cavernous sinus), clinical correlations and other anatomical curiosities. Radiological images will not be tested. At the end of the test the student is required to draw and annotate one of the required schemes (see below).

 

Clinical correlations

Unilateral damage to the phrenic nerve leads to ipsilateral paresis of the diaphragm and unilateral respiratory impairment.

Transverse lesions of the spinal cord above C4 often lead to respiratory arrest. This is common in vertebral column trauma.

Paresis of the long thoracic nerve impairs the function of the serratus anterior. The scapula no longer strongly adheres to the thorax and so protrudes from the patient's back as what is called a winged scapula (scapula alata). Abduction of the arm above the horizontal plane is impaired.

The median nerve is most commonly damaged in the carpal tunnel, which results in carpal tunnel syndrome. Shoulder dislocation and stab and bullet wounds can damage the nerve in the axilla. Colles' fracture and slash wounds on the wrist may damage the nerve in the forearm. Damage to the median nerve results in impaired pronation of the forearm (pronator teres and quadratus), impaired opposition and flexion of the thumb (opponens pollicis and the deep head of the flexor pollicis brevis) and impaired flexion of the second and third finger. Flexion of the fourth and fifth finger is preserved, due to the part of the flexor digitorum profundus innervated by the ulnar nerve. The clinical sign of impaired flexion of the second and third finger is called the ‘hand of benediction’ or ‘preacher's hand’.

The ulnar nerve is most commonly harmed in narrow anatomical spaces such as the cubital and ulnar canals. Fractures of the elbow and forearm often damage both the ulnar and median nerves. Injury to the ulnar nerve results in impairment of motor function giving rise to the clinical appearance of a ‘claw hand’.

Axillary nerve palsy can occur due to a fracture of the surgical neck of the humerus. It manifests as impaired abduction of the arm and predisposes to subluxations of the shoulder due to instability of the head of the humerus.

Radial nerve injuries are fairly common. They are usually caused by fractures of the distal part of the humerus affecting the groove for the radial nerve (Holstein-Lewis fractures). Fractures of the forearm may also affect the radial nerve. Radial nerve injuries manifest as an absent the tricipital reflex and weakness in elbow extension. Another prominent sign of radial nerve palsy is an inability to extend the wrist causing a condition called wrist drop.

Pelvic fractures, hip dislocations, surgeries and incorrectly applied intramuscular injections can harm the femoral nerve. Induration of the inguinal lymphatic nodes and aneurysms of the femoral artery are the most common non-traumatic causes of femoral nerve injury. The motor symptoms of femoral nerve injury occur as are a result of impaired function of the quadriceps femoris. Walking without support and walking upstairs become difficult. It is impossible for the patient to raise his or her lower limb when lying supine. Standing and squatting becomes unstable. In certain cases, hyperextension of the knee (genu recurvatum, back knee) may occur when standing.

Damage to the tibial nerve usually occurs in the malleolar canal and can result from cut wounds, ankle fractures and incorrectly applied plasters. It presents as impaired plantar flexion (an inability to stand on tiptoes) due to impaired function of the triceps surae. The ankle jerk reflex is absent. Due to the predominance of the unantagonised dorsal flexion of the tibialis anterior, the heel is the first part of the foot to touch the ground while walking. This results in a syndrome called ‘club foot’ (pes calcaneus).

The common fibular nerve is covered only by skin where it lies near the head of the fibula. In this location the nerve is most vulnerable to trauma and compression from incorrectly applied casts. Dislocations and sprains of the knee may also harm this nerve. The anterior muscles of the leg become impaired, resulting in a ‘flapping foot’ and the inability to stand on the heels. The arches of the foot gradually deteriorate due to dysfunction of the fibularis longus.

Head's zones are areas of skin innervated by the same spinal cord segments as particular internal organs. Due to the rich interconnections of neurones within the spinal cord (a phenomenon known as convergence), diseases of the internal organs may cause pain or hyperesthesia of certain skin areas. This is called referred or radiating pain. The skin of the anterior thorax and left upper limb corresponds to the heart. Cardiac pain can radiate further to the ulnar side of the left forearm and the little finger. This occurs in angina pectoris and myocardial infarction.

Proper knowledge of dermatomes helps determine the level of spinal cord injury.

Intervertebral disc herniation is the most common cause of harm to the spinal nerves. It typically occurs at the spinal roots of L5 or S1. The pain typically radiates into the corresponding dermatomes. This differs from pain in peripheral nerve fields caused by injury to a peripheral nerve.

Pain in the L5 dermatome spreads on the lateral side of the thigh and leg down to the dorsal side of the foot and big toe. This segment innervates several muscles. It results in weakness of plantar extension and the inability to walk on heels or extend the big toe.

Pain in the S1 segment radiates on the posterior thigh and calf towards the fifth toe. The function of the gluteus maximus is impaired and the patient has difficulties standing up from sitting and struggles to walk on tip toes.

Unilateral paresis of the oculomotor nerve causes divergent strabismus, a dilated pupil (mydriasis), impaired accommodation (cycloplegia), ptosis and double vision (diplopia).

Unilateral paresis of the trochlear nerve causes diplopia while looking down and laterally.

Trigeminal neuralgia / prosopalgia is characterised by intensive unbearable episodic facial pain in areas innervated by branches of the trigeminal nerve (the supraorbital, infraorbital and mental nerves). The most common cause is impingement of the trigeminal nerve by an aberrant loop of the superior cerebellar artery at the junction between the CNS and PNS, where oligodendrocytes and Schwann cells meet (the Obersteiner-Redlich zone). This may be treated by microvascular decompression. Non-lesional causes can be treated by gamma-knife surgery (radiosurgery) of the trigeminal ganglion.

Central paralysis of the facial nerve presents as a drop of the contralateral angle of the mouth. However, innervation of both eyelids remains unaltered. The most common cause is a stroke.

Peripheral paralysis of the facial nerve may arise from a lesion at any point of the nerve. It presents as dysfunction of both the upper and lower branches, so that one whole half of the face is affected. Other than paresis of the orbicularis oris it has other manifestation, including:
Lagophthalmus – the inability to close the eye, caused by dysfunction of the orbicularis oculi.
Xerophthalmia – conjunctival and corneal dryness caused by dysfunction of the lacrimal gland. It increases the risk of corneal scaring and subsequent blindness and eye infections.
Xerostomia – dryness of the mouth due to dysfunction of the sublingual and submandibular glands.
Hyperacusis – pain when hearing loud sounds due to an impaired stapedial reflex.
Hypogeusis – decreased perception of taste caused by dysfunction of the chorda tympani.

Unilateral paresis of the vagus nerve causes dysphagia, hoarseness and divergence of the uvula.

Bilateral paresis of the vagus nerve causes rhinolalia, speech defects (dysarthria) and paresis of both vocal cords.

Impairment of the sympathetic internal carotid plexus (plexus caroticus internus) may be caused by dissection of the internal carotid artery (usually traumatic or postoperative separation of the intima from the media) or by damage to the cervicothoracic ganglion (by tumors in the apex of the lungs). The clinical manifestation is called Claude Bernard-Horner syndrome. It presents as ptosis (drooping of the upper eyelid), miosis (constricted pupil), enophthalmos (posterior displacement of the eyeball) and anhidrosis in the face (absence of sweating).
 

Anatomical curiosities

The most commonly assessed reflexes of the upper extremity:


  • the bicipital reflex (C5)
  • the brachioradial reflex (C5–C6)
  • the tricipital reflex (C7)
  • the finger flexors (C8)
     

The most commonly assessed reflexes of the abdomen:


  • the epigastric region is stimulated for assessing spinal segments T7-8
  • the mesogastric region is stimulated for assessing spinal segments T9–10 

  • the hypogastric region is stimulated for assessing spinal segments T11–12
     

The most commonly assessed reflexes of the lower extremity:

  • the patellar reflex (L2-L4)
  • the Achilles tendon reflex (L5-S2)
     

The Cannon-Böhm point corresponds to a developmental border between the midgut (innervated by the vagus nerve) and hindgut (innervated by the sacral splanchnic nerves).

The solar plexus is an obsolete term for the coeliac plexus
 

Required schemes for drawing

  • Scheme of the spinal nerve
  • Dermatomes of the trunk and extremities
  • Sensory innervation of the head and extremities
  • The brachial plexus
Created: 27. 3. 2017 / Modified: / Responsible person: MUDr. Azzat Al-Redouan