Lower motor neuron

Anatomy and physiology

• Location: anterior horn of the spinal cord + motor nuclei of cranial nerves

• Each LMN is connected (through arborization of the terminal part of its efferent fiber) to a group of muscle fibers (few to 100) = MOTOR UNIT (MU)

• MU – central concept of peripheral nervous system organization and function

MU

• Variations in force, range, rate and type of movements are related to the number and size of the MU recruited in contraction and to the sequence and frequency of firing into each MU

• Muscles are innervated according to the segments of the spinal cord in a “metameric “ distribution.

• Motor nerve fiber of each ventral root participate together with the neighboring roots to the plexuses formation

• So each large muscle can be innervated by 2 or more roots

• But a single peripheral nerve innervates completely a muscle or a group of muscles

•

• Tendon reflex activity and muscle tone are controlled by – large (alpha) motor neurons– Muscle spindles– Afferent fibers of the muscle spindles– Small anterior horn cell (gamma) motor neurons –

axons are connected with the spindles.

Myotatic stretch reflex

• Tap on the tendon – activates nuclear bag fibers inside the spindles

• Afferent projections synapse with the alpha motor neurons (in the same and adjacent segments)

• Impulses to skeletal muscles – monosynaptic muscle contraction = strech reflex

• Antagonist muscles are inhibited through disynaptic connections in the same time.

LMN lesion – clinical picture

• Motor deficit – restricted to the muscle fibers that are innervated by the injured LMNs

• Diminished or loss of muscle tone – hypotonia in the same territory.

• Muscle fibers amyotrophic changes - refers to the muscle fibers part of the affected MU – we can have “group amyotrophy”

• Tendon and cutaneous reflexes controlled by the injured LMNs– diminished or lost

• Fasciculations – involuntary rippling of muscles without moving segments due to pericarional and axonal membrane instability and generation of spontaneous, aberrant, action potentials (very prominent in pericarional and radicular lesions)

Radicular syndrome• Motor deficit – metameric segmental• sensory deficit if sensory root is involved• Sensory deficit and pain on a dermatoma (root

territory • tendon reflexes controlled by the root involved –

abolished• Multiple roots involvement – change in composition

of CSF• Elongations maneuvers positive• Disorders affecting roots: GBS, disc herniation,

tumors

Brachial plexus involvement

Clinical syndrome• Monomelic motor deficit affecting the UL

ipsilateral to the injury – hangs uselessly at the side

• Lost tendon reflexes – affected UL – unilateral• Total sensory loss below a line extending from

the shoulder to the middle third of the upper arm.

• Causes: trauma, irradiation, neoplastic invasion, viral (Parsonage-Turner sy), alergic (vaccination), electrical injury, narrowed thoracic outlet

Median nerve

• Origin C5-T1• Union of the median and lateral cords of the

brachial plexus• Clinical sy:

– Innability to pronate the forearm and flex the hand in radial direction

– Paralysis of the flexion of the index finger and terminal phalanx of the thumb,

– weakness of flexion of the remaining fingers

• Weakness of opposition and abduction of the thumb in the plane at the right angle to the palm

• Sensory impairment over the radial two thirds of the palm and dorsum of the distal phalanges of the index and third fingers

• Causes: dislocation in the axila, wounds occuring on along the nerve, paravenous infusions at the bicipital groove.

• Nerves of theupper limb

medianulnar

Carpal Tunel Syndrome

• Compression of the nerve at the wrist• The most frequent nerve entrapment

syndrome• Clinical syndrome:

– Dysesthesia and pain in the fingers– Paresthesias are characteristically worst during

the night– Sensory loss affecting the thumb, index and

middle fingers

• Causes: – Excesive use of hands and occupational exposure

to repeated trauma– Infiltration of carpal ligament with amyloid (in

multiple mieloma)– Connective tissue disorders (reumatoid arthritis)– Hypothyroidism

Ulnar nerve• C8-T1 origin• Clinical syndrome:

– Claw-hand deformity– Wasting of the small hand muscles– Deficit of the wrist flexors, the ulnar half of the

deep fingers flexors– Deficit of abduction and adduction of the fingersCauses: pressure in axilla, injured at the elbow –

fracture, dislocation, “delayed ulnar palsy” (years after the injury at the elbow”, ulnar tunnel at the wrist (prolonged pressure – ex.on bycicle driving)

Radial nerve• C6- C8 (mainly C7), posterior cord of the

brachial plexus• Clinical syndrome:Paralysis of

• extension of the elbow• Flexion of the elbow with the hand perpendicular to floor

(brachioradialis muscle)• Supination of the forearm• Extension of the wrist and fingers• Extension and abduction of the thumb in the plane of the

palm• Sensory impairement on radial aspect of the dorsal hand

• Radial nerve

• Causes:– Compression in the axilla “crutch palsy”– Around the humerus bone (fractures, pressure

palsies during alcoholic stupor – Lead intoxication– Part of the clinical picture of neuralgic

amyotrophy.

Lumbosacral plexus

• Extends from the upper lumbar area to the lower sacrum

• Clinical syndrome:– Weaknes or paralysis of all leg muscles– Amiothrophy– Areflexia – Anesthesia from the toes to the perianal region– Autonomic loss with warm, dry skin, edema of the

leg– No sphicterian involvement

• Causes:– Trauma– Neoplasms in the pelvis– Adenopathy– Irradiation – Surgical lesions during hysterectomy of childbirth– Neuralgic amyotrophy

Femoral nerve

• Origin L2,3,4 roots• Clinical syndrome:

– Weaknes of the extension of the lower leg– Wasting of the quadriceps muscle– Failure of fixation of the knee– Knee tendon reflex – abolished– Injury in the initial part before emerging the

braches for iliopsoas muscles associates weakness of hip flexion

Femoral nerve and lumbar plexus

• Causes– Commonest: DM– Pelvic tumors - red flag for a process that

happens inside the pelvis and may compress the nerve directly or indirectly

– Bleeding into the iliacus muscle or the retroperitoneal in patients receiving anticoagulants and hemophiliacs

Sciatic nerve• Origin L4-5 and S1-2• Complete paralysis:

– Abolished knee flexion– Weaknes of gluteal muscles– All the muscles below the kneeare paralysedPartial compressions are more common and tend to involve peroneal innervated muscles

• Causes:– Disc herniation - compression at the root level– Fractures of the pelvis or femural bone– Injection in the lower gluteal region– Total arthroplasty– Tumors of the pelvis– Lying flat on a hard surface in stupor state– Neurofibromas– Infections– Ischemic necrosis– DM, PAR– cryptogenic

Common peroneal nerve

• Lateral or external popliteal nerve• Clinical syndrome:

– Weakness of the dorsiflexion of the foot– Weakness of eversion– Numbness of the dorsum of the foot– the foot in “Var equin” position– Stepage – peculiar gait

• Causes :– Pressure palsy spontaneous or associated to HNPP

(hereditary neuropathy with pressure palsies)– Diabetic neuropathy– Fibular bone fractures– Hematomas around the fibular head

Tibial nerve

• The sciatic nerve terminal division that continues its trajectory in the posterior compartment of the calf muscles

• Clinical syndrome:– Calcaneovalgus deformity of the foot– Weakness of plantar flexion and inversion– Loss of sensation over the plantar aspect of the

foot– Loss of achilean tendon reflex

• Causes:– Disc herniation at the root level– DM– Baker Cyst in the popliteal fossa - compression– Tarsal tunnel entrapment