Motor pathways

• Motor control requires 2 neurones
• The left side of the brain controls the right side of the body and vice versa
• The anterior and lateral corticospinal tracts carry the motor fibres

  • the lateral tract is bigger and innervates the limbs
  • the anterior tract is smaller and innervates the trunk
• Lesions in the upper or lower motor neurones cause a specific constellation of signs and symptoms.
• For the cranial nerves that are motor, the upper motor neuron synapses with the lower motor neuron in the brainstem: so cranial nerves are lower motor neurones.
• Motor cortex - distinct area anterior to central sulcus - the precentral gyrus
• Blood supply of motor cortex - anterior/middle/posterior cerebral arteries.
• Lateral corticospinaltract - fibres decussate in the medulla.
• Anterior corticospinal tract - fibres decussate at exit level
• Aa fibres are responsible for movement in skeletal muscle and stretch in muscle spindles. They are large, myelinated fibres with fast conduction to allow rapid adjustment/stability.
• AB fibres are responsible for touch and pressure sensation, and position of muscle spindles. They are large, myelinated fibres with fast conduction to allow rapid adjustment/stability.
• Cutaneous sensory innervation is arranged by spinal root: dermatomes
• Muscle innervation can be arranged by spinal level: myotomes
• Contraction of a muscle in isolation results in a coarse, uncontrolled action. Precise, controlled movement involves other processes, including

  • cerebellum
  • basal ganglia
  • stretch reflex
• • Muscle groups work generally in pairs
  • contraction of one stretches the other passively - a reflex arc that tenses the stretched muscle
  • provides muscle tone and stability
  • 2 nerves/1 synapse - very fast (monosynaptic stretch reflex)
• Muscle spindles

  • special sensory organ in muscles that respond to stretch and position of muscle
  • sensitivity is adjusted by special intrafusal muscle fibres innervated by Ay efferents
• Stretch reflex in knee

  • muscle spindle in knee extensors stretched
  • rapid, monosynaptic reflex causes contraction of extensors (and inhibition of flexors)
  • initiated by contraction of knee flexors
• The knee jerk reflex can be reinforced by strong muscle activity in the upper body - the Jendrassik maneouvre
• Nerve damage impairs or changes the normal function of the fibres. In motor fibres this causes weakness or paralysis initially. In sensory fibres this causes paraesthesia or numbness.
• Upper Motor Neurone Lesion symptoms

  • increased muscle tone
  • unchanged muscle bulk (or disuse atrophy)
  • increased reflexes and clonus
  • upgoing plantar reflex (babinski sign)
  • no fasciculationss
• Lower motor neurone lesion symptoms

  • decreased muscle tone
  • reduced muscle bulk (wasting)
  • decreased or absent reflexes
  • downgoing plantar reflex
  • fasciculations
• Spinal cord transection produces an upper motor neuron lesion for all levels below damage
• Spinal injury at C4 causes quadriplegia/tetraplegia, complete paralysis below the neck
• Spinal injury at C6 results in partial paralysis of hands and arms as well as lower body
• Spinal injury at T6 results in paralysis below the chest
• Spinal injury at L1 causes paraplegia, paralysis below the waist
• Brachial plexus injury from stab wound - upper or lower motor neurone lesion?

  Lower
• Stroke from middle cerebral artery thrombosis - upper or lower motor neurone lesion?

  Upper
• Compression of median nerve at wrist (carpal tunnel syndrome) - upper or lower motor neurone lesion?

  Lower
• C5 nerve root irritation from spondylosis of the cervical spine - upper or lower motor neurone lesion?

  Lower
• Transection of the spinal cord at C6 after a cycle accident causing a fractured spine - upper or lower motor neurone lesion?

  Upper
• Inclusion-Body Myositis - characterised by muscle fibres that contain empty, bubble-like spaces (vacuoles) and clumps of cellular material (inclusion bodies). Inflammatory cells can be seen between the fibres.
• Polymyositis produces muscle weakness. It can often be treated by drugs like corticosteroids or immunosuppressants.
• Inclusion body myositis is a slowly progressive disease that produces weakness of hand grip and straightening of the knees. No effective treatment is known.
• Dermatomyositis produces muscle weakness and skin changes. The skin rash is reddish and most commonly occurs on the face, especially around the eyes, and over the knuckles and elbows. Ragged nail folds with visible capillaries can be present. It can often be treated by drugs like corticosteroids or immunosuppressants.
• Glucocorticoid myopathy is caused by this class of steroids increasing the breakdown of the muscle proteins leading to muscle atrophy
• Drug-induced myopathy

  • glucocorticoid myopathy
  • alcoholic myopathy
  • myopathy due to other toxic agents
• Congenital myopathies do not show evidence for either a progressive dystrophic process i.e. muscle death or inflammation, but instead characteristic microscopic changes are seen in association with reduced contractile ability of the muscles.
• Examples of congenital myopathies

  • nemaline myopathy
  • multi/minicore myopathy
  • centronuclear (myotubular) myopathy
• Nemaline myopathy is characterised by the presence of nemaline rods in the muscle