Weakness of one or more of our limbs is most often neurological.Classically, there are two distinct patterns of neurological weakness, and identifying the one you're dealing with is rather useful. I'm referring toupper motor neuron vs lower motor neuron weakness, of course.
The ‘lower motor neuron’ is simply the last neuron to touch the relevant muscle.It always starts in the spinal cord and ends in a muscle.Damage to any of the neurons in the motor pathway before this point (e.g. spinal cord, brainstem, or cerebrum) will give you an ‘upper motor neuron’ pattern of weakness.
Although both syndromes present with muscle weakness, they thereafter part company in the following ways:
Upper Motor Neuron
Lower Motor Neuron
Increased, with ‘clasp knife’ quality
Absent, but disuse atrophy eventually results
Present (within 2-3 weeks)
Increased.Extensor plantar reflexes.
Decreased or absent.Flexor plantar reflexes.
Whole limbs, with more weakness in the upper limb extensors and lower limb flexors
Specific muscle groups affected (e.g. in the distribution of a spinal segment, or just the proximal muscles, etc.)
Of course, there are a few caveats and qualifications to be made to this handy list.
Although the deep tendon reflexes follow the above protocol, there are also superficial tendon reflexes (such as the abdominal or cremasteric) that obey the opposite pattern.It is rare to test for these reflexes, though.
The pattern with cranial nerves is a little more complicated, since most of the muscles supplied by them receive bilateral innervation.For instance, the tongue is supplied by the XII cranial nerve from both the left and the right side, and so damage to only one side won’t produce any discernible weakness.The one large exception to this rule is the lower 2/3 of the facial nerve’s supply, which follows convention and is unilateral.Therefore it is not uncommon to see a stroke patient with a drooping face – but with a forehead that is mysteriously spared.
Lastly, the lopsided distribution of weakness found with upper motor neuron lesions produces a particular 'spastic posture'. Since the weakness is greatest in the upper limb extensors, the limb tends to become flexed. The reverse is true for the lower limb, which is consequently extended. (Frustratingly, I can't seem to find a nice picture of this important clinical sign...)