Motor System Examination
The examination of the motor system includes inspection (for atrophy, hypertrophy, fasciculations, and tremor), palpation (for cutaneous reflexes and tone), percussion (for myotonia and stretch reflexes), full flexion and extension of the elbows and knees (for abnormal tone and non-neurologic restrictions to movement e.g. from contractures and joint disease), and strength testing.
Motor System
The following systems participate in the creation and coordination of muscle movement
- Motor system: both in its pyramidal and extrapyramidal components (for power)
- Cerebellar system: for rhythmic movement and posture
- Vestibular system: for balance and coordination of the eye, head, and body movements
- Sensory system: for afferent input to the spinal axis
Motor System Components
The motor system is made up of the pyramidal and extrapyramidal components. The pyramidal and extrapyramidal levels of the motor system converge on the "final common pathways" which are the lower motor neurons of the brain stem (cranial nerves) and spinal cord, whose axons descend to skeletal muscles.
Pyramidal component: this is the corticospinal level of the motor system. It consists of
- Upper motor neurons. These exert direct or indirect supranuclear control over the lower motor neurons. Upper motor neurons are found in the motor cortex and brain stem.
- Pyramidal tracts (i.e. the descending corticospinal pathways)
Extrapyramidal component: The nuclei reside in the basal ganglia and their complex connections. There is a complex neural organisation that works closely with other levels of the motor system to achieve neuromotor control.
Upper and Lower Motor Neuron Signs
Upper motor neuron dysfunction: weak and flaccid muscles that eventually become spastic, hypertonic, and hyperreflexive. There is an association with pathologic reflexes (such as Babinski's and Hoffmann) and induced clonus of the ankle or wrist. Spasticity is prominent in the antigravity muscles (flexors of the upper extremities and extensors of the lower extremities). Spasticity is associated with clasp-knife finding due to a variable degree of resistance to passive motion.
Lower motor neuron dysfunction: weakness and paralysis of affected muscles. Flaccidity, hypotonia, diminished or absent stretch reflexes, and eventually atrophy. Fasciculations occur which are visible twitches of small groups of muscle fibres. There are no pathologic reflexes.
Atrophy and Hypertrophy
Atrophy: muscular wasting caused by damage to the lower motor neurons or their axons. The interruption of flow of trophic factors to muscles leads to atrophy of the dependent myofibres, plus fasciculations can also occur.
Causes of atrophy include:
- Damage to the supplying nerve
- Congenital muscular disease
- Disuse: e.g. from trauma or arthritis
Examples of atrophic muscles are the flat thenar eminence of carpal tunnel syndrome, the prominent metacarpals with loss of interossei in polyneuropathy, and calf atrophy in radiculopathy.
Atrophy is tested by assessing the muscles three S's which are: size, symmetry, and shape. Atrophy, hypertrophy, and abnormal bulging or depressions are important findings in identifying various muscular diseases, especially when it is asymmetric. Shape is important for example in tendon rupture.
Hypertrophy:
