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The motor neurons whose axons terminate at neuromuscular junctions and which activate voluntary skeletal muscles contraction upon stimulation are the lower motor neurons. These are located in the anterior horns of the spinal cord grey matter and the motor cranial nerve nuclei.
The upper motor neurons are situated in the cerebral cortex, and their axons leave the cortex to control the lower motor neurons. These are distributed in the primary motor cortex, supplementary motor area, and premotor cortex.
The effect of lower motor neuron lesion is the weakness (ultimately paralysis) of affected muscles with marked wasting. The effects of upper motor neuron lesions are more variable, depending on the pathways affected. There can be a loss of dexterity, weakness, and spasticity.
Bulk and Nutrition of Muscles
- Expose the limbs and compare corresponding parts of both sides.
- To check for wasting use the bony points on the limbs to compare both sides using a measuring tape.
- For forearms measure the distance from the styloid process to a point at forearm where the muscle bulk is maximum and measure the circumference of forearm at that site. Repeat on the other side by measuring circumference at the same distance from the styloid process.
- For upper arms mark the midpoint between the acromion of scapula and olecranon of ulna (with elbow flexed at 90˚). Measure the circumference of the upper arm at that site on both sides.
- For legs measure the distance from medial malleolus to a point at calf where muscle bulk is maximum and measure the circumference of calf at that site. Repeat on the other side at the same distance.
- For thighs measure the distance from the lateral tibial condyle to a point at thigh where muscle bulk is maximum and measure the circumference of thigh at that site. Repeat on the other side at the same distance.
Wasting of muscle is a common sign of disease but does not always have a neurological basis as atrophy can also occur due to misuse but tends to be mild. Cachexia involves diffuse muscle wasting. Thyrotoxicosis can lead to diffuse muscle wasting eventually resulting in weakness. Many myopathic disorders can lead to muscle wasting in which the specific pattern of wasting is diagnostic.
When considering a neurological basis of wasting, peripheral nerve disease which affects motor neurons present with muscle wasting. Denervated muscles waste markedly indicating motor nerve terminal or axon loss. In mononeuropathies, the wasting only affects muscles supplied by the affected nerves range of pathologies of the spinal cord can also cause wasting, for example, poliomyelitis and syringomyelia.
Fasiculations can also be seen accompanying muscle wasting. These are involuntary contractions of a group of muscle fibers innervated by a single motor neuron. These appear as fine, flickering, irregular twitching movements that occur spontaneously. Widespread fasciculations are characteristic of amyotrophic lateral sclerosis and can also be seen in peripheral neuropathies and radiculopathies.
Tone of Muscles
The tone is defined as the resistance felt when a joint is moved passively.
- The patient should lie supine and should be relaxed and comfortable.
- First, grasp the patient’s forearm and shake it. Observe the movement of the wrist (this indicates the tone around the wrist). Compare both sides.
- Repeat the act in the lower limb by grasping the leg below the knee and moving it from side to side while supported on the bed. Observe the movements of the ankle. Compare both sides.
- Then carry out a full range of passive movements at wrist, elbow, hip, knee and ankle joints. At first rapidly and then slowly assessing the resistance encountered during these movements. Make sure the movements do not elicit any pain or are uncomfortable for the patient. Also be wary of degenerative diseases (especially in the elderly) that can lead to stiff movements.
The tone may either be increased, referred to as hypertonia, or decreased, referred to as hypotonia. Hypertonia will present as greater resistance to movement and hypotonia will present as decreased resistance to movement.
There are two types of hypertonia: spasticity and rigidity. In spasticity, the resistance increases with the speed of passive movements. Spastic hypertonia in the upper limbs affects flexor muscles more than