Motor Unit & It ’ s Excitation

1. Motor Unit & It’s Excitation By: Baljit Brar

2. What Is a Motor Unit? • A Motor Unit is described as being a motor neuron plus the muscle fibres that it contracts. What Is a Motor Neuron? • Also known as Efferent, or effectorneurons, are described as being nerve fibres that are located in the Central Nervous System and the nerve fibres that elongate to the muscle fibres.

3. The Central Nervous System • In the spinal cord we have two types of matter • White Matter • Grey Matter • In the Grey matter of the spinal cord, we have two dorsal horns and two ventral horns. • The Motor neurons coming out of the spinal cord are located in the ventral horns, therefore they can be also named as Ventral Horn Cells.

4. The Central Nervous System • The Ventral Horn cells give rise to the motor neuron that runs towards the muscle fibres. • Motor neurons can only come out of the ventral aspects from the ventral horn cells. • Here is what this looks like:

5. Motor Unit

6. Motor Unit Properties: Stimulation • The Motor Unit is stimulated generally by the brain. The brain will send a stimulus to the ventral horn cells. • The ventral horn cells then receive this stimulation from the brain, and in turn become activated to further stimulate their end muscle fibres to respond. • The Motor Unit can also be stimulated by something called the Reflex Arc • The Reflex Arc occurs when the sensory receptors receive great stimuli. In turn the sensory nerves stimulate interneuron's to further stimulate the Motor Unit to activate.

7. Motor Unit Properties: Excitation • The Motor units which lead to Skeletal Muscle each are excitatory only. • This means any stimuli will cause the neuromuscular junction to release neurotransmitters into the muscle fibres • This will ultimately cause a contraction in the muscle fibre itself.

8. Motor Unit Properties: Excitation • As for Cardiac muscle and Smooth muscle, the response given from a stimuli can be either excitatory or inhibitory. • This means that stimuli may be sent to these muscles but the muscle does not have to respond to that stimuli.

9. Motor Unit Properties: Precision • The total area of Muscle Fibres a single Motor Unit innervates determines how precise those muscle fibres movements will be. • If a single Motor Unit innervates less muscle fibres, those muscle fibres will be able to have more precise movements. The more muscle fibres a single Motor Unit innervates, the less precise the muscle movements will be. • Eg) Motor Units in leg muscles and back muscles must innervate more muscle fibres. Eye muscles however have less muscle fibres to innervate.

10. Motor Unit Properties: Muscle Contraction • Some muscles have many motor units innervating different muscle fibres of that same muscle. We can see these certain motor units excite their muscle fibres at the same time. • When one motor unit for a certain muscle is stimulated, this motor unit in turn often will stimulate motor units running to the same muscle. • This property is known as “Motor Unit Recruitment”

11. Clinical Aspects • Lower Motor Neuron Lesion • This is described as being damage to the motor neurons running from the spinal cord to the muscle fibres that it innervates. • This damage will first cause the muscle paralysis, along with atrophy of the muscle. • In place of the muscle shrinkage we can see formation of fibrous tissue or fibrous muscle. • Fibrillations, or invisible fine irregular muscle contractions in skeletal muscle, can also be seen in LMNL. • Also Fasciculation’s can be seen, which are visible by the naked eye on the skin, can occur as well.

12. Muscular Dystrophy • Is described as a condition where a defect in the Dystrophin gene leads to complications in the Dystrophin glycoprotein complex. • The Dystrophin glycoprotein complex is described as being a large protein which connects thin filaments of skeletal muscle fibres to the tissue anchors in the extra cellular fluids which help the structural integrity of the muscle fibres to each other. • This condition comes with many symptoms generally dealing with chronic muscle weakness and is hereditary with different degrees is severity. • There are two main types • Duchenne's • Becker’s

13. Muscular Dystrophy • Duchenne’s– This X-linked recessively transferred condition is the most severe of the two types. • Patients with this type experience an onset of muscle weakness as early as infancy due to complete lack of the Dystrophin glycoprotein complex. • Eventually this muscle weakness will increase and lead to complications of the heart muscles eventually leading to death around the ages of 20-40.

14. Muscular Dystrophy • Becker’s– This type is less severe in contrast to Duchenne’s Muscular Dystrophy. • Muscle weakness can be seen in this type but the weakness is less severe because the Dystrophin glycoprotein complex can be seen in small amounts, or in an altered manner. • Patients with Becker’s muscular dystrophy however usually will live the normal life span.

15. Myotonia • This condition is described as being the presence of abnormal genes on chromosomes 7, 17, and 19. They may be hereditary or can also be acquired. • These abnormal genes lead to difficulty in relaxing muscles after contraction. These difficulties in relaxation vary in degree depending on how mutated these genes are. • The difficulty in relaxation is caused by the mutation of sodium, chloride, or potassium ion channels from the mutated genes themselves.

16. McArdle’s Syndrome • Also known as the glycogen storage disease V. • Is described as the deficiency of the enzyme myophosphorylase which breaks down glycogen into glucose which creates energy for the muscles. • Patients usually present generalized symptoms such as pain in muscles during exercise, muscle cramping, and/or weakness in muscles when exercising.