Discuss and explain the anatomical parts of a sarcomere and its function. How do muscles contract? How do muscles relax? How would different diseases affect this movement? Describe at least two muscular diseases that would affect the physiology of the sarcomere?

A sarcomere is the basic functional unit of the muscle tissue. the parts include:

- A band -contains myosin filaments

-I band - contains the actin filaments. Shortens during muscular contraction

- Z line - anchors the actin filaments on the I bands

- M line - anchors myosin filaments in place

- H zone - area along the A band where there is no overlapping of the I band

Muscle contraction  occurs when the Z lines in most if not all sarcomeres move closer together and I bands become smaller. This happens through the sliding filament theory which has the following steps:

> AP is generated to stimulate contraction of muscle

> Calcium ions are released from sarcoplasmic reticulum

> Calcium ions bind to troponin C which causes tropomyosin to move away from active binding sites on actin filaments thus exposing them

> Myosin bridges attach on the active sites

> ATP is hydrolyzed to ADP and Pi causing power stroke i.e bending of the myosin heads thus pulling the actin filaments hence shortening the I bands

> Myosin heads detach from the actin filaments and rejoin the actin filaments at a new point

> ATP is hydrolyzed thus producing energy for another power stroke which is repeated over and over until the muscle contracts to the desired point

Muscle relaxation occurs when the Z lines move away from each other which occurs through the following steps:

> Ca2+ ions are reabsorbed back to sarcoplasmic reticulum

> Ca2+ detaches from troponin C thus tropomyosin goes back to its resting position covering active sites on actin

> Myosin heads detach from the actin filaments 

> Muscle relaxes

Diseases that impair the transmission of neural signals, release of calcium ions and gross movement of the myofilaments result in impaired movement of the muscle resulting in a situation called flaccid paralysis. On the other hand, diseases that amplify the neural signals and stimulate increased release of calcium ions cause a continuous muscle contraction causing spastic paralysis.

Muscular diseases include:

-Myositis - autoimmune disease characterized by muscle inflammation that causes difficulty in muscle movements as well as increased fatigue of muscles,

- Myasthenia gravis - a disease that impairs neural conduction of impulses required for muscular contraction and thus there's impaired release of calcium ions hence less muscular contraction.