Muscle Biology

Muscle twitch is the response of a muscle to a single electrical stimulation. It is divided into three phases: latent period - time between stimulation and beginning of shortening, contraction phase - time of shortening, relaxation phase- time when muscle is returning to resting length (longest phase).

Rigor mortis is stiffening of the body caused by contraction of muscles after death. In humans it occurs four hours after death. Due to absence of oxygen after death, the ATP is not formed which is required for separation of actin and myosin. When oxygen is no longer present, the body may continue to produce ATP via anaerobic glycolysis. When the body's glycogen is depleted, the ATP concentration diminishes, and the body enters rigor mortis because it is unable to break those bridges. So, the correct answer is 'Contraction of muscles after death'.

The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are the strongest and fastest contracting muscles due to the constant blinking action they have to do. Eye movements do require high speed movements, and eye muscles are exercised continuously even during rapid eye movement of sleep. 

At the maximal stimulus, all motor units are recruited. All muscle cells in the whole muscle are contracting. Any stimulus above the maximal stimulus is called a supramaximal stimulus (voltage) and the tension will not increase beyond that elicited by a maximal stimulus. Thus, the correct answer is option A.

• Each myofibril(a unit of the muscle cell) has alternate dark and light bands on it.
  
• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.
  
•  In the center of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line.
  
• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. 
  
• The sarcomere is considered as the functional unit of contraction. The portion of the myofibril between two successive ‘Z’ lines is considered as a sarcomere. Hence sarcomere is the distance between two Z lines.
• So, the correct answer is 'Two Z-lines'.

• Each myofibril(a unit of the muscle cell) has alternate dark and light bands on it.
  
• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.
  
•  In the center of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line.
  
•  The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. 
  
• The sarcomere is considered as the functional unit of contraction. The portion of the myofibril between two successive ‘Z’ lines is considered as a sarcomere. Hence The functional unit of the contractile system of a striated muscle is sarcomere.
• So, the correct answer is 'Sarcomere'.

Syncytial cell is a cell that is multinucleated. It contains more than one nuclei. It is resulted from multiple cell fusions of uninuclear cells. The muscle fibre is a syncytial cell as it has multiple nuclei, in which each nucleus has originated from single myoblast.

• Aryepiglotticus is a muscle of the larynx running in the aryepiglottic fold from the arytenoid cartilage to the epiglottis.
• Hiltons' muscle is a muscle in which the nerve supplying the muscles extending directly across and acting at a given joint not only supplies the muscle but also innervates the joint and the skin overlying the muscle.
• Aryepiglotticus muscle is a Hiltons' muscle.
• So, the correct answer is 'Aryepiglotticus'.
  

• The 650 muscles in the body support movement  controlling  walking, talking, sitting, standing, eating and other daily functions that people consciously perform .
•  also help to maintain posture and circulate blood and other substances throughout the body.  so , option A is  correct

• Muscle is a  bundle of fibrous tissue in a body that has the ability to contract, producing movement in or maintaining the position of parts of the body.
• The total number of muscles in the human body comes in the range of 640-850.
• Latissimus dorsal muscle is a shoulder muscle.
• The latissimus dorsi muscle is responsible for extension, adduction, the transverse extension(horizontal abduction), flexion from an extended position, and (medial) internal rotation of the shoulder joint.
• So, the correct answer is 'Shoulder joint'.

Unstriated muscles consists of muscle fibres which are long flattened and uninucleate because these muscles are smooth muscle fibres. They are present in the form of thin sheets and spindle shaped fibres wth only one nucleus in them.

It helps in adduction, extension and medial rotation of the shoulder as in swimming, rowing, climbing, pulling folding the arm behind back and scratching the opposite scapula. It also helps in violent expiratory efforts as coughing, sneezing, etc.

Arrector pili muscles are small muscles attached to hair follicles in the skin. They are smooth muscles and are controlled by the autonomic nervous system, making them involuntary.

Cardiac muscle is immune to fatigue because it has more mitochondria than skeletal muscles. It contains up to 35% mitochondria whereas skeletal muscles contain only 1-2%. Mitochondria is necessary for ATP production that provides energy to the cells. Hence even if the heart pumps for a lifetime it resists fatigue due to presence of cardiac muscles. Thus the correct answer is option B.