Locomotion and Movement Question Answers: NCERT Class 11 Biology

Tetany is the rapid spasms in muscles due to low calcium in body fluid.

Yes, in certain cases calcium ion concentration in the blood leads to tetany. A decrease in calcium is associated with increase in permeability of neuronal cells to sodium ions causing a progressive depolarization, which increases the possibility of action potentials causing contraction of peripheral skeletal muscles which leads to tetany. Calcium plays an important role in muscle contraction. Parathyroid hormone secreted by parathyroid gland is responsible for increasing the Ca²⁺ ions in the blood stream.

Calcium ion concentration in the blood affects muscle contraction.

The bones of the forearm are humerus, radius and ulna. The humerus forms joints at the shoulder and elbow. Radial bone extends from the other side of elbow to the side of thumb and it runs parallel to ulna. The ulna stretches from elbow to the long finger. 

A slipped disc is a condition in which there is displacement of invertebral disc from their normal position. Effects:

1. Muscular weakness
2. It can cause permanent nerve damage and paralysis.
3. It can cause neck or lower back pain and pain in muscles of arms and legs.
4. In would cause burning and tingling sense along the severe pain.

The sliding filament theory states that contraction of muscle fibre occurs through the sliding of thin filaments over the thick filaments. Actin is a thin filament and myosin is the thick filament. A – bands are the anisotrophic bands of the sarcomere. A- bands appears as dark bands under the microscope. I- bands are the isotropic bands of sarcomere. I- bands appears as light bands under the microscope. Light and dark bands has contractile protein actin and myosin. A Z line elastic fibre is present in the fibre which is attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e. contraction. The I- band contains only thin filaments and also shortens. The A- band does not shorten, it remains the same length but A bands of different sarcomere move closer together during contraction.

The above diagram shows the contraction of muscles fibres according to sliding filament theory by sliding of the thin filaments over thick filaments.

When a sarcomere contracts, myosin heads attach to actin to forms cross- bridges. Then, the thin filaments slide over the thick filaments as the heads pull the actin. Thus result in sarcomere shortening, creating the tension of muscle contraction. The steps followed during relaxation are:

1. At the beginning when the neural signal reaches the neuromuscular junction then neurotransmitter acetylcholine which creates an action potential in the sarcolemma.
2. Calcium ions are released and spread through muscle fibres due to this action potential.
3. Calcium ions bind to the subunit of troponin on the actin filament due to the increase in the concentration of the calcium ions which further remove the active sites in the myosin.
4. Cross- bridge formed when myosin bind with actin.
5. This causes the attached actin filaments to move towards the center of A- band. A concentration of the sarcomere occurs in the muscles as the Z- line moves inwards, the I- band gets reduced while the A- band retains its original length.  
6. ADP and Pi are released when the cross bridge breaks and myosin goes back to its relaxed state and new ATP binds which breaks the cross- bridge.
7. The muscle relaxation occurs after contraction when the calcium ions are pumped back to the active sites on actin myofilaments. The Z- line returns to original position, i.e. relaxation.

Role of Ca²⁺ ions in muscle contraction are:

1. Sarcoplasmic reticulum releases stored Ca²⁺ which binds into the specific sites of troponin component of thin filament.
2. Due to this binding F- actin molecule become exposed and the confirmation of the troponin molecule changed.
3. The active sites of F- actin are specific to myosin head which have ATP activity depend on magnesium ion.
4. During the relaxation of muscle calcium ion is pumped back to the sarcoplasmic reticulum and causes the active site of myosin masking and the muscles get relaxed.

Gout is a skeletal disorder which is caused by the accumulation of metabolic waste like uric acid crystals or sodium urate crystals in the joints when lead to inflammation of bone and joints thereby causing pain.

The source of energy in muscle contraction is ATP (Adenine Triphosphate).

In the presence of magnesium and calcium ions, the myosin molecule breaks down ions ADP and inorganic phosphate and releases energy in head of the myosin.

The components of pectoral girdle are clavicle and scapula. It is the glenoid cavity of pectoral girdle in which head of humerus articulate by forming a ball and socket joint.

The components of pelvic girdle are ileum, ischium and pubis which is articulated by a cavity known as acetabulum.

The diagrammatic representation of a sarcomere is as follows:

(a) All mammals (except a few) have seven cervical vertebra.

(b) The number of phalanges in each limb of a human is 14.

(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins, namely troponin and tropomyosin.

(d) In a muscle fibre, Ca⁺⁺ is stored in the sarcoplasmic reticulum.

(e)11^th  And 12^th  pairs of ribs are called floating ribs.

(f) The human cranium is made up of eight bones.

The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments, which shortens the myofibril.

Each muscle fibre has an alternate light and dark band, which contains a special contractile protein, called actin and myosin respectively. Actin is a thin contractile protein present in the light band and is known as the I-band, whereas myosin is a thick contractile protein present in the dark band and is known as the A-band. There is an elastic fibre called z line that bisects each I-band. The thin filament is firmly anchored to the z line. The central part of the thick filament that is not overlapped by the thin filament is known as the H-zone.

During muscle contraction, the myosin heads or cross bridges come in close contact with the thin filaments. As a result, the thin filaments are pulled towards the middle of the sarcomere. The Z line attached to the actin filaments is also pulled leading to the shortening of the sarcomere. Hence, the length of the A- band remains constant as its original length and the I-band shortens and the H-zone disappears.

During skeletal muscle contraction, the thick filament slides over the thin filament by a repeated binding and releases of myosin head along the filament. This whole process occurs in a sequential manner.

Step 1:

Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. Neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result, Acetylcholine (a neurotransmitter) is released into the synaptic cleft by generating an action potential in sarcolemma.

Step 2:

The generation of this action potential releases calcium ions from the sarcoplasmic reticulum in the sarcoplasm.

Step 3:

The increased calcium ions in the sarcoplasm leads to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin, wrapped around actin filaments. Hence, active actin sites are exposed and this allows myosin heads to attach to this site.

Step 4:

In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilizing energy from ATP hydrolysis. The actin filaments are pulled. As a result, the H-zone reduces. It is at this stage that the contraction of the muscle occurs.

Step 5:

After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate. ATP molecules bind and detach myosin and the cross bridges are broken.

Step 6:

This process of formation and breaking down of cross bridges continues until there is a drop in the stimulus.As a result, the concentration of calcium ions decreases, thereby masking the actin filaments and leading to muscle relaxation.

(a) Answer: True

(b) Answer: False

H -zone of striated muscle fibre is the central part of the thick filament that is not overlapped by the thin filament.

(c) Answer: True

(d) Answer: False

There are 12 pairs of ribs in a man.

(e) Answer: True

(a) Actin and Myosin

(b) Red and White muscles

(c) Pectoral and Pelvic girdle

Movement is a characteristic feature of living organisms. The different types of movement exhibited by cells of the human body are:

• Amoeboid movement: Leucocytes present in the blood show amoeboid movement. During tissue damage, these blood cells move from the circulatory system towards the injury site to initiate an immune response.

• Ciliary movement: Reproductive cells such as sperms and ova show ciliary movement. The passage of ova through the fallopian tube towards the uterus is facilitated by this movement.

• Muscular movement: Muscle cells show muscular movement.

(a) atlas/axis: Pivotal joint

(b) carpal/metacarpal of thumb: Saddle joint

(c) between phalanges: Hinge joint

(d) femur/acetabulum: Ball and socket joint

(e) between cranial bones: Fibrous joint

(f) between pubic bones in the pelvic girdle: Ball and socket joint