Human Physiology - Locomotion and
Movement
(NEET Syllabus): Origin
Types of movement- ciliary, fiagellar, muscular; Skeletal muscle- contractile
proteins and muscle contraction; Skeletal system and its functions (To be dealt
with the relevant practical of Practical syllabus); Joints; Disorders of
muscular and skeletal system-Myasthenia gravis, Tetany, Muscular dystrophy,
Arthritis, Osteoporosis, Gout.
HUMAN
PHYSIOLOGY (Locomotion and Movement)
Theoretical
Questions - TQ 3 (Q. No.12 - 15)
Very
Long Answer Type Questions
Question.12: How does
calcium affect the process of muscle contraction?
Question.13: Explain
sliding filament theory of muscle contraction.
Question.14: Explain the
bones of fore limbs.
Question.15: Describe
the various types of joints present in human body with examples.
Answer 12. Mechanism of muscle contraction is
best explained by the sliding filament theory which states that contraction of
muscle fibres takes place by the sliding of the thin filaments over the thick
filaments.
Muscle contraction is initiated by a
signal sent by the central nervous system (CNS) via a motor neuron. A motor
neuron along with the muscle fibers connected to it constitutes a motor
unit. The junction between a motor neuron and the sarcolemma of the muscle
fibre is called the neuromuscular junction or motor-end plate.
A neural signal reaching this junction
releases a neurotransmitter (Acetylcholine) which generates an action potential
in the sarcolemma. This spreads through the muscle fibre and causes the release
of calcium ions into the sarcoplasm.
Increase in Ca2+ level
leads to the binding of calcium with a subunit of troponin on actin filaments
and thereby remove the masking of active sites for myosin.
Utilising the energy from ATP
hydrolysis, the myosin head now binds to the exposed active sites on actin
to form a cross bridge.
This pulls the attached actin
filaments towards the centre of "A" band. The "Z" line
attached to these actins are also pulled inwards thereby causing a shortening
of the sarcomere, i.e., contraction. It is clear from the above steps, that
during shortening of the muscle, i.e., contraction, the I bands get reduced,
whereas the "A" bands retain the length.
The myosin, releasing the ADP and Pi
goes back to its relaxed state. A
new ATP binds and the cross-bridge is broken. The ATP is again hydrolysed by
the myosin head and the cycle of cross-bridge formation and breakage is
repeated causing further sliding.
The process continues till the Ca2+
ions are pumped back to the sarcoplasmic cisternae resulting in the masking of
actin filaments. This causes the return of "Z" lines back to their
original position, i.e., relaxation. The reaction time of the fibres can vary
in different muscles.
Answer 13. Two groups of workers (A.F.Huxley
and Ralph Niedergerke 1954; H.E.Huxley and Jean Hanson 1954) proposed the
sliding filament theory. The essential features of this theory are as
follows:
1. During
muscle contraction, the thin myofilaments slide inward towards the H-zone.
2. The
sarcomere shortens, but the lengths of thin and thick myofilaments do not
change.
3. The
crossbridges of the thick myofilaments connect with portions of actin of the
thin myofilaments. The myosin cross bridges move on the surface of thin
myofilaments and the thin and thick myofilaments slide past each other.
4. As the
thin myofilaments move past the thick myofilaments, the H-zone narrows and even
disappears when the thin myofilaments meet at the centre of the sarcomere.
Thus, the length of the sarcomere decreases during contraction. Size of I band
also decreases.
5. The
lengths of the thick and thin myofilaments do not change during muscle
contraction.
Answer 14. Each arm consists of the following
30 bones:
1 humerus, 1 radius, 1 ulna, 8 carpal,
5 metacarpal bones, 5 digits (14 phalanges). Phalangeal formula: 2,3,3,3,3.
Upper rounded end of the humerus is
called head which articulates into the glenoid cavity of the pectoral
girdle. A greater and a lesser tubercles occur near the head.
The shaft of the humerus has a
V-shaped deltoid ridge at about its middle. A pully like trochlea
is present between two ridges. Its upper end has a larger olecranon process that
forms the eminence of our elbow. The head of the radius articulates with
the humerus. Each wrist is composed of eight carpals which are arranged
in two rows: scaphoid, lunate, triquetrum and pisiform in proximal row and
trapezium, trapezoid, capitate and humate in distal row.
Answer 15. The structural arrangements of
tissues by which bones are joined together are called joints. According to the
mobility they are classified as fibrous or fixed or immovable joints,
cartilaginous or slightly movable joints and synovial or freely movable joints.
1. Fibrous or Immovable Joints: In this type of joints there is no
movement between the bones concerned. As the name suggests, there is white
fibrous tissue between the ends of the bones. Examples of this type include
- the joints between the bones of skull called sutures and the joints
between the teeth and the maxilla and teeth and mandible.
2. Cartilaginous or Slightly Movable
Joints: In this type
there is a pad of white fibrocartilage between the ends of the bones
taking part in the joints which allows for very slight movement. Movement is
only possible because of compression of pad of cartilages. Examples of
cartilaginous joints include the pubic symphysis of pubis and the joints
between the vertebrae (intervertebral discs).
3. Synovial or Freely Movable Joints: A considerable movement is possible at
all synovial joints. Synovial joints are of six types:
(i). A gliding joint- It is the
simplest of the synovial joints. The articular surfaces of two bones are
usually flat, permitting only back-and-forth and side-to-side movements.
Gliding joints are found between the carpal bones and between the tarsal bones.
(ii). A hinge joint - It allows
movement primarily in one plane. In a hinge joint spool (reel) surface of one
bone fits into the concave surface of another bone. The elbow, the knee, ankle
and interphalangeal hints are examples of hinge joints.
(iii). A pivot joint- This joint also
allows movement in only one plane. In a pivot joint rounded or pointed bone
fits into a shallow depression in another bone. The primary movement at a pivot
joint is rotation.
(iv). Condyloid or ellipsoid joint-
Allows movement in two planes, back & forth & side-to side. The joints
between the metacarpals and phalanges (metacarpo-phalangeal joint) of the
fingers are examples of ellipsoid joints.
(v). A saddle joint allows the same
movements as an ellipsoid joint, but the movements are free. The joint between
the carpal and metacarpal of thumb of the hand is an example of saddle joint.
(vi). A ball-and-socket joint- One bone
of this joint forms a rounded head while the other bone forms a cup shaped
structure into which the head fits. It allows free movement in all directions.
It is most movable joint. Examples: hip joint and shoulder joint.
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