Locomotion and movement

Locomotion is the voluntary movements resulting in a change in location.

Types of movement in human being

· Amoeboid movement: By pseudopodia. E.g. Macrophages, leucocytes, cytoskeletal elements like microfilaments etc.

· Ciliary movement: By cilia. E.g. ciliary movements in trachea (to remove dust particles and foreign substances), ciliary movements in oviducts (for the passage of ova).

·  Muscular movement: By muscles. E.g. movements of limbs.

HUMAN MUSCULAR SYSTEM

-    Muscles are mesodermal in origin.

-    40-50% of the body weight.

-    Have excitability, contractility, extensibility &elasticity.

-    Based on location, muscles are 3 types:

Striated or voluntary muscles	Non-striated (Visceral) muscles	Cardiac muscles
Striations present	Absent	Present
Voluntary	Involuntary	Involuntary
Attached to skeleton	In visceral organs	In heart wall
Rich blood supply	Poor blood supply	Rich blood supply
Fatigue muscle	Non-fatigue	Non-fatigue
Multinucleate	Uninucleate	Uninucleate
More mitochondria	Less mitochondria	More mitochondria

STRUCTURE OF STRIATED (SKELETAL) MUSCLE

-    Skeletal muscle is made of muscle bundles (fascicles) held together by collagenous connective tissue layer (fascia).

-    Each fascicle contains number of muscle fibres (muscle cells). Muscle fibres are lined by plasma membrane

(sarcolemma) enclosing the sarcoplasm.

-    Each muscle fibre contains myofilaments (myofibrils).

-    Each myofibril has alternate dark (Anisotropic or A-band) and light striations (Isotropic or I-band). This is due to the presence of 2 fibrous contractile proteins- thin Actin filament and thick Myosin filament. I-bands contain actin. A-bands contain actin and myosin.  They are arranged parallel to each other.

-    A-band bears a lighter middle region (H band) formed of only myosin. A thin dark line (M-line) runs through the centre of H-zone.

-    I-band is bisected by a dense dark band called Z-line. Region between two Z-lines is called sarcomere. They are the structural and functional units of a muscle.

Structure of contractile proteins

-    Each actin filament is made of 2 filamentous (F) actins which form double helix.

-    F-actin is a polymer of monomeric Globular (G) actins.

-    Actin contains 2 other proteins (tropomyosin & troponin).

-    Two filaments of tropomyosin run along the grooves of the F-actin double helix.

-    Troponin (having 3 subunits) is seen at regular intervals on tropomyosin. In the resting state a subunit of troponin masks the binding sites for myosin on the actin filaments.

-    Each myosin filament is a polymer of many monomeric proteins called Meromyosins.

-    A meromyosin has 2 parts:  

o  Heavy meromyosin or HMM or cross arm (globular head + short arm): It projects outwards.

o  Light meromyosin or LMM (tail).

-    The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

Mechanism of muscle contraction

According to sliding filament theory, contraction of a muscle fibre takes place by the sliding of actin over the myosin.

The steps are given below:

-    An impulse from the CNS reaches the neuromuscular junction (Motor-end plate) via motor neuron. (A motor neuron + muscle fibres = a motor unit).

Neuromuscular junction is the synapse between a motor neuron and the sarcolemma of the muscle fibre. 

-    The synaptic vesicles release a neurotransmitter (Acetylcholine) that generates an action potential in the sarcolemma. It spreads through the muscle fibre and causes the release of Ca²⁺ ions into the sarcoplasm.

-    Ca binds with a subunit of troponin on actin filaments and removes the masking of active sites for myosin.

-    Using energy from ATP hydrolysis, myosin head binds to exposed active sites on the actin to form a cross bridge. This pulls actin filaments on both sides towards the centre of A-band. The actin filaments partially overlap so that H-zone disappears.

-    The Z- line attached to actins is also pulled inwards. It causes a shortening (contraction) of sarcomere. I-bands get shortened, whereas A-bands retain the length.

-    Myosin releases ADP and Pi and goes back to its relaxed state.  A new ATP binds and the cross-bridge is broken.

-    The ATP is again hydrolyzed by the myosin head and the above process is repeated causing further sliding.

-    When Ca²⁺ ions are pumped back to sarcoplasmic cisternae, actin filaments are again masked. This causes the return of Z-lines back to their original position, i.e., relaxation.

The reaction time of the fibres varies in different muscles. Repeated activation of the muscles leads to the accumulation of the lactic acid causing muscle fatigue.

This is due to anaerobic breakdown of glycogen in muscles.

Red muscle fibres and white muscle fibres

Red (Aerobic) muscles	White muscle
Red coloured due to myoglobin.	White coloured due to the lesser amount of myoglobin
More mitochondria	Less mitochondria
Aerobic metabolism	Anaerobic metabolism
Slow & sustained contraction	Fast contraction for short period

HUMAN SKELETAL SYSTEM

Consists of a framework of bones (206) and few cartilages.

Human skeletal system has 2 parts: axial & appendicular.

1.  Axial skeletal system (80 bones)

  Includes Skull, Vertebral column, Sternum and Ribs.

a.    Skull (29 bones): It includes

o Cranial bones (8):  Include Frontal (1), Parietals (2), Temporals (2), Occipital (1), Sphenoid (1) & Ethmoid (1).

o Facial bones (14): Include Nasals (2), Maxillae (2), Zygomatics (2), Lacrimals (2), Palatines (2), Inferior nasals (2), Mandible (1) and Vomer (1).

o Hyoid bone (1): U-shaped bone seen below buccal cavity.

o Ear ossicles (3 x 2 = 6): Malleus (2), Incus (2) & stapes (2).

b.   Vertebral column

-    Formed of 26 vertebrae. Includes Cervical vertebrae (7), Thoracic vertebrae (12), Lumbar vertebrae (5), Sacral vertebrae (1-fused) and Coccygeal vertebrae (1-fused).

-    Skull articulates with First vertebra (atlas) with the help of 2 occipital condyles (dicondylic skull).

-    Vertebra has a central hollow portion (neural canal) through which the spinal cord passes.

-    Number of cervical vertebrae are 7 in almost all mammals.

-    The vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back.

c.    Sternum or Breast bone (1)

-    Flat bone on the ventral midline of thorax.

d.   Ribs (12 pairs)

o True ribs (first 7 pairs):  They are attached to thoracic vertebrae and ventrally connected to sternum with the help of Hyaline cartilage.

o Vertebrochondral (false) ribs (8^th, 9^th & 10^th pairs):  They do not articulate directly with the sternum but join the 7^th rib with the help of Hyaline cartilage.

o Floating ribs (11^th & 12^th pairs): They are not connected ventrally (no connection with sternum or other ribs).

-    Each rib has 2 articulation surfaces on its dorsal end and is hence called bicephalic.

2.  Appendicular skeletal system (126 bones)

a.    Bones of fore-limbs (30 x 2 = 60)

Include Humerus (1), Radius (1), Ulna (1), Carpals (wrist bones-8), Metacarpals (palm bones-5) & Phalanges (digits-14).

b.   Bones of hind-limbs (30 x 2 = 60)

Include Femur (thigh bone- 1), Patella (knee cap- 1), Tibia (1) & fibula (1), Tarsals (ankle bones-7), Metatarsals (5) & Phalanges (digits-14).

c.     Pectoral girdle (2x2=4)

-    Include clavicle (2) & scapula (2).

-    Scapula is a large triangular flat bone situated in the dorsal part of the thorax between the second and 7^th ribs.

-    Scapula (shoulder blade) has a slightly elevated ridge (spine) which projects as a flat, expanded process (acromion). The clavicle (collarbone) articulates with this.

-    Below the acromion is glenoid cavity which articulates with the head of humerus to form the shoulder joint.

d.    Pelvic girdle (2)

-    Formed of 2 coxal bones. Each coxal bone is formed by the fusion of 3 bones- Ilium, Ischium & pubis.

-    At the point of fusion of Ilium, Ischium and Pubis is a cavity (Acetabulum) to which the thigh bone articulates.

-    The 2 halves of the pelvic girdle meet ventrally to form pubic symphisis containing fibrous cartilage.

JOINTS

Joints are points of contact between bones, or between bones and cartilages. 3 types.

1.    Fibrous (Immovable) joints: E.g. sutures b/w skull bones.

2.    Cartilaginous joints (Slightly movable joints): Bones are joined together with the help of cartilages. E.g. Joints between the adjacent vertebrae.

3.    Synovial (movable) joints: They have a fluid filled synovial cavity between articulating surfaces of 2 bones.

Types of synovial joint

¨     Ball & socket joint: E.g. Shoulder joint & hip joints.

¨     Hinge joint: E.g. Knee joint, elbow joint etc.

¨     Pivot joint: E.g. Joints b/w atlas & axis.

¨     Gliding joint: E.g. Joints b/w carpals.

¨     Saddle joint: E.g. Joints b/w carpal & metacarpal of thumb.

DISORDERS OF MUSCULAR & SKELETAL SYSTEMS

·   Myasthenia gravis: Auto immune disorder. Affects neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscles.

·   Muscular dystrophy: Progressive degeneration of skeletal muscles. Mostly due to genetic disorder.

·   Tetany: Rapid spasm in muscle due to low Ca²⁺ in body fluid.

·   Arthritis: Inflammation of joints.

·   Osteoporosis: Age-related disorder characterized by decreased bone mass and increased chances of fractures. Decreased level of estrogen is a common cause.

·   Gout: Inflammation of joints due to accumulation of uric acid crystals.