Locomotion and Movement: NEET Prep

Introduction to Locomotion and Movement

• Muscles pull bones to facilitate movement, highlighting the interdependence of muscles and bones in locomotion.
• Both muscles and bones are essential for movement and daily functions.
• Movement and locomotion are vital topics in NEET (National Eligibility cum Entrance Test) preparation.

Types of Movement

• Movement involves an action that results in the movement of external or internal body parts.
• Locomotion is movement that results in a change of place or location.
• Walking, running, flying, and swimming are types of locomotion.
• All locomotion are movements, but not all movements are locomotion.
• Voluntary movements are controlled by the central nervous system.

Types of Movements Possible in the Human Body

• Movements can be muscular, amoeboid, ciliary, or flagellar.
• Muscular movement involves contraction and relaxation.
• WBCs (White Blood Cells) exhibit amoeboid movement.
• Ciliary movement is observed in the fallopian tubes and respiratory tract.
• Flagellar movement occurs in sperm cells.

Muscles

• Muscles are primarily derived from the mesoderm.
• Muscles constitute 40 to 50 percent of body weight.
• Muscles are categorized into skeletal, visceral (smooth), and cardiac muscles based on location.
• Muscles exhibit properties such as excitability, contractility, extensibility, and elasticity.
• Excitability is the ability to receive and respond to stimuli.
• Contractility is the ability to contract and shorten muscle length.
• Extensibility is the ability to relax and lengthen.
• Elasticity is the ability to return to the original shape after contraction.

Types of Muscle

• Skeletal muscles are striated, attached to the skeleton, and controlled by the central nervous system.
• Skeletal muscles facilitate voluntary movement, locomotion, and changes in body posture.
• Smooth muscles are found in the walls of internal organs, are unstriated, and are controlled by the autonomic nervous system.
• Smooth muscles facilitate peristalsis.
• Cardiac muscles are striated, located in the heart, and controlled by the autonomic nervous system, and are branched.

Skeletal Muscle Structure

• The entire muscle is covered by a connective tissue layer called epimysium (fascia).
• Muscle fibers are arranged in bundles called fascicles.
• Each muscle fiber contains cytoplasm called sarcoplasm.
• Muscle fiber plasma membrane is called sarcolemma.
• Myofibrils are cellular organelles composed of actin and myosin proteins and known as myofilaments.
• Myofilaments play a crucial role in muscle contraction.
• Endomysium covers each muscle fiber and helps in muscle contraction.

Myofilaments

• Myofilaments are proteins such as actin and myosin.
• Actinin, myomesin, and dystrophin are additional proteins.
• Actin and myosin are contractile proteins.

Myofibril Structure

• Myofibrils have a dark A band (anisotropic) constituted by thick Myosin.
• Myofibrils have a light I band (isotropic) constituted by thin Actin
• The H zone is present in the middle of the A band that has portion of myosin with no overlapping actin.
• The Z line bisects the I band and defines the sarcomere for one Z line to the next.
• Sarcomeres shorten during muscle contraction.
• During Muscle Contraction: A band remains the same and I band shortens.

Actin Structure

• Thin filaments include actin, tropomyosin, and troponin.
• Globular actin (G-actin) polymerizes into filamentous actin (F-actin).
• Tropomyosin is a fibrous protein arranged helically.
• Troponin has three subunits: TnC (binds calcium), TnI (inhibits binding), and TnT (binds tropomyosin).

Myosin Structure

• Thick filaments are made of meromyosin.
• Meromyosin has a globular head with actin-binding and ATP-binding sites.
• Meromyosin exhibits ATPase enzyme activity and is responsible for ATP hydrolysis.
• Heavy meromyosin (HMM) consists of the globular head and short arm.
• Light meromyosin (LMM) consists of the tail.
• Multiple meromyosin units form the myosin filament.

Mechanism of Muscle Contraction

• A motor neuron transmits a signal to the muscle fiber.
• The signal is in the form of a neurotransmitter, such as acetylcholine.
• The neuromuscular junction is equivalent to the motor end plate according to NCERT.
• Acetylcholine binds to receptors on the sarcolemma, causing depolarization.
• Depolarization releases calcium ions from the sarcoplasmic reticulum.
• Calcium binds to troponin, unmasking the active sites on actin.

Muscle Contraction Cycle

• ATP binds to the myosin head.
• ATP is catalyzed by ATPases in the globular head into ADP + Pi
• The myosin head binds to actin.
• Phosphate is released, initiating a power stroke.
• ADP is released, and actin slides over myosin, contracting the sarcomere.

Muscle Relaxation

• ATP binds to the myosin head to dissociate from actin.
• Calcium is pumped back into the sarcoplasmic reticulum, allowing relaxation.

Types of Muscle Fibers

Red Muscle Fibers:

• Have high myoglobin and mitochondrial content.
• Undergo aerobic respiration.
• Do not fatigue easily.
• Ex: Extensor muscles of the back.

White Muscle Fibers:

• Have low myoglobin.
• Undergo anaerobic activity, fatiguing easily, because of more lactic acid.
• Have more sarcoplasmic reticulum.
• Ex: Extrinsic muscles of the eyeball.