Locomotion and Movement

Movement is an essential trait of all living organisms. From the protoplasmic movement within a cell and unicellular organisms to the movement of organs in multicellular organisms, movement plays a vital role in the functioning of living organisms.

The movement that results in a change of position is known as locomotion.

There are three types of movement in a cell and organs:

1. Cytoplasmic streaming

2. Cell motility

3. Organelle motility

4. Amoeboid Movement: Pseudopodia, which is seen in amoeba, is similar to the movement of macrophages, leukocytes, and cytoskeletal microfilaments, which all show ameboid movement.

5. Ciliary and Flagellar Movement: Ciliary movement is observed in the epithelial lining of the trachea, reproductive tract, etc. Meanwhile, sperms exhibit flagellar movement.

6. Muscular Movement: Muscles play a major role in the movement of multicellular organisms. From breathing, to the function of the heart, digestion, and movement of appendages and locomotion, muscles are responsible for performing these functions in our bodies. Locomotion, in particular, requires the coordination of skeletal, neural, and muscular systems.

Muscles originate from the mesodermal germinal layer, and they possess unique properties such as contraction, extension, excitation, and elasticity.

We all know, there are three types of muscles:

• Skeletal Muscle
• Smooth Muscle
• Cardiac Muscle

Cardiac Muscles: Striated and Involuntary, found in the heart.

Visceral Muscles: Smooth and non-striated, these muscles are involuntary and support various internal organs, as well as taking part in functions such as digestion and reproduction.

Skeletal Muscles: Striated and voluntary, these muscles are responsible for the locomotion and movement of appendages.

Let’s dive deeper into skeletal muscles, exploring their structure and how they contract.

The Anatomy of a Muscle Fiber and Sarcomere

Skeletal muscles are the most abundant muscles, consisting of bundles of muscle fibres wrapped in connective tissue.

Fascicles (Muscle Bundles)

A muscle such as a biceps is composed of multiple muscle bundles (fascicles) bound together by fascia, a connective tissue layer. Each fascicle contains numerous muscle fibers.

Muscle Fibers

Muscle fibres are elongated cells which are organized into bundles called fascicles. The characteristic features of muscle fibres are:

Each skeletal muscle fibre is long, cylindrical, and striated.

It is a syncytium, which contains many nuclei.

The Sarcolemma is the plasma membrane of the muscle fibre.

Sarcoplasm is the cytoplasm of the muscle fibre.

The Sarcoplasmic Reticulum is the endoplasmic reticulum of the muscle fibre, and is the storehouse of Ca ions.

Myofibrils

Each muscle fibre has multiple myofibrils running lengthwise and parallel to each other in the sarcoplasm. The alternating dark and light bands present in myofibrils give the muscle a striated appearance. Each myofibril consists of even simpler structures called myofilaments.

Myofilaments

There are two types of myofilaments present in a myofibril: thin and thick filaments. For muscle contraction, thin and thick filaments must attach to each other.

A. Thin Filament or Actin Filaments are composed of three different proteins.

It contains two polymeric filamentous, or ‘F’ actin filaments, which are wound together. These filaments are a polymer of globular, or ‘G’ actin monomers.

Tropomyosin filaments are coiled lengthwise around the actin filaments.

Troponin, located at specific points on tropomyosin, obscures the active attachment sites for myosin.

Troponin and tropomyosin play an important role in muscle contraction by regulating the binding of actin and myosin filaments.

![Thin Filament (Actin filament)]()")

b. Myosin filaments are made up of myosin.

**It is a polymeric protein consisting of monomeric units of a protein known as meromyosin.

It has three parts: tail, short arm or neck, and a globular head.

The head and cross arm protrude outward from the filament at regular intervals.

The globular head contains binding sites for ATP and Actin.

The head of the ATPase enzyme acts as an enzyme.

The Functional Unit of Muscle Contraction: Sarcomere

A myofibril contains hundreds of sarcomeres, which are connected end to end.

A sarcomere is the basic unit of muscle contraction, consisting of repeating units of actin and myosin filaments arranged in an orderly fashion.

Sarcomeres are joined at the end by interweaving filaments, forming the ‘Z’ line, which is the limiting membrane of a sarcomere.

Actin filaments (or thin filaments) are attached to the Z-line at regular intervals.

Thick filaments, or myosin filaments, are situated between the thin filaments. These filaments are held in place by a very thin fibrous membrane known as the ‘M’ line.

The actin and myosin filaments overlap each other in a specific pattern, resulting in the striation pattern of muscles. This pattern consists of three types of bands.

The ‘I’ band or Isotropic band consists of actin filaments that are attached to two adjacent sarcomeres, where the thick and thin filaments do not overlap. This band is referred to as the light band.

The ‘A’ band (or anisotropic band) is the dark band that contains overlapping actin and myosin filaments.

The ‘H’ zone is located in the center of the thick filaments and is not overlapped by the thin filaments.

In summary, the sarcomere is the fundamental building block of muscle and is composed of thick and thin myofilaments. Hundreds of sarcomeres form a myofibril, multiple myofibrils form a muscle fibre, and a bundle of muscle fibres is referred to as a fascicle. Multiple fascicles are surrounded by a fascia, forming a muscle.

Let’s Dive into the Complex Physiology of Muscle Contraction

Mechanism of Muscle Contraction

Muscle contraction occurs when actin and myosin filaments slide past each other.

The Sliding Filament Model was proposed by Andrew Huxley and Hugh Huxley.

The contraction of muscles is a result of sliding, which causes an increase in the overlap of thick and thin filaments and shortening of the sarcomere.

Steps of Muscle Contraction

1. Motor neurons transmit a signal from the brain or spinal cord (CNS) to initiate muscle contraction.

2. The neural signal triggers the release of Acetylcholine at the neuromuscular junction’s synaptic cleft. When Acetylcholine binds to receptors on the muscle fibre, it causes the sarcolemma to depolarize.

1. The action potential generated propagates through the muscle fibre.

2. Calcium ions (Ca2+) are then released from the sarcoplasmic reticulum into the sarcoplasm.

3. This release of Ca2+ is regulated by the protein dystrophin, which is the longest gene found in the human body.

4. The binding of Ca²⁺ ions to troponin triggers conformational changes, which exposes the myosin-binding sites on actin filaments.

5. Myosin head contains a binding site for ATP, where ATP binds and undergoes hydrolysis via ATPase activity.

6. The energised myosin head (cocked) binds to the active binding sites on actin, forming a cross bridge.

7. The attachment of the myosin head to the actin filament is followed by the release of phosphate, which triggers the ‘power stroke’. This causes the myosin filaments to bend and pull the actin filaments towards the centre of the sarcomere, resulting in the shortening of the sarcomere and muscle. In the process, ADP is also released.

8. The detachment of myosin head is also driven by ATP.

In the presence of sufficient Ca²⁺ ions, the process repeats repeatedly.

Muscle Relaxation

Once the neural signal ceases, Acetylcholinesterase inactivates acetylcholine in the synaptic cleft, causing the muscle fibres to come to a resting state. Ca2+ ions are then pumped back into the sarcoplasmic reticulum, where the absence of Ca2+ ions causes the troponin-tropomyosin complex to cover the myosin-binding sites on actin filaments. Finally, the Z line of sarcomere returns to its original position and muscles become relaxed.

Muscle Fatigue

Muscle contraction is powered by ATP, which is obtained from the storage of creatine phosphate and glycogen. Under normal conditions, glycogen is converted to glucose, and ATP is released from cellular respiration.

In strenuous exercise, muscles need energy in large amounts. Since the body cannot provide enough oxygen, glucose is broken down without oxygen, leading to the buildup of lactic acid, resulting in muscle fatigue.

Rigor Mortis, the temporary stiffening of muscles after death, is caused by the depletion of ATP when cellular respiration stops. The detachment of myosin heads requires ATP, and in its absence, the cross-bridges remain attached in the muscles that were contracting. This phenomenon can be used to estimate the time of death.

There are two types of muscle fibres based on the amount of oxygen-binding pigment, myoglobin, they contain:

• Type 1 muscle fibres which have a high myoglobin content
• Type 2 muscle fibres which have a low myoglobin content

Red Fibres or Aerobic Muscles: Reddish in color, these muscles contain more myoglobin and mitochondria.

White Fibres or Anaerobic Muscles: Pale or white in colour, they contain less myoglobin and mitochondria but more sarcoplasmic reticulum.

Let’s take a look at the bones and joints of the skeletal system in brief.

Skeletal System

The skeletal system provides the structural framework to our body and helps in movement and locomotion. It provides protection to the internal organs. Our skeletal system comprises special kinds of connective tissues, i.e. bones and cartilage.

Bones

An adult human being has a total of 206 bones. Bones are hard because of the calcium salts present in their matrix, while cartilage contains chondroitin salts.

The human skeleton consists of 80 bones in the axial skeleton, and 126 bones in the appendicular skeleton.

Axial Skeleton (80 bones):

• Skull
• Vertebral Column
• Ribs
• Sternum

The Appendicular Skeleton consists of 126 bones and includes the Pectoral and Pelvic Girdle as well as the Limbs.

Given below is a table of the main bones for quick reference.

| Axial Skeleton (80) | Skull (22) | 8 cranial bones | Frontal |