Human Physiology - Dr. Manisha Majumdar (De) (little readers .TXT) 📗

Locomotion

 

 

Locomotion

 

Learning objectives

 

After studying this chapter you can understand about

 

Joints

    Immovable or fixed or fibrous joint

    Slightly movable or cartilagenous joint

    Freely movable or synovial joint       

        Planar joint

        Hinge joint

        Pivot joint

        Condyloid joint

        Saddle joint

        Ball and socket joint

 

Knee joint

    Skeletal muscle 

        Anatomy and physiology

        Structure of skeletal muscles

        Process of muscle contraction 

             Sliding-filament theory of muscle contraction

             Control of muscle contraction

                  Role of Ca++ in contraction + +

             Classification of skeletal muscular system

             Properties of muscles

                  Red and white muscle

             Types of skeletal muscle contraction

                  Muscle fiber twitch

                  Summation

                  Treppe

                  Tetanus

                  Muscle fatigue

                  Rigor mortis

                  Isotonic and isometric contraction

 

 

Movement is a characteristic of all living things. There is movement within plant and animal cell. The movement of whole organisms from place to place is a somewhat different phenomenon, known as locomotion. The purpose of animal locomotion includes the search for food, avoidance of predators and other dangers, the search for a mate and in reproduction, migratory movements and the search for a more favorable environment. In many motile multicellular animals the support required is provided by the skeleton. The bones of the skeleton protect many internal organs, and the whole skeleton provides numerous points of attachment for the voluntary, skeletal muscles of the body. Locomotion is made possible by the contraction of these muscles, acting across joints. The movement is a complex series of coordinated functions of the skeletal, muscle and nervous system.

 

Joint

 

The skeletal movements of the body are produced by contraction and shortening of muscles. Skeletal muscles are generally attached by tendons to bones, so when the muscles shorten, the attached bones move. This movement of skeleton occurs at joints, or articulations, where one bone meets another. There are three main classes of joints.

 

 

1. Immovable or fixed or fibrous joints

 

These contain connective tissue with collagen fiber.

They contain no space between articulating joints and the synovial cavity.

They permit little or no movement.

Sutures uniting the bones of the skull (Fig. 14.1(a).

 

2. Slightly movable or cartilagenous joints

 

These joints are fixed or slightly movable.

There is no synovial cavity.

Bones are held together with cartilage.

This type of joint is found in vertebral bones at the spine which are separated by pads of cartilage called intervertebral disc. These types of joints allow some movement, primarily flexibility, while acting as efficient shock absorbers. (Fig. 14.1(b)).

 

3. Freely movable joints or synovial joints

 

The articulating ends of the bones are located within a synovial cavity or capsule filled with lubricating fluid.

The ends of the bones are capped with cartilage.

The synovial capsule is strengthened by ligaments that hold the articulating bones in place (Fig. 14.1(c)).

The articulate capsule has two layers :

Fibrous capsule : the outer layer

Synovial membrane: the inner layer that secretes synovial fluid which lubricates, reduces friction, supplies nutrients and move metabolic waste. It contains phagocytic cells that remove microbes and many debrises created from normal wear of the joint.

 

Fibrous capsule may contain bundles of fibers called ligaments, these are arranged in parallel bundles to help resist excess strain and prevent damages. Ligaments may lie inside or outside the articular capsule.

 

Ligaments are tough fibrous bands containing elastin fibers that allow the ligament to stretch. Ligaments are attached to both ends of the articulating bone to help keep the two articular cartilages together.

 

To reduce friction in joints that lie close to the skin, fluid-filled structures called bursae cushion. Bursae are filled with a fluid similar to synovial fluid and are present in the knee and shoulder joint.

 

Types of synovial joint

 

Synovial joints are classified into six sub-types, according to the range of movements they allow (Fig. 14.2).

 

a) Planar joints

 

This is also called gliding joint. The articulating surfaces of the bones are flat or slightly curved. These allow side to side and back and forth gliding movements.

 

b) Hinge joints

 

The articulating surfaces of the bones consist of one concave surface and one convex surface, where one bone fits into the other. A hint joint produces an open and close movement similar to the action of a hinge on a door.

 

c) Pivot joints

 

In this joint the end of one bone is rounded and the other has a ring or hole made of bone and ligament. A pivot joint allows rotational movement.

 

d) Condyloid joints

 

The articulating surface of one bone is an oval-shaped projection that fits into an oval-shaped depression of the other. This joint allows up and down and side to side movement.

 

e) Saddle joints

 

The articulating surface of one bone is saddle shaped with the articular surface of the other bone-shaped to fit into the saddle. This type of joint allows side to side and up and down movement.

 

f) Ball and socket joints

 

In this joint a rounded ball-shaped surface fits into a cup-shaped depression that allows movement in several directions.

 

 

   

Knee joint

 

The knee joint is the largest joint in the body

and it requires stabilizing by ligaments and

tendons. The knee joint is a hinge joint formed

by the condyles of the femur and tibia and the

posterior surface of the patella. The joint allows

flexion and extension and a small degree of side

to side movement when the knee is flexed. The

joint has a joint capsule and extra-capsular

and inter-capsular ligaments (cruciate ligaments )

to strengthen it by limiting movement. The joint

is further strengthened it by limiting movement.

The joint is further strengthened by two crescent

wedge-shaped pieces of fibrous tissue called the

menisci. The patella is a sesamoid bone that

lives within the joint capsule. If slides on the

patellar surface of the distal femur and its function

is to reduce friction during extension and protect

the knee joint (Fig. 14.3).

 

 

Capsular ligament helps to prevent dislocation, stabilizes joint.

Synovial membrane secretes synovial fluid to lubricate and supply nutrients. Phagocytic cells are present to keep fluid free debris.

Articular cartilage reduces friction and act as shock absorber.

Cruciate ligaments strengthen and limit movement.

Menisci – fibrous tissue to ensure tight fit between joint surfaces of different shapes.

Patella tendon helps prevent dislocation of patella, stabilizes joint.

Patella – sesamoid bone.

Prepatella bursa – sac of synovial fluid.

Bursa – sac of synovial fluid.

 

Skeletal muscle

 

Most skeletal muscle lies just below the skin and there are approximately 600 named muscles in the body. Skeletal muscle is the only muscle tissue that may be controlled voluntarily, although in many cases this control operates through reflexes.

 

Anatomy and physiology

 

A whole muscle is made up of numerous muscle fibers and enclosed in a layer of connective tissue. Skeletal muscles are well supplied with nerves and blood vessels. Skeletal muscle is stimulated by nerves of the peripheral nervous system and can produce rapid forceful contraction needed for movement. In most cases skeletal muscle has two ends that attach to other tissues and a wide middle section called the belly. Muscles connect to bone by tendons. Tendons are tough strands or cords of fibrous tissue.

 

Structure of skeletal muscle

 

Skeletal muscle or voluntary muscle mainly occurs attached to the skeleton in the trunk, limbs and head, either directly to the bone or indirectly via tendon. Skeletal muscle consists of thousands of elongated, cylindrical, multinucleated muscle fibers, lying parallel to one another. Skeletal muscle is also called striated muscle because the highly regular arrangement of its actin and myosin filaments gives it a striped appearance. This muscle is enclosed by a sheet of connective tissue – the deep fascia that separates and holds muscle together. The outer covers of the muscle fibers also extend and form tendons that are connective tissue that attaches the bone to the muscle. Under the deep fascia, there are bundles of muscle fiber which are called fascicles that are covered by perimysium. Each individual muscle fiber is covered with endomysium. The muscle fibers contain a series of transverse stripes of muscle protein., Each muscle fiber is covered with a plasma membrane called sarcolemma from which extend small vessels called transverse tubules. Sarcoplasm (the name of muscle cytoplasm ) in each muscle fiber stores glycogen and oxygen (myoglobin ) to provide energy during muscle contraction. Along the length of each muscle fiber are tube-like structure called myofibrils. The myofibrils consist of interlinking thick (myosin protein ) and thin (actin protein ) filaments. Thick and thin filaments overlap in pattern and form a functional unit of muscle. The units are called sarcomeres. Sarcomeres are separated from each other by a zigzag band of dense material that is called z band. One sarcomere is the region between two Z lines. Sarcomeres have bands of filaments. The A band extends along the length of the thick filament and its center is narrow band which is called the H zone. At each of the A band, thick and thin filaments overlap. Thin filaments create the I-band on either side of the A band. The I band is divided in half by a Z band.

 

 

Process of muscle contraction

 

The process of muscle contraction is stimulated by nerve impulses conducted via motor neurons. Neurons and muscles meet at the neuromuscular junction (NMJ). The NMJ provides a space/synapse across which message from the impulses will travel.

At the axon terminal there are vesicles containing neurotransmitter, i.e. Acetylcholine (Ach). When a nerve impulse is received the vesicle fuses with the cell membrane and released Ach.

The Ach is moved across the synapse due to active transport promoted by high concentration of sodium and potassium.

The Ach attached to Ach receptors on the muscle cell membrane (Sarcolemma) and this open up a channel to allow the high concentration levels of sodium to flood into the cell.

The change in sodium concentrations within the cell causes the shape of the troponin molecule and in turn the tropomyosin molecule, allowing the sliding filament mechanism of muscle contraction to take place.

To relax the muscle, Ach is removed from the synaptic cleft by the action of Acetylcholinesterase (AchE).

 

The sliding-filament theory of muscle contraction

 

Huge Huxley and Andrew Huxley proposed a molecular mechanism of muscle contraction. This theory is called the sliding-filament theory of muscle contraction. The repeating structure sarcomere is the smallest subunit of muscle contraction.

 

The thin filaments stick pathway into, and overlap with thick filaments on each side of an A band but in a resting muscle, do not project all the way to the center of the A band. As a result, the center of an A band (called an H band) is lighter than each side, which its interdigitating thick and thin filaments. These appearances of the sarcomeres change when the muscle contracts.

 

A muscle contracts and shortens because of myofibrils contract and shorten; instead, the thin filaments slide deeper into the A bands (Fig. 14.4). This makes the H band narrower until, at maximal shortening, they disappear entirely. It also makes I bands narrower, because the dark A bands are brought closer together. This is sliding filament mechanism of contraction.

 

Electron micrographs reveal cross-bridges that extend from the thick to the thin filaments, suggesting a mechanism that might cause the filaments to slide. Each thick filament is composed of many myosin proteins packed together, and every myosin molecule has a “head” region that protrudes from the thick filament (Fig. 14.5). These myosin heads form the cross bridges seen in electron micrographs.

 

 

Thin filaments are composed of globular actin protein. Two rows of actin proteins are twisted together in a helix to produce the thin filaments (Fig. 14.6). Other proteins tropomyosin and troponin, associate with the strands of actin and are involved in muscle contraction.

 

The interactions of thick and thin filaments in striated muscle sarcomere (molecular level) are depicted in figure (14.7(a)). The heads on the two ends of the thick filaments are oriented in opposite directions, so that the cross-bridges pull the thin filaments and the Z lines on each side of the sarcomere toward the center (Fig. 14.7(a)). The sliding of the filaments produces muscle contraction.

 

The heads on the two ends of the thick filaments are oriented in opposite directions, so that the cross-bridges pull the thin filaments and the Z lines on each side of the sarcomere toward the