THE SKELETAL SYSTEM AND MOVEMENT IN MAMMALS

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

·        Explain the concept skeleton and mention the types of skeletons.

·        Describe the general plan of the mammalian skeleton.

·        Identify the different vertebrae in the vertebral column.

·        Identify the limb bones and their girdles.

·        Define the term joint and identify the different types of joints.

·        Identify the types of muscles.

·       Explain the sliding-filament model of muscle contraction.

·       Describe how muscles bring about movement.

MAMMALIAN SKELETON                   

Skeleton is the hard parts of an animal that forms the framework for the body. Mammalian skeleton is composed of three types of organs; bones, cartilages and ligaments, tightly joined together.

Types of skeletons

1.  Hydrostatic Skeleton: is found in soft body animals e.g. hydras, flatworms, annelids.

2.   Exoskeletons: are hard structures found outside the body of an animal.  They enclose and protect soft tissues and organs of the body. E.g. beetle, cockroach, snails, crab etc.

3.     Endoskeletons: are rigid, jointed and lie within the body of an animal. They are composed of living tissues (bone and cartilage). E.g. fish, toad, lizard, birds, mammals.

Functions of the Mammalian skeleton

1.         Red blood cells are manufactures in the bone marrow. 

2.         Forms the rigid support of the body

3.         Provides point for muscles attachment

4.         Provides series of levers for movement

5.         Gives shapes to the body

6.         Protects the delicate internal organs

7.         It stores minerals such as calcium and phosphorus

General Plan of Mammalian Skeleton

The bones of the skeleton consist of two groups:

1.  Axial Skeleton made up of the skull, vertebral column, ribs, and sternum.

2.  Appendicular Skeleton made up of forelimbs and hind limbs

Skull

The skull protects the brain and sense organs. Cranial bones are flat bones that surround and protect the brain.

Vertebral Column

The vertebral column (back bone) consists of a series of irregular bones called vertebrae, separated from each other by fibro-cartilaginous called intervertebral discs. The vertebrae are held together by ligaments which prevent their dislocation. Between the vertebrae are opening called intervertebral foramina that allow passage of spinal nerves.

There are five different types of vertebrae in human. They are named according to location:

1.      Cervical vertebrae - neck region

2.      Thoracic vertebrae - in the chest region

3.      Lumbar vertebrae - lower back

4.      Sacral vertebrae - in pelvic region

5.      Coccygeal vertebrae- form the coccyx or tail

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General Structure of Vertebrae

1.  Centrum: it forms the body of the vertebra. The centrum fits into the intervertebral dics on both sides. The centrum carries the neural arch and neural spine.  

2.   Neural arch: is attached to the posterior surface of the body. It forms an arch over the centrum and surrounds the neural canal. The neural arch extends dorsally upwards to form neural spine. The neural arch gives off transverse processes; the anterior process called prezygapophyses and those at the posterior part are called post-zygapophyses.  

3.  Neural canal: is a central canal which serves as passageway for the spinal cord.

Cervical Vertebrae

The cervical vertebrae are the smallest of the bones. There are seven cervical vertebrae in the neck region. They have two vertebral canals through which blood vessels of the neck pass.  The first and the second cervical vertebrae are peculiar in shape.

Characteristics of cervical vertebral

1.      centrum is small and oblong in shape

2.      neural canal is large

3.      presence of a vertebraterial canal or foramen in each transverse process

4.     reduced or short neural canal 

Atlas

The atlas is the first cervical vertebra.  It is almost ring like. It provides up and down or nodding movement to the skull.

Characteristics

1.  absence of centrum

2.  large neural canal

3.  short rounded neural spine

4.  flat and broad transverse process

5.  presence an anterior and posterior arch. 

6.  it has articular facets which articulate with the oval occipital condyles of the skull to form atlanto-occipital joint

Axis

The second cervical vertebra is termed as axis. Its centrum bears an odontoid process, which allows side to side or turning movement of the skull.

Characteristics

1.   presence of large and flat neural canal

2. large neural spine

3. broad and flat centrum

4.  presence of odontoid process which articulate with the atlas

5. has two postzygapophyses at the posterior end

The atlas and axis are specialized to support the head and allow it to nod “Yes” and shake “No”

Function of cervical vertebra

1.       form the framework of the neck and support the head region. 

2.        provides surfaces for the attachment of the neck muscles

3.        vertebrate rial canal provides pathway for blood vessels and nerves in the neck region

Thoracic Vertebrae

Thoracic vertebrae are larger than cervical vertebrae. The twelve vertebrae in the thoracic region constitute the thoracic vertebrae. Each thoracic articulate with a pair of ribs using its capitular and tubercular facets.

 Characteristics

1.    have heart-shaped centrum with articular facets for attachment of the ribs. 

2.     neural arch is relatively small.

3. neural spine is long and is directed downwards 

4.  thick and strong transverse processes articular facets which support the ribs

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Lumbar Vertebrae

Lumbar Vertebrae are located in the abdomen. They are the largest vertebrae of the vertebral column. Lumbar vertebrae have projections for attachment of powerful back muscles.

Characteristics

1.      very large thick centrum

2.      the neural spine is broad and hatchet-shaped

3.      the transverse processes are long and slender

4.      presence of projections, anapophysis and metapophysis

5.      the fifth lumbar vertebra articulates with the sacrum at the lumbo-sacral joint

The sacral vertebrae

It consists of five sacral vertebrae, which are fused into a single bone called sacrum. The wedge-shaped sacrum provides a strong foundation for the pelvic girdle.

Characteristics

1.   broad and flat centrum

2.   neural canal is narrow

3. transverse processes of the first sacral vertebra are large and wing-like.

Coccygeal or Caudal Vertebrae

These are 4 in number and occur in the vestigial tail. They are very small and fused to form a curved, triangular bone called the coccyx or tail bone. The first vertebra of the fused coccyx attached to the sacrum by ligaments.  The coccyx flexes anteriorly, acting as a shock absorber, when a person sit. It also supports the tail.

Characteristics

1.   Absence of neural canal neural arch, neural spine

2.   Lack transverse process

3.   Thick centrum

Caudal vertebrae progressively decrease in size from the base of the sacrum to the tip of the tail.

Functions of Vertebral Column

1.    The vertebrae enclose and protect the spinal cord
2.    Support the skull and allow for its movement
3.      Articulate with the rib cage

4.    Provide for the attachment of trunk muscles.

5.   The intervertebral discs between the vertebrae act as shock absorbers.

6.    Vertebrae allow passage of the spinal nerves.

Rib Cage or Thoracic Cage

The cone-shaped, flexible rib cage consists of the 12 thoracic vertebrae, 12 paired ribs, the sternum (or breast bone) and costal cartilages. The costal cartilages secure the ribs to the sternum. The sternum lies in the anterior midline of the thorax. It is elongated, flattened bone plate. All the 12 pairs of ribs attached posteriorly to the thoracic vertebrae. The upper six pairs of the ribs (true ribs) are attached directly to the sternum by their costal cartilages. The seventh through tenth pairs of ribs (false ribs) are attached indirectly to the sternum through their costal cartilages by attaching to the costal cartilage of the sixth rib. The 11^th and 12^th pair of ribs (floating ribs) do not attach to the sternum.

Functions

1. It protects vital organs (heart, lungs and blood vessels)

2. It supports the pectoral girdle 

3. It involves in respiration (breathing) 

4. Bone marrow in certain rib bones produce blood cells

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Appendicular Skeleton

The appendicular skeleton is divided into

1.      pectoral girdle and the pelvic girdle

2.  upper (Fore) limbs and lower (Hind) limbs

Pectoral (Shoulder) Girdle

The pectoral girdle consists of the clavicles or collarbone and the scapula (shoulder blade). The clavicle is a slender, curved or S-shaped bone that joins with the scapula. It has a flattened lateral acromial end, which articulates with the scapula. Scapula is a thin triangular flat bone that forms a movable joint with the upper arm bone. It has a shallow cavity called the glenoid cavity which articulates with the head of the humerus.

Functions    

1.      clavicle holds the scapula in position

2.   supports the arm

3.  scapula provides surfaces attachment of shoulder muscles

4. the acromion process articulate with the clavicle to allow sliding movement 

5. glenoid cavity of scapula articulates with head of humerus to allow movement in any plane

Pelvic (Hip) Girdle

Pelvic girdle or hip girdle is formed by a pair of hip bones, each called innominate bone. Each large, irregularly shaped hip bones consist of three separate bones; the ilium, ischium and pubis. In adults these bones are firmly fused and their boundaries are indistinguishable. At the point of fusion of the ilium, ischium and pubis is a deep hollow called acetabulum on the lateral surface of the pelvic. The acetabulum receives the head of the femur at this hip joint. The pubis and ischium enclose a large hole called obturator foramnen. The deep basin-like structure formed by the hip bones together with the sacrum and coccyx is called the bony pelvis.

Functions                            

1.    protects the female reproductive organs

2.    supports the legs
3.    ilium has large surface area for attachment of the hip muscles
4.    Acetabulum articulates with head of femur to form the hip joint

5.  Obturator foramen provides passage for blood vessels and nerves

Upper Limb (Fore-Limb)

The forelimb consists of: Humerus, radius and ulna, carpals, metacarpals, digits or phalanges.

Humerus (upper arm): is the bone of the arm. It articulates with the scapula at the shoulder joint and with the radius and ulna (forearm bones) at the elbow. It consists of three sections:

1.  Proximal part consisting of a rounded head, a narrow neck, and two short processes called tuberosities or t   Cylindrical body or shaft   

2.  Distal part consisting of two condyles, processes (trochlea & capitulum), and fossae

Functions of Upper limbs

1. The smooth rounded head fits into the glenoid cavity of the scapula. 

2. The tuberosity is a site for muscles attachment. 

3.  The trochlea articulates with the sigmoid notch of the ulna.  

4. The capitulum articulates with the radius.

5. The supratrochlear foramen provides a passage for blood vessels and nerves 

Radius and Ulna: the radius and the ulna form the bones of the lower arm. Their proximal ends articulate with the humerus and their distal ends form joints with bones of the wrist. The radius and ulna articulate with each other at a small joint called radioulnar joints. The ulna is slightly longer than the radius. The proximal end of the ulna is large and is known as Olecranon process. This has trochlear notch which articulates with the trochlea of the humerus and radial notch which articulate with the head of the radius.

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Bones of the wrist and hand

Lower Limb (Hind Limb)

The hind limbs have a similar basic pattern to the forelimb. They consist of femur, tibia and fibula, tarsals, metatarsals, digits or phalanges. These bones form the three segments of each hind limb; the thigh, the leg and the foot.

Femur: is the longest, heaviest and the strongest bone in the body. The proximal rounded head of the femur articulates with the acetabulum of the pelvic girdle to form hip joint. The constricted region supporting the head is called the neck. The body (shaft) has several features for muscle attachment. The distal end of the femur is expanded for articulation with the tibia. 

Tibia and Fibula: these are the bones of the leg. The tibia articulates proximally with the femur at the knee and distally with the talus of the ankle. It also articulates both proximally and distally with the fibula.

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Joint and Muscles

Joints or Articulations are sites where two or more bones meet. Joints give skeleton mobility and also hold bones together

Classification of Joints

Joints are classified based on the material binding the bones together. 

1.   Fibrous (or Fixed) Joints: are immovable.  The articulating bones are joined by fibrous tissues. No joint cavity is present. E.g. Sutures of the skull.

2.   Cartilaginous (or Slightly Movable) Joints: are slightly movable. The articulating bones are united by cartilage. They lack a joint cavity. E.g.  intervertebral joints. 

3.       Synovial (or Movable) Joints: are freely movable. The articulating bones are separated by a fluid-containing-joint cavity. This arrangement, permits freely movement synovial joints.  E.g. pivot joint, hinge joint. 

Types of Synovial Joints

1.     Plane (or Gliding) joints: allow only short slipping and gliding movements e.g. the intercarpal and intertarsal joints.

2.    Hinge joints: the movement is along a single plane and is similar to that of a mechanical hinge. E.g. knee joint and elbow joint.

3.   Pivot joints: allow rotation of the bone around its own axis or over the other. E.g. is the joint between the atlas and the axis.

4.    Ball and socket joints: allow a wide range of movement. E.g. the joints at the shoulder and hip

General Structure of Synovial Joints

1.     Articular Cartilage: smooth articular (hyaline) cartilage covers the surface of the bones. This prevent wearing of bone surfaces.  

2.     Joint (synovial) cavity: One of the unique features of the synovial joints is the joint cavity and thereby keeps the bone ends from being crushed. 

3.  Articular capsule: The joint cavity is enclosed by a double-layered articular (joint) capsule. The external layer is a tough, flexible fibrous capsule. The inner layer of the joint capsule is called synovial membrane composed of loosed connective tissues. Synovial membrane secretes the synovial fluid to fill the cavity.   

4.     Synovial fluid: A small amount of slippery synovial fluid occupies all free spaces within the joint capsule. Synovial fluid provides a slippery film that reduces friction between the cartilages. Synovial fluid also contains phagocytic cells that rid of microbes or cellular debris.    

5.   Ligaments: Synovial joints are held in position and strengthen by a number of fibrous ligaments called capsular ligament.  

Muscle tissues are made up of elongated, elastic cells called muscle fibers. The elongated and elastic feature helps muscle to contract and relax. Contraction and relaxation of muscles help in movement. Muscles usually work in pairs. When one contracts the other relaxes and vice versa. Pairs of muscles that work like this are called antagonistic muscles. The biceps and triceps muscles of the arm are an example of an antagonistic pair.

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Types of Muscle

The 3 types of muscle tissue are skeletal, cardiac and smooth.

Skeletal, Voluntary or Striated Muscle

It is attached to and covers the bony skeleton. Skeletal muscle cells or fibers are long, threadlike, with dark cross-markings or stripes called Striations. Each fiber contains many nuclei located just beneath its cell membrane (sarcolemma). Skeletal muscle is called voluntary muscle because it is the only type subject to conscious control.

The cytoplasm (sarcoplasm) of the fibers contains:

1.    Bundles of myofibrils which consist of fine filaments (or sarcomeres) of contractile proteins including actin and myosin.

2.      Many mitochondria that generate energy (ATP) from glucose by cellular respiration. 

3.     Glycogen, a store carbohydrate which is broken down into glucose when required. 

4. Myoglobin, a unique oxygen-binding protein that stores oxygen within muscle cells.

For more on muscle fiber types, refer to this article

Skeletal muscle consists of a large number of fibers. Each fiber is enclosed in a fine fibrous connective tissue called endomysium.  Small bundles of fiber are enclosed in epimysium.  The fibrous tissue enclosing the fibers, the bundles and the whole muscle extends beyond the muscle fibers to become the tendon which attaches muscle to bone or skin.

For more details, visit:

• Anatomy and Physiology of Skeletal Muscles
• Muscle Contraction and Movement

Cardiac Muscle Tissue: Structure and Function

Cardiac muscle is only found in the wall of the heart.  It is striated, but it is not voluntary.  The muscle fibers are branched and interconnected to form complex networks. The fibers are composed of cells which are joined end to end. Each cell has a single nucleus.  Each cell is attached another cell at a specialized intercellular junction called an intercalated disk. A wave of contraction spreads from cell to cell across the intercalated discs.

Function

Contraction and Coordination: A wave of contraction spreads from cell to cell across the intercalated discs. This coordinated contraction ensures that the heart pumps blood efficiently throughout the body.

For more detailed information on cardiac muscle tissue, visit these resources:

• NCBI: Cardiac Muscle Tissue Overview
• Khan Academy: Cardiac Muscle Structure
• Mayo Clinic: Cardiac Muscle Function

Smooth Muscle Tissue

Smooth muscle is found in the walls of the hollow internal organs such as the stomach, urinary bladder, blood vessels and the respiratory passages. It is called smooth because its cells lack striations. It is described mostly as visceral, nonstriated and involuntary. Contractions of smooth muscle fibers are slow and sustained. The cells are spindle-shaped with only one central nucleus.   The cells are shorter than those of the skeletal muscle.

Function 0f smooth muscle

1. Movement and Control of Substances

2. Regulation of Blood Flow and Pressure

3. Control of Airflow in Respiratory Tract

For more detailed information on smooth muscle tissue, visit these resources:

• NCBI: Smooth Muscle Overview
• Khan Academy: Smooth Muscle Structure and Function
• Mayo Clinic: Smooth Muscle Function

The Sliding-Filament Model of Muscle Contraction

The sliding filament theory of muscle contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement. Muscles contract when sarcomeres shorten. The thin and thick filaments that compose sarcomeres, slide past one another, causing the sarcomere to shorten while the filaments remain the same length.

Thick filaments are composed of the protein myosin. Thin filaments are composed of the protein actin.  During contraction, myosin heads bind to actin molecules of the thin filaments. Flexing of the myosin heads pulls the thin filaments over the thick filaments - shortening the sarcomere.

Movement of the Fore Limb

A single muscle is fat in the middle and tapers towards the ends (i.e., the origin and the insertion). The middle part, which gets fatter when the muscle contracts, is called the belly of the muscle.  

Skeletal muscles usually work in pairs. When one contracts the other relaxes and vice versa. Pairs of muscles that work like this are called antagonistic muscles. Example of antagonistic muscle is the biceps and triceps in the upper forearm. Together they bend and straightened the forearm. 

The origin of the biceps is attached to the scapula (the bone that moves the least when muscle contract) while the insertion is attached to the radius (movable part). When the biceps contracts (and the triceps relaxes) the lower forearm is raised and the angle of the joint is reduced. This kind of movement is called flexion. When the triceps is contracted (and the biceps relaxes), the angle of the elbow increases (i.e., straightening the lower arm). The term for this movement is extension.

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References 

1. Skeletal System Overview

• NIH Osteoporosis and Related Bone Diseases National Resource Center offers comprehensive information on bone biology, development, and disorders.

2. Educational Resources on the Skeletal System

• Khan Academy offers free educational materials on the skeletal system, including videos and articles on bone anatomy and physiology.

3. Structure and Function of the Skeletal System

• National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)  provides comprehensive information on bone health and osteoporosis, which is crucial for understanding the skeletal system.

   4. Skeletal System in Animals

• Encyclopædia Britannica offers detailed articles on the skeletal system in humans and other animals, including the structure and function of bones.

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