THE MUSCULAR SYSTEM

Agriscience 332

Animal Science

#8646-D

TEKS: (c)(2)(A)

Introduction

In terms of human consumption, muscles are the lean portions of the carcasses of meat animals.

Photo by M. Jasek.

As with other animal systems, the muscle system follows a hierarchical structure.

Organism (pig)

System (muscle system)

Organs (muscles)

Tissues (muscle tissues)

Cell (muscle cells)

The functions of the muscle system:

• Movement of the animal or locomotion, and

• Movement of materials within the animal’s body for life support.

Classification of Muscles and Muscle Tissues

There are two classifications of muscles:

• Voluntary, and

• Involuntary.

Voluntary Muscles – muscles that the animal can control.

Example: leg muscles (biceps and triceps)

Involuntary Muscles – muscles that the animal cannot control.

Example: heart muscles

There are three types of muscle tissue:

• Striated (skeletal),

• Smooth, and

• Cardiac.

Physiology of Muscles

As mentioned, there are three types of muscle tissues: striated (skeletal), smooth, and cardiac.

All voluntary muscles are striated.

Involuntary muscles may be striated, smooth, or cardiac.

Striated Muscle Tissue – muscle tissue that consists of light and dark fiber bands, which create a striated or striped appearance.

Photo from U.S. Federal Government courtesy of Wikipedia.

Smooth Muscle Tissue – muscle tissue that is non-striated (not striped) and is found in the “walls” of hollow organs.

Photo from U.S. Federal Government courtesy of Wikipedia

Cardiac Muscle Tissue – striated, involuntary muscle found in the heart.

Photo from U.S. Federal Government courtesy of Wikipedia

Individual muscle fibers are surrounded by connective tissue called endomysium.

Bundles of muscle fibers are enclosed in connective tissue called perimysium.

The entire muscle is then enclosed by connective tissue called epimysium.

Structure of Skeletal Muscle and Its Fibers

Action of Voluntary Muscles

Muscle action is complex.

Energy for muscle contraction comes mainly from sources such as glycogen and body fats through the breakdown of adenosine triphosphate (ATP).

Insufficient food intake can cause protein sources to be used for energy.

Muscle contraction involves the movement of small filaments that make up the bands of light and dark fibers of striated muscle.

Remember, all voluntary muscles are striated.

The light, or isotrophic, bands contain the protein actin.

The dark, or anisotrophic, bands contain the protein myosin.

Muscle contraction involves the longitudinal movement of the actin filaments relative to the myosin filaments.

As mentioned, ATP (adenosine triphosphate) is the energy source for muscle contractions.

An enzyme called adenosine triphosphatase, which is formed from myosin and calcium ions, splits the ATP molecules and releases energy in the process.

If ATP is not available, the muscle will become stiff.

Rigor mortis is the term used to describe this stiffening of muscles in dead animals.

If sufficient ATP is present, but is not being split, the muscles relax.

This muscle relaxation is caused by alphaglycerophosphate, along with magnesium ions and the enzyme activity of myosin, which together make the ATP ions stop splitting.

A muscle will contract to a maximum of 50% of its resting length.

Motor neurons stimulate the muscle fibers to contract.

When a muscle contracts, 25% of the available energy is used for the contraction and the remaining energy is heat.

This heat is important in maintaining body temperature, when environmental temperature decreases.

There are two types of heat involved in muscle contraction.

• Initial heat – heat given off during muscle contraction; and

• Recovery heat – heat given off when muscles are

rebuilding energy.

Involuntary muscles have a variety of functions, such as:

• moving food through the digestive tract,

• aiding sperm in reaching the ovum in the uterus of the reproductive tract,

Action of Involuntary Muscles

• aiding in the birthing process,

• regulating blood flow by causing heart and blood vessels to contract, and

• forcing secretions out of the body from secretory organs.

Involuntary muscles can be smooth or striated and are spindle-shaped, with a centrally-located nuclei.

Involuntary muscle cells vary in size and can increase in number through mitosis and development of mesenchymal cells into involuntary muscle cells.

Each smooth involuntary muscle cell is surrounded by connective tissue, which is joined to other connective tissue of that involuntary muscle.

In the hollow organs, only a small fraction of the smooth involuntary muscle cells receive the nerve impulse for contraction and they spread the message.

In some cases, such as with the eye and skin, each smooth muscle cell appears to receive its own nerve stimulation.

Nerve impulses, chemicals, hormones, or electrical stimulation cause smooth muscles to contract.

This contraction of smooth muscle is usually in a wave-like motion (rhythmic).

Involuntary muscles do not generally contract from gradual stretching (Ex. Bladder).

However, sudden or extreme stretching can cause involuntary muscles to contract.

Action of Cardiac Muscles

Cardiac muscle is the striated involuntary muscle of the heart.

Heart muscle fibers interconnect to form a network with each other.

The heart beats automatically beginning in early prenatal life and continues until death, but the rate and force of the heart’s contractions are influenced by the autonomic nervous system.

Cardiac muscle requires a large blood supply and therefore contains many blood vessels, as well as, lymph vessels.

Heart problems result from an increase in size of heart muscles, which is caused by overexertion or high altitude.

Anatomy of Voluntary Muscles

Muscles can be classified based on their function and involvement in action.

Muscles involved in more than one joint may have different classifications depending on the joint involved.

Flexor – a muscle that decreases the angle between two bones when it contracts.

Example: biceps of the forearm.

Extensor – a muscle that increases the angle between two bones when it contracts.

Example: triceps of the forearm.

Agonist – muscle that produces a desired action.

Antagonist – muscle that produces the opposite of a desired reaction.

Example: If desired reaction is to bend the elbow, then the biceps muscle is the agonist and the triceps is the antagonist.

If the desired action is to straighten the elbow, then the triceps is the agonist muscle and the biceps is the antagonist muscle.

Adductors – muscles that pull a limb towards the middle of the animal’s body.

Example: pectoral muscles.

Abductors – muscles that tend to pull a limb away from the middle of the animal’s body.

Example: deltoid muscles.

Sphincter muscles – smooth or striated muscles surrounding the openings in the body.

Examples:

Smooth – muscles surrounding valve-like opening between the stomach and small intestine (pyloric sphincter).

Striated – muscles around the eyelid.

Cutaneous muscles – muscles, located just under the skin, that are responsible for movement of the skin.

Cutaneous muscles are joined to connective tissue covering the major skeletal muscles.

Muscle names are descriptive of the muscle’s action, attachments, shape, position, direction, function, kind of tissue, or a combination of these factors.

Example: transversus (movement) thoracis (position) muscle is located in the thorax of the body and moves during the breathing process.

Muscle shapes – muscle shapes can be triangular, quadrilateral, fan-shaped, long, or short.

Muscle fiber arrangements – muscle fiber arrangements can be sheet, bundle, spindle-shaped, or feather-like.

Size of voluntary muscle fibers vary.

In general, males have larger muscle fibers than females.

Animals on full feed have larger fibers than animals on restricted diets.

Exercise increases the size of muscle fibers, not the number.

An animal is born with the maximum number of muscle cells that it can have.

Voluntary muscle cells grow in size, not number.

If a voluntary muscle’s nerve supply is damaged, the muscle will atrophy because it cannot regenerate.

Muscle attachments – many voluntary muscles connect to bones by tendons, while some are attached to cartilage, ligaments, fascia (connective tissue) or skin.

One attachment is usually less moveable than the other.

Origin – the muscle attachment site that is least moveable.

Insertion – the muscle attachment site that is more moveable.

Example: the bicep muscle’s origin is the attachment to the scapula, which has less movement than the other attachment site, or insertion, at the radius.

Tendons – tissues that connect muscles to bones.

Ligaments – tissues that connect bones and/or supporting organs.

Synovial structures - connective tissue membranes that produce a thick, adhesive fluid; they function to reduce friction and allow for free movement of body parts.

Capsules, bursae, and synovial sheaths are examples of synovial structures.

Capsules – synovial structures that produce fluid for smooth operation of the joints.

Arthritis – a condition in which excess synovial fluid is produced due to inflammation of the joint, which results in pain and swelling.

Bursa – a synovial sac located between two body structures to reduce friction.

Example: bursa located just under the skin at the elbow to reduce friction between skin and ulna when the elbow moves.

Bursitis – condition that results from excess fluid production in the bursa.

Synovial Sheath – membranes with fluid (similar to bursa) that are wrapped around the tendon to provide protection.

Synovitis – inflammation of the synovial sheath.

Muscles of the Front Limb

The following are muscles of the front limb, which will be identified by function, origin and insertion, and shape (if available).

Name: Trapezius

Function:

Swings the scapula forward and backward.

Shape: Triangular, flat muscle.

Origin: Midline of the back.

Insertion:

Along the spine or raised ridge of the scapula.

Name: Rhomboid

Function:

Movement of scapula.

Shape: Heavier, diamond-shaped muscle.

Origin: Midline of the back.

Insertion:

Deeper inside the face of the scapula.

Name: Serratus Ventralis

Function:

Serves as a sling to support the body trunk between the front legs. It also swings the lower part of the scapula forward and backward.

Shape: Fan-shaped muscle.

Origin: Raised protrusions of the cervical vertebrae.

Insertion:

Inside of the top of the scapula.

Name: Brachiocephalicus

Function:

Main extensor of the shoulder; flexor of the neck.

Shape: Single-part muscle in horse; three-part in cattle, sheep, and dogs.

Origin: Occipital bones of the skull and protrusions of the cervical vertebrae.

Insertion:

Outside top part of the humerus.

Name: Latissimus Dorsi

Function:

Strong flexor of the shoulder; pulls the front leg backwards.

Shape: Wide, triangular muscle.

Origin: Protrusions on the thoracic and lumbar vertebrae.

Insertion:

Inside of the humerus.

Name: Pectoral Muscles

Function:

Primary muscles in the brisket.

Shape:

Origin: Sternum.

Insertion:

Top part of the humerus.

Name: Deltoid muscles

Function:Shape:

Origin: Scapula

Insertion:

Humerus

Name: Biceps

Function: Main flexors of the elbow. In animals with a separate radius, they tend to rotate the forearm outward.

Shape:

Origin: In front of the joint surface of the scapula.

Insertion: Top end of the radius.

Name: TricepsFunction: Main extensors of the

elbow.Shape:Origin: One part on the lower

scapula and the other two parts on the inside and outside of humerus .

Insertion:

Along the spine or raised ridge of the scapula.

Name: Extensor Carpi Radialis

Function: Main extensor of the carpus or knee joint. Most common muscle of the forearm.

Shape:

Origin: Bottom end of humerus.

Insertion:

Top end of the cannon bone.

Name: Flexor Carpi Radialis

Function:

Primary flexor of the carpus.

Shape:

Origin:

Insertion:

Name: Common Digital Extensor

Function:

Extensor of all digits in the foreleg.

Shape:

Origin:

Insertion:

Name: Deep and Superficial Digital Flexors

Function:

Main digital flexors in all animals.

Shape:

Origin:

Insertion:

Muscles of the Rear Limb

The following are muscles of the rear limb, which are identified by function, shape, origin and insertion.

Name: Hamstrings – group that includes the biceps femoris, semitendinosus, and semimembranous muscles.

Function: Main extensor of the hip; flexor of the stifle.

Shape: Divisions of these three muscles are visible as vertical grooves in thigh.

Origin: Pin bones behind hip joint.

Insertion:

Tibia or fibula.

Name: Middle Gluteus Muscle

Function:

Strong extensor of hip.

Shape:

Origin: Wing of pelvic girdle.

Insertion:

Femur.

Name: Iliacus

Function:

Important flexor of hip, along with psoas major.

Shape:

Origin: Wing of pelvic girdle.

Insertion:

Front of femur.

Name: Psoas Major

Function:

Import flexor of hip, along with iliacus.

Shape: Psoas major and psoas minor make up the tenderloin.

Origin: Extension of lumbar vertebrae.

Insertion:

Front of femur.

Name: Deep Gluteus Muscle

Function:

Abductor of the hindleg.

Shape: Run lengthwise over hip.

Origin: Front of pelvic girdle.

Insertion:

Top of femur.

Name: Gracilis Muscle

Function:

Inward pull on the hindleg.

Shape:

Origin:

Insertion:

Name: Adductor Muscle

Function:

Pulls leg toward middle of body.

Shape: Largest muscle on the inside of the thigh.

Origin:

Insertion:

Name: Quadriceps Femoris

Function:

Extension of the stifle and hock.

Shape: Four-part muscle.

Origin: Longest part from the pelvic girdle; other three parts from the femur shaft.

Insertion:

Patella.

Name: Gastrocnemius

Function:

Extensor of the hock.

Shape:

Origin: Bottom backside of femur.

Insertion:

Point of the hock by the Achilles tendon.

Name: Superficial Digital Flexor

Function:

Extensor of the hock.

Shape:

Origin: Bottom backside of femur.

Insertion:

Point of the hock by the Achilles tendon.

Name: Digital Extensor

Function:

Extensor of the hind digits.

Shape: Long; one-part muscle in horse, two-part muscle in cattle and sheep, four-part muscle in pig, dog, and cat.

Origin: Lower part of femur.

Insertion:

Third phalanx.

Name: Superficial and Deep Flexor Muscles

Function:

Flexors of the hind leg digits.

Shape: Similar to digital flexors in front leg.

Origin:

Insertion:

Muscles of the Trunk, Neck, and Head

The following muscles are associated with the trunk, neck, and head and are identified by name, function, shape, origin, and insertion.

Name: Longissimus DorsiFunction:

Extension and lateral flexion of the vertebral column; also allows slight twisting of vertebral column.

Shape: Long, loin muscle consisting of many bundles of muscle fibers.

Origin: Transverse processes of the lumbar vertebrae.

Insertion:

Transverse processes of the thoracic vertebrae.

Name: Splenius, complexus, rhomboids, dorsal oblique, and dorsal straight.

Function:

Extensor of the neck (raise the head).

Shape: Group of neck muscles.Origin: Vertebrae in the area of

the withers.Insertion:

Occipital bones of the skull.

Name: Ligamentum Nuchae

Function:

Assists in raising head.

Shape: A heavy ligament, not a muscle.

Origin: Withers.

Insertion:

Skull.

Name: Sterno-cephalicus

Function:

Assists gravity in the flexion (lowering) of the head by the neck.

Shape:

Origin: Sternum.

Insertion:

Mandible.

Name: Sterno-thyro-lyoideus, longus coli, and ventral straight muscles.

Function:

Flexors of the head.

Shape: Group of muscles.

Origin:

Insertion:

Name: Gastrocnemius

Function:

Extensor of the hock.

Shape:

Origin: Bottom backside of femur.

Insertion:

Point of the hock by the Achilles tendon.

Abdominal muscles support the digestive and reproductive organs, assist in defecation, urination, and gestation (female), and assist with regurgitation and rapid expiration of air during sneezing or coughing.

Functions specific to a muscle are listed with that muscle.

Name: External Abdominal Oblique

Function:

Forms the outside of the inguinal canal.

Shape: Fan-like layer of muscle.

Origin: Last few ribs and heavy sheet of connective tissue over lumbar area of the back.

Insertion:

Ligament at white line (linea alba) of abdominal cavity.

Name: Internal Abdominal Oblique

Function:

Forms inner wall of inguinal canal and inguinal ring preventing protrusion of intestines into scrotum; rear fibers (cremaster muscle) extend through inguinal canal attaching to outer testicle coverings .

Shape: Fan-shaped muscle layer.

Origin:Insertion:

Middle of abdominal cavity.

Name: Transversus Abdominis

Function:

Deepest abdominal muscle layer.

Shape: Fan-shaped muscle layer.

Origin: Back.

Insertion:

White line of the abdomen.

Name: Rectus Abdominis

Function:

Forms floor of abdomen.

Shape:

Origin: Cartilage of ribs and sternum.

Insertion:

Pubis of the pelvic girdle.

Name: Diaphram

Function:

Drawing air into the lungs for inspiration by pushing abdominal organs downward, which increases chest capacity and creates a vacuum.

Shape: Convex-shaped sheet of muscle that separates the chest cavity from the abdominal cavity.

Origin:Insertion:

Name: External Intercostal Muscle

Function:

Aid inspiration by pushing the ribs upward and outward, expanding thoracic cavity.

Shape: Extend between the ribs.

Origin: Ribs.Insertion:

Ribs.

Name: Internal Intercostal Muscle

Function:

Assist expiration by pushing the ribs downward and inward, decreasing the thoracic cavity and forcing air from the lungs.

Shape: Extend between the ribs.

Origin: Ribs.

Insertion:

Ribs.

ALL RIGHTS RESERVED

Reproduction or redistribution of all, or

part, of this presentation without

written permission is prohibited.

Instructional Materials Service

Texas A&M University

2588 TAMUS

College Station, Texas 77843-2588

http://www-ims.tamu.edu

2007